It is well to use salt water for the gasometer, as acetylene is but slightly soluble therein.

Lequeux-Wiesnegg Apparatus (Figs. 5, 6, and 7).—The apparatus represented in Fig. 5 is capable of being used in lecture courses. It consists of a tank, B, and a holder, A, which is provided at the top with a wide aperture closed by a hydraulic plug, F. When the apparatus is at the bottom of its travel and ready to be filled with acetylene, the plug, F, as well as the basket, D, and the bucket, E, are removed. The quantity of carbide necessary to fill the gasometer is introduced into the basket. After care has been taken to put a certain quantity of water into the gutter forming the hydraulic joint of the plug, F, the parts, E, D, F, are introduced into the tube, C, in operating rapidly enough to prevent the loss of gas. The holder immediately rises as a consequence of the production of acetylene. The gas redescends through a tube to the bottom of the tank and rises laterally in a column by serving as a guide to the holder and as a support to the cocks designed to send the gas to the points of utilization. A cock, H, placed at the lower part of the apparatus, permits of clearing the piping in case a condensation of water occurs.



The apparatus represented in Figs. 6 and 7 is continuous. It consists of an apparatus with two holders, that is to say, so arranged as to put the least liquid possible in contact with the gas produced, and to thus prevent absorptions and losses. This gasometer consists of a tank, A, of a movable holder, C, and of a stationary holder, B. The generator, E, is formed of a cylinder, at the bottom of which there is a bucket, F, designed for the reception of the greater part of the lime resulting from the reaction. It is closed by a cover, G, arranged with a simple or multiple joint, according to the precision that it is desired to obtain and that may reach 30 centimeters of water. The figure represents the holder at the bottom of its travel.

Mr. Edward N. Dickerson's Apparatus (Figs. 8 to 13).—Mr. Dickerson, of New York in June, 1895, patented several arrangements permitting of automatically regulating the production of acetylene in measure as it is consumed. In the apparatus represented in Fig. 8 the water is led from a sufficiently high reservoir, A, through the pipe, B, into the gas generator, D, and over the carbide, C, placed upon a grate, O. The acetylene forms when the water reaches the carbide, and its disengagement ceases when the pressure forces the water back. The gas passes through the intermedium of a cock, e, into the pipe, W, provided with a cock, Z, into the automatic regulator, G, and then into the gasometer, P R. Between the regulator, G, and the gasometer, Mr. Dickerson interposes an arrangement consisting of an engine, H, actuating an air pump, K, through the pressure of the gas when it is desired to introduce a mixture of acetylene and air into the gasometer. This arrangement is evidently useless when it is desired to collect the acetylene alone. The gas upon making its exit from the gasometer flows through the pipe, T, to the burners, V.



When the holder, R, is filled, the cord or chain, a, passing over the pulley, b, revolves the sector, c, until the pin, g, meets the counterpoised lever, d, of the stopcock, e. In the return of the chain, the other pin, o, carries the lever back to the position shown in the figure.

The gas generator, D, is provided with a discharge cock, E, and a charging aperture, m.

Figs. 9 to 13 show another of Mr. Dickerson's apparatus that permits of an intermittent automatic distribution either of the water upon the carbide or of the carbide in the water in regulating such distribution through the displacement of the holder of a gasometer that collects the excess of gas necessary for the consumption.



Mr. Dickerson rightly remarks that it is disadvantageous to directly control the distribution of the water upon the carbide by means of the holder of the gasometer. In fact, the water cock may remain open before the holder has moved, and there may thus fall upon the carbide an excess of water, giving rise to a production of acetylene greater than the capacity of the holder warrants.

The object of the Dickerson apparatus is to prevent such overproduction and to furnish water or carbide to the gas generator only as long as the gasometer will have been emptied of the desired quantity of gas.

Fig. 10 shows a modification of the gas generator relative to the introduction of the carbide into the water; but the same letters designate the same parts. We shall describe the operations corresponding to the figures.



1 represents the gasometer; 4, the gas generator; 11, the funnel through which the water is introduced into the generator through the pipe, 13; 12, the pipe that connects the generator with the gasometer; 5, a stopcock with counterpoise that alternately opens and closes the communication between the funnel and the generator; 10, a lever connected with the cock, 5; 2, a chain that moves with the holder and maneuvers the lever, 10.

The plug, 6, of the cock, 5, is provided with two conduits, 7 and 8, at right angles. This plug turns 90 degrees, when it is maneuvered by the chain of the gasometer. In the position shown in Fig. 13 the holder is at the top of its travel, and the counterpoise, 9, of the cock is in the position marked by dotted lines in Fig. 9.



In this case, a charge of water fills the chamber 7 and 8 of the cock. This chamber may be oblong, as shown in Fig. 12, in order to increase its capacity. On the contrary, in the position of the counterpoise, 9, marked in continuous lines in Figs. 9 and 11, the channel, 8, communicates with the pipe, 13; the charge of water of chamber, 7 and 8, has fallen upon the carbides, but another quantity of water has not been able to enter, because the revolution of the cock has cut off all communication between the funnel, 11, and the generator, 4.

The acetylene produced by the reaction of the water upon the carbide raises the gasometer holder, which then actuates the plug, 6, of the cock, 5, and allows a new charge of water to enter the chambers, 7, 8. It is only when the holder descends anew to the position, 1, that the water in the chamber, 7, 8, can fall upon the carbide. The quantity of water that the cock is capable of containing is not sufficient to produce a quantity of gas exceeding the capacity of the gasometer, and, as it is impossible to introduce another quantity of water as long as the gasometer has not been emptied anew, any overproduction of gas is thus rendered impossible.

Fig. 10 applies to the introduction of the carbide into the water. It is necessary in this case that the carbide shall have been previously reduced to powder. The funnel, 11, is then closed by a cover, 21, in order to prevent any accidental escape of the gas. The carbide falls into the generator, the bottom of which is open. The latter enters a tank into which flows a current of water, escaping through the waste pipe, 19, in carrying along the lime formed. The height of the water in the tank is sufficient to furnish the pressure necessary to allow the gas to enter the gasometer through the pipe, 12.

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DEVICE FOR THE DISPLAY OF LANTERN SLIDES.

Those who would wish to have a little extra shop window attraction by way of displaying slides for the season now at hand might do worse than resort to something of the following style. The appliance can hold any number of slides, according to the diameter of the wheel portion, but in the diagram herewith it is for holding a dozen. The slides can be changed readily, hence a little time would be expended in making a complete change at least once a day.



The relative portions of the sketch being to scale, particulars as to the making of the revolving wheel need not be entered into, as any mechanic could grasp the whole idea at a glance. The edge of the wheel should, of course, be placed facing the window, and a band on the pulley wheel, A, attached to a clockwork or electric motor would supply all the driving power necessary.

In order to get good illumination on the slides, it will be necessary to have a piece of white cardboard or opal glass, B, hung on the axle, the lower side being the heavier, so that although the wheel revolves, it will remain stationary.

Various devices may be resorted to for hanging the slides on the cross rods, but perhaps the method shown at C will prove as simple as any, and consists of small springs which grip the slide at both sides.



By the judicious arrangement of shielded lights placed at side of reflector, a pretty effect is produced as each slide is gradually brought to view.—The Optical Magic Lantern Journal and Photographic Enlarger.

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THE FECULOMETER.

The selling price of beets naturally depends upon their yield in sugar, and what gives potatoes their value is their yield in fecula or starch, a product that serves to nourish man and animals and that is also used in the manufacture of alcohol and glucose. No account, however, is taken of this important coefficient in business transactions, potatoes containing proportions of starch varying from 13 to 23 per cent. being sold at the same price. Nevertheless, it is of the greatest interest to cultivators to make such measurements, since, in order to increase the value of their product, they might thereby be led to make a judicious selection in their planting.



Mr. A. Allard, starting from the fact that the richness in starch increases along with the density, has constructed a simple apparatus that gives both these data at once, with sufficient precision, and without calculations, tables, etc. It is, upon the whole, a large areometer with constant weight and variable volume that is plunged into a cylindrical vessel 0.5 m. in depth and 0.3 m. in diameter, filled with water. The instrument itself consists of three parts: (1) A lower receptacle in which is placed a weight to assure the equilibrium; (2) a central float into which is put a kilogramme of very clean and very dry potatoes; and (3) a rod graduated for density and feculometric richness. The deeper the apparatus sinks, the more valuable is the potato. How much more?

The degree to which the rod sinks shows this. The same principle and the same instrument might be applied to the determination of the density of various agricultural products, such as beets, cider fruits, grain, etc. It would suffice to graduate a special scale each time.

For each variation of a thousandth in density, the areometer sinks about 5 millimeters—that is to say, it presents a sensitiveness that is more than sufficient in practice.—Le Monde Illustr.

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THE COMING LIGHT.

There is no more eager contest than that which has been going on for some time between gas and electricity. Which of these two systems of lighting will triumph? Will electricity suppress gas, as gas has dethroned the oil lamp? A few years ago, the answer to this question would not have been doubtful, and it seemed as if gas in such contest must play the role of the earthen pot against the iron one. At present the case is otherwise.

The Auer burner has re-established the equilibrium, and the Denayrouse burner is perhaps going to decide the fate of electricity.

As naturalists say, the function creates the organ, and it is truly interesting to observe that in measure as the need of an intenser and cheaper light grows with us, science makes it possible for us to satisfy it by giving us new systems of lighting or by improving those that we already have at our disposal.

What a cycle traversed in twenty years! What progress made! Let us remember that the electric light scarcely became industrial until the time of the Exposition (1878), and that the Auer burner obtained the freedom of the city only five or six years ago. Is there any need of recalling the advantages of these two lights? In the first, a feeble disengagement of caloric, automatic lighting and a steadier light; in the second, a better utilization of the gas, which gives more light and less heat.

A description of the Auer burner will not be expected from us. It is now so widely employed as to render a new description useless. As an offset we think that our readers will be more interested in a description of the Denayrouse burner, the industrial application of which has but just begun. This burner has been constructed in view of the best possible utilization of the gas, in approaching a complete theoretical combustion. In order that it may give its entire illuminating power, gas, as we know, must be burned in five and a half times its volume of air. In the Denayrouse burner the gas burns in four and four-tenths its volume of air. The result reached is, consequently, very appreciable.



The apparatus consists essentially of a bronze or brass box in which revolves a fan keyed upon an axle that passes through the box. The axle is revolved by means of a small electro-magnetic machine mounted upon one of the external sides of the box. The motor may also be a hydraulic or compressed air one. Upon the axle is arranged a speed regulator. The air enters at the bottom of the box and the gas at the center. The exit of the mixture takes place through a chimney arranged at the top and to which is fixed a luminous mantle. The apparatus operates as follows: The motor causes the fan to make about 1,200 revolutions a minute. There is thus formed a strong draught of air, which mixes with the gas that enters at the side. The ignition occurs at the upper aperture of the chimney.

Although in this competition of gas and electricity the intensity of the light and, its quality are important factors, it is certain that what will decide the victory will be the price. This is why we are going to establish the net cost of the different lights; for, although up to the present the contest has seemed to be limited to gas and electricity (oil and kerosene not being capable of having any other pretension than to preserve their position), a new competitor—acetylene—will perhaps soon put gas manufacturers and electricians in accord, to the great benefit of the public, by furnishing a brilliant light at a price that defies competition.



In all systems of lighting, save electricity, the unit of light is the carcel. This represents the light produced for one hour by 10 wax candles, or, better still, it is the illuminating power given by the combustion of 42 grammes of pure colza oil for one hour in what is called a carcel lamp.

In electricity we count by watts. The watt, like the kilogrammeter, of which it represents nearly a tenth, is not a unit of light, but a unit of energy. What is called a kilogrammeter is the force capable of lifting 1 kilogramme to 1 meter in height during 1 second. Further along we shall estimate the watts in carcels.

This stated, let us ascertain the net cost of the unit of light in each system of lighting. We shall take as a basis the Paris prices, which are generally higher than those of other countries, owing to taxes, and shall confine our researches to the eight following systems:

Electricity (incandescent and arc lamps), gas (butterfly, Auer and Denayrouse burners), lamp oil, kerosene and acetylene.

1. Oil Lamp.—This method of lighting has become more and more neglected because it is the most troublesome. The mean price of the kilo is 1.6 francs. As the carcel hour consumes 42 grammes, it consequently amounts to 0.06, say 6 centimes.

2. The Incandescent Lamp.—In the scale of prices one of the oldest processes of lighting is closely followed by one of the most recent—the incandescent lamp. We shall base our calculations upon the Edison 16 candle electric lamp, which is the one most widely used. In this it takes 35 watts to obtain a carcel. As the hectowatt, the mean price of which is 15 centimes, gives approximately 3 carcels, the price of the carcel will, consequently, be 5 centimes.

3. Gas.—Gas, with the butterfly burner, burns from 125 to 130 liters to furnish the carcel. As the price of a cubic meter is 30 centimes, the carcel will cost 0.39, that is to say, 4 centimes.

4. Kerosene, the decline of which is perhaps beginning, costs about 0.75 centime per kilo. The consumption per carcel is nearly 40 grammes. It amounts, therefore, to 3 centimes.

5. The arc lamp is of very varied model. We shall take as a type those used for lighting the large boulevards. They are of 8 amperes and 50 volts; that is to say, of 4 hectowatts, and are presumed to give an illuminating power of 300 carcels. The carcel is consequently obtained with 13 watts and its net cost is 0.0195, or, approximately, 2 centimes.

6. Acetylene.—This new system of lighting has hardly as yet made its exit from the laboratory. So we must not be greatly astonished at the variations in the price at which it is claimed that it can be obtained on the two sides of the Atlantic. As a kilo of carbide of calcium gives 300 liters of acetylene, and as the minimum price of the carbide is 40 centimes per kilo in France, a cubic meter of the gas costs 1.35 franc. As it requires about 7.5 liters to give the carcel, the latter will consequently amount to 0.01; say 1 centime.

7. The Denayrouse Burner.—This burns nearly 300 liters of gas to produce 30 carcels, normally. As the photometric experiments are recent, let us suppose that it gives but 25 carcels. As 300 liters of gas represent an approximate expense of 10 centimes, we shall obtain the carcel at the price of 0.004, or at less than half a centime.

8. The Auer Burner.—This burns nearly 115 liters of gas to produce 5 carcels. The expense per carcel, with the cubic meter of gas at 30 centimes, is therefore 0.0069; say 0.7 of a centime.

Finally, in the United States, thanks to particularly favorable hydraulic installations, it is claimed that it is possible to produce acetylene at a very low price, say at 33 centimes per cubic meter. Under such conditions, the carcel would cost no more than 0.0025, say of a centime. It seems, however, that these are hypotheses as yet. If they chanced to be realized, it is certain that acetylene would be the light of the future; but those who are best informed in the matter assert that they never will be realized.

In order to establish still more accurately the net cost of each of these systems of lighting, it is necessary to take into account the wear of the mantles of the incandescent lamps and the carbons of the arc ones. As regards these latter, it is customary to estimate the wear of the carbons at 8 centimeters an hour.

As for the mantles, we shall base our calculations upon the data furnished by those interested; say 1,000 hours for the Edison lamp, 1,200 for the Auer burner and 400 hours for the Denayrouse burner. It must be remarked that in practice such duration generally drops to a half. The price of the mantles in these different systems is approximately 2.5 francs.

1. As the Edison 16 candle lamp gives 1.6 carcels and its filament burns 1,000 hours, the wear will increase the price of the carcel by 0.0015.

2. As the Auer burner gives 5 carcels and its mantle burns 1,200 hours, the wear will increase the price of the carcel by 0.0004.

3. As the Denayrouse burner gives 25 parcels, and its mantle burns but 400 hours, the wear will increase the price of the carcel by 0.0002.

Finally, if we compare the butterfly, Auer and Denayrouse burners with each other, in taking into account the cost of replacing the mantles of the two latter and the actuating of the Denayrouse burner, we find the following figures per carcel hour:

Butterfly burner, consumption 0.04 /consumption 0.0069 Auer burner, wear of mantle 0.0004 /consumption 0.04 Denayrouse burner, { wear of mantle 0.0002 expense of motor 0.0003 Say 4 centimes per carcel hour Butterfly burner. 0.7 " " " Auer " 4.5 " " " Denayrouse "

For the same sum, the Auer burner, therefore, burns six times more and the Denayrouse nine times more than the butterfly. These figures may give an idea of the surprising intensity of the Denayrouse light.

Upon the whole, if the experiments that are being made publicly at this moment confirm the data of the laboratory, the Denayrouse burner will be destined to play a considerable role in the lighting of public gardens, streets and buildings, for the very intensity of the light that it gives renders it unfitted for private use. Moreover, it must not be forgotten that it requires a motor to actuate its fan, and everyone has not the necessary motive power in his house.

This new burner will likewise prove very valuable for the righting of theaters.—L'Illustration.

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AN AIR BATH.

By J.H. COSTE.

This has been found useful for drying substances at temperatures above 100 C. It is usually difficult to obtain a temperature much above, say, 120 in the ordinary air oven without using a large burner, which is generally difficult to regulate. The temperature also varies considerably at different heights in the oven. If the substance is attacked by air at high temperatures or gives off other substances than water, an estimation of the water is difficult.



The apparatus figured—which is made from a square "tin" or copper box, with a lid perforated at the top to take a thermometer (T), the bulb of which is level with the tubes (A and B) passing through the sides of the box—is heated by an Argand burner and supported on a retort stand. Dry air (or other gas) passes through the tube, B, where it undergoes a preliminary heating, and then through the drying tube, A. The substance to be dried is placed in a porcelain boat, or in a tube passing through the cork of A (by the latter means precipitates on filter tubes can be dried). It is usually sufficient to estimate the loss in weight of the substance in the boat; but, if necessary, drying tubes can be used to collect the water, or special absorbing apparatus for other volatile substances.

A temperature of over 200 C. can be easily obtained with an ordinary Argand flame and maintained fairly constant. When a thermometer was placed inside as well as one outside the drying tube, it was found that the temperatures only differed by a few degrees when a water pump was drawing air through the system at the rate of about 8 liters per hour. If this bath is protected from draught, any temperature can be maintained within a few degrees easily.—Journal of the Society of Chemical Industry.

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FIREDAMP TESTING STATION AT MARCHIENNE-AU-PONT.[1]

[Footnote 1: H. Schmerber, Genie Civil, xxix, No. 11.—From the Colliery Guardian.]

In a previous paper[2] a description was given of the experimental gallery at the St. William pit of the Kaiser-Ferdinands-Nordbahn Colliery at Mahrisch-Ostrau (Moravia). In the present article a similar experimental station, designed for the same purpose, but presenting certain considerable advantages on the score of economy by reason of the moderate expense of its installation, will be described.

[Footnote 2: Reproduced in the Colliery Guardian, vol. lxxi, p. 317.]

Some few years ago the Socit des Explosifs Favier obtained permission from the proprietors of the Marchienne-au-Pont, near Charleroi, Belgium, to construct there an experimental station for testing the explosives manufactured by the company. Though of but modest proportions, this station is well designed, and many valuable researches and tests have been made on the explosives used in the fiery pits of Belgium, thanks to which investigations one is able to readily determine in a practical manner the degree of security offered by any explosive intended for use in pits containing coal-dust in suspension or firedamp.

In order to avoid the expense of constructing a large gallery above ground, recourse was had to the cylindrical shell of a disused boiler of large dimensions—some 5 m. in length by 1 m. internal diameter—one end of which was taken out, and the shell made to do duty for a testing gallery. With this object it was mounted on two settings of brickwork (Fig. 2), and the further end backed by a brick wall of very substantial construction, being 1 m. thick and 2 m. in height, and forming the base of a high bank of earth. The boiler, as may be seen in Figs. 1 and 2, was let into the ground a little, in order that in case of an explosion there might be less chance of the debris being projected to a distance. On one side the boiler was pierced by six rectangular openings 20 cm. in height fitted with thick glass panes in caoutchouc frames, to prevent their becoming fractured by the aerial vibrations resulting from explosions. These windows enable the operators to observe the phenomena occurring within the chamber at the moment the explosion is produced. At the top of the boiler, two circular apertures, each 50 cm. diameter, were made for the purpose of acting as safety valves. By means of two rabbets, one fixed at the open end of the gallery and the other in the center, the testing chamber could be made either large or small by means of paper disks pasted on to the first or second rabbet. The capacity of the large chamber was double that of the smaller one, and the cubical area of each was known beforehand.



In the backing wall was fitted a large mortar of cast steel, which in carrying out the tests served to replace the borehole used in actual mining operations. A pipe for conveying the gas and another for steam were laid on the floor of the chamber, the latter for heating purposes, in order to ascertain whether, in certain cases, an increase in temperature exerts any sensible influence on the inflammability of the explosive mixture. The temperature of the chamber is read off from a thermometer placed at the top of the boiler, its position being indicated by T in Fig. 2.

In view of the possibility of the boiler, notwithstanding its strength, bursting, in the event of a violent explosion of the gas, it became necessary to make special arrangements for allowing the operators to observe everything occurring in the testing chamber without being themselves exposed to the consequences of any accident that might ensue. A special shelter was, therefore, erected for occupation by the operators at the moment of the explosion. This shelter, at about a dozen yards away from the boiler, consisted of a chamber protected on the side next the gallery by a stout bank of earth, in which a longitudinal aperture was provided (by means of a lining of boards) at about the height of the face, through which the operators could observe the progress of the tests, without danger. It may be stated, however, that hitherto no accident has occurred, the boiler effectually resisting the force of the explosions. The chamber of shelter likewise contained the gasometer for regulating the supply of gas to the testing apparatus, and the electrical machine for firing the cartridges under test.

There being no continuous current of firedamp at disposal, use was made of illuminating gas in preparing the explosive mixtures for the tests. The borehole is charged with the explosive to be fired, and the temperature is regulated by means of the steam pipe. The entrance of the chamber and the two safety apertures in the roof having been closed by disks of paper fastened by paste, the gas is turned on until the desired percentage, has been introduced; the mixture of the air and gas takes merely a short time to effect by diffusion, the difference in density causing the gas to rise on issuing from the jet, which is on the floor of the chamber. The detonating cap is then ignited by the passage of the electric current and the shot fired. The operator, placed in his shelter, can observe, by means of the small lateral windows, whether any flame is produced, and indeed, a little experience will enable him to determine by the sound alone, whether an explosion has ignited the mixture or not.

Fig. 1 is a front view of the testing chamber with transverse section of the shelter. Fig. 2 is a longitudinal section of the chamber along CD, and Fig. 3 a view, half in plan, half in section, along AB. The following are the references: M, backing wall; C, boiler; G, gas pipe; V, steam pipe; M, mortar; E, electric wires; A, shelter; RG, gasometer; ME, electrical machine; R', protective bank; R", backing of earth; R, glazed windows; S, apertures serving as valves; T, thermometer.

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PHOTOGRAPHY FOR CHEMISTS.

LANTERN SLIDES BY REDUCTION.

When a negative happens to be of larger size than a quarter plate, it rarely happens that we can print a small portion by contact on a lantern plate without spoiling the composition of the picture. This is assuming, of course, that the operator has composed a picture and not put his camera down anywhere. There is no great difficulty in making lantern slides by reduction; the exposure is the only bugbear, as usual.

There are two distinct methods of reduction: (1) daylight; (2) artificial light. There is nothing to choose between them, and the question of time and opportunity must decide which is to be adopted. The apparatus required is not expensive. It can be made in odd moments for a few pence, and is applicable to day and artificial light. It consists of a printing frame the size of the large negative, four pieces of bamboo a quarter of an inch in diameter, some black twill, the ordinary camera and lens, and a carrier to take lantern plates 3 X 3 inches.

The negative is placed in the printing frame upside down and kept in position by four little slips of wood, or better still, a frame such as the gold slip used in picture frames, which will fit tightly into the frame and hold the negative securely. Of course, brads may be driven into two sides of the frame and the negative slipped behind them, but in this case it is necessary to safe edge the negative. This is done by cutting strips of tinfoil just wide enough to cover the rabbet of the negative so that no clear glass can be seen; these should be pasted and stuck on the glass of negative round the four sides. The strips of bamboo are either nailed to the printing frame or merely fastened together by stout copper wire, the shape being exactly that of the printing frame. The other end of the bamboos are tied with stout string to a piece of cardboard tube, postal tube, which slips over the lens. The length of the bamboos depends upon the focus of the lens and the amount of reduction. It will sometimes be found convenient to have the bamboo in two lengths; thus, supposing we want as a general rule 36 inches, two pieces, 24 inches each, should be obtained, and by fastening these together in the middle by two loose rings of copper wire we can extend them to 48 inches or reduce them to 24 inches.

The black twill or the focusing cloth (or even a dark table cloth may be used) must also depend for its size on the length of bamboo, but sufficient should be obtained to completely cover over the space between lens and negative, and hang down on each side.

Of course, two laths of wood can be used, merely resting them on the top of printing frame and camera, but the other plan is preferable, the arrangement being more complete and adaptable to both day and artificial light, and also more rigid, especially when the camera is sloped toward the sky.

The ordinary camera may be used, but a carrier to take lantern plates must be used in the dark slide. The ordinary lens may be used unless of inordinately long focus, when it becomes inconvenient on account of the great distance between negative and lens. To find the required distance there is a simple rule, which is as follows:

(a) Divide the longer base of the plate by the longer base of the image required, to the quotient add 1, and multiply by the focus of lens used; the result will be the distance between negative and lens.

(b) Divide the distance found as above by the quotient obtained in the first rule, and the result will be the distance between lens and plate.

Example.—What are the relative distances in reducing a whole plate negative, 8 X 6 inches, to lantern, size with an 8 inch focus lens?

Now that the whole of the lantern plate is not used, we reckon that 3 inches is all that can be used, because of the mask, hence:

(a) 8 3 = 17/6 = the amount of reduction. 17/6 + 1 8 = 23/6 8 = 30-2/3 inches. (b) 30-2/3 17/6 = 11 inches (practically).

Therefore, if we place our lens about 30 inches from the negative and rack the camera out to about 11 inches, we shall have an image on the ground glass which merely requires a little adjustment of the camera screw to be sharp and of the right size. In focusing, it is always advisable to temporarily affix to the outside of the focusing screen a square mark, this being, of course, accurately placed as regards the center of the screen, and to use a focusing magnifier to obtain critical sharpness.

Having satisfactorily arranged our image as regards composition by shifting the camera nearer to or farther from the negative—because it will be obvious that the nearer the lens to the negative, the less of the negative we shall include, and vice versa—we fill our dark slide and are ready for exposure.

For daylight work the arrangement of frame and camera should be placed near a window, and if anything but sky is seen opposite the negative, place outside the window a large sheet of white cardboard at an angle of 45. This will reflect equal skylight through all parts of the negative. Now cover over the space between negative and lens, insert your dark slide, in front of the negative place an opaque card, draw the shutter of the dark slide, and remove the opaque card from negative and expose.

Very little assistance can really be given as to exposure, but with a negative of average density, which will give a good silver print, and using a lens working at F/11 and a Mawson lantern plate at midday in May, ten seconds will give a good black slide.

There is but one little point that has been missed—the diaphragm; always use the largest diaphragm which will give satisfactory definition, this will usually be F/11 or F/16.

Be very careful while exposing not to shake the camera—it is quite sufficient for anyone weighing about eleven or twelve stones to walk across the room to give double outlines.

Daylight is not a constant quantity, and although visually the same on two different days, the actinic power of the light varies enormously; therefore we prefer artificial light.

Precisely the same apparatus can be used for artificial light with one or two additions. In some such arrangement in use the printing frame containing the negative is fastened to the side of a cube sugar box in which a hole is cut.

Opposite to the negative on the other side of the box is placed a sheet of white cardboard bent slightly to the arc of a circle. The lights, etc.—two incandescent gas burners do well with tin reflectors behind them—are placed one on each side of the negative inside the box, so that the light is reflected on to the card and thence on to the negative, and no direct light reaches the negative. Absolutely even illumination, even of a large negative, is thus obtained, and the exposure, using the same conditions as stated for daylight, is only twenty seconds.

Of course, the light may be placed directly behind the negative, but in this case a diffuser, such as a sheet of opal glass, must be placed between light and negative, and even then, unless great care is exercised, uneven illumination of the negative and consequent unequal density of the slide must ensue.

We may use magnesium ribbon, and a diffuser of opal is then necessary, and the ribbon must be kept in motion the whole of the time. Magnesium is objectionable because the particles of magnesia form a voluminous cloud, which tastes and smells unpleasantly and settles down on everything. Still, for those who wish to work with this substance, about 18 inches burnt close to the opal and moved about all over it will be about sufficient to obtain good results under above mentioned conditions. An ordinary oil lamp or gas may also be used, provided the light is diffused.

Only the bromide lantern plates are suitable for reduction, the exposure, especially with the chloride emulsions, being so long as to place them out of court. The chloro-bromide may be used for daylight and magnesium ribbon.

After development and fixing, which may be performed in the developers recommended by the makers of the plates used, the lantern slide must be well washed and cleared in an alum and acid bath, then again well washed and finally given a gentle rub with a piece of cotton wool under the tap, and set up to dry.

The finishing off of a slide is not a difficult matter, but one which wants doing properly. Place the slide film downward upon a piece of white paper, and with a box of assorted masks try various shapes till the one most suitable to the picture is found, and frequently a mask with a comparatively small opening will give the best results pictorially. Having found the most suitable mask, lay it on the slide, on the top of this a cover glass well cleaned, and it is ready for binding. Binding strips can be purchased commercially in long strips, but personally we prefer to use 3 strips, as somewhat easier to apply. Wet 3 in. of the strip, lay it flat on the table, pick up the slide and cover glass and adjust on the wetted slip so that there is an equal width on either side; now press the glasses firmly on to the strip and lift from the table and with a handkerchief or soft duster wipe the strip on to the glass of the slide and cover, taking care that these do not slip; when it adheres firmly, that is, does not immediately rise up, lay the whole on one side and go on with next slide; by the time half a dozen have been thus treated a second side may be stuck down, and thus with the third and fourth. By working in this way a far neater and safer job is made of it than if all four sides are bound at once.

The final operation is tilting and spotting. There are several makes of masks on the market on which a blank white space is left for the title, and it is just as well to write the title on the mask, as it is then protected by the cover glass. If the ordinary masks are used, Chinese white may be used for the titles.

"Spotting" the slides is affixing to them two marks, by means of which the lantern operator can tell which side is to be placed next the lantern, and these marks usually take the form of two white circles. Such "spots" can be bought commercially already gummed, or postage stamp edging may be used.

A few minutes' thought will show that the projecting lens of the lantern will reverse an image just as the lens of the camera does, so that we must insert the slide into the lantern carrier upside down and wrong way round, and as the spots are used to indicate this, they must be placed at the top of the slide, when the view appears to us as we saw it in nature. If it be a subject with lettering in it, the spots must be placed at the top of the slide, when we can read the lettering the right way as the slide is looked at against a piece of white paper.

* * * * *



PRECIOUS STONES.[1]

[Footnote 1: Lecture delivered before the Society of Arts, from the Journal of the Society.]

By Prof. HENRY A. MIERS, M.A., F.R.S.

LECTURE I.

The object which I have proposed to myself in these two lectures is to consider, not the history nor the artistic interest of precious stones, but simply some of their curious properties. In the first place, then, I will ask you to accompany me in the inquiry as to those characters of precious stones to which they owe their beauty and their value, and next to pursue the inquiry a little farther and to see how, by means of these characters, the same stones may be studied, and hence, also, identified with accuracy.

From the earliest times certain minerals, which are conspicuous for their beauty, have been prized for decorative purposes; the brilliant green hue of malachite, the deep blue of lapis lazuli and the rich color of red jasper would naturally attract early attention. But these particular minerals are not numbered among the true precious stones; they do not possess the remarkable qualities which endow the diamond, the ruby or the topaz with their peculiar attractiveness. The two essential qualities, namely, brilliancy and hardness, are only possessed by certain rare minerals; a brilliancy which makes them unrivaled for ornamental purposes and a hardness which protects them from wear and tear and makes them practically indestructible.

It is difficult in a town like London, where every jeweler's shop is ablaze with diamonds, to realize that large and good stones possessing these qualities are so rare; that thousands of natives are toiling in the river beds of India, Burma and Ceylon washing out from the gravel or the sand the little blue and red pebbles which are to be converted by the lapidary's art into brilliant jewels of sapphire and ruby. Even in that wonderful pit at Kimberley, where half the diamonds of the world seem to have been crowded together for the use of man, although, perhaps, ten tons of diamonds, worth more than 50,000,000, have been extracted in twenty-five years, yet those which weigh more than an ounce each may be counted on the fingers.

It is in the qualities of hardness and brilliancy that such minerals as malachite and lapis lazuli fail; owing to their comparative softness, they would not, if cut and polished, possess the sharp edges and brilliant surface of the emerald or sapphire, and would soon become dull and rounded by friction, even by the friction of ordinary dust. Again, since they are opaque, they can never flash like the sapphire or the emerald; and yet it is quite a mistake to suppose that the necessary qualities are confined to those few stones which are familiar to everyone, such as the diamond, ruby, sapphire, emerald, garnet and amethyst. There are many others, though they are not so well known. I think we may fairly assert that such minerals as tourmaline, jargoon, peridote, spinel and chrysoberyl, though their names may be familiar, are not stones which would be recognized by any but those who are in some sense experts; while other minerals, such as sphene, andalusite, axinite, idocrase and diopside, are possibly almost unknown to most people, even by reputation. Yet all these minerals possess qualities of transparency, hardness and beauty of color which render them extraordinarily interesting and attractive as precious stones. (A number of faceted stones cut from the less known minerals were thrown upon the screen by reflected light.)

Take first the hardness. A few years ago the hardness of stones was a very important character in the eyes of the mineralogist; it was one of the characters by which they were invariably identified, and a distinguished German mineralogist drew up a table by means of which the hardness of minerals can be compared. Any stone is said to be harder than the minerals of this scale which it can scratch, and softer than those by which it can be scratched. In the right hand column the gem stones are arranged according to their hardness.

MOHS' SCALE OF HARDNESS.

1. Talc. 2. Gypsum. 3. Calcite. 4. Fluor. 5. Apatite. / Sphene. Opal. 6. Feldspar. / Diopside. Moonstone. / Epidote. Idocrase. Peridote. Axinite. 7. Quartz. / Quartz. Tourmaline. Cordierite. Garnet. / Andalusite. Zircon. Emerald. Phenacite. 8. Topaz. / Spinel. Topaz. Chrysoberyl. 9. Corundum. / Ruby. Sapphire. 10. Diamond. Diamond.

Among precious stones diamond stands out pre-eminent as the hardest of all known substances. Ruby and sapphire are scratched by diamond alone, while chrysoberyl, topaz and spinel scratch all the remaining stones, although they do themselves yield to the scratch of ruby and sapphire. The hardness is a character still generally utilized by the expert when he is in doubt; in experienced hands it has some value. By long practice it is possible to form a very close estimate of the hardness of a given stone, and that often, not by the scratch of the other minerals in the scale, but by the feel of the stone against a file; the resistance offered by the stone to the file is taken as a measure of its hardness. It is not a character capable of any accurate measurement, neither is it to be recommended for use by inexperienced persons.

I hope to show, as I go on, that we have now accurate methods of testing at our disposal which render the trial of hardness quite unnecessary. But, none the less, the character is one of great importance, as investing the stone with durability. All the precious stones, except moonstone, opal and sphene, have at least the hardness of quartz, and can barely be scratched by metals, even by hard steel.



Take next the quality of brilliancy. This depends upon two things—first, the manner in which rays of light are affected when they enter or leave the stone, and, secondly, the manner in which this action can be intensified by the art of the lapidary.

When light passes from one transparent substance to another it is bent or refracted, as every one knows from the bent appearance of a stick plunged into water. Consider, now, a ray of light falling upon the surface of a transparent stone; a portion of the light is reflected, but a portion enters the stone. In passing from air into the stone it is refracted inward. When, on the other hand, it passes from a transparent stone into air, its course is reversed and the emerging ray is refracted outward or toward the surface. It is, however, with the emerging as with the entering light, the beam is subdivided, only a portion is refracted out, another portion of the light is reflected within the stone.

Consider next successive rays within a piece of glass or a stone which are about to emerge with different inclinations. (See Fig. 1.) As their course approaches more nearly to the surface, so will the emerging rays issue more nearly along the surface of the stone; but the obliquity of the emerging rays increases much more rapidly than that of the internal rays, until for one ray in the series the direction of the light (C in the figure) refracted out coincides with that surface. What, then, will happen to the light within the stone, which falls still more obliquely? It cannot be refracted out, and, as a fact, it is entirely reflected within the stone. Imagine, then, how much greater is the brilliancy of the beam of light, c, e, d, which is completely reflected, than that of the intermediate portion of the reflected light, a, b, c, which has lost a large part of its rays by refraction. The difference is easily seen by looking at a glass of water held above the head; the brilliant silvery appearance of the surface, when viewed obliquely, is due to total reflection. The light, c, d, e, is said to have been totally reflected; and half the angle between C and c is called the "angle of total reflection." This angle depends upon the refractive power of the stone. The angle of total reflection for diamond is about 25; in no other stone is the corresponding angle less than 30; for most of them it is much greater; while for heavy glass it is about 40. Light striking the internal surface more obliquely is reflected without losing any of its rays by refraction.



It is very clear, then, that of the light traveling in directions within a diamond, a far larger proportion is internally reflected than is the case with any other stone. We shall see presently that it is this property which gives the diamond its consummate brilliancy.

Another effect produced by refraction is, as every one knows, the separation of ordinary light into rays of different colors—it is seen in any prism of glass. This property is known as the "dispersion" of light; and a stone which possesses great dispersion will exhibit a beautiful play of spectral colors—will exhibit a high degree of what is called fire. In this respect again the diamond is pre-eminent; its dispersion is nearly twice as great as that of other stones.

All these optical properties are beautifully shown by those unworked jewels of which the smooth facets have been produced by nature; I mean the crystals of the various minerals. The beauty of natural crystals of transparent minerals is largely due to the optical effects which I have just been describing.

The beautiful specimens of rock crystal, calc spar, topaz, emerald, and other stones which adorn mineral collections are sufficient evidence of these properties. But it is very certain that natural crystals, although they possess a beauty of form which is all their own, are not by a long way so brilliant as the faceted stones which are cut from them by the art of the lapidary; that a natural diamond is not so lustrous as a faceted brilliant.

In fact, many of the finest gem stones present a very mean and sordid aspect before they have passed through the hands of the lapidary; one has only to compare the dull and unattractive appearance of a parcel of rough rubies, sapphires or rough diamonds with the finished jewels displayed in the jewelers' windows to see how much these owe to the lapidary's art.

In recutting the Koh-i-noor it was thought advisable to spend 8,000 on the process and to reduce its weight from 186 to 106 carats. When the great Pitt diamond was cut, its weight was reduced from 410 carats to 137; and the fragments and dust removed were valued at 8,000; but the extent to which the stone was improved is indicated in the fact that having been purchased for 20,000, it was after cutting sold for 135,000.

To understand how the cutting of a precious stone adds to its brilliancy, we have only to trace the course of the rays within the stone, and consider how it can best be faceted in order that the light which enters in various directions on the upper side, or crown, may be reflected internally from facet to facet on the under side of the stone with as little loss as possible, and may be thrown out from the front of the stone. For this purpose the facets must be so arranged that as much of the light as possible within the crystal shall meet the facets at an inclination exceeding the angle of total reflection. A brilliant with its 58 facets is one of the forms which experience has shown to be best adapted for the purpose. How little of the light gets through a stone so faceted, and, therefore, how much of it is totally reflected internally, is easily shown by holding the stone in a strong beam of light; first so that the light is so reflected, and then so that the light shall, if possible, be transmitted. In the latter case, the stone merely throws a dark shadow, indicating that little light, if any, has passed through it.

A faceted stone is always cut from a single crystal, and not from an ordinary lump of the mineral, which is generally a mass of crystals. The chief reason why jewels are cut from natural crystals is that these, by virtue of their crystalline nature, are remarkably homogeneous, and therefore clear and limpid when free from cracks and flaws. A stone which is not homogeneous can never have the purity and limpid brilliancy of a single crystal, for at every point of contact of one part with another reflection takes place. Among minerals used as precious stones which are not crystals may be mentioned the opal. The opal probably owes its peculiar beauty to the very fact that it is filled with minute cracks or cavities, each of which contributes some tint of color by reason of its extreme thinness, just as the colors of the soap bubble are due to the thinness of its film.

Or take the agate. Here the stone consists of layers of different materials differently colored. Its beauty is of a different nature from that of clear crystals, which it can never rival in brilliancy. Stones like the agate are generally classed apart as semi-precious stones, and their interest depends upon beauty of structure or color, or possibly to a large extent upon their rarity. The turquois, for example, is a very rare stone, which is apparently absolutely uncrystallized, but possesses great beauty of color, and is therefore much prized. The same is true of carnelian. On the present occasion we are not concerned with those opaque or curiously structured minerals whose beauty resides almost solely in their color.

Those who have had no practical acquaintance with minerals have little idea how variable and accidental are their colors. They may scarcely realize that the ruby and the sapphire are the same mineral, and that this mineral also occurs, and is used in jewelry, absolutely colorless, when it is known as lux sapphire, green as the so-called Oriental emerald, and yellow as the so-called Oriental topaz; that topaz itself may be yellow, brown, blue, or colorless; that zircons range from colorless through almost all conceivable shades of brown and green, and that even diamond has been found green, red and blue.

When we come to consider the properties by which precious stones are recognized, I shall say little or nothing about color, for it is of little value as a criterion. There are, for example, certain red stones which the most skillful experts cannot by their color alone refer with certainty to ruby, garnet or spinel. It might be expected that a noteworthy difference in chemical composition would accompany this difference of color, or that the pigment could be ascertained by analysis. In reality this is scarcely ever the case. It is fairly certain that the emerald owes its color to the presence of chromium, but the variation in the analyses of precious stones cannot generally be attributed to anything indicated by the variation of color.

The chemical composition, though of great general importance in mineralogy, is of little practical value in the discrimination of precious stones, since it is usually impossible to sacrifice a sufficient quantity for chemical analysis. If we are dealing with a faceted stone, not even the smallest portion can be utilized, for fear of injuring it.

There is, however, one remarkable optical property, which is ultimately related to the chemical composition. As is well known, many substances possess the property of absorbing certain rays of light. When the solar spectrum produced by admitting ordinary daylight through a slit, and transmitting it through a prism, is passed through the glowing vapor of certain substances, particular rays of light are absorbed, and their absence from the emerging fight is manifested by corresponding dark bands in the spectrum. The instrument by which the observations are made is the spectroscope. It is well known to most people that the solar spectrum itself contains certain dark bands of this sort, which are produced by vapors that can be identified by the position of the bands in the spectrum; and thus it is possible to ascertain something regarding the chemical constitution of the sun and certain of the heavenly bodies. Now, a precisely similar effect is produced by certain elements if present in a mineral, by merely transmitting the light through a piece of it. Thus, transparent minerals which contain the rare element didymium betray the presence of that element as soon as they are viewed through a spectroscope by ordinary daylight; the spectrum is seen to be traversed by black bands in the green, which are quite characteristic.

Among gem stones there are two which possess this curious property. One is the variety of red garnet known as almandine, and the other is the jargoon. The almandine produces characteristic bands in the green and the jargoon in the red, green and blue portion of the spectrum. To see these remarkable absorption spectra, to which attention was first called, I think, by my friend, Prof. Church, it is not necessary to look through the stone, it is quite sufficient to place it in a strong light, and look at it through an ordinary pocket spectroscope; the light which enters the instrument consists largely of rays which have penetrated the stone, and been reflected from the facets at the back. These rays produce the absorption spectrum. In this way we are enabled to identify a jargoon or an almandine merely by looking at it. There is no test so simple or so easy of application. It is curious that the almandine, or iron aluminum garnet, is the only garnet which presents an absorptive spectrum, and it is not yet certain to what element the bands are due. In the case of jargoon, they are supposed to be caused by the presence of some uranium compound in the mineral. All the almandine garnets which I have examined, and nearly all the jargoons, show these characteristic absorption spectra.



By way of summary, I have thought it desirable to indicate the general characters of precious stones in a diagram, which exhibits some of their relationships and also some of their differences in a graphic manner.

Opal, which is a comparatively light mineral, has a low refractive power; zircon or jargoon is a heavy mineral, and has a high refractive power. Let now the refractive power of any mineral (as measured by its refractive index for yellow light) be represented by a corresponding length set off from left to right, and let its density (as measured by its specific gravity) be represented by a corresponding length measured downward. Fixing in this way a point corresponding to opal, and another representing the character of zircon, draw a straight line from the one to the other. It will then be found that the points which, by their position on the diagram, represent the specific gravity and refractive index of the various minerals will be very nearly upon this line; that is to say, as the refractive index of precious stones increases, so also does their density, and the two increase together in a remarkably regular manner.

It appears from this table that those minerals which, by their high refractive power, possess the greatest brilliancy, possess also the highest specific gravity or weightiness; that the precious stones are therefore all heavy minerals. There is also a rough general correspondence between these characters and the hardness of the stones; the brilliant heavy minerals are also generally speaking hard.

Two remarkable exceptions display themselves. Sphene lies far to the right of the position which it should occupy according to its specific gravity; it possesses an extraordinarily high refractive index, and is, therefore, an extremely brilliant gem stone. On the other hand, a glance at the scale of hardness shows that it is, unfortunately, one of the softest of the possible gem stones, and that in this respect it is not very well fitted for jewelry.

Diamond is still more remarkable; its refractive index places it at the extreme right of the diagram, with a refractive power, and therefore a brilliancy, greater than that of any other stone; at the same time its hardness exceeds that of any mineral, and this combination of qualities renders it the chief among gem stones, unequaled for brilliancy and durability, although not a heavy mineral. Moreover, in dispersion, and therefore in fire, it stands alone. Minerals which are heavier than zircon, such as the metallic sulphides and iron glance, are unsuitable for gem stones, since they are nearly opaque, but they follow the same law, and possess a refractive power still greater than that of zircon or even diamond.

There is one other stone which is exceptional, but in less degree and in the other direction, namely, topaz, whose refractive index is not 1.7, as it should be by its position on the line due to the specific gravity, but 1.62; the point corresponding to topaz must therefore be placed a short distance to the left of the line. It is curious that these three exceptional stones lie on the same horizonal line, having all the same specific gravity, 3.5.

In mentioning the specific gravity I have introduced a property which is not essential to win esteem for a precious stone, but one which is of great value in its identification.

We have next then to consider those properties by which precious stones may in practice be most readily recognized. The table shows very clearly that specific gravity is one such property. The meaning of specific gravity is easily explained. A piece of tourmaline of any size weighs three times as much as an equal volume of pure water at 4 C., the specific gravity of tourmaline is therefore said to be 3; a piece of almandine garnet of any size weighs four times as much as an equal volume of water under the same conditions, and the specific gravity of garnet is therefore 4.

Now any substance immersed in water loses in weight by an amount exactly equal to that of the water displaced. Hence, to ascertain the specific gravity it is only necessary to suspend the stone by a fine thread to the beam of a balance and weigh it first in air, and then immersed in water. The first weighing gives the weight of the stone itself, the difference between the first weighing and the second gives the weight of the displaced water; hence the specific gravity is found at once by dividing the weight of the stone by this difference. For very small stones, where the weights concerned are slight, it is necessary to use a refined chemical balance. But for ordinary stones a well made Westphal balance is sufficient.

The Westphal balance is constructed on the principle of the common steelyard. At one end of the beam is a counterweight, at the other end the stone is suspended; the beam is divided into ten equal parts. A weight can be suspended on the beam, and its action, of course, varies with its position on the beam; at the tenth division from the center it has a value ten times as great as at the first division.

The specific gravity is then found as follows: First, counterpoise the counterweight. Let this require a weight, A, on the right hand side of the beam. Next, find the weight necessary to restore equilibrium when the stone is suspended from the beam. Let this be B. Then A-B is the weight of the stone in air. Next raise the vessel of distilled water below the stone until it is immersed. If C be the weight now required to restore equilibrium, C-B is the loss of weight in water, and, finally, the specific gravity is (A-B)/(C-B).

This process is known as "hydrostatic weighing," and can be applied to any stone, except such as are very small. Great precautions must be taken, in order to determine the specific gravity with accuracy. Especially it is necessary to free the stone from all adhering bubbles of air. For this reason the process of hydrostatic weighing is a somewhat laborious one.

Now, in order to identify a mineral, it ought to be unnecessary to determine exactly the specific gravity, provided that means can be devised for showing that its specific gravity is the same as that of some known substance. For purposes of identification, a comparative method is often quite as efficacious, and much more easy than actual measurement. This may now be done by means of certain heavy liquids.

Wood floats in water because it is lighter than water; iron sinks because it is heavier; but a substance which possessed exactly the specific gravity of water would neither float nor sink, but would remain suspended in the water like a balloon in midair. Taken, then, a liquid which is heavy—the most convenient is methylene iodide, whose specific gravity is 3.3—a fragment of zircon will sink in this, and a fragment of tourmaline will float, but a fragment of the mineral augite, whose specific gravity is also 3.3, will remain exactly suspended.

This liquid, then, enables one to say with certainty whether a given stone has a specific gravity greater or less than 3.3; in the one case it will sink, in the other it will float.

But methylene iodide further possesses the valuable property of mixing easily with benzene, which is a very light liquid. Every drop of benzene added reduces the specific gravity of the mixture, which can thus easily be made to range between that of chrysolite and that of opal.

To identify any one of the stones which lie between those limits on the diagram, it is only necessary to drop it into a test tube or small vessel containing methylene iodide—the stone will float—benzene is added drop by drop, the mixture being kept well stirred until a point is reached at which the stone neither sinks nor floats. Then different fragments of mineral possessing specific gravities between 3.3 and 2.5 are taken in order of increasing density and dropped into the liquid; the stone under examination possesses a specific gravity between that of the last which floated and the first which sinks, and the limits may, if necessary, be further narrowed by comparing it with other mineral fragments of known density intermediate between those two. One great advantage of this method is that the size of the fragment does not affect the result; a minute fragment only just large enough to be visible is equally convenient; in fact, more convenient than a larger one.

If a stone in the rough is under examination, a minute chip can easily be taken from it, and used for the experiment in the most satisfactory manner. The method is, moreover, extremely sensitive; a mere drop of benzene added to a considerable volume of the liquid is sufficient to send to the bottom a stone which was previously floating.

So much for stones whose density is less than that of chrysolite. As regards the denser minerals, it was until a short time back impossible to test them by any such method; they all sank in the heaviest liquid available. But now, thanks to the fortunate discovery by Dr. Retgers of the remarkable properties of thallium silver nitrate, all the known gem stones may be distinguished by a similar process.

This salt, which may be prepared by fusing together in equal molecular proportions nitrate of silver and nitrate of thallium, possesses the remarkable property of fusing at a temperature far below that of either of its constituents, and well below that of boiling water, while at the same time the fused salt possesses a specific gravity greater than that of zircon. The salt fuses at 75 C. to a clear colorless liquid in which zircon just floats; it further possesses the useful property of being miscible in all proportions with water, so that the specific gravity can be reduced to any desired extent by adding water, just as that of methylene iodide, was reduced by adding benzene. The substance can be kept liquid by maintaining it at a temperature above 75 C., and this may easily be done by immersing the vessel in which it is contained in water heated to near the boiling point.

In these two liquids then we have the means of producing a liquid of any required density for the discrimination of gem stones, since we can obtain from one or the other a liquid in which any precious stone will be exactly suspended.

The nitrate might be used by itself to include the whole series, but it is more convenient to use the methylene iodide when possible, both because it can be employed at ordinary temperatures and because it is cheaper than the nitrate.

Both substances darken on exposure to light, and should be both kept and used in the dark as far as possible: they are easily freed from the liquid employed to dilute them. The benzene readily evaporates spontaneously from the methylene iodide, and the water can be driven off from the diluted thallium silver nitrate by boiling.

(To be continued.)

* * * * *



A RESEARCH ON THE LIQUEFACTION OF HELIUM.[1]

[Footnote 1: Translated from the original paper, by Prof. K. Olszewski, in the Bulletin de l'Academie des Sciences de Cracovie for June, 1896, "Ein Versuch, das Helium zu verflunigen," by Morris Travers, and published in Nature.]

My experiments on the liquefaction of helium were carried out with a sample of that gas, sent to me by Prof. Ramsay from London, in a sealed glass tube holding about 140 c. cm. I take this opportunity of rendering him my most sincere thanks. In his letter Prof. Ramsay informed me that the gas had been obtained from the mineral clevite, and that it was quite free from nitrogen and other impurities, which could be removed by circulation over red hot magnesium, oxide of copper, soda lime, and pentoxide of phosphorus. The density of the gas was 2.133 and the ratio of its specific heats (Cp/Cv) 1.652, the latter figure indicating that the molecule of helium was monatomic, as had already been found to be the case with argon. Prof. Ramsay further informed me that the gas was only very slightly soluble in water, 100 c. cm. of water dissolving scarcely 0.7 c. cm. of helium.

From the results of my earlier experiments I had been led to expect that it would be only possible to liquefy helium at a very low temperature; the small values obtained for the density and solubility of the gas, together with the fact that its molecule is monatomic, indicating a very low boiling point. For this reason I did not consider it necessary to use liquid ethylene as a preliminary cooling agent, but proceeded directly to conduct my experiments at the lowest temperature that could be produced by means of liquid air. The apparatus employed in these investigations is figured in the accompanying diagram.

The helium was contained in the glass tube, c, of the Cailletet's apparatus, C. The tube, c, reached to the bottom of a glass vessel, a, which was intended to contain the liquid air. The vessel, a, was surrounded by three glass cylinders, b, b' and b", closed at the bottom and separated from one another. The outer vessel, b", was made just large enough to fit into the brass collar, o, which supported the lid, u, of the apparatus. The tube, a, fitted into an opening in the center of the lid; the tube, t, connected with an apparatus delivering liquid oxygen, passed through a hole on the right. The vessel, b, was also connected with a mercury manometer and air pump by means of a T tube, p, v, one arm of which passed through the third hole in the lid of the apparatus. The tube, a, was closed by a stopper, through which passed the tube, c, of the Cailletet's apparatus, a tube connected with the drying apparatus, u, u', and one limb of a T tube, by means of which the manometer and air pump could be put in connection with the interior of the vessel. The lower part of the whole apparatus was inclosed in a thick walled vessel, e, containing a layer of phosphorus pentoxide.

By turning the valve, k, the vessel, b, could be partially filled with liquid oxygen, which, under a pressure of 10 mm. of mercury, boiled at about -210 C. Almost immediately the gaseous air began to condense and collect in the tube, a; a supply of fresh air was constantly maintained through the drying tubes, u and u', which were filled with sulphuric acid and soda lime respectively. When the quantity of liquid air ceased to increase, the tap on the U tube, u, was closed, the T tube, p' v', was connected with the manometer and air pump, and the liquid air was made to boil under a pressure of 10 mm. of mercury. In order to protect the liquid air from its warmer surroundings, a very thin, double wall tube, f, reaching to the level of the liquid in the outer vessel, was placed inside the tube, a. When, as in some of my experiments, liquid oxygen was used in the inner vessel, this part of the apparatus was dispensed with.



Using the apparatus I have just described, I carried out two series of experiments, in which liquid air and liquid oxygen were employed as cooling agents. The tube of the Cailletet's apparatus was thoroughly exhausted by means of a mercury pump, and then carefully filled with dry helium. In the first series of experiments the helium, confined under a pressure of 125 atmospheres, was cooled to the temperature of oxygen boiling, first under atmospheric pressure (-182.5), and then under a pressure of 10 mm. of mercury (-210). The helium did not condense under these conditions, and even when, as in subsequent experiments, I expanded the gas till the pressure fell to twenty atmospheres, and in some cases to one atmosphere, I could not detect the slightest indication that liquefaction had taken place. The first time that I compressed the gas I had, indeed, noticed that a small quantity of a white substance separated out and remained at the bottom of the tube when the pressure was released. Possibly this may have been due to the presence of a small trace of impurity in the helium, but it could not have constituted more than 1 per cent. of the total volume of the gas.

In the second series of experiments I employed liquid air, boiling under a pressure of 10 mm. of mercury. The helium was first confined under a pressure of 140 atmospheres, and then allowed to expand till the pressure fell to twenty atmospheres, or, in some cases, to one atmosphere. The results of these experiments were also negative, the gas remained perfectly clear during the expansion, and not the slightest trace of liquid could be detected. The boiling point of liquid air was taken, from my previous determination, to be -220 C. (Comptes Rendus, 1885, p. 238). This number cannot, however, be taken as a constant, as the liquid air, boiling under reduced pressure, becomes gradually poorer in nitrogen. Further, the quantity of nitrogen lost by the liquid air on partial evaporation varies not only with the rate of boiling, but even according to the manner in which it has been liquefied.

If air, under high pressure, be cooled first to the temperature of boiling ethylene, and then to -150 C., it liquefies, and, on reducing the pressure slowly, liquid air is obtained boiling under atmospheric pressure. During the process a considerable quantity of the liquid air evaporates, and the proportion of nitrogen to oxygen in the remaining liquid is less than in air liquefied under high pressure. If the liquid air obtained by this process be made to boil under a pressure of 10 mm. of mercury, the proportion of nitrogen in the mixture continues to decrease, but, on account of the large quantity of oxygen present, the liquid does not solidify, although its temperature is some six degrees below the freezing point of nitrogen. When, as in some of my former experiments, the air was liquefied under normal pressure by means of liquid oxygen boiling under a pressure of 10 mm. of mercury, the ratio of nitrogen to the oxygen in the liquid air was the same as in the gaseous air from which it had been produced. The liquid air, obtained by direct condensation at normal pressure, appeared to lose oxygen and nitrogen with about equal rapidity, and at the end of the experiment a considerable quantity of liquid nitrogen remained behind in the apparatus. On reducing the pressure to 10 mm. of mercury the nitrogen solidified. Prof. Dewar has stated that liquid air solidifies as such, the solid product containing a slightly smaller percentage of nitrogen than is present in the atmosphere. My experiments have proved this statement to be incorrect; liquid oxygen does not solidify even when boiling under a pressure of 2 mm. of mercury.

After carrying these experiments to a successful conclusion, I found that it was yet necessary to prove that, on reducing the vapor pressure of boiling oxygen, to a minimum, no corresponding fall of temperature takes place. The vessel, e, was partially filled with liquid oxygen, and, by means of a small siphon, a small quantity of the liquid was allowed to flow into the tube, a. The inner vessel, a, was then connected with the air pump and manometer, and the pressure was reduced to 2 mm. of mercury. The oxygen remained liquid and quite clear. In a second experiment the temperature of the liquid oxygen, boiling under 2 mm. of mercury pressure, was measured by means of a thermometer. The temperature indicated lay above -220 C., a temperature easily arrived at by means of liquid air. I, therefore, concluded that liquid air was a much more efficient cooling agent than liquid oxygen, and that it would be quite unnecessary to make further experiments on the liquefaction of helium.

In every single instance I have obtained negative results, and, as far as my experiments go, helium remains a permanent gas, and apparently much more difficult to liquefy than even hydrogen. The small quantity of the gas at my disposal, and, indeed, the extreme rarity of the minerals from which it is obtained, compelled me to carry out my investigation on a very small scale. Using a larger apparatus, and working at a much higher pressure, I could have submitted the gas to greater expansion. Further, I should have been able to measure the temperature of the gas at the moment of expansion by means of a platinum thermometer, as I did when working with hydrogen; but to make such experiments I should have required 10, if not 100, liters of the gas. As I was unable to determine the temperatures to which I cooled the gas, by any experimental means, I have been obliged to calculate them from Laplace's and Poisson's formula for the change of temperature in a gas during adiabatic expansion.

T/T1 = (p/p1)^{(k - 1/k)}

Where:

T, p are the initial temperature and pressure of the gas.

T1, p1 are the final temperature and pressure of the gas.

k is the ratio (cp/cv) which, for a monatomic gas, is 1.66.

In the first series of experiments the gas, under a pressure of 128 atmospheres, was cooled down to -210 C.

p T p1 T1 At. Deg. At. Deg. Deg. 125 -210 C. 50 -229.3 C. 43.7 A. ... ... 20 -242.7 C. 30.3 A. ... ... 10 -250.1 C. 22.9 A. ... ... 5 -255.6 C. 17.4 A. ... ... 1 -263.9 C. 9.1 A.

The results of these calculations tend to show that the boiling point of helium lies below -264 C., at least 20 lower than the value I have found for the boiling point of hydrogen. If the boiling point of a gas be taken as a simple function of its density, helium, which, according to Prof. Ramsay's determination, has a density of 2.133, more than double that of hydrogen, should liquefy at a much higher temperature. Both argon and helium have much lower boiling points than might be expected, judging from their densities. This anomalous condition may be accounted for by the fact that in each case the molecular structure is monatomic, as shown by the values obtained for the ratios of their specific heats.

The permanent character of helium might be taken advantage of in its application to the gas thermometer. The helium thermometer could be used to advantage in the determination of the critical temperature and boiling point of hydrogen. To determine whether the hydrogen thermometer is of any value at temperatures below -198 C. I carried out a series of experiments, in which I measured the temperature of liquid oxygen boiling under reduced pressure. I made use of the identical thermometer tube employed by T. Estreicher (Phil. Mag. [5] 40, 54, 1898) as a hydrogen thermometer for the same purpose, and applied the same corrections as were made in his experiments.

Temperature. Pressure Helium Thermometer. Hydrogen Thermometer. Mm. Deg. Deg. 741 -182.6 C. -182.6 C. 240 -191.8 C. -191.85 C. 90.4 -198.7 C. -198.75 C. 12 -209.3 C. -209.2 C. 9 -210.57 C. -210.6 C.

The results of these experiments prove that the coefficient of expansion of hydrogen does not change between these limits of temperature, and that the hydrogen thermometer is a perfectly trustworthy instrument even when employed to measure the very lowest temperatures.

I have already pointed out (Wied. Ann., Bd. xxxi, 869, 1887) that the gas thermometer can be used to measure temperatures which lie even below the critical point of the gas with which the instrument is filled. For instance, the critical temperature of hydrogen, which I have found to be -234.5 C. (Wied. Ann., 56, 133; Phil. Mag. [5] 40, 202, 1898) can be determined by means of a hydrogen thermometer. The helium thermometer could be used at much lower temperatures, and would probably give a more exact value for the boiling point of hydrogen than it is possible to obtain by means of a platinum thermometer.

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SOME NOTES ON SPIDERS.

By Rev. SAMUEL BARBER.

The instinct of spiders in at once attacking a vital part of their antagonist—as in the case of a theridion butchering a cockroach by first binding its legs and then biting the neck—is most remarkable; but they do not always have it their own way. A certain species of mason wasp selects a certain spider as food for its larv, and, entombing fifteen or sixteen in a tunnel of mud, fastens them down in a paralyzed state as food for the prospective grubs.

Perhaps the most entertaining points in connection with spiders are their concentration of energy, their amazing rapidity of action, and their inscrutable methods of transition and flotation.

During the past autumn large numbers of these creatures appeared at intervals. Thus I observed a vast network of lines that seemed to have descended over the town of Whitstable, in Kent, and which were not visible the day before or the day after. Many were fifteen to twenty feet long; they stretched from house to lamppost, from tree to tree, from bush to bush; and within six or seven feet of the ground I counted, in a garden, twenty-four or more parallel strands. The rapidity with which spiders work may be gathered from the fact that, while moving about in my room, I found their lines strung from the very books I had, a moment before, been using.

Insect life, as might have been expected after so mild a winter and so dry a spring and summer, is (1896) intensely exuberant. The balance is preserved by a corresponding number of Arachnida. On May 25 and 26 the east wall of the vicarage of Burgh-by-sands was coated with a tissue of web so delicate that it required a very close scrutiny to detect it. I could find none of the spinners. Every square inch of the building appeared coated with filmy lines, crossing in places, but mostly horizontal, from north to south.

Walking by the edge of a wheatfield in Suffolk on May 14, I observed all over the path, which was cracked with the drought, dark objects flitting to and fro. They were spiders—mostly of the hunting order. Tens of thousands must have occupied a moderate space of the field, and the cracks in the parched soil afforded them a handy retreat.

In reference to the visitation of spiders at Whitstable during the autumn and winter of 1895-6, it is right to note that the people of that place regard them as a sign of an east wind. In this connection we can note the fact of the phenomenal clouds of flies occurring at times on the east coast of England; and it would be interesting if observers could ascertain whether spiders ever cross the Channel and accompany such visitations of insects.

The production of the flotation line, and its method of attachment, are the two points to which I ask the attention of observers.

Is it not evident that air (and probably at a high temperature) must be inclosed within the meshes of the substance forming the line when it passes from the spinnerets into the atmosphere? The creature with this substance within its body drops to the ground at once by force of gravitation; yet, when emitted, the very same substance lifts it into the air. It has been usual to explain the ascent by the kite principle, i.e., the mechanical force of the contiguous atmosphere. But air movements, especially on a small scale, are so capricious and uncontrollable that, without a directive force, the phenomena seem quite inexplicable.

Moreover, all my own observations lead me to accept the theory of a direct propelling force, and I can hardly accept the conclusions on this point of Mr. Blackwall, though he is an authority on the subject. The intense rapidity with which the initial movements are made cannot be reconciled with any theory of simple atmospheric convection; and illustrations such as the following go to prove that spiders possess the faculty of weighting or condensing the ends of their threads, and throwing them, within limited distances, to a point fixed upon.

I was writing, and had two sheets of quarto before me. Perceiving a small spider on the paper I rose and went to the window to observe it. To test its power of passing through the air, I held another sheet about a foot from that on which the creature was running. It ascended to the edge, and vanished; but in a moment I saw it landing upon the other sheet through midair in a horizontal direction, and picking up the thread as it advanced.

In this case there was no air movement to facilitate, nor any time to throw a line upward, which, indeed, would not have solved the difficulty. Propulsion appears the only explanation.

The next illustration is more marvelous, and seems to indicate that some species, at any rate, have the power of movement through the air in any direction at will.

Some years ago, at a dinner party in Kent, four candles being lighted on the table, I noticed a thread strung from the tip of one of the lighted candles close to the flame, and attached to another candle about a yard off; and all the four lights were connected in this way, and that by a web drawn quite tight. No little surprise was caused among the guests on finding that the diamond form of the web was complete.

No satisfactory explanation of this has been offered, and I can only suggest that the spinner was suspended at first by a vertical line from above, and thus swayed itself to and fro, from tip to tip of the candles. It was certain that the spider could not have ascended from the table; and it was equally certain that aerial flotation of the line from a fixed point was impossible, as it involved floating in four opposite directions. I have seen a creature of this or a nearly allied species moving laterally through the air of a room in this way.—Knowledge.

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ENGINEERING NOTES.

AUSTRIA is turning out a new variety of Mannlicher repeating rifle for its army, which is the lightest rifle in the world, weighing 3.3 kilogrammes, seven pounds and four ounces, instead of 4.4 kilogrammes, nine pounds eleven ounces, the weight of the old pattern. All the individual parts in the new rifle, including the locking box, the magazine and the barrel, are lighter than in the old. The bayonet and sheath are also made lighter.

A TROLLEY express car system is now in successful operation in Brooklyn, N.Y. The trolley system of Brooklyn is one of the most extensive in the world, and many of the outlying districts are now served with great dispatch. Parcels are collected by wagons, they are then brought to the cars, and, after being carried to the nearest express station to their destination, they are then transported again by wagons. On Sundays the cars are run to carry bicycles.

IN STANISLAU oil gas is being a good deal used for incandescent lighting, says the Gas World. The gas is used at a pressure of from 1.1 in. to 1.2 in. When 1.7 cubic feet per hour is used the Welsbach mantle gives 69 candles at first, 65 candles after 120 hours, 48 candles after 500 hours. The fall in lighting power is comparatively slow with oil gas, and the mantles are not so much worn by lighting the gas, for the kind of oil gas is not as explosive as that of coal gas. The mantles are found to last from 400 to 600 hours.

DURING THE construction of the Simplon tunnel every possible alleviation will be made for the workmen employed, says the Railway Review. On leaving the tunnel when they are hot and wet through they will go at once to the douche and bathrooms provided for their accommodation, where, after a refreshing shower bath, they will resume their dry clothes. The sheds from which the workmen leave the tunnel are to be covered in and closed at the sides so as to protect them from cold. Water will be taken at intervals to the workmen who may require it, either from the pipe which feeds the drills or from that which brings water for cooling. No provision has been made as regards workmen's lodgings, because it is supposed that they will easily find accommodation in the neighborhood. As it is believed that the temperature of the rock of the Simplon tunnel may reach a maximum of 104 F., costly measures will have to be taken to cool the air in many parts where the works are to be carried on.

"RECENT DEVELOPMENTS IN LIGHTHOUSE ENGINEERING" was the title of a paper read recently at the Institution of Civil Engineers, by Mr. N.G. Gedye, says the Colliery Guardian. The author pointed out the marked development which has of late years taken place in the direction of reducing the length of flash emitted by lighthouse apparatus to a minimum, and the consequent increase obtained in intensity. The apparatus now being erected at Cape Leeuwin, Western Australia, gives a flash of one-fifth of a second duration every five seconds. It is the most powerful oil light in the world, the flash being over 145,000 candle power emitted from a pair of dioptric lenses mounted on a mercury float revolving once every ten seconds. Each of the two lenses is 8 feet in diameter. The powers of these oil lights are far exceeded by electric lighthouse lights, there being several in France up to 23,000,000 candle power, while there has recently been established at Fire Island, at the entrance to New York Harbor, an electric light, of French design and construction, of 123,000,000 candle power; this is the most powerful lighthouse light in the world.

DISCUSSING THE use of potassium cyanide for steel-hardening purposes, T.R. Almond, of Brooklyn, N.Y., suggests that this salt assists the hardening process because of its powerful deoxidizing properties, and also because it forms a liquid film on the surface of the steel, which causes a more perfect contact between the steel and the water, thereby permitting a more rapid abstraction of heat. The inevitable formation of a thin coat of oxide is unfavorable to the process of rapid cooling; and as rapid cooling seems to be the one thing necessary for success in hardness, any means used for the removal of a bad conductor of heat, like the black oxide, will be of advantage, and more especially if this means also results in the formation of a liquid film on the steel surface having the affinity for water which, it is well known, is peculiar to potassium cyanide. Mr. Almond recommends the removal of all scale or oxide from the surfaces of steel to be hardened, either by pickling or by the cyanide. Steel covered with a very thin film of oxide will take the heat less quickly when immersed in hot lead than if the steel be bright before being immersed. This being the case, it would seem to follow that, because of a film of oxide, heat will leave steel more slowly when being cooled by water.

THE GIGANTIC WHEEL, now being erected on the site of the old bowling green in a corner of the Winter Gardens, Blackpool, was commenced on December 1, 1895, says the Building News. The work of erecting the supports was not finished until the third week in March, and then the most difficult portion of the work, viz., that of hoisting the axle, was commenced. The axle, a steel forging weighing over 28 tons and measuring nearly 41 ft. long and 26 in. in diameter, was forged at the works of Messrs. W. Beardmore & Company, of Glasgow. The axle and bearings being fixed complete, the work of building the rims of the wheel will be pushed forward rapidly under the direction of Mr. Walter B. Basset, who also built the Earl's Court wheel. The carriages, thirty in number, and each capable of carrying forty persons, are rapidly approaching completion in the works of Messrs. Brown, Marshall & Company, of Birmingham. The driving engines and most of the intermediate gearing are already in position in the engine house. These engines will operate two steel wire ropes, one on either side of the rim of the wheel, and arrangements have been made and provided for in such gearing to enable the wheel to be turned at a quicker speed than that at Earl's Court. The Blackpool wheel will be able to carry more passengers per hour than its predecessor in London. The particulars of the great wheel are: Total height above sea level, 250 ft.; total diameter (across centers of pins), 200 ft.; total weight, 1,000 tons. The solid axle is of a diameter through the journals of 2 ft. 2 in., a diameter across the flanges of 5 ft. 3 in., length over all 41 ft., and weight 28 tons.

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ELECTRICAL NOTES.

PORTRAITS of Morse and Fulton are printed on the reverse of the new two dollar silver certificate, affording a relief to the dreary monotony of ex-presidents, generals and statesmen.

A MONSTER electric elevator is to be erected at Allegheny, Pa. It will be large enough to carry up several wagons at once. The new elevator will save a trip of a mile and a quarter.

AN EXCURSION TROLLEY car on the Milwaukee Street Railway has 700 incandescent lights. The car is 32 feet over all. The platforms are 5 foot. The floor of the car is carpeted and a few tables for refreshments are provided.

AMSTERDAM will have next year an international exhibition of hotel arrangements and accommodations for travelers. Among the features of the exhibition will be an "electric restaurant," without waiters, in which visitors will be served automatically with a complete dinner on pressing an electric button.

PROF. FLEMING has shown by experiments that with a 2,000 volt alternating current with a water resistance, that the latter is quite non-inductive, and that the readings of the amperes may be taken, says the Electrical World, as a measurement of the voltage, and the product of the volts and amperes will represent correctly the power consumed.

OUR contemporary, The Engineer, suggests doing away with windsails on board steamers entirely and substituting electric fans. In warships the fan ought to be placed where room can be found for it low down in the ship, far below the water line. An electrically driven horizontal fan, with its motor, can be got into the thickness of a deck with its beams, if needs be. This would clearly be better than depending on a flimsy construction, which would certainly be greatly damaged, if not entirely shot away, in action. If clear decks are wanted, the windsail is about as inconvenient as it is ugly, and that is saying a great deal.

SINCE January 1, last, a new and reduced telephone tariff has been in force in Switzerland, and from reports to hand it appears to have worked satisfactorily all round. The former charge per annum for a telephone, with an annual limit of 800 conversations, was 80 francs (3 4s.) The new tariff now in force is 40 francs (1 12s.) per annum, plus an additional charge of 5 centimes for each local connection. The charges for interurban connections, with a time limit of three minutes, are as follows: Up to a distance of thirty-one miles, 3 d.; up to sixty-two miles, 5 d.; and above sixty-two miles, 7 d. The telephone system throughout Switzerland is owned by the government, and the service, says the Electrician, is first class in every respect.

"THERE ARE three ways by which high temperature may be measured," says the Electrical Engineer, London. "The first uses an air thermometer of refractory material; the second depends on the change in the resistance of a platinum wire with change in temperature; and the third is based on the employment of a thermo couple of relatively infusible metals. According to Messrs. Holborn and W. Wein, in a paper published in Wiedemann's Annalen, the air thermometer method was valueless until recently, as suitable vessels could not be made. But now these are produced from refractory clays, and permit of measurements up to 1,500 C. (2,732 F.) The results are, however, vitiated by the effects of capillarity in the interior of the vessel. The resistance method has also its disadvantages. At high temperatures the resistance generally increases, but the temperature coefficient is irregular. The presence of free hydrogen also affects the resistance. The third or thermopile method is favored by the authors, who prefer a circuit of platinum and an alloy of platinum with ten per cent. of rhodium. Temperatures up to 1,600 C. (2,912 F.) can be measured by it, and it is remarkably constant under various conditions."

THE LONDON ELECTRICIAN states that at a special meeting of the South African Philosophical Society held on August 2, a lecture on the above subject was delivered by Mr. A.P. Trotter, Government Electrician and Inspector. Toward the end of the lecture the lecturer rang up the Capetown Telephone Exchange, and asked if any of the longer post office telegraph lines were clear. The Port Elizabeth line was then connected up, and by means of a Wheatstone bridge on the lecture table, the resistance of the line was measured. The lecturer then observed that, with the extremely sensitive instrument used in the Government Electrical Laboratory, it was not necessary to use ordinary electric batteries for signaling to such a distance as to Port Elizabeth. He disconnected the battery, and, plunging a steel knife and silver fork into an orange, sent signals by means of the feeble current thus generated. He then asked the front row of the audience to join hands, and, putting them in the circuit, sent signals through their bodies to Port Elizabeth and back by means of the orange cell. As a concluding experiment an omelette was made "under some disadvantages," and the cost of the electrical energy was stated to be only two cents.

"THE QUESTION of injury to the eyes from the electric light is being prominently discussed by scientists, oculists, and laymen throughout the country," says the American Journal of Photography. "While opinion widely differs as to the ultimate injury likely to result from the rapidly increasing use of electricity, the consensus of opinion is that light from uncovered or uncolored globes is working damage to eyesight of humanity. In a discussion of the subject a London electric light journal in defending its trade feels called upon to make some important admissions. It says: 'It is not customary to look at the sun, and not even the most enthusiastic electrician would suggest that naked arcs and incandescent filaments were objects to be gazed at without limit. But naked arc lights are not usually placed so as to come within the line of sight, and when they do so accidentally, whatever may result, the injury to the eye is quite perceptible. The filament of a glow lamp, on the other hand, is most likely to meet the eye, but a frosted bulb is an extremely simple and common way of entirely getting over that difficulty. The whole trouble can be easily remedied by the use of properly frosted or colored glass globes. In any case, however, the actual permanent injury to the eye by the glowing filament is no greater than that due to an ordinary gas flame.'"