Chapter XVI: TECHNOLOGY.--Apparatus for the Manufacture of Acetylene (3)

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 Illustré.

* * * * *

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 ¼ 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.

* * * * *

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.

* * * * *

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 Société 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 1½ 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 1½ 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.

* * * * *

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 3¼ X 3¼ 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, 8½ X 6½ 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) 8½ ÷ 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 3¼ strips, as somewhat easier to apply. Wet 3¼ 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.