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their thicknesses; and the effect produced by plates of different substances placed together is independent of the order in which they are arranged.

Effect of the Screens employed.-The calorific rays which pass through one or more diathermous substances undergo a modification which renders them more or less proper for transmission through other diathermous substances. Thus, by comparing the results obtained by means of an Argand lamp, whose flame is surrounded with glass, with those obtained by means of a Locatelli lamp, whose flame is not so surrounded, M. Melloni found that out of 100 incident rays, the following were the numbers of rays, or the quantities of heat, respectively transmitted by the two lamps :

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From these experiments we conclude that the heat, which in the Argand lamp has already passed through the glass, is more easily transmitted through other substances. Rock-salt alone always permits the same quantity of the incident rays to pass through it.

Effect of the Nature of the Source.-The nature of the source of heat, generally speaking, considerably modifies the diathermous power of bodies, as shown by the results obtained by M. Melloni in employing four different kinds, as in the following table, the number of incident rays of heat being 100, as before:

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same phenomenon.

Variety in the Calorific Rays.-The properties which heat presents in its passage through bodies, led M. Melloni to form concerning caloric, a hypothesis analogous to that which has long been held respecting light. Thus Newton showed that there were seven different kinds of rays of light, viz. red, orange, yellow, green, blue, indigo, and violet, which are unequally transmissible through diaphanous bodies, and which can either be combined or isolated; in like manner, M. Melloni has shown the existence of several kinds of calorific rays, which are emitted simultaneously, in variable proportions, from diffe. rent sources of heat, and which are endowed with the property of passing more or less easily through diathermous substances. These substances possess, therefore, a real calorific coloration; that is, they absorb certain rays and allow others to pass, in the same way that a blue glass, for example, is traversed by the colour blue, and not by other colours. The theory of M. Melloni is very well explained by the system of undulations, by admitting that the properties of different kinds of heat are due to the different numbers of the vibrations, or to the calorific waves of unequal length.

The properties of diathermous bodies have been employed to separate the light and heat which radiate together from the same source. Rock-salt blackened with smoke completely stops the rays of light, but allows those of caloric to pass through it. On the contrary, plates or solutions of alum stop the rays of heat and give passage to those of light. This latter process is usefully employed in apparatus illuminated by the rays of the sun or by the electric light, when it is necessary to prevent too great a heat. In gardens, the use of bell glasses, which are employed to shelter certain plants, is founded on the diathermous property of glass indicated in the preceding tables; this substance is traversed by the solar rays which have a high temperature, but not by the dark heat which radiates from the sun.

Diffusion.-It has already been remarked that the heat which falls on the surface of a body is not wholly reflected according to the laws of reflection above mentioned. A part of this heat is irregularly reflected, that is, in all directions round the point of incidence. This phenomenon is known under the names of diffusion, dispersion, or irregular reflection of caloric; and the name of specular reflection is given to that which follows the regular laws of reflection. The phenomenon of diffusion from the surfaces of bodies was the discovery of M. Melloni.

Regular reflection takes place only in polished surfaces; on the contrary, irregular reflection takes place in dull or rough surfaces, as in plates of wood, glass, or metal, ground or unpolished. The diffusive power varies according to the nature of the source and of the reflecting substances. White bodies are very dispersive in the case of the caloric which radiates from an incandescent source. The metals unpolished are still more dispersive than white bodies.

CONDUCTIBILITY OF SOLIDS, LIQUIDS, AND GASES. Conductibility of Solids.-The property which bodies possess of transmitting caloric more or less easily through the interior of their mass is called conductibility. It is considered that this kind of propagation of heat takes place by internal radiation from particle to particle. As caloric is not conducted in the same manner through all bodies, those which transmit it easily and readily are called good conductors, as the metals in general; and those which present more or less resistance to the propa gation of heat are called bad conductors, such as glass, rosin, wood, and especially the liquids and the gases.

In order to compare the conducting power of solids, Ingenhousz, a Dutch physician, who died at the end of the last century, constructed a small apparatus which bears his name, and which is represented in fig. 170.

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It is composed of a box made of tin plate, to which are fixed, by means of short tubes and corks, rods of different substances, as iron, copper, wood, and glass. These rods penetrate the interior of the box a very little way, and are covered with Instances of the Diathermous Power.-Although no direct ex-white wax which melts at 65° Centigrade. The box being periment has been made on the diathermous power of the filled with boiling water, the wax on some of the rods will gases, it cannot be doubted that air is very diathermous, since soon be observed melting at a greater or less distance, whilst it is in this fluid that all the phenomena of radiant heat take on others there will appear no trace of fusion whatever. The place. It is on account of their great diathermous power that conducting power of each is evidently greater in proportion as upper strata of the atmosphere are always at a low tempe- the part on which the wax melts is more remote from the box. rature, notwithstanding the solar rays which pass through M. Despretz measured the conducting power of solids with them. Water being little diathermous, the contrary phenome- the apparatus represented in fig. 171. non takes place in the bosom of seas and lakes. The upper strata alone partake of the variations of temperature, according to the seasons, while at a certain depth the temperature is always the same.

the

It consists of a prismatic bar of metal, in which are formed a series of cavities at equal distances, which are filled with mercury, and in each of these cavities is placed a thermometer. This bar being exposed at one of its extremities to a constant

source of heat, the mercury in the thermometers is seen successively rising in each, according to its distance from the source; and then indicating a fixed temperature, but diminishing in height as these distances increase. By this process, M. Despretz verified the following 1w, which was first announced by M. Lambert of Berlin, viz.,-If the distances from the source increase in arithmetical progression, the excess of temperature above that of the surrounding air decreases in geometrical progression. This law, however, holds true only for the good conductors among metals, as gold, platinum, silver, and copper; it is only approximately true for iron, zinc, lead, and tin, and not at all applicable to non-metallic bodies, as marble, porcelain, &c. If the conducting power of gold be represented by 1000, that of the following substances will be!

only be ascribed to the diathermous power of the liquid, however feeble it may be.

Mode of heating Liquids.-When liquids are heated by the application of the source at their under surface, it follows, from their feeble conductibility, that it is only by the ascending and descending currents which take place in their inferior mass that their heating is effected. These currents are explained by the expansion of the lower strata of the liquid, which, becoming less dense, rise in the liquid, and are replaced by the upper strata, which are cooler, and consequently more dense. These currents are rendered visible by throwing into water some saw-dust, which rises and falls along with them. This experiment is arranged in the manner represented in fig. 173. Conductibility of Gases.-We cannot, in a direct manner,

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represented as in the table, according to the experiments of determine the conducting power of the gases, on account of M. Despretz:

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their great diathermous power, and the extreme mobility of their particles; but when they are restrained in their motions, their conductibility is almost null. It is observed, indeed, that all substances, between whose filaments the air remains Fig. 172.

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Organic substances are bad conductors of heat; as to wood, M. De La Rive, of Geneva, has shown that its conductibility is greater in the direction of the fibres than across the length, and that the most dense is the best conducting. Bran, straw, wool, and cotton, which are neither dense nor uniform, but composed of discontinuous parts, are very bad conductors.

Conductibility of Liquids.-The conductibility of liquids is extremely small, as may be proved by the following experiment:-A piece of ice being kept at the bottom of a glass tube filled with water, and the apparatus arranged as shown in fig. 172, the water is made to boil at the upper part of the tube, by heating it with the flame of a spirit-lamp, and it is then observed that while the column of liquid is at the boiling point at one of its extremities, the ice is scarcely begun to melt at stationary, present great resistance to the propagation ass the other extremity. Mercury is the only liquid which is caloric, such as straw, eider-down, fur. When a gasect with a a good conductor of caloric, and this is owing to its metallic is being heated, it takes place chiefly by its contact with a the hand is immersed in it, at the ordinary temperature, we expansion, in the same manner as in liquids. warm body, and by the ascending currents which arise from

nature. It is in consequence of its conductibility that when

experience a sensation of cold more striking than in any other liquid at the same temperature. The conductibility of liquids, serve a liquid warm for a length of time, we enclosed with however, is not null, as some philosophers ity of serve having double walls, the interval of which is di charcoal, Applications of Conductibility.-When it is required to prefact, if we place on the surface of a liquid a small vessel non-conducting matters, as saw-dust, glass, pounded charcoal, full of boiling water, it is observed that a thermometer placed and straw. The same means are employed to prevent a body at a small distance below it, will, at the end of a certain time, from absorbing caloric; hence, in order to preserve ice, in I warm weather, it is enveloped in straw, or with a covering of

indicate a slight increase in temperature, an effect which can

wool. In our dwellings, the stone-paving appears to be cooler than the wooden flooring, because it is a better conductor of caloric. The sensation of heat or of cold which we feel when we come in contact with certain bodies, is due to their conductibility. If their temperature is lower than ours, they appear to us colder than they are, because they take caloric from us in consequence of their conductibility, as is the case with marble; if, on the contrary, their temperature is higher than ours, they seem to us warmer than they are, because they impart to us caloric at various points of their mass. This phenomenon is exemplified in the case of an iron bar exposed to the rays of the sun.

Fig. 173.

ever care the process may be conducted, performed by the
oldest, the most experienced operators, the results of cupella-
tion are always more or less discordant with the truth,
partly from losses experienced in placing the alloy on the
cupel and removing it from the same, and partly by the
evaporation of minute quantities of silver (for silver is sensibly
volatile at high temperatures), and partly from the "
already described, a result which may be diminished within a
spitting"
very narrow range, but which scarcely admits of being altoge-
ther prevented.

As the subject of cupellation is strictly a practical one, which may be of use to the student, especially at this time, (for it applies also to the estimation of gold, under which head we shall have to review it,) I append a practical table of the discrepancies between the results of cupellation and the more correct process of mint analysis. The table, it must be remarked, refers exclusively to alloys of silver, copper, and lead, and so based on the assumption that all possible care has been taken in order to avoid unnecessary causes of loss.

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LESSONS IN CHEMISTRY.-No. XXXI.

RESUMING the subject of silver assaying by cupellation, it is well to explain the meaning of the term "standard silver," which signifies a silver alloy of the purity legalised by the legislature for the purposes of coinage. When engaged in the performance of experiments on the metal silver, you could not have failed to remark its quality of softness, whereas the silver of coins is moderately hard; the hardness is produced by alloying or incorporating it by fusion with copper. Standard silver, then, is a compound of eighteen parts by weight (say pennyweights) of pure silver alloyed with two parts by weight (pennyweights) of pure copper, and according as the alloy is richer or poorer in silver than the above proportion, so is it said to be better or worse than proof. As standard silver is a mixture of the precious metal and copper in the rates of eighteen to two, so therefore is it spoken of as being 18 pennyweights fine.

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350

346.73

3.27

325

322.06

2.91

300

297·40

2.60

275

272 42

2.58

250

247-44

2.56

225

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In our previous operations with the cupel, or its substitute, we have merely taken cognizance of the quality possessed by lead of oxidation, fusion, and final absorption of the oxide by means of bone earth; in other words, we have treated of the cupelling operation as though it could only apply to alloys of the precious metals and lead. It remains, therefore, to state at this time that the powers of the operation are far more extensive. Not only has lead the quality of oxidation, fusion, and final absorption by bone earth, but it promotes these results in most other metals, especially copper; otherwise the cupelling operation would not be of the slightest practical service in the routine of mint operations. Suppose, for example, our object to be the assay of a silver coin, we take it, or rather a part of it, not usually more than 24 grains, envelope it in about three or four times its weight of pure sheet-lead, sold on purpose for the operation; place the enve- In concluding the subject of the silver assay by cupellation, loped mass on the cupel with all the precautions already it may be remarked, that if the operator take 24 grains of the indicated, and proceed as described. Not only under these alloy, he will have as many twentieths of a grain as there are circumstances will the lead become oxidised and finally half pennyweights in the troy pound; and since no smaller absorbed, but the copper along with it, leaving the silver fraction than half a pennyweight fine is reported on by silver assayers, the convenience of commencing operations on 24 grains will be obvious. Nevertheless some assayers prefer 12 grains, in which case the representative of half a pennyweight will obviously be half a grain.

pure.

From the remarks I have made on cupellation, and from what the experimentalist will himself have seen, he will not fail to discover that the process is imperfect in the chemical sense of the term, inasmuch as it only indicates the amount of base metal contained in any alloy; nor is this all; with what

Extraction of Silver from its Ores.-Silver occurs in several states of combination, and occasionally "native," or metallic

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Having dissipated every portion of volatile matter from the fragment under treatment, next powder it, rinse it with a small portion of washing soda and borax, and fuse it by means of the strongest portion of the blowpipe jet into a metallic bead, using, during the operation, a charcoal support. Finally, this bead may be cupelled by means of the blowpipe flame directed down upon some bone earth rammed into a tobaccopipe as described already. The student will not do ill in preparing several beads of this kind. One of these he may subject to the process of cupellation, the others he may analyse by as many variations of the moist procees as suggest themselves. At least two distinct processes of accomplishing the separation of silver from lead will at once occur to him if he have studied, practically, certain facts already detailed. I shall leave him with the hint to consider the distinctive properties of chloride of lead and chloride of silver. He will surely now experience no difficulty in effecting the moist analysis.

Wand'rer, seek thy Father's throne,
Hasten there, thy errings own;

Turn-thy faults will be forgiv'n!
Turn-then shall thy songs of praise
Mingle with angelic lays.

Wand'rer! they have charms for thee;
Remember this, and "follow me."

si at C. P. ILLSLEY, (amended)

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