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their thicknesses; and the effect produced by plates of The properties of diathermous bodies have been employed
Rock-salt blackened with smoke completely Effect of the Screens employed. The calorific rays which pass stops the rays of light, but allows those of caloric to pass through one or more diathermous substances undergo a through it. On the contrary, plates or solutions of alum stop modification which renders them more or less proper for trans- the rays of heat and give passage to those of light. This latter mission through other diathermous substances. Thus, by process is usefully employed in apparatus illuminated by the comparing the results obtained by means of an Argand lamp, rays of the sun or by the electric light, when it is necessary to whose flame is surrounded with glass, with those obtained by prevent too great a heat. In gardens, the use of bell glasses, means of a Locatelli lamp, whose name is not so surrounded, which are employed to shelter certain plants, is founded on M. Melloni found that out of 100 incident rays, the following the diathermous property of glass indicated in the preceding were the numbers of rays, or the quantities of heat, respectables ; this substance is traversed by the solar rays which tively transmitted by the two lamps :
have a high temperature, but not by the dark heat which
radiates from the sun, Subatances.
Locatelli, 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 Plate Glass
of this heat is irregularly reflected, that is, in all directions Smoky Topaz
round the point of incidence. This phenomenon is known Alabaster
under the names of diffusion, dispersion, or irregular reflection of Alum
caloric; and the name of specular reflection is given to that
which follows the regular laws of reflection. The phenomenon From these experiments we conclude that the heat, which in of diffusion from the surfaces of bodies was the discovery of M. the Argand lamp has already passed through the glass, is more Melloni. easily transmitted through other substances. Rock-salt alone Regular reflection takes place only in polished surfaces; on always permits the same quantity of the incident rays to pass the contrary, irregular reflection takes place in dull or rough through it.
surfaces, as in plates of wood, glass, or metal, ground or un-
CONDUCTIBILITY OF SOLIDS, LIQUIJS, AND GASES.
Locatelli Incandescent Copper at Copper at
Conductibility of Solids. The property which bodies possess of
92 transmitting caloric more or less easily through the interior
0 of their mass is called conductibility. It is considered that this
0 kind of propagation of heat takes place by internal radiation
0 from particle to particle. As caloric is not conducted in the
0 same manner through all bodies, those which transmit it easily
In order to compare the conducting power of solids, Ingen. l'ariety in the Calorific Rays.---The properties which heat housz, a Dutch physician, who died at the end of the last presents in its passage through bodies, led M. Melloni to form century constructed a small
apparatus which bears his name, concerning caloric
, a hypothesis analogous to that which has and which is represented in fig. 170. long been held respecting light. Thus Newton showed that
Fig. 170. 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 different 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 coloratron; 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.
interior of the box a very little way, and are covered with
This bar being exposed at one of its extremities to a conatan
source of heat, the mercury in the thermometers is seen suc- only be ascribed to the diathermous power of the liquid, how-
represented as in the table, according to the experiments of determine the conducting power of the gases, on account of M. Despretz:
their great diathermous power, and the extreme mobility of
their particles ; but when they are restrained in their motions,
Conducting Power. their conductibility is almost null. It is abserved, indeed,
that all substances, between whose filaments the air remains Platinum
11 Organic substances are bad conductors of heat; as to wood, M. De La Rire, of Genera, 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 ceither 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 experi. ment:-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 of the other extremity: Mercury is the only liquid which is caloric; such as straw, eider-down, fur. When a gaseous mass a good conductor of caloric, and this is owing to its metallic is being heated, it takes place chiefly by its contact with a nature. It is in consequence of its conductibility that when warm body, and by the ascending currents which arise from the hand is immersed in it, at the ordinary temperature, we expansion, in the same manner as in liquids. experience a sensation of cold more striking than in any other liquid at the same teinperature. The conductibility of liquids, serve a liquid warm
for a length of time, we enclose it with
Applications of Conductibility.--When it is required to prehowever, is not null, as some philosophers have asserted. In vessel having double walls, the interval of' which is filled with fact, if we place on the surface of a liquid a small vessel non-conducting matters, as sawadust, 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 indicate a slight increase in temperature, an effect which can warm weather, it is renveloped in straw, or with a covering of
wool. In our dwellings, the stone-paving appears to be cooler ever care the process may be conducted, performed by the
which may be of use to ihe student, especially at this time,
(for it applies also to the estimation of gold, under which head Fig. 173.
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,
LESSONS IN CHEMISTRY.–No. XXXI. Resuming the subject of silver assaying by cupellativn, 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 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 zicher 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 pennywe have merely taken cognizance of the quality possessed by
In our previous operations with the cupel, or its substitute, 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 i 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; other wise 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 precautiors already i 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 absorbed, but the copper along with it, leaving the silver fraction than half a pennyweighi fine is reported on by silver eircumstances will the lead become oxidised and finally half pennyweights in the troy pound; and since no smaller what the experimentalist will himself have seen, he will not | 12 grains, in which case the representative of half a penny
From the remarks I have made on cupellation, and from grains will be obvious. Nevertheless some assayers prefer sense of the term, inasmuch as it only indicates the amount of fail to discover that the process is imperfect in the chemical weight will obviously be half a grain.
assayers, the convenience of commencing operations on 24 babe metal contained in any alloy, nor is this all; with what states of combination, and occasionally “native," or metallic
Extraction of Silver from its Ores.-Silver occurs in several