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their thicknesses; and the effect produced by plates of The properties of diathermous bodies have been employed
different substances placed together is independent of the to separate the light and heat which radiate together from the
order in which they are arranged.

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.

Argand.

Locatelli, Diffusion. It has already been remarked that the heat
Rock-salt
92

which falls on the surface of a body is not wholly reflected
Calcared

62

39

according to the laws of reflection above mentioned. A part Plate Glass

62

39

of this heat is irregularly reflected, that is, in all directions Smoky Topaz

57

37

round the point of incidence. This phenomenon is known Alabaster

20

14

under the names of diffusion, dispersion, or irregular reflection of Alum

9

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-
Eject of the Nature of the Source:--The nature of the source polished. The diffusive power varies according to the nature
of heat, generally speaking, considerably modifies the diather of the source and of the reflecting substances. White bodies
mous power of bodies, as shown by the results obtained by are very dispersive in the case of the caloric which radiates
M. Melloni in employing four different kinds, as in the follow- from an incandescent source. The metals unpolished are still
ing table, the number of incident rays of heat being 100, as more dispersive than white bodies.
before:

CONDUCTIBILITY OF SOLIDS, LIQUIJS, AND GASES.
Substances.

Locatelli Incandescent Copper at Copper at
Lamp. Platinum. 400° cent.

Conductibility of Solids. The property which bodies possess of
Rock-salt .. 92

92
92

92 transmitting caloric more or less easily through the interior
Calcareous Spar 39 28

6

0 of their mass is called conductibility. It is considered that this
Plate Glass

39
24
6

0 kind of propagation of heat takes place by internal radiation
Alabaster 14

5
0

0 from particle to particle. As caloric is not conducted in the
Alum

9
2
0

0 same manner through all bodies, those which transmit it easily
This table shows that the proportion of heat transmitted and readily are called good conductors, as the metals in general;
through solids, with the exception of rock-salt, diminishes gation of heat are called bad conductors, such as glass, rosin,
with the temperature of the souree of heat, and becomes zero | wood, and especially the liquids and the gases.
when the source is at 100° Centigrade. Liquids exhibit the
same phenomenon.

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.
Melloni is very well explained by the system of undulations, It is composed of a box made of tin plate, to which are fixed,
by admitting that the properties of different kinds of heat are by means of short tubes and corks, rods of different substances,
due to the different numbers of the vibrations, or to the calorific as iron, copper, wood, and glass. These rods penetrate the
waves of unequal length.

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
the upper strate 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 ihe bosom of seas and lakes. The upper It consists of a prismatic bar of metal, in which are formed a
strata alone partake of the variations of temperature, according series of cavities at equal distances, which are filled with
to the seasons, while at a certain depth the temperature is mercury, and in each of these cavities is placed a thermometer.
always the same.

This bar being exposed at one of its extremities to a conatan

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source of heat, the mercury in the thermometers is seen suc- only be ascribed to the diathermous power of the liquid, how-
cessively rising in each, according to its distance from the ever feeble it may be.
source; and then indicating a fixed temperature, but diminish- Mode of heating Liquids.--When liquids are heated by the
ing in height as these distances increase. By this process, application of the source at their under surface, it follows, from
M. Despreiz verified the following liw, which was first an their feeble conductibility, that it is only by the ascending and
nounced by M. Lambert of Berlin, viz., --If the distances from descending currents which take place in their inferior mass
the source increase in arithmetical progression, the excess of that their heating is effected. These currents are explained by
temperature above that of the surrounding air decreases in the expansion of the lower strata of the liquid, which, becoming
geometrical progression. This law, however, holds true only for less dense, rise in the liquid, and are replaced by the upper
The good conductors among metals, as gold, platinum, silver, strata, which are cooler, and consequently more dense. These
and copper ; it is only approximately true for iron, zinc, lead, currents are rendered 'visible by throwing into water some
and tin, and not at all applicable to non-metallic bodies, as saw.dust, which rises and falls along with them. This ex-
marble, porcelain, &c. If the conducting power of gold be periment is arranged in the manner represented in fig. 173.
represented by 1000, that of the following substances will be ! 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:

their great diathermous power, and the extreme mobility of

their particles ; but when they are restrained in their motions,
Substances.

Conducting Power. their conductibility is almost null. It is abserved, indeed,
Gold

1000

that all substances, between whose filaments the air remains Platinum

981
Silver
973

Fig. 172,
Copper

898
Iron

374 Zinc

363 Tin

301 Lead

179 Marble

2+ Porcelain

12 Brick

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

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wool. In our dwellings, the stone-paving appears to be cooler ever care the process may be conducted, performed by the
than the wooden flooring, because it is a better conductor of oldest, the most experienced operators, the results of cupella-
caloric. The sensation of heat or of cold which we feel when tion are always more or less discordant with the truth,
we come in contact with certain bodies, is due to their con. partly from losses experienced in placing the alloy on the
ductibility. If their temperature is lower than ours, they cupel and removing it from the same, and partly by the
appear to us colder than they are, because they take caloric evaporation of minute quantities of silver (for silver is sensibly
from us in consequence of their conductibility, as is the case volatile at high temperatures), and partly from the spitting
with marble; if, on the contrary, their temperature is higher already described, a result which may be diminished within a
than ours, they seem to us warmer than they are, because very narrow range, but which scarcely admits of being altoge-
they impart to us caloric at various points of their mass. ther prevented.
This phenomenon is exemplified in the case of an iron bar As the suhject of cupellation is strictly a practical one,
exposed to the rays of the sun.

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,

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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- |

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In concluding the subject of the silver assay by cupellation,

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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

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