and when equally compressed under the same volume, evolve equal quantities of heat; and the rise of temperature thereby produced varies inversely as the relative heat of the gas referred to a constant volume.

1358

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If the relative heat of common air at 0.7405 metre external pressure be equal to 1, the relative heat of air under a pressure of 1.0058 met. (the density of which will be to that of the former = 1.3583: 1) will be 1-2369, according to Delaroche & Bérard,-while the specific heat of this condensed air will, according to these numbers, be 1336 0.9126, the specific heat of air at the ordinary pressure being supposed 1. If the relative heat of air under a pressure of 0.758 met. be assumed = 1, that of air at a pressure of 0.379 met. will, according to Clement & Desormes, be 0.693; at 0.189 met. it will be 0·540, and at 0·095 met., 0.368. If the specific heat of air at 20° 1, that of air at + 52° will be 1.206; hence the specific heat of gases increases with the temperature (Gay-Lussac, Ann. Chim. 83, 108. Comp. Suermann, Ann. Chim. Phys. 63, 327.)

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In the two tables which follow (pp. 241, 244) Av. denotes Avogadro,C. D. Clement and Desormes,-Cf. Crawford, -Dl. Dalton,-Dz. Despretz,-D. M. De la Rive & Marcet,-Hs. Hess,-Kw. Kirwan,-Hm. Hermann, Nm. Neumann,-P. D. Petit &. Dulong,-Pr. Potter,-Rg. Regnault.

Specific Heat of Liquid and Solid Elementary Bodies that of Water

= 1.0000.

Diamond..... 0-1192 DM Molybdenum .. 0.0659 D M Lead...

0.0314 Rg

0.1469 Rg

0.0722 Rg

0.0320 Pr

Graphite, nat... 0-2019

0.0400 DI

From gas-retorts.

0.0364 Rg

0-0299 Hm Iridium ... 0.0368

2 From Anthracite from Wales containing 3 per cent. of ash. 'From cannel coal containing 4-5 per cent. ash. Passed through a red-hot tube.

"

R

VOL. I.

The capacity for heat of any given body increases with its temperature. If the specific heat of the following substances be determined, first by heating them to 100°, then plunging them into cold water and observing the temperature of the water, secondly, by heating them to 300°, and repeating the same process, the following differences of specific heat will be found.

Heated to Iron. Mercury. Zinc. Antimony. Silver. Copper. Platinum. Glass. 100° 0.1098 0.033 0.0927 0.0507 0.0557 0.0949 0.0335 0.177 300° 0.1218 0.035 0.1015 0.0549 0.0611 0.1013 0.0355 0.190

Those metals whose rate of expansion increases most rapidly when they are heated, likewise increase most in specific heat; the relative heat also increases in so far as when the expansion amounts to, the increased capacity for heat is about. (Dulong and Petit.)

The specific heat of copper is reduced by violent hammering from 0.095 to 0·0935, but raised again by ignition to 0·0949: lead and tin, on the contrary, which do not increase in specific gravity under the die, likewise suffer no diminution of specific heat by pressure. (Regnault.)

¶ Regnault also finds that soft steel, the density of which at 14° C. is 7.8609, has a specific heat of 0·1165; hard steel of density 7.7982 has a specific heat of 0.1175.-The specific heat of soft bell-metal (80 Cu + 20 Sn), which has a density of 8.6843, is 0·0862, while the same metal hardened, in which state its density is 8:5797, has a specific heat of 0.0858. (Pogg. 62, 50.)

In the same memoir Regnault gives the specific heats of several metals in the finely divided state, as determined by the method of cooling. The following are the results.

The greater the atomic weight of any substance the smaller will be the number of atoms of it required to make up a given absolute weight.

Since, for example, an atom of hydrogen weighs 1, an atom of sulphur 16, and an atom of silver 108, a pound of sulphur must contain and a pound of silver as many atoms as a pound of hydrogen. If then the specific heat (capacity for heat referred to a given weight) of any substance be multiplied into its atomic weight, the product will give the capacity for heat referred to a given number of atoms. This has been done in the following table, in which the data employed are those of De la Roche & Bérard, Regnault, Neumann, and De la Rive & Marcet, as given in the tables (pp. 239 and 241).

Capacity for Heat of the Atoms of Elementary Substances.

From their exact determinations of the specific heats of several elementary bodies, Petit & Dulong deduced the law, that the specific heats of these bodies vary inversely as their atomic weights, so that an atom of any simple substance, whether its volume be great or small, has the same capacity for heat, and requires the same quantity of heat to raise its temperature through a given number of degrees, as an atom of any other elementary substance. The exceptions which they found to this rule have been for the most part removed by the later observations of Regnault, as given in the preceding table. In most substances, the product of the specific heat into the atomic weight is nearly 3.2. Exact agreement is not to be expected, inasmuch as the specific heat of a body varies with its density, and undoubtedly increases to a great degree when the body passes from the solid to the liquid or gaseous state.

With regard to the non-gaseous substances of the foregoing table, the following circumstances may be noticed. The deviation in the case of manganese perhaps arises from the presence of carbon in the metal examined by Regnault, and that in the case of iridium from the impurity of the metal.-Phosphorus, iodine, arsenic, antimony, silver, and gold, exhibit twice as much capacity for heat in the same number of atoms as most other substances. This exception might be made to disappear by halving, as is done by many chemists, the atomic weights of these substances. But if the atomic weight of iodine be reduced one half, the same must necessarily be done with regard to hydrogen and nitrogen, and then the

specific heat of these bodies would also be reduced one half. Moreover, this halving would render the chemical formula unnecessarily complex.

The capacity for heat of liquid bromine appears to be three times as great as that of most elements; but from the analogy between this substance and iodine, it may be supposed that the capacity of solid bromine is only twice as great as that of the majority of simple substances. The capacity for heat of carbon in the form of diamond is, in that of graphite, and in that of charcoal about the ordinary amount. These exceptions cannot be explained away; we cannot treble or quadruple, nor even double, the atomic weight of carbon without incurring great inconveniences.

Among gaseous bodies, nitrogen conforms most closely to the law: for the larger product which it gives is referable to the increase of specific heat arising from the gaseous form. Hydrogen, whose specific heat was probably estimated too low by De la Roche & Bérard, is also in conformity with the law. The atoms of oxygen, on the contrary, appear to have only half the ordinary capacity for heat.-If we suppose, with Haykraft, De la Rive, and others, that all simple gases have the same relative heat, and that the law of Dulong & Petit holds good without exception,the law of Berzelius (p. 45) will be established, according to which the elementary gases all contain the same number of atoms in the same volume; and the division (p. 53) of gases into 6, 2, and 1-atomic must be considered as incorrect. But the preceding table shows that exceptions to the law of Dulong & Petit occur even among gases. Indeed, if we merely compare oxygen and hydrogen with one another, making the atomic weight of hydrogen 1, and that of oxygen 16, the capacity for heat of the atom of hydrogen will be 3·2936. 1 = 3·2396, and that of the atom of oxygen 0·2361. 16 3.7776,-products which approximate pretty closely to those of other substances. But if the atomic weight of oxygen be made = 16, that of sulphur must be increased to 32, and those of the metals must also be doubled, and then their product will not be 3.2, but 6'4; or if the atomic weight of sulphur be made = 16, of oxygen = 8, and of hydrogen = 0.5, the last two bodies will give nearly 16, or half the product given by sulphur.

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=

All this being considered, it is impossible to get rid of all the exceptions to the law of Dulong & Petit; and since this law is not universally applicable, it would be useless to remove a few only of the exceptions by altering the atomic weights, particularly when such alterations would entail unnecessary complexity on chemical formulæ. But these exceptious bear a simple relation to the general law. Thus, if the capacity for heat of an atom of sulphur, and of most other substances, be assumed = 1, that of an atom of diamond will be: 4, of oxygen = , of bromine iodine, phosphorus, arsenic, antimony, silver, and gold = 2.

=

Specific Heat of Liquid and Solid Compounds, that of Water = 1·0000.