232 



Dr. Kopp on the Specific Heat of Solid Bodies. [May 12, 



cases it is less. It is near G*4 in the case of those compounds which only 

 contain elements whose atomic heats, in accordance with Dulong and 

 Petit's law, are themselves approximately =6*4. It is less in those com- 

 pounds containing elements which, as exceptions to Dulong and Petit' s law, 

 have a considerably smaller atomic heat than 6*4, and which are found to 

 be exceptions, either directly, by determinations of their specific heat in the 

 solid state, or indirectly, by the method to be subsequently described. 



After Dulong and Petit had propounded their law, Neumann showed 

 that a similar regularity existed in the case of compounds, that is, that the 

 atomic heats of analogous compounds are approximately equal. Regnault, 

 as is known, has confirmed Dulong and Petit' s, as well as Neumann's law, 

 to a considerably greater extent, and for a larger number of compounds, 

 than had been previously done. And Regnault's researches have more 

 especially shown that the elementary atoms, now regarded as monequivalent, 

 are, as regards the atomic heat of their compounds, comparable with the 

 elementary atoms which are to be considered as poly equivalent. Thus, as re- 

 gards atomic heat, arsenious acid, A.s^ O3, and sesquioxide of iron, O3, or 

 chloride of silver and subchloride of copper, €u CI, may be classed together. 

 Of the applicability of Neumann's law, as hitherto investigated and found 

 in the case of chemically analogous compounds, the author's experimental 

 determinations have furnished a number of new examples. But more 

 interest is presented by his results in reference to the applicability of this 

 law to compounds to which it had not hitherto been supposed to apply. 



In comparing compounds as regards their atomic heat, their chemical 

 character has been taken into account, as represented by the formulae 

 hitherto adopted. Sulphates and chromates, for instance, were looked 

 upon as comparable, but they would not have been classed with perchlorates, 

 or with permanganates. According to more recent assumptions for the 

 atomic weights of the elements, the following salts have analogous for- 

 mulae, and the adjoined atomic heats have been determined : — 



Chromate of lead ]gbCrO, 29-0 



Sulphate of lead ?b8 0, 25-8 



Permanganate of potass KMnO^ 28 "3 



Perchlorate of potass KCIO^ 26-3 



The atomic heats of carbonates, S € O3, of silicates, E 8i O3, of metaphos- 

 phates, R P O3, of nitrates, E, N O3, are also very near. 



But not even a common chemical behaviour, such as the bodies in this 

 group possess — that is, a common haloid character — is necessary in order 

 that compounds of analogous atomic composition shall show the same atomic 

 heat. No one would think of considering magnetic oxide of iron as analo- 

 gous to chromate of potass ; and yet both have the same atomic structure, 

 and determinations of their specific heat have given approximately the 

 same atomic heat for both. 



Magnetic oxide of iron ^^CgO^ 37*7 



Chromate of potass K^CrO 36-4 



