﻿ATOMIC HEAT TO CRYSTALLINE FORM. 119 



The isomorphous and calorific correspondences between the different bodies com- 

 posing each of the several groups of compounds, are sufficiently obvious to require no 

 comment. 



In most of the standard chemical works, the elements have been classified isomor- 

 phously, not so much from any proof that they individually present, as from the iso- 

 morphism of their compounds. The law promulgated by Mitscherlich naturally leads 

 to such supposition and consequent classification. It must not be forgotten, however, 

 that comparatively few of the elementary bodies are crystallizable, and that, therefore, 

 we are in great measure deprived of the positive and demonstrative proof of the 

 isomorphism of all the elements. Nevertheless, two-thirds of the crystallizable 

 elements belong to the regular system, and are hence isomorphous the one with the 

 other. Of these, however, some crystallize in other forms, under which they are 

 isomorphous with elements belonging to other groups, as the Rhombohedral and Right 

 Prismatic. Hence, from their arrangement into various isomorphous classes, and the 

 resemblance between and apparent correlation of these classes, we fairly, though 

 indirectly, infer, as highly probable, that the chemical elements, under certain favor- 

 able conditions, are capable of assuming the same form. Now, the approximation of 

 many of these elements, in atomic heat and volume, certainly seems to confirm the 

 reality of this isomorphism. It will thus be seen that the capacity for heat of ele- 

 mentary atoms is related through isomorphism only. Moreover, it will be observed 

 among compound atoms, that although similarity in chemical constitution is, in 

 general, accompanied by sameness in capacity for heat, yet examples exist of differ- 

 ently constituted bodies also presenting this similarity. This is an important fact 

 and one worthy of close consideration, since it probably explains why one atom in a 

 compound body will replace two or more atoms in another with which it is isomor- 

 phous. Thus Cu 2 S and AgS are isomorphous. Here two atoms of copper replace 

 one of silver, and yet retain the same form. Now the atomic heat of copper is about 

 half that of silver. The atomic heats of the two sulphurets are as follows : — 



At. Heat. At. Volume. 



Cu 2 S . ... . . 9.647 . . . 13.528 



AgS .... 9.258 . . . 18.116 



Even if we adopt Regnault's suggestion, before alluded to, and write both substances 

 according to the formula R 2 S, the numbers for atomic heat will still be nearly the 

 same. 



Bisulphuret of iron Fe 2 S 4 is dimorphous. In the mineral Spirkise it is isomorphous 

 with arsenide of iron Fe 2 AsS 2 . In the pentagonal dodecahedron of the regular system 

 it is isomorphous with cobalt glance, C0 2 AsS 2 . Again, Cd 2 S 2 and Ni 2 S 2 are each 

 isomorphous with Ni 2 As. Thus it would appear that one equivalent of arsenic is 

 isomorphous with two of sulphur. Now the table shows us that the atomic heat of 



31 



