﻿132 MEIGS ON THE RELATION OF 



to conclude that any variation in the ponderable substratum or medium upon which 

 a definite amount of caloric is made to act, or in the attending circumstances by 

 which the body is surrounded, must necessarily produce a variation in calorific effects. 

 Hence the want of absolute identity in the crystalline form of many bodies which 

 are really isomorphous. Hence, too, the discrepancies occasionally met with between 

 the atomic volume of a body and that of the other members of the group to which it 

 belongs. For atomic volume is calculated from relative weight and specific gravity; 

 as the latter two vary, so must the former. Now the specific gravity of a body may 

 vary according as it is adulterated or not with matters lighter or heavier than itself, 

 and having naturally different quantities of heat; or the body itself may, in different 

 states, possess different quantities of combined heat. This view will explain the 

 fortuitous coincidences in form accompanying dissimilarity of properties and compo- 

 sition; and, also, why certain elements manifest isomorphism in their analogous 

 compounds, and yet cannot be shown to be isomorphous in their elementary state. 

 For the mathematico-chemical investigations of Kopp lead us to suppose that the 

 primitive atomic volume of an element suffers a considerable change in its different 

 combinations, — the combining volume bearing in most instances a simple or multiple 

 relation to the primitive. Hence a body may have two or more atomic volumes as 

 well as crystalline forms, which, indeed, we should infer a priori. 



We know very well that a certain elevation of temperature may so affect the 

 molecular structure of a body as to destroy its identity or individuality, — changing 

 its form, color, properties, &c. This molecular change is nearly always accompanied 

 by a simple multiple change in the equivalent of a body, and of course in the atomic 

 volume. It seems very probable, therefore, that the different forms which certain 

 elementary bodies are known to assume, are intimately connected with diversity of 

 equivalent volume. We may suppose that sulphur has a larger and more intricate 

 volume when it takes the complex form of bisulphate of potassa, than in its ordinary 

 simple form. So, likewise, we may suppose that when one equivalent of chlorine 

 replaces one of cyanogen, or two of manganese in combination, it must have a 

 varying volume, and consequently a varying form, in compound bodies. According 

 to Graham, the tendency of discovery is to bring all the elements into one class, 

 either as isomorphous, atom to atom, or with the relation to the others which sodium, 

 chlorine and arsenic exhibit. In this connection, the sameness of atomic heat of the 

 elements, the similarity of many of the atomic volumes, and the simple relationship 

 of the others, as shown in the table, are significant facts. 



If we ignore the materiality of heat, and deny that it is as essential to the compo- 

 sition of bodies as ponderable matter itself, the- changes in form, properties, &c, which 

 these bodies undergo, are without explanation. If, on the contrary, we follow the 

 evidence of our senses, and acknowledge the substantiality of caloric and the extent 



