432 ON THE THEORY OF CHEMICAL CHANGES. [XVI. 



if now the relation between C and be what we have sup- 

 posed, it may be expected that mineral species will exhibit the 

 same relations as those of the carbon series, and the principle 

 of homology be greatly extended in its application. Such is 

 really the case, and the history of mineral species affords many 

 instances of isomorphous silicates whose formulas differ by 

 n0 2 M 2 , as the tourmalines, and the silicates of alumina and 

 magnesia, while the latter, with many zeolites, exhibit a similar 

 difference of n 2 H 2 . The relation is in fact that which exists 

 between neutral and surbasic or hydrated salts. 



Laurent has asserted that salts of the same base, with homol- 

 ogous acids of the type (C 2 H 2 )n0 4 , may be isomorphous when 

 they differ by 2 H 2 , and has pointed out, besides, several in- 

 stances of what he has called hemimorphism in species thus 

 related, as well as in others differing by n C1 2 . The observations 

 of Pasteur and Nickles have greatly extended the application of 

 these cases, which assume a new importance in connection with 

 the views here brought forward, and demand further study.* 



But to return : we have seen that in gases and vapors the 

 specific gravity of a species enables us to fix its equivalent, 

 which is often a multiple, by some whole number, of that cal- 

 culated from the results of ultimate analysis. As the equiva- 

 lents of non-volatile species are generally assumed to be those 

 quantities which sustain the simplest ratio to certain volatile 

 ones, the real equivalent weight corresponding to four volumes 

 of vapor, and consequently the theoretical vapor-density of such 

 species, is liable to a degree of the same uncertainty as those 

 deduced from ultimate analysis. Having, however, determined 

 the true equivalent of a species from the density of its vapor, 

 the inquiry arises whether a definite and constant relation may 

 not be discovered between its vapor-density and tjie specific 

 gravity of a species in the solid state. Such a relation being 

 established, and the value of the condensation in passing from 

 a gaseous to a solid state being known, the equivalents of 



* See Laurent, Comptes Rendus de 1'Acad., Tom. XXVI. p. 353 ; and p. 257 

 of Laurent and Gerhardt's Comptes Rendus des Travaux de Chimie for 1848; 

 also Pasteur, ibid., p. 165 ; and Nickles, ibid, for 1849, p. 347. 



