THEORY OF ISOMORPHISM. 
503 
may assume the crystalline forms of hornblende or augite 
indifierently, according to the more or less rapid cooling of 
the melted mass, it is nevertheless certain that the variety 
commonly called augite, and recognized by a peculiar crys¬ 
talline form, has usually more lime in it, and less alumina, 
than that called hornblende, although the quantities of these 
elements do not seem to be .always the same. Unquestiona¬ 
bly the facts and experiments above mentioned show the 
very near affinity of hornblende and augite ; but even the 
convertibility of one into the other, by melting and recrys¬ 
tallizing, does not perhaps demonstrate their absolute iden¬ 
tity. For there is often some portion of the materials in a 
crystal which are not in perfect chemical combination with 
the rest. Carbonate of lime, for example, sometimes carries 
with it a considerable quantity of silex into its own form of 
crystal, the silex being mechanically mixed as sand, and yet 
not preventing the carbonate of lime from assuming the form 
proper to it. This is an extreme case, but in many others 
some one or more of the ingredients in a crystal may be ex¬ 
cluded from perfect chemical union; and after fusion, when 
the mass recrystallizes, the same elements may combine per¬ 
fectly or in new proportions, and thus a new mineral may be 
produced. Or some one of the gaseous elements of the at¬ 
mosphere, the oxygen for example, may, when the melted 
matter reconsolidates, combine with some one of the compo¬ 
nent elements. 
The different quantity of the impurities or the refuse above 
alluded to, which may occur in all but the most transparent 
and perfect crystals, may partly explain the discordant re¬ 
sults at which experienced chemists have arrived in their 
analysis of the same mineral. For the reader will often find 
that crystals of a mineral determined to be the same by 
physical characters, crystalline form, and optical properties, 
have been declared by skillful analyzers to be composed of 
distinct elements. This disagreement seemed at first sub¬ 
versive of the atomic theory, or the doctrine that there is a 
fixed and constant relation between the crystalline form and 
structure of a mineral and its chemical composition. The 
apparent anomaly, however, which threatened to throw the 
whole science of mineralogy into confusion, was reconciled 
to fixed principles by the discoveries of Professor Mitscher- 
lich at Berlin, who ascertained that the composition of the 
minerals which had appeared so variable was governed by a 
general law, to which he gave the name of isomorphism (from 
nroc, isos^ equal, and morphe^ form). According to this 
law, the ingredients of a given species of mineral are not ab- 
