464. THEOKY OF ISOMOKPHISM. [Ch. XXVHI 



were frequent, the hornblendic entirely absent ; hence it was conjec- 

 tured that hornblende might be the result of slow, and augite of rapid 

 cooling. This view was confirmed by the fact, that Mitscherlich and 

 Berthier were able to make augite artificially, but could never succeed 

 in forming hornblende. Lastly, Gustavus Rose fused a mass of horn- 

 blende in a porcelain furnace, and found that it did not, on cooling, 

 assume its previous shape, but invariably took that of augite. The 

 same mineralogist observed certain crystals in rocks from Siberia which 

 presented a hornblende cleavage, while they had the external form of 

 augite. 



If, from these data, it is inferred that the same substance may assume 

 the crystalline forms of hornblende or augite indifferently, 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 crystalline 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. Unquestionably the facts and ex- 

 periments above mentioned show the very near affinity of hornblende 

 and augite ; but even the convertibility of one into the other, by melting 

 and recrystallizing, does not perhaps demonstrate their absolute identity. 

 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 excluded from 

 perfect chemical union ; and after fusion, when the mass recrystallizes, 

 the same elements may combine perfectly or in new proportions, and 

 thus a new mineral may be produced. Or some one of the gaseous 

 elements of the atmosphere, the oxygen, for example, may, when the 

 melted matter reconsolidates, combine with some one of the component 

 elements. 



The different quantity of the impurities or refuse above alluded to, 

 which may occur in all but the most transparent and perfect crystals, 

 may partly explain the discordant results at which experienced chemists 

 have arrived in their analysis of the same mineral. For the reader will 

 find that crystals of a mineral determined to be the same by physical 

 characters, crystalline form, and optical properties, have often been de- 

 clared by skilful analyzers to be composed of distinct elements. (See 

 the table at p. 475.) This disagreement seemed at first subversive of 

 the atomic theory, or the doctrine that there is a fixed and constant re- 

 lation between the crystalline form and structure of a mineral and its 

 chemical composition. The apparent anomaly, however, which threat- 

 ened to throw the whole science of mineralogy into confusion, was in a 

 great degree reconciled to fixed principles by the discoveries of Professor 

 Mitscherlich at Berlin, who ascertained that the composition of the min- 



