MINEEALOGY ; WITH A CLASSIFICATION OF SILICATES. 29 



It was therein maintained that chemistry is to mineralogy what biotics is to organography ; 

 that both physics (or dynamics) and chemistry, which together preside over the genesis 

 of inorganic species, mnst be taken into account in their stïidy, and that chemical charac- 

 ters must be greatly depended upon in mineralogical classification ; while it was added 

 that " in its wider sense the chemical history of bodies takes into consideration all those 

 characters " upon which the natural-history system is based.' 



§ 12. After discussing in this connection the question of the densities of certain sul)- 

 stances of high equiA'alent, alike in vapor and in liquid and solid forms, it was said : 

 " Starting from these high equivalent weights of liquid and solid hydrocarbonaeeous spe- 

 cies, and their correspondingly complex formulas, we are prepared to admit that other 

 orders of mineral species, such as oxyds, silicates, carbonates, and sulphids, have formulas 

 and equivalent weights corresponding to their still higher densities ; and we proceed to 

 apply to these bodies the laws of substitution, homology, and polymerism, which have so 

 long been recognized in the chemical study of the members of the hydrocarbon series. 

 The formulas thus deduced for the native silicates and carbon-spars shew that these poly- 

 basic salts may contain many atoms of different bases, and their frequently complex and 

 varying constitution is thus rendered intelligible. In the application of the principle of 

 chemical homology we find a ready and natural explanation of those variations within 

 certain limits, occasionally met with in the composition of certain crystalline silicates, sul- 

 phids, etc. ; from which some have conjectured the existence of a deviation from the law 

 of definite proportions in what is only an expression of that law in a higher form. The 

 principle of polymeri?m is exemplified in related mineral species, such as meionite and 

 zoisite, dipyre and jadeite, hornblende and pyroxene, calcite and aragonite, opal and 

 quartz, in the zircons of different densities, and in the various forms of titanic acid and of 

 carbon, whose relations become at once intelligible if w^e adopt for these species high 

 equivalent weights and complex molecules. The hardness of these isomeric or allotropie 

 species, and their indifference to chemical reagents, increase with their condensation or, 

 in other words, vary inversely as their empirical equivalent volumes ; so that we here 



find a direct relation between chemical and physical properties " 



§ 13. "Chemical change implies disorganization, and all so-called chemical species are 

 inorganic, that is to say, unorganized, and hence realljr belong to the mineral kingdom. 

 In this extended sense, mineralogy takes in not only the few metals, oxyds, sulphids, 

 silicates, and other salts which are found in nature, but also all those which are the pro- 

 ducts of the chemist's skill. It embraces not only the few native resins and hydrocarbons, 

 but all the bodies of the carbon series made known by the researches of modern chemistry. 

 The primary object of a natural classification, it must be remembered, is not, like that 

 of an artificial system, to serve the purpose of determining species, or the convenience 

 of the student ; but so to arrange bodies in orders, genera, and species, as to satisfy inost 

 thoroughly natural affinities. Such a classification in mineralogy will be based upon a 

 consideration of all the phj^sical and chemical relations of bodies, and will enable us 

 to see that the various properties of a species are not so many arbitrary signs, but the 

 necessary results of its constitution. It will give for the mineral kingdom what the 



' Read before the American Academy of Sciences, Jan., 18G7, and published in the Amer. .Tonr. Science of tlie 

 same vear, xhii. 203-20G : aJso in ilie author's Clicmical and Geological Essays, pp. 4.53-4.58. 



