8o2 THE POPULAR SCIENCE MONTHLY. 



of high temperature ; and m either case only the more stable com- 

 pounds are likely to be formed. Simplicity of chemical structure is 

 therefore to be presumed ; and the reverse should be the exception 

 rather than the rule. Great complexity may sometimes exist ; but in 

 most cases it may be traced to the commingling of isomorphous bodies, 

 or to impurities which have been overlooked. 



Suppose, now, that for a given mineral the true chemical composi- 

 tion has been made out and expressed in a definite formula. All errors 

 due to inlerminglings of other bodies may have been eliminated, and 

 yet something still remains to be done before we can truly understand 

 the nature of the substance. Here we must draw from the stock of con- 

 ceptions furnished to science by organic chemistry. Two or more 

 compounds, identical in percentage composition, may be widely different 

 in other respects. Among organic compounds this fact is one of the 

 commonplaces, but among minerals it is rarer and less easily explained. 

 For example, calcite and aragonite, differing in crystalline form and in 

 physical properties, are alike in composition, both being simply car- 

 bonate of lime. The differences lie within the molecule, and arise 

 from the fact that the atoms are differently grouped or arranged. 

 Partly from physical evidence, and partly from ultimate composition, 

 the organic chemist infers the number of atoms in an organic mole- 

 cule, and by a study of the changes which a substance can undergo he 

 draws conclusions as to the position of these atoms with reference to 

 each other. These conclusions are expressed in terms of chemical 

 structure. By reasoning, too special for review here, he accounts for 

 the differences between two "isomeric" compounds, and by means of 

 *' structural formulae " he symbolizes the relations of each to the other. 

 Can similar reasoning be applied to mineral species ? 



It is easy enough to devise structural formulae, even with all the 

 limitations which chemical science imposes. Given a certain number 

 of atoms, built up into a molecule, and we can represent them as ar- 

 ranged in a variety of ways. But, to have value, that way must be 

 chosen which shall represent the relations of the substance under con- 

 sideration to other substances, and which shall, therefore, fulfill a defi- 

 nite scientific purpose in the interpretation of known facts. Formulae 

 so devised are of great utility ; they shed much light upon the changes 

 which bodies undergo, and upon their possible modes of generation ; 

 and this they do independently of all speculative considerations as to 

 their ultimate meaning. The simpler chemical formula? express com- 

 position only ; the structural formulre indicate function also. The lat- 

 ter, equally with the former, is essential to the discussion of our fun- 

 damental problem, namely, to determine the nature of the substances 

 with which we deal. 



At the present moment mineralogy is just entering upon this higher 

 field of chemical study. Some mineralogists are vaguely distrustful 

 of the new structural conceptions ; some are indifferent to them ; 



