8oo THE POPULAR SCIEWCE MONTHLY. 



gards the gathering of more data of like kind, with possible refine- 

 ments of method. The relations and hearing of mineralogy toward 

 other sciences have been, with rare exceptions, slighted ; and a general 

 theory of its nature and purpose has hardly been considered at all. 



Of late years, however, an improvement has been noticeable. New 

 lines of investigation are open, new modes of thought are recognized, 

 and philosophical treatment is in order. In mineralogy, as in most 

 other sciences, a central stem of growth is perceivable, around which 

 the facts are grouping themselves to obvious advantage. Lithology 

 (which mainly deals with the association of minerals), the study of 

 pseudomorphs and alterations, and the synthesis of many mineral spe- 

 cies, all furnish lines of evidence which converge toward certain gen- 

 eral conceptions dominating the entire subject, and linking it intimately 

 with other divisions of scientific thought. The discovery of new spe- 

 cies is no longer the main object of the mineralogist, who is learning 

 to look upon the correlation of known minerals as much more impor- 

 tant. 



In the study^of minerals as such, apart from their geological rela- 

 tions, two main avenues of research are followed : First, the physical 

 method, which is now mainly devoted to morphological considera- 

 tions ; and, secondly, the chemical method, which discusses composition. 

 Philosophically regarded, the former method is subordinate to the lat- 

 ter, for physical properties, including form, are undoubtedly functions 

 of chemical structure, which alone is fundamental and determining. A 

 mineral species is best denoted as a definite chemical substance occur- 

 ring in the crust of the earth ; and its integrity depends upon the 

 sharpness with which its constitution can be made out. In any given 

 case, the claims of a species to recognition depend upon definiteness 

 of composition, together with, in less degree, definiteness of form. The 

 latter consideration, however, is only approximately general ; for in 

 some cases it can not be determined, and some minerals are amorphous ; 

 whereas the former test applies without exception. Crystalline form 

 is but one property of a substance — more important to the mineralo- 

 gist, doubtless, than any other physical condition, yet governed ulti- 

 mately by chemical determinations. The nature of the substance is 

 the one fundamental fact in the description of any mineral species. 



It is not always easy, however, to prove definiteness of composi- 

 tion. A mineral may, to all outward a))pearance, be uniform in texture, 

 and yet, seen in thin section under the microscope, it may be found to 

 contain several different things. The microscope, therefore, with its 

 adjunct the polariscoj)e, is an important weapon in the hands of the 

 mineralogist. By its aid he determines the mechanical purity of a 

 given specimen ; and upon examination under polarized light he can 

 tell something as to its crystalline system, even though distinct crystals 

 as such may not be visible. Few minerals are wholly free from me- 

 chanical admixtures, which complicate analysis, and vitiate the con- 



