THE PRESENT STATUS OF MINERALOGY. 805 



also bear upon the subject of alterations ; and definite alteration prod- 

 ucts are artificially obtained. So far, little has been done toward 

 generalizing upon this class of observations ; but, as the facts accumu- 

 late, new relations will appear and reasoning must follow. Each ex- 

 periment suggests new experiments ; each discovery points toward 

 others, and the connecting theories will grow up around the concep- 

 tion of chemical structure. It is the only conception yet clearly recog- 

 nized which is general enough to cover the whole field. 



It has already been argued that the physical study of minerals is 

 subordinate to their chemical investigation, for the reason that all 

 properties depend upon composition. Physical researches, neverthe- 

 less, have great value in mineralogy, and a paper under the caption of 

 this essay would be wretchedly incomplete if it failed to consider them. 

 Physical data, moreover, aid in the discussion of chemical structure, 

 and point out analogies of weighty significance. Specific gravity, for 

 instance, is always an important datum in the study of a species ; and 

 the ratio between it and the molecular weight of a compound tells us 

 something of the condensation which the elementary material has un- 

 dergone in combining. One eminent mineralogist is now using this 

 ratio as a basis for mineral classification, especially among the silicates ; 

 and his results are likely to emphasize the conclusions drawn from 

 quite different sources. This method of study, however, presupposes 

 a knowledge of true chemical composition. With the latter it means 

 much ; alone it signifies little. 



Upon the thermal and electrical properties of minerals compara- 

 tively little has been done ; and that little has slight reference to min- 

 eralogy in general. The optical constants, on the other hand, are 

 elaborately studied by mineralogists, on account of their direct rela- 

 tions to crystalline form. Indeed, optical and crystallographic work 

 is a dominant feature of modern mineralogical investigation, although 

 a great part of it never rises above the plane of mere descriptive de- 

 tail. In its higher aspect it deals with the internal molecular struct- 

 ure of crystals, and so furnishes data which may some day be con- 

 nected with the broader general conceptions of the chemical field of 

 research. The question of how the atoms -are grouped has a mechani- 

 cal as well as a chemical side ; and some time it will be systematically 

 attacked from both directions. At present we only see the future 

 possibility of so handling the physical evidence ; but the expectation 

 is philosophically just. To-day a knowledge of crystalline form is 

 mainly useful in the identification of minerals ; for by it we may de- 

 termine a species without destroying the specimen ; but its deeper 

 potential significance is none the less apparent. Along these lines we 

 may safely prophesy progress, which can only end in the complete cor- 

 relation of all mineralogic facts, and therefore in the solution of the 

 fundamental problem. 



Looked at from the descriptive side alone, mineralogy is a small 



