8o4 THE POPULAR SCIENCE MONTHLY. 



we trace oat the left-over material Avliich was last deposited ; we find 

 secondary growths built up from more primitive substances. Through- 

 out we gather evidence bearing upon the life-history of each mineral, 

 and. this may be directly correlated with the conception of chemical 

 structure. When we can determine the conditions under which a com- 

 pound can be formed, we shall have made a long step forward in un- 

 derstanding the nature of the substance. 



The second of our three lines of investigation is closely allied to the 

 first, and, indeed, overlaps it somewhat. It is the study of alterations. 

 A mineral has not only an origin and growth, but also a process of 

 decay, during which its material, disintegrated, is made over into 

 new forms. It is very common to find a crystal with its nucleus un- 

 changed, and its surface transformed into some other species. Some of 

 these alterations are easy to understand ; as when, by oxidation, a 

 cube of iron pyrites becomes a cube of the brown hydroxide, limonite ; 

 or when an arseniate or sulphate is derived from an arsenide or sul- 

 phide. Other changes, however, are less simple, such as the transforma- 

 tion of topaz into mica, or of corundum into margarite ; but all of 

 them tell something as to the nature of the substance altered, and help 

 to elucidate the problems of structure and function. An alteration 

 product is the record of a chemical reaction, which may be traced and 

 reasoned about ; and the evidence which it offers is quite analogous to 

 that used by the organic chemist to determine the structure of a car- 

 bon compound. In the latter case alteration products — that is, deriva- 

 tives — are produced artificially ; in the former the mineralogist finds 

 them ready formed in Nature. Unfortunately, hoAvever, such altera- 

 tion ])roducts ai-e not attractive specimens ; and the ordinary collector 

 throws them aside as worthless. An altered crystal has lost its per- 

 fection and beauty, and is valuable only for v.^hat it signifies. But, 

 from a scientific point of view, its value is real and considerable, if 

 only it be studied thoroughly, apart from superficial appearances, and 

 without jumping at conclusions. Here, again, the microscope and the 

 chemical analysis are necessary coadjutors. 



One line of research yet remains to be considered. The two 

 already disposed of deal with material as gathered in the field ; the 

 third is an affair of the laboratory. Of late years man}'^ mineralogists 

 have been actively at work upon the synthesis of minerals, building 

 up their crystals by artificial means, and reproducing in a rapid way the 

 slower processes of Nature. Many species have thus been formed in 

 well-defined condition ; and other compounds, different from but 

 closely analogous to well-known minerals, have also been produced. 

 Every year there are great advances in this field of work, and every 

 step which is taken is in the direction of the main problem. Some- 

 times results are attained by methods unlike those which grew the 

 native crystal ; but even then new light is shed upon its nature, and 

 wo know more of its possible modes of genesis. Some experiments 



