RELATIONS TO OTHER SCIENCES 597 



trial rocks, the chemist was called upon to discover those particular 

 reactions best suited to show the presence of those elements, most 

 sharply to separate them from one another, and to determine them 

 quantitatively with the greatest accuracy. Such investigations 

 undertaken, as, for example, were those by Hillebrand, at the com- 

 mand of petrography for her own profit, have thus proved a benefit 

 to the whole science of analytic chemistry. While serving petro- 

 graphy Gooch discovered the new methods for the separation of 

 titanium, lithium, and boron; and the chemist owes to his inventive 

 ingenuity the perforated platinum filter-cone and tubulated crucible 

 used in making water determinations. The mineral riches of the 

 Stassfurt rock-salt deposits inspired van't Hoff to undertake long- 

 continued and important researches into the conditions of equilib- 

 rium, the solubility curves, and conditions of formation of the 

 hydrates, the double salts, and the products of double reactions. 



In recent years the science of micro-chemistry has grown up and 

 developed alongside the ordinary macro-chemistry. In this new 

 science the eye, armed with the microscope, attempts to recognize 

 both the changes produced in the subject under examination, and 

 also the newly formed product of the chemical reaction. After apply- 

 ing the reagents to a very minute particle of the mineral, or drop of 

 a solution, it is endeavored to secure by evaporation a product of 

 the reaction which, though microscopic, shall be so characteristic 

 optically and crystallographically, that it may serve to identify 

 beyond doubt the presence of its particular elements in the original 

 specimen. These special micro-chemical methods, which have proved 

 most satisfactory for numerous elements, and are frequently em- 

 ployed, are now in the service of ordinary qualitative analysis. An 

 historical review must, however, emphasize the fact that they were 

 first introduced solely for the uses of petrography. It was Boricky 

 who, in 1877, in the course of his studies in lithology, hit upon the 

 idea of treating mineral particles with hydrofluosilicic acid in order 

 to obtain fluor-silicates of the alkalies, alkaline earths, etc., which 

 betray the presence of suspected elements by distinct and character- 

 istic crystal forms. 



There is a constantly growing conviction that a large number of 

 petrographic problems will find their explanation among the future 

 results of the science called physical chemistry, a science which has 

 won by its recent successes the right of actual independence, although 

 its name suggests that it occupies an intermediate position. The 

 following broad outline shows that its principles, laws, and methods 

 may be most profitable to, and have already been in part applied in, 

 petrographic fields. 



It is peculiarly interesting that the concept of " solid solutions," 

 which excited great interest in chemistry upon being put forward 



