METHODS OF PETROGRAPHICAL RESEARCH. 213 



special cases. Doubtless one reason for this is that the 

 terms "soluble'' and "insoluble," as applied to the be- 

 haviour of mineral substances with acids, are strictly of 

 relative rather than absolute significance. One of the most 

 successful attempts on this line was that of Fouque (1876), 

 who in his investigation of the Santorin lavas made use 

 of the action of concentrated hydrofluoric acid upon the 

 pounded rock. This readily dissolved the glassy matter 

 and felspars, leaving the magnetite, pyroxene, and olivine 

 almost intact. To separate the olivine from the pyroxene, 

 he further employed weak sulphuric acid, which attacks 

 only the former mineral. In his complete scheme of 

 "mediate analysis" these chemical methods were supple- 

 mented by the use of the electro-magnet to separate the 

 iron-bearing- from the white constituents. Various other 

 chemical devices have been adopted by other workers. 



One main object of all methods of separation is to 

 obtain specimens of the individual minerals pure enough 

 for chemical analysis. The various improvements in the 

 processes of analytical chemistry, which have given in- 

 creased precision and value to results obtained in recent 

 years, scarcely fall within our special province. 



The event which has more than any other impressed a 

 character upon modern petrographical research, and largely 

 determined its direction during the last thirty or forty 

 years, is, of course, the invention by Sorby (1850) of thin 

 slices of rocks, followed by the systematic application of 

 the polarising microscope to the study of the optical pro- 

 perties of rock-forming minerals. The rapid growth of 

 interest in this branch of study and the rich harvest of 

 results attained by its aid form an interesting chapter in 

 the history of geology. Its progress, especially in the 

 laboratories of the German Universities, has been sketched 

 by Fouque (8), Zirkel (9), Stelzner (10), Williams (11), 

 Cohen (6), and others. In this place it will be sufficient 

 to note a few of the more recent advances in this line of 

 research. 



Improved appliances for the preparation of thin slices 

 of rocks need not be specially alluded to, most workers 



