218 HISTORY OF MINERALOGY. 



that though the sides of the hexagonal crystal may 

 vary, the angles are not changed. And Dominic 

 Gulielmini, in a Dissertation on Salts, published in 

 1707, says 7 , in a true inductive spirit, "Nature does 

 not employ all figures, but only certain ones of 

 those which are possible ; and of these, the deter- 

 mination is not to be fetched from the brain, or 

 proved a priori, but obtained by experiments and 

 observations." And he speaks 8 with entire decision 

 on this subject : " Nevertheless since there is here a 

 principle of crystallization, the inclination of the 

 planes and of the angles is always constant." He 

 even anticipates, very nearly, the views of later 

 crystallographers as to the mode in which crystals 

 are formed from elementary molecules. From this 

 time, many persons laboured and speculated on this 

 subject ; as Cappeller, whose Prodromus Crystallo- 

 graphice appeared at Lucern in 1723; Bourguet, 

 who published Lettres Philosophiques sur la For- 

 mation de Sels et de Cristaux, at Amsterdam, in 

 1729; and Henckel, the "Physicus" of the elector 

 of Saxony, whose Pyritologia came forth in 1725. 

 In this last work we have an example of the de- 

 scription of the various forms of special classes of 

 minerals, (iron pyrites, copper pyrites, and arsenic 

 pyrites ;) and an example of the enthusiasm which 

 this apparently dry and laborious study can excite : 

 "Neither tongue nor stone," he exclaims 9 , "can 

 express the satisfaction which I received on setting 

 7 p. 19. 8 p. 83. 8 p. 343. 



