of Mineral Species. 75 



intrusive nature of the new compounds, in prejudice of those 

 which existed before. 



The facts met with in nature, are at all events highly in- 

 teresting, and deserve the particular attention of naturalists, 

 who should have an opportunity of ascertaining the circum- 

 stances under which they take place ; this may eventually com- 

 plete the series in which they are here considered, beginning 

 with the simplest case, when the substance formed has the 

 same chemical composition as the one destroyed, and termi- 

 nating in those where the composition of the two is so different, 

 that even the analogies of the cases will not suffice for removing 

 every doubt concerning their formation in the manner described. 

 One remarkable result, however, we obtain by this comparison, 

 that a new species is always produced, though its individuals be so 

 small, that they are beyond the reach of natural-historical exa- 

 mination. 



I. Changes in substances having the same composition. 



The chemical mixture, essential to the common vitriol of 

 zinc, is a dimorphous one, or one of those which are capable 

 of crystallizing in two different kinds of forms, incompatible 

 with each other. The most common of them is derived from 

 a scalene four-sided pyramid, which has its three axes per- 

 pendicular to each other, and is comprised in the prismatic 

 system. It is deposited from solutions not sufficiently concen- 

 trated to form a crystalline skin on their surface, and at tem- 

 peratures below 126 Fahrenheit. Above that temperature, 

 a highly concentrated liquid yields crystals of another spe- 

 cies, whose forms are derived from a scalene four-sided pyra- 

 mid, having its axis inclined on the base, and belonging to 

 the hemi-prismatic system. The chemical composition of both 



K2 



