76 DECREMENTS. 



part of the coloured plane, having apparently begun 

 to form at the edge </, and to have proceeded from 

 that edge over part of the coloured plane #, but 

 scarcely touching the colouring matter. 



Mr. Beudant has collected together many facts 

 relative to the influence which particular circum- 

 stances are supposed to exert upon the formation of 

 crystals in the laboratory; but these facts are insuf- 

 ficient to explain the causes which determine the 

 particular crystalline form of a mineral, or to account 

 for its modifications. His memoirs, however, are 

 interesting, and may be found in the Annals of Phi- 

 losophy, vol. xi. p. 262; and in the RoyaHnstitution 

 Journal. 



Decrements appear to be sometimes influenced, as 

 will appear in the next section on the symmetry of 

 crystals, by the capacity of a body to become electric 

 by heat, but I am not aware that any explanation 

 has been given of the manner in which this influence 

 is exerted. 



