152 



DOMAIN II. SILICEOUS. 



second, the grains will appear distinct, small or 

 gross, coarse or fine, according to the nature of 

 the disturbance, whether by the interruption of the 

 process, or the accession of foreign matter ; this 

 latter generally produces coarse or rude grains, in 

 proportion to the quantity. 



" If the disturbance only takes place in the 

 third stage, we shall have fibres or strias, as com- 

 plete surfaces cannot be formed ; the striae having 

 more extension in breadth than the fibres or fila- 

 ments, argue a smaller degree of disturbance than 

 the mere fibrous appearance. 



" If, during the third stage, the strias be forced 

 into contact by the gradual dereliction of the fluid 

 that kept them suspended, they will form lamellas 

 in proportion as they are deserted, which will 

 either adhere to each other, and then fall con- 

 fusedly, being too heavy to be supported by the 

 menstruum, or, if supported, will be superimposed 

 on each other. 



" But if the process of crystallisation be dis- 

 turbed only in the fourth stage, then the form and 

 shape only of the crystals will be more or less 

 altered. 



" All these steps are noticed and described by 

 chemical writers; and particularly by the cele- 

 brated Rouelle, in the Memoirs of the Academy of 

 Paris on the crystallisation of salts." 



