MODE I. QUAfcTZ. 



simple mode of attraction, which may with great 

 propriety be called simple homogeneous attraction 

 of aggregation. Of this kind are, granulated 

 quartz, granulated carbonate of lime, &c. the dif- 

 ferent kinds of which substances differ from each 

 other only by the fineness or coarseness of their 

 grain." 



Sometimes this takes place along with the crys- 

 talline, whence small crystals, &c. Sometimes 

 the molecules are precipitated in a detached but 

 confused manner, so as to form earthy or compact 

 substances. In aggregate stones there is the 

 attraction of aggregation, as in granite, sand- 

 stone, and others. 



But long before, Mr. Kirwan* had made the 

 following observations : 



" The first step in the process of crystallisation 

 is the formation of grains ; the second is the in- 

 crease in one dimension ; the third in two dimen- 

 sions ; and the fourth in three dimensions : the 

 grains themselves, however, to be visible, must 

 receive accretions in the three dimensions. If the 

 process be uninterrupted, no traces of distinction 

 will be perceived, and the whole will appear per- 

 fectly uniform ; but, if it be disturbed in the first 

 step, no crystallisation can take place ; if in the 



Min. i. 21, edit. 



