BISTORT of the SOCIEtr. ir 



ties of glafs, being opaque, white, very hard and refractory, 

 and wholly compofed of a fet of cryftals, which fhot into fome 

 cavities in a determined form. When a piece of this fubftance 

 was melted by the violent heat of a blowpipe, and was allowed 

 to cool inftantly, it recovered all the properties of glafs. We 

 may conclude from this example, that if the glafs produced by 

 the fufion of granite had been allowed to cool with fufficient 

 flownefs, it might have cryftalized, producing a granite fimilar 

 to that which was originally melted. 



The fame principle feems to point out the theory of all kinds 

 of granite, and mows their connection with one another, and 

 with all the other unftratified bodies. If quartz, felt-fpar, 

 fchorl, mica, garnet, fcfc. happen to be melted together, the 

 moft fufible fubftance of them all may be confidered as the 

 menftruum in which all the reft are diffolved, and we may fup- 

 pofe, that thefe various diifolved fubftances may differ amongft 

 themfelves in their properties of folution, as falts differ from 

 one another ; fo that fome of them may be more foluble in 

 the menftruum when very much heated, than when it is com- 

 paratively cold, and others may be as foluble in it, when little 

 warmer than its point of congelation, as when raifed to a much 

 higher temperature. If then we fay, for example, that the 

 congealing point of the folvent is iooo degrees of Fahrenheit, 

 and if the folution is at the temperature of 2000, we may con- 

 ceive one portion of the matters diffolved, as held by the fim- 

 ple diffolving power of the menftruum, and another portion 

 as held by means of its elevated temperature. When therefore 

 a mafs of this kind is allowed to cool very ilowly, as we may 

 fuppofe muft be the cafe with liquid granite in the bowels of 

 the earth, thofe fubftances, held in folution by the heat of the 

 folvent, will firft feparate, and being formed in a liquid, will 

 affume their cryftaline forms with perfect regularity ; whereas 

 thofe fubftances which were held by the menftruum Amply as 

 a fluid, will not feparate till the congelation of the folvent itfelf 



(B 2) takes 



