HISTORY of the SOCIETY. rr 
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 fub{tance 
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. 
TueE fame principle feems to point out the theory of all kinds 
of granite, and fhows their connection with one another, and 
- with all the other unftratified bodies. If quartz, felt-{par, 
fchorl, mica, garnet, &c. 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 diffolved 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 1000 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 flowly, 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 perfeét regularity ; whereas 
thofe fubftances which were held by the menftruum fimply as 
a fluid, will not feparate till the congelation of the folvent itfelf 
(B 2) takes 
