10 HISTORY of th SOCIETY. 
fally molded upon the cryftals of felt-fpar. Now, were it true, 
that all granite is formed by fufion, the very contrary, it 
would feem, ought always to take place, as felt-fpar is very ea- 
fily melted, and quartz refifts the greateft efforts of heat that 
have hitherto been applied to it. 
Tuts difficulty is obviated thus: It is well known, that when 
quartz and felt-fpar are pounded and mixed together, the mix- 
ture may, without difficulty, be melted and run into a kind of 
glafs, the felt-fpar ferving as a flux to the quartz. The fame 
fact may be ftated in another way, by confidering the felt-fpar, 
when melted, as a fluid in which, as in a menftruum, the 
quartz is diffolved ; and in this view, we may expect, by ana- 
logy, that phenomena, fimilar to thofe of the folution of falt in 
water, fhould take place. Now, it is certain, that when ex- 
ceflive cold is applied to falt water, the water is frozen to the 
exclufion of the falt, the ice obtained yielding frefh water when 
melted, and the falt, when the experiment is pufhed to the ut- 
moft, feparating from it in the form of fand. Why fhould not 
the fame thing happen in the folution of quartz in liquid felt- 
fpar, when the mafs is allowed to cool below the point of con- 
gelation of the menftruum? The felt fpar may cryftalize fepa- 
rately from the quartz, as we have feen pure ice formed fepa- 
rately from the falt ; in both cafes, the congelation of the folvent 
being fimultaneous to that of the diflolved fubftance. Hence 
the cryftals may mutually interfere with each other’s forms, 
and we-may as naturally expect to fee quartz molded on cryftals. 
of felt {par as the reverfe. 
In anfwer to an objection which might be urged againft this 
reafoning, viz. that the refult of the fufion of granite is a glafs in 
which no cryftalization can be feen, an accidental experiment was 
produced, which had happened at one of the Leith Glafs-houfes 
a few weeks previous to the reading of this paper. A quantity of 
common green glafs having been allowed, in a great mafs, to cool 
gradually and very flowly, it was found to have loft all the proper- 
3 ties 
