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taining heat be in a greater ratio than the fpecific 
gravities •, then, during the time of cooling after be- 
ing fufficiently heated, there will be an inftant, when 
the heavier body will become the lighter, and fwim 
upon the other. This feems the cafe in the buff co- 
vering of inflamed blood, the fkum of heated milk, 
and the criftallization of fome falts : for if thefe 
effects were from the evaporation of the thinner 
parts at the furface, they fhould happen during the 
greatefl evaporation, or when boiling ; but, on the 
contrary, they are all done in the greatefl; degree 
when the liquor has for fome time began to cool. 
Laftly, If the quicknefs of acquiring heat be in a 
greater proportion than their fpecific gravities (the 
power of expanfion being equal), then, during the 
time of their acquiring heat, there will be an inftant, 
when the body, that was heavier when cold, will now 
become the lighter. From one or more of which 
principles, I apprehend, the volatility or fixity of all 
minerals, and many other bodies, takes its origin. 
It is no part of my defign to account to you, 
gentlemen, in what manner fuch an expanfion of 
the parts of bodies can be brought about by the 
adtion of fire. Tho’ perhaps a rotatory motion only 
of each particle on its own center might be fufficient 
to produce fuch a rarefaction ; and the more fo, if 
fuch parts were any other figures than fpheres, as 
by the percufllon of their angles they muft refult 
further from each other. Nor is the exiftence of fuch 
a rotatory motion without fome probability, when we 
obferve the verticillary motion given to charcoal-duft 
thrown on nitre in fufion, or the wonderful agitation 
of the parts of burning phofphorus, or even of a 
common 
