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engine, a jet of cold water, we find, inflantly condenfes 
that immenfe rarefaction ; which I apprehend could 
not be, if it was conflicted of efcaped elaflic air. 
And altho’ this fleam mufl be acknowledged to put 
on fome properties of air ; fuch as ventilating a fire ; 
or that a taper blown out by it, is capable of being 
again lighted immediately, and that without a crack- 
ling noife, which occurs when touched with water ; 
this does not in the leafl invalidate our opinion, tho’ 
it has certainly conduced very much to propagate the 
former one : fince from this way of reafoning, the 
whole mufl be air, and we fhould have no water at 
all in vapour. 
From confidering this power of expanfion, which 
the conflituent parts of fome bodies acquire by heat ; 
and withal, that fome bodies have a greater affinity to 
heat, that is, acquire it fooner and retain it longer than 
others ; which affinity appears from experiments, and 
which, I apprehend, is in fome ratio of their fpecific 
gravities and their powers of refradion, reflexion, or 
abforption of light ; or at leafl in fome ratio much 
greater than that of their fpecific gravities alone. From 
confidering thefe, I fay, many things, before utterly 
inexplicable, became eafily underflood by me. Such 
as, Why when bifmuth and zinc are fuied together, 
and fet to cool, the zinc, which Is fpecifically heavier, 
is found above the bifmuth ? Why the bufi covering 
of inflammatory blood, the fkum of heated milk, the 
fedative fait of borax, which are all fpecifically hea- 
vier than the liquids in which they are formed, are 
flill formed at the furface of them ? How benzoin, 
fulphur, and even the ponderous body mercury, may 
be raifed into vapour, again to be condenfed unaltered ? 
I i 2 And 
