the Congelation of Qniclf Infer, 
nomenon in quedion may depend upon that circumdance : for 
if, from whatever caufe, the mercury in the thermometer 
fhould begin to congeal as foon as it was cooled down to -39* 
or - 40°, whild that which furrounded it would fudain a cold 
of - 43 0 or - 44 0 without becoming folid ; it is evident, that 
the whole of the former might be congealed, and yet no part 
ot the latter, though the real freezing point of both were the 
fame, that is, though the furrounding quickfilver as foon as it 
came to (hoot its crydals would rife immediately to — 39 0 , the 
point at which that in the thermometer froze. 
As this is undoubtedly the mod obfcure part of our know- 
ledge relative to the congelation of quickfilver, I endeavoured 
to illudrate it by fome experiments on the freezing of water. 
The pured water I could obtain bore to be cooled to -f 21 0 , no 
lefs than eleven degrees below the temperature to which it in- 
dantly rofe as foon as the crydals of ice fhot through it. This 
was didilled water very recently boiled ; it is a midake, there- 
fore, that boiling necedarily renders water not fo capable of 
being cooled below- the freezing point. In proportion as the 
water was lefs pure, it leemed to congeal the fooner ; and the 
kind of impurity which had the mod effedl appeared rather 
to be extraneous matter diffufed through the water, fo as to 
trouble its tranfparency, than fuch as was chemically didolved 
in it *. The fmalled particle of ice, alfo, whenever the water 
was below the freezing point, either added from without, or 
by any means formed in it, would indantly caufe a crydalliza- 
tion, by which the whole came immediately up to +32 0 . 
Likewife a crack in the bottom of the containing glafs vedel 
* I take this to be the reafon that boiling has been thought to render water in- 
capable of being cooled below the freezing point. In moll kinds of water, the 
application of heat occafions the precipitation of earthy fubftances which were 
befoie held in folution ; hence the water conies to be in the Hate of having extra- 
neous matter diffufed through it, and therefore readily congeals. 
p.fFedrnaf! v 
