cooling of Water below its freezing Point . 135 
evolution of the latent heat from the very particles that 
congeal. 
Many of the circumftances attending the greater or lefs 
cooling of water below its freezing point depend upon this 
principle. In a calm day, when the temperature of the air 
was about 20°, 1 expofed two veflels with diftilled water to 
the cold ; one of them w 7 as (lightly covered with paper, the 
other was left open : the former bore to be cooled many 
degrees below the freezing point, whilft a cruft of ice 
always formed on the furface of the other before the thermo- 
meter immerfed in the middle of it came to the freezing 
point. This phenomenon, which other obfervers have re- 
marked without being able to account for it, appears to me 
clearly owing to frozen particles, which in frofty weather are 
almoft always floating about in the air, often perceptibly to the 
fenfes. They come moft commonly either from clouds pafting 
over head, or from (now or hoar-froft lying upon the earth ; 
and when they touch the cooled furface of the water, inftantly 
make it freeze. That the effedt does not depend (imply on 
the con tad! of cold air, is plain from the following experiment. 
I expofed to the cold a glafs jar, with fome diftilled water, and 
placed in it tw 7 o thermometers ; one immerfed in the water, 
the other fufpended a little above its furface, in the empty part 
of the jar. The latter funk fafter than the former ; but after 
a certain time, the thermometer above the furface was at 25 0 , 
and that in the water at 23°!, yet the water continued un- 
frozen. I perceive too by M. Wilcke’s experiments, that 
in much more intenfe cold than we ufually experience in this 
country, veftels of water (landing within doors in a laboratory 
are often cooled fo far below the freezing point as to become 
almoft full of ice upon being made to (hoot, though the 
furface 
