bis Thermometer with the common mercurial ones. 383 
‘been hinted at, which experiments have decidedly eftablifhed, 
‘from which the phenomena may perhaps be equally accounted 
for, and which, even though the other alfo.is received, muft 
be fuppofed to concur fer fome part of the effet; I mean, that 
“evaporation produces cofd; both vapour and fteam carrying off 
{ome proportion of heat from the body which produces them. 
If, therefore, evaporation be made to take place upon the fur- 
face of ice, the contiguous ice will thereby be rendered colder ; 
and as it is already at the freezing point, the fmalleft increafe 
of cold will be fufficient for frefh congelation. It feems to be 
on this principle that the formation of ice is effected in the 
Eaft Indies, by expofing water to a ferene air, at the coldeft 
feafon of the year, in fhallow porous earthen veflels: part of 
the water tranfudes through the veflel, and evaporating from. 
the outfide, the remainder in the veffel becomes cold enough to. 
freeze ; the warmth of the earth being at the fame time in- 
tercepted by the veffels being placed upon bodies little difpofed 
to conduct heat*. If ice is thus producible in a climate where 
natural ice is never feen, we need not wonder that congelation 
fhould take place where the fame principle operates amidft 
actual ice. 
It has been obferved above, that the heat emitted by the 
eongealing vapour probably unites with and liquefies conti- 
guous portions of ice; but whether the whole, either of the 
heat fo emitted, or of that originally introduced into the fun- 
nel, is thus taken up; how often it may unite with other por- 
tions of ice, and be driven out from other new congelations ; 
whether there exifts any difference in its chemical affinity or 
* See a defcription of this procefs in the Philofophical Tranfactions, vol. 
LEV. p. 253. 
elective 
