EXPERIMENTS on EVAPORATION. 131 
of its heat this {moking ceafes, but if removed toa cold 
er place, (as the outfide of the window on a frofty day;) 
it will fmoke again. Many other familiar fas tend to 
fhow, that vifible evaporation or {moking, does not de- 
pend upon any pofitive degree of heat, but merely upon 
an excefs of it in the moift body, when compared with the 
air to which it is expofed. 
The {moking of water has been afcribed by Mr. de Luc, 
to the paflaze of heat or fire, from the moi body iato the 
air around it: he fuppofes thts fire to carry fome water 
diffolved in it into the air, thus forming {moke. 
Without entering into the circumftances of this union 
of water and heat, I think it may be aflumed as a general 
fact, that whenever water and air are in conta&t, and the 
heat of the water exceeds that of the air in any confidera- 
ble degree, the paflage of heat from the water to the air 
is attended with {moking, or the afcent of inelaftic vifible 
vapour. 
If this motion of heat and fmoking are infeperably con- 
nected, the reafon why ice {moked when firft introduced in-~ 
to the cold vefiel, is very clear, as its temperature was 32° 
above that of the air in the veffel. 
Ido not pretend that this paflage of heat from moift 
iubfances into air is the only caufe of evaporation, we 
have already obferved that water will evaporate into air 
which is warmer than itfelf as in the {pecies of evaporati- 
on firft defcribed, andin the third fpecies, the elaftic va- 
pour forms at the bottom of boiling water without any 
contact with air, But the vifible {pontaneous evaporation 
appears different from thefe, and I think that the hypothe- 
fis which fuppofes it to depend upon the paflage of heat, 
is rendered -probable by the following fats which occurred 
during the above experiments. 
1. The ice {moked for a few minutes only after it was 
drapped into the cold air. 
KR 2. rhe 
