EXPERIMENTS on EVAPORATION. 127 
with hoar froft. To prevent deception, I varied this ex- 
periment by placing a tumbler inverted in the cold veffel— 
It remained there a long time, and its furface, both with- 
in, and without, continued free from any moifture or 
frofti—I then introduced under it a piece of ice, and ina 
few minutes, the whole internal furface was covered with 
froft. 
This proved clearly that the vapour arofe from the ice 
alone; and during this experiment, another fact of the 
fame nature occurred.—When the mirrors or tumblers 
were removed from the cold veflel into the air of the room, 
which was 34°, they foon attracted moifture from it, which 
appeared on their furfaces in the form of ice or froft; they 
were replaced in the veflel when thus encrufted, and the 
ice foon difappeared, their furfaces becoming as bright as 
before. ! 
The whole of this procefs was pleafiag,—while the 
mirror remained in the cold veffel, its furface continued 
bright, very foon after it was placed in the air of the room, 
it became dull, as if breathed upon, this dullnefs increafed 
to an evident moifture confifting of {mall drops of water, 
a fibre of ice then formed fuddenly in the moifture, a fe- 
cond appeared to fhoot from this, a third from the fecond, 
and fo on, untilthe whole was congealed. Whenthisconge- 
lation was completed, the mirror was returned to the cold 
veflel, and the ice difappeared in about the fame fpace of 
time in which it had formed. 
This colle&tion of moifture on the furfaces of bodies 
cooled to o®, and then expofed to air of 34°, is analo- 
gous to the formation of drops of water on the furfaces of 
cool bodies expofed to the warm air of fummer, it proves, 
that even in cold weather, a large quantity of moifture 
exiits in our atmofphere. 
When the ice was in the cold veffel, I obferved that it 
{moked but about twelve or fifteen minutes, and fufpedcted 
that 
