+728 EXPERIMENTS on EVAPORATION. 
that perhaps the evaporation continued no longer, to de- 
termine this point, I placed two tumblers in the cold vef- 
fel, and when they were cooled, placed alump of ice in 
the fame fituation and inverted one of them over it—this 
tumbler became encrufted with froft as before; it remain- 
edtwenty minutes and then being removed, the other was 
inverted over the ice in its place, but although the fecond 
tumbler remained a long time in this fituation, its furface 
continued perfectly free from any moifture or ice whatever. 
This refult appeared to me a full proof that the actual, as 
well as the apparent evaporation, ceafed in a few minutes 
after its commencement; but from the whole of the ex- 
periments I was induced to believe that, while the evapo- 
ration went on, it was much more rapid in the cold veffel, 
than in the open air which was fo much warmer—to deter- 
mine this accurately, two lumps of ice of the fame weight 
and form, fhould have been expofed a given time, one to 
the air of the veflel, and the other to the air of the room, 
and then weighed accurately; but having no nice {cales, 
I was reduced to another expedient much lefs exa@.—As 
mioifture is very confpicuous on mirrors or polifhed furfac- 
es, I thought of comparing one of them which had been 
moiftened and placed in the cold veffel, with another which 
had been equally moiftened, but placed in warmer air,— 
for this purpofe I took two razors highly polifhed, and, 
after expofing them to my breath fo that each was equally 
dull, I placed one of them in the cold veffel, and at the 
fame time, held the other in air of 34°—1n feveral inflances 
the razor in the cold air loft its moifture fooneft, and in 
fome other inftances, both of them loft their moifture fo 
guickly, that it was difficult to compare them. 
I refrain however from drawing a conclufion from thefe 
refults, becaufe when the fame razors were expofed to my 
breath, and then placed, both of them in air of 34°, one 
loft its moifture in lefs time than the other—although this 
circumftance 
