Mr. Daniell on Evaporation. 50 
shewn, that a surface of 2:7 inches diameter, only lost 1:24 grains 
in half an hour. This would have been 1:41 grains at the tempe- 
rature of 490. The surface of the wet thermometer could not have 
exceeded ;1,th of that of the evaporating vessel, and the maxi- 
mum effect was produced in ten minutes, or 4 of the time, so that 
the weight of water evaporated in this case was not more than 
(:0094 grains) one-hundreth of a grain. It will be seen, that the 
depression increased with the rarefaction of the air, but in the pro- 
portion only of the terms of an arithmetical progression to those of 
a geometrical. The increase is attributable, not to the augmented 
quantity of the evaporation, but to the decreased heating power 
of the atmosphere. MM. Du Long and Petit, in their experi- 
ments upon the cooling power of air, determined it to be nearly 
as the square root of the elasticity; but whether the heat which it 
is capable of communicating to a cold body, follow the same pro- 
gression, the experiments above detailed are not sufficient to de- 
termine with precision, We may, however, certainly conclude 
from them, that the temperature of an evaporating surface is not 
affected by the mere quantity of evaporation. 
It is right to remark that, in the last experiment, care was 
always taken to station the evaporating thermometer in .the same 
place in the receiver, for I found that, when the air was highly 
rarefied, a greater degree of cold could be produced by approxi- 
mating the wet bulb to the surface of the acid. No difference, 
however, could be perceived from such a change at the full atmo- 
spheric pressure. I also ascertained that no change of relative 
position in the surfaces of the acid and water produced any al- 
teration in the dew point under any circumstances. 
_ The few simple facts above determined appear to me to be in- 
timately connected with the solution of some very important at- 
mospheric phenomena, and I shall endeavour briefly to indicate 
their relation. 
_ The aqueous fluid is so abundantly spread over the face of the 
earth, that there can be no doubt that the permanently-elastic 
atmosphere, which surrounds it, would very speedily be saturated 
With its steam, did notgome cause, analogous to the sul phuric acid 
