Mr. Daniell on Evaporation. 47 
something appears to be still wanting to complete the investiga- 
tion, and to follow up the results to their ultimate consequences. 
The following observations, however inadequate to fulfil this 
desirable purpose, may possibly attract some attention to the sub- 
ject, and may be the means of indicating the points which most 
require elucidation. 
_ It is a well-known fact that water, under all circumstances, is 
- endued with the power of emitting vapour, of an elastic force 
proportioned to its temperature. It is also well understood, that 
the gaseous atmosphere of the earth, in some degree, opposes the 
diffusion, and retards the formation of this vapour; not, as Mr. 
Dalton has shewn, by its weight or pressure, but by its vis iner= 
tie. What is the amount of this opposition, and by what pro- 
gression it is connected with the varying circumstances of density 
and elasticity, have never yet been experimentally explained. 
It may facilitate the comprehension of the subject, to distinguish 
three cases with regard to the evaporating fluid: the first, when 
its temperature is such as to give rise to vapour equivalent in elas- 
ticity to the gaseous medium, and when it is said to boil; the 
second, when the temperature is above that of the surrounding 
air, but below the boiling point; and the third, when the tempe- 
rature is below that of the atmosphere. 
_ With regard to the first, all the phenomena have been accurately 
appreciated. The quantity evaporated from any surface, under 
any given pressure, is governed, in some measure, by the in- 
tensity of the source of heat, and is in no way affected by the 
motions of the aérial fluid. The elasticity of the vapour is ex- 
actly equivalent to that of the air, which yields en masse to its 
lightest impulse. When disengaged, it is immediately precipi- 
tated in the form of cloud, giving out its latent caloric to the 
ambient medium; and under that form is again exposed to the 
process of evaporation, according to the laws of the third division 
of the process. All the phenomena attending the process of 
boiling, have been ably investigated by Gay-Lussac, Dalton, Ure, 
and Arch-deacon Wollaston; but, as they have but little con- 
nexion with the atmospheric relations, which are the particular 
