114 Prof. Clausius on the Nature of the Motion 
with such violence from its neighbouring molecules that it has 
already receded from the sphere of their action before losing all 
its velocity under the influence of their attracting forces, and 
thus that it continues its flight into the space above the liquid. 
Conceive this space to be enclosed, and at the commencement 
empty ; it will gradually become more and more filled with these . 
expelled molecules, which will now deport themselves in the space 
exactly as a gas, and consequently in their motion strike agaist 
the enclosing surfaces. The liquid itself, however, will form one 
of these surfaces ; and when a molecule strikes against the same, 
it will not in general be driven back, but rather retained, and, as 
it were, absorbed in consequence of the renewed attraction of the 
other molecules into whose vicinity it has been driven. A state 
of equilibrium will ensue when the number of molecules in the 
superincumbent space is such, that on the average as many 
molecules strike against, and are retained by the surface of the 
liquid in a given time, as there are molecules expelled from it in 
the same time. The resulting state of equilibrium, therefore, is 
not a state of rest or a cessation of evaporation, but a state im 
which evaporation and condensation continually take place and 
compensate each other in consequence of their equal intensity. 
The density of the vapour necessary for this compensation, 
depends upon the number of molecules expelled from the surface 
of the liquid in the unit of time; and this number is again evi- 
dently dependent upon the activity of the motion within the 
liquid, that is to say, upon its temperature. I have not yet suc- 
ceeded in deducing from these considerations the law according to 
which the pressure of vapour must increase with the temperature. 
The preceding remarks on the deportment of the surface of 
the liquid towards the superincumbent vapour, apply in a similar 
manner to the other surfaces which enclose the space filled with 
vapour. The vapour is in the first place condensed on these 
surfaces, and the liquid thus produced then suffers evaporation, 
so that here also a state must be attained in which condensation 
and evaporation become equal. The requisite quantity of con- 
densed vapour on these surfaces depends upon the density of the 
vapour in the enclosed space, upon the temperature of the vapour 
and of the enclosing surfaces, and upon the force with which the 
molecules of vapour are attracted towards these surfaces, In 
this respect a maximum will occur when the enclosing surfaces 
are completely moistened with the condensed. liquid; and as - 
soon as this takes place, these surfaces deport themselves exactly 
like a single surface of the same liquid. 
8. The reason why the presence of another gas above the liquid 
cannot impede the evaporation of the same may now be immedi- 
ately explained, 
