VIII
ON THE EVAPORATION OF LIQUIDS, AND
ESPECIALLY OF MERCURY, IN VACUO
(Wiedemann's Annalen, 17, pp. 177-193, 1882.)
WHEN a liquid evaporates into a gas whose pressure is greater
than the pressure of the saturated vapour of the liquid, the vapour
near the surface is always exceedingly near the state of satura-
tion; and the rate of evaporation is chiefly determined by the
rate at which the vapour formed is removed. The removal of
the vapour, at any rate through the layers nearest the surface,
takes place by diffusion. Starting with this conception, the
evaporation of a liquid into a gas has been frequently discussed.
But hitherto no attention seems to have been paid to the con-
ditions which determine the rate of evaporation in a space
which contains nothing but the liquid and its vapour. In
this paper evaporation under these conditions will be considered.
In the first place evaporation in vacuo is affected by the rate at
which the vapour formed can escape, in SO far as this escape may
under certain circumstances be greatly retarded by viscosity;
but clearly this is a matter of very little importance. For if
we imagine the evaporation to take place between two plane
parallel liquid surfaces, then as far as this is concerned
the rate of evaporation might be infinite. Again we may
specify the rate at which heat is supplied to the surface
of the liquid as the condition of evaporation. When the
stationary state is attained, the amount of liquid which
evaporates is just so much that its latent heat is equal
to the amount of heat supplied. But this explanation