VIII
195
EVAPORATION OF LIQUIDS
this rule which probably occurs, and which is of interest from
the theoretical point of view.
II. Let us now consider a process of steady evaporation
taking place between two infinite, plane, parallel liquid surfaces
kept at constant, but different temperatures. We shall sup-
pose that the liquid which evaporates over can return to its
starting-point by means of canals or similar contrivances.
All the particles of vapour will move from the one surface to
the other in the direction of the common normal and, neglecting
radiation, we may with sufficient accuracy assume that in
passing over they neither absorb nor give out heat. On this
assumption it follows from the hydrodynamic equations of
motion that during the whole passage from the one surface
to the other, whatever the distance between them may be,
the pressure, temperature, density, and velocity of the vapour
must remain constant. From this it follows that the process
is completely known to us when we know the following
quantities:-
1. The temperatures T₁ and T₂ of the two surfaces.
2. The temperature T, the pressure p, and the density d
of the vapour which passes over. We must suppose the
temperature to be measured by means of a thermometer
which moves forward with the vapour and with the same
velocity. In the same way the pressure p is to be supposed
measured by a manometer moving with the vapour, or deter-
mined by the equation of condition of the vapour. We may
approximately take as the latter the equation of a perfect gas,
RT = p/d.
3. The velocity u and the weight m which passes over in
unit time from unit area of the one surface to the other.
Clearly mud.
4. The pressure P which the vapour exerts upon the
liquid surfaces. This is necessarily the same for both surfaces,
and is different from the proper pressure p of the vapour
itself.
But we can calculate P if the other quantities men-
tioned are known. For let us suppose the quantity m spread
over unit surface, the pressure upon one side of it being P
and on the other side p, and its temperature T maintained
constant. It will evaporate just as before; after unit time
it will be completely converted into vapour, which will occupy