VIII
199
EVAPORATION OF LIQUIDS
greatly rarefied vapours nearly the same amount will impinge
upon the liquid boundary-surface, for the molecules at their
mean distance from the surface will be removed from the
influence of the latter. Now, as the amount of the saturated
vapour neither increases nor decreases, we may conclude that
an equal amount is emitted from the liquid into the vapour.
The amount thus emitted from the liquid will be approximately
independent of the amount absorbed; thus evaporation, i.e.
diminution of the amount of liquid, takes place when for any
cause a smaller amount than that above mentioned returns
from the vapour to the liquid. In the extreme case in which
no single molecule is returned to the liquid, the latter must
lose the above amount in unit time from unit surface. This
amount is therefore an upper limit for the rate of evaporation.
It is somewhat narrower than the one first deduced. Calcu-
lation shows that for mercury at 100° this limit is 0.54
mm./min., whereas from our earlier assumptions we could
only conclude that the rate of evaporation must be less than
0.70 mm./min. Similar reasoning can be applied to the
maximum amount of energy which can proceed from an
evaporating surface; we thus find that the velocity of the
issuing vapour can never exceed the mean molecular velocity
of the saturated vapour corresponding to the temperature of
the surface, e.g. for mercury at 100° it cannot exceed 215
m./sec. Finally, since the pressure of a saturated vapour upon
its liquid arises half from the impact of the molecules entering
the liquid and half from the reaction of those which leave the
surface, and since the number and mean velocity of the latter
approximately retain their original values, it follows that the
pressure upon an evaporating surface cannot be much smaller
than half the saturation-pressure.
These considerations enable us to fix limiting values, but
they will not carry us further unless we are willing to accept
the assistance of very doubtful hypotheses.