VIII
197
EVAPORATION OF LIQUIDS
can only be diminished, not increased. (2) The vapour arising
from an evaporating surface is either saturated or unsaturated,
never supersaturated. For it appears perfectly transparent,
which could not be the case if it carried with it substances in a
Now m =
liquid state. The first statement asserts that P<p₁, the second
that d<d, if we denote by p, the pressure of the saturated
vapour at the temperature T₁, and by d, the density of the
saturated vapour at the pressure p.
√gd(P-p),
and therefore m</gd,(Pi-p). But the right-hand side of
this inequality is zero when p = 0 and when p-p₁; between
these it attains a maximum value which m cannot under any
circumstances exceed for a surface-temperature T₁. But if in
spite of an adequate supply of heat the evaporation cannot
exceed a finite limit, the hindrance can only lie in the nature
of the fluid; and hence every fluid must have a specific
evaporative power.
The existence of such a constant is there-
fore as probable as the assumptions on which our reasoning
is based. From the above equation I have calculated the
limits for m, assuming the Gay-Lussac - Boyle law to be
applicable to the vapour, and taking for the relation between
the pressure and temperature of the saturated vapour the
equation log p = 10.59271-0·847 log T-3342/T, which is
established elsewhere.' By dividing the values of m by the
density of mercury we get values for the maximum depth of
the layer of liquid which can evaporate in unit time from a
surface at the given temperature.
T= 100 110 120 130 140 150 160 170 180 °C
h< 0.70 111
u< 2110 2192
1.86 3.01 4.50 6.73 9.82 14.31 20.42
mm.
min.
P > 0.046 0.07
d/d> 0.0034 32
30
28
24
2294 2400 2522 2668 2823 2980 3145
0.09 0.14 0.20 0.27 0.38 0.53 0.71 mm.
26
18
m.
sec.
22
20
h> 0.03 0.13 0.21
0.32
0.47
0.65 0.88 1.21 1.67
u > 7.5
7.5 74 7.3
7.1
6.9
6.8 6.6 6.5 6.2
mum.
min.
m.
sec.
|(P-p)/p₁>0·0034|
32
30
28
26
24
22
20
22
20
18
¹ See IX. p. 204.