152 Scientific Proceedings, Royal Dublin Society. 
for this increase of pressure by saying that the energy put into 
the vapor in the form of heat to raise its temperature, has gone, 
at least in part, to increasing the kinetic energy of translation of 
the molecules, so that when the vapor is hotter its molecules fly 
about more violently, strike harder and oftener on the walls of the 
enclosing vessel, and so exert a greater pressure. 
“« Now suppose that a part of the enclosing wall consists of 
the surface of the liquid from which the vapor has been sent off. 
If we raise the temperature of the vapor without raising that of 
the liquid, we cannot have a state of equilibrium, and we can- 
not speak of a definite pressure of the vapor, or of a definite vapor 
density ; we must always keep the liquid and the vapor at the same 
temperature, and imagine them heated or cooled simultaneously. 
““¢ Suppose then that we heat a liquid in contact with its 
vapor, the two being enclosed in an envelope of invariable volume. 
According to the kinetic hypothesis, the vapor molecules fly about 
more violently ; they strike the surface of the liquid harder and 
oftener; and we should naturally expect more of them to get 
caught among the molecules of the liquid, so that the vapor 
density would decrease, and we should have, in effect, a conden- 
sation of vapor, and a decrease in the vapor density. 
“¢¢ But we have been considering only the vapor without 
paying any attention to the liquid, and this upsets our former 
conclusion. It is true that more molecules of vapor may get 
caught in the liquid, and so be condensed or become a part 
of it. But, on the other hand, the kinetic hypothesis assumes 
that the molecules of a liquid also are in a state of motion, 
although they move through shorter distances, and with less 
freedom than the molecules in the vapor. Heat applied to 
raising the temperature of the liquid increases the violence of 
motion of the molecules of the liquid too. Hence more particles 
are likely to arrive at the surface of the liquid, from within it, 
with sufficient velocity to tear themselves away from the attraction 
of their fellow-molecules and fly away freely into the space filled 
with the vapor. 
«‘¢There are thus two opposing tendencies: one for the 
molecules of the liquid to fly off into the vapor space, and the 
other for the molecules of vapor to get entangled in the liquid. 
But there is no way of seeing d priori why the one 
tendency should increase faster than the other, because we do not 
