Sutron—Study of Evaporation from Water-Surfaces. 153 
know enough about the internal structure of liquids, or the way 
in which the mutual attraction of their molecules is influenced by 
temperature. We may, to be sure, say that we know that the 
cohesion of liquids decreases as the temperature rises, because we 
know experimentally that the surface-tension (which is an ex- 
pression of the cohesion) does decrease with rising temperature.? 
Hence we may say that, with rising temperature, the violence of 
motion of the molecules in both liquid and vapor increases, but 
the restraining attractions in the liquid decrease, and there is 
nothing on the side of the vapor to offset this. And so, on the 
whole, we might expect just what actually happens.’ 
“Under date of May 27, 1902, Prof. Ernest Merritt, of 
Cornell University, Ithaca, N.Y., writes as follows :— 
“<T'o make the point raised by your correspondent perfectly 
definite, let us suppose that a certain quantity of water is placed 
in a closed vessel, the space above the water containing saturated 
vapor. The presence of air would, of course, not modify the 
conditions essentially. If the temperature of the whole mass 
is raised, it is a matter of observation that some of the water 
evaporates and the vapor becomes more dense, yet the pressure 
of the vapor is increased, and therefore the tendency of vapor 
molecules to go back into the liquid is greater than at the lower 
temperature. From the standpoint of the kinetic theory, I think 
the explanation is somewhat as follows :— 
“«< Owing to the rapid motion of the molecules, some of the 
more rapidly-moving water molecules are continually escaping 
from the attraction of their neighbors and passing out into the 
vapor, while some of the vapor molecules are at the same time 
continually returning again to the liquid. When these two pro- 
cesses just balance one another, the vapor is said to be saturated. 
Now if the temperature is raised, the motions of the molecules 
become more rapid. This is true not merely in the vapor, where 
their increased speed leads to greater pressure, but also in the 
liquid, where the result is an increased tendency for molecules to 
escape into the vapor. More molecules return to the liquid each 
second than before, but more molecules also leave the liquid each 
second. At first glance it is impossible to tell which of these 
1 [That is, increase of evaporation is, in a sense, synonymous with decrease of 
surface-tension. At the same time, other things being equal, drops will evaporate 
faster than plane-surfaces, because of surface-tension.—J. R. S.] 
SCIENT. PROC. R.D.S., VOL. XI., NO. XIII. U 
