Surron—Study of Evaporation from Water-Surfaces. 151 
strike each other and the boundary surface, that is to say, we 
increase the general expansive pressure of the vapor. If no 
liquid is present, and therefore if no evaporation throws more 
vapor into the space, then this increase of pressure corresponds to 
that due to the ordinary coefficient of expansion of the gas; but 
if we allow heat to break up liquid molecules into gaseous ones, 
and evaporation to increase the number of molecules in the 
gaseous space until it is saturated at the new higher temperature, 
then the increase of pressure is the sum of two causes, namely, 
the increased momentum of the old molecules, and the added 
momentum of the new ones... . 
“In order that our correspondent and readers may have the 
choice of several methods of looking at this question, we give the 
following extracts from recent correspondence : 
“Under date of April 4, 1902, Professor J. S. Ames, of 
Johns Hopkins University, Baltimore, Md., writes as follows :— 
“«My understanding of the reason why an increase in 
temperature increases the evaporation of a liquid has always been 
along the lines that the effect of temperature upon the liquid is 
such as to so increase the velocity of the particles that more of 
them are able to escape from the surface, and that therefore the 
evaporation is increased. At the same time, of course, there is 
an increase in the velocity of the particles of vapor. But the 
question as to these particles reaching the liquid surface is more 
one of the mean free path than of anything else, and this is 
not affected to any such extent as to increase the rate of 
condensation to so great an extent as the evaporation.’ 
“Under date of April 15, Professor J. Willard Gibbs, of 
Yale University, writes as follows :— 
“In regard to your correspondent’s question, we must 
remember that the average velocity of molecules in the liquid is 
increased as much as the average velocity in the vapor (when 
the temperature is increased). The restraining power of the 
attractions in the liquid will evidently have less effect in 
these greater velocities.’ 
“Dr. Edgar Buckingham, now physicist in the Department 
of Agriculture, under date of May 10, 1902, writes as follows :— 
“«« Heating a gas or vapor in an enclosed space of fixed 
volume increases its pressure. If we accept the hypothesis that 
matter is made up of molecules, or separate particles, we account 
