REVIEWS 521 



Putting aside all special and technical senses of the term "vapor 

 pressure," the facts of the case seem to be these: 



If a granule of snow be placed in a suitable receptacle whose inner 

 surface is such as to throw the evaporated particles back, an equilibrium 

 state between the thrown-out and the thrown-back will be reached in 

 time. In this state neither growth nor depletion takes place. The out- 

 throw (evaporation) of a small granule with a high curvature is greater 

 than that of a large granule with low curvature. When, therefore, the 

 small granule is shut up by itself in a receptacle until it attains equilib- 

 rium, it develops a higher throw-back; i.e., a higher vapor pressure about 

 itself, than does a large granule of less curved surface. But looked at in 

 this way, it is seen that it is the relative evaporation that is the actuating 

 agency and that it is the superior evaporation of the small granule of higher 

 curvature that gives it the higher vapor pressure when it is suitably con- 

 fined. The vapor pressure is merely a secondary effect and is dependent 

 on the confining contrivance. 



If a granule be placed in a perfect vacuum of unhmited extent^ — the 

 open sky would be an admirable illustration if the atmosphere about the 

 earth were absent^ — no such vapor pressure on the granule would arise. 



If, instead of either of these ideal cases, we consider the most common 

 of actual cases — that of a granule within a layer of snow — we find it 

 surrounded by the interspaces between itself and its neighboring granules. 

 These interspaces are more or less affected by the varying pressures of 

 the wind and the states of the barometer, and are directly affected by 

 the evaporation of all the granules opening upon them. Out of this 

 complexity let us select for illustration the simple typical case of a large 

 granule opposite a small one. The latter evaporates faster than the 

 former. The first effect then is that each granule receives the particles 

 thrown off from the other, i.e., the large granule receives more granules 

 thrown off by the small granule than the small granule receives from 

 the large granule. After the first action there follows an indefinite 

 series of secondary actions of the to-and-fro type which give rise to a 

 common vapor pressure to which all the other granules opening on the 

 cavity also contribute. This common pressure is controlled ultimately 

 by the gravity of the earth and becomes a factor in the partial pressure 

 of the vapor of the atmosphere. Taken as a whole, it is thus clear that 

 the process of growth under these conditions is primarily that of evapora- 

 tion and re-accretion, and in this the small granule loses most on the 

 average and gains least. The special vapor pressure on the surface of 



