364 PROCEEDINGS OF THE AMERICAN ACADEMY. 



It was thought possible that there might be a change in the character 

 of the surface as evaporation proceeded. The spheres were hard on 

 the surface, and quite smooth as they came from the water, but they 

 undoubtedly consist of a mass of very small irregular crystals and any 

 roughening that might appear during the course of the experiment 

 would lead to a considerable increase in surface. That such changes 

 do not occur in disturbing amount is shown by the fact that the deter- 

 minations made with small spheres fresh from formation fall accurately 

 on the curve of measurements on spheres which have been evaporating 

 for some hours. Microscopic examination corroborates this and shows 

 also that the spherical shape is maintained practically unchanged until 

 the sphere finally disappears completely. 



In these experiments the spheres were supported on a nearly flat 

 scale-pan of thin glass. This may introduce a variation in the surface 

 exposed to the air, due to difference in the surface of contact between 

 sphere and glass, and especially to be expected if the particles are not 

 closely spherical. This factor is also shown to be negligible by the 

 closeness with which the spherical form is kept during evaporation and 

 also by the fact that turning the particle over has no measurable effect 

 on the rate of evaporation. 



Measurements on three spheres of different radii are given below. 



These observations are plotted in the curve of Figure 1. 



There is plenty of evidence that in any system made up of smaller 

 and larger particles of the same substance, whether solid or liquid, the 

 smaller particles are relatively unstable. So far, however, all of our 

 knowledge about solids is of a purely qualitative nature, and no definite 

 relation has ever been obtained based on vapor pressure or surface ten- 

 sion, and expressing quantitatively the change of vapor pressure or 

 surface tension with change of radius. It has been many times noticed 

 that, in a sealed tube containing iodine crystals of various sizes, the 

 larger crystals grow at the expense of the smaller ones, which gradually 

 disappear. In a few days this can be clearly proved, and the same 

 effect has been noticed for water drops and for camphor and other 

 rather volatile substances. 



In the case of liquids it is possible to set up a definite relation be- 

 tween vapor pressure and curvature of drop. This has been done for 

 water and a few other liquids, and the theory has been tested with some 

 accuracy by experiments on the formation of fog by the expansion of 

 saturated water vapor. For water the difference in vapor pressure be- 

 tween a drop of radius 0.001 millimeter and a flat surface is of the 

 order of 0.001 mm. of mercury, so that the effect becomes almost in- 

 sensible for drops of any size. 



