30 Lieut. Hunt on Cohesion of Fluids, 



layers, or each term has approximately only one-half of its 

 value for the interior. Hence the value of x is approximately 

 only one-half of the interior value, or the cohesion along a sur- 

 face is about one-half what it is within the mass. But as this 

 value gives a rarefication also along the surface as well as 

 along the normal, it will therefore be much diminished, so as 

 to become less than one-half the general value. Thus both 

 along the normal and along the surface, a weak cohesion is a 

 necessary characteristic of the bounding layers of material 

 masses, both fluid and solid. The result thus reached in re- 

 spect to a mass in vacuo, would not be greatly affected in the 

 ordinary atmosphere. 



It is somewhat remarkable that Poisson's capillary theory, 

 as stated by Mossotti, in Taylor's Scientific Memoirs, is 

 based essentially on an analysis of the fluid surface, in 

 which the halving of the normal layer is totally overlooked, 

 and the cohesion along the surface is declared to be the same 

 as in the mass, the surface layer only having been taken into 

 account. I have not seen Poisson's work, but it is singular 

 that Mossotti should either have made such an oversight, or 

 have failed to detect it in Poisson, if he really committed it. 

 It is a radical defect — even using Poisson's own hypothe- 

 sis — and must directly affect, or even invalidate, his whole 

 theory. 



I come now to an important deduction from the preceding 

 discussion. Fluid surfaces are in a state of weak cohesion 

 as compared with fluid interiors ; hence a partially atmo- 

 spheric condition of rarefaction exists along such bounding 

 surfaces. If, then, we assimilate heat to a molecular repul- 

 sion, as is customary, we see at once that as the temperature 

 is raised the weak cohesion in the surface layer will be 

 wholly overcome long before the mass is heated to that point 

 which will overmaster its internal cohesion. Hence the sur- 

 face molecules will freely pass off as vapour, while a strong 

 cohesion still exists throughout the entire mass. Evapora- 

 tion thus goes on at surfaces, at all temperatures above 

 that which just suffices to overcome the weak surface cohe- 

 sion. This constitution or structure necessarily characteriz- 

 ing the limiting layers of fluids, is the true and full explana- 



