294 Prof. J. II. Poynting on Osmotic Pressure. 



theorem shows that there is ultimately a balance between 

 evaporation and condensation at each surface, or that the 

 vapour-tension is less at the surfaces a and b than the normal 

 amount existing at c, while at d, e, and / it is greater. In 

 other words, the surface mobility gradually increases as we 

 go downwards. This is usually connected with the curvature 

 of the liquid surface, but, as I have tried to show in a former 

 paper (Proc. Phys. Soc. vol.iv.p. 271, Phil. Mag. July 1881), 

 it should rather be connected with the increased pressure of 

 the liquid just under the surface as we descend ; the curva- 

 ture of the surface is a non-essential accompaniment. 



Taking the pressure of the vapour at the flat surface c in 

 fig. 1 as -or, and the densities of liquid and vapour as p and a 

 respectively, then at a level h below or above c the hydro- 

 static pressure is greater or less than at c by gph, =P say, 

 while the vapour-pressure is greater or less than at c by 



gah — — ; or the increase in vapour-pressure at a surface as 



r 



we descend is proportional to the increase in hydrostatic pres- 

 sure just under that surface. This is accounted for if we 

 suppose that the increased hydrostatic pressure results in 

 increased mobility, and therefore increased evaporation from 

 the surface. The vapour-pressure increases from -or to 



, Pot A P<A 



P \ ™PJ 



or the coefficient of increase of its mobility is — per unit of 



hydrostatic pressure, and this is the coefficient we must 

 assume for the increase of internal liquid mobility to account 

 for the facts on this theory. 



We have no direct evidence that increase of pressure does 

 thus increase liquid mobility. The justification is to be 

 sought in such explanations of known facts as that just 

 given *. 



It is perhaps worth noting that we obtain the true state of 

 affairs externally if we picture the liquid in fig. 1 as a kind 



* Liquid viscosity should decrease if mobility increases, and should 

 therefore, in our view, decrease with increase of pressure were mobility 

 alone concerned. But rigidity also comes in, and we must ascribe to this 

 complication the result that, in water, pressure lessens the viscosity 

 while in turpentine it increases it (Cohen, Wied. Ann. No. 4, 1892). 

 But it would appear lair to seek support for the supposition of increased 

 mobility in the "fluwing" of solids under great stresses, as in the stamp- 

 ing and wiredrawing of metal, when the molecules undoubtedly change 

 their positions with very greatly increased rapidity when under great 

 strain. 



