Prof. J. H. Poynting on Osmotic Pressure. 



293 



greater than that at the surface for the reason already given ; 

 viz. that in the one case the neighbouring molecules entirely 

 surround one which tends to get free, while at the surface they 

 are all on one side and so tend to pnll back and retain a mo- 

 lecule which may be inclined to move away. If, however, 

 the internal mobility at a given temperature is altered, say 

 by the pressure, or by the presence of some substance in 

 solution, the surface mobility will be altered too. We shall 

 assume that it is altered in the same ratio as the internal 

 mobility, an assumption which appears to be justified by the 

 account which it will enable us to give of the effect of pres- 

 sure and of solution. 



Let us now apply this idea to the familiar case of rise in a 

 capillary tube standing in a liquid having only its own vapour 

 above it. Or let us take the more general case of a liquid in 

 a vessel with tubes which are wet rising above the flat sur- 

 face, and with tubes which are not wet coining out of the side 



Fiff.l. 



a - 







5 







• ■ 







c 









d 





r 



e 





■ \ 



and turning upwards, and of such diameters that the liquid 

 does not rise to the top of the tube, as in fig. 1. Thomson's 



