Prof. J. H. Poynting on Osmotic Pressure. 295 



of open framework, or as a spongy structure through which 

 the molecules of vapour pass freely so that they are at the 

 same pressure within and without the liquid at the same level. 

 But this conception must bo used only to give us the net 

 result, and not as representing the actual condition. 



If, in addition to the vapour, any soluble gas is present in 

 the vessel, it too will exist both inside and out in quantities 

 increasing as we descend, and it must be in equilibrium at all 

 levels. So that if near the flat surface the density of the gas 

 in solution is n times the density at the same level outside, 

 the same ratio will hold at all depths. Again the net 

 external result is the same as if we picture to ourselves a 

 spongy structure through which the gas passes freely. 



As a further illustration of the change of mobility with 

 pressure, we may take the alteration of the melting-point 

 which I have 'discussed in the paper mentioned above. Thus, 

 in the case of water, water and ice are in equilibrium under 

 1 atmo. at 0°, and therefore have equal vapour-tensions and 

 equal surface mobilities. If, however, we put on pressure, 

 the coefficients of increase of mobility are, as we have just 



seen, — and — 7 , where p and p' are the densities of water 

 tn p tz p n r r 



and ice, and a and vr the density and pressure of the vapour 

 respectively. Since p is greater than p f the water mobility 

 is increased less than the ice mobility, and so at the surface 

 of contact the ice sends more molecules to the water than it 

 receives in return, that is to say, it melts. Below 0° the 

 vapour-pressures and mobilities at atmospheric pressure are 

 different, the mobility of water being greater than that of ice. 

 But if we put on sufficient pressure we may once more equa- 

 lize the mobilities and so lower the melting-point to the new 

 temperature. Thus if ot and vr* are the vapour-pressures of 

 water and ice at — dd, and P is the pressure making the 

 mobilities equal, or the pressure reducing the melting-point 

 to -dd, 



or „ ^/-P 



tzr — txr =Jr 





a formula equivalent to that of Kirchhoff deduced by purely 

 thermodynamic considerations. For using the ordinary for- 

 mula for lowering of melting-point, 



*G-3-* w 



