October %, 1914] 



SCIENCE 



463 



which move back from the adsorption layer 

 are reduced owing to this deposit, thus 

 necessitating an increase in pressure for 

 equilibrium. If either or both of these 

 effects really exist, it would seem to require 

 that the pressure should be higher for equi- 

 librium of the molecular surface transfer 

 than if there were no adsorption layer and 

 the unaltered solution were to touch the 

 medium, but at the same time it should be 

 ■remembered that there is a second surface 

 where equilibrium must also exist — that is, 

 the surface of separation of the adsorption 

 layer and the solution itself. It is just 

 possible that the two together cancel each 

 other's action. 



Quantitative determinations of adsorp- 

 tion by solid media from solution are hard 

 to carry out, but with a liquid medium it 

 is not so difficult. Ether constitutes an 

 excellent semi-permeable medium for use 

 with sugar solution, because it takes up or 

 dissolves only a small quantity of water and 

 no sugar. A series of experiments using 

 these for medium and solution has shown 

 (1) that the absorption of water from a 

 solution diminishes with the strength of 

 the solution; and (2) that the absorption of 

 water for any given strength of solution 

 increases with the pressure. This increase 

 with pressure is somewhat more rapid than 

 if it were in proportion to the pressure. 

 On the other hand, from pure water ether 

 absorbs in excess of normal almost in pro- 

 portion to the pressure. Certainly this is 

 so up to 100 atmospheres. This would go 

 to confirm the suggestion already made that 

 the departure from proportionality in the 

 osmotic pressure is attributable to absorp- 

 tion. 



By applying pressure ether can be thus 

 made to take up the same quantity of water 

 from any given solution as it takes up from 

 pure water at atmospheric pressure. It is 

 found by experiment that this pressure is 



the osmotic pressure proper to the solu- 

 tion in question. 



Decidedly the most interesting fact con- 

 nected with the whole question of osmotic 

 pressure, the behavior of vapor pressures 

 from solution, and the equilibrium of mo- 

 lecular transfer of solutions with colloids, 

 is that discovered by Van't Hoff, that the 

 hydrostatic pressure in question is equal 

 to what would be produced by a gas having 

 the same number of particles as those of the 

 introduced salt. Take the case of a mass of 

 colloid or semi-permeable medium placed 

 in a vessel of water; the colloid when in 

 equilibrium at atmospheric pressure holds 

 what we will call the normal moisture. By 

 increasing the pressure this moisture can 

 be increased to any desired amount. Now, 

 on introducing salt the moisture in the col- 

 loid can be reduced at will. The question 

 is, what quantity of salt must be introduced 

 just to bring back the amount of the mois- 

 ture in the colloid to normal? Here we 

 get a great insight into the internal mech- 

 anism of the liquid state. The quantity of 

 salt required turns out to be, approxi- 

 mately at least, that amount which if in 

 the gaseous state would produce the pres- 

 sure. So that normality can be either di- 

 rectly restored by removing the pressure or 

 indirectly by introducing salt in quantity 

 which just takes up the applied pressure. 

 That this is so naturally suggested that the 

 salt, although compelled to remain within 

 the confines of the liquid, nevertheless pro- 

 duces the same molecular bombardment as 

 it would were it in the gaseous state, 

 though of course the free path must be 

 viewed as enormously restricted compared 

 with that in the gaseous state. 



Many have felt a difficulty in accepting 

 this view of a molecular bombardment oc- 

 curring in the liquid state, but of recent 

 years much light has been thrown on the 

 subject of molecular movements in liquids, 



