OCTOBEB 2, 1914] 



SCIENCE 



461 



some salts under the circumstances men- 

 tioned above, the first of the solution to 

 come through the sand filter is stronger in- 

 stead of weaker. This, as already men- 

 tioned, is because water, or at least a 

 weaker solution, forms the adsorption 

 layer. 



Most of the alkali chlorides as the tem- 

 perature is raised show this anomalous ad- 

 sorption, provided the strength of the so- 

 lution is below a certain critical value 

 differing for each temperature. For 

 strengths of solution above these values the 

 normal phenomenon takes place. 



No investigations seem to have been 

 made on the effect of pressure on adsorp- 

 tion. These data are much to be desired. 



The investigation of adsorption and ab- 

 sorption should throw light on osmosis, as 

 in the first place the phenomenon occurs 

 across a surface necessarily covered with 

 an adsorption layer, and in the second 

 place, as we shall see, the final condition is 

 an equilibrium between the absorption of 

 water by the solution and that by the mem- 

 brane. 



The study of the conditions of absorp- 

 tion of water throughout the mass of the 

 colloidal substance of which osmotic mem- 

 branes are made is of much interest. Little 

 work has been done on the subject as yet, 

 but what little has been done is very prom- 

 ising. 



It is convenient to call the material of 

 which a semi-permeable membrane is made 

 the semi-permeable medium. The ideal 

 semi-permeable medium will not absorb 

 any salt from the solution, but only water, 

 but such perfection is probably seldom to 

 be met with. If a semi-permeable medium 

 such as parchment paper be immersed in a 

 solution, say, of sugar, less water is taken 

 up or absorbed than is the case when the 

 immersion is in pure water. The diminu- 

 tion in the amount absorbed is found to in- 



crease with the strength of the solution. It 

 is at the same time found that the absorp- 

 tion or release of water by the semi-perme- 

 able medium according as the solution is 

 made weaker or stronger is accompanied by 

 a swelling or shrinkage greater than can be 

 accounted for by the water taken up or re- 

 jected. 



The amount of water absorbed by a semi-, 

 permeable medium from a solution is found 

 by experiment to depend upon the hydro- 

 static pressure. If the pressure be in- 

 creased the amount of water absorbed by 

 the semi-permeable medium is increased. 

 It is always thus possible by the applica- 

 tion of pressure to force the semi-perme- 

 able medium to take up from a given so- 

 lution as much water as it takes up from 

 pure water at atmospheric pressure. 



It is not possible for a mass of such a 

 medium to be simultaneously in contact 

 and in equilibrium with both pure water 

 and with a solution all at one and the same 

 pressure, seeing that the part of the me- 

 dium in contact with the pure water would 

 hold more water than that part in contact 

 with the solution, and consequently diffu- 

 sion would take place through the mass of 

 the medium. 



If, however, the medium be arranged so 

 as to separate the solution and the water 

 and provided the medium is capable of 

 standing the necessary strain, it is possible 

 to increase the pressure of the solution 

 without increasing the pressure of the 

 water on the other side. Thus the part of 

 the medium which is in contact with the 

 solution is at a higher pressure than that 

 part in contact with the pure solvent ; con- 

 sequently the medium can be in equilib- 

 rium with both the solution and the sol- 

 vent, for if the pressures are rightly ad- 

 justed the moisture throughout the medium 

 is everywhere the same. 



The ordinary arrangement for showing 



