i 4 SCIENCE PROGRESS. 



but not to dissolved substance, is now spoken of as a 

 semi-permeable membrane and the phenomena to which 

 it gives rise when it separates a solution from pure 

 solvent are commonly termed osmose, whilst phenomena 

 such as those involved in the older experiments in which 

 dissolved substance as well as solvent pass through the 

 membrane are referred to as diosmose. 



OSMOTIC PRESSURE. 



Traube noticed that different pressures were set up 

 on opposite sides of the precipitates obtained in his 

 experiments, but as the septa were much too fragile to 

 withstand any appreciable difference of pressure he was 

 unable to make quantitative observations. Such results 

 were first obtained in 1877 by W. Pfeffer, who put into 

 practice the happy idea of depositing the precipitate 

 within the wall of a small porous clay cell such as that 

 employed in a voltaic battery. If such a cell be 

 lowered into a solution of copper sulphate, while at the 

 same time a solution of potassium ferrocyanide be poured 

 into its interior, the solutions meet within the wall of 

 the cell. Little diaphragms of copper ferrocyanide are 

 thus formed which stretch across the interstices in the 

 wall, sufficient support being thus afforded to enable the 

 precipitate to withstand a pressure of several atmospheres. 



After such a cell has been washed free from the 

 liquids producing the membrane, on introducing into it a 

 quantity of solution and lowering it into pure water, just 

 as in the older experiments water passes into the solution 

 and pressure begins to increase on the solution-side of 

 the membrane. And if instead of allowing the head of 

 liquid to rise considerably within the cell, the top of the 

 latter be closed in, a quantity of air being thus im- 

 prisoned over the solution, pressure may be established 

 by the compression of this quantity of air, and the value 

 of the pressure may be ascertained by having the air- 

 enclosure connected with a manometer. 



The striking result here arrived at is that water enters 

 the cell until the pressure difference reaches a maximum, 



