96 PHYSICAL SCIENCE 



membrane, indicated by a dotted line, where they 

 meet inside the thickness of the walls. This 

 process can be hastened, and the resulting 

 membrane improved, by forcing the salts into 

 the porous material by means of an electric 

 current. The solutions are washed away, and 

 the wide glass tube is drawn out and sealed to 

 a smaller tube in the manner shown in the figure. 



Inside a cell thus prepared let us place the 

 solution of some substance, such as sugar in 

 water, and surround the outside with a large 

 volume of the pure solvent, in this case, water. 

 Water will gradually force its way into the cell, 

 and, by placing mercury in the glass tube to use 

 as a pressure gauge, it will be found that this 

 influx will continue till a definite internal pressure 

 is reached — a pressure greater than that without. 

 This gives a measure of what is called the osmotic 

 pressure of the solution as it finally exists in the 

 cell after the entrance of the additional quantity 

 of water. 



Pfeffer found that this osmotic pressure was 

 proportional to the concentration of the solution, 

 at all events between the concentrations of i and 

 6 per cent, of sugar. For a i per cent, solution, 

 the excess of pressure at 6°. 8 C. was equal to 

 that of a column of mercury 505 millimetres high, 

 the normal atmospheric pressure being equivalent 

 to 760 millimetres. 



Many membranes within animal and vegetable 

 organisms are semi-permeable, or, at all events, 

 are more permeable to solvent than to solution. 

 The permanent or temporary differences of 

 pressure, which are thus set up, are being 

 investigated extensively by physiologists, and 



