142 



Mr. H. E. Roaf. The Vapour-Pressure 



A m'odel to illustrate the above principles was made (22) by enclosing 

 four rubber balloons in silk bags and suspending them from a disc ; a rubber 

 cylinder was placed round these, extending from the supporting disc to a 

 similar disc below. The whole was filled with water : a certain amount of 

 water was removed from the cylinder surrounding the balloons and the same 

 volume was injected into the balloons. The total volume was thus unchanged 

 and the model contracted to 50 per cent, of its original length. By more 

 careful construction there is no doubt that a greater degree of contraction can 

 be produced. 



The diagram shows, to scale, the contraction when ellipsoids 0'9 /x long 

 and - 3 //, in diameter become spheres of the same circumference, the light 

 bands before contraction being of the same length as the dim band. 



The results of the calculations in this section are independent of the actual 

 dimensions, but they depend on the relative dimensions. The results in the 

 following section, however, depend on the absolute values. 



Mate of Contraction. 



Graham pointed out that " in minute microscopic cells the osmotic move- 

 ments should attain the highest velocity, being mainly dependent upon the 

 extent of the surface " (7). This statement can be amplified by calculating 

 the rate at which the ellipsoids would swell to form spheres. 



The amount of liquid that is absorbed (column 13) is the difference between 

 the volume of the sphere (column 6) and the volume of the ellipsoid 

 (column 2). This amount of liquid passes through a surface which can be 

 taken to be of the same extent as the surface of the sphere (column 7). 



To calculate the rate at which absorption of water may take place through 

 the membrane we may utilise the data which I obtained in some direct 

 determinations of the osmotic pressure of haemoglobin solutions (17, 20). 

 The surface area of the parchment paper was 19 sq. cm. and osmotic 

 equilibrium was attained in about three days by the absorption of about 

 5 c.c. of water. 



If the rate of diffusion into the anisotropic substance is the same as the 

 rate of diffusion through the parchment paper, the length of time until 

 equilibrium will be reached will be directly proportional to the volume of 

 liquid passing in, and inversely proportional to the surface area through 

 which the liquid can pass. The increase in volume when the ellipsoids 

 become spheres (column 13) and the surface of the spheres (column 7) are 

 given in the Table. 



In order to compare the increase in volume and surface of the osmometer 

 with the corresponding measurements of the ellipsoids, the measurements in 



