Mr Vegard, On the Free Pressure in Osmosis. 13 



On the Free Pressure in Osmosis. By L. Vegard, Cand. 

 real. (Research Student at the Cavendish Laboratory), [Com- 

 municated by Sir J. J. Thomson.] 



[Read 9 November 1908.] 



1. In the introduction to their paper on osmotic pressure* 

 Berkeley and Hartley mention that a method for studying osmotic 

 phenomena is given by the determination of what they call the 

 osmotic force, and this method " depends on the determination of 

 the rate at which the solvent will flow through a semipermeable 

 membrane into a large quantity of solution when there is no 

 pressure on the latter. A knowledge of this rate together with 

 the frictional resistance will enable the osmotic force to be 

 calculated in absolute units." 



The authors, however, do not enter further into the question, 

 and we get from this no clear idea as to the strict definition of the 

 osmotic force. 



The following paper contains some experiments done in this 

 direction as well as some conclusions to be drawn from the 

 experiments. The experiments consist in some determinations 

 of the frictional resistance and- of velocities produced by osmosis 

 when solution and solvent are exposed to the same pressure. The 

 cell used is the same as used in the experiments upon osmosis 

 described in an earlier paper f. 



Determination of the Frictional Resistance. 



2. The frictional resistance depends on the velocity and is 

 measured by the pressure that must be applied in order to press 

 the solvent through the membrane with the velocity under con- 

 sideration. If pressure and velocity are denoted by Q and V 

 respectively we have Q=y'(F) provided temperature is kept con- 

 stant. To find this function we have simply to determine the 

 velocity corresponding to different pressures. 



The measurements were made with the same osmometer as 

 described in an earlier paper and the way of proceeding the same 

 as used for the determination of osmotic velocities only that now 

 the solution was replaced by the pure solvent. The relation 

 between Q and V has been determined in two sets of measure- 



* The Earl of Berkeley and E. G. J. Hartley, Phil. Trans. Roy. Soc. 1906, 

 p. 481. 



t L. Vegard, Phil. Mag. Vol. xvi. No. 92, 93, 1908. 



