Notes on Osmotic Theory. 261 



is an ideal limiting case of the experimental fact that good 

 conductors of electricity are also good reflectors of radiant 

 energy. The electric vector accompanying a ray of light 

 gives rise to a finite charge on the surface of the super- 

 conductor and a finite current within the conductor. The 

 charge exerts a negative pressure on the surface, while the 

 current, in presence of the field of the magnetic vector 

 accompanying the ray, produces a mechanical force in the 

 contrary direction. The resultant of the two ; when averaged 

 statistically, yields the radiation pressure. How far these 

 theories are consistent with the theory of stresses may form 

 a subject for interesting investigations. 



My best thanks are due to Prof. D. N. Mallik for his 

 kind help and encouragement. 



XXV. Notes on Osmotic Theory. 

 By the Earl of Berkeley, F.R.S.* 



ONE of the chief difficulties in understanding osmotic 

 theory seems to lie in the unavoidable complexity of 

 the notation. This complexity arises from the fact that there 

 are two sets of osmotic pressures, depending on which com- 

 ponent is regarded as the solvent, and each of these sets is 

 again capable of differentiation into another two sets accord- 

 ing as the pressure on the solution or the pressure on the 

 solvent is to be regarded as constant. Again, from the defi- 

 nition of osmotic pressure — namely, that it is the difference 

 between the pressure on the mixture and that on the pure 

 component when the two are in equilibrium through a semi- 

 permeable membrane, — it is easy to see that, replacing the 

 solution by a mixture of vapours and the pure solvent by 

 pure vapour, there will be yet another eight sets of osmotic 

 pressures. We can even extend the definition to cover the 

 case of any liquid (or solution) which, when under pressure, 

 is in equilibrium through a membrane with its own vapour 

 under a different pressure. 



In fig. 1 all but two of the possible states of osmotic equi- 

 librium are shown f . 



The dotted iines represent semi-permeable membranes ; 

 the left-hand membranes are permeable to component A 

 oidy and the right to B only. The left-hand half of the 

 horizontal membrane separating compartment (2) from (5) 



* Communicated by the Author. 



f The two that are omitted are those between compartment (2)/(4) 

 and (2)/(6). 



