478 TRANSACTIONS OF SECftoN A. 



would be overcome, as sucli direct measiiremeut would ali'oid the most useful data 

 by means of wliich to obtain further light on the much-vexed question of the 

 nature of solutioua. I remember, also, that it was the general opinion of those 

 who had given attention to this matter that the experimental difficulties were 

 insuperable. 



I am glad, therefore, to have this opportunity of stating my high appreciation 

 of the manner in which Lord Berkeley and Mr. Hartley have grappled with the 

 difficulties of this investigation. They have proved that the osmotic pressure 

 obtained by direct measurement agrees with that derived from vapour-pressure 

 observations to within less than 5 per cent.* This agreement is of great 

 importance, as it diminishes our doubts as to the extent to which the 

 imperfections of semi-permeable membranes may affect the validity of results 

 dependent upon their behaviour, and points to the possibility of determining the 

 osmotic pressures of concentrated solutions by measurement of their vapour 

 pressures. 



I trust it will not be thought out of place if 1 here refer to the interesting 

 correspondence which has recently appeared in ' Nature ' on the thermodynamic 

 theory of osmotic pressure, and the allied, but by no means identical, problem of 

 the difference between electrolytic and non-electrolytic solutions. 



On the one side we have Professor Armstrong, whose chief desire appears to 

 be the vindication of the moral character of what he terms ' the poor molecule ' ; 

 and jNIr. Campbell, whose doubts concerning the second law of thermodynamics 

 are closely connected with a lurking belief in the existence of Maxwell's ' sorting 

 demons ' ; and by way of reserves we have Professor Kahlenberg, who contends 

 that ' thermodynamic reasoning cannot be applied to actual osmotic processes ' on 

 account of the ' selective action of the membrane ' and ' insists that the formation 

 of crystals from a solution or the concentration of a solution by evaporation are 

 not osmotic processes.' 



On the other hand we have Mr. Whetham, who, I confess, seems to me to be 

 capable of holding his own without need of reinforcements. He has pointed out 

 that confusion has arisen from the use of the term ' osmotic pressure ' to denote 

 the actual pressure experimentally realised in certain conditions, as well as the 

 ideal pressure required in thermodynamic theory. With regard to the theory of 

 electrolytic dissociation, Mr. AVhetham shows that the fact that the velocities 

 of the ions are constant in dilute solutions and decrease slowly with 

 increasing concentration, while the conductivity of a dilute solution is at most 

 proportional to the first power of the concentration, appears irreconcilable with 

 any assumption as to the existence of the active part of an electrolyte in the 

 form of combined molecules when in solution. I would here join with Mr. 

 Whetham in the request that those who oppose the theory of ionic dissociation 

 would state their views as to the mechanism of electrolysis, and their reasons for 

 supposing that the application of the principles of thermodynamics to the 

 phenomena of .solution is unjustifiable. 



Professor Armstrong remarks that it is unfair to ' cloak the inquiry by restrict- 

 ing it to thermodynamic reasoning, a favourite manoeuvre with the mathematically 

 minded.' He adds that such a course may satisfy the physicist, but ' is repulsive 

 to the chemist.' 



The inquiry, ' Why is the application of thermodynamic reasoning repulsive 

 to the chemist?' naturally suggests itself. I confess that at one time I regarded 

 the extreme advocates of the theory of ionic dissociation with a certain amount of 

 suspicion, but I think that most of those who have studied the evidence now at 

 our disposal, or who have been engaged in experimental investigation into this 

 interesting branch of physics, cannot fail to agree with Mr. Whetham that, as 



1 Concentration. Direct O.P. at 0° C. ^■'^- ^''^'""''^ ^^"^ .. 



vapour pressure at 0° C. 



540 grains per lit. solution , . 075 69-4 



060 „ „ „ . . 100-8 101-9 



750 „ „ „ . . 133-7 13G0 



Proc. Moy. Soc, June 1900. 



