528 Prof. A. B. Macallum. Acineta tuberosa : [Feb. 19, 



view, the same pressure and, in consequence, the number of molecules per 

 given volume of the solution in any portion of it would be uniform. 



On this conception of the force determining the distribution of salts in 

 fluids there were based a number of views which have played a part in 

 explaining physiological processes. Of these the most important is that 

 which postulated that all diffusion, whether in the cytoplasm of a cell or 

 through a living membrane, is due to osmotic pressure acting as a driving 

 force, the ultimate result of whose action would be to equalise the pressure 

 throughout the cytoplasm or on both sides of the membrane. This reduced 

 the processes of secretion and excretion, as well as the diffusion into and from 

 cells, to the operation of gas laws. 



This explanation of the force and the conditions that make for the diffusion 

 of solutes, not only in physical solutions but also in living matter, has 

 obtained and still obtains a wide acceptance. The very simplicity of it, the 

 support it derived from a considerable range of experimental evidence, and 

 the unifying effect it appeared to exercise in a large number of phenomena 

 manifested by solutions and gases, told very strongly in its favour, and 

 eventually revolutionised the aspect from which all the problems of osmosis 

 and diffusion were viewed. 



Criticism, however, was not silenced. It was seen that there were 

 phenomena which not only could not be explained in that way but also were 

 irreconcilable with the explanation. These were physical and physiological. 

 Of the physiological only is there concern here. In one of these, that 

 observed in renal excretion, the concentration of the urine is much greater, 

 ordinarily, than that of the blood plasma from which it is derived through 

 the activity of the kidney tubules. In other words, the osmotic pressure of 

 the product of renal action is greater than that of the blood. This cannot 

 be explained by the van 't Hoff-Arrhenius theory, the only conceivable result 

 of which would, in such a case, be approximately an equality of pressure or 

 that the urine formed would not exceed in concentration, and, therefore, in 

 osmotic pressure, the blood plasma itself. 



The failure of the van 't Hoff-Arrhenius theory to explain this and other 

 physiological results of a like character does not put the theory out of court 

 in explaining many physical phenomena. It still may be regarded as of 

 value in accounting for these, though even in this respect it may be looked 

 upon as beset with limitations. In the physiological sphere its application is 

 of much less service and, were it here the last word in the way of an 

 explanation, the causation of a few physiological phenomena would ever 

 remain an insoluble problem. 



In recent years the aid of other factors in explanation of certain physio- 



