28 ELEVENTH REPORT. 



OSMOTIC THEORIES (WITH SPECIAL REFERENCE TO VAN'T 



HOFF'S LAW). 



[Abstract.] 

 J. B. Daxdexo. 



There is no subject in plant physiology of more fundamental importance 

 than osmosis. It is not confined to plants alone, but is universal as far as 

 physiological operations are concerned, whether of jilant or animal. Indeed, 

 wherever a living cell is active, either in the matter of intake, or outflow, 

 osmosis is a prominent feature of the operation. Yet it is not in itself a 

 biological operation, though it is almost universally found in connection 

 with such activities. It is purely physical, or physico-chemical in its nature, 

 and it is from these sources that we may expect a reply to the question as to 

 the true nature of the phenomena. 



A considerable amount of work in physiology has been done, which Ijears 

 more or less closely upon osmosis, yet very little experimental facts have 

 been produced either in Ijiology or physical chemistry which can be said 

 to throw much light upon the fundamental principles. The most important 

 investigators were Pfeffer, de Vries and Morse with associates. Pfeffer's 

 researches were made with a view to determine the magnitude of osmotic 

 pressure, and he developed an apparatus — the Pfeffer cell — which has ever 

 since borne his name. A description of this may be found in Goodale's 

 Plant Physiology, p. 227, and also in other works. This ai^paratus can be 

 used as the basis of quantitative experiments with almost any kind of solu- 

 tions. 



The researches of de A'ries were physiological, mainly. He aimed to 

 determine the relative osmotic pressure in the cells of different plants, and, 

 so far as the relative pressm-e was concerned, his conclusions might be con- 

 sidered quantitative. The results of Pfeffer and de Vries were taken up later 

 by van't Hoff, and from these data he (van't Hoff) formulated a law or 

 theory, since known as van't Hoff's Law. 



This law has a remarkably wide acceptance, particularly among physicists, ' 

 and is given in all the more prominent texts as the only explanation of osmotic 

 pressure, and of osmosis in general. The law, put in simple form, is this: 

 "Osmotic pressure is gas pressure, the result of the impact of molecules of 

 solute upon the bounding surface of solution;" and "the osmotic pressure of 

 a solute is the same as it would be if in the form of a gas and occupying the 

 same volume." The idea involved in this law has very frequently l^een 

 used in an attempt to explain osmotic phenomena, but the difficulty of the 

 matter is that there are so many features it does not explain, and there are 

 several experimental results which are entirely opposed to it. 



The most important argument brought against the validity of van't Hoff's 

 law is the experimental quantitative results of Morse, which go to show 

 that the pressure in a sugar solution, for a grammolecular Aveight per liter is 

 more (nearly as 3:2) than what it would be theoretically according to van't 

 Hoff's law. There has been quite a number of experimental facts deduced 

 by the writer and others, which go to show that van't Hoff's theory is no 



