RESPIRATION 175 



a definite electrical charge, that determines the phenomena of 

 electrolysis and the exact quantitative relationship between-the 

 current passed through a cell containing an electrolyte in solution 

 and the splitting up of the electrolyte into its constituents. Van't 

 Hoff and Arrhenius brought Faraday's conception into relation 

 with osmotic pressure and various other phenomena connected 

 with solutions. 



Osmotic pressure was first measured accurately by the botanist 

 Pfeffer. 7 He used a semi-permeable membrane (i.e., a membrane 

 which allowed the solvent water, but not the dissolved substance, 

 to pass) which had been originally discovered by Moritz Traube 

 in i867, 8 though Traube had not seen how to apply this membrane 

 for measuring osmotic pressures. Some years later van't Hoff 9 

 made the brilliant discovery that in dilute solutions of sugar and 

 other substances, the osmotic pressure is practically the same as 

 the pressure which the solute would have if its molecules were 

 present alone in the gaseous form at the same temperature. There 

 must thus be a fundamental connection between molecular con- 

 centration, osmotic pressure, and gas pressure ; also between mo- 

 lecular concentration and the vapor pressures, boiling points and 

 freezing points of solutions, as had already been empirically shown 

 by the investigations in particular of Raoult. Van't Hoff believed 

 that osmotic pressure, etc., were due in some way to the molecular 

 bombardment of the solute molecules, and therefore vary as their 

 concentration per liter of solution ; and this theory has served for 

 the building up of the theory of solutions as it is still represented 

 in current textbooks of physical chemistry. In reality this theo- 

 retical interpretation was not even justified by Pfeffer's data if 

 concentration per liter is considered, and breaks down entirely for 

 concentrated solutions. The theory is also quite unintelligible 

 mechanically, since the bombardment pressure of the solute mole- 

 cules would be in the wrong direction for explaining the phe- 

 nomena. Hence many persons regarded van't Hoff's theory with 

 the greatest suspicion ; but the fact that it seemed to answer ad- 

 mirably as a means of prediction in the case of dilute solutions, 

 and to cover an enormous mass of facts, has led to its very general 

 acceptance, though other attempts have been made to substitute 

 for it some more intelligible conception. 



In iQiS 10 I showed quite clearly, as I think, that van't Hoff's 



T Pfeffer, Osmottsche Untersuchungen, 1877. 



8 Traube, Archiv f. (Anat. .) Physiol., p. 87, 1867. 



9 Van't Hoff, Zeitschr. f. physik. Chemie, I, p. 481, 1887. 

 "Haldane, Bio-Chemical Journal, XII, p. 464, 1918. 



