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osmotic pressures observed with salts, strong acids, and bases 

 on the basis of the hydrate theory, 1 which we owe more 

 particularly to Mendeleeff 2 and Pickering, 3 and which postulates 

 that in aqueous solution the (undissociated) solute enters into 

 chemical combination with the solvent ; but this theory has 

 proved inadequate to account quantitatively for the observed 

 phenomena. 



In recent years attention has been directed more particularly 

 to direct measurements of osmotic pressure, to the question of 

 combination between solvent and solute, to complex ions, and 

 to the investigation of other solvents than water ; each of these 

 points will now be shortly considered. 



Osmotic Pressure 



As we have already seen, there is an intimate connection 

 between the phenomenon of osmotic pressure and the electrolytic 

 dissociation theory, since Arrhenius was able, on the basis of 

 the latter, to account quantitatively for the abnormal osmotic 

 pressures shown by solutions of electrolytes. As regards the 

 experimental basis of his theory of solutions, Van 't Hoff chiefly 

 relied upon the direct measurements of osmotic pressure made 

 by Pfeffer, with the help of a semi-permeable membrane of 

 copper ferrocyanide, as well as on the indirect measurements 

 by the freezing-point and boiling-point methods carried out by 

 Raoult. Within the last two or three years very careful 

 measurements of the osmotic pressure of cane sugar solutions 

 have been made by Morse and Frazer 4 in America with a 

 semi-permeable membrane deposited by an electrical method ; 

 they find that "cane sugar in aqueous solution exerts an osmotic 

 pressure equal to that which it would exert if it were gasified 

 at the same temperature and the volume of the gas were reduced 

 to that of the solvent in the pure state." 



Van 't Hoff 5 was further able to prove thermodynamically 

 that the osmotic pressure and gas pressure must have the same 

 absolute value if the solution is sufficiently dilute. The prin- 

 ciples underlying the thermodynamical proof have been put very 

 clearly by Larmor. 6 According to his view, " the change of 



1 For a clear account of the hydrate theory, see Watts's Dictionary of 

 Chemistry, Art. "Solution." 



2 Principles of Chemistry, i. 66, etc. 3 Watts's Dictionary, loc. cit. 

 4 Amer. Chem.J. 1905, 34, 1. 5 Lectures, ii. 33. 



6 Encyc. Brit. 10th ed. vol. 28, p. 170; Phil. Trans. 1897, 190 A, 205. 



