EXPERIMENTAL WORK ON OSMOTIC PRESSURE 629 



differences between the directly observed osmotic pressures 

 and the theoretical values calculated on the basis of the validity 

 of the simple equation PV = RT. The following Table IV. 

 shows that the observed osmotic pressure is found in all cases 

 to be greater than the theoretical pressure, and that the 

 deviation between the two sets of values increases with. 

 increasing concentration of the cane sugar solutions : 



Osmotic pressure in atmospheres. 



Calculated. 



n-8 

 19-6 



27*5 

 35'4 

 43'2 



49" 1 



The solutions used by Messrs. Morse and Frazer, although 

 the most of them can hardly be called dilute, furnish more 

 suitable material for an examination into the validity of van't 

 Hoff's propositions, and it is interesting therefore to learn that 

 the results obtained by these investigators are in harmony 

 with the view that osmotic pressure obeys the laws of gases. 

 This statement, however, is subject to the reservation that the 

 standard osmotic volume is not 1 litre of solution, but the 

 volume occupied by 1,000 grams of water. It is further found 

 that cane sugar, dissolved in water, 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. It must be remembered 

 that this is not the first attempt to allow for deviations from 

 normal behaviour which are due to the volume occupied by the 

 solute in a solution. Cohen 1 has proved that the increase in 

 the velocity coefficient for sugar inversion with rising concen- 

 tration of the sugar is conditioned by the gradually increasing 

 fraction of the total solution volume which is occupied by the 

 dissolved sugar molecules. 



How far the important result reached by Morse and Frazer 

 is borne out by their experimental work will be clear from a 

 study of Table V. In this table the concentrations of the 

 solutions are recorded in two ways, firstly as gram molecules 



1 Zeitschrift fur physikalische Chemie, 23, 442. 



