MEASUREMENT OF OSMOTIC PRESSURE 571 



seen to be capable of giving only a very approximate expres- 

 sion of the actual facts. In his latest paper Morse has shown 

 that at temperatures higher than atmospheric each solution in 

 turn reaches a point at which a modified form of van't Hofl's 

 equation gives a correct value for the osmotic pressure but it is 

 not yet clear whether this agreement is only momentary or 

 whether it persists over a large range of higher temperatures. 

 At temperatures from o° to 25 the deviations recorded by Morse 

 amount to 6 to 12 per cent, even when using the modified form 

 of van't Hoff s equation, whilst Lord Berkeley has recorded at 

 the freezing-point an osmotic pressure nearly three times as 

 great as the values calculated from the equation in its original 

 form. 



The various attempts to calculate the osmotic pressures 

 of cane-sugar solutions are summed up by Findlay {Scientia 

 191 2) in the following table for 20 C. : 



Table III 



The concentrations are shown (1) in gramme-molecules per 100 

 grammes of water and (2) in gramme-molecules per litre. The 

 observed osmotic pressures are shown in the third column, 

 whilst the remaining columns show the values calculated by 

 means of four different formulae. 



Van't Hoff s equation PV = RT may be thrown into the form 



RT RT n^ RT 

 P= V* = Vo N ~ V Q * 



in which V° is the molecular volume of the solvent and x is the 

 ratio of the number of gramme-molecules n of the solute to 



