EXPERIMENTAL WORK ON OSMOTIC PRESSURE 625 



laid on this conclusion, since the scope of the comparison is 

 limited. 



Indirect Determination of the Osmotic Pressures of Cane 



Sugar Solutions. 



As is well known, there are certain properties of solutions 

 which are intimately related to their osmotic pressure. Thus 

 it is possible, from the difference in vapour pressure of the 

 solvent and the solution at the same temperature, and from the 

 difference in boiling point or freezing point of solvent and solution, 

 to calculate the osmotic pressure. The difficulty is that the 

 methods usually available for the determination of the vapour 

 pressure, the boiling point, and the freezing point of solutions are 

 not very accurate, for dilute solutions at least, and consequently 

 most of the data obtained by these methods are of little use in 

 checking the values of the osmotic pressure determined by direct 

 observation. In the case of a cane sugar solution, for example, 

 containing yi^th of a gram molecule per litre, the osmotic pressure 

 at o° would be 17-18 cm. of mercury, a very considerable and easily 

 measurable quantity, whereas the freezing point of the solution 

 would be only o - o2° lower than the freezing point of water ; the 

 boiling point of the solution would be only 0*005° higher than 

 the boiling point of water, and the vapour pressure of the solution 

 would, even at ioo°, be only 0*15 mm. lower than the vapour 

 pressure of water. 



By a dynamical method, however, it is possible to find the 

 ratio of the vapour pressures of solvent and solution, by deter- 

 mining differences of weight instead of differences of pressure. 

 Such a method was employed some years ago by Walker, who 

 adopted the device of simply bubbling air for some time through 

 a series of four absorption tubes or bulbs. If, in such an 

 arrangement, the first and second tubes contain the aqueous 

 solution for which the vapour pressure is to be determined, 

 the third tube contains pure water, and the fourth tube contains 

 strong sulphuric acid, then, on the assumption that in the first 

 and second tubes the air becomes charged with water vapour up 

 to the vapour pressure of the solution, the loss of weight in 

 the third tube, observed after a current of air has passed through 

 the apparatus for some time, is a measure of the difference 

 between the vapour pressures of solvent and solution, while the 

 gain in weight of the fourth tube is a measure in equivalent 



