Mr Barlow, On the Osmotic Pressure of Aqueous Solutions. 229 



On the Osmotic Pressure of Dilute Aqueous Solutions. 

 By P. S. Barlow, St John's College. 



[Read 27 November 1905.] 



This paper is an account of a few experiments that were 

 made with dilute solutions during compulsory intervals in work 

 with aqueous solutions of alcohol. The work was thus of an 

 intermittent and preliminary character rather than exhaustive. 

 However, the results that have been obtained are definite in their 

 general bearing. 



In experiments with dilute solutions in water there seem to 

 be two reasons why the osmotic pressure set up does not reach its 

 theoretical value. These are due to the impurities absorbed by 

 the outer water from the air and to the slight solubility of the 

 clay of the pot. The pressure set up thus becomes a difference 

 between two osmotic pressures, that of the solution within and 

 that of a solution without. The difficulty arising from contami- 

 nation of the outer water by the air can be avoided by closing the 

 outer vessel ; but the difficulty arising from the solubility of some 

 portion of the pot cannot be overcome for aqueous solutions, 

 where the latter are very weak. For solutions which should give 

 an osmotic pressure above an atmosphere the solubility of the cell 

 is probably too small to interfere appreciably with the theoretical 

 values. 



It will thus be seen that this method of direct experiment 

 cannot be used to determine with any accuracy the osmotic 

 pressures of dilute solutions. Sufficient purity of the liquids can- 

 not be ensured under these conditions. 



.In ihe experiments given below three cells were used, XIV, 

 XIII and e. Each of these had been used shortly before in other 

 experiments and had shown itself capable of bearing internal 

 pressures above three atmospheres. In the interval they were' 

 standing in distilled water. Their soundness is assumed from the 

 results of the experiments referred to. 



The experiment with cell XIV is with a cane-sugar solution of 

 strength "056 normal. This is much stronger than the other 

 solutions, but I add it to show that it takes considerable time for 

 a cell to be freed from substances of previous experiments. 



Cell XIV. 056 N cane-sugar solution. Theoretical pressure 

 = 1"26 atmos. The cell after being used for an alcohol solution 

 was washing in distilled water for three days (Oct. 26 to 29). It 

 was then filled with the sugar solution, and after a short time 

 emptied and refilled. 



VOL. XIII. PT. iv. 16 



