492 Professor H. L. Gallendar [Feb. 26, 



which I have employed for this purpose, and some of the conclusions 

 at which I have so far arrived. 



It is often a difficult matter, when the difference of vapour- 

 pressure between a solution and the solvent is small, to measure the 

 pressure difference directly to a sufficient degree of accuracy. A 

 method very commonly employed, which has been brought to a high 

 degree of accuracy by Lord Berkeley and his assistants, depends on 

 the observation of the losses of weight of two vessels, containing 

 solution and solvent I'espectively, when the same volume of air is 

 aspirated slowly through them in succession. To secure accurate 

 results, the air must pass very slowly. One complete observation 

 takes about a week to perform successfully, and involves many 

 difficult manipulations. I have endeavoured to avoid this difficulty 

 by measuring the temperature difference in place of the pressure 

 difference, since the temperature difference remains nearly constant, 

 while the pressure difference tends to diminish in geometrical pro- 

 gression with fall of temperature. The method adopted for this 

 purpose is that indicated in the diagram of the Vapour-Temperature 

 Balance. The temperatures of solution and solvent, contained in 

 separate vessels communicating through a tap, are adjusted, until, on 

 opening communication between them, there is no flow of vapour 

 from one to the other, as indicated by a change in the reading of a 

 pair of thermo-junctions immersed in the solvent respectively. The 

 corresponding difference of temperature is observed, and since the 

 vapour-pressures of the solvent are known, it is easy to calculate the 

 required ratio or difference of the vapour-pressures of solvent and 

 solution at the same temperature. When the vapour-pressures are 

 very small, it may be difficult to observe the change of temperature 

 on opening the tap, unless the apparatus is very carefully exhausted. 

 A more delicate method in this case is to observe the direction and 

 magnitude of the current of vapour from solution to solvent, or vice 

 versa, by means of the " Yapour-Current Indicator," illustrated in the 

 companion diagram. This consists of a delicately suspended vane, 

 the deflections of which are read by a mirror, and will readily 

 indicate a difference of pressure less than the thousandth part of a 

 miUionth of an atmosphere. 



The vapour-current indicator is so constructed that its deflections 

 are very accurately proportional to the pressure difference, much more 

 so in fact than any form of electric galvanometer. It can also be 

 employed for direct measurements of small differences of vapour- 

 pressure. The chief difficulty in this case is to ensure the absence of 

 air or other disturbing factors. A method of avoiding this difficulty 

 is to work at atmospheric pressure, and to measure the pressure 

 difference between two vertical columns of air saturated with the 

 vapours of the solvent and solution respectively.* The temperature 



* I first showed this experiment ten years ago, in illustration of the delicacy 

 of the apparatus, at a Friday Evening Lecture at the Eoyal Institution. 



