314 THE EARL OF BERKELEY, MESSRS. E. G. J. HARTLEY AND C. Y. BURTON: 
Empirical formulae were then obtained which represented the experimental values of 
the densities and the compressibilities (Table VI.) for different concentrations, and 
from these formulae the values s 1 were computed as a function of c 2 and p 0 . In 
Porter’s equation* has to be integrated over a range of pressure from one 
atmosphere to the osmotic pressure of the particular solution ; it was considered 
sufficiently accurate to regard it as a linear function of the pressure and the mean 
value, (equals half the sum of the value at the limits) is given in Table VII. 
PABT II. 
The Vapour-Pressure Measurements. 
Description of Apparatus and Method .—Although the method employed has 
already been described it may not be out of place to recall it to mind. 
Dry air is led through a number of vessels containing the solution, then through a 
vessel containing the solvent (water), and finally over sulphuric acid in the end vessel 
of the train. The air is supposed to flow slowly enough to be saturated up to the 
vapour pressures of the liquids. In these circumstances, if we call the sum of the 
losses of weight of solution and solvent l 0 , and the loss in the solution alone Z 1} then 
the observed ratio of the vapour density of the solvent to the vapour density of the 
solution is lfl Y . 
This statement is however subject to various corrections ; but, before considering 
these, it will be convenient to describe the complete installation. 
Fig. 3 represents in diagram form the arrangement for passing the air stream over 
the vessels, as finally modified so as to eliminate the various sources of error, which 
several years’ experience has shown to be possible. 
Filtered air, drawn from outside the laboratory, is passed through soda-lime, and 
then over the surface of sulphuric acid in vessel A, where it is deprived of almost all 
its moisture. (It may here be mentioned that the chief aim of this final arrangement 
has been to avoid, as far as possible, any sudden alteration of pressure ; the air 
passages are therefore made as wide as may be and at no point does the air bubble 
through any liquid.) On leaving A the air passes through another sulphuric 
acid vessel B which is in, and at the constant temperature of, the bath ; it then 
enters the train of weighed vessels 1, 2, 3, 4, and 5, which contain the various 
liquids under examination; the last of these, No. 5, is again a sulphuric acid 
vessel. 
From 5 it goes to the regulating valve C. This valve, which was designed by 
Berkeley and Thomas for another research, consists of a biscuit-porcelain filter (f) 
surrounded to a variable height by mercury. It will be seen that the volume of 
* See p. 343. 
