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V. On Osmotic Pressures Derived from Vapour-Pressure Measurements: 
Aqueous Solutions of Cane Sugar and Methyl Glucoside. 
By The Earl 0 / Berkeley, F.R.S. ; E. G. J. Hartley, B.A. (Oxon); and 
C. V. Burton, B.Sc. (London). 
Received February 3,—Read March 1, 1917. 
Introductory. 
Since our previous communications on cane sugar (‘ Boy. Soc. Proc.,’ A, vol. 77, 
1906) and on calcium ferrocyanide (‘ Phil. Trans.,’ A, vol. 209) we have endeavoured 
to improve the apparatus and method therein described. In these endeavours, two 
objects, besides that of determining osmotic pressures, have been in view : (l) to find 
out what are the best conditions for making the method both accurate and of general 
applicability; (2) to obtain data for the construction of suitable apparatus for the 
determination of the absolute vapour density of liquids in air or other gas. 
As far as accuracy is concerned, the close agreement between osmotic pressures 
observed directly, and those here calculated from the vapour pressures, shows that, 
for solutions at 0° C. the method is susceptible of considerable precision. The 
experiments at 30° C., however, are on the whole not quite so concordant; this is 
probably because the quantity of water vapour in the air stream is some six times 
the amount carried at 0° C., so that small variations in conditions which would be 
without effect at the lower temperature, now make themselves apparent. This, 
indeed, is shown clearly in the magnitude of the correction to be applied for the 
expansion of the air between the solution and water vessels (see p. 316). In the early 
stages of the work (at 0° C.) our results indicated that the successful application of 
the method depends largely on the control of the experimental conditions ; 
accordingly we made as many changes as possible, but as each experiment takes 
several days to complete, a long time is required to determine the effect of any one 
change. The departure of Mr. Hartley for the front prevented a further 
investigation of the question but we think that no new changes will alter sub¬ 
stantially the results now presented. 
The importance of measurements of the absolute vapour density of water vapour 
in air has been realized generally for some time past; the differences between 
Begn ault’s values for air and for a vacuum (at 0° C., especially) require 
VOL. CCXVILI.-A 565. 2 R [Published May 20, 1919. 
