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X. On the Osmotic Pressures of some Concentrated Aqueous Solutions. 
By the Earl of Berkeley and E. G. -J. Hartley, B.A. ( Oxon ). 
Communicated by W. C. D. Whetham, M.A., F.R.S. 
Received May 28,—Read June 7, 1906. 
Introduction. 
In a communication* to the Royal Society by one of us it was stated that an 
attempt would be made to determine directly the osmotic pressures of strong 
solutions. The purpose for which these determinations were required was to obtain 
data for the tentative application of Van der Waals’ equation of state to solutions, 
but we propose, in this communication, to restrict ourselves to the actual results 
obtained, reserving for a future occasion the theory of the subject. 
Some tune ago, in the proceedings of the Royal Society,! we gave an account of 
some preliminary experiments. We there described the method and apparatus we 
used, and at the same time stated that we hoped to get more accurate results bv 
- modifying the apparatus. This hope has been fulfilled, and we trust that a 
somewhat full description of the methods used both in the actual determinations 
and in the preparation of the membranes will, therefore, not be out of place. 
Before proceeding to describe these matters, it will be advisable to discuss briefly 
what is meant by osmotic pressure in the case of concentrated solutions, for it 
appears from recent discussions} that there is some ambiguity about the meaning of 
the expression. 
There seem to be two methods of investigating directly the osmotic phenomena of a 
solution, and these will, in general, give two different results:— 
(1) One, which we may call the osmotic “force” method, depends on the 
determination of the rate at which the solvent will flow through a semi-permeable 
membrane into a large quantity of solution when there is no pressure on the latter. 
A knowledge of this rate together with the frictional resistance to the flow will 
enable the osmotic “force” to be calculated in absolute units. 
* ‘Phil. Trans. Roy. Soc.,’ vol. 203, pp. 189-215. 
t ‘Roy. Soc. Proc.,’ vol. 73, pp. 436-443. 
f Cf. a letter in ‘Nature,’ vol. 74, p. 6. 
3 Q 
VOL. CCVI.—A 411. 
22.10.06 
