PHILOSOPHICAL TRANSACTIONS 
VII. The Bakerian Lecture. — On Osmotic Force. 
By Thomas Graham, F.R.S. &c. 
Received June 15, — Read June 15, 1854. 
The expression “ Osmotic Force ” (from wafxbc,, impulsio) has reference to the endos- 
mose and exosmose of Dutrochet. 
We may succeed in covering a solution of salt occupying the lower part of a glass 
jar by a stratum of pure water without much intermixture of the two liquids. A force, 
however, is thereby brought into action which carries up the salt in a gradual manner, 
dispersing it and ultimately producing a uniform mixture of the salt with the whole 
volume of water. The molecules of salt have the liquid condition when in solution 
as well as those of water itself, and we have in the experiment the contact of two 
different liquids, which must of necessity diffuse through each other, the molecules 
of a liquid being self-repellent, or subject to a force the same in kind but less in 
degree as that which gives to gases their elasticity and diffusibility. 
The force of liquid diffusibility will still act if we interpose between the two liquids 
a porous sheet of animal membrane or of unglazed earthenware ; for the pores of 
such a septum are occupied by water, and we continue to have an uninterrupted 
liquid communication between the water on one side of the septum and the saline 
solution on the other side. 
To impel by pressure any liquid through the pores of such a septum may be 
extremely difficult, from the interference of frictional resistance and the attraction of 
capillarity. But these last forces act on masses and not on molecules, and the ultimate 
particles of water and salt which alone diffuse, appear really to permeate the channels 
of the porous septum with little or no impediment. A comparative experiment on 
diffusion, with and without septa, is easily made by means of a wide-mouthed phial, 
which is filled completely with the saline solution and then immersed in water, in one 
experiment with the mouth of the phial open, and in the other experiment with the 
mouth covered by membrane. In a fixed time, such as seven days, a certain quan- 
2 A 
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