178 
PROFESSOR GRAHAM ON OSMOTIC FORCE. 
tity of salt leaves the phial by diffusion. This quantity was reduced to one-half when 
the strong and thick membrane of the ox-gullet was used to cover the mouth of the 
phial ; and it was not affected in a sensible degree by passing through a thinner mem- 
brane, consisting of ox-bladder with the outer muscular coat removed. In the last 
experiment the actual diffusates were 0*631 gramme common salt in the absence of 
the membrane, and 0*636 gramme common salt with the membrane interposed, which 
may be considered as the same quantity. The diffusion of a salt appears to take 
place, therefore, without difficulty or loss through the substance of a thin membrane, 
although the mechanical flow of a liquid may be nearly stopped by such an obstacle. 
It is well to bear in mind the last fact in the consideration of what is seen in an 
endosmotic experiment. An open glass tube, with one end expanded into a bell form 
and covered by tight membrane, forms a vessel which may be filled with a saline solu- 
tion and immersed in a jar of pure water. The volume of liquid in this osmometer 
soon begins to increase and is observed to rise in the tube, while the simultaneous 
appearance of salt in the water of the jar may easily be verified. M. Dutrochet de- 
scribed the result as the movement of two unequal streams through the membrane 
in opposite directions, the smaller stream being that of the saline solution flowing 
outwards, and the larger that of pure water flowing inwards. The double current 
has been always puzzling, but the expression of the fact becomes more conceivable 
when we say (as we may do truly) that the molecules of the salt travel outwards by 
diffusion through the porous membrane. It is not the whole saline liquid which 
moves outwards, but merely the molecules of salt, their water of solution being 
passive. The inward current of water, on the other hand, appears to be a true sensible 
stream or a current carrying masses. The passage outwards of the salt is inevitable, 
and being fully accounted for by diffusibility, requires no further explanation. It is 
the water current which requires consideration, and for which a cause must be found. 
This flow of water through the membrane I shall speak of as osmose, and the 
unknown power producing it as the osmotic force. It is a force of great intensity, 
capable of supporting a column of water many feet in height, as shown in Dutrochet’s 
well-known experiments, and to which naturalists are generally disposed to ascribe a 
wide sphere of action, both in the vegetable and animal kingdoms. 
Cannot liquid diffusion itself, it may first be asked, contribute to produce osmose ? 
Diffusion is always a double phenomenon, and while molecules of salt pass in one 
direction through the membrane, molecules of water no doubt pass by diffusion in 
the opposite direction at the same time, and replace the saline molecules in the osmo- 
meter. Water also is probably a liquid of a high degree of diffusibility, at least it 
appears to diffuse four times more rapidly than alcohol, and four or six times more 
rapidly therefore than the less diffusive salts. A possible consequence of such in- 
equality of diffusion is, that while one grain of a certain salt diffuses out of the osmo- 
meter, four or six grains of water may diffuse into the osmometer. Liquid diffusion, 
I believe, generally tends to increase the volume of liquid in the osmometer, and a 
