372 ABSORPTION. 



its greater affinity for this substance, would pass freely 

 into the water. 



Various opinions have been advanced in regard to the 

 nature of the force by which fluids of different chemical 

 composition thus tend to mix through an intervening 

 membrane. According to some, this power is the result 

 of the different degrees of capillary attraction exerted by 

 the pores of the membrane upon the two fluids. Prof. 

 Graham, however, believes that the passage or osmose of 

 water through membrane may be explained by supposing 

 that it combines with the membranous septum, which thus 

 becomes hydrated, and that on reaching the other side it 

 partly leaves the membrane, which thus becomes to a 

 certain degree de-hydrated. For example, a membrane 

 such as that used in the endosmometer, is hydrated to a 

 higher degree if placed in pure water than in a neutral 

 saline solution. Hence, in the case of the endosmometer 

 filled with the saline solution and placed in water, the 

 equilibrium of hydration is different on the two sides ; 

 the outer surface being in contact with pure water tends 

 to hydrate itself in a higher degree than the inner surface 

 does. ' * When the full hydration of the outer surface ex- 

 tends through the thickness of the membrane, and reaches 

 the inner surface, it there receives a check. The degree 

 of hydration is lowered, and water must be given up by 

 the inner layer of the membrane." Thus the osmose or 

 current of water through the membrane is caused. The 

 passage outwards of the saline solution, on the other hand, 

 is not due, probably, to any actual fluid current ; but to a 

 solution of the salt in successive layers of the water con- 

 tained in the pores of the membrane, until it reaches the 

 outer surface and diffuses in the water there situate. 



Thus, "the water movement in osmose is an affair of 

 hydration and of de-hydration in the substance of the 

 membrane or other colloid septum, and the diffusion of the 

 saline solution placed within the osmometer has little or 



