PHYSICAL PKOCESSES IN CELLS. 53 



the current of water to the colloid is very slow, possibly because the 

 large molecules of the colloids readily stop the pores of the membrane. 

 Albumen in solution passes more readily through a membrane to mix 

 with salt solution than with water. A very concentrated solution of 

 salt, however, prevents osmosis of albumen by simply removing the water 

 from the albuminous solution. 



When a solution of a diffusible substance, together with a solution 

 of a colloid, is placed on one side of a membrane and pure water on the 

 other, at first none of the colloid passes through the membrane, but 

 simply water to the colloid and the diffusible substance to the water; 

 hence, albumen may be freed of its salts by diffusion (dialysis), or, if a 

 mixed solution of sugar and gum is subjected to dialysis, only the sugar 

 passes through the membrane. 



An exception to this is seen when albumen and salts dialyze with water. 

 First the salts pass, then the albumen passes into the salt solution. 



Just as we found there were two kinds of imbibition, — the capillary 

 and molecular, — so are there two kinds of osmosis, which differ somewhat 

 according as the partition between the two fluids is a porous, inorganic 

 solid, or an organic tissue capable of absorbing liquid by imbibition. In 

 the case of a porous, inorganic partition the phenomena of osmosis are 

 entirely governed by capillarity and the laws of diffusion of liquids. 

 Certain liquids have a greater tendency to enter capillary tubes than 

 others. When, therefore, two miscible liquids are separated by a porous 

 solid, which may be regarded as a collection of capillary tubes, the liquid 

 which has the greater affinity for the walls of the tube will enter to a 

 greater extent than the other, and will meet the other fluid advancing in 

 the opposite direction, but with less force on account of its lesser affinity. 

 The two liquids thus coming into contact with each other will diffuse into 

 each other, and the pores will be occupied with a mixture of two liquids, 

 for one of which the walls of the tube will have a greater affinity than 

 ' for the other. Then, although diffusion will take place from this mixture 

 into the liquid of greater affinity, the latter continually forcing out some 

 of the mixture, the liquid of lesser affinity will continually increase in 

 volume. 



In the case of organic membranes, the power possessed by the mem- 

 brane of imbibing different liquids enables osmosis to take place, while 

 the direction of the current is governed by the affinity of the liquids for 

 the membrane. Whichever liquid has the greater affinity for the mem- 

 brane will pass in greatest amount. Here, for the sake of simplifying 

 the matter, we may assume that the liquid which has entered the inter- 

 molecular spaces of the membrane is, to a certain extent, governed by 

 the same conditions which apply to the entrance of liquids into capil- 

 lary tubes. 



