PHYSICAL PROCESSES IN CELLS. 51 



capable of diffusion. In most cases, however, these fluids are in con- 

 stant motion, and the purely physical phenomena of diffusion are of 

 little importance, particularly when the extreme slowness of ordinary 

 diffusion is remembered. As the composition of the fluids of the body 

 is continually changing, diffusion, greatly aided by the motion of the 

 fluids, will, nevertheless, serve to maintain a certain degree of constancy 

 of composition. 



It has been already shown that when watery solutions are found on 

 different sides of a membranous partition, as in the case of the cell-con- 

 tents of two neighboring cells whose contents are more or less fluid, the 

 membrane does not serve to keep the fluids apart. For, the membrane 

 being capable of absorbing liquids by imbibition, the liquids fill the 

 intermolecular spaces of the membrane, and so come in contact with each 

 other; the phenomena of diffusion then commence, though the process is 

 very greatly modified by the behavior of the intervening membrane. As 

 already mentioned, diffusion, with the exception of the part it plays in 

 distributing fluids uniformly through the interior of cells, fills quite a 

 secondary role in the physical processes of the animal economy. Dif- 

 fusion as modified by the passage of liquids through an animal membrane 

 occupies a much higher position in point of importance. 



8. OSMOSIS. When two liquids capable of mixing are separated by a 

 membrane which possesses the power of imbibing these liquids, a gradual 

 union of the two liquids takes place through the membrane. 



This interchange, which is called osmosis, continues until the two 

 liquids are equally mixed ; consequently, the final result is the same as if 

 no membrane separated the two liquids, though the process is essentially 

 different ; for the diffusion currents must be modified by the molecular 

 forces which the molecules of the membrane exert on the liquids in con- 

 tact with them. If two liquids are 

 poured into the arms of a U-tube 

 (Fig. 38) so that they are in con- 

 tact at A they will mix, but the 

 level will remain the same in both 

 tubes, i.e., equal portions of each 

 liquid pass into one another in equal 

 time ; if, on the other hand, a mem- FIG. 38. 



brane is placed at A the liquids will 



mix, but the column of liquid will rise in one tube above the original 

 level and sink to a corresponding amount in the other. From which it 

 follows that in the mixing of liquids through a membrane the interchange 

 is unequal, i.e., more of one liquid passes than of the other. The current 

 through the membrane is a double one. Thus, if a saturated salt solu- 

 tion is placed in one arm of the tube and an equal quantity of distilled 



