IMPORTANCE OF WATER 109 



of a substance of colloidal character move much slower than the 

 molecules of crystalloids. 



If a diffusing substance meets a membrane in its path, dif- 

 fusion becomes more or less complicated. If this membrane is 

 porous and the pores are filled with the same solvent, then there is 

 less complication. In this case, however, the rate of diffusion is 

 still more reduced. The difference between the speed of the 

 moving substance and the size of its particles increases. Only 

 the largest colloidal aggregates are retained by the membrane. 

 Membranes of this type are used in dialysis, by means of which 

 colloids may be separated from crystalloids. The cellulose of the 

 cell wall is a membrane of this character. It is permeable to 

 almost all substances dissolved in water. 



A sac made of a membrane, such as animal bladder, parchment 

 paper, or a hardened film of collodion, is filled with a solution of 

 cane sugar and tightly closed. If placed in water, this sac will swell 

 rapidly, its walls will become turgid, and finally it may rupture 

 when it is unable to withstand the pressure from within. If 

 instead of closely tying up the opening, a glass tube is inserted 

 into it, we shall soon see that under the influence of internal pres- 

 sure the level of the liquid in the tube begins to rise, first rapidly, 

 then slower and slower, until it stops at a certain definite level, 

 after which it begins to fall again. 



Following the changes in the contents of the sac, it is found that 

 at first a rapid entrance of the water takes place, which leads to 

 a considerable increase in volume. Afterwards, the diffusion of 

 the dissolved substance from the sac into the surrounding water 

 becomes more and more obvious, and finally the concentration of 

 the outer and of the inner solutions becomes almost equal, and the 

 walls begin to collapse. The entrance of the water into the sac is 

 called "endosmosis," the outward escape of the solute, "exos- 

 mosis." The whole apparatus, the sac with the inserted tube, 

 has been called an osmometer by Dutrochet, the French botanist, 

 who was the first (1826) to study this phenomenon. The diffusion 

 of liquid and dissolved substances through a membrane has been 

 given the general name, osmosis, while the hydrostatic pressure, 

 developing as a result of an increase of endosmosis over 

 exosmosis and swelling of the sac has been called "osmotic 

 pressure." 



Further investigations have shown that besides minutely por- 



