352 



ABSORPTION OF THE DIGESTED FOOD. 



If both miscible fluids are simply placed over one another in a vessel, without 

 the intervention of a porous septum, an interchange of particles of the liquids also 

 takes place, until the entire mass has undergone homogeneous admixture. This 

 interchange is designated diffusion. 



The rapidity of diffusion is influenced: i. By the nature of the fluids. Acids 

 pass over most rapidly, alkaline salts more slowly; liquid albumin, gelatin, gum, 

 dextrin, and starch-solutions most slowly. The latter, in part, do not crystallize, 

 and also in part do not represent true solutions, but only suspensions. 2. The 

 more concentrated the solutions, the greater is the diffusion. 3. Heat promotes, 

 cooling retards, diffusion. 4. If the solution of a body difficult of diffusion is 

 mixed with a readily diffusible solution, the former diffuses with even greater 

 difficulty. 5. Dilute solutions of various substances diffuse into one another 

 without difficulty, while concentrated solutions mutually retard diffusion. 6. 

 Double salts, of which one constituent diffuses more readily, and the other with 

 greater difficulty, may even be separated chemically by diffusion. 



In the endosmotic interchange of fluids, the passage of the fluid-particles 

 takes place independently of the hydrostatic pressure. 

 Fig. 129 is a simple illustration of endosmotic exchange. 

 A glass cylinder is filled with distilled water (F). A flask 

 (J) is kept immersed in the water to a suitable height, and 

 closed by a membrane (m) replacing its broken bottom. 

 From the neck of the flask, in which it is tightly corked, 

 projects a glass tube (R) . The flask is filled with concen- 

 trated salt-solution up to the level of the lower extremity of 

 the tube. The flask is introduced into the glass cylinder to 

 such a distance that both fluids stand at the same level (x). 

 In a short while the fluid rises in the tube (R), because 

 particles of water pass through the membrane into the 

 concentrated salt-solution in the flask, and independently 

 of the hydrostatic pressure. The fluid rises in the tube as 

 high as the attraction of the water causes it to. The height 

 of the fluid thus indicates the osmotic pressure. 



Conversely, also, particles of the concentrated salt- 

 solution pass from the flask into the interior of the cylinder, 

 mixing with the water (F) . This interchange of current 

 continues until an entirely uniform mixture is present in 

 the flask and in the cylinder. Under these circumstances 

 the level of the fluid will to the last always have risen 

 higher in the tube (to y). 



The circumstance that the level of the liquid within the 

 tube can rise so high and be kept at such a height depends 

 upon the fact that the pores of the membrane are too fine to 

 permit of the action of hydrostatic pressure through them. 

 Therefore endosmosis is defined as an interchange of par- 

 ticles of fluid independently of the hydrostatic pressure. 



Reflection will show that if, in an endosmosis-experi- 

 ment of similar kind, the water in the cylinder is renewed 

 from time to time, the solution in the flask must become 

 progressively more dilute, until, finally, the flask (J) and 

 the cylinder (F) contain only pure water. 



Endosmotic Equivalent. It has been found that in endomosis-experiments, 

 equal parts by weight of different fluids or soluble substances (which soon coalesce 

 on the moist surface of the membrane within the flask to form concentrated solu- 

 tions, as, for example, sodium chlorid) being present in the flask, a varying amount of 

 distilled water passes through the membrane, so that, finally, if the water in the 

 cylinder is constantly renewed, a variable amount of distilled water will be pre- 

 sent in the flask. In other words, it has been found that a definite part by weight 

 of a soluble substance in the flask has been exchanged by endosmosis for a 

 definite part by weight of distilled water. The figure that indicates how many 

 parts by weight of distilled water pass over in the endosmosis-flask for a 

 definite part by weight of a soluble substance has been designated by Jolly as the 

 endosmotic equivalent. For i gram of alcohol, 4.2 grams of water are exchanged; 

 for i gram of sodium chlorid, 4.3 grams of water. The endosmotic equivalents 

 for the following substances are: 



FIG. 129. Apparatus for 

 Diosmosis. 



