FORCES CONCERNED IN ABSORPTION. 



303 



the fluids occurs until both fluids have the same composition. This exchange of 

 fluids is termed endosmosis or diosmosis. 



If we remember that within the intestinal tract, there are relatively concen- 

 trated solutions of those substances which have been brought into solution by 

 the digestive juices peptone, sugar, soaps, and solutions of the salts while 

 separated from these by the porous mucous membrane and the walls of the blood- 

 and lymph-capillaries is the blood, which contains relatively less of these sub- 

 stances, it is clear that an endosmotic current must set in towards the blood and 

 lymph-vessels. 



Diffusion. If the two mixible fluids are placed in a vessel, the one fluid over 

 the other, but without being separated by a porous septum, an exchange of the 

 particles of the fluids also occurs, until the whole mixture is of uniform composi- 

 tion. This process is called Diffusion. 



Conditions Influencing Diffusion. Graham's investigations showed that the 

 rapidity of diffusion is influenced by a variety of conditions: (1) The nature of 

 the fluids themselves is of importance; acids diffuse most rapidly; the alkaline 

 salts more slowly; and most slowly, fluid albumin, gelatin, gum, dextrin. These 

 last do not crystallise, and perhaps do not form true solutions. (2) The more 

 concentrated the solutions, the greater the diffusion. (3) Heat accelerates, while 

 cold retards, the process. (4) If a solution of a body which diffuses with difficulty 

 be mixed with an easily diffusible one, the former diffuses with still greater 

 difficulty. (5) Dilute solutions of several substances diffuse into each other 

 without any difficulty, but if concentrated solutions are employed, the process is 

 retarded. (6) Double salts, one constituent of which diffuses o 



more readily than the other, may be chemically separated by 

 diffusion. 



The exchange of the fluid particles takes place independently 

 of the hydrostatic pressure. Fig. 156 represents an endosmo- 

 meter. A glass cylinder is filled with distilled water, and 

 into this is placed a flask, J, without a bottom, instead 

 of which a membrane, m, is tied on. A glass tube, R, is fixed 

 firmly by means of a cork into the neck of the flask. The 

 flask is filled up to the lower end of the tube with a concen- 

 trated salt solution, and is then placed in the cylindrical 

 vessel until both fluids are on the same level, x. The fluid 

 in the tube, R, soon begins to rise, because water passes 

 through the membrane into the concentrated solution in the 

 flask, and this independently of the hydrostatic pressure. 

 Particles of the concentrated salt solution pass into the 

 cylinder and mix with the water, F. These outgoing and 

 ingoing currents continue until the fluids without and within 

 J are of uniform composition, whereby the fluid in R always 

 stands higher (e.g., at y), while it is lowered in the cylinder. 

 The circumstance of the level of the fluid within the tube 

 being so high and remaining so, is due to the fact that the 

 pores in the membrane are too fine to allow the hydrostatic 

 pressure to act through them. 



Endosmotic Equivalent. Experiment has shown, that 

 equal weights of different soluble substances attract different 

 amounts of distilled water through the membrane i.e., a 

 known weight of a soluble substance (in the flask) can be 

 exchanged by endosmosis for a definite weight of water. The term endosmotic 

 equivalent indicates the weight of distilled water that passes into the flask of 

 the endosmometer, in exchange for a known weight of the soluble substance (Jolly). 

 For 1 gram, alcohol 4'2 grams, water were exchanged; while for 1 gram. NaCl, 4'3 



Fig. 156. 

 Endosmometer for 



Diffusion. 



