DIFFUSION AM) OSMOSIS 29 



been introduced. In the same spirit we take up tlie subjects of dif- 

 fusion and osmosis. 



Diffusion and Osmosis. — Although the non-electrolytes do not ion- 

 ize to any considerable extent, and therefore are relatively inactive 

 chemically, the crystalloidal non-electrolytes, of which sugar and urea 

 are the two chief examples among the cell constituents, possess in 

 common with the electrolytes the important property of diffusion. 

 By this process the exchange of chemical substances between the blood 

 and the cell is brought about, by it the chemical composition of the 

 different parts of the cell and between different cells is equalized, 

 and without it chemical change would be practically impossible. Dif- 

 fusion occurs most simply between two solutions of mil ike nature, 

 or between a solution of a substance and the solvent alone, when 

 placed directly in contact with one another. If we place in tlie bot- 

 tom of a cylindrical vessel a solution of copper sulphate and above 

 it some water, carefully avoiding mixing, it will be found after some 

 time tliat the fluid has become equally blue throughout. This is 

 brought about by the movement of the dissolved particles which 

 gradualh' carries them through the entire mass of fluid, and as their 

 migration is against the force of gravity, they evidenth' accomplish 

 work. This process is not dependent upon ionization, for a solution 

 of cane-sugar or of urea will show the same diffusion. A solution of 

 protein or other colloid does so much more slowly, however, indeed 

 quite imperceptibly. 



If we were to introduce a piece of filter-paper between the water 

 and the copper sulphate solution, the diffusion would go on the same, 

 the pores of the paper permitting the passage of the molecules with- 

 out hindrance. If, instead of filter-paper, there were introduced a 

 sheet of some substance free from pores, the diffusion would be much 

 more affected. If the septum was of such a nature that the sub- 

 stances in solution were insoluble in it (e. g., glass), diffusion would 

 of necessity stop ; but if it were something in which the solvent or the 

 solute was soluble, such as a gelatin plate, then these substances would 

 dissolve in it, and diffusing through its substance escape into the 

 fluid on the other side. The last example indicates the conditions 

 afforded in the animal cell, and also in the usual laboratory diffusion 

 experiments when the membrane is generally either an animal mem- 

 brane or a parchment paper, both of which are composed of colloids. 

 Crystalloids are generally soluble in colloids and hence pass through 

 such diffusion membranes; colloids dissolve but slightly in colloids, 

 and hence they do not pass through a diffusion membrane readily, 

 and are, therefore, but very slightly diffusible. 



The process of diffusion, if uninterrujited, always continues until 

 the solution is of exactly the same composition throughout. If on one 

 side of the diffusion membrane there is a substance that passes through 

 the membrane rapidly, and on the other a substance that passes 



