THE CHEMISTRY AND PHYSICS OF THE CELL 35 



The applications in pathology will be brought out as the sub- 

 ject under discussion in subsequent chapters necessitates, and it 

 is largely to facilitate the understanding of such references that 

 this brief summary of the subject of ionization has been 

 introduced. In the same spirit we take up the subjects of 

 diffusion and osmosis. 



Diffusion and Osmosis. Although the non-electrolytes 

 do not ionize to any considerable extent, and therefore are rela- 

 tively 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. Diffusion 

 occurs most simply between two solutions of unlike nature, or 

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

 placed in contact with one another. If we place in the bottom 

 of a cylindrical vessel a solution of copper sulphate and above 

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

 some time that the fluid has become equally blue throughout. 

 This is brought about by the movement of the dissolved parti- 

 cles, which gradually carries them through the entire mass of 

 fluid, and as their migration is against the force of gravity, they 

 evidently 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 proteid 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 without hindrance. If, instead of filter-paper, 

 there were introduced a sheet of some substance free from 

 pores, then diffusion .would be much more affected. If the sep- 

 tum was of such a nature that the substances 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 dis- 

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

 fluid on the other side. The last example indicates the condi- 

 tions afforded in the animal cell, and also in the usual labora- 

 tory diffusion experiments when the membrane is generally 

 either an animal membrane or a parchment paper, both of 



