DIFFCSION AND OSMOSIS 27 



work. 'Hiis process is not dependent upon ionization, for a s(jluti(ni 

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

 l)rotein or other colloid does so nuich more slowly, however, indeed, 

 ([uite 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. 

 Crj^stalloids are generally soluble in colloids and hence pass through 

 such diffusion membranes; colloids dissolve but slightlj' 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 uninterrupted, 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 

 through slowlj' or not at all, there will soon be an unequal condition on 

 the two sides of the membrane, for the diffusible substance would ac- 

 cumulate in equal amounts on each side, while the non-diffusible 

 would remain where it was. On one side there would then be more 

 material exerting osmotic pressure than on the other, and if the mem- 

 brane were flexible, it would bulge toward the opposite side. The 

 pressure is supposed to be due to the bombardment of the containing 

 walls by molecules or ions of the substances in solution, and hence the 

 more molecules and ions in solution, the more pressure. When equal 

 numbers of particles are on each side of the partition, the pressure is 

 equalized. It is quite possible to have membranes readily permeable 

 to one substance and almost entirelj^ impermeable to another; such 

 membranes are called semipermeable. To produce osmotic pressure 

 it is not necessary that the membrane be absolutely impermeable to 

 any of the substances — it may only be relatively less permeable for 

 the solute than for the solvent. If, for example, we fill a parchment 

 bag with concentrated sugar solution, tie up the top tightly and throw 

 it into water, it will swell up rapidly and eventually burst. But if the 

 parchment is in the form of a tube, open at the top, and the lower 

 end is placed in water, the amount of fluid inside the tube will in- 



