226 THE PLANT. 



plant. If not so utilised they remain in the sap and thus 

 prevent the entrance by diffusion of additional matter of the 

 same kind. 



A few words may here be said about the phenomena of 

 diffusion and osmotic pressure. Diffusion, as is well known, 

 refers to the process by which a substance dissolved in a 

 solvent, moves from the more concentrated to the less concen- 

 trated portions of the liquid. This movement is an indication 

 of a motion which is constantly occurring in the particles of a 

 dissolved substance, but which is only readily apparent when 

 more of the substance moves in one direction or into a given 

 space than moves in the other direction or out of it. 



Graham in 1850 investigated the rates of diffusion of various 

 compounds in aqueous solution and found great differences 

 with different substances. Many crystallizable substances 

 moved comparatively rapidly, while colloidal bodies, e.g., gum, 

 albumen, tannin, &c., moved with extreme slowness. He 

 found, moreover, that colloidal bodies, either in solution or 

 when thoroughly wetted with water, allowed crystalloids to 

 diffuse freely, but practically stopped all diffusion of dissolved 

 colloids. 



The practical application of this phenomenon to the separa- 

 tion of crystalloids from colloids is known as <7/^///.s/'.s and is 

 usually accomplished by means of a vessel, the bottom of 

 which is composed of animal or vegetable parchment. The 

 mixture of the two dissolved substances is placed within this 

 vessel, which is then floated upon pure water ; the crystalloid 

 passes through the membrane, but the colloid does not. The 

 diffusion of the crystalloid goes on \vith diminishing speed 

 until the concentration of the solution within and without the 

 dialyser becomes equal. Even then, it is to be clearly under- 

 stood, the action probably does not cease; but, since just as 

 much leaves the inner vessel in a given time as enters it, the 

 process does not readily show itself. 



By repeatedly renewing the water in the outer vessel the 

 whole of the crystalloid may be removed from the liquid within 

 the dialyser, while the amount of colloid is not appreciably 

 diminished. 



