CHAPTER VII. 



DIFFUSION, OSMOSIS, AND ABSORPTION OF LIQUIDS. 

 DIFFUSION AND OSMOSIS. 



598. WHEN two liquids which are not miscible for instance, 

 oil and water are shaken together, and then left at rest, they 

 will separate sooner or later, according to their specific gravity. 

 But if two miscible liquids are shaken together, they remain as a 

 homogeneous mixture no matter what their specific gravity may 

 be. Also when two miscible liquids are left in contact, without 

 any agitation they become thoroughly commingled, and constitute 

 a uniform mixture ; this uniform commingling of two or more 

 miscible fluids is termed diffusion. 1 



599. Furthermore, if two miscible liquids are separated by 

 a membrane which can be moistened by them, they will diffuse 

 through it and make a uniform mixture. This latter kind of 

 diffusion, in which the contact between the two liquids is not 

 direct, but takes place through a septum of some substance, is 

 known as osmosis. In the plant and in its surroundings the 

 two kinds of diffusion play such an important part that they 

 must receive special attention. 



600. Diffusion of liquids. The rate of diffusion varies with 

 the nature of the liquids and the temperature. The statements in 

 the following paragraphs are substantially as given by Graham. 2 



1 Pfaundler applies this term to the commingling whether it is or is not 

 brought about by agitation (Miiller's Lehrbuch, 1877, i. 162). 



2 They are based upon two series of experiments conducted with very sim- 

 ple apparatus. In the first series a small, wide-mouthed vial containing one 

 liquid was placed in a jar holding the other liquid, allowed to stand a few 

 days, withdrawn, and the amount of diffusion noted. In the second series 

 Graham pursued the plan of placing in a cylindrical glass jar, 152 mm. high 

 and 87 mm. wide, seven tenths of a liter of pure water, and then carefully car- 

 rying to the bottom of the jar, by means of a fine pipette, one tenth of a liter 

 of the liquid to be diffused. The jar was then left at rest in an apartment 

 where the temperature was nearly constant, and after a certain time its contents 

 were drawn off carefully in portions of fifty cubic centimeters, each portion 

 evaporated separately, and the residue remaining after evaporation weighed. 



