1023 



tremely imperfect. The latter consist in microphysical and micro- 

 chemical reactions and not in phenomena which might more rightly 

 find a place in metaphysics. While the work in molecular physics, 

 such as that of DeVries, Van't Hoff and Fischer, has gone far to clear 

 up the nature of these processes, it must be admitted that our explana- 

 tions are still based upon generalities. 



Diffusion, Osmosis, Dialysis. The term' diffusion is applied to 

 the spreading about or scattering of molecules through media allowing 

 this movement. Thus, if a solution of a salt is placed in a receptacle 

 and a layer of water is carefully allowed to run over it, it will be found 

 after a time that a certain number of the mole- 

 cules of the salt have entered the overlying 

 water and have established a medium of uniform 

 composition throughout. This spreading out 

 also takes place if two solutions of different 

 salts are brought into contact with one another. 

 A uniform mixture is the final result. Next we 

 proceed to interpose between the solution and 

 the water an animal membrane, such as a 

 piece of intestine, urinary bladder, swim- 

 bladder, or an artificial membrane made by 

 allowing ferrocyanide of potassium to come in 

 contact with cupric sulphate in an unglazed 

 piece of porcelain. The result of this interac- 

 tion is a layer of ferrocyanide of copper. l Such 

 membranes may be absolutely impermeable, 

 completely permeable, or partially permeable 

 to water and its constituents. The first allows 

 no diffusion at all, whereas the second permits 

 it to occur freely in both directions. This 

 narrows this discussion down to membranes of 

 semi-permeable character, namely, to those 





FIG. 519. A SIMPLE 



OSMOMETER. 



The receptacle contains 

 water, and the cell a solu- 

 ... , tion of magnesium sul- 



which allow a free interchange of water but not pna te. As the molecules 



Of the dissolved Substances. Consequently, if of water are drawn through 



the water and the salt solution are separated brane^t'h^bv 1 !' 16 f^th" 

 by a membrane of this kind, the molecules of MgSO< solution rises, 

 water will gradually pass through its pores into 



the solution. This phenomenon is called osmosis. Quite similarly, 

 we may fill a thistle tube with a solution of magnesium sulphate, 

 close its large orifice with an animal membrane, and place it in water 

 so that the level of the latter corresponds precisely with that of the 

 said solution. Water then passes into the thistle tube, causing the 

 level of the magnesium sulphate solution to rise until its height indicates 

 a considerable back pressure against the membrane. This pressure 



1 Morse and Frazer, Am. Chem. Jour., xxxiv, 1905, 1, also Hedin, Pfliiger's 

 Archiv, Ixxviii, 1899, 205, Hober, ibid., Ixxi, 1898, 624, and Denis, Am. Jour, of 

 Physiol., xvii, 1906, 35. 



