EVOLUTION OF COLLOIDS 151 



part of the proper amount of alkali to neutra- 

 ize it, which it would have needed, if supposed 

 to be composed of mono-molecules. 



The huge molecules were unable to pass 

 through the parchment paper in the method 

 of separation mentioned above, although the 

 acid and other crystalloids had passed through 

 readily. This is a general method of separa- 

 tion of colloids and crystalloids. A colloid 

 in solution is unable to penetrate a film of a 

 hydrogel of another colloid placed in its way. 

 The living cell makes great use of this property 

 by possessing such a film. Inside the film 

 of hydrogel it holds dissolved colloids or 

 hydrosols so constituted that they can unite 

 with certain crystalloids (such as the dissolved 

 food-constituents, amino acids, sugars, fatty 

 acids, glycerine, etc.). The hydrosols are 

 kept within the cell by the hydrogel membrane 

 and the crystalloids diffuse in. Once within 

 they are either broken down by the hydrosols 

 to yield energy, or built up into their substance 

 as described above. This relieves pressure 

 within and more crystalloid diffuses in. 

 Certain inorganic salts also combine with the 

 hydrosols of the cell, and so accumulate 

 preferentially within it, such, for example, 

 as potassium and phosphates. 



This property of indiffusibility of the, 



