CHAPTER II 



THE COLLOIDAL STATE 



Many of the substances of which the plant is built up exist in the living 

 cell in the colloidal state, and it is therefore important that some account 

 should be given of this condition of matter. 



There are many organic products found in the plant (and also in the 

 animal), such as starch, various proteins, gums, etc., that apparently dis- 

 solve in water, giving a solution which, as a rule, only differs from an 

 ordinary solution by being opalescent. In addition, it has been known 

 for a long time that various inorganic substances, such as sulphides of 

 arsenic and antimony, hydroxide of iron, and also certain metals (gold, 

 silver), can, by special methods, be obtained in " solution," though in 

 ordinary circumstances they are quite insoluble. The above examples 

 are representative of colloidal solutions. 



A property which all the above solutions possess is that the substance 

 dissolved will not pass through a parchment membrane, i.e. will not 

 dialyze, whereas if a solution of sodium chloride in water is separated 

 from pure water by a parchment membrane, the salt will pass through 

 the membrane until the concentration of the sodium chloride is equal 

 on either side of it. 



The conclusion drawn from investigations of various kinds is that in 

 the colloidal solutions the substances dissolved exist in the state, either 

 of aggregates of molecules, or of very large molecules, and hence are 

 unable to pass through the pores of the parchment. 



Moreover, certain distinctions can be drawn between colloidal solu- 

 tions : some, like those of gold, silver, metallic sulphides, hydroxides and 

 in fact most inorganic substances, are very sensitive to the presence of 

 small amounts of inorganic salts, i.e. electrolytes, and are precipitated 

 by them, but will not as a rule go into solution again. Also such col- 

 loidal solutions are very little more viscous than pure water. The organic 

 substances, on the other hand, are only precipitated from colloidal solu- 

 tions by comparatively large quantities of electrolytes. The viscosity, 

 moreover, of these solutions is greater than that of water, and is, in fact, 

 considerable, even if the percentage of dissolved matter is small. 



