APPENDIX TO PROTEINS. 

 COLLOIDS AND COLLOIDAL SOLUTIONS. 1 



The proteins, also the fats and soaps and the polysaccharides, the 

 principal substances with which physiological chemistry has to deal, 

 are colloids. Their properties depend so much upon this fact that 

 it is necessary to examine the nature of colloids and colloidal solutions. 



Crystalloids and Colloids. 



Thomas Graham between 1861 and 1864, whilst studying the 

 diffusion of dissolved substances through organic membranes, such as 

 parchment paper, found that some substances dialysed, or passed freely 

 through the membrane, but that other substances did not pass through 

 or passed through very slowly. The substances belonging to the first 

 class were salt, sugar, urea, etc., which crystallised well : the substances 

 belonging to the second class like gelatin, albumin, gum, starch, did 

 not crystallise. He distinguished the two classes as crystalloids and 

 colloids. 



Natural and Artificial Colloids. 



The substances belonging to the group of colloids show amongst 

 hemselves many differences : 



Hot solutions of gelatin or agar on cooling form jellies which re- 

 dissolve on warming. Solutions of albumin on heating coagulate, i.e. 

 form an insoluble precipitate. Solutions of gum neither set to a jelly 

 nor coagulate, but always form more or less viscous solutions. 



Graham found that substances like silicic acid, ferric hydroxide, 

 etc., substances which are usually insoluble, could be made to form true 

 solutions in their appearance to the eye, and that the solid matter in 

 apparent solution did not diffuse through parchment membranes. 



These artificial solutions had one peculiar property : they under- 

 went a marked and irreversible change on the addition of a small 

 quantity of an electrolyte. The solid matter was either precipitated or 

 the solution set to a jelly ; neither the precipitate nor the jelly could 

 be redissolved to form a solution. 



1 An excellent description is given by Hatschek, " An Introduction to the Physics and 

 Chemistry of Colloids," from which book most of these notes have been compiled. 



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