PROTEINS 



AND 



THE THEORY OF 

 COLLOIDAL BEHAVIOR 



CHAPTER I 

 HISTORICAL INTRODUCTION 



1. THE ALLEGED DIFFERENCE BETWEEN THE CHEMISTRY OF 

 COLLOIDS AND OF CRYSTALLOIDS 



The distinction between crystalloids and colloids was proposed 

 by Graham in 1861, the crystalloids being characterized by a 

 tendency to form crystals when separating from a watery solu- 

 tion, and the colloids by a tendency to separate out in the form 

 of " gelatinous" (or amorphous) masses. Graham found that 

 these two groups of substances differ also in two other respects, 

 first, in their " diffusive mobility/' and second, in a peculiar 

 '" physical aggregation." The crystalloids diffuse readily through 

 different kinds of membranes (e.g., pig's bladder, parchment) 

 through which colloids can diffuse not at all or only very slowly. 

 The second peculiarity is the tendency of the colloids to form 

 aggregates when in solution while this property is lacking or less 

 pronounced in crystalloids. A brief quotation from a paper by 

 Graham will illustrate these definitions: 



" Among the latter [i.e., the substances with low order of diffusibility] 

 are hydrated silicic acid, hydrated alumina, and other metallic peroxides 

 of the aluminous class, when they exist in the soluble form; and starch, 

 dextrin and the gums, caramel, tannin, albumen, gelatine, vegetable and 

 animal extractive matters. Low diffusibility is not the only property 

 which the bodies last enumerated possess in common. They are 

 distinguished by the gelatinous character of their hydrates. Although 



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