2 THEORY OF COLLOIDAL BEHAVIOR 



often largely soluble in water, they are held in solution by a most feeble 

 force. They appear singularly inert in the capacity of acids and bases, 

 and in all the ordinary chemical relations. 1 But, on the other hand, 

 their peculiar physical aggregation with the chemical indifference 

 referred to, appears to be required in substances that can intervene in 

 the organic processes of life. The plastic elements of the animal body 

 are found in this class. As gelatine appears to be its type, it is proposed 

 to designate substances of the class as colloids, and to speak of their 

 peculiar form of aggregation as the colloidal condition of matter. Opposed 

 to the colloidal is the crystalline condition. Substances affecting the 

 latter form will be classed as crystalloids. The distinction is no doubt 

 one of intimate molecular constitution." 2 



It is therefore obvious that there are according to Graham at 

 least two essential differences between colloids and crystalloids, 

 the difference in diffusion through membranes, and the difference 

 in the tendency to form aggregates in solutions. We shall see 

 in this volume that the chief if not all the characteristics of 

 colloidal behavior can be explained mathematically from the 

 difference in diffusibility between colloids and crystalloids, while 

 the tendency of the protein molecules to form aggregates plays 

 only an indirect role, namely, by immobilizing one kind of ions 

 without interfering with the mobility of other ions. 



In modern colloid chemistry it has, however, become custom- 

 ary to consider the tendency of colloids to form aggregates as 

 the fundamental property, for the reason that the precipitation 

 of colloids was the chief topic of research and discussion in 

 colloid chemistry, and precipitation is, of course, due to the 

 formation of aggregates. The colloidal state is defined by colloid 

 chemists as that state of matter in which the ultimate units in 

 solutions are no longer isolated molecules or ions, but aggregates 

 of molecules for which Naegeli had introduced the term micella 

 (small crumb). Thus Zsigmondy states, 



"that the essential and characteristic constituents of colloidal solutions 

 are very small ultramicroscopic particles the dimensions of which lie 

 between molecular and microscopic size. . . . These ultramicroscopic 



1 This is no longer correct, as we shall see. 



2 GRAHAM, T., Phil Trans., pp. 183-224, 1861. Reprinted in "Chemical 

 and Physical Researches," p. 553, Edinburgh, 1876. 



