HYDROPHILIC SOLS AND GELS 145 



in the case of proteins are molecules, though they may be micelles 

 or aggregates of molecules. Around this question have centered 

 the most severe of the controversies in colloid chemistry. The 

 concept of a colloidal particle, a micelle, as the unit of structure 

 in colloidal systems, has dominated the thoughts of most colloid 

 chemists. That such particles exist in lyophobic systems {e.g., 

 colloidal gold) there can be no doubt, but the term micelle is 

 applied primarily to the structural unit of lyophilic systems 

 (i.e., gelatin). An attempt at a solution of this problem in so 

 far as it applies to gel structure will be reserved for a later chapter. 

 For our present purpose (i.e., as an explanation of slow diffusion 

 rate and low osmotic pressure), it is sufficient that the particle 

 is large, whether micelle or molecule, and this condition is met 

 by the very large size of the protein molecule. This does not 

 preclude the possibility of these molecules being grouped together 

 to form colloidal aggregates. 



Little is known of the nature of the structural units of jellies. 

 Coagula are in general granular as is true of the silica gel. Here 

 we undoubtedly have to do with a very finely porous framework 

 built up of colloidal granules (Fig. 90). In the case of the sols 

 and gels of metallic oxides, there is also little doubt but that 

 typical colloidal particles are the structural units. 



Hysteresis. — Hysteresis, a word of Greek origin meaning a 

 "deficiency" or a "coming after," is applied to two quite distinct 

 physical phenomena, one in electrical engineering and one in 

 colloid chemistry, which have, however, one thing in common, 

 viz., a lagging of one process behind another. When gelatin is 

 repeatedly melted and allowed to resolidify, the temperature 

 of the melting point and also of the solidifying point becomes 

 progressively lower. This behavior is known as hysteresis. 

 It is an expression of the past history of a substance, and upon 

 it do a number of properties, such as hydration and viscosity, 

 depend. 



An extraordinary case of hysteresis is the effect on a gel of the 

 degree of previous swelling. A 10 per cent gelatin jelly will, if 

 dried to 97 per cent gelatin and allowed to swell again in water, 

 swell to a 10 per cent gel when it otherwise would have taken up 

 much more water. A 30 per cent gelatin gel under the same 

 conditions swells again to approximately a 30 per cent gel. 

 We see, therefore, that the present behavior of a colloidal system 



