GENERAL CHEMISTRY OF THE PROTEINS 3 



of dispersion into a gaseous phase. Is it not possible, therefore, 

 to build up a process of separation and purification of dissolved 

 substances on these differences ? Graham's observation that 

 substances which diffused with difficulty were more or less held 

 back by parchment paper or by animal membranes indicated 

 the method of purifying substances by dialysis, as he called the 

 process. In this way salts and other substances which diffuse 

 easily are removed without fundamental changes or chemical 

 complications. It is well known how important dialysis has 

 become, particularly in biochemical investigation. In Graham's 

 hands it became the first general method of preparing pure 

 colloids, a name which he gave to those substances which are 

 distinguished by the following three criteria : I. They diffuse 

 very slowly or too slowly for the rate to be measured. II. They 

 are unable to penetrate certain membranes, particularly some 

 of animal origin. III. They lack the power to appear in the 

 crystalline form. To these a fourth has .since been added. 

 G. Bredig, in particular, has shown that the work required to 

 be done to separate the colloid from the solvent is very small or 

 practically nil, as is shown by measurements on the evaporation 

 or freezing out of the solvent. We shall see that all these 

 criteria depend on one single property, which now appears to 

 us as the significant characteristic of the colloid state. 



The greatest progress which has been made since the time of 

 Graham lies in the following straightening-out of ideas. Graham 

 divided substances themselves into colloids and crystalloids. 

 In which of these forms it appeared would thus seem to depend 

 on the chemical nature of a substance. We now know, however, 

 that the colloid or crystalloid state is no more than a physical 

 manifestation of a dependent condition of matter. The same 

 chemical substance can be obtained, according to circumstances, 

 with colloid or crystalloid characteristics. This development 

 is, in the first place, a result of improvements in methods of 

 preparation. It is possible to obtain in the colloidal state all 

 kinds of substances of low molecular weight, e.g., metals, salts 

 of alkalis and of the alkaline earths, etc. It has, moreover, been 

 shown that for the production of the colloidal condition a state 

 of fine subdivision is necessary, but not division into single 



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