CATALYSIS: THE GUIDE OF LIFE 93 



possess double the velocity of diffusion of sulfate of potash, and sul- 

 fate of potash again double the velocity of sugar, alcohol, and sulfate 

 of magnesia. But the substances named, belong all, as regards dif- 

 fusion, to the more 'volatile' class. The comparatively 'fixed' class, as 

 regards diffusion, is represented by a different order of chemical sub- 

 stances, marked out by the absence of the power to crystallize, which 

 are slow in the extreme. Among the latter are hydrated silicic acid, 

 hydrated alumina, and other metallic peroxides of the aluminous 

 class, when they exist in the soluble form; with starch, dextrin, and 

 the gums, caramel, tannin, albumen, gelatin, 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 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 reactions. 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 gelatin appears to be its type, it is pro- 

 posed 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. Sub- 

 stances affecting the latter form will be classed as crystalloids. The 

 distinction is no doubt one of intimate molecular constitution." 



While it is impossible here to attempt to follow historically the 

 development of colloid chemistry and of catalysis, the following 

 brief quotation from a paper by Professor Leonard Thompson 

 Troland of Harvard 5 clearly shows his appreciation of the 

 problem: 



"As a matter of fact, in the school of the physical chemists there 

 has been in preparation, since the days of Thomas Graham, a sys- 

 tem of knowledge which, even in its present unfinished form, has 

 a most important and distinct bearing upon mooted biological 

 problems. This is the science of the colloidal state. The difficult 

 abstractions and elaborate classificatory scheme, in terms of which 

 the theory is now stated, will tend to be cleared up as our study of 

 colloids comes definitely under the dominion of the general 

 electro-molecular theory of matter. Intimate contact with the 

 latter has already been established, indeed, through recent remark- 

 able contributions by Langmuir, dealing with the atomic consti- 



