Mr. T. Graham on Liquid Diffusion applied to Analysis. 205 



tion. So similar in character to volatility is the diffusive power 

 possessed by all liquid substances, that we may fairly reckon 

 upon a class of analogous analytical resources to arise from it. 

 The range also in the degree of diffusive mobility exhibited by 

 different substances appears to be as wide as the scale of vapour- 

 tensions. Thus hydrate of potash may be said to possess double 

 the velocity of diffusion of sulphate of potash, and sulphate of 

 potash again double the velocity of sugar, alcohol, and sulphate 

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

 diffusion, to the more " volatile " class. The comparatively 

 " fixed ;; class, as regards diffusion, is represented by a different 

 order of chemical substances (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, dextrine and the gums, caramel, tan- 

 nin, 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 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. 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 aggre- 

 gation 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. 



Although chemically inert in the ordinary sense, colloids 

 possess a compensating activity of their own arising out of their 

 physical properties. While the rigidity of the crystalline struc- 

 ture shuts out external impressions, the softness of the gelatinous 

 colloid partakes of fluidity, and enables the colloid to become a 

 medium for liquid diffusion, like water itself. The same pene- 

 trability appears to take the form of cementation in such colloids 

 as can exist at a high temperature. Hence a wide sensibility on 

 the part of colloids to external agents. Another and eminently 

 characteristic quality of colloids is their mutability. Their 

 existence is a continued metastasis. A colloid may be compared 

 in this respect to water while existing liquid at a temperature 

 under its usual freezing-point, or to a supersaturated saline 



