Mr. T. Graham on Liquid Diffusion applied to Analysis. 377 



Gummic acid, for instance, may be represented by C ,2 H U 11 ; 

 but, judging from the small proportions of lime and potash 

 which suffice to neutralize this acid, the true numbers of its 

 formula must be several times greater. It is difficult to avoid 

 associating the inertness of colloids with their high equivalents, 

 particularly where the high number appears to be attained by 

 the repetition of a smaller number. The inquiry suggests itself 

 whether the colloid molecule may not be constituted by the 

 grouping together of a number of smaller crystalloid molecules, 

 and whether the basis of colloidality may not really be this 

 composite character of the molecule. 



With silicic acid, which can exist in combination both as a 

 crystalloid and colloid, we have two series of compounds, silicates 

 and cosilicates, the acid of the latter appearing to have an equi- 

 valent much greater (thirty- six times greater in one salt) than 

 the acid of the former. The apparently small proportion of acid 

 in a variety of metallic salts, such as certain red salts of iron, is 

 accounted for by the high colloidal equivalent of their bases. 

 The effect of such an insoluble colloid as prussian blue in carry- 

 ing down small proportions of the precipitating salts, may admit 

 of a similar explanation. 



Gelatine appears to hold an important place as a colloidal base. 

 This base unites with colloidal acids, giving a class of stable 

 compounds, of which tanno-gelatine only appears to be hitherto 

 known. Gelatine is precipitated entirely by a solution of meta- 

 phosphoric acid added drop by drop, 100 parts of gelatine 

 uniting with 3'6 parts of the acid. The compound formed is a 

 semitransparent, soft, elastic, and stringy solid mass, presenting 

 a startling resemblance to animal fibrine. It will be an interest- 

 ing inquiry whether inetaphosphoric acid is a colloid, and enters 

 into the compound described in that character, or is a crystal- 

 loid, as the small proportion and low equivalent of the acid 

 would suggest. Gelatine is also precipitated by carbolic acid. 



The hardness of the crystalloid, with its crystalline planes and 

 angles, is replaced in the colloid by a degree of softness, with a 

 more or less rounded outline. The water of crystallization is 

 represented by the water of gelatination. The water in gelati- 

 nous hydrates is aptly described by M. Chevreul as retained by 

 "capillary affinity," that is, by an attraction partaking both of 

 the physical and chemical character. While it is here admitted 

 that chemical affinity of the lowest degree may shade into capil- 

 lary attraction, it is believed that the character of gelatinous 

 hydration is as truly chemical as that of crystalline hydration. 

 Combination of a colloid with water is feeble, it is true ; but so 

 is combination in general with the colloid. Notwithstanding 

 this, anhydrous colloids can decompose certain crystalloid 



