132 ANIMAL PROTEINS 



tanning sol is not chromium hydrate at all, but a basic 

 salt of chrome also in colloidal solution, and to contend 

 that this salt, like most substances, forms a negative sol, 

 but in practice not negative enough, hence the desirability 

 of alkali, divalent anions, etc. From this point of view 

 the analogy with vegetable tannage becomes more com- 

 plete and the stabilizing effect of the soda salts of organic 

 acids becomes easy to understand. 



It is highly probable that the electrical properties of 

 the chrome sol need closer investigation on account of the 

 complexity due to the prominent effect of multivalent 

 ions. It is desirable to bear in mind the remarkable 

 phenomenon observed by Burton (Phil. Mag., 1905, vi, 

 12, 472), who added various concentrations of aluminium 

 sulphate to a silver sol (negative). He observed (i) a zone 

 of non-precipitation due to protection ; (2) a zone of pre- 

 cipitation due to the trivalent kation ; (3) a second zone 

 of non-precipitation due to protection after the sol has 

 passed through the isoelectric point and become a positive 

 sol ; (4) a second zone of precipitation due to the pre- 

 cipitating effect of the anion on the now positive sol. It 

 seems to the writer that similar phenomena may possibly 

 occur in chrome tanning, for whatever the sol actually 

 is, it is not far from the isoelectric point. 



A few observations on the vegetable-chrome combina- 

 tion tannages will not be out of place at this stage. Wilson 

 refers to the well-known practical fact that chrome leather 

 can take up about as much vegetable tan as if it were 

 unchromed pelt, and considers this evidence that the two 

 tannages are of fundamentally different nature. " In 

 mineral-tanned leathers the metal is combined with carboxyl 

 groups, while in vegetable-tanned leather the tannin is 

 combined with the amino groups. This strongly suggests 

 the possibility that the two methods of tanning are to 

 some extent independent of one another, and that a piece 

 of leather tanned by one method may remain as capable 

 of being tanned by the other method as though it were 

 still raw pelt " (Collegium (L,ondon), 1917, iio-m). To 



