240 ANIMAL PROTEINS 



All the above decolorizing materials are insoluble and 

 hydrophobe, and act in virtue of their finely divided con- 

 ditions, which causes them to have a large specific surface ; 

 but there is another type or branch of substances, whose 

 effect is due to surface action of rather a different type. 

 These are the hydrophile gels. In a gelatine sol the colloid 

 particles have largely adsorbed the colouring matters which 

 it is desired to remove. This adsorption, which is after all 

 only an equilibrium, is reduced by introducing another very 

 strong adsorbent. This latter, by adsorption from the 

 continuous phase, reduces the adsorption of colouring matters 

 by the gelatine particles. In the case under discussion 

 another lyophile colloid is introduced, and after bringing 

 about such an action is removed by appropriate means. 

 The use of albumin has long been known for such a purpose, 

 its special advantage being that after its admixture and 

 adsorptive action, it may easily be removed by raising the 

 temperature above 70 C., when coagulation takes place, and 

 by subsequent mechanical filtration. The coagulated albumin 

 takes down the adsorbed colouring matters. Albumin 

 has been used in this way not only for gelatine and glue 

 liquors, but also for tanning extracts (Part I., Section III., 

 p. 37) and other commercial preparations. Into this class 

 of decolorizing agents fall the insoluble inorganic gels which 

 have been advocated by W. Gordon Bennett, e.g. alumina 

 cream. Freshly precipitated alumina hydrate is a colloid 

 gel with very considerable adsorptive powers. It has also 

 the advantage that it is quite insoluble, easily removed in 

 filtration, and has a powerful adsorptive action upon other 

 objectionable impurities, especially the poisonous metals, 

 arsenic, copper, zinc and lead. Its use is an undoubted 

 advantage when in addition to the other clarifying agents and 

 adsorbents. It is conceivable, in some cases, that when alum 

 is employed as clarifying agent in an alkaline gelatine liquor, 

 some alumina may be formed, and as such contribute to 

 the total effect. 



