236 ANIMAL PROTEINS 



the well-known practical fact that it is more satisfactory to 

 filter a dilute gelatine sol. Further, the electric charge may 

 be reduced also by causing the adsorption of an ion of 

 opposite charge. This is the principle underlying the 

 precipitation (of any colloid) by adding electrolytes. It is 

 essential here to consider which ions are most likely to be 

 adsorbed, and also to bear in mind what charge they carry. 

 Now the hydrion (H+) of acids and the hydroxyl ion (OH~) 

 of alkalies are most strongly adsorbed, so that to precipitate 

 a negative sol, acid is very effective, whilst with a positive 

 sol an alkali is an appropriate precipitant. Further, it is 

 known that organic ions are usually more strongly adsorbed, 

 hence when precipitating from an alkaline sol (negative sol), 

 one should preferably select an inorganic or mineral acid 

 rather than an organic acid. Thus in clarifying an alkaline 

 gelatine sol, hydrochloric or sulphuric acid is to be preferred 

 to acetic or lactic acid. Again, it is necessary to remember 

 that a divalent ion carries twice the charge of a univalent ion, 

 hence the precipitating power of an electrolyte depends upon 

 the valency of the ion whose electric charge is opposite to 

 that on the sol (Hardy's valency rule). Thus a negative 

 sol is most easily precipitated by a monobasic acid. Thus 

 hydrochloric acid is better than sulphuric, on account of the 

 stabilizing effect of the divalent 864 ion on a negative sol. 

 In such a sol, also, the valency rule indicates that the multi- 

 valent kations, e.g. iron, Fe+ + + ; chromium, Cr+ + + ; and 

 aluminium, A1+ ++, should have great precipitating and 

 clarifying effect. This of course is known to be the case, 

 aluminium salts having long been used. The rule indicates, 

 also, that aluminium chloride would be better than the 

 sulphate or than potash alum. Another feature of precipita- 

 tion worthy of mention is the phenomenon of " acclimatiza- 

 tion." This describes the fact that when the precipitating 

 reagent is added very slowly, or a little at a time, a larger 

 amount must be used, and the slower the addition the greater 

 the excess required. Hence in precipitating matters from 

 an alkaline gelatine sol the acid, if practicable, should be 

 added all at once. In any case it is clear that one should 



