210 ANIMAL PROTEINS 



Before leaving this point, it is desirable to note the effect 

 on the swelling of gelatine of the extremes of this lyotrope 

 influence. Substances like iodides, thiocyanates and urea 

 prevent a gelatine sol from setting to a gel at all, and a piece 

 of gelatine in such solutions swells rapidly until it solates. 

 On the other hand, sulphates, tartrates, etc., make a stiffer 

 gel on account of the enhanced compression. Gelatine in 

 such solutions may swell, but at a much slower rate than in 

 water and with a decreased maximum extent. A gelatine 

 gel may in such solutions not only fail to swell at all, but 

 actually contract and in some cases, indeed, be practically 

 dehydrated. If a gel be in a very concentrated solution of 

 such a substance, it may be that the lyotrope compression 

 in the external solution is greater than the compression in the 

 dispersion medium of the gel ; in which case the surface 

 tension effect is reversed, and the external solution tends to 

 increase in volume and the gel to contract. Hence we find 

 that the saturated solutions of such substances as ammonium 

 sulphate and potassium carbonate will dehydrate a gel almost 

 completely, and will also, by a similar action on pelt, make a 

 kind of white leather. It is important to remember this 

 contractile effect of strong solutions of salts, because it is 

 very easy to confuse this effect with a similar result produced 

 in another manner, viz., by a reduction of the force tending 

 to swell. 



2. THE DISPERSE PHASE 



A very important feature of the colloid state is that the 

 particles of the disperse phase appear to possess an electric 

 charge, and if this charge be removed a colloid sol no longer 

 remains such, but precipitates, floculates, coagulates, etc. 

 As to the origin of this charge several theories have been 

 advanced, but the most generally accepted is that it is a 

 result of the adsorption of electrically charged ions by the 

 particles of the disperse phase. The enormous specific 

 surface possessed by this phase renders it particularly liable 

 to such adsorption. This view harmonizes well also with 

 the general behaviour, of colloid sols and gels, in endosmosis, 



