208 ANIMAL PROTEINS 



Also the osmotic pressure of gelatine sols is markedly lowered 

 by neutral electrolytes in lyotrope sequence : 



Cl>SO 4 >NO 3 >Br>I>CNS 



Similarly lyotrope influences are shown in the modulus 

 of elasticity : substances which favour gelation increase 

 elasticity, whilst substances which favour solation decrease 

 elasticity. The order is again lyotrope. 



The permeability of the gel is affected by lyotrope in- 

 fluences ; alcohol and glycerol reduce diffusion through 

 gelatine (or agar) ; and urea, chloride and iodide increase it. 

 (Similarly the diffusion of sols through " semipermeable " 

 membranes is affected by lyotrope influence.) The lyotrope 

 series also influence the optical activity of gelatine sols and 

 the double refraction of strained gels. 



The swelling of gelatine (and other gels) is very strongly 

 influenced by the lyotrope substances and merits more 

 attention than it has received. Hence this lyotrope influence 

 exerts a profound effect in the manufacture of gelatin, and 

 perhaps even greater in the manufacture of leather. This 

 is only to be expected. If a gel comprise a continuous 

 network of compressed water, as suggested above, the 

 presence of other substances in the gel which cause increases 

 or decreases in the compression must modify accordingly 

 the properties which depend upon this state of compression, 

 such as the viscosity of the melted gel, the rate of gelation, 

 the elasticity of the gel, and the rate and extent of its 

 imbibition. This indeed we find to be the case. Now 

 the substances which affect the compressibility, surface 

 tension, etc., of water least, i.e. the substances producing 

 little or no compression of water, are just those which 

 reduce the compression of water in a gelatine jelly, and cause 

 a decreased viscosity, elasticity, surface tension, etc., and 

 which therefore naturally allow the gel to swell more than 

 in pure water. Conversely, the substances which cause the 

 greatest compression of water, the greatest increase in its 

 surface tension and viscosity, are also the substances which 

 increase the compression, viscosity, elasticity, and surface 



