GELS 403 



[d) Gelatinization by Altering the Concentration. — If a dilute 

 solution of gelatine in water be concentrated until it is about 

 5 per cent strength it will set to a jelly on cooling to the 

 atmospheric temperature. Solutions of agar will gelatinize 

 at much greater dilution. The change is, in both cases, 

 reversible, for, by raising the temperature, or by adding more 

 water, the gel goes into solution again. 



A gel, however, once set will require a higher temperature 

 to liquefy than its original setting temperature. Thus a 5 per 

 cent gelatine gel setting at about 18° C. melts at about 26° C, 

 while an agar solution which sets at about 35-40° C. will re- 

 quire to be heated to over 90° C. before it melts. 



GENERAL PROPERTIES OF GELS. 



Gels partake of some of the properties of both solids and 

 liquids. With solids they share the property of maintaining 

 their shape and of being more or less elastic, on the other 

 hand their compressibility is very low like that of water of 

 which they are very largely composed. 



Owing to their rigid nature they lend themselves well for 

 experiments on diffusion, and many interesting results have 

 been obtained. One experiment, originally due to Liesegang, 

 consists in placing a drop of silver nitrate solution on a 

 gelatine gel containing a dilute solution of potassium bichro- 

 mate ; after a short time concentric rings of silver chromate 

 are deposited around the original drop ; this experiment has 

 given rise to much experimental work with other reagents 

 under varying conditions and there is much speculation re- 

 garding the true explanation of the phenomenon. There is, 

 however, no doubt that the experiment illustrates the possi- 

 bility of diffusion of a crystalloid such as silver nitrate in a gel. 



The bringing about of such periodicity, as is exhibited by 

 the alternating layers of deposit and clear solution in an 

 inanimate system without variations in external conditions 

 such as temperature changes, has an important bearing on 

 biological and other natural problems ; it would appear to 

 offer a possible explanation of the stratification observable in 

 agate and its possible significance in connection with the 



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