GEL FORMATION 277 



For a fuller account of these phenomena works on phys- 

 ical chemistry should be consulted. 



C. Change of State of Colloids or Gel Formation. 



It will be seen from what has gone before that, in addition 

 to the normal colloidal solutions produced by dissolving typi- 

 cal colloids such as glue, gelatine, or albumen in water, there 

 are a number of artificial colloidal solutions, some of which are 

 produced from normally insoluble substances, such as silicic 

 acid, gold, silver, and other inorganic substances, and some of 

 which are produced from crystalloids such as sodium chloride. 



With such widely different solutions it is not surprising to 

 find a very considerable difference in stability, and, as a matter 

 of fact, it is found that all colloidal solutions undergo with 

 greater or less readiness a change of state resulting in their 

 precipitation in the form of a gel. The resulting gel may be 

 one which cannot be got into solution again, in which case the 

 change is said to be irreversible ; if, on the other hand, the 

 change of state is only temporary, so that a slight change of 

 conditions causes the re-solution of the gel, the change is said 

 to be reversible. 



The various types of reversible or irreversible change of 

 state which colloidal solutions may undergo can be classified 

 as follows : — 



(a) Spontaneous Precipitation, (f)) Gelatinization, {c) Heat 

 Coagulation, {d) Coagulation by Enzymes, [e) Precipitation by 

 Electrolytes, (/) Precipitation by other Colloids. 



{a) Spontaneous Precipitation. — The decomposition of a col- 

 loidal solution of silicic acid after keeping for a few days may 

 be quoted as an illustration of spontaneous precipitation. 



(b) 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 even more readily. The change is, in both 

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

 more water, the gel goes into solution again. 



{c) Heat Coagulation. — This change, which may be illus- 

 trated by the coagulation of egg white in boiling water, is 

 irreversible, 



