402 THE COLLOIDAL STATE 



than for others. The phenomenon can be noticed on agar 

 culture tubes, etc., and is famihar to bacteriologists. 



GEL FORMATION. 



Many colloidal solutions are able, under certain conditions, 

 to undergo a change of state known as gel formation, in which 

 the sol loses its liquid properties and becomes more or less 

 rigid. 



In some cases the change is reversible, meaning that by 

 suitably altering the conditions the gel will return to a solu- 

 tion, and in other cases the change is irreversible. 



Examples of such changes are given below : — 



[a) Spontaneous Precipitation. — A silicic acid sol prepared 

 by the addition of acid to a solution of sodium silicate will, on 

 keeping, set spontaneously to a bluish almost transparent gel. 

 This change is irreversible. 



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

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

 irreversible. 



An instructive experiment, due to Hardy, consists of 

 boiling side by side in separate beakers a fairly strong and a 

 very dilute solution of egg white in water. The strong one 

 coagulates while the dilute one becomes turbid only ; on the 

 addition of a small quantity of barium chloride, however, a 

 precipitate is produced. The explanation of this phenomenon 

 is that, owing to the dilution of the solution, the particles of 

 coagulated protein are too small to unite together, and there- 

 fore remain apart, forming a suspensoid which is, however, pre- 

 cipitated by the electrolyte. 



[c] Coagulation by Enzymes. — The curdling of milk by 

 rennet is a familiar example of this type of irreversible gel 

 formation ; so also is the coagulation of pectic bodies occurring 

 in fruit juices by the enzyme pectase with the formation of 

 gelatinous calcium pectate. 



Enzymes capable of coagulating milk also occur in many 

 plants, such as Lolium perenne, Anthriscus vulgaris, Geranium 

 molle, Ranunculus bulbosus, Medicago lupiilina, Ricinus, Datura, 

 Pisum, Lupinus, etc. 



