300 THE COLLOIDAL STATE 



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 lupulina, Ricinus, Datura, 

 Pisum, Lupinus, etc. 



(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- 



