288 COLLOIDS 



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. 



(<f) 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 gela- 

 tinous 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, 

 Pt'sum, Lupinus, etc. 



(e) Precipitation by Electrolytes. The effect of electrolytes 

 on colloidal solutions differs very markedly according to the 

 nature of the solution, since the inorganic colloidal solutions, 

 or suspensoids, are extremely sensitive to the addition of only 

 a small amount of an electrolyte, whilst, on the other hand, 

 the so-called emulsoids, or organic colloidal solutions, are only 

 affected by considerable quantities. 



This is due to the fact that emulsoids have less well- 

 defined electrical characteristics as compared with suspensoids 

 (see next section on Electrical Properties). 



The electrolytes which precipitate colloids from solution 

 may be classified under two heads : 



(i) Sodium, potassium, lithium, ammonium, and magne- 

 sium salts. 



(ii) Calcium, barium, strontium salts, and salts of the 

 heavy metals, such as mercury, copper, lead, zinc, etc. 



The former class produce reversible precipitation, while 

 the precipitation produced by the second class is, as a rule, 

 irreversible. Thus, for example, on saturating a solution of 

 soap or gelatine, with ordinary salt, the colloid is precipitated 

 in a form which can be redissolved in water at will. 



