EMULSOIDS 



potassium thiocyanate, which have a strong influence on the 

 swelling of colloids, also accelerate the rate of cell division of 

 Paramcedum, while calcium chloride and sulphates, which 

 reduce swelling, retard cell division. 



Many of the epidermal tissues in plants and animals 

 are cuticularized or ptherwise hardened ; this prevents their 

 swelling when brought in contact with water, thus enabling 

 them to maintain their shape, and it is a commonplace in 

 histological technique to harden tissues by immersion in 

 formaldehyde or other solutions so as to counteract and pre- 

 vent this same tendency. The hardening of gelatine by 

 means of bichromate is another example of the same principle. 



Syneresis. This is the name given by Graham to a 

 phenomenon which may be regarded as the reverse of swelling. 

 Most gels, on keeping, squeeze out a small quantity of liquid 

 which is not pure water but a dilute solution of the colloid in 

 question. The amount of liquid thus extended varies with 

 the concentration of the gel, and is greater for some colloids 

 than for others. The phenomenon can be noticed on agar 

 culture tubes, etc., and is familiar 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 



