400 THE COLLOIDAL STATE 



SWELLING OF COLLOIDS OR IMBIBITION. 



Whereas a water-soluble crystalloid commences to dissolve 

 as soon as it is brought in contact with water, the same is not 

 true for most emulsoid or lyophilic colloids. Before going into 

 solution, these substances undergo a preliminary swelling, 

 sometimes known as imbibition ; this is accompanied by the 

 disappearance of a certain volume of water. According to an 

 experiment described by Hatschek, i gram of gum-tragacanth 

 covered with water in a specific gravity bottle kept under 

 water for a week had increased in weight by O-p gram at the 

 end of this period ; this means that in the process of imbibi- 

 tion the gum had succeeded in drawing in to the flask 0-9 c.c. 

 of water. In view of the resistance which water is known to 

 offer to compression, it is clear that enormous force must have 

 been exerted during the process. 



Direct measurement of the pressures produced during 

 swelling were made by Reinke,* on Laminaria contained in 

 an apparatus known as the Oedometer. 



Only by the application of an opposing pressure of 41 

 atmospheres was he able to reduce the amount of water im- 

 bibed to one-twentieth of the amount it would normally have 

 taken up. 



Conditions Affecting Imbibition. — [a) Temperature. — Heat 

 is evolved during swelling, as may be seen from the following 

 table taken from Taylor's " Chemistry of Colloids " : — 



This being so, heat hinders imbibition, while cold and 

 pressure favour it. For this reason it is best in making a 

 solution of a colloid such as agar or gelatine to allow it to 

 swell for some time in cold water without applying any 

 heat. 



[b) Presence of Impurities. — The swelling of colloids is very 

 considerably increased by the presence of small quantities of 



* Reinke : " Hanstein's bot. Abhandl.," 1879, 4, i. 



