Gelation in Beversihle .Colloidal Systems. 



103 



Effect of Temperature upon the Composition of the Phases. 



This was determined by running a large mass of the hydrosol into 

 a number of glass vessels of the same shape and size. Each vessel 

 held 600 c.c. of the hydrosol. They were close stoppered and allowed 

 to cool to the room temperature. After forty-eight hours they were 

 placed in chambers of known temperature, where they were kept for 

 five to seven days before the contents were subjected to pressure. In 

 these experiments, as is obvious, the internal changes are those which 

 follow on raising or lowering the temperature of the hydrogel from 

 the air temperature. In other experiments the hydrosol was cooled 

 down to, but not below, the temperature of observation. This dis- 

 tinction is important, because it was found that the composition of the 

 phases varied for a given temperature according to whether that tem- 

 perature was the lowest of a descending series or the highest of an 

 ascending series. This is shown clearly in the two curves AB AB', 

 fig. 1, The arrows indicate the direction, ascending or descending, of 

 the changes of temperature. 



No. I. — Agar content of Mixture 1*6 per cent. 





Temperature. 



Agar in 

 expressed fluid. 



Agar in 

 solid. 



33 

 50 

 14 

 33 



per cent. 

 0-14 

 0-29 

 0-80 



0- 14 



1- 10 



per cent. 



No. II. — Agar content of Mixture 2 '23 per cent. 





Temperature. 



Agar in 

 expressed fluid. 



Agar in 

 solid. 



13° 

 36 

 5 

 13 

 36 



per cent. 

 0-12 

 0-25 

 0-09 

 0-12 

 0-47 



per cent. 

 4-7 

 5 



3- 



4- 7 

 3-2 



Putting on one side for a moment the difi'erent effect of an ascending 

 or a descending temperature change, these experiments show that 

 (1) a hydrogel of agar is a structure form of a more solid part and a 

 fluid, and (2) each of these two phases is a mixture of agar and water, 



