420 



tigated temperature region. For the 1 "/„ solution a more i-apid rise 

 sets in at + 30°, and the 5 "/o solution exhibits a similar gradual 



£^l 



\£|a 



iir 



3^' 



IfO' 



So' 60' 



\o' 



do' 



Fig. 1. Variation of the inleusily of the Tyndall- phenomenon 

 with the temperature in a Vi o/l> 1 '','0. and 5 7o gelatin 

 glycerin solution. 



change of direction already at + 55°. It is clear that the change of 

 intensity does not only depend on the temperature, but also on the 

 concentration ic) of the solution. And the number of large particles 

 beinf estimated from the intensity, the formula may be extended to: 



L 



It is also indicated in fig. 1 at what temperature the transition 

 from sol into gel will take place. When the decision whether the 

 solution is sol or gel is never given before the system is in equilibrium, 

 it can be accurately determined whether a liquid or a solid state 

 belongs to a temperature. The V4 "/o solution is always a sol in the 

 examined temperature region ; the J "/o solution is a gel below 19°, 

 the 5 7o solution below^ 40°. F'or every concentration therefore, a 

 limiting value of the number of large particles can be assumed; the 

 solution is a gel if this number is greater than that limiting 

 value, and a sol in the opposite case. This limiting vahie evidently 

 does not lie where the more pronounced curvature (in fig. 1) begins, 

 but only at a somewhat lower temperature. 



The variations of the number of large particles may be estimated, 

 besides by means of the TYNDAi.i.-iiitcusily, also by means of the 

 viscosity. A uiodiliration of the laitcr is namely always attended 



