162 



COLLOIDS IN BIOLOGY AND MEDICINE 



to 100 C., the relative internal friction (according to E. VON SCHROE- 

 DER) falls from 1.75 (at the end of one-half hour) to 1.22 (at the end 

 of 16 hours). Possibly this is due to the increasing hydrolytic cleav- 

 age. The following figures give some idea of the relations: 



These solidification temperatures are markedly shifted by elec- 

 trolytes and, in fact, the anions have the greatest influence, whereas 

 the cations are of less moment. 



The solidification temperature is raised by ) 864 > CH 3 CO 2 > 

 The solidification time is shortened by ) tartrates. 



benzoates and salicy- 



The solidification temperature is lowered by 

 The solidification time is lengthened by 



lates>SCN>I>Br 

 > NO 3 > Cl. 



The following data (from H. BECHHOLD and J. ZiEGLER* 2 ) serve 

 as an example: 



Melting point. 



10 per cent gelatin 31.6 



10 per cent gelatin + 1 mol. NaCl 28 . 5 



10 per cent gelatin + 2 mol. Na 2 SO 4 34. 2 



10 per cent gelatin + 1 mol. Nal 10.0 



Nonelectrolytes also influence the melting point of gelatin. 

 Glycerin and sugar (mannit, cane sugar, etc.), in contradistinction 

 to agar, raise the temperature and increase the rate of gelatinization, 

 while furfurol, urea, alcohols, resorcin, hydrochinon and pyrogallol 

 lower them. Nongelatinizing colloids have no influence on gelatini- 

 zation. 



The following figures from H. BECHHOLD and J. ZiEGLER* 2 serve 

 to make this clear: 



Melting point. 



10 per cent gelatin 31.66 



10 per cent gelatin + 1 mol. grape sugar 32 . 25 



10 per cent gelatin + 2 mol. glycerin 32. 17 



10 per cent gelatin + 2 mol. alcohol 30. 



10 per cent gelatin + 1 mol. urea 26. 3 



Precipitation of the gelatin sol must be sharply differentiated from 

 gelatinization. Precipitation is induced by electrolytes, whereas 

 nonelectrolytes usually interfere with it. Precipitation corresponds 



