175 



times determined by the age and previous history of the 

 material. 



Bottazi and Bergami (1924) studied coagulation by 

 freezing in ox and dog sera and in Octopus blood. The 

 material, in glass containers with parallel faces 15 mm. 

 apart, was left for a given time in a freezing mixture, then 

 allowed to thaw and its turbidity was judged by the degree 

 to which it obscured a series of dark lines drawn against 

 the back wall of the container. They found that a con- 

 gelation of ox serum at -20° gave, after thawing, a liquid 

 as clear as before, except sometimes for a trace of opales- 

 cence. So, temperatures wiiich solidify all the water of 

 the proteins do not denaturate and coagulate them irre- 

 versibly. At higher, non-freezing temperatures, for ex- 

 ample, at -0.9°, the sera would coagulate partly if they 

 were dialyzed for 10 days and so deprived of the major 

 part of their electrolytes, but the coagulation process was 

 entirely reversible on rewarming. This partial coagula- 

 tion was easier to obtain if the sera were diluted with 

 water. The addition of NaCl to coagulable sera pre- 

 vented coagulation. The addition of hydrochloric acid 

 caused the sera to become turbid when the isoelectric point 

 was approached ; coagulation was, however, always rever- 

 sible. The serum, it is concluded, is a rather stable col- 

 loidal system, which becomes labile when it loses its electro- 

 lytes and when it is acidified near the isoelectric point. 

 The hemocyanin of the blood of Octopus macropus was 

 found still more stable. It did not coagulate at -3° or -4° 

 even after a month of dialysis. 



According to Callow (1925), repeated freezing irrever- 

 sibly destroys the structure of gelatin gels. 



Moran (1925) observed that frozen egg yolk loses its 

 fluidity on thawing and takes on a stiff pasty consistency. 

 This change, however, occurs only if the following con- 

 ditions are fulfilled: 1. The yolk must have been cooled 

 below -6° (called by Moran the ''critical temperature of 

 freezing"); a stay of several months above -6° did not 

 change the volk consistencv ; 2. Congelation must actually 



