424 C. M. HILLIAKD AND MILDRED A. DAVIS 



The first extensive investigations were made by Prudden in 

 1887. Pure cultures of B. prodigiosus and B. proteus were found 

 to be sterile after fifty-one days at temperatures ranging between 

 — 10° to 1°C. B. typhosus survived for at least one hundred and 

 three days at a temperature between 14° and 30°F. It will be 

 of interest in relation to the data presented later to reproduce 

 the results obtained by alternate freezing and thawing. 



By coating tubes with sweet oil to prevent the crystallization 

 of the water, and comparing the death rates in these tubes with 



Before freezing 40,896 



Frozen 24 hours 29,780 



Frozen 3 days 1,800 



Frozen 4 days 950 



Frozen 5 days 2,490 



Before freezing 40,896 



Frozen 3 times 90 



Frozen 5 times 



Frozen 6 times 



B. prodigiosus 



Before freezing 339,516 



Frozen 24 hours 36,410 



Frozen 30 hours 41,580 



Frozen 48 hours 14,440 



Frozen 96 hours 4,850 



Before freezing 339,516 



Refrozen 1 time 2,570 



Refrozen 2 times 275 



Refrozen 3 times 15 



Refrozen 4 times 



* Frozen at 2° to 5° F., then kept just below 32° F. 



those in which solid freezing had occurred, the former showed 

 greater fatality. Prudden concludes that "the greatest reduc- 

 tion occurs during, or shortly after the sudden reduction of 

 temperature to freezing, and, if after this the bacteria remain in 

 ice, a comparatively gradual destruction goes on; if bacteria are 

 thawed out and immediately refrozen another large increment 

 is destroyed." The killing action of cold as such is chiefly 

 emphasized. 



Ravenel, (1899) Macfayden, and others have obtained very 

 low temperatures with liquid air and liquid hydrogen (—252) 



