VI. 



INFLUENCE OF PHYSICAL AGENTS. 



Heat. We have already seen (Section II. , Part Second) that the 

 temperature favorable for the growth of most bacteria is between 20 

 and 40 C. ; that some species are able to multiply at the freezing tem- 

 perature, and others at as high a temperature as 60 to 70 C. ; that, 

 as a rule, the parasitic species require a temperature of 35 to 40; 

 and that low temperatures do not kill bacteria. 



Frisch (1877) exposed various cultures to a temperature of 87 C., 

 which he obtained by the evaporation of liquid CO 2 , and found that 

 micrococci and bacilli, after exposure to such a temperature, multi- 

 plied abundantly when again placed in favorable conditions. Prud- 

 den has also made extended experiments upon the influence of 

 freezing. He found that while certain species resisted the freezing 

 temperature for a long time, others failed to grow. Thus Bacillus 

 prodigiosus did not grow after being frozen for fifty-one days ; Pro- 

 teus vulgaris was killed in the same time, and a slender, liquefying 

 bacillus obtained from Croton aqueduct water was killed in seven 

 days. Staphylococcus pyogenes aureus withstood freezing for sixty- 

 six days, a fluorescent bacillus from Hudson River ice for seventy- 

 seven days, and the bacillus of typhoid fever for one hundred and 

 three days. Cultures made at intervals showed, however, a dimi- 

 nution in the number of bacteria. A similar diminution would per- 

 haps have occurred in old cultures in which the pabulum for growth 

 was exhausted, independently of freezing ; for bacteria, like higher 

 plants, die in time which varies for different species as a result of 

 degenerative changes in the living protoplasm of the cells, and con- 

 tinued vitality in a culture depends upon continued reproduction. 



Repeated freezing and thawing was found by Prudden to be 

 more fatal to the typhoid bacillus than continuous freezing. Cul- 

 tures were sterilized by being thawed out at intervals of three days 

 and again ref rozen, after repeating the operation five times. 



Cadeac and Malet kept portions of a tuberculous lung in a frozen 

 condition for four months, and found that at the end of this time 

 tuberculosis was still produced in guinea-pigs by injecting a small 

 quantity of this material. 



