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 arule, the parasitic species require a temperature of 35° to 40°; 
and that low temperatures do not kill bacteria. 
Frisch (1877) exposed various cultures toa temperature of —87° C., 
which he obtained by the evaporation of liquid CO,, 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 refrozen, after repeating the operation five times. 
Cadeéac 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. 
