684 
As a rule, bacteria are attenuated and lose their power to infect 
before they lose their ability to vegetate upon artificial culture media. 
It is therefore safe to assume that a micro-organism that will not grow 
in artificial media under favorable conditions is “ dead.” The tuber- 
cle bacillus is an exception to this rule, for reasons given further on. 
The methods used in the tests recorded below were planned to imi- 
tate the actual conditions of pasteurization, so far as practicable, in 
laboratory experiments. 
The test tubes in which the infected milk was heated were open to 
the air, and scum formation was disregarded in all instances, my ob- 
ject being to determine the thermal death point against natural diffi- 
culties, so that the results might be applied with confidence to prac- 
tical pasteurization. 
BACILLUS TUBERCULOSIS. 
Certain special difficulties are met with in determining the thermal 
death point of the tubercle bacillus. This organism does not grow 
readily upon artificial media. The few experiments made to deter- 
mine its thermal death point by cultural methods have no significance, 
because its vegetability upon artificial media does not correspond to 
its power of growing in the animal organism. It is therefore neces- 
sary to inoculate animals in order to determine whether or not the 
tubercle bacillus is alive and virulent. Here again we meet with 
complications. Dead tubercle bacilli have a certain amount of patho- 
genic power and produce lesions, including tubercle formation, ab- 
scesses, and coagulation necrosis. However, while we lack a criterion 
to determine with precision the exact point when the tubercle bacillus 
dies, we are able by means of animal inoculations to determine just 
when the tubercle bacillus is so enfeebled that it is no longer able to 
infect. This, after all, is the important practical point. 
In my own experiments, in order to avoid the confusion resulting 
from the effects produced by dead tubercle bacilli, doubtful lesions 
were carried over into another animal. 
From these experiments it is evident that the tubercle bacillus in 
milk loses its infective properties for guinea pigs when heated to 
60° C. and maintained at that temperature for twenty minutes or 
to 65° C. for a much shorter time. 
It should be remembered that the milk in these tests was very 
heavily infected with virulent cultures, indicated by the prompt 
deaths of the control animals. Milk would practically never contain 
such an enormous amount of infection under natural conditions. It 
is justifiable to assume that if 60° C. for twenty minutes is sufficient 
to destroy the infectiveness of such milk - when injected into the 
peritoneal cavity of a guinea pig, any ordinary market milk after 
