9 
bacteria which may have escaped the heat, or the fluid upon the side 
of the test tube may afterwards flow back. 
As a rule, bacteria are attenuated and lose their power to infect 
before they lose their abilit}" to vegetate upon artifical culture media. 
It is, therefore, safe to assume that a micro-organism that vdll not 
grow in artificial media under favorable conditions is ‘‘dead.” The 
tubercle bacillus is an exception to this rule, for reasons given fur- 
ther on. 
METHODS. 
The methods used in the tests recorded below *were planned to 
imitate the actual conditions of pasteurization, so far as practi- 
cable, in laboratory experiments. 
Except in the case of the tubercle bacillus, all the organisms were 
first planted in sterilized milk and not subjected to the tests until 
an abundant growth was obtained. In order to obtain pure cul- 
tures it was first necessary to sterilize the milk, which was done by 
the fractional method. It is believed that this does not appreci- 
ably influence the results obtained. By growing the micro-organ- 
isms in milk we perhaps imitate the natural conditions more closely 
than by using suspensions prepared for the purpose. 
♦All the tests were made by placing the test tube containing the cool 
infected milk into the water bath at the desired temperature, usually 
60° C. The time required for the milk to reach the temperature 
stated is always given. 
The illustration shows how the apparatus was arranged for all the 
tests. The temperature was controlled within a fraction of a degree; 
it never varied more than half a degree, and, as a rule, was kept 
within one or two tenths of a degree. The test tubes containing 
about 10 to 15 c. c. of milk were immersed so that the level of the 
milk in the tube was well below the level of the water. 
. . . N 
The reaction of the milk was determined by titration with NaOH 
solution, using phenolphthalein as indicator, and the results are 
expressed in terms of lactic acid. The reaction of the milk was 
always taken *at the time of testing the thermal death points.® 
Enrichment methods were used only in the case of cholera, but 
did not materially alter the results of the simpler methods. 
The tests were made with both whole and skimmed milk. As a 
rule, the milk was first partly skimmed in accordance with the usual 
I am indebted to Dr. Joseph H. Kastle, Chief of the Division of Chemistry, for 
making these tests, as well as for determining the chemical composition of the milks 
used in the work on B. tuberculosis. I am also indebted to my colleagues in the 
Division of Pathology and Bacteriology, especially Doctors McCoy, Stimson,. Miller, 
King and Anderson, for assistance with the tests thorughout this work. Further I 
deshe to express my special obligations to Doctor Stimson for assistance in collecting 
the literature upon the thermal death point of the turbercle bacillus. 
