206 PRINCIPLES AND PRACTICE OF MILK HYGIENE 
more divergent. Woodhead reports that while in some 
experiments a temperature of 140° F. (60° C.) killed 
tubercle bacilli in 25 minutes, in others an exposure of 
8 hours was required. Yersin, Bitter, Bonhoff, and other 
investigators report that although an exposure to 140° 
F. (60° C.) for one hour was nearly always fatal to 
tubercle bacilli, the effect was uncertain when the time 
of exposure was much shorter. In Foster and Rull- 
mann’s experiments, the bacilli remained alive after 45 
minutes at 140° F. (60° C.). DeJong asserts that tu- 
bercle bacilli will survive heating at 159.8 to 161.6° F. 
(71 to 72° C.) for % hour, while Van der Sluis de- 
clares that a temperature of 176° F. (80° C.) for 1 hour 
is necessary to kill tubercle bacilli in naturally infected 
milk. Hittcher considers 1 hour at 140 to 145.4° F. (60 
to 68° C.), 45 minutes at 147.2 to 149° F. (64 to 65° C.), 
or 80 minutes at 150.8 to 158° F.. (66 to 70° C.) neces- 
sary to destroy tubercle bacilli. On the other hand, Hew- 
lett, in England, found that tubercle bacilli did not sur- 
vive 80 minutes exposure to 140° F, (60° C.) and, in 
this country, Rosenau found 20 minutes at 140° F., 
Theobald Smith 15 minutes at 140° F., and Russell and 
Hastings 10 minutes at that temperature sufficient to 
kill the organisms. Hewlett and the American investi- 
gators used in their experiments milk artificially in- 
fected with tubercle bacilli, while nearly all, if not all, 
of the other experiments mentioned were made with 
naturally infected milk. 
The conditions were therefore by no means the same. 
In naturally infected milk, the tubercle bacilli are em- 
bedded in masses of mucus, clots of fibrin, or shreds of 
tissue, and this albuminous covering protects them from 
the action of the heat. It is true that in those experi- 
