EFFECT OF TEMPERATURE UPON BACTERIA. 149 
cidal power as hot, dry air at the same temperature. 
Esmarch found that anthrax spores were killed in 
streaming steam in four minutes, but were not killed 
in the same time by superheated steam at a temperature 
of 114° C. It should also be remembered that dry 
heat has but little penetrating power. Koch and 
Wolffhiigel found that registering thermometers placed 
in the interior of folded blankets and packages of vari- 
ous kinds did not show a temperature capable of kill- 
ing bacteria after three hours’ exposure in a hot-air 
oven at 133° C. and over. 
Fractional Sterilization (Tyndalization). Certain nutri- 
ent media, such as blood-serum and the transudates of 
the body cavities, as well as certain fluid food-stuffs, 
such as milk, need at times to be sterilized, and yet 
cannot be subjected to temperatures high enough to 
kill spores without suffering injury. The property of 
spores, when placed under suitable conditions, to germi- 
nate into the non-spore bearing form, is here taken 
advantage of by heating the fluids up to 55° to 70° C, 
for one hour on each of six consecutive days. By this 
means we kill, upon each exposure, all bacteria in the 
vegetative form, and allow, during the intervals, for 
the development of any still remaining in the spore 
stage, or which have reproduced spores, to change again 
into the vegetative form. Experience has shown 
that, with but few exceptions, an exposure for six con- 
secutive days will completely sterilize the fluids so 
exposed. 
Pasteurization. It is sometimes undesirable to expose 
food, such as milk, to such a temperature as will destroy 
spores, because of the deleterious effects of such high 
temperatures, and yet where a partial sterilization is 
