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 



