MEANS FOR KILLING BACTERIA. 663 



The other conditions of life of the bacteria do not 

 produce any such marked effect. Pressure and elec- 

 tricity only appear to destroy bacteria when carried to 

 extremes (p. 535) ; intense sunlight is said to have a 

 comparatively energetic action, but these experiments 

 must be repeated with more careful attention to the 

 possibility of other hurtful factors coming into play. 



A very active agent for the destruction of the lower High tempera- 

 fungi is high temperature, while, on the other hand, low 

 temperatures, even carried to an extreme degree, only 

 prevent development, but never cause the death of 

 the organism. The effect of heat depends on the 

 degree of temperature, and on its duration ; con- 

 tinued action of relatively low temperatures produces 

 the same result as a short action of high temperatures. 

 The temperature necessary to cause the death of the Varying 

 organisms varies also very greatly according to the other 

 couditions of life, and more especially according to the 

 resisting power of the species in question. The chief of spores, 

 difference is found between bacteria which do not form 

 spores and those which are spore-bearing. The former 

 are, as a rule, killed when in a moist state or in fluids by 

 exposure for one to two hours to a temperature of 48 to 

 60 C. ; where they are dry the temperature must as a 

 rule be continued for a longer time. Even spore-form- 

 ing bacteria may be killed by these relatively low 

 degrees of temperature if the heat is applied on repeated 

 occasions, and if in the intervals between its applica- 

 tion the organisms are placed under the most favour- 

 able conditions of existence, so that any spores that are 

 present may sprout and form bacilli. If the latter are 

 killed by the subsequent application of heat before new 

 spores are formed, we may be certain that after the 

 heat has been applied five or six times no living spores 

 exist, and all the adult organisms are destroyed. For 

 example, blood serum may be freed from all organisms, 

 although the heat employed has not been sufficient to 

 cause coagulation of the albumen ; the material is placed 

 for an hour daily for five or six days in succession at a 

 temperature of about 56 C., and is in this way com- 



