IX EXPERIMENTS ON BIOGENESIS 101 



proved to be in the right, by the important discovery that 

 the spores of bacteria and monads are not killed by a tem- 

 perature many degrees higher than is sufficient to destroy the 

 adult forms : that in fact while the fully developed organisms 

 are killed by a few minutes' exposure to a temperature of 

 70 C. the spores are frequently able to survive several 

 hours' boiling, and must be heated to 130 150 C. in 

 order that their destruction may be assured. It was also 

 shown that the more thoroughly the spores are dried the 

 more difficult they are to kill, just as well-dried peas are 

 hardly affected by an amount of boiling sufficient to reduce 

 fresh ones to a pulp. 



This discovery of the high thermal death-point or ultra- 

 maximum temperature of the spores of these organisms has 

 necessitated certain additional precautions in experiments 

 with putrescible infusions. In the first place the flask and 

 the cotton-wool should both be heated in an oven to a 

 temperature of 150 C., and thus effectually sterilized. The 

 flask being filled and plugged with cotton-wool is well boiled 

 and then kept for some hours at a temperature of 32- ~38C., 

 the optimum temperature for bacteria. The object of this 

 is to allow any spores which have not been killed by boiling 

 to germinate, in other words to pass into the adult con- 

 dition in which the temperature of boiling water is fatal. 

 The infusion is then boiled again, so as to destroy any such 

 freshly germinated forms it may contain. The same process 

 is repeated once or twice, the final result being that the 

 very driest and most indurated spores are induced to ger- 

 minate, and are thereupon slain. It must not be forgotten 

 that repeated boiling does not render the fluid incapable of 

 supporting life, as may be seen by removing the cotton-wool 

 plug, when it will in a short time swarm with microbes. 



Experiments conducted with these precautions all tell the 



