PLAXTS WHIVn CAUSIf: DEVAY 



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hay infusions produce resistant bodies known as 

 spores (compare Fig. 205, g) which are not killed by 

 this temperature. On standing a few hours the spores 

 germinate and pass into the vegetative condition, when 

 they may be killed by steam heat. Hence the value 

 of sterilizing for three successive days. Ordinarily, 

 however, one sterilization of half an hour answers 

 every purpose. 



It was formerly supposed that the growth which 

 occurs in infusions, etc., was due to spontaneous 

 generation, i. e., to the 

 origin of living organisms 

 from lifeless matter. This 

 view I'eceived confirma- 

 tion from the fact that an 

 infusion may be boiled 

 and growth nevertheless 

 occur. With the discov- 

 ery of resistant spores 

 and of the fact that after 

 several sterilizations no 

 growth occurs, the doc- 

 trine of spontaneous 

 generation was over- 

 thrown. 



Let us now take three tumblers, place a slice of 

 boiled potato in each, cover with cotton held in place 

 by an elastic band as shown in Fig. 207 (or use 



207. Tumbler containing a slice of boiled po- 

 tato, closed by a cotton plug held in place 

 by an elastic band. 



