SPONTANEOUS GENERATION 107 



bacillus were proven to be exceptionally resistant 

 to heat, especially in an alkaline medium, but all 

 of Tyndall's protected tubes "remained for more 

 than three months as clear and healthy as they 

 were on the day the infusion was poured into 

 them." 



That several kinds of micro-organisms remain 

 alive in boiling water is a fact that has proven a 

 fertile source of error in experiments on spon- 

 taneous generation. It was but natural that in the 

 earlier experiments on this subject it was generally 

 assumed that boiling must be fatal to all living 

 substance. Gradually it became manifest that dif- 

 ferent species vary greatly in their power to resist 

 destruction by heat. The bacteria causing the sour- 

 ing of milk, for instance, are easily killed, and in 

 Pasteurizing milk it is only necessary to heat it to 

 about 6o° C. Other organisms in milk are not 

 killed by this temperature, and some of these may 

 cause milk to putrefy without becoming sour. Only 

 a few forms can withstand boiling, and they do so 

 commonly in the form of spores. Their protection 

 by a resistant coating which prevents their proto- 

 plasm from being softened is probably one reason 

 for the high resistance of the spores of several 

 forms. For a similar reason a high resistance to 



