196 Life and Matter [chap. x. 



are rigorously excluded, the attempt 

 hitherto has been a failure : so far, no life 

 has made its appearance under observation, 

 except from antecedent life. 



But, to exclude all trace of antecedent life, 

 it is necessary not only to shut out floating 

 germs, but to kill all germs previously 

 existing in the material we are dealing with. 

 This killing of previous life is usually 

 accomplished by heat ; but it has been argued 

 that strong heat will destroy not only the 

 life but the potentiality for life, will break 

 up the complex aggregate on which life 

 depends, will deprive the incubating solution 

 not only of life but of livelihood. There is 

 some force in the objection, and it is an 

 illustration of the difficulty surrounding 

 the subject. But Tyndall showed that ante- 

 cedent life could be destroyed, without any 

 very high temperature, by gentle heat 

 periodically applied : heat insufficient to 

 kill the germs, but sufficient to kill the 

 hatched or developed organisms. Periodic 

 heating enables the germs of successive 



