66 CULTURES IN LIQUID MEDIA. 



mal death-point, consists in the application of heat and thus destroy- 

 ing all except the most resistant species. This method io especially 

 applicable when one of the species, only, forms spores. By subject- 

 ing the mixed culture to a temperature which is sufficient to destroy 

 all the vegetative cells in it, the more resistant spores are left and, 

 under favorable conditions, may subsequently vegetate and give us 

 a pure culture of the species to which they belong. 



