THE VITAL PHENOMENA OF BACTERIA. 67 



If such a mixture is heated these other organ- 

 isms and the vegetative forms of the bacteria 

 are the first to be killed ; by this means such 

 spores as have retained their vitality can be sep- 

 arated from other germs. In this way Roberts ' 

 secured for the first time pure cultures of the 

 so-called hay bacillus by boiling hay infusion 

 for an hour. We do not, to be sure, always 

 obtain in this way a single species in " pure 

 culture," but simply a culture of all those 

 germs which happen to be present and possess 

 spores of approximately equal resistance. So 

 it happens that under the names " hay bacil- 

 lus" and " potato bacillus" are commonly in- 

 cluded not a single species but a mixture which, 

 by the aid of other methods, may be resolved 

 into several true species. 



Sterilisation. 



By selecting temperatures so low that germs, 

 cannot develop, putrescible substances can be 

 preserved from spoiling. This is what is accom- 

 plished by the use of ice, by refrigerating cham- 

 bers for the cold storage of meat, and by the 

 chilling of milk. For effectual conservation 

 it is better, however, to destroy the germs 

 utterly. This process of making a sub- 

 stance free from all germs is generally called 



1 Cf. Zopf. Die Spaltpilze, 1885, p. 74. 



