SYSTEMATIC STUDY OF SPECIES OF BACTERIA. I05 



plugs removed and burned and the tubes filed with 5 per cent, 

 carbolic acid between the second and third sterilizations. In 

 taking these measures, the same kind of reasoning applies as 

 that which induces engineers to give bridges from four to six 

 times the strength they need to bear the greatest strain Ukely to 

 be put upon them, or to make the boiler of a steam engine strong 

 enough to bear six times the greatest pressure which it is expected 

 that the steam contained in it will exert. 



