216 THE FLOATING-MATTEK OF THE AIR. 



By this method very instructive comparative experi- 

 ments might be made, and the resistant power of dif- 

 ferent germs might be expressed in terms of the heatings 

 necessary for their sterilization. I possess, for example, 

 two test-tubes, containing the same infusion and asso- 

 ciated with the same closed chamber, one of which has 

 been heated five times and the other six. The former 

 is quite turbid, while the latter is perfectly clear. In 

 this case five heatings had left some of the more resist- 

 ant germs still unkilled, which were destroyed by the 

 sixth heating. Of two other tubes charged with a 

 different infusion, one has been heated seven times and 

 is now full of life ; the other has been heated eight 

 times and is perfectly barren. 



With due care the method of sterilization here de- 

 scribed is infallible, however highly infective the sur- 

 rounding atmosphere may be. But here, as elsewhere in 

 these difficult inquiries, the sagacity which comes in 

 great part from nature, the skill which comes from 

 training, and the care which ought to root itself in his 

 moral constitution, are all necessary to save the experi- 

 menter from error and to lead him to the truth. 



§ 23. Mortality of Germs through defect of Oxy- 

 gen ^produced by Exhaustion with the Sprengel 

 Pump. 



An equally striking mode of sterilization is now to 

 be described. The crowding together of the organisms 

 so as to form in a multitude of cases a heavy, corrugated, 

 fatty scum upon the surface of the infusions obviously 

 indicated that air was a necessity of their life. In some 

 cases the oxygen dissolved in the infusions sufficed to 

 enable the Bacteria to cloud them from top to bottom ; 

 but in many cases they gathered at the top, and formed 



