8o METHODS USED IN THE STUDY OF BACTERIA 



incipient charring of cotton occurs at about this temperature; 

 and it is thus only necessary to run the sterilizer until cotton 

 in the flask or test tubes, or cotton placed in for this special 

 purpose, is slightly browned. 



This method of sterilization has a very limited use, since it 

 cannot, evidently, be used for sterilizing liquids ; and because 

 of the charring of vegetable and animal matter at the above 

 temperature, this method cannot be used to sterilize sub- 

 stances made of these and similar materials. 



Moist Heat. Steam for the purpose of sterilization has 

 been used since the time of Schultz and Schwann's experi- 

 ment, and is applied in the form of either streaming steam or 

 as steam under pressure. 



Streaming Steam. The temperature of streaming steam 

 depends, of course, upon the barometric pressure, but approxi- 

 mates 100 C. It is much more efficient than dry heat at the 

 same temperature. This is probably due to the fact that the 

 walls, or membranes, of the bacteria become moistened and 

 swelled by the steam and thus permit the more ready entrance 

 of the heat to the interior of the cell. By means of streaming 

 steam materials may be sterilized without injury which would 

 not stand dry heat. It is, however, practically impossible to 

 kill bacteria when they are in the spore condition by this 

 means, for it has been found that an exposure of sixteen hours 

 is necessary to kill certain spore bacteria found in the soil. A 

 method which overcomes the difficulty of long exposure was 

 suggested by Tyndall and is known as the intermittent method 



