268 SACTEBIOLOGY. 



scopic methods are deceptive ; cloudiness of the media 

 or the presence of bacteria microscopically does not 

 always signify that the organisms possess the property 

 of life. 



Inoculate in the same way a third flask of bouillon 

 with a very small drop from one of the old cultures upon 

 which the pellicle has formed ; mix it well and subject 

 it to the action of steam for two minutes ; then place it 

 to one side for from twenty to twenty-four hours, and 

 again heat for two minutes ; allow it to stand for another 

 twenty-four hours, and repeat the process on the third 

 day. No pellicle will be formed, and yet spores were 

 present in the original mixture, and, as we have seen, 

 the spores of this organism are not killed by an exposure 

 of five minutes to steam. How can this result be ac- 

 counted for ? 



Saturate several pieces of cotton thread, each about 2 

 cm. long, in the original decomposed bouillon, and dry 

 them carefully at the ordinary temperature of the room ; 

 then at a little higher temperature — about 40° C. — to 

 complete the process. Regulate the temperature of the 

 hot-air sterilizer for about 100° C, and subject several 

 pieces of this infected and dried thread to this temper- 

 ature for the same lengths of time that we exposed the 

 same organisms in bouillon to the steam, viz., five, ten, 

 thirty, and sixty minutes. At the end of each of these 

 periods remove a bit of thread, and prepare a set of 

 plates or Esmarch tubes from it. Are the results anal- 

 ogous to those obtained when steam was employed? 



Increase the temperature of the dry sterilizer and 

 repeat the process. Determine the temperature and 

 time necessary for the destruction of these organisms 

 by dry heat. These threads should not be simply 



