METHODS OF CULTIVATION. 163 



serum with but little chance of contamination by 

 floating atmospheric germs. 



Blood-serum obtained without these special pre- 

 cautions may also be used by resorting to the 

 method of Koch for sterilizing it subsequently to 

 its separation from the clot. This is accomplished 

 by introducing it into test-tubes from which at- 

 mospheric germs are excluded by a plug of cotton, 

 or into hermetically sealed culture-flasks, like those 

 described on page 176, and exposing it for an hour 

 daily to a temperature of 58 C. (136.4 Fahr.) for 

 a period of six days. This method insures the de- 

 struction of living germs contained in the blood- 

 serum without coagulating the albumen, which 

 would destroy its value as a culture-fluid. If a 

 solid culture-medium is desired, the blood-serum 

 is subsequently subjected to a temperature of 

 65 C. (149 Fahr.) for several hours. A solid, 

 transparent, jelly is produced by this method, 

 which is the material upon which Koch cultivated 

 the tubercle bacillus in his experiments relating to 

 tuberculosis. 



Milk. The experiments of Lister, Roberts and 

 Cheyne have demonstrated that milk, as it exists 

 in the udder of the cow, is free from the germs 

 of fermentation or putrefaction, and may be pre- 

 served indefinitely without undergoing change, if 

 proper precautions are taken to introduce it into 

 sterilized flasks without contamination by organ- 

 isms detached from the external surface of the 



