CULTURE-MEDIA. 7 1 



During the filling, the neck of the test-tube where it comes in 

 contact with the cotton must not be wet with the medium. 

 Ordinarily about 7 to lo c.c. are placed in a test-tube. For 

 Esmarch's roll-tubes a somewhat smaller quantity is desirable. 

 The sterilization of tubes containing culture-media is 

 always done by steam, and has been sufficiently described. 

 It is to be remembered that the soUdifying power of gelatin 

 is impaired by too prolonged heating, while heating is less 

 likely to damage other culture-media. The media which 

 are sterihzed at a low temperature (70° C.) should be tested 

 for two days in the incubator to determine whether steriUzation 

 has been effective. It is the universal experience in bacterio- 

 logical laboratories that occasionally culture-media will become 

 contaminated with extremely resistant spores which fail to be 

 steriUzed by the ordinary processes, an occurrence which 

 causes great annoyance and calk for the ■ exercise of much 

 patience. Sometimes, also, moulds attach themselves to the 

 plugs, especially if they are moist, and send their filaments 

 down through the cotton; finally, having reached the lower edge 

 of the cotton, their spores may fall upon the medium, grow 

 there and ruin it. 



