172 TECHNOLOGY OF BACTERIA. 



before the fluid is introduced, to destroy all germs 

 adhering to their inner surface. The culture-fluid 

 may be sterilized before or after its introduction 

 into these tubes. In the former case, the opera- 

 tion must be performed expeditiously, in as pure 

 an atmosphere as possible ; and the mouth of the 

 tube is to be closed at once with a plug of cotton- 

 wool. It is evident that this method admits of the 

 entrance of floating atmospheric germs while the 

 tubes are being filled, and, therefore, that a certain 

 proportion are likely to break down. The per- 

 centage of failures will depend upon the skill of 

 the operator and upon the purity of the atmos- 

 phere in wfyich the operation is performed. The 

 liability to failure from contamination by floating 

 germs is not, however, as great as is commonly 

 imagined; and experience proves that contact with 

 instruments or surfaces e. g. the lip of the vessel 

 from which the culture-fluid is poured which are 

 not perfectly pure, is a more frequent cause of the 

 breaking-down of the culture-fluid. 



Sterilization of the culture-fluid after its intro- 

 duction into the tubes, offers greater security, and 

 the following method of manipulation is recom- 

 mended : Test-tubes, or wide-mouthed bottles 

 having a capacity of half an ounce or more, are 

 washed clean, and the mouth of each is covered 

 with several layers of cotton-wadding. This is 

 secured in position by means of a strong linen 

 thread, or a piece of copper wire, tied about the 

 neck. The wide-mouthed bottles have the advan- 



