METHODS FOR THE MICROSCOPIC STUDY OF BACTERIA 193 



The Distribution of Media. The clarified media are either stored in 

 flasks or transferred to smaller containers for immediate use. Then 

 they are sterilized. Most media are used in test-tubes. Test-tubes 

 are filled from a reservoir, usually a large funnel, the smaller end of 

 which is provided with a short length of rubber tubing, into which a 

 short glass tube, constricted somewhat at the outer end, is introduced. 

 The flow is controlled by a pinch cock, which constricts the rubber 

 tubing midway between the funnel and the delivery tube. The cotton 

 plug is removed from a test-tube and the delivery tube is introduced 

 into the open end of it to a depth of about two inches. The pinch 



STERILIZING CHAMBER 



^ * M 





FIG. 15. Arnold steam sterilizer. 

 (Abbott.) 



FIG. 16. Arnold steam sterilizer. 

 Ordinary type. (Park.) 



cock is opened somewhat and the desired volume is allowed to flow in. 

 The pinch cock is then released to stop the flow, the delivery tube 

 removed, care being taken that no media touches that part of the 

 test-tube where the cotton fits, so that it will not adhere to the sides 

 of the tube, and the cotton plug is replaced. Usually about 8 to 10 

 c.c. of media are added to a tube. 



Sterilization of Media. Media which do not contain coagulable 

 proteins, gelatin or carbohydrates are sterilized for fifteen minutes in 

 an autoclave at a live steam pressure of fifteen pounds (121.3 C.). 

 Media containing gelatin or carbohydrates are sterilized at a lower 

 temperature by discontinuous sterilization half an hour on three 



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