2io Appendix. 



If the medium is to be used in tubes the sterilized 

 azolitmin solution shall not be added until just before the 

 final sterilization. 



If the medium is to be used in Petri dishes the steril- 

 ized azolitmin solution shall not be added to the medium 

 until it is ready to be poured into the dishes. 



MILK. 



The milk to be used as a culture medium shall be as 

 fresh as possible, "Certified Milk" being ordinarily the 

 best obtainable in city laboratories. It shall be placed in 

 a refrigerator over night to allow the cream to rise and 

 the suspended matter to settle. The skimmed milk shall 

 be siphoned off into a flask for use. It will be found 

 more convenient, however, to allow the milk to stand in 

 a separating funnel. Should the milk be too acid the 

 reaction shall be corrected to + i per cent by the addition 

 of normal sodium hydrate. It is then ready to be tubed 

 and sterilized. Litmus milk shall be prepared as above, 

 with the addition of sterile i per cent azolitmin. As it 

 is impossible to make each lot of litmus milk with the 

 same shade of color, it is recommended that a control 

 tube be always exposed with the inoculated tubes for 

 purposes of comparison. 



NITRATE BROTH. 



Dissolve one gram pepton in one liter of tap water, 

 and add 0.2 grams of nitrite-free potassium nitrate. 

 It is convenient to prepare a stock solution of potas- 



