44 HERBERT W. HAMILTON 



its color. The residue is dissolved in water. This is evaporated 

 to dryness, first over a free flame and finally on the water bath. , 

 The yield is about one twenty-fifth of the original weight of the 

 cubes. (Azolitmin may be purchased from certain dealers.) 



2. A satisfactory powder may be prepared by extracting the 

 cubes with hot water, filtering and evaporating the filtrate to 

 dryness. The residue is treated with 95 per cent alcohol over- 

 night. In the mornmg the alcohol is decanted off and the 

 residue dried on the water bath. 



THE MILK 



Skimmed milk powder is purchased from a bakers' supply 

 company or grocer. Care should be taken to see that it is 

 clean. Quantities in small containers are more satisfactory 

 than bulk purchases. 



The litmus powder so prepared is mixed with the milk powder. 

 In the case of the litmus powder prepared by the first method 

 (or purchased azolitmin powder) one part of the dye is added to 

 52.7 parts of the skimmed milk powder. When the dye is 

 prepared by the second method one part of the dye powder is 

 added to 49.6 parts of the milk powder. 



The resulting mixture is dissolved in distilled water in the 

 proportion of one part of the htmus milk powder to 9.5 parts of 

 water. 



Care should be taken in the mixing that the Utmus powder is 

 weU ground and well dispersed in the milk powder. 



All media made in this way have been sterilized perfectly in 

 the Arnold Sterilizer. When autoclaved at 15 pounds for fi^-e 

 minutes the medium resumes its true color on cooling. The 

 mixed powder has kept perfectly in a glass stoppered bottle for 

 two years. 



Cultures of Bad. coli, B. Welchii, Bad. acidi-ladici, Bad. 

 alcaligenes, Pseudomonas fluorescens, B. mesentericus-vulgatiis, 

 Staph, pyogenes-albus, and Proteus vulgaris all grew well and gave 

 typical reactions in the normal time. 



