26 AMERICAN MINERAL WATERS. 



Milk. The best available milk is obtained for this medium. Cer- 

 tified milk is the most desirable, but it is not always obtainable. The 

 milk is heated in the Arnold sterilizer for about 30 minutes, and then 

 placed in the refrigerator overnight, the skim milk being removed on 

 the following day by siphoning. Litmus milk is prepared by using a 

 1 per cent c. p. litmus solution. Generally about 5 cc of this 1 per 

 cent solution to 100 cc of milk is required to give the desired shade 

 of color. 



Nitrate 'broth. Dissolve 1 gram of peptone in 1 liter of water, to 

 which add 0.2 gram of nitrate-free potassium nitrate. (The first 

 committee report calls for 2 grams, which was evidently a mistake, 

 and was corrected in their later report of progress.) 1 Tube and 

 sterilize. 



Indol solution is made according to the formula of Dunham, as 

 follows: 10 grams of Witte's peptone; 5 grams of sodium chlorid; 

 1,000 cc of distilled water; boil, filter, tube, and sterilize. 



APPARATUS. 



Test tubes 150 mm in length and from 12 to 16 mm in diameter 

 are used for ordinary work. Culture tubes of larger diameter are 

 used for potato medium. 



The fermentation tube used is the one recommended by the Com- 

 mittee. For presumptive fermentation tests, the small inverted, 

 straight tube, within a larger test tube, is used instead of the regular 

 fermentation tube. This inverted tube requires less space, is not so 

 easily broken, and is more desirable in many respects. The petri 

 plates used are 100 mm in diameter; the tops and the bottoms should 

 be well matched. Pipettes of various capacities may be employed, 

 varying from 1 to 10 cc, having an outflow not to exceed 5 seconds. 

 The small-caliber pipettes used by chemists do not permit a flow of 

 sufficient rapidity for bacteriological work. 



All glassware should be well cleansed with suitable soap mixtures, 

 rinsed with clean water, dried, and sterilized. Sterilization may be 

 considered complete when glassware is exposed for one hour at 160 

 C., or when the cotton plugs are slightly browned. 



TECHNIQUE OF PLATING. 



Shake well, at least twenty-five times, the bottle containing the 

 sample, withdraw 5 cc of the sample with a sterile pipette, and 

 deliver 1 cc into each of two sterile petri dishes, 1 cc into a fermenta- 

 tion tube, and 1 cc into a small sterilized Erlenmeyer stoppered flask 

 containing 9 cc of sterile water for a dilution of 1 to 10. Kepeat this 



i Progress report of the Committee on Standard Methods for the Bacteyal Examination of Water and 

 Sewage. Reprinted from the American Journal of Public Hygiene, v. 18, No. 4, November, 1908. 



