EXAMINATION OF AIR, SOIL, AND WATER 239 



tion must not be delayed longer than possible after the 

 water has been collected. Every precaution must be taken 

 in the way of cleanliness to prevent contamination; sterilized 

 flasks with glass stoppers, pipets, and plugs must be at hand, 

 glassware sterilized in autoclave at 120 C. for fifteen minutes, 

 or dry heat at 160 C. for one hour, and the gelatin tubes or 

 agar dishes be inoculated on the spot. If this cannot be done, 

 the sample should be packed in ice until it arrives at the 

 laboratory. If it is necessary to send the sample by rail, 

 the bottle containing the sample should be wrapped in steril- 

 ized cloth, or the neck covered with tinfoil and the bottles 

 placed in tin boxes (about 4 ounces 100 c.c. is sufficient 

 for bacterial analysis), and then packed in cotton or paper 

 to prevent breakage and surrounded by plenty of ice until 

 it reaches its destination. As soon as it arrives at the lab- 

 oratory the sample is placed in a sterilized glass flask, and 

 the flask then closed with a sterile cotton plug. A sterilized 

 pipet is then dipped into the flask, and i c.c. of the water 

 withdrawn and added to a Petri dish. To a second dish, a 

 dilution of i c.c. of the sample with sterile distilled water is 

 added, and other dilutions made if desired. To each plate 10 

 c.c. of standard agar at a temperature of 40 C. is added. 

 Mix the water and media thoroughly by tipping the dish 

 back and forth, and place in incubator at 37 C. for twenty- 

 four hours. The incubator should be in a dark, well- venti- 

 lated, and moist place. Then count all the colonies present 

 on each plate, which will give the number per cubic centi- 

 meter. 



Water that is very rich in germs requires dilution with 

 sterilized water fifty to one hundred times. Fewer colonies 

 will be found on agar than on gelatin, even at the same tem- 

 perature. 



Special Media and Preparation. In the preparation of 

 media for water analysis, sodium chlorid must not be used. 

 The reaction of most culture-media should be -f-i per cent, 

 to phenolphthalein. 



Sugar broths should be neutral, and must be sterilized care- 



