/o - 



To sterilized beef tea or bouillon tubes add 1 c. c., ^ c. 

 c., and one drop of the water by means of a sterilized 

 pipette. Set aside in the incubator at 37 to 39 C. for 24 

 hours. If no growth occurs at this temperature it is at 

 once sufficient evidence that the water is free from disease- 

 producing organisms. On the other hand, if growth 

 develops, injections of 1 c. c. of the culture are made intra- 

 peritoneally into white rats by- means of a sterilized Koch 

 syringe. The recovery of the animal indicates the absence 

 of pathogenic bacteria. If death occurs the toxic or 

 pathogenic form can be found arid isolated from the organs 

 and tissues of the animal (see anthrax). 



When it is desired to examine snow or ice this should 

 be melted in a sterilized flask and the water thus obtained 

 is examined as above. 



The number of bacteria present in water from various 

 sources is subject to the greatest variation. Thus spring 

 water may be sometimes wholly free of microorganisms, 

 but as a rule the number is less than 50 per c. c., and may, 

 in exceptional case, contain 3,000 per c. c. In well 

 waters considerable variation has been observed, but 

 usually the number is less than 500 per c. c. The water 

 of deep wells may be said to be free of microorganisms. 

 The same may be said to be true of the water of lakes. 

 The number of bacteria present in river water varies from 

 a few hundred to as many thousand, but in the neighbor- 

 hood of large cities it may reach hundreds of thousands 

 per c. c. In Paris the river water in the wool-washing 

 stations has been shown to contain from 12 to 40 millions 

 per c. c. 



LABORATORY WORK. Make plate cultures of two 

 samples of water tap-water and well-water. Also Petri 

 dishes of milk. The gelatin tubes are inoculated with 

 milk in the same manner as with water. 



