EXAMINATION OF WATER. 133 



Water collected for bacteriologic examination should be ex- 

 amined as soon as possible, as the contained bacteria multiply 

 very rapidly. An examination made twenty-four hours after 

 collection will not give the same results as an examination 

 made within a few hours. Warm water should be examined 

 immediately. The sample may be placed on ice, but it has 

 been found that extreme cold is fatal to some of the germs. 

 Ice taken from water which contains bacteria usually contains 

 the same germs as the water. The quality of the water is not 

 affected by the presence of large numbers of non-pathogenic 

 germs, but a few pathogenic organisms suffice to make the 

 water an .element of danger. Lake-water contains less bac- 

 teria than river-water. Wells having a very deep supply 

 contain very few bacteria, unless contaminated by the surface- 

 flow. Very deep wells and springs may contain no bacteria 

 at all. Sewer-water, of course, contains immense numbers 

 of bacteria. 



Bolton showed that two varieties of non-pathogenic bacteria 

 occur in water which has been sterilized as often as six times. 



FIG. 40. 



Glass bulb for collecting samples of water. 



The most important pathogenic bacteria found in water are 

 the typhoid and the cholera organisms. These germs find 

 their way into the water-supply from the ground-water. The 

 feces and other excreta containing these germs are not disin- 

 fected before they are disposed of, and when they are thrown 

 on the ground the germs make their way into the substrata 

 of the soil, whence they are carried by the ground-water to 

 the water-supply. These organisms retain their vitality in 

 water for from seven to thirty days, but do not multiply. 

 The non-pathogenic bacteria contained in water soon destroy 

 these pathogenic bacteria, The typhoid bacillus will retain 

 its vitality for thirty days in ice. 



Samples of water may be collected in sterilized Erlenmeyer 



