CHAPTER XVI. 



BACTERIOLOGICAL EXAMINATION OF WATER AND 

 AIR THE CONTAMINATION AND PURIFICATION 

 OF DRINKING WATERS. 



Bacteriological Examination of Water. The bacterio- 

 logical examination of water is undertaken with two 

 purposes : First, to discover the number of living bac- 

 teria present in the water, and, second, the varieties 

 that may be present. In order to roughly determine 

 the number of living bacteria in water, we thoroughly 

 mingle certain definitely measured quantities of water 

 with suitable quantities of melted but sufficiently cooled 

 nutrient agar or gelatin, the mixtures being immedi- 

 ately poured into Petri dishes, or retained in Esmarch 

 tubes and allowed to quickly harden. The bacteria in 

 the water are thus scattered throughout the solidified 

 media. If nutrient gelatin is employed care must be 

 taken to keep it cool during transportation. The agar 

 is usually allowed to remain at the body-temperature, 

 while the gelatin is necessarily kept at the usual room- 

 temperature (about 70 F.). If nutrient agar alone 

 is used two sets of plates are made, one being kept at 

 body-heat the other at the usual room-temperature. 

 After a suitable length of time has been allowed for 

 the development of the colonies we examine the plate 

 cultures, and by counting the number of colonies de- 

 veloped by means of a low power lens and Wolffhiigel's 

 apparatus (Fig. 29) or some equivalent, we are enabled 

 to find approximately the number of living bacteria 



