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 
