EXAMINATION OF WATER AND AIR. 249 
presence of the colon bacilli in water is, except possibly 
in rare cases, only of importance as an indication of fecal 
contamination, and, therefore, of the possibility of dan- 
gerous infection with other bacteria. Formerly it was 
considered that its presence indicated human fecal con- 
tamination, but we now know that many of the animals 
contain in their intestines colon bacilli so similar to those 
found in human beings that we cannot differentiate 
the one variety from the other; therefore, the finding of 
colon bacilli in water must always be judged according 
to the conditions surrounding the water-supply. Thus, 
it may indicate cattle contamination from the barn or 
surface water, human contamination, or, in certain 
conditions, simply the accidental contamination of the 
stream by wandering cattle or animals. Properly 
judged, however, the examination for the colon bacilli 
may yield results of considerable practical impor- 
tance. Whenever, in water examinations, any special 
variety of bacteria is found in unusual abundance, the 
fact should be noted, for sometimes it may be the cause 
of some prevailing infectious disease ; thus the bacillus 
pyocyaneus has been found in water producing diar- 
rheea with greenish discharges. 
The Obtaining of Water for Examination. Whenever 
possible the inoculation of the gelatin or agar tubes 
and the pouring of their contents into the Petri dishes 
should be done immediately after gathering the samples, 
otherwise the actual and relative numbers of the differ- 
ent organisms will change. Asa rule, the pathogenic 
bacteria will decrease and the harmless water species 
will increase. When the plates cannot be made imme- 
diately. the water held in the sterile vials should be 
kept nearly at the freezing-point. Even at very low 
