WATER ANALYSIS. II5 
purposes. Water containing more than 500 per 
cubic centimeter should be looked upon with sus- 
picion. 
2. Qualitative Analysis of Water.—The quali- 
tative examination of water requires not only the iso- 
lation of the several species present, but also their 
cultivation and the determination of their patho- 
genic or non-pathogenic properties. Such an 
examination takes a long time, and under most 
favorable circumstances it is very difficult to 
recognize the presence of pathogenic forms. 
The principal value of the qualitative analysis of 
water is in the detection of contamination by sew- 
age. Sewage is always liable to contain the evacu- 
ations of patients sick with typhoid fever or other 
transmissible diseases. The germs of typhoid fever 
are not easily identified, but there are certain bacteria 
common in human and other animal evacuations 
and in sewage (B. coli, B. vulgaris, B. cloace, B. 
sporogenes, Bact. aérogenes) whose presence is 
easily detected. Consequently the presence of such 
forms, though harmless in themselves, always indi- 
cates contamination. 
3. Laboratory Work in Water Analysis.—All 
samples should be collected in sterile flasks, and 
cultures should be made immediately to secure ac- 
curate results. If transportation is necessary, the 
samples should be packed in ice. Tap water should 
be allowed to run a few minutes before the sample 
is taken ; if spring or well water is to be examined, 
the sample should be collected from about a foot 
below the surface. 
