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water has been specially sterilized or distilled
and received and kept in sterile vessels, it always con-
tains some bacteria. The number will bear a very dis-
tinct relation to the amount of organic matter in the
water, though experiment has shown that certain patho-
genic and non-pathogenic bacteria can remain vital in
perfectly pure distilled water for a considerable length of
time. Ultimately, owing to the lack of nutriment, they
undergo a granular degeneration.
The majority of the water-bacteria are bacilli, and as a

FlG. 46.—Wolf hiigel's apparatus for counting colonies of bacteria upon plates.

rule they are non-pathogenic. Wright,1 in his examina-
tion of the bacteria of the water from the Schuylkill
River, found two species of micrococci, two species of
cladothrices, and forty-six species and two varieties of
bacilli. Of course, at times the most virulent forms of
pathogenic bacteria—those of cholera and typhoid fever
—occur in polluted water, but this is the exception, not
the rule.

The method of determining quantitatively the number

1 Memoirs of the National Academy of Sciences, Third Memoir.