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fluenza, and sometimes of pneumonia, causes the specific
bacteria of tliese diseases to be far from uncommon in

Giinther points out that the majority of the bacteria
which occur in the air are cocci, sarcina being very
abundant. Most of them are chromogenic and do not
liquefy gelatin. It is unusual to find a considerable
variety of bacteria at a time ; generally not more than
two or three species are found.

It is an easy matter to determine whether bacteria are
present in the air or not, all that is necessary being to
expose sterile plates or Petri dishes of gelatin to the air
for a while, close them, and observe whether or not bac-
teria grow upon them.

To make a quantitative estimation is, however, much

FIG. 43.—Hesse's apparatus for collecting bacteria from the air.

more difficult.    Several methods have been suggested, of
which the most important may be considered.

The method suggested by Hesse is simple and good.
It consists  in making a measured quantity of the air to