168 PA THOGENIC BA CTERIA.

of bacteria is considerably increased: if the person be a
woman with skirts, more bacteria will probably be raised
from the floor than would be disturbed by a man; if the
room be swept, the increase is enormous. From these
and similar contingencies it becomes very difficult to
know just when and how the air is to be examined,
and the value of the results is correspondingly lessened.
The most valuable examinations are those which aim
at the discovery of some definite organism or organisms
regardless of the number per cubic meter.