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212                 PATHOGENIC BACTERIA.

The physician will secure a better result if the exam-
ination be made on the same day than if he wait a num-
ber of days, because if the bacilli are few they occur
most plentifully in the small caseous flakes to be de-
scribed farther on, which are easily found at first, but
which break up and become part of a granular sediment
that always forms in decomposed sputum.

The  fresh sputum when  held over a black surface '
generally shows a number of grayish-yellow, irregular,
translucent granules somewhat smaller than the head of
a pin.    These consist principally of the caseous material
from tuberculous tissue, and are the most valuable part
of the sputum for examination.    One of the granules is.
picked up with a pointed match-stick and spread over
the surface of a perfectly clean cover-glass.    If no such
fragment can be found, the purulent part is next best for
examination.     The mucus itself rarely contains bacilli
when free from scraps of tissue and pus.

In cases in which this ordinary procedure fails to reveal
bacilli whose presence is strongly indicated by the clin-
ical signs, the exact method of searching for them is to
partially digest the sputum with caustic potash, and then
collect the solid matter with a centrifugal apparatus. If
a very few bacilli are present in the sputum, this method
will often secure them.

The material spread upon the cover-glasses should not
be too small in amount. Of course a massive, thick
layer will become opaque in staining, but should the
layer spread be, as is often advised, u as thin as possible,'*
there may be too few bacilli upon the glass to enable one
to make a satisfactory diagnosis.

As usual, the material is allowed to dry thoroughly,
and is then passed three times through the flame for
purposes of fixation,

Ehrlictts Method, or the Koch-Ehrlich Method.—The
cover-glasses thus prepared are floated, smeared side-
down, upon, or immersed, smeared side up, in, a small
dish of Ehrlich's anilin-water gentian-violet solution :