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2i6                PATHOGENIC BACTERIA.

A special arrangement of the microscope was devised
for the purpose, and the number of bacilli in each drop
estimated with extreme care. The number varied from
472 to 240,000. To estimate the number of bacilli in a
given quantity the number of drops to a cubic centimeter
is multiplied by the number of bacilli in the drop, and
then by the number of cubic centimeters to be estimated.

The method is an ingenious one, but a glance down
the columns of figures in the original article will be
sufficient to show that the counting of the bacilli is
devoid of any particular value.

This is only to be expected when one considers the
pathology of the disease and remembers that accidents,
such as unusually violent cough one day, modified by
the use of sedatives the next, may cause wide variations
in the quality if not in the quantity of the sputum.

When the tubercle bacilli are to be sought for in sections
of tissue, considerable difficulty is at once encountered,
partly because of the thickness of the section and partly
because of the presence of nuclei which color intensely.

Again, Ehrlich's method must be recommended as the
most certain and best method of staining a large number
of bacilli.

The sections of tissue, if imbedded in celloidin or par-
affin, should be freed from the foreign substances. Like
the cover-glasses, they are placed in the stain for twelve
to twenty-four hours at a temperature of 37 C. Upon
removal they are allowed to lie in water for about ten
minutes to wash away the excess of stain and to soften
the tissue, which often shrinks and becomes brittle. The
washing in nitric acid (20 per cent.) which follows may
have to be continued for as long as two minutes. Thor-
ough washing in 60 per cent, alcohol follows, after which
the sections can be counter-stained, washed, dehydrated
in 95 per cent, and absolute alcohol, cleared in xylol,
and mounted in Canada balsam.

A method which has attained great and deserved praise
is Unna's. It is as follows: The sections are placed in