TUBERCULOSIS. 217

a dish of twenty-four-hours-old, newly-filtered Ehrlich's
solution, and allowed to remain twelve to twenty-four
hours at the room-temperature or one to two hours in
the incubator. From the stain they are placed in water,
where they remain for about ten minutes to wash. They
are next immersed in acid (20 per cent nitric acid) for
about two minutes, and become greenish-black. From
the acid they are placed in absolute alcohol, and are
gently moved to and fro until the pale-blue color returns.
They are then washed in three or four changes of clean
water until they become almost colorless, and are then
removed to the slide by means of a section-lifter. The
water is absorbed with filter-paper, and then the slide is
heated over a Bunsen burner until the section becomes
shining, when it receives a drop of xylol balsam and a
cover-glass.

It is said that sections stained in this manner do not
fade as quickly as those stained by Ehrlich's method.

The tubercle bacillus also stains well by Gram's method,
but as this is a general method by which many different
bacteria are colored, it is ill adapted for purposes of differ-
entiation, especially when the prosecution of the charac-
teristic methods is not more difficult.

So far as is known, the tubercle bacillus is a purely
parasitic organism. It has never been found except in
the bodies and excretions of animals affected with tuber-
culosis, and in dusts of which these are component parts.
This purely parasitic nature greatly interferes with the
isolation of the organism, which cannot be grown upon
the ordinary culture-media. Koch first achieved its arti-
ficial cultivation by the use of blood-serum. When
planted upon this medium the bacilli are first apparent
to the naked eye in about two weeks, and occur in the
form of small dry, whitish flakes, not unlike fragments
of chalk. These slowly increase at the edges, and grad-
ually form scale-like masses of small size, which under
the microscope are seen to consist of tangled masses of
bacilli, many of which are in a condition of involution.