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METHODS OF OBSERVING BACTERIA. 99
Spirillum of cholera and of chicken-cholera;
Bacillus mallei (of glanders);
Bacillus of malignant edema ;
Bacillus pneumonise of Friedlander;
Micrococcus gonorrhoeae of Neisser;
Spirochaete Obermeieri of relapsing fever;
Bacillus of typhoid fever;
Bacillus of rabbit-septicemia.
. Gram's method is a method of staining bacteria in
tissues, but the fact that the method colors some but not
all bacteria is one of considerable importance from a dif-
ferential point of view ; and as the difficulty of separating
the species of bacteria is so great that every such point
must be eagerly seized for assistance, this method be-
comes one much employed for cover-glass preparation
where it is more easily performed than for sections.
Gram's Method for Cover-glass Preparations.—A
thin layer of the bacteria to be examined is spread upon
the cover-glass, dried, and fixed. The cover, held in the
grip of a cover-glass forceps, is flooded with Ehrlich's
solution. By holding the cover flooded with stain over
a small flame for a moment or two the solution is kept
warm, and the process of staining is continued from two
to five minutes. If the heating causes the stain to
evaporate, more of it must be dropped upon the glass,
so that it does not dry up and incrust.
The stain is poured off, and the cover placed in a small
dish of Gram's solution and allowed to remain one-half
to two minutes, the solution being agitated. It is pos-
sible to apply the Gram solution in the same manner
in which the stain is used, but as a relatively larger
quantity should be employed, the dish seems preferable.
The cover is next washed in 95 per cent, alcohol until
the blue color is wholly or almost lost, after which it can
be counter-stained with eosin, Bismarck brown, vesuviu,
etc., washed, dried, and mounted in Canada balsam.
Given briefly, the method is: