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98                  PATHOGENIC BACTERIA.

lodin crystals,                                   i ;

Potassium iodid,                               2 ;

Water,                                          3░░-

While the specimen is in the Gram's solution it
appears to turn a dark blackish-brown color. When
removed from the solution it is carefully washed in 95
per cent, alcohol until no more color is given off and
the tissue assumes a grayish color. If it is simply
desired to find the bacteria, the section is dehydrated
in absolute alcohol for a moment, cleared up in xylol,
and mounted in Canada balsam. If it is necessary to
study the relation between the bacteria and the tissue-
elements, a nuclear stain, such as alum carmin or Bis-
inarck brown, may be subsequently used. Should a
nuclear stain requiring acid for its differentiation be
desirable, the process of staining must precede the Gram
method altogether, so that the acid shall not act upon
the stained bacteria.

The success of Gram's method rests upon the fact that
the combination of mycoprotein, basic anilin, and the
iodids forms a compound insoluble in alcohoL

The process described may be summed up as follows :'

Stain in Ehrlich's anilin-water gentian violet five

to thirty minutes;
Wash momentarily in water;
Immerse two to three minutes in Gram's solution ;
Wash in 95 per cent, alcohol until no more color

comes out;

Dehydrate in absolute alcohol;
Clear up in xylol;
Mount in Canada balsam.

This method stains a large variety of bacteria very
beautifully, but, unfortunately, does not stain them all,
and as some of those which do not stain are important,
it seems well to mention theŚ