METHODS OF OBSERVING BACTERIA. 97

Some bacteria, as the typhoid-fever bacillus, decolorize
so rapidly as to contraindicate the use of acid for the dif-
ferentiation, washing in water or alcohol being sufficient.

Gram's Method of Staining Bacteria in Tissue.—
Gram was the fortunate discoverer of a method of stain-
ing bacteria in such a manner as to saturate them with
an insoluble color. It will be seen at a glance what a
marked improvement this is on the method given above,
for now the stained tissue can be washed thoroughly in
either water or alcohol until its cells are colorless, with-
out fear that the bacteria will be decolorized. Its prose-
cution is as follows : The section is stained from five to
ten minutes in a solution of a basic aniliii dye—pure
anilin (anilin oil) and water. This solution, first devised
by Ehrlich, is known as Ehrlich's solution. The ordinary
method of preparing it is to mix the following:

Pure anilin, 4;

Saturated alcoholic solution of gentian violet, 11 ;
Water, 100.

Instead of gentian violet, methyl violet, fuchsin, or any
basic anilin color may be used. The mixture does not
keep well—in. fact, seldom longer than six to eight weeks,
sometimes not more than two or three ; therefore it is
best to prepare it in very small quantity by pouring
about i c.cm. of pure anilin into a test-tube, filling
the tube about one-half with distilled water, shaking
the mixture well, then filtering as much as is desired
into a small dish. To this the saturated alcoholic solu-
tion of the basic dye is added until the surface becomes
distinctly metallic in appearance.

Friedlander recommends that the section remain from
fifteen to thirty minutes in warm stain, and in many cases
the prolonged process gives better results.

From the stain the section is given a rather hasty wash-
ing in water, and then immersed from two to three min-
utes in Gram's solution (a dilute LugoPs solution):