192 THE AMERICAN MONTHLY [ October, 
is this :—because in preparations which have not been heated the above-de- 
scribed precipitation occurs, while in preparations which have been passed 
through the flame only once or twice, the fixation of the elements, especially in 
those containing much albumen, is not sufficient for all cases. In those forms 
passed through the -flame three times, while the forms themselves are not 
materially altered, the capacity for staining is retained, and the albuminoid 
material has become so insoluble that precipitation no longer takes place. 
Passing the preparations through the flame a yet greater number of times 
destroys the susceptibility of the bacteria to the staining fluid. 
The want of success in making preparations, which many beginners ex- 
perience, seems to be generally due to the fact that the preparations are 
generally heated before they have completely dried in the air. If the prepa- 
ration still contains any water, coagulation of the albuminoid material occurs 
when heated, while in those completely free from water this does not happen, 
and the albumen is rendered homogeneous by heating. 
The preparations dried in air, and then drawn three times through the 
flame, are now stained. The cover-glasses, with the prepared side upward, 
are laid on a piece of filter-paper, and, by means of a glass rod, a cap pipette, ° 
or the glass-stopper, with the capillary tube, a few drops of a staining solu- 
tion are placed upon the preparation. The staining fluid should remain 
about twenty minutes, or until it is seen by an inclination of the cover-glass 
that the preparation has already taken up the color. If the action of the stain- 
ing solution ought to be prolonged, then it should not be placed, drop by 
drop, on the cover-glass, because, in drying, the staining solution forms a 
ring of color at the edge which it is difficult to remove. In this case a suff- 
cient quantity of the staining solution is placed in a watch-glass, or crystali- 
zation-glass, and the cover-glass is then taken, with the prepared side down- 
ward, between the thumb and index, or middle, finger, and allowed to fall 
flat upon the surface of the staining solution, so that it swims with the pre- 
pared side upon the surface of the fluid. To prevent evaporation, the disk is 
covered with a glass plate. 
For the removal of the excess of coloring matter, a stream from a wash- 
bottle is thrown obliquely from above upon the cover-glass, taking care not 
to strike the surface of the preparation directly ; or the cover-glass, held in 
pincettes, is moved to and fro in a beaker filled with distilled water; or 
the excess of the fluid may be soaked up with filter-paper, a few drops of 
water added to be soaked up anew, and so on until none of the coloring mat- 
ter is given up to the filter-paper. Then the cover-glass preparation is ex- 
amined in a drop of distilled water. 
The upper side of the cover-glass is freed from every particle of water by 
soaking it up with filter-paper, because on it must be placed a drop of oil for 
the homogeneous immersion lens. 
If the cover-glass preparations are to be preserved, the oil is removed with 
filter-paper, and chloroform, the water by careful warming or evaporation 
(protected from dust), and the dried preparation is directly imbedded in 
Canada balsam. ; 
Different dyes are used for each variety of bacteria, since some stain only 
the bacteria; others, at the same time, the fine gelatinous sheath ; others, the 
capsule. On this account the corresponding pictures in all the methods of 
staining are not absolutely similar; so that it ought to be self-evident that 
always, and in comparison, only preparations should be used which have been 
treated in exactly the same manner. These considerations must guide one in 
the choice of a staining solution. We must therefore distinguish between 
staining for a special purpose, 2. e., for the establishment or employment of 
coloring methods which have been described, or proved to be best, in partic- 
