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92                  PATHOGENIC BACTERIA.


glasses can never be too clean. It is best to immerse
them first in a strong mineral acid, then to wash them in
water, then in alcohol, then in ether, and keep them in
ether until they are to be used. Except that it some-
times cracks, bends, or fuses the edges of the glasses, a
better and more convenient method of cleaning them is to
wipe them as clean as possible, seize them in fine-pointed
forceps, pass them repeatedly through a small Bunsen
flame until it becomes greenish yellow, then slowly ele-
vate the glasses above the flame, so as to allow them to
anneal. This maneuvre removes the organic matter by
combustion. It is not expedient to use covers twice for
bacteriological work, though if well cleaned they may
subsequently be employed for ordinary microscopic ob-
jects. **~S

To return: After the material spread upon the cover
has dried, it must be fixed to the glass by immersion for
twenty-four hours in equal parts of absolute alcohol and
ether, or, as is much easier and more rapid, be passed
three times through a flame. Three is not a magic
number, but experience has shown that when drawn
through the flame three times the desired effect seems
best accomplished. The Germans recommend that a
Bunsen burner or a large alcohol lamp be used, that the
arm holding the forceps containing the cover-glass in-
scribe a circle a foot in diameter, and that, as each revo-
lution occupies a second of time, the glass be made to pass
through the flame from apex to base three times. This
is supposed to be exactly the requisite amount of heating.
The rule is a good one for the inexperienced.

After fixing, the material is ready for the stain. Every
laboratory should be provided with several stock-solutions
of the more ordinary dyes. These stock-solutions are
saturated alcoholic solutions made by adding- 25 grams
of the dye to 100 of alcohol. Of these it is well to
have fuchsin, gentian violet, and methylene blue always
made up. The stock-solutions will not stain, but are the
standards for the manufacture of the working stains.