n Ermengem's Method.~--This method, being merely a precipitation of
a silver salt on the micro-organisms and not a true stain, creates a
false impression as to the relative proportions of bacteria and
flagella.

(a) _Fixing Fluid._--

Osmic acid, 2 per cent. aqueous solution 10 c.c.
Tannic acid, 20 per cent. aqueous solution 20 c.c.
Acetic acid, glacial 1 c.c.

The fixing fluid should be prepared some days before use and
filtered as required. In colour it should be distinctly
violet.

(b) _Sensitising Solution._--

Silver nitrate, 0.5 per cent. aqueous solution.

This solution must be kept in a dark blue glass bottle or in
a dark cupboard.

Filter immediately before use.

(c) _Reducing Solution._--

Weigh out

Gallic acid 5 grammes
Tannic acid 3 grammes
Potassium acetate, fused 10 grammes

and dissolve in

Distilled water 350 c.c.

Filter.

This solution will keep active for several days, but fresh
solution must be used for each preparation.

METHOD.--

1. Prepare emulsion, make and fix films as above in the
preceding method, steps 1 to 4.

2. Pour on the film as much of the fixing solution as the
cover-glass will hold, heat carefully over the flame till
steam rises, and allow the steaming fixing fluid to act for
five minutes.

3. Wash well in water.

4. Wash in absolute alcohol.

5. Wash in distilled water.

6. Pour some of the sensitising solution on the film and
allow it to act for from thirty seconds to one minute; blot
off the excess of fluid with filter paper.

7. Without washing, transfer the film to a watch-glass
containing the reducing solution and allow it to remain
therein for from thirty seconds to one minute; blot off the
excess of fluid with filter paper.

8. Without washing, again treat the film with the
sensitising solution, this time until the film commences to
turn black.

9. Wash in distilled water.

10. Dry and mount.

~To Stain Nuclei of Yeast Cells.~

1. Prepare and fix film in the usual manner.

2. Soak in ferric ammonia sulphate 3 per cent. aqueous solution for two
hours.

3. Wash thoroughly in water.

4. Stain in haematoxylin solution (see page 95)