ide of
the hanging-drop slide.

3. Place a drop of water on the centre of the cover-slip by means of the
platinum loop.

4. Obtain a small quantity of the material it is desired to examine, in
the manner detailed above (pages 74-76, steps 2 to 11 must be followed
in their entirety and with the strictest exactitude whenever tube
contents are being handled), and mix it with the drop of water on the
cover-slip.

5. Raise the cover-slip in the points of the forceps and rapidly invert
it on to the ring cell of the hanging-drop slide, so that the drop of
fluid occupies the centre of the ring. (Carefully avoid contact between
the drop of fluid and either the ring cell or the layer of vaseline.
Should this happen, the now _infected_ hanging-drop slide and its
cover-slip must be dropped into the pot of lysol and a new preparation
made.)

6. Press the cover-slip firmly down into the vaseline on to the top of
the ring cell. (This spreads out the vaseline into a thin layer, and
besides ensuring the adhesion of the cover-slip, seals the cells and so
retards evaporation.)

7. Examine microscopically.

The examination of a "fresh" specimen or a "hanging-drop" preparation is
directed to the determination of the following data:

1. The nature of the bacteria present--e. g., cocci, bacilli, etc.

2. The purity of the cultivation; this can only be determined when gross
morphological differences exist between the organisms present.

3. The presence or absence of spores; when present, spores show their
typical refrangibility exceedingly well by this method.

4. The presence or absence of mobility. In a hanging-drop specimen some
form of movement can practically always be observed, and its character
must be carefully determined by noting the relative positions of
adjacent micro-organisms.

(a) Brownian or molecular movement. Minute particles of solid matter
(including bacteria), when suspended in a fluid, will always show a
vibratory movement affecting the entire field, but never altering the
relative positions of the bacteria. (Cocci exhibit this movement, but
with the exception of the Micrococcus agilis, the cocci are non-motile.)

(b) Streaming movement. This is due to currents set up in the hanging
drop as a result of jarring of the specimen or of evaporation, or to the
fact that the cover-slip is not perfectly level, and although the
relative positions of the bacteria may vary, still the flowing movement
of large numbers