" and is heated from below by a Bunsen
burner controlled by a thermo-regulator. The more elaborate forms
resemble the hot-air oven (Fig. 26) in shape and are provided with
adjustable shelves so that any desired obliquity of the serum slope can
be obtained.

8. Place the tubes in the incubator at 37 deg. C. for forty-eight hours
in order to eliminate those that have been contaminated. Store the
remainder in a cool place for future use.

_Alternative Method._

_Steps 1-5 as above._

6. Sterilise the serum by the fractional method--that is, by exposure in
a water-bath to a temperature of 56 deg. C. for half an hour on each of
six consecutive days; store in the fluid condition.

7. Coagulate in the inspissator when needed.

[Illustration: FIG. 110.--Serum inspissator.]

~Serum Water.~--

This forms the basis of many useful media, and is prepared
as follows:

1. Collect blood in the slaughterhouse (see page 168) and
when firmly clotted collect all the expressed serum and
measure in a graduated cylinder.

2. For every 100 c.c. of serum add 300 c.c. distilled water
and mix in a flask.

3. Heat the mixture in the steamer at 100 deg. C. for thirty
minutes. (This destroys any diastatic ferment present in the
serum and partially sterilises the fluid.)

4. Filter if turbid.

5. If not needed at once complete the sterilisation of the
serum water by two subsequent steamings at 100 deg. C. for
twenty minutes at twenty-four hour intervals.

~Citrated Blood Agar. Guy's.~--

1. Kill a small rabbit with chloroform vapour, and nail it out on a
board (as for a necropsy); moisten the hair thoroughly with 2 per cent.
solution of lysol.

2. Sterilise several pairs of forceps, scissors, etc. by boiling.

3. Reflect the skin over the thorax with sterile instruments.

4. Open the thoracic cavity by the aid of a fresh set of sterile
instruments.

5. Open the pericardium with another set of sterile instruments.

6. Sear the surface of the left ventricle with a red-hot iron.

7. Take a sterile capillary pipette (Fig. 13, c); break off the sealed
extremity with a pair of sterile forceps.

8. Steady the heart in a pair of forceps and thrust the point of the
pipette through the wall of the ventricle and through the seared area,
apply suction to the plugged end of the pipette and fill it with blood.

9. Transfer the entire quantity of blood collec