nutrient agar (+10 reaction),
liquefy the medium by heat and cool in a water bath to 50 deg. C.

5. Add 200 c.c. ascitic or hydrocele fluid (horse or sheep serum may be
employed, but is not so good) to the liquid agar and mix carefully to
avoid formation of air bubbles.

6. Fill about 20 c.c. of the ascitic agar into each of the sterilised
tubes which already contains a piece of sterile rabbit's tissue, stand
all the tubes upright in racks or a jar, and allow agar to set.

7. After solidification pour sterile paraffin oil on the surface of the
medium in each tube to the depth of 3 centimetres.

8. Incubate tubes at 37 deg. C. for several days and discard any which
prove to be contaminated.

9. Store such tubes as are sterile for future use.

XIII. INCUBATORS.

[Illustration: FIG. 113.--Incubator.]

An incubator (Fig. 113) consists essentially of a chamber for the
reception of cultivations, etc., surrounded by a water jacket, the walls
of which are of metal, usually copper, and outside all an asbestos or
felt jacket, or wooden casing. The water in the jacket is heated by gas
or electricity and maintained at some constant temperature by a
thermo-regulator. The cellular incubator (Fig. 114) which was made for
me[7] some years ago is of the greatest practical utility. Here the
central cavity is subdivided by five double-walled partitions (in which
water circulates in connection with the water tanks at the top and base
of the incubator) and again by iron shelves to form twenty-four pigeon
holes. Into each of these slides an iron drawer 35 cm. long x 12 cm.
wide x 22 cm. high forming a self-contained incubator. The drawer is
fitted with a wooden form to which is fixed a handle and a numbered
label. The thermo-regulating apparatus is the well-known Hearson
capsule.

[Illustration: FIG. 114.--Cellular incubator.]

Two incubators at least are required in the laboratory, for the
cultivation of bacteria the one regulated to maintain a temperature of
37 deg. C., and known as the "hot" incubator; the other, 20 deg. C. to
22 deg. C., and known as the "cool" or "cold" incubator.

Two other incubators, regulated to 42 deg. C. and 60 deg. C. respectively,
whilst not absolutely, necessary very soon justify their purchase.

~Thermo-regulators.~--The thermo-regulator is the most essential portion
of the incubator, as upon its efficient working depends the maintenance
of a constant temperature in the cultivation chamb