in a water-bath to
58 deg. C. and keeping it at that point for ten minutes. The mixture is
then planted on to fresh media and incubated, when the resulting growth
will be found to consist entirely of the B. mesentericus.

(b) _Sporing Bacteria._--This method finds its chief practical
application in the differentiation of a spore-bearing organism from one
which does not form spores. In this case the mixture is heated in a
water-bath at 80 deg. C. for fifteen to twenty minutes. At the end of this
time the non-sporing bacteria are dead, and cultivations made from the
mixture will yield a growth resulting from the germination of the spores
only.

Differential sterilisation at 80 deg. C. is most conveniently carried out
in a water-bath of special construction, designed by Balfour Stewart (Fig.
140). It consists of a double-walled copper vessel mounted on legs, and
provided with a tubulure communicating with the space between the walls.
This space is nearly filled with benzole (boiling-point 80 deg. C.; pure
benzole, free from thiophene must be employed for the purpose, otherwise
the boiling-point gradually and perceptibly rises in the course of
time), and to the tubulure is fitted a long glass tube, some 2 metres
long and about 0.75 cm. diameter, serving as a condensing tube (a tube
half this length if provided with a condensing bulb at the centre will
be equally efficient). The interior of the vessel is partly filled with
water and covered with a lid which is perforated for a thermometer. This
latter dips into the water and records its temperature. A very small
Bunsen flame under the apparatus suffices to keep the benzole boiling
and the water within at a constant temperature of 80 deg. C. The bath is
thus always ready for use.

METHOD.--To use the apparatus.

1. Place some of the mixture itself, if fluid, containing the spores, or
an emulsion of the same if derived from solid material, in a test-tube.

2. Immerse the test-tube in the water contained in the benzole bath,
taking care that the upper level of the liquid in the tube is at least 2
cm. beneath the surface of the water in the copper vessel.

3. The temperature of the water, of course, falls a few degrees after
opening the bath and introducing a tube of colder liquid, but after a
few minutes the temperature will have again reached 80 deg. C.

4. When the thermometer again records 80 deg. C., note the time, and
fifteen minutes later remove the tube containin