filtered toxin from living
bacteria.

4. In the event of a fatal termination make complete and careful
post-mortem examinations.

5. Repeat the experiments and, if the results are positive, ascertain
the minimal lethal dose of toxin for each of the susceptible animals.

The estimation of the _m. l. d._ of a toxin is carried out on lines
similar to those laid down for living bacteria (_vide_ page 316) merely
substituting 1 c.c. of toxin as the unit in place of the unit "loopful"
of living culture.

It frequently happens, during the course of casual investigations that a
bouillon-tube culture is available for a toxin test whilst a flask
cultivation is not. In such cases, Martin's small filter candle and tube
(Fig. 161) specially designed for the filtration of small quantities of
fluid, is invaluable. This consists of a narrow filter flask just large
enough to accommodate an ordinary 18 x 2 cm. test-tube. The mouth of the
tubular Chamberland candle 15 x 1.5 cm. is closed by a perforated rubber
cork into which fits the end of the stem of a thistle headed funnel,
whilst immediately below the butt of the funnel is situated a rubber
cork to close the mouth of the filter flask. When the apparatus is fixed
in position and connected to an exhaust pump, the cultivation is poured
into the head of the funnel and owing to the relatively large filtering
surface the germ free filtrate is rapidly drawn through into the
test-tube receiver.

~Raising the Virulence of an Organism.~--If it is desired to raise or
"exalt" the virulence of a feebly pathogenic organism, special methods
of inoculation are necessary, carefully adjusted to the exigencies of
each individual case. Among the most important are the following:

1. _Passage of Virus._--The inoculation of pure cultivations of the
organism into highly susceptible animals, and passing it as rapidly as
possible from animal to animal, always selecting that method of
inoculation-e. g., intraperitoneal--which places the organism under
the most favorable conditions for its growth and multiplication.

[Illustration: FIG. 161--Martin's filtering apparatus for small
quantities of fluid.]

2. _Virus Plus Virulent Organisms._--The inoculation of pure
cultivations of the organism together with pure cultivations of some
other microbe which in itself is sufficiently virulent to ensure the
death of the experimental animal, either into the same situation or into
some other part of the body. By