132! Proceedings of the Royal Physical Society. 
follows that it cannot be accepted that blood-serum has any 
general or universal attenuating property any more than a 
bactericide power. 
When it was ascertained that a microbe, inoculated into a 
refractory animal, might survive for some time at the point 
of inoculation, the question. came to be asked if the blood- 
serum did not rather exercise its action by neutralising or 
destroying the substances produced by the microbe than by 
acting directly upon the microbe itself. This idea led to the 
discovery of what is now generally known as the antitoxic 
action of the blood-serum and other fluids in the blood. 
In this relation it is necessary to state that microbes act 
in two different ways when introduced into the body. In 
one case the microbe multiplies and invades the body 
generally, this being what is known as infection by the 
microbe. In the other case, instead of the microbe multiply- 
ing and becoming generalised, it remains localised, and 
produces its injurious effects by the poisons, or toxines, 
which it produces, and which, by their absorption, produce 
what is technically known as an intoxication. 
Two examples may be taken, in which the micro-organism 
plays a minor part to its toxines, viz., tetanus and diphtheria. 
To understand aright the bearing of this on our general 
subject, it is necessary to inform you that animals can be 
vaccinated against these two affections, and that a further 
interesting and important fact has been demonstrated, 
namely, that the blood-serum of animals so vaccinated can 
be used as a vaccine material for the prevention of these 
diseases in other animals. It is also necessary that I should 
remind you of the fact that microbes can be filtered out of 
the fluid in which they have been cultivated, the filtered 
fluid containing the toxines produced by the microbe. This 
toxic fluid may be investigated, and its powers and reactions 
tested. 
The influence of blood-serum, when brought into contact 
with these toxines, was one of the first points to be investi- 
gated. It was found that toxin mixed with a small quantity 
of serum from an immunised animal could be injected with 
impunity into the body of an animal susceptible to extremely 
