138 Proceedings of the Royal Physical Society. 
this may be an additional mode of attenuation, which might 
have some advantages if it proved to be equally efficacious 
with the original method. Many other examples might be 
given illustrating the fact that when animals are artificially 
rendered immune to disease, the blood-serum of the animal 
may be utilised for vaccinating other animals, and thereby 
rendering them immune. However interesting the details of 
these may be, I cannot attempt to take you over the whole 
field. I may, however, mention to you that this line of 
investigation has been followed in such important and well- 
known diseases as pneumonia, typhoid fever, cholera, and 
tetanus. In all these, animals which are specially susceptible 
to the action of the microbes associated with these diseases 
have, by this process of vaccination by attenuated serum, been 
rendered immune. In all of them the interesting question as 
to the precise mode of action of the curative or preventive 
serum arises. 
One of the first theories as to the mode of action of the 
serum was that it is antitoxic, the idea, as I have already 
indicated, being that the antitoxin counteracted the toxin, 
and hence the use of the term “antitoxin,’ which has now 
become general. Experiments in another direction tended 
to support this view. It was found that animals could be 
readily rendered able to withstand very large doses of certain 
vegetable toxalbumins by giving to them small doses to 
begin with. It was further found that if the blood-serum 
from such animals was mixed with the poison, the mixture 
could be injected into fresh animals without causing them 
any inconvenience. From this it was naturally enough con- 
cluded that the blood of the immunised animal had attained 
the power of destroying the poison. 
It is well enough known to you that the human body can 
become habituated to and proof against the action of poisonous 
substances, and no better example can be given of this than 
is seen in the innocuousness of tobacco in those accustomed 
to its use. The startling side of this action is the marked 
effect which is evidently produced on the blood, the blood- 
serum evidently becoming possessed of a new and special 
property. Whether the vaccination with blood from a smoker 
