134 Proceedings of the Royal Physical Society. 
even causing a fatal termination. A marked difference is 
thus shown to exist in the constitution of the serum in 
different species, and indicates very clearly the vital, rather 
than the inert nature of it, and its complex and variable 
composition. 
As illustrating the differences in the serum of different 
animals, and the extent to which it is capable of being 
modified experimentally, it is necessary to refer to a number 
of diseases which have been experimentally investigated. 
The first of these to which I may refer is produced by 
an organism known as the Staphylococcus pyosepticus, an 
organism which very rapidly produces death in rabbits when 
injected into their tissues. To this microbe the dog is com- 
pletely refractory, that is to say, its injection into the dog 
produces no appreciable effect. Some of the blood of a dog 
injected into the peritoneum of a rabbit renders the rabbit. 
proof against the action of the microbe, and the dog’s serum 
is still more efficacious if the natural immunity of the dog 
has been reinforced by inoculating it with the microbe in 
question. 
This observation, as you can understand, suggested most 
important possibilities in the direction of utilising the serum 
of animals, refractory to certain microbes, as a means of 
preventing their action in other animals. This, however, has 
not been found so applicable as this special case seemed to 
indicate. The existence, however, of a relative, if not of an 
absolute immunity, has been utilised by augmenting it in 
the manner I have indicated, and in using the serum of 
animals thus both naturally and artificially immunised for 
curative purposes. 
Other means were also found of producing an artificial 
immunity, and one of the first diseases in which this was 
done was a disease known as hog-cholera, also due to a 
special microbe. This microbe injected into rabbits rapidly 
proved fatal, the microbe rapidly developing and multiplying 
throughout the rabbit’s body. Notwithstanding the great 
susceptibility of the rabbit to this organism, it was found 
comparatively easy to vaccinate it against the microbe. To 
attain this, all that was necessary was to heat to a certain 
