448 
TWENTY-SEVENTH ANNUAL MEETING 
are now experimenting with the chemical substances produced during the growth 
of the microbes. We have found that vaccination and inoculation, as applied 
in hog cholera, are uncertain and insufficient in results, and have certain disad¬ 
vantages which make these methods of little value for the prevention of this dis¬ 
ease. We turn, therefore, to the method of prevention by means of the chemi¬ 
cal substances formed during bacterial multiplication. 
This question is one of the most difficult ones to study that has confronted 
the bacteriologist. When we first demonstrated the principle we found that the 
culture liquid, when freed from living germs, had the power of conferring im¬ 
munity when injected into the tissues. But this very discovery suggested a 
host of questions which must be answered before the method could be of prac¬ 
tical use. The culture liquid, considered chemically, is a complex substance. 
Which of its constituents has this wonderful power ? What are its properties ? 
How can it be separated from the useless matter with which it is combined ? 
Is it a single alkaloidal substance, or does the result depend on two or more of these 
combined ? At what period of bacterial growth is it most abundant ? Will it pro¬ 
duce the desired effect when administered in other ways than by hypodermic in¬ 
jection ? What is the dose required to produce satisfactory immunity in the hog, 
and how often, and at what intervals must this dose be repeated? These are some 
of the most important questions which pressed themselves upon us for solution; 
and when we attempted to solve them we found other questions of a secondary 
interest which we were compelled to grapple with and solve before the others 
could be undertaken. 
We had no test for this mysterious and hidden substance, no way to recog¬ 
nize its presence, except its effect in the production of immunity, and this test 
it required weeks to make, and even then the experiments were liable to fail. 
How much more patience and skill does such an investigation require than the 
ordinary chemical methods where a re-agent is added to a solution, and the im¬ 
mediately formed precipitation shows the presence of the sought-for constituent! 
Our first experiments were made with pigeons, and while they served to demon¬ 
strate the principle, we soon found, to use chemical language, that the reaction 
with them was not a sufficiently delicate one to enable us to solve, without great 
loss of time, the other questions that came before us. The difficulty was, that 
pigeons are not easily killed by the hog cholera germ, especially in summer, and, 
consequently, after we had treated some to produce immunity, and afterwards 
inoculated them, together with other pigeons, to test their comparative resist¬ 
ance, none of the inoculated birds would die, the untreated ones resisting, as 
well as those that had received treatment. Such an experiment was, of course* 
a failure, and must be repeated. 
In short, it was necessary for us to select an animal to experiment with that 
in its natural condition of resistance would always die when inoculated with a 
moderate dose of virus, but which, with a slight addition to its natural power of 
resistance, would always recover from such an inoculation. The pig was out of 
the question—its resistance to virus is too variable and the possibility of pro¬ 
ducing immunity too doubtful. After considerable experimentation we found 
the guinea pig to answer these conditions to our satisfaction. It was here that 
our first discovered principle came to our assistance. We so graduated the dose 
