Part IV* 
THE TOXIC PRINCIPLE. 
At one time we made efforts to isolate the active principle in horse 
serum which causes the symptoms, but as soon as we realized that 
the toxic principle present in horse serum exerts its action in quan- 
tities so minute as to place it almost in the category of the ferments, 
and, further, when we concluded from our work that this toxic prin- 
ciple is doubtless one of those highly organized and complex proteid 
substances belonging to the “haptin” group, we recognized how 
hopeless it would be with present technique to isolate this substance. 
Nevertheless, we devoted much time and study to the relation of 
this toxic principle to various chemical, physical, and electrical influ- 
ences. The practical importance of eliminating or neutralizing this 
toxic principle from horse serum is at once evident. 
IS THE POISON SPECIFIC ? 
Whether the poisonous substance in horse serum is specific may 
be determined in several ways. We first endeavored to determine 
whether guinea pigs treated with horse serum are susceptible to the 
serums of other aimals; and, secondly, whether guinea pigs treated 
with the serums of other animals are susceptible to subsequent injec- 
tions of horse serum. 
The following is a list of animals treated with horse serum and sub- 
sequently tested with the blood serums of other animals. 
It will be noticed that in this list some of the guinea pigs were 
treated with donkey serum instead of horse serum, but as the donkey 
serum in our hands has shown itself to act the same as horse serum 
we have used them interchangeably. 
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