( 785 ) 
entering into the detailed theoretical considerations put up with 
reference to this, I only wish to point out that in more than one 
way serum can be deprived of its toxicity for sensitized animals, 
without at the same time losing its sensitizing power. This also holds 
good especially for the fluids described in my last communication : 
serum treated with washed guineapig-blood and the filtrate of dialysed 
serum, which have both lost their toxicity for sensitized guinea pigs, 
are nevertheless able to render normal animals sensible to a later 
injection of horse-serum. 
The same is the case with the new method, to be described here 
after, to deprive the horseserum of his toxicity with the aid of 
bariumsulphate. Therefore this may be laid down as a general rule. 
A priori there is no doubt something to be said for the opinion 
that the sensitizing and the intoxicating principle of horse-serum are 
represented by two different substances, which apparently may be 
dissociated with the help of divers biological and physico-chemical 
processes. I myself, however, should for the present prefer to take an 
cther standpoint, and that for the following reason: taking for granted 
the fact that for the sensitizing action only traces of serum are 
required, we may safely assume that in the different methods of 
destroying the toxic serum principle so much of the active matter 
is destroyed or fixed, that intoxication can no longer be brought 
about, but that there is a sufficient quantity left to cause sensibility. 
Because (and here we have a parallel): for not a single immunebody, 
whose function can be made to disappear from the serum by heat, 
there exists a limit for which holds good that after a certain time 
and for a definite temperature the substance should be altogether 
destroyed. Even though we can no more prove the presence of such 
a material by a reaction in vitro, yet the curve, of which the ordinate 
represents the time and the abscis the temperature, theoretically never 
reaches zero. And such theoretical possibilities ought to be taken 
into consideration (as moreover experience has shown in heating 
horse-serum), when — as appears from serum-sensitization — we 
can cause a biological reaction with such extremely small quantities 
of a substance. Therefore I do not see why we should in this respect 
depart from our generally accepted opinions and why we should 
not identify the sensitizing antigen with the toxical, if only we bear 
in mind that the organism reacts upon small doses otherwise than 
upon large doses. But in this communication we do not want to 
enter into a more detailed description of the theoretical part of the 
problem and we will now revert to the facts. 
The serum rendered atoxie in one of the above mentioned ways 
