29 
In studying these spectra, it must be remembered that one immu- 
nity unit theoretically neutralizes 200 minimal lethal doses of a pure 
poison, containing only toxin. It is self-evident that such toxin ‘^val- 
ences” mav be substituted by other substances having the same com- 
bining affinity but less poisonous or not at all poisonous, such as toxoid 
and toxone. 
As toxines containing onh^ toxin are practically never obtained, it is 
evident that the immunit}’ unit will be found in actual practice to neu- 
tralize a less number of minimal lethal doses than 200- the remainino' 
number being satisfied with the toxoids and toxones. 
In order to obtain a correct understanding of these spectra, it is 
necessary to know how one is cast. The most instructive results are 
obtained by using the L® dose of toxine. The dose is that quantity 
of toxine which will just neutralize one immunit}" unit. The dose 
should contain just 200 MLD's of a theoretically pure toxine, contain- 
ing onh" toxin. 
Let us make a spectrum of an impure toxine containing the fol- 
lowing: 
50 combining units oi jy'ototoxin. 
100 combining units of toxin ( = 100 MLD's). 
50 combining units of toxone. 
200 
Then: 
L” dose of toxine =100 MLD's, i. e., contains enough toxin 
. to kill 100 guinea pigs. 
L^+gl^o immunit}" unit = 100 MLD's. 
L®+/o\ inimunity unit=100 MLD's. 
L^+gVo immunit}^ unit = 100 hILD's. 
Up to this point we see that the addition of fifty two-hundredths of 
the immunity unit has no efiect upon the toxicity {toxin) of the poi- 
son. In other words, the antitoxin is being used up b}^ combining 
with some nonpoisonous substance, viz, prototoxoid. The first 50 
segments of our spectra will therefore consist of this modified toxin. 
If now we add more antitoxin to our toxine we will find that for the 
addition of each one two-hundredths of the immunity unit the toxine 
loses one MLD — 
L”+gVo inimunity unit=99 MLD’s. 
L^-j-gVo inimunity unit = 80 ^ILD’s. 
J immunity unit= 3 MLD’s. 
inimunit}^ unit= 2 MLD’s. 
L®+y§ immunity unit= 1 MLD. 
That is, the L® dose of toxine plus one hundred and forty-nine two- 
hundredths of the inimunity unit contains in the mixture just enough 
