29 
In studying these spectra, it must be remembered that one immu- 
nity unit theoreticall}" neutralizes 200 minimal lethal doses of a pure 
poison, containing only toxin. It is self-evident that such toxin ‘‘val- 
ences” may 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 only 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 remaining 
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 dose of toxine. The L® dose is that quantity 
of toxine which will just neutralize one immunity unit. The dose 
should contain just 200 MLD’s of a theoretically pure toxine, contain- 
ing only toxin. 
Let us make a spectrum of an impure toxine containing the fol- 
lowing: 
L®== 50 combining units oi j^)7'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 toxhi 
to kill 100 guinea pigs. 
L*’+ 2 ^o immunit}^ unit=100 MLD’s. 
L^-h/oV iuim unity unit =100 MLD’s. 
L®+//o immunity unit = 100 MLD’s. 
Up to this point we see that the addition of fift}^ two-hundredths of 
the immunity unit has no efi'ect upon the toxicit}^ {toxw) of the poi- 
son. In other words, the antitoxin is being used up by 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 INILD — 
L'^+gVo iuimunity unit=99 MLD’s. 
L^-f-gVo immunitv unit=80 MLD’s. 
immunity unit = 3 MLD’s. 
L”T-|t§ immunity unit= 2 MLD’s. 
immunity unit= 1 MLD. 
That is, the L® dose of toxine plus one hundred and forty-nine two- 
hundredths of the immunity unit contains in the mixture just enough 
