234 BOTANICAL GAZETTE [SEPTEMBER 
When the solution contains more than two components, we 
may follow a method similar to that already outlined for equally 
toxic solutions containing more than two components. 
In most cases the calculations described above may be dis- 
pensed with, as the error (if any) in proceeding by the method 
described in the two preceding papers’ is so small as to be negligible. 
Calculations such as are here discussed will be necessary only in 
those cases in which equally toxic solutions acquire, when diluted 
to the same degree, a very marked difference in toxicity, or in cases 
where a mixture is made of unequally toxic solutions which have 
dilution curves of very dissimilar character. 
Summary 
In most cases two solutions which are equally toxic remain so 
(at least approximately) when both are diluted to the same 
degree; this allows the additive effect to be easily determined. 
But in exceptional cases, where this does not hold, a value may be 
assigned to the additive effect. 
Similar considerations apply to unequally toxic solutions. 
LABORATORY OF PLANT PHYSIOLOGY 
HARVARD UNIVERSITY 
5 Op. cit. 
