1899] TOXIC EFFECT OF DELETERIOUS AGENTS 401 
20. That an element is necessary for the nutrition of a plant | 
does not indicate whether it would or would not bea poison in 
greater concentration. See iron, oxygen, etc. 
21. That an element is not necessary for the normal devel- 
opment of a plant does not imply that it would be injurious even 
in relatively great concentration. See chlorin, calcium, etc. 
22. The ionization of the molecule of electrolytes in aqueous 
solution has a very important bearing on the study of the 
physiology of poisons. It is of especial value in determining 
the element or group of elements in a compound to which its 
toxic properties are to be attributed. 
23. In this study no new evidence has been adduced sup- 
porting the theory that the chemical activities of a substance are 
due wholly or chiefly to the ionized portion. 
24. Evidence has been adduced to the effect that in the 
case of several acids ionization /essens the chemical activities 
toward the substances involved in the life processes of the plant. 
25. In the case of the eight acids investigated six were found 
tobe much more toxic in the molecular form than after ioniza- 
tion. The toxic properties of the un-ionized molecules vary from 
épproximately 2.8 times that of ionic H in the case of acetic acid 
t f : 
to 76.6 times that of H in hydrocyanic acid. 
26. The substitution of Cl for H in the acetic acid radical 
has a double effect. In the first place it increases the toxicity 
of the un-ionized molecules to a greater or less extent depend- 
Ing on the number of H atoms so replaced. In the second place, 
_ it increases the ionization of the acid. The amount of the ioniza- 
tion is also dependent on the amount of H so replaced, being 
- Steatest, as are the toxic properties of the whole molecules, 
When all three H atoms have been replaced by Cl. 
i __ 27. These two factors to a great extent counterbalance each 
: other, Which has the greater influence in any given solution 
depends altogether on the concentration, the increased toxicity 
, % the molecules having the predominating influence at the 
: Steater concentrations, and the ionization being more effective 
: at the greater dilutions. 
