1896 | TOXIC EFFECT OF ACIDS AND SALTS UPON PLANTS 135 
gram molecule to the liter. Now the H,SO, solution which con- 
tains the same amount of ionic H as the other acids kills at the 
same point of dilution, so that this again points to the toxic 
effect of the H* ions. 
The action of HNO, may be discussed in a similar manner. 
Ca(NO,), is one of the common compounds by which a plant 
receives calcium and nitrogen and ata dilution relatively the 
same as that at which the acid killed the seedlings, it is without 
any harmful effect. It is presented to the plant in the form of 
*Ca* ions and NO; ions. The HNO, and Ca (NO,), contain 
NO, in common and since the NO~, ions are non-poisonous it 
leaves the H* ions again as the agent which produces the toxic 
effect. A large part of the nitrogen contained in a plant is sup- 
plied to it in the form of nitrates, so that here again the plant is 
constantly subjected to the action of NO~, ions. The non- 
poisonous character of the NO-, ions may be shown by other 
nitrates as well. 
It may seem doubtful at first whether HBr can be considered 
in the same way as the other acids, but Dirck’ has found that 
KBr in dilute solution produces no harmful effect. Now since 
this would dissociate as K+ ions and Br- ions, it follows that in 
dilute solution Br-ions are non-poisonous, and hence play no 
part in the toxic effect of the HBr, at least not at the dilution at 
which the HBr killed the roots of seedlings. It will also be 
seen from the tables that the HBr kills the seedlings at the same 
degree of concentration as the other acids; now since it has been 
shown that the toxic effect of HCl, H,SO, and HNO, is due to 
the H- ions, we should expect HBr to kill at a different degree 
of concentration if the Br was poisonous also, for then we should 
have the sum of the effect of H* ions and Br~ ions. Here 
again the entire toxic action is produced by the H* ions. 
It has been clearly shown by the above experiments that in 
the case of poisoning by acids, the harmful effect is produced 
entirely by the H* ions. By putting the results of the experi- 
ments in a different form we can get a better idea of the 
7 Bericht d. Verhdlg. d. siichs. Ges. d. Wiss. zu Leipzig 21: 20. 1869. 
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