1896 | TOXIC EFFECT OF ACIDS AND SALTS UPON PLANTS 145 
twenty-four hours of the experiments in which the roots were 
killed. The nickel and cobalt are quite poisonous, but the toxic 
effect is not felt by the plant as soon as in the case of acid or 
copper poisoning. The toxic action of nickel is seen to be 
greater than cobalt, by the results on both Pisum sativum and 
Zea Mais. . 
~ In the case of the NiSO, the dissociation will be into 
*Ni* ions and ~SO;z ions. The CoSO, will form *Co* ions 
and ~SO; ions. At the dilution used for both of these salts 
the dissociation will be practically complete. It has already 
been shown that the SO; ion is non-poisonous, so the *Ni* 
ions and the *Co* ions are left to bring about the toxic action. 
The Ni(NO,), will form in dilute solution *Ni* ions and 
two NO; ions. The Co (NO,), will dissociate to form *Co* ions 
and two NO; ions. Here also the dissociation will be practically 
complete at the dilutions used, so that we have to deal simply 
with the resulting ions. The NO; ion has been shown to be 
Non-poisonous, so that in this case also the toxic action must 
be due to the *NO+ ions and to the *Co* ions. 
The small amount of ionic Ni and Co necessary to kill the 
roots is shown below: 
PISUM SATIVUM. 
Nickel—z5}59 mol.=1 part Ni to 435,374 H20. 
Cobalt—yz459 mol.=1 part Co to 217,687 H,0. 
ZEA MAIS. 
Nickel—y5}55 mol.=r part Ni to 435,374 H.0. 
Cobalt—3s!55 mol.=1 part Co to 54,421 H,O. 
Even in the case of the cobalt and Zea Mais, which shows 
the greatest concentration for poisoning, the amount of ionic 
Co is very small when compared with the amount of H,O. 
6. SILVER SALTs. 
In testing the toxic action of silver two salts were used, 
Silver sulfate and silver nitrate. For these experiments the 
Same two seedlings were used. The results obtained for the 
Seedlings of Pisa sativum are given in the following table: 
