200 Mineral Poisons [398 
tops for wheat. The total concentration of the solution corre-. 
sponded to an osmotic pressure of 1.75 atmospheres at 25° C. 
The technique was essentially the same as that employed by 
Shive. : 
The minimum concentrations at which the various poisons 
began to produce clearly marked injury, as indicated by 
smaller dry weights of tops, are given in the following table. 
Concentrations are given in parts of the poisonous element 
per million parts of the nutrient solution. In most cases the 
concentration at which injury begins is not sharply marked, 
and, therefore, the figures given have only approximate quan- 
titative value. 
Toxic 
Concentration 
Element Compound used of element. 
p. p.m. 
Arsenic ............ trioxide (As,0,)........... 1 
Boron ............. borax (Na,.B,O,;).........- 10 
Cobalt: 2. as ec sulphate (CoSO,).......... 7 
Copper ............ sulphate (CuSQ,).......... 1 
Manganese ......... sulphate (MnSO,)......... 1000 
Mereury) jsowcce saree bichloride (HgCl,)........ 40 
Nickel sicocceniscisdens sulphate (NiSO,).......... 5 
Vanadium.......... chloride (VCI,)............ 20 
ZANE Ssesaiend ond ecae twee sulphate (ZnSO,).......... 100 
Experiments were made also with lead, as lead nitrate 
(Pb(NO,),), and with uranium as uranyl nitrate 
(U0.(NO;),), but both of these elements were precipitated 
by the constituents of the nutrient solution. The maximum 
concentrations obtainable in the solution were approximately 
100 parts per million in case of lead and 20 parts per million 
in the case of uranium. Neither of these was toxic. 
A slight stimulating effect, indicated by greater produc- 
tion of tops, was observed with manganese, between the con- 
centrations of 4 and 20 parts per million, and with vanadium, 
between 2 and 7 parts per million. There was a clear stimu- 
lation in the uranium cultures above a concentration of 50 
parts per million of uranium, but it is possible that this was 
