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 2 3 ) 1 



Boron borax (Na,B 4 T ) 10 



Cobalt sulphate (CoSOJ 7 



Copper sulphate ( CuSO 4 ) 1 



Manganese sulphate (MnS0 4 ) 1000 



Mercury bichloride (HgCl 2 ) 40 



Nickel sulphate (NiS0 4 ) 5 



Vanadium chloride ( VC1 2 ) 20 



Zinc sulphate (ZnS0 4 ) 100 



Experiments were made also with lead, as lead nitrate 

 (Pb(ISr0 3 ) 2 ), and with uranium as uranyl nitrate 

 (U0 2 (N~0 3 ) 2 ), 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 



