372 MANUAL OF VEGETABLE-GARDEN INSECTS 



may be used with equally good results and only one half the 

 quantity is necessary. 



Zinc arsenite. 



Arsenite of zinc, Zn(As02)2, if^ a light fluffy powder and con- 

 tains the equivalent of 40 per cent arsenious oxid. It is some- 

 times used as a substitute for arsenate of lead. It has a greater 

 tendency to burn the foliage when used in water but is fairly 

 safe if combined with bordeaux mixture. It is probable that 

 foliage injury by zinc arsenite is due to the solubility of this 

 poison in water containing a small quantity of carbonic acid ; 

 the latter is usually present on the leaves, being derived from 

 the respiration of the plant. One pound of zinc arsenite is 

 equivalent in effectiveness to about three pounds of arsenate 

 of lead paste. 



Calcium arsenate. 



Commercial calcium arsenate consists of a variable mixture 

 of acid calcium arsenate, CaHAs04, and basic calcium arsenate, 

 Ca3(As04)2. It is sold in two forms — a thick paste and a 

 powder. It may be used as a substitute for arsenate of lead 

 and is considerably cheaper. Unfortunately, when used alone 

 there is some danger of burning the foliage. This may be 

 overcome by adding an equal weight of quicklime, or by using 

 it with bordeaux mixture. Calcium arsenate contains a larger 

 percentage of arsenic than arsenate of lead and it is, there- 

 fore, not necessary to use so large a quantity. 



Hellebore. 



Hellebore is a light brown powder made from the roots of 

 the white hellebore plant {Veratrum album), one of the lily 

 family. It is applied both dry and in water. In the dry 

 state, it is usually applied without dilution, although the ad- 

 dition of a little flour will render it more adhesive. In water. 



