INSECTICIDES AND THEIR APPLICATION 157 



ing from the latter in a tnistlike spray. This spray is di- 

 rected upon the leaves of the plant to be protected, covering 

 each leaf with the liquid. 



As soon as the spraying ceases, the water begins to evap- 

 orate and rapidly disappears, leaving the particles of poison 

 more or less evenly distributed over the foliage. There 

 they remain for some time, so that in case a biting insect 

 feeds upon the poisoned surface it will eat some of these 

 particles and be killed. 



Fortunately the insoluble poison is not absorbed into 

 the tissues of the plant, so that the process can gener- 

 ally be carried on without danger to the consumers of 

 the crop. In the course of a few weeks the particles of 

 poison are washed away by rain or dew, or blown away 

 by the wind, or dissipated through the action of the sun- 

 shine. 



A substance that is used to destroy insects is called an 

 insecticide or insect killer. Thus Paris green is an insecti- 

 cide. So also is lead arsenate, hellebore, powdered to- 

 bacco, and insect powder. Each of these substances may 

 be applied to plants in mixture with water, or most of them 

 may be dusted on in the form of a dry powder. One may 

 sometimes use for this purpose simply a porous bag,''out of 

 • which the powder is sifted. It is better, however, to apply 

 the material by means of one of the many kinds of powder 

 bellows manufactured for this purpose. With one of the 

 better forms of these, a pound of Paris green can be evenly 

 distributed over an acre of potato vines. 



Paris green is so powerful a poison that it is often de- 

 sirable to dilute it with some substance like land plaster, 

 air-slaked lime, or flour. The proportion of poison to dilu- 

 ent varies greatly with different users. One part of poison 

 to twenty parts of diluent serves the purpose if the mixing 



