SUMMARY 295 



insects which eat the foliage it is only necessary to spray 

 the leaves with a stomach poison and for this purpose the 

 so-called arsenicals — lead arsenate and Paris green — are used. 

 Hellebore is employed to a less extent. 



Paris green is an aceto-metarsenite of copper and unless 

 mixed with lime is apt to injure foliage because of the ease 

 with which soluble arsenous acid is formed bv the action of 

 water and carbon dioxide in the air. Lead arsenate is usually 

 a mixture of triplumbic and acid lead arsenates, although 

 the better grades are now composed very largely of the 

 triplumbic arsenate only. The latter does not dissolve or 

 decompose under any ordinary conditions, whereas the acid 

 arsenate may break up to form arsenic acid, soluble and 

 injurious to leaves. 



For insects which puncture the surface of plant leaves or 

 stems and suck the juices, a stomach poison on the surface 

 does no good. It is necessary to use a material which kills 

 by contact, either on account of its caustic action, or because 

 it enters their pores and stops up their breathing apparatus. 

 Such external insecticides are principally hydrocyanic acid 

 gas, kerosene emulsion, lime-sulphur boiled, and miscible 

 oils. 



Hydrocyanic Acid is a liquid at ordinary temperatures, 

 but is volatile and exceedingly poisonous, especially to man, 

 so it must be used with great care. It is generated by mixing 

 potassium cyanide and sulphuric acid. If the acid is too 

 strong, or if the cyanide contains chloride as it frequently 

 does, loss of hydrocyanic acid occurs by decomposition. 

 Kerosene emulsion is a mechanical mixture of kerosene and 

 soap in which the kerosene is suspended in very minute 

 globules. It will not do this in pure water. Kerosene itself 

 is harmful to vegetation and the emulsion is a convenient 

 way of applying it in dilute form. Lime-sulphur boiled is a 

 very important insecticide and is in addition an excellent 

 fungicide. It is a solution of tetra- and pentasulphides of 

 calcium, and calcium thiosulphate. After application the 

 sulphides gradually oxidize to sulphite of calcium and free 

 sulphur; and the thiosulphate changes to sulphite and free 

 sulphur. This formation of sulphite and sulphur increases 



