MISCELLANEOUS 231 



is heavy, and very poisonous to animals, including insects. On 

 this account its vapour is particularly well adapted for killing 

 subterranean insects or larvae. It is often used for destroying 

 ants. One or two ounces of the liquid poured down the holes, 

 which should then be covered, will evolve a poisonous vapour 

 which may penetrate into all parts of the nest. 



It can also be used for the destruction of weevils in mealies 

 or other grain. For this purpose the grain is placed in bins 

 or tanks, sufficient carbon disulphide is either poured on to 

 the grain or placed in a vessel on the top, and the bin or tank 

 closely covered. 



Insects on low-growing shrubs or trees may be destroyed by 

 surrounding the trees with boxes to enclose the heavy vapour 

 given off from a small quantity — half an ounce to an ounce — 

 of the liquid placed in a saucer. 



Sulphur Dioxide^ SOj (see p. 213), cannot be used to 

 destroy insect pest on plants, but is often employed for the 

 destruction of bugs, cockroaches and other household insects. 



Tobacco Smoke, or, better, the fumes from tobacco extract, 

 is often used as an insecticide in greenhouses, (fee. 



Hydrocyanic Acid, HON, "prussic acid," is a gas with a 

 curious and characteristic, though not strong, odour. It is in- 

 tensely poisonous to animals, but as, in small quantities, it is 

 not fatal to plants in the dark, it can be and is extensively 

 used for the destruction of insect pests on shrubs and trees. 



The gas is made as required by the action of diluted sulphuric 

 acid upon potassium cyanide. The reaction is : 



KCN + H,SO, = KHSO, 4- HON. 



Potassium Sulphuric Potassium acid Hydrocyanic 

 cyanide acid sulphate acid 



Fairly pure cyanide is now easily obtainable (98 per cent, 

 potassium cyanide), and ordinary oil of vitriol is suitable for 

 the purpose. Before use the acid should be diluted with about 

 one and a half or twice its volume of water. The proportions 

 to use are about one part by weight of potassium cyanide to 



