INSECTICIDES. 377 



about 1 part of the solid suspended in from 1000 to 2000 parts 

 of water is generally employed. The addition of lime is said 

 to prevent the corrosive action of arsenic compounds upon 

 foliage. 



For sucking insects, contact poisons as distinguished from 

 food poisons have to be used. Soap of any kind, but particu- 

 larly potash or soft soaps, in from 5 to 20 times its weight of 

 water, is effective on the small scale. Pyrethrum, the ground 

 flowers of the plant of that name, is also effective. 



Flowers of sulphur, too, is useful for the purpose, as is also 

 a solution of calcium or sodium sulphide, made by boiling 

 sulphur with lime and water or with a solution of caustic soda. 



Petroleum, either alone or, better, in most cases, as an 

 emulsion with soap arid water or with sour milk, is highly 

 recommended. The proportions used are 2 gallons of petro- 

 leum to 1 gallon of water containing half a pound of (preferably) 

 whale-oil soap, or to 1 gallon of sour milk. The emulsion is 

 made by means of a force-pump. For use the emulsion is 

 diluted with 15 or 20 times its volume of water and applied 

 with a sprayer. 



Fumigation. Tobacco smoke is often used in green-houses 

 as a means of destroying insect pests. For shrubs and trees 

 extensive use is now being made in America of hydrocyanic 

 acid. The tree or shrub is enclosed in a tent made of canvas 

 rendered gas-tight by treatment with boiled linseed oil, and the 

 gas is evolved by the action of dilute sulphuric acid upon 

 potassium cyanide. From 30 to 40 minutes' treatment is all 

 that is necessary, and the quantity of pure cyanide to be used 

 appears to be about 1 to If ounces per 100 cubic feet of 

 enclosed space. The cyanide should be dropped into a glass 

 or earthenw r are jar containing about three times its weight of 

 water and its own weight of commercial sulphuric acid. It is 

 hardly necessary to say that the greatest care has to be taken 

 to avoid breathing air containing any hydrocyanic acid. 

 Plants are not readily injured by the gas in the dark, but 

 when in leaf are quickly killed by it in sunlight. 



Carbon disulphide, CS 2 , is an effective poison for most 

 insects. It is used for low-growing plants which can be 



