XVII. MISCELLANEOUS PRODUCTS USED IN AGRICULTURE 371 



Paris green, London purple. They are best applied in suspension in 

 water, as a spray ; but sometimes they are used in the dry state, either 

 alone or mixed with flour. In some cases they are employed as 

 poisoned bait, mixed with sugar and bran. For spraying, 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 particularly potash 

 or soft soap, in from five to twenty 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. 



Lime and sulphur dip, i.e., a liquid formed by boiling finely 

 divided sulphur with lime and water, is extensively used as a sheep 

 dip for the destruction of scab. The active constituents are the sul- 

 phide and polysulphides of calcium. 



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

 with soap and water or with sour milk, is highly recommended. 

 The proportions used are 2 gallons of petroleum 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 fifteen or twenty times its volume of 

 water and applied with a sprayer. 



Fumigation. Tobacco smoke is often used in greenhouses 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 thirty to forty minutes' 

 treatment is all that is necessary, and the quantity of pure cyanide to 

 be used appears to be about 1 to 1^ ounces per 100 cubic feet of en- 

 closed space. The cyanide should be dropped into a glass or earthen- 

 ware 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 surrounded by boxes 

 to enclose the heavy vapour given off from a small quantity (5 or 

 10 c.c.) of the liquid placed in a saucer. It is, however, particularly 

 well adapted for the destruction of subterranean insects or their larvae. 

 For this purpose a hole from 6 to 12 inches deep is made, not too 

 near the tree whose root is affected by the insects, and from 15 to 

 30 c.c. of carbon disulphide poured in. The hole is then quickly 



