232 ELEMENTARY AGRICULTUEAL GHEMISTRY 



one and a half parts of sulphuric acid and two or three parts of 

 water. The water should be placed in a glass or earthenware 

 vessel, the sulphuric acid poured gradually in, with continuous 

 stirring, and lastly, when all is ready, the cyanide should bo 

 dropped in, and the building or tent at once vacated. 



For trees a tent made of canvas, rendered gas-tight by 

 treatment with boiled linseed oil, is used to retain the gas, and 

 the fumigation done at night. Usually from thirty to forty 

 minutes' exposure to the gas is sufficient. The greatest care 

 should be taken in dealing with such intensely poisonous sub- 

 stances as potassium cyanide and prus;?ic acid. Caution to 

 avoid breathing air containing the acid is particularly neces- 

 sary. This method is also very successful in ridding houses, 

 mills, (fee, of insect pests of all descriptions. For trees the 

 amount of cyanide to use is from 10 to 25 grams — i.e., from J to 

 J ounce — per 100 cubic feet of air space, depending upon the 

 kind of trees. For buildings about 1 ounce per lOQ cubic feet 

 will generally be sufficient. 



(C) The so-called Contact Poisons. 



These are intended particularly for sucking insects, which, 

 deriving their food from the interior of the host plant or 

 animals, cannot be killed by poisoning their food. They, there- 

 fore, are destroyed either by clogging the breathing pores by 

 some liquid or solid, or in some cases, perhaps, by absorption 

 of the poison through the skin. Soap of any kind, and parti- 

 cularly potash or soft soap, is effective, being usually applied 

 in from five to twenty times its weight of water. 



Kesin soaps are also employed, being made as required by 

 boiling resin with caustic soda or potash solution, or with 

 sodium carbonate (washing soda) solution. Generally some fish- 

 oil or tallow is also used. Thus a common wash is made by 

 taking : 



Resin . . . ^ . . . 20 lb. 



Fish-oil . s i to 1 gallon 



Caustic soda 8 lb. 



