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HOUSEHOLD INSECTS 



tion a house should be vacated. There may be danger in 

 fumigating one house in a solid row of houses, if it should 

 happen that there was a crack in the walls through which 

 the gas might find its way. It also follows that the fumiga- 

 tion of one room in a house might endanger the occupants 

 of an adjoining room, if the walls between the two rooms 

 were not perfectly tight. It is absolutely essential to keep 

 all of these points in mind and to do the work deliberately 

 and thoughtfully. 



Generation of the gas. — Hydrocyanic acid gas is gener- 



Fig. 151. — Materials used in fumigation. 



ated from the salt, potassium cyanide, by treating it with 

 sulphuric acid diluted with water. Experiments and 

 experience have shown that the cyanide should be at 

 least 98 per cent pure in order to give satisfactory results. 

 Potassium cyanide is presented to the trade in varying 

 grades of purity, from 40 per cent to 100 per cent actual 

 cyanide. When diluted, a useless salt, usually sodium 

 carbonate or sodium chloride, is used, which is of no value 

 in fumigation. Indeed, the sodium chloride may be of 

 positive detriment. The purchaser of cyanide should, 

 therefore, insist on its being at least 98 per cent pure, 

 and it ought to be bought for not more than forty cents 



