24 



CIRCULAR 369, U. S. DEPT. OF AGRICULTURE 



As the dust may be blown about while the building is being ven- 

 tilated, it should be removed as soon as possible. To obviate this 

 difficulty, as well as for other reasons, the gas is sometimes liberated 

 in a special apparatus (fig. 41). Air is forced by a small motor into 

 a rubberized sack containing the calcium cyanide dust, which is kept 

 agitated until all the hydrocyanic acid gas is liberated. The gas 

 enters the space to be fumigated through a filtering device, which 

 frees it of the dust. 



Figure 24. — Warehouse being fumigated with hydrocyanic acid gas by the powder method. 

 Calcium cyanide in dust form is spread on the floor, usually from tin cans with 

 perforated covers. 



The Dosage 



To figure the quantity of calcium cyanide needed for a fumigation, 

 it is necessary to know the percentage of available h} T drocyanic acid 

 it contains. This percentage will be found on the label of the can. 

 If, for example, the dust contains 50 percent of available hydrocyanic 

 acid, 1 pound will give off as much gas as 8 ounces of liquid hydro- 

 cyanic acid or 1 pound of sodium cyanide. Calcium cyanide ranges 

 in price from $1.60 per pound in 1-pound cans to $1.20 per pound 

 in 25-pound cans. 



FUMIGATION WITH CHLOROPICRIN 



Chloropicrin, although not so popular as hydrocyanic acid, is 

 sometimes used as a general fumigant for mills and warehouses. It 

 is a colorless or slightly yellowish liquid, a little more than one 

 and a half times as heavy as water. It has a boiling point of 233.6° 

 F., and on exposure to air evaporates slowly, forming a mixture of 

 air and chloropicrin vapor that is 1.1458 times as heavy as air at 

 the same temperature and pressure (77° F., 29.9 inches). It can be 

 purchased in cylinders of from 1 to 100 pounds' capacity at a cost 

 of $1.20 per pound for 1-pound cylinders down to 85 cents per pound 

 in 100-pound lots on contract. The gas is nonexplosive and nonin- 

 flammable as ordinarily used, is very toxic to insects and also to 



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