INSECT CONTROL IN FLOUR MILLS. 15 



belting passes and similar openings in the floor can be sufficiently 

 well closed by crowding pieces of sacking firmly into them. 



Chemicals Necessary eok Fumigation. 



The chemicals necessary for the generation of hydrocyanic-acid 

 gas are potassium or sodium cyanid, 3 sulphuric acid, and water. 



Cyanid. — Either potassium cyanid or sodium cyanid can be used 

 in fumigation, but trade conditions have made sodium cyanid more 

 available for use and, as it is just as good and slightly cheaper, it is 

 recommended. For a full discussion of the relative values of potas- 

 sium and sodium cyanid for fumigating purposes, see Bulletin 90 

 of the Bureau of Entomology. Sodium cyanid for fumigation pur- 

 poses should be 96 to 99 per cent guaranteed purity. The volume 

 of gas liberated is in direct proportion to the purity of the cyanid, 

 hence too much stress can not be placed on purchasing high-grade 

 cyanid. Protection can be had by purchasing a cyanid guaranteed 

 under the Federal insecticide act; the analysis on the label should 

 show that the cyanid is at least as pure as follows : 



Sodium cyanid (XaCN), 96 to 99 per cent, analysis. 



Per cent. 



Cyanogen (CN), not le.:s than . :2 — 51.3 



Sodium (Na), not more than 43.7 



Inert substances, not more than 4. 



Chlorids, not more than 1. 4 



High-grade cyanid can be purchased in tin cans containing 50, 100, 

 or 200 pounds. For ease in handling and preparing doses, it is con- 

 venient to purchase brands of cyanid in which the material is divided 

 into lumps weighing approximately 1 ounce each. Cyanid decomposes 

 slowly when exposed to the air, hence such cyanid as remains unused 

 after a fumigation should be protected by being placed in an air- 

 tight receptacle. Where this can not be done, the filling in of the 

 original container with firmly packed sacking will greatly retard 

 decomposition. 



Sulphuric acid. — Commercial sulphuric acid (H 2 S0 4 ) 92 to 94 

 per cent pure (66° Baume), free from nitric acid, arsenic, lead, and 

 zinc, should be used. It may be made from pyrites or pure brim- 

 stone, so long as impurities are eliminated. Acid is usually pur- 

 chased in iron drums containing 800 to 1,500 and 2,000 pounds, though 

 when used in smaller quantities it can be had in glass carboys of about 

 100 pounds capacity. Pure acid is colorless and about twice as heavy 

 as water; its specific gravity is 1.83. When stored in iron drums it 



3 Recent developments in liquid hydrocyanic acid indicate that sooner or later its use wilt 

 replace this long established method of generating gas by the mixing of chemicals in 

 containers, 



