22 



Hydro cyanic- Acid Gas, HCN. 



This insecticide is used largely in the fumigation of nursery stock. 

 It is also used for destroying scales on orchard trees and for ridding 

 mills, stores, and elevators of grain pests and rodents. The applicability 

 of it was first demonstrated in California, where it was found useful in 

 combatting the cushiony scale affecting citrus trees, but it has since 

 found a very extended use against other insect enemies. 



The gas is not bought as such, but is prepared at the time of use 

 from a substance known as potassium cyanide (KCN). The cyanide is 

 a solid body and when treated with sulphuric acid (HgSO^), is decom- 

 posed or broken up and the gas liberated as : 



KCN , -r H 2 S0 4 = HKS0 4 + HCN 



Pot. cyanide. Sulphuric acid. Pot. acid sulphate. (Hydrocyanic 



acid gas). 



The gas at low temperature is condensed to a liquid and is then called 

 prussic acid. The liquid boils at 26. 5 C. , and thus is easily changed 

 into the gas again. Being quite light, the gas rapidly diffuses and pene- 

 trates to every little nook and corner of the fumigating enclosure. For 

 this reason it is very effective, and, when supplied in sufficient quantity, 

 leaves nothing undone. 



Fumigation of trees is best done while in the dormant state ; if trees 

 in foliage are treated, night should be chosen as the time of action, since 

 the actinic or light-giving rays of the sun have a very damaging effect 

 oa leaves for some time after they have been surrounded by the gas. 



As the gas is extremely poisonous, great care should be taken that 

 it be not inhaled ; and before a building or tent is entered after the opera- 

 tion, a thorough airing should be given. 



For generating the gas, an open* glazed vessel is used, an ordinary 

 crock serving the purpose admirably. The water is first placed in the 

 vessel, the sulphuric acid is then added, and last, the potassium cyanide 

 is dropped in and the door quickly closed. All ventilators, cracks and 

 openings should be tightly closed to prevent any leakage or waste of gas. 

 The amounts of the different materials employed are as follows : 



Potassium cyanide (98 per cent.) 1 ounce. 



Sulphuric acid (1.83 specific gravity) 1 fluid ounce. 



Water 3 fluid ounces. 



Enough will be supplied by these quantities to fill 150 cubic feet of 

 confined space. If there are 300 cubic feet of space, then twice the 

 quantities given will have to be employed ; if 450, then two and a half 

 times ; and so on up. The factor to be used can always be found by 

 dividing the cubical contents by 150. 



It is interesting to know that the residue left in the vessel after the 

 action is completed is a valuable fertilizer, and should not be wasted. 

 It should either be placed at some depth in the manure or compost heap 

 or buried near the base of some tree or shrub. At any event, do not 

 leave it lying around, as it is both acid and poisonous. 



