358 journal of economic entomology [vol. 12 



Comparative Effectiveness of Pot-generated Gas and Liquid 



Hydrocyanic Acid 



The purity of sodium cyanide used in California since its introduc- 

 tion for fumigation by the writer in 1909 has averaged about 97 per 

 cent, according to analyses made of representative samples from time 

 to time. In pot-generation not all of the cyanogen is given off as 

 gas but many analyses made by the Bureau of Chemistry of the United 

 States Department of Agriculture have shown that 90 to 95 per cent 

 (average 93 per cent) of the total available hydrocyanic acid is evolved. 

 A 93 per cent gas evolution from a 97 per cent sodium cyanid is equiv- 

 alent to 20.2 cubic centimeters of 100 per cent liquid hydrocyanic 

 acid (60° F., specific gravity .6969), or 20.9 cubic centimeters of 96 

 per cent hydrocyanic acid this last purity being considered a 

 standard for field use. The schedules in common use for fumiga- 

 tion in CaHfornia are based on the gas delivery from sodium 

 cyanid generated in pots; therefore, to deliver a gas equal in amount 

 would require approximately 20.9 cubic centimeters of 96 per cent 

 liquid hydrocyanic acid for each ounce of solid sodium cyanid in 

 any given dosage. It happens that the machine commonly used 

 this past season for applying liquid hydrocyanic acid in field fumiga- 

 tion was graduated to deliver 16.56 cubic centimeters as equivalent 

 to each ounce of solid sodium cyanid, which is approximately 21 per 

 cent below requirements as based on gas delivery in pot-generation. 

 In short, where commercial fumigation in California during the past 

 season was based on the same dosage schedule in liquid as in pot- and 

 machine-generation there was applied approximately 21 per cent less 

 gas to the trees under the former method than the latter. 



It was shown by the writer in a paper presented at the thirty-first 

 annual meeting of this Association that the gas distribution is essen- 

 tially different in pot- or machine-generation from that obtained with 

 liquid hydrocyanic acid under the present method of application. It 

 is a matter of common knowledge that the best scale-kill on trees 

 treated with pot- or machine-generated gas is toward the top of the 

 tent, whereas in the case of trees treated with liquid hydrocyanic acid 

 at the warmer temperatures of fumigation it was demonstrated that 

 the killing is the best toward the bottom of the tree. Since very much 

 the larger proportion of insects, especially the black and purple scales, 

 is toward the bottom of the tree, the very desirable condition exists 

 of the heaviest infestation of insects and the greatest concentration of 

 gas being distributed at the same place. This is the ideal for effective 

 fumigation, and is, very probably, the principle reason for the increased 

 efficiency of liquid hydrocyanic acid over pot-generated gas. 



