120 JOURNAL OF ECONOMIC ENTOMOLOGY [Vol. 12 



methods. Furthermore, the gas from hquid hydrocyanic acid is cooled 

 almost to the freezing point on formation and consequently is decidedly 

 heavier than the hot machine or pot-generated product (temperature 

 of pot-generated gas is 100° to 153° in accordance with dosage).^ 



Since the molecular activitiy of gases increases proportionately as 

 the temperature, it is evident that we have a very differently acting 

 product in the new than in the old practice. The molecular activity of 

 gas from liquid hydrocyanic acid is least when first generated but 

 increases as it. attains the temperature of the air, while in pot or ma- 

 chine generated gas quite the inverse is true, the initial molecular 

 velocity of these being the greatest and decreasing as the gas cools to 

 the lower air temperature. Thus it is evident that initial diffusion 

 would be slower in the case of liquid hydrocyanic acid, and attained 

 throughout the bottom of the tent sooner than the top. Differences in 

 diffusion and molecular activity would signify a difference in tent 

 leakage between the generated and atomized gas and that such takes 

 place under field condition is readily detectable by careful observation. 



The destruction of scale pests on citrus trees is proportional to gas 

 concentration and hydrocyanic acid gas being lighter than air, it is 

 natural to presume that the greatest density, signifying the best scale- 

 kill, is toward the top of the tent. This presumption has been sup- 

 ported by the observation of practical fumigators and the experiments 

 of investigators, of whom one of the first to give definite proof in sup- 

 port of this condition was MorrilP in 1908, working in Florida with the 

 citrus white fly, Dialeurodes citri. More recently Quayle^ conducted 

 tests under tent forms with the Bean Weevil (Acanthoscelides ohtectus) 

 and the Granary Weevil {Calandra granaria) arriving at the same gen- 

 eral conclusion, that the highest mortality is toward the top of the 

 tent. Quayle, however, concluded that the percentage killed in the 

 case of a tall tree is better toward the center than the top, a conclusion 

 not in keeping with the data presented. 



The writer has been conducting an investigation of the use of liquid 

 hydrocyanic acid with special reference to dosage requirements and 

 during the past season has closely followed the fumigation of more 

 than 500 acres of citrus trees. It was felt that the difference in physi- 

 cal properties between gas obtained from liquid and field-generated 

 hydrocyanic acid is sufficient to demand a careful revision of the dosage 

 schedule originally prepared for pot-generated gas, but subsequently 

 also adopted for machine and liquid hydrocyanic acid fumigation. 



Accordingly gas diffusion was one of the first problems taken up, 



1 U. S. Dept. Agr. Bur. Ent. Bull. 79, p. 37. 



2 U. S. Dept. Agr. Bur. Ent. Bull. 76, p. 51. 

 »Jr. Econ. Ent. 11, 3, 1918. 



