April, '08] JOURNAL OF ECONOMIC ENTOMOLOGY 111 



As we had no basis from which to compute quantities of some of 

 these gases, guesswork had to be employed, but we sought to kill the 

 scale, and expected that many of the trees would be injured. I am 

 now convinced that further tests, especially with carbon tetrachloride, 

 in smaller quantities, are desirable, and possibly it may prove to have 

 some value as an insecticide. 



Volatile Liquids 



Carbon disulphide. — Preliminary tests taught us that there is more 

 or less trouble in volatilizing the liquid rapidly at ordinary temper- 

 atures. Where a large quantity is used, the period necessary for vol- 

 atilization is so long that the roots may dry out and the trees are in- 

 jured. Cast iron stew pans were adopted as generating dishes, and 

 after heating to nearly 200° F., one was placed near each end of the 

 box above the trees. A small hole through the cover of the box en- 

 abled us to pour the liquid through a funnel into the heated dish. 

 The holes were stopped with corks and the carbon disulphide aU vol- 

 atilized in a few seconds or minutes, according to the quantity, while 

 without the heated dishes several hours were necessary in some cases. 

 Quantities of the liquid varying from ten to eighty fluid ounces per 

 one hundred cubic feet of space were given periods of time varying 

 from one to four hours. At the rate of ten ounces per one hundred 

 cubic feet for one hour, 4.3 per cent of the scales survived, and one 

 tree died. Similar proportions for the same time at 59° F. (the dish 

 not being heated) gave a surviving percentage of 19.2 per cent. Aside 

 from these all scales were killed, and no trees injured until given a 

 period of three hours with a quantity of carbon disulphide equivalent 

 to sixty ounces. Above this about half of the trees failed to grow. 



Carbon tetrachloride. — The quantities used of this liquid varied 

 from one to eight ounces, and the fumigating period from two to six 

 hours. All scales were killed, and no trees were injured, where thirty 

 ounces per one hundred cubic feet or less of the liquid was used, with 

 a fumigating period of two hours. Greater quantities of the liquid 

 caused injury, and killed some of the trees. This liquid was also 

 volatilized by means of heated pans. It is non-inflammable, and is 

 not very poisonous to the higher forms of animal life. 



Gases Generated by Chemical Action 



Hydrocyanic acid gas. — This gas was generated in the usual way 

 with one ounce potassium cyanide, two ounces sulphuric acid and four 

 ounces water for each one hundred cubic feet of space. In each case 

 all scales were killed. In one case where these qauntities were used 



