41 



SUPPLEMENTARY REPORT ON THE GAS TREATMENT FOR SCALE 



INSECTS. 



By D. W. COQUILLETT. 



Since writing my ''Keport on the gas treatment for scale insects," 

 which forms part of Professor Riley's annual report, published in the 

 Report of the Department of Agriculture for the year 1887 (pp. 123-142), 

 I have made a number of observations on this treatment, the more im- 

 portant of which are recorded below. 



There is a great difference in the relative strengths of the best grade 

 of the various brands of potassium cyanide. In this city (Los Angeles) 

 the two brands most commonly offered for sale are the Powers & 

 Weightman, manufactured at Philadelphia, Pa., and the Malline- 

 krodt, manufactured at Saint Louis, Mo. A careful analysis of each 

 of these brands by Prof. E. M. Wade, an analytical chemist of this city, 

 showed the Powers & Weightman cyanide to contain fully 99 per 

 cent, of pure potassium cyanide (KON), while the Mallinekrodt con- 

 tained only a fraction over 93 per cent. ; and several experiments which 

 I have made with each of these brands fully confirm the correctness of 

 the above analyses in favor of the Powers & Weightman cyanide. 



Either of these brands of cyanide will dissolve in a few hours in cold 

 water, only requiring to be frequently stirred. When thus dissolved, 

 the solution does not emit the disagreeable odor of ammonia, which is 

 emitted by the solution made by boiling. The solution made without 

 heat is evidently stronger than the other, since heat decomposes the 

 cvanide; and several tests which I have made with the cold solution 

 indicate that it is stronger than when made by boiling. Moreover, the 

 cold solution is more liable to be of a uniform strength, since in the 

 other the boiling water will take up more of the cyanide than it can 

 retain, and upon cooling will deposit the excess in the bottom of the 

 vessel. 



The Powers & Weightman cyanide will dissolve in about half the 

 time required by the Mallinekrodt. The latter solution, after standing 

 a few hours, assumes a reddish-brown color, but the solution of the 

 Powers & Weightman cyanide does not change color, remaining of a 

 light, olive-gray color for an indefinite length of time. 



The proportions are as given in my report, namely, 5 pounds of the 

 cyanide to 1 gallon of water. When dissolved, the solution will measure 

 about 1G8 fluid ounces, each pound of the cyanide having added 8 fluid 

 ounces to the solution. Two fluid ounces of the solution will contain 

 about 1 ounce by weight of the cyanide, and will require 1 fluid ounce 

 of sulphuric acid (commercial) to evolve the gas from them. One ounce 

 by weight of the dry cyanide will require about 2^ fluid ounces of sul- 

 phuric acid to evolve the gas from it. 



