286 INSECTICIDES AND FUNGICIDES 



The soluble copper compound called cuprammonium hydrox- 

 ide is written graphically, 



/NH 3 — NH 3 — OH 



Cu< 



\NH3-NH3-OH 



and is the same thing as Schweitzer's reagent used for 

 dissolving cellulose (Section 58). The gypsum is left as an 

 insoluble residue. So far as adulterations go, however, the 

 Insecticide Act of 1910 protects the farmer by limiting the 

 minimum amount of arsenous oxide to 50 per cent. 



II. EXTERNAL INSECTICIDES 



(a) Hydrocyanic Acid Gas is a very violent poison to 

 man as well as to insects. The acid at ordinary tempera- 

 tures is a mobile, volatile liquid, with an odor of bitter 

 almonds and a boiling point of 26.5° C. It kills insects by 

 entering their breathing apparatus and putting a stop to 

 their vital functions. As used for fumigation it is prepared 

 by treating sulphuric acid with potassium cyanide. The 

 reaction is: 



KCN + H2SO4 = KHSO4 + HCN. 



Potassium sulphate, K 2 S0 4 , is not formed with the excess 

 of sulphuric acid necessary to get the maximum evolution 

 of gas. It is necessary to use enough water to hold the acid 

 potassium sulphate in solution after the reaction, especially 

 in the generators used for the purpose of producing the gas 

 on a large scale. The best proportion of cyanide, acid, and 

 water is a 1-1-3 formula, or 1 part potassium cyanide to 1 

 part of concentrated sulphuric acid (commercial concentrated 

 acid is about 93 per cent, pure) and 3 parts of water. Much 

 stronger sulphuric acid results in the decomposition of 

 hydrocyanic acid and the formation of ammonia and formic 

 acid or carbon monoxide depending on the strength of the 

 sulphuric acid. The ammonia forms ammonium sulphate 

 with the acid. 



