EXTERNAL INSECTICIDES 287 



The use of sodium cyanide is preferable in some ways, since 

 more hydrocyanic acid gas can be liberated from a pound 

 of sodium cyanide than from a pound of potassium cyanide, 

 due to the lower atomic weight of sodium. 100 parts of 

 sodium cyanide is equal to 132 parts of potassium cyanide 

 in theoretical effectiveness. The reaction is similar, but 

 the sodium acid sulphate is more soluble than the corre- 

 sponding potassium salt and a 3-4-6 formula is recommended. 



The trouble which is sometimes experienced in obtaining 

 good results from the use of potassium cyanide is due in 

 part to the presence of sodium chloride. Sulphuric acid 

 sets free hydrochloric acid, and the latter forms ammonia, 

 later ammonium chloride, and formic acid. This reaction 

 of course reduces the vield of hydrocyanic acid gas. It is 

 not sufficient that the potassium cyanide have a guarantee 

 of 98 per cent, pure, for a mixture of potassium and sodium 

 cyanides would show 98 per cent, expressed as potassium 

 cyanide and yet have considerably more than 2 per cent, of 

 sodium chloride present. If possible the amount of sodium 

 chloride present should be known, and it is recommended 

 not to have more than 1 per cent. 



Since the use of hydrocyanic acid gas is attended with 

 great danger to the operator, extreme pains should be taken 

 by him not to inhale the gas himself, or generate it where 

 others may run any risk. It is altogether too dangerous to 

 handle with impunity, and too much carelessness is displayed 

 in its use. 



(b) Kerosene Emulsion. — Kerosene, or coal oil, as it is 

 sometimes called, is an excellent contact insecticide, killing 

 by entering the pores of the insect. Used in the pure state, 

 however, it is apt to harm vegetation, and is not much 

 employed. It is one of the fractional distillation products 

 from crude petroleum and consists of a mixture of paraffine 

 or methane hydrocarbons whose boiling points lie between 

 150° and 300° C. Kerosene is insoluble in water and hence 

 cannot easily be diluted, -although attempts have been made 

 to agitate the two together so as to obtain a mechanical 

 mixture suitable for application to trees, but the mixture 

 separates too rapidly to be satisfactory. By thoroughly 



