648 AGRICULTURAL ANALYSIS 



timeters. Place a portion of the distillate in a 40 centimeter 

 tube and take a polariscopic reading, calculate the result on the 

 value of one degree on the sugar scale as previously established 

 from chemically pure nicotin. From this calculate the percen- 

 tage of nicotin in the weight of sample originally taken. 



Method II for nicotin can only be used by those who have had 

 long enough experience with the polariscope to make an exceed- 

 ingly close reading. 



542. Potassium Cyanid. Potassium cyanid is not used by it- 

 self, nor in solution in water for spraying purposes. It is used 

 however as a reagent in preparing hydrocyanic acid gas, which 

 in its turn is of value in destroying scale insects, controlling in- 

 sect pests in green houses and cold frames, destroying insects 

 and vermin in homes, etc. In examining this compound chem- 

 ically it is the amount of cyanogen present that is of interest 

 to the entomologist. A very accurate method of determining 

 this constituent is given below. 



Cyanogen. Solutions Required. Prepare a twentieth-nor- 

 mal solution of silver nitrate. 



Determination. Weigh a large quantity of the sample from a 

 weighing bottle, dissolve in water and make up to a definite 

 volume. To an aliquot add twentieth-normal silver nitrate, a 

 drop at a time, with constant stirring, until one drop produces a 

 permanent turbidity. In calculating the results, one equivalent 

 of silver is equal to two equivalents of cyanogen, according to 

 the following equation: 



2KCN+AgNO 3 =KCN.AgCN-fKNO 3 . 



543. Carbon Disulfid. The vapors of carbon disulfid are often 

 used to destroy such insects as plant lice on melons and squash 

 vines, insects in granaries, and insects and vermin in homes. 

 Special chemical methods for examining the purity of this com- 

 pound have not been approved. 



544. Insecticides for Subterranean Insects. Insecticides for 

 this purpose must either be of such a character that they will dis- 

 solve in water and be carried down to the insects beneath the 

 ground, or must be volatile, so that they will smother the insects. 



80 Bureau of Chemistry, Bulletin 107, 1907 : 30. 



