The Tea insects of India. 67 



of pure potassium cyanide. It is, therefore, of the utmost importance that the 

 operator should know the exact percentage of pure potassium cyanide that his cyanide 

 contains, and when large quantities of it are purchased at one time it would be advis- 

 able to obtain one or more analyses of it by a reliable analytical chemist ; or if it is 

 not possible to submit the cyanide to such person, an analysis of it could be made by 

 almost any person accustomed to the use of chemicals or drugs, 



" The only substance required for this purpose is the crystals of nitrate oE silver 

 (AgNOj), which may be obtained at almost any well-stocked drug-store. Dissolve 

 the nitrate in cold water contained iu a glass or earthen vessel, using one-fourtli of 

 an ounce (Troy) of the crystals to one pint of water ; this dissolves in a few minutes, 

 forming a whitish, semi-transparent solution. The cyanide, when dissolved in water, 

 forms a transparent, nearly colourless solution ; when a small quantity of the nitrate 

 of silver solution is added to this it at first spreads out in a white cloud, like milk, 

 but it soon breaks up into small, white, ftoccy pieces which gradually disappear upon 

 being agitated, leaving the solution nearly as transparent as at first ; when more of 

 the nitrate of silver solution is added from time to time, the above process is repeated 

 except toward the last, when the cyanide solution becomes somewhat milky, but it 

 still remains semi-transparent, permitting the operator to see quite clearly the bottom 

 of the vessel containing the solution. As soon as a sufficient quantity of the nitrate 

 of silver solution has been added to the cyanide eolation, the latter immediately 

 becomes white and opaque, like milk, completely concealing from view the bottom 

 of the vessel containing it. This completes the operation, and the quantity of nitrate 

 of silver solution used will indicate the strength of the cyanide tested. When abso- 

 lutely pure, 5 1 grains of thS' potassium cyanide dissolved in water will require one 

 fluid ounce of the above nitrate of silver solution before the turbidity occurs, indicating 

 that the cyanide is 100 per cent, strong ; if only one-half of a fluid ounce of the 

 nitrate of silver solution produces this turbidity, this indicates that the cyanide is only 

 half strength, or 50 per cent, strong ; if only one- fourth of a fluid ounce is required, 

 then the cyauide is 25 per cent, strong ; and so forth. The nitrate of silver solution 

 -should be added to the cyanide solution very slowly, the latter being agitated by 

 gently shaking it each time that any of the nitrate solution is added. Wherever any 

 of the nitrate of silver solution comes into contact with the skin or nails of the hand 

 it produces a reddish or black stain, which can easily be removed by washing the 

 stained part in a solution of cyanide and water ; this will quickly remove the staia 

 without causing any injury to the parts afiFected, except, of course, when the stains 

 occur upon a sore or cut in the hand, in which case it would be dangerous to apply 

 the cyanide to these places. 



" It sometimes happens that the percentage of syanogen (CN or Cy) is given instead 

 of the percentage of potassium cyanide (KCN or KCy) ; but in cases of this kind 

 the percentage of cyanide can be readily ascertained by always bearing in mind that 

 two-fifths of a given quantity of potassium cyanide is cyanogen. Thus, if a certain 

 brand of cyanide contains 24 joer cent, of cyanogen, this is equivalent to 60 per cent, 

 of pure potassium cyanide. Potassium cyanide when absolutely pure (equal to 100 

 per cent.) contains 40 per cent, of cyanogen ; and, therefore, no grade of cyanide 

 could contain a larger percentage of cyanogen than this. 



" The potassium cyanide used for producing the hydrocyanic acid gas is princi- 

 pally manufactured by two firms : Power and Weightman, of Philadelphia, Pa., and 

 the Mallinkrodt Chemical Works, of St. Louis, Mo, That made by the first-named 

 firm is the most largely used ; when purchased by the ton the price is 36 cents per 

 pound for the grade containing about 57 per cent, of pure potassium cyanide, packages 

 and carriage extra. It is put up in tin cans holding 10 pounds each, and also in 



