DETECTION OF ACIDS. 133 



drop of amnionic sulphide, saturated with sulphur, 

 in its centre, over the first one ; after a few min- 

 utes evaporate this drop of amnionic sulphide to 

 dryness wath a gentle heat, and moisten the dry- 

 residue with ferric chloride. A deep red color 

 appears. Cyanogen. 13 



13. To a portion of the aqueous solution or extract of 

 the substance, add two or three drops of potassic 

 dichromatc, or enough to give a pale yellow color 

 to the liquid, and then a few drops of concentrat- 

 ed hydrochloric acid. A drop of this mixture on 

 starch-paper colors it blue. Iodine. - - - - 14 



14. A portion of the first solution gives, ^yhh ferric 

 chloride, a deep blue precipitate, unaffected by di- 

 lute acids, but decomposed by sodic hydrate with 

 the conversion of the blue color into a reddish 

 one. Feekocyanogen. 15 



15. a. No very decided reaction was obtained in 12 



for cyanogen. 16 



5. A decided reaction v^as obtained for cyanogen. 

 To the dilute nitric acid extract of the substance, 



or the aqueous extract acidified with nitric acid, 

 add argentic nitrate as long as a precipitate is 

 formed, filter, wash the precipitate a little, dry it, 

 and ignite it gently in a porcelain crucible until it 

 is fused, pour a little water and a few drops of di- 

 lute sulphuric acid over the fused mass after it is 

 cool, put a piece of zinc in contact with it, and let 

 the whole stand some time ; finally filter, acidify 

 the filtrate with nitric acid, and add argentic ni- 

 trate. 



a. No precipitate. - 17 



1). A Avhite precipitate is formed ; no iodine or 

 ferrocyanogen has been found. Chlorine. - - IT 

 c. A precipitate is foi-med, but it is not white, 



