988 CYANOGEN COMPOUNDS. 



the four contained in 2 equivalents of the double salt ; and the 

 residuary insoluble double cyanide is 2FeCy + KCy. 



The proportion of hydrocyanic acid in the acid prepared by 

 the preceding process may be ascertained by accurately weighing 

 a portion of it, amounting to about 100 grains ; adding to this 

 portion nitrate of silver in excess, collecting the white insoluble 

 precipitate of cyanide of silver which falls on a weighed filter, 

 drying and weighing together precipitate and filter. Five parts 

 of the precipitate correspond to 1 part of hydrocyanic acid. Or 

 red oxide of mercury may be used to test the strength of the 

 aqueous acid. A drop or two of caustic potash is added to a 

 weighed quantity of the dilute acid, and then a known weight of 

 the red oxide of mercury in fine powder is agitated with it. 

 The metallic oxide dissolves readily as cyanide of mercury, 

 every 4 parts of oxide dissolved corresponding with i of anhy- 

 drous acid. The hydrocyanic acid may then be reduced to any 

 degree of strength desired, by the addition of water. 



The dilute acid, when absolutely pure, soon decomposes, 

 becoming brown and then black, but has stability imparted to it 

 by the smallest trace of sulphuric acid; an addition which 

 should be made when it is intended to preserve hydrocyanic 

 acid for medical use. The solution has the taste, odour and 

 poisonous qualities of the anhydrous acid. 



Hydrocyanic acid may be detected by its odour, or by adding 

 a few drops of sulphuric acid to the liquid containing it, and 

 covering the vessel with a glass plate having its lower surface 

 moistened by a solution of nitrate of silver. The hydrocyanic 

 acid rises from its volatility, and produces a white precipitate in 

 the nitrate of silver. 



But the most delicate and satisfactory indication of hydro- 

 cyanic acid is the production of Prussian blue from it, by a 

 process which is known as Scheele's test. If the liquid to be 

 examined contains much organic matter, as the contents of the 

 stomach in a case of poisoning, it is mixed with about one 

 sixth of its bulk of oil of vitriol and distilled. The test is then 

 applied to the distilled liquid. 



1. A few drops of protosulphate of iron are added to the 

 liquid to be tested, with a slight excess of caustic potash so as 

 to precipitate the oxide of iron. 



2, The alkaline liquid, after a few minutes' exposure to air, 



