CYANOGEN AND ITS COMPOUNDS. 339 



potassium ferrocyanide, the change is actually more complete, 

 as the atoms arrange themselves differently, the iron passing 

 also from the ferrous to the ferric state. 



Potassium ferricyanide crystallizes in red prisms, soluble in 

 water. It forms, with ferrous solutions, a blue precipitate of 

 ferricyanide of iron or Turnbull's blue: 



K 6 Fe 2 Cy 12 + 3(FeSO 4 ) = 3(K 2 SO 4 ) + Fe 3 Fe 2 Cy 12 . 



With ferric solutions no precipitate is produced by potassium 

 ferricyanide, but the color is changed to a dark olive green. 



Nitrocyanmethane, CH 2 .CN.N0 2 (Falminic acid). This substance 

 may be looked upon as a derivative of methane, CH 4 , in which 

 two atoms of hydrogen are replaced by cyanogen and NX) 2 

 respectively. It is not known in the separate state, but its 

 combinations with metals are well known, especially mercuric 

 fulminate, which is manufactured and used as an explosive in 

 percussion caps, etc. It is made by adding alcohol to a solution 

 of mercury in nitric acid. Silver fulminate can be obtained by 

 a similar process. 



. QUESTIONS. 



471. What is cyanogen, what is dicyanogen, and how is the latter 

 obtained ? 



472. How does cyanogen occur in nature, and which non-metallic 

 elements does it resemble in the constitution of various compounds ? 



473. State the composition and properties of hydrocyanic acid. 



474. Mention some reactions by which hydrocyanic acid is formed, 

 and state the two processes by which the officinal diluted acid is ob- 

 tained. 



475. What strength and what properties has the diluted hydrocyanic 

 acid? 



476. State the composition of pure potassium cyanide and of the 

 commercial article. How is the latter made? 



477. Give reactions for hydrocyanic acid and cyanides. 



478. Explain the constitution and give the composition of ferro- and 

 ferricyanides. 



479. Give composition, mode of manufacture, and tests of potassium 

 ferrocyanide. 



480. What is red prussiate of potash, how is it obtained, and by 

 what reaction can it be distinguished from the yellow prussiate ? 



