XIT THE HALOGENS 255 



showed that it was inflammable and gave carbon dioxide when 

 burnt. 



Berthollet (1787), by acting on prussic acid successively with 

 chlorine and an alkali, oxidised prussic acid to carbonic acid and 

 ammonia, and concluded that it was a compound of hydrogen, 

 carbon, and nitrogen. The action is expressed by the following 

 equations : 



HCN + C1 2 -> CNC1 + HC1 



(Prussic (Cyanogen 



acid.). chloride.) 



CNCl + KOH-f H 2 O->CO 2 + NH 3 + KC1. 



Gay-Lussac (1811), by distilling the mercury salt of the acid 

 with hydrochloric acid, obtained anhydrous prussic acid as a 

 liquid boiling at 26-5 C. and freezing at - 15 C. In 1815, he 

 exploded the vapour with oxygen and found that it gave i vol. 

 carbonic anhydride, and \ vol. nitrogen, and that an additional 

 j vol. of oxygen was used to burn \ vol. of hydrogen to water. 

 The actual figures for 100 vols. of vapour were : 



Contraction ... ... ... = 78*5 vols. 



Contraction by potash ... ... = ici'ovols. 



Nitrogen .. ... ... ... = 46*0 vols. 



Oxygen used ... ... ... about 125 vols. 



These numbers agree approximately with those required by 

 the equation : 



4HCN + 5O 2 -> 4CO 2 + 2N 2 + 2H 2 O Contraction 



4 vols. 5 vols. 4 vols. 2 vols. liquid 3 vols. 



Gay-Lussac was struck by the analogy of prussic acid with 

 hydrochloric and hydriodic acids and suggested that it contained 

 a "compound radical" cyanogen which acted the part of the 

 simple radicals chlorine and iodine. Prussic acid was therefore 

 called hydrocyanic acid and its salts became known as cyanides. 

 Typical formulas are : 



Hydrocyanic acid (compare HC1), HCN 



Potassium cyanide (sol. like KC1), KCN 



Silver cyanide (insol. like AgCl), AgCN 



Mercuric cyanide (compare HgCl 2 ), HgC 2 N 2 . 



These formulas are often written, HCy, KCy, AgCy, HgCy 2 , 

 to bring out their close analogy with HC1, KC1, AgCl, HgCl 2 , 



