COMPOUNDS OF CARBON WITH OXYGEN AND NITROGEN 41 B 



factured on a large scale, and is generally used as the source of the 

 other cyanogen compounds. 



If four parts of yellow prussiate. be mixed with eight parts of water 

 and three parts of sulphuric acid, and the mixture, be heated, it decom- 

 poses, volatile hydrocyanic acid separating. This was obtained for the 

 first time by Scheele in 1782, but it was only known to him in solution. 

 In 1809 Ittner prepared anhydrous prussic acid, and in 1815 Gay- 

 Lussac finally settled its properties and showed that it contains only 

 hydrogen, carbon, and nitrogen, CNH. If the distillate (a weak solu- 

 tion of HCN) be redistilled, and the first part collected, the^ anhy- 

 drous acid may be prepared from this stronger solution, In order to do 

 this, pieces of calcium chloride are added to the concentrated solution, 

 when the anhydrous acid floats as a separate layer, because it is not 

 soluble in an aqueous solution of calcium chloride. If this layer -be 

 then distilled over a new portion of calcium chloride at -"the lowest 

 temperature possible, the prussic acid may be obtained completely free 

 from water. It is, however, necessary to use the greatest caution in 

 work of this kind, because prussic acid, besides being extremely 

 poisonous, is exceedingly volatile. 44 



Anhydrous prussic acid is a very mobile and volatile liquid ; its 

 specific gravity is 0'697 at 18 ; at lower temperatures, especially when 

 mixed with a small quantity of water, it easily congeals ; it boils at 26\ 

 and therefore very easily evaporates, and at ordinary temperatures 

 may be regarded as a gas. An insignificant amount, when inhaled or 

 brought into contact with the skin, causes death. It is soluble in all 



44 The mixture of the vapours of water and hydrocyanic acid, evolved on heating yellow 

 prussiate with sulphuric acid, may be passed directly through vessels or tubes filled with, 

 calcium chloride. These tubes must be cooled, because, in the first place, hydrocyanic 

 acid easily changes on being heated, and, in the second place, the calcium chloride when 

 warm would absorb less water. The mixture of hydrocyanic acid and aqueous vapour 

 on passing over a long layer of calcium chloride gives up water, and hydrocyanic acid 

 alone remains in the vapour. It ought to be cooled as carefully as possible in order to- 

 bring it into a liquid condition. The method which Gay-Lussac employed for obtaining- 

 pure hydrocyanic acid consisted in the action of hydrochloric acid gaa on mercuric 

 cyanide. The latter may be obtained in a pure state if a solution of yellow prussiate be 

 boiled with a solution of mercuric nitrate, filtered, and crystallised by cooling; the- 

 mercuric cyanide is then obtained in the form- of colourless crystals, Hg(CN) 2 . 



If a strong solution of hydrochloric acid be poured upon these crystals, and the mix- 

 ture of vapours evolved, consisting of aqueous vapour, hydrochloric acid, and hydrocyanic 

 acid, be passed through a tube containing,- first, marble (for 'absorbing the hydrochloric 

 acid), and then lumps of calcium chloride, - on cooling the hydrocyajnic acid will be con-" 

 densed. In order to obtain the latter in an anhydrous- form, the decomposition 

 of heated mercury cyanide by hydrogen sulphid>may be made use of. Here the sulphur 

 and cyanogen change places, and hydrocyanic acid and mercury sulphide are formed 

 Hg(CN) 3 + HaS = 2HCN + HgS. 



