402 PRINCIPLES OF CHEMISTRY 



quarter part 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 con- 

 tains only hydrogen, carbon, and nitrogen, CNH. If the distillate be 

 redistilled (a weak solution of HCN), and the first part be collected, 

 the anhydrous acid may be prepared from this stronger solution. In 

 order to do this, pieces of calcium chloride are added to the concentrated 

 solution, and 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. On this account the greatest caution is 

 necessary in dealing with it, as an insignificant amount, when inhaled 

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

 all proportions in water, alcohol, and ether ; weak aqueous solutions 

 are used in medicine. 45 



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

 prussiate with sulphuric acid, may be directly passed 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 obtain- 

 ing pure hydrocyanic acid consisted in the action of hydrochloric acid gas 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, then 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 hydrocyanic acid will be con- 

 densed. In order to obtain the hydrocyanic acid in an anhydrous form, the decomposition 

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

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

 Hg(CN) 2 + H 2 S = 2HCN + HgS. 



45 A weak (up to 2 p.c.) aqueous solution of hydrocyanic acid is obtained by the dis- 



