(4.) The reaction usually called Schönbein’s, or Pagenstecher and Schönbein’s[252] (but long known,[253] and used before the publication of their paper), consists of guaiacum paper, moistened with a very dilute solution of sulphate of copper (1 : 2000). This becomes blue if exposed to the vapour of hydrocyanic acid. Unfortunately, the same reaction is produced by ammonia, ozone, nitric acid, hypochlorous acid, iodine, bromine, chromate of potash, and other oxidising agents, so that its usefulness is greatly restricted. [252] _Neues Repert. de Pharm._, 18, 356. [253] This reaction (with tincture of guaiacum and copper) has been long known. “I remember a pharmaceutist, who attended my father’s laboratory, showing me this test in 1828 or 1829.”--Mohr’s _Toxicologie_, p. 92. (5.) A very delicate test for prussic acid is as follows:--About one-half centigrm. of ammonia, ferrous sulphate (or other pure ferrous salt), and the same quantity of uranic nitrate, are dissolved in 50 c.c. of water, and 1 c.c. of this test-liquid is placed in a porcelain dish. On now adding a drop of a liquid containing the smallest quantity of prussic acid, a grey-purple colour, or a distinct purple precipitate is produced.[254] [254] M. Carey Lea, _Amer. Journ. of Science_ [3], ix. pp. 121-123; _J. C. Society_, 1876, vol. i. p. 112. (6.) A hot solution of potassic cyanide, mixed with picric acid, assumes a blood-red colour, due to the formation of picro-cyanic acid. Free HCN does not give this reaction, and therefore must first be neutralised by an alkali. (7.) =Schönbein’s Test.=--To a few drops of defibrinated ox-blood are added a few drops of the carefully-neutralised distillate supposed to contain prussic acid, and then a little neutral peroxide of hydrogen is added. If the distillate contains no prussic acid, then the mixture becomes of a bright pure red and froths strongly; if, on the other hand, a trace of prussic acid be present, the liquid becomes brown and does not froth, or only slightly does so. (8.) =Kobert’s Test.=--A 1-4 per cent. solution of blood, to which a trace of ferridcyanide of potassium is added, is prepared, and the neutralised distillate added to this solution. If hydric cyanide be present, then the liquid becomes of a bright red colour, and, examined spectroscopically, instead of the spectrum of methæmoglobin, will be seen the spectrum of cyanmethæmoglobin. Kobert proposes to examine the blood of the poisoned, for the purpose of diagnosis, during life. A drop of blood from a healthy person, and a drop of blood from the patient, are examined side by side, according to the process just given.