38 MR JOHNSTON ON PARACYANOGEN 



rally in slight excess. Tliat paracyanic acid is formed is proved by precipitating 

 the broAvn solution with water, filtering the acid Uquid, and adding a solution of 

 nitrate of mercury, when a yellow precipitate falls more or less copiously of para- 

 cyanate of mercury. 



If the solution in sulphuric acid be boiled for a sufficient length of time, it be- 

 comes colourless, and water gives no longer any precipitate ; the paracyanogen, 

 therefore, is wholly decomposed. 



The black residue obtained by decomposing the bicyanide can rarely be freed 

 entirely from the minute globules of mercury which adhere to it. If mercury be 

 present in sufficient quantity when the paracyanogen is treated with sulphuric 

 acid, the entire solution on cooling congeals into a mass of minute transparent 

 crystalhne needles of a brown colour, which are possibly a compound of the sul- 

 phates of oxide of mercury and of paracyanogen. They are decomposed by water, 

 and have hitherto baflled my attempts to obtain them free from the great excess 

 of concentrated sulphuric acid. 



The action of this acid on paracyanogen seems to indicate, that the black 

 substance is possessed of basic properties, and that it may be in some measure 

 analogous to the carbo-hydrogens. 



IV. — Action of Nitric Acid on Paracyanogen — Paracyanic Acid. 



1. Paracyanogen, after heating to redness, as it is obtained from bicyanide 

 of mercury, is very slightly acted upon by nitric acid. Boiled in this acid, a small 

 portion is dissolved, and if the boiling be long continued, the insoluble matter gra- 

 dually assumes a lighter colour, and ultimately becomes of an orange yellow. 

 The acid solution, dQuted largely with water, becomes turbid, and deposits a yel- 

 low precipitate in small quantity. 



2. When the black deposit formed in an alcohohc solution of cyanogen, in 

 strong prussic acid, &c. is treated with nitric acid in the cold, it dissolves slowly 

 without any sensible evolution of gas, giving a dark reddish brown solution, from 

 which water again precipitates it apparently unchanged. If, however, heat be ap- 

 plied to the solution, if a current of sulphuretted hydrogen be passed through it, or 

 more rapidly and certainly, if the black matter be gradually added to the hot acid, 

 as each successive portion dissolves, and the dark colour caused by it disappears, 

 copious red fumes are evolved, and a transparent reddish yellow solution is ob- 

 tained. From this solution water precipitates a bulky bright yellow powder, a 

 further portion falls on saturation with an alkali, and a third, though much smaller 

 quantity, by evaporating to dryness the supernatant yellow solution and washing 

 the residue. 



3. If the dark brown solution of paracyanogen in sulphuric acid be heated, 

 and nitric acid gradually added, red fumes are evolved, and a yellow precipitate 



