AND THE PARACYANIC ACID, 41 



V. — Properties of the Paracyanates. 



1. In powder they are all of a pale yellow or orange yellow colour, the salt 

 of oxide of silver crystallizes in bright red crystals. 



2. The salts of the metallic oxides are insoluble or sparingly so in water. They 

 dissolve readily when heated in concentrated acids, and are in great part preci- 

 pitated without apparent change by cooling, or by copious dilution with water. 

 Acidulated solutions dissolve them more sparingly, and, in all cases, they dissolve 

 more largely in hot than in cold solutions. 



3. The alkaline salts are soluble in water, and the acid is precipitated in yel- 

 low flocks on the addition of a stronger acid. The metallic salts, if decomposed, are 

 so with difficulty, and only by protracted boiling in solutions of the hydrates and 

 carbonates of the fixed alkalies, or in liquid ammonia. They are partially soluble 

 in such menstrua, giving yellow solutions from which they are in great measure 

 deposited on cooling. By such action the salt of mercury becomes of a dark 

 grey or chocolate colour. 



4. Heated in close vessels they give off carbonic acid and nitrogen gases, 

 leaving a black mass, which, at a full red heat, is generally resolved more or less 

 completely into cyanogen gas. If the salt is moist, instead of carbonic acid and 

 nitrogen, it gives carbonate of ammonia, 



5. Heated in the air they become brown between 800° and 400° Fahr. 

 Above that temperature they blacken and give off white fumes of carbonate of 

 ammonia, which are more or less dense according to the hygrometric state of the 

 atmosphere. At a duU red heat they burn like tinder, leaving only the metal, 

 or if easily oxidizable its oxide. 



VI. — Paracyanate of Mercury. 



1. The paracyanic acid, as already stated, has a strong affinity for the oxides 

 of mercm-y. A solution of nitrate of protoxide of mercury, though largely di- 

 luted, causes an immediate precipitate in solutions of the paracyanic acid, or of 

 any of its soluble salts. It forms, in fact, an excellent test for this acid, where 

 there is reason to suspect its presence. 



2. The precipitate is of a pale, rarely of a bright yellow colour, and nearly 

 insoluble in water. When thrown down from solutions of the acid in strong 

 nitric or sulphuric acids, the yellow colour is generally more decided. 



3. There appear to be several paracyanates of the oxides of mercury, not to 

 be distinguished by their external characters, yet differing in composition as the 

 basic and neutral salts of these oxides with the nitric acid do. Hence it requires 

 great precautions and the use of carefully prepared nitrates, to insure precipitates 

 of uniform composition. 



VOL. XIV. PART I. F 



