NITRIC ACID.
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and one of strong sulphuric acid, or about an equivalent of each of theingredients. As the water of the sulphuric acid is not sufficient in quan-j tity to form liquid nitric acid with all the acid set free from the nitrate,a part of the nitric acid is decomposed into nitrous acid and oxygen. Itis the presence of nitrous acid which communicates the red colour to thecommercial or fuming nitric acid.
The generation of nitrous acid is greatest at the commencement andtowards the close of the operation : for at the commencement the excessof uncombined sulphuric acid attracts water from the small quantity ofnitric acid then set free, in consequence of which the latter is resolvedinto nitrous acid and oxygen: about the middle of the process, when thequantity of free nitric acid has increased while that of sulphuric acid hasdiminished, the former passes over with water, unchanged: but towardsthe end of the process, owing to the volatilization of the nitric acid, thesulphuric acid becomes again predominant, and the red vapours of nitrousacid then make their appearance.
Properties. — Strong liquid nitric acid is colourless, and has apeculiar odour, and an acrid, intensely sour taste. In the air it evolveswhite fumes, formed by the union of the acid vapour with the aqueousvapour of the atmosphere ; these fumes redden litmus, and become muchwhiter when mixed with the vapour of ammonia, owing to the formationof the nitrate of ammonia. The sp. gr. of the acid prepared according tothe Pharmacopoeia is T5033 to 1'504, and Mr. Phillips believes this tobe the strongest procurable ; but Proust obtained it as high as 1'62,Kirwan T554, Davy 1'55, Gay-Lussac P510, Thenard 1'513. The acidsold in the shops as double aquafortis has a sp. gr. of P36. Singleaquafortis is of sp. gr. 1-22. It has a powerful affinity for water, and,when mixed with it, heat is evolved.
Nitric acid is easily deprived of part of its oxygen. Thus exposure tosolar light causes the evolution of oxygen and the production of nitrousacid, which gives the liquid a yellow, orange, or reddish brown colour.The acid thus coloured may be rendered colourless by the appli-cation of a gentle heat, to drive off' the nitrous acid. Several of thenon-metallic combustibles rapidly decompose nitric acid, as charcoal,phosphorus, sugar, alcohol, volatile oils, resins, &c. The acid is unactedon by leaf gold, platinum, &c. Some of the metals also act powerfullyon it, as copper (in the form of turnings) and tin (in the state of foil). Alittle water added to the acid facilitates, in some cases, the action of me-tals on it. For an account of the anomalous relations of this acid andiron, I must refer the reader to Becquerel’s Trait'e de VElectricite, tom. v.p. 8. The liydracids (as hydrochloric acid) decompose and are decom-posed by nitric acid. -
Characteristics. —Nitric acid is known by the following characters:it stains the cuticle yellow or orange : mixed with copper tilings, effer-vescence takes place, owing to the escape of binoxide of nitrogen, and agreenish blue solution of nitrate of copper is obtained: the binoxideforms ruddy vapours in the air, by uniting with oxygen to foim nitrousacid gas ; and passed into a solution of the protosulphate of iron, formsa dark olive-brown coloured liquid, which has a strong affinity foroxygen,and has in consequence been employed for analysing atmospheric air :but the results obtained by it are uncertain. Morphia, brucia, or com-mercial strychnia, communicates a red colour to nitric acid, which is