"Take of nitrate of potassa dried, and sulphuric acid, each two pounds; mix them in a glass retort; and distil the nitric acid from a sand bath."

Edinburgh.

" Take of nitrous acid any quantity. Put it into a retort, and having fitted a receiver, which must be kept cold, apply a very gentle heat, until the reddest part shall have passed over, and the acid which remains in the retort, already almost free from colour, have become nitric acid."

Dublin.

"Take of nitrate of potassa, one hundred parts; sulphuric acid of the market, ninety-seven parts. Mix in a glass retort, and distil into a receiver fitted with an apparatus for receiving the elastic gas, until the residue in the retort concretes and again liquefies.

" The specific gravity of this acid is, to that of distilled water, as 1.400 to 1.000." 3

1 Annates de Chim. xxvii. p. 200.

2 Acidum nitrosum, P. L. 1787. Spiritus nitri Glauberi. Aquafortis, P. L. 1745. Aquafortis simplex et duplex, P. L. 1720.

3 For the preparation of this acid on a large scale in this country, rough nitre, with half the weight of sulphuric acid, is employed. These are put into a large glass or earthenware body, to which a glass pipe is luted, communicating with an empty receiver, which is connected by means of pipes also, with several other receivers half filled with water.

Acidum Nitrosum. Edin. Nitrous acid

"Take of nitrate of potassa bruised, two pounds; sulphuric acid, sixteen ounces. Pour the acid upon the nitrate of potassa in a glass retort, and distil from a sand bath with a gradually augmented heat, until the iron pot becomes obscurely red hot.

"The specific gravity of this acid is, to that of distilled water, as 1.520 to 1.000."

Syn. Acide nitrique (F.), Salpeter saure (G.), Zalpeterzuur Skerkwater (Dutch), Skedwatter (Swed.), Acido Nitrico (I.), Pottle Ooppoo travagum (Tarn.), Areki Shora (Pers.), Maulabker (Arab.).

In performing these processes it is advisable to use a retort with an adopter, and to place a small tube in the receiver, which should be tubulated for this purpose. The nitric acid is separated from its combination with potassa, in the nitrate, by the superior affinity of the sulphuric acid for the potassa; which, however, requires, to be aided by quantity, a larger portion of sulphuric acid than is necessary for saturating the potassa of the nitrate being used; and also by heat, which volatilizes the nitric acid as it is disengaged. As soon as the materials are heated, orange-yellow vapours are disengaged, which in a short time, as the heat increases, become paler, and continue so until the ingredients in the retort are nearly dry, and the heat is augmented to 500°; when, owing to a partial decomposition of the acid next disengaged, nitrous gas comes over in deep red fumes, with a quantity of permanently elastic, pure oxygen, which may be collected in an inverted receiver filled with water, placed in a pneumatic trough, and connected with the last of the receivers by means of a bent tube. The nitrous gas combines with the condensed acid in the receiver, deepens its colour, and gives it that form which, according to the Edinburgh College, constitutes nitrous acid.

It is with the view of preventing this, that the London College has ordered so large a portion of sulphuric acid to be employed, the principal use of which appears to be to contribute a sufficient portion of water to preserve the constitution of the nitric acid. The Edinburgh College orders the acid to be kept in its coloured state; and as a medical agent it answers the same purposes as the colourless acid; for when both are diluted with water they have the same colourless limpid appearance, and are brought to the same state, the addition of the water expelling completely the nitrous gas, which is only loosely united with the nitric acid. The quantity of acid obtained by the Edinburgh process is about half the weight of the nitrate employed; and the residue is a white, spongy, saline cake of sulphate of potassa, mixed with a bisulphate, which may be dissolved out of the retort by hot water.

By the London process the nitric acid is obtained not altogether free from nitrous gas. In the expulsion of the nitrous gas, to change the nitrous acid into nitric acid, according to the directions of the Edinburgh College, a portion of the acid is carried over with the gas, which should not be wasted, but should be condensed by a small portion of water being put into the receiver, and thus it will form a diluted acid. Dr. Murray l justly remarks, that the heat of a water bath is best adapted for this operation, being sufficient for the purpose, and not too great to produce the decomposition of the acid. A completely colourless acid, however, is not obtained, unless the acid be redistilled from a small portion of black oxide of manganese; but this is not at all necessary for medical purposes.2

1 System of Materia Medica, ii. 184.

2 Nitric acid was first obtained by Raymond Lully, in the 13th century, by distilling a mixture of nitre and clay : a process still employed on the Continent. The name nitric acid was imposed in 1787, by the French chemists.

As nitre sometimes contains a small portion of chloride of sodium, nitric acid, in whatever method it has been procured, may be contaminated with a minute portion of hydrochloric acid, or it may contain some sulphuric acid, if the acid have not been introduced into the retort by means of a retort funnel : the presence of the first is detected by dropping in nitrate of silver, which forms an insoluble chloride of silver; while the formation of a precipitate on the addition of nitrate of baryta discovers the second; but both the acid to be tested and the nitrate of baryta should be previously diluted with distilled water: if any undecomposed nitrate be present, it will be detected by evaporating a portion of the acid, when, if it contain no nitre, it will have no residue. These contaminations do not affect the medicinal virtues of the acid.