438

Acids.

PART MI.

oxide, which is only loosely united with the nitric acid toform the nitrous. The quantity of acid obtained is about halfthe weight of the nitrate employed; and the residue is a whitespongy saline cake of sulphate of potash with an excess of sul-phuric acid, which may be dissolved out of the retort by hotwater.

By the London process the nitric acid is at first obtainedLolerably free from nitrous oxide; but in general the re-distil-lation, as directed, will be found necessary. In the expulsionof the nitrous oxide, to change the nitrous into nitric acid, ac-cording to the directions of the Edinburgh College, a portionof the acid is carried over with the gas, as nitrous acid va-pour ; which should not be wasted, but be condensed by asmall portion of water being put into the receiver, and thusform a diluted acid. Mr. Murray* justly observes, that theheat of a water-bath is best adapted for this operation, beingsufficient for the purpose, and not too high to produce thedecomposition of the acid. A completely colourless acid,however, is not obtained, unless the acid be re-distilled froma small portion of black oxide of manganese, but this is notat all necessary for medical purposes. z

As nitre sometimes contains a small portion of muriate ofsoda, nitric acid, in whatever method it has been procured,may be contaminated with a minute portion of muriatic acid;or of sulphuric acid, if a large proportion of this has been usedfor decomposing the nitre: the presence of the first is detectedby dropping in nitrate of silver, which forms an insolublemuriate of silver; while the formation of a precipitate on theaddition of muriate of barytes discovers the second. Thesecontaminations do not affect the medicinal virtues of the acid.

Qualities. — Nitrous acid, as the term is understood in theEdinburgh Pharmacopoeia, is a yellow or orange-colouredfluid, emitting, when exposed to the air, deep-orange-co-loured extremely suffocating fumes. In its chemical affini-ties and other qualities it agrees in many respects with nitricacid. It consists of nitrous gas loosely combined with nitricacid and water; and the colour varies according to the propor-tion of nitrous gas which is present. From experiments madeby Sir H. Davy 3 on this subject, the following appear to be theproportions in the three states in which nitrous acid is usuallyprocured for pharmaceutical purposes.

> System nf Materia Medica, ij. 184.

* Nitric acid was first obtained by Raymond Luily, in the I3tli century, by dis-tilling a mixture of nitre and clay, a process still employed on the continent. Thename Nitric acid was imposed in 1787, by the French chemists,

3 Researches; 3J,