This section is from the book "A Treatise On Therapeutics, And Pharmacology Or Materia Medica Vol1", by George B. Wood. Also available from Amazon: Part 1 and Part 2.
Origin, etc. This combination first attracted notice as a solvent for gold, whence it received the name of aqua regia. It is said to have been known to Geber, who lived in the seventh century; but its introduction into medicine dates only from the early part of the present. According to the strict chemical nomenclature of the day, it is named nitro-chlorohydric, or nitro-hydrochloric acid. It is prepared, according to the present Pharmacopoeia, by mixing three troyounces of nitric acid with five troyounces of muriatic acid. When the acids are of sufficient strength, a reaction takes place, which Davy supposed to result in the production of nitrous acid, water, and chlorine. But M. Baudri-mont found, upon collecting the vapours rising from the mixture, that they did not consist of chlorine, but of a combination of this element with hyponitrous acid, in the proportion of two equivalents of the former to one of the latter; and he gave the name of chlorazotic gas (chloro-nitric gats) to the new compound. Gay-Lussac afterwards investigated the subject, and states as the result of his examination, that two new products are formed, consisting of nitric oxide and chlorine, which may be deemed to have been produced, the one from hyponitrous acid by replacing one equivalent of its oxygen with one of chlorine, the other from nitrous acid by a similar substitution of two equivalents of chlorine for two of oxygen; but Gay-Lussac admits also the evolution of free chlorine. It is seen, therefore, that the compound is no longer a mixture of the nitric and muriatic acids, but of certain new substances, having distinct chemical properties, and probably an entirely different physiological action. As the medicine is officinally prepared, it contains a considerable excess of nitric acid; so that its effects must be those conjointly of that acid and the new products.
It is a fact worthy of particular attention, that nitric and muriatic acids will not react on each other so as to produce the changes above referred to. which are essential to the distinctive medicinal character of the compound, unless in a certain degree of concentration. If the acids employed be weak, they will still remain nitric and muriatic acids in the mixture, and will exercise on the system only the effects of these acids. It is probably from this cause, in part at least, that the disappointment in the effects of nitromuriatic acid has proceeded, which has led to its abandonment by many practitioners, and to the slighting notices of it given by some of the recent English writers. I have used it much, have taken care to guard against this source of failure, and have had every reason to be satisfied of its great efficiency. Indeed, I consider nitromuriatic acid as among our most valuable remedies.
When the strong acids cannot be obtained, reaction may be speedily brought about by the addition of a little sulphuric acid, which probably operates by concentrating the weak acids through its affinity for water. At present there is little occasion for this expedient, as the acids are generally to be found of sufficient strength; but it was not always so; and I have repeatedly made this addition with satisfactory results.
When reaction has but partially taken place between the constituents, the colour of the mixture is yellow; but it deepens as the changes go on; and at length, when they are completed, is reddish or orange. The odour closely resembles that of chlorine, but is somewhat different. The taste, upon dilution so as to render the medicine supportable in the mouth, is intensely sour, and somewhat peculiar. Care should be taken that the preparation is kept in a cool place, and excluded from the light.