It is in the form of nitre that this acid is offered to our notice; in the language of modern chemistry, nitrat of potash. As it occurs from the hand of nature, it is far from pure. A large proportion has often, as might be expected, a calcareous basis; and sea salt generally forms a portion of it. We find nitre also in many plants, particularly the parietaria and bugloss; in fact, in all those whose extracts are liable to ferment. We need not enlarge on the methods of purifying nitre. We receive it in the form of a neutral, crystallized in prismatic octoedrons, terminated by dihedral summits. It is fusible on ignited coals, and its acid is decomposed. It does not deliquesce, but dissolves in seven parts of cold water, and its own weight of boiling: 100 grains of the crystals are usually said to contain 30 of acid, 63 of alkali, and 7 of water. This salt is cooling, and highly useful in all inflammatory diseases, those of the bladder sometimes excepted. Nitre is decomposed by alum, Epsom salt, tartar, spirit of vitriol, vitriol of zinc, copper, and iron. From the usual laws of affinity, it should be decomposed by Glauber's salt; but this takes place only in a slight degree, and at a temperature of 32°. Its chief utility in the arts for the process of making gunpowder is well known. It is the principal ingredient also in fulminating powder; but even modern fancy has not yet enrolled this among the articles of the materia medica.

Nitrat of soda is styled cubic, quadrangular, and rhomboidal nitre, from the form of its crystals, which, however, are not cubic, but rhomboidal. It is somewhat more bitter than common nitre, and grows moist in the air. Cold and boiling water equally dissolve about two parts of the salt: 100 grains contain, it is said, 28.80 of acid, 50.09 of alkali, and 21.11 of water; but by others the proportions assigned are very different, and Tromsdorff thinks that 100 grains contain 43 of acid, 32 of alkali, and 25 of water. It is generally the product of art, though it has been suspected in some mineral waters. We know not that it has been used in medicine; but should it be employed, the prescriber should recollect that it may be decomposed by potash, alum, Epsom salt, vitriolic acid, and the vitriols.

Nitrat of ammonia is the production of art, and its crystals are described as resembling needles; the taste is cooling, but it has been little attended to, and has never been employed in medicine.

The marine acid has hitherto resisted every attempt to ascertain the nature of its basis. Yet, from some Galvanic experiments, it is suspected that it may be water; and this acid is only water with a diminished proportion of oxygen. We shall first describe this acid and its properties in the common way.

When pure, it is colourless; but has generally a yellowish hue, and exhales white suffocating fumes. It is lighter than either of the other acids; but its most distinguishing property is the very great change it experiences from an excess of oxygen. Though this be undoubtedly the principle of acidity, yet when added to the muriatic acid in an over proportion, its volatility is increased, its acidity and power of attraction for alkalis weakened, and instead of reddening vegetable colours, it destroys them. It is procured from common salt by the addition of sulphuric acid, when it assumes a gaseous form; and in this state has been recommended for destroying infection, and purifying infected rooms. As these fumes are very suffocating, the vapour of the nitric acid has been preferred; and the discovery of each has occasioned a controversy which perhaps might have been spared, and which we must afterwards more particularly notice, though we shall not pretend to decide the dispute.

The most delicate test of the presence of muriatic acid is the nitrate of silver; for the metal when united with the muriatic acid is insoluble, but the oxygenated acid occasions no precipitation; so that if any doubt arises, the acid should be previously exposed to the light, which separates in every instance the oxygen. The oxygenated acid forms, with different bases, fulminating powders, whose detonation is extremely violent, and takes place on trituration only, without heat. The use of the oxy-muriatic acid in bleaching is well known; and in its union with alkalis it furnished the active ingredient of De Morveau's box, which, on opening, diffuses round the person who holds it a pure atmosphere, which destroys infection. The ingredients are now altered. See Contagion.

The experiments lately alluded to are, however, too striking to be wholly passed over, since they offer such a clear simple view of the different operations of nature as will greatly elucidate many unaccountable phenomena. In attempting to ascertain the basis of this acid, even Berthollet erred, and Mr. Lambe was misled by a remote analogy; nor some months since should we have given a more favourable account of Girtanner's hypothesis, who derived it from hydrogen. On the 23d of April, 1805, Mr. W. Peel of Cambridge, announced, in the Philosophical Magazine, his discovery of muriat of soda from the decomposition of water by the Galvanic apparatus. In June he repeated his communication, and mentions his having formed water from its elements to repeat the experiment; but this water, from a little inaccuracy in the proportions, being acidulous, he neutralized the acid with lime, and distilled it. But, after the Galvanic process, he found muriat of potash.

By a singular coincidence, about the same time Pac-chioni of Pisa made a similar discovery. His letter is dated the 9th of May, sixteen days only later than Mr. Peel's, and both speak of recent discoveries. Pacchioni's apparatus is not described; and at the first glance it excites a suspicion that he procured oxygen only in the decomposition of the water, while others have obtained, exclusively, hydrogen; but as both are ingredients of water, evolved separately on different sides of the pile, this occasions little difficulty. On separating the oxygen, he found the gold wire dissolved, and the oxygenated muriatic acid evidently produced. Water then is hydrogen, with its full proportion, perhaps, of oxygen. When the proportion of the latter is diminished, it becomes oxymuriatic acid; and, when still further, the common muriatic acid. Mr. Henry has repeated these experiments with some success, but suggests a source of fallacy, which leads us at least to hesitate. We shall have frequent opportunities of resuming the subject in different parts of the work, as in the articles Muriatic acid, Muriats, and Sea salt, when we shall give whatever the labours of chemists have added to the 9tock. Even while this sheet was passing through the press, we find in the Journal de Physique, vol. lxi. some experiments instituted by the Galvanic Society at Paris, which throw considerable doubts on the supposed discovery.