Muriat of potash has been styled the febrifuge salt of Sylvius, and, from one process by which it was prepared, the regenerated marine salt. Its crystals are cubic, but not regular; its taste pungent and bitter; and it dissolves in three times its weight of cold water. It is supposed by some authors to contain 29.68 of acid, 63.47 of alkali, and 6.85 of water. It is found occasionally in sea water, and in some mineral waters; but, when wanted as a medicine, which scarcely ever happens, it is prepared by art.

Muriat of soda is the common salt so generally diffused not only in the earth, but by the ocean, of which it constitutes the chief ingredient. Should the suspicions just stated be confirmed, its source will be well understood: at present it is unknown. There is every reason, however, to suppose, that all the fossile muriat of soda has been deposited from an ocean which once covered the highest continents, perhaps the whole globe, when the principal inhabitants were fish, and where animals, now known only from their debris, were the tyrants of this watery world. Common salt is highly necessary to the health of men and many other animals; it is a condiment most congenial to their constitution, and though some are, from necessity, obliged to live without it, yet all find its advantages when it can be procured. In America, animals from a vast distance repair to those regions where the salt effloresces on the surface, which they "lick"with great avidity, and the spots are distinguished by this appellation.

Common salt is pungent, with a slight bitterness. It dissolves equally in cold and hot water, in a little more than twice its weight of the menstruum: 100 grains have been said to contain 33.3 of acid, 50 of alkali, and 16.7 of water. Its crystals are usually cubical, but they seem to be sometimes octoedral. It scarcely ever forms an article of extemporaneous prescription, except in the form of sea water. Barytes, lead, and sometimes iron, seem to decompose this salt, and are employed to separate the soda for the artist; but could have no extemporaneous effect in mixture, were it even necessary to combine them in a medicine. Vitriolic and nitric acids are, apparently, the only instances from which any inconvenience could arise. Potash would, however, have a similar effect; and some metalline salts, particularly nitrat of silver, would decompose it; but these are not likely to meet in a prescription.

Muriat of ammonia was originally prepared in Egypt from the dung of camels, and is met with in commerce in round cakes, concave on one side, and convex above, from the shape of the glasses into which it is sublimed. When in conical masses, it is debased by other salts, particularly earthy ones. Its taste is acrid and pungent, with a flavour not very distant from the urinous alkaline. It dissolves in about three times its weight of cold, and about an equal weight of boiling, water; and, during its solution, produces nearly 32° of cold. Its crystals are four or six sided pyramids, generally aggregated in a plumose form, and it is said to consist of 42.75 of acid, 25 of ammonia, and 32.25 of water. It is not often used internally, yet we shall find that it is more commonly useful than has been supposed. Externally it is often employed; but, as it is decomposed by the sulphuric and nitric acids, by barytes, potash, soda, strontia, and lime, as well as by different salts composed of these acids or their bases, and by some metalline salts, the practitioner should be very careful of the ingredients with which he joins this salt. It is soluble in little more than four parts of alcohol.

These are the acids which chiefly claimed the attention of chemists within the last twenty years. An acid from vegetables was indeed well known, and it seemed to be varied in its forms and its properties; but modern chemistry has only ascertained their different sources, and their distinguishing affinities. We find also in the animal system a variety of peculiar acids which claim our attention, and in both vegetables and animals there are various substances denominated oxides, which, with the addition of oxygen, assume acid properties. These we shall not consider in the present article, unless they have become peculiar objects of our attention, by having been introduced into the practice of medicine.

The principal vegetable acid is the acetous, known chiefly in common life by the appellation of vinegar; but nature offers us also that of lemons (the citric), of apples (the malic), that of galls (the gallic), that of acetosella (the oxalic), that of benzoin (the benzoic), that of tartar (the tartarous), and that of borax (the bo-racic acid, or sedative salt). With a little exertion we separate from cork, the suberic; from camphor, the camphoric; from sugar, the saccharine; from gum, the mucous: but these are, perhaps, more strictly oxides. Fire separates the pyrotartarous acids, the ligneous and the pyroligneous acids. The carbonic acid we have already noticed.

Of these, the chief acid, perhaps that from which every other with some modifications proceeds, is the acetous. The tartarous and boracic acids will indeed merit our particular attention, on account of their medicinal formulae; but the malic, the citric, the oxalic, acids, do not essentially differ. The benzoic acid, Mr. Hatchett, in the Philosophical Transactions for 1805, (Part II.), considers, with some reason, as a production of fire; in his own strictly philosophical language, a product rather than an educt. The saccharine, the camphoric, the mucous, the suberic, are certainly modifications of the acetous acid; and the ligneous, the pyroligneous, and the pyrotartarous, acids, are only vegetable acids disguised by an empyreumatic oil.

We have thus confined our views to the acids of vinegar and tartar. The former is the product of fermentation, or of that new combination, which, by the "play of affinities"from the slow intestine motion of the particles of a saccharine, and afterwards of a spirituous, fluid takes place. See Fermentation.