Salts are either acid or alkaline. The acid salts, or acids, are mineral, vegetable, and animal; though various acids are not exclusively confined to the class in which they are arranged.

The mineral acids, or those usually styled so, are three, the vitriolic, the nitrous, and the muriatic; but the succinic, the boracic, and several others, have an equal claim to the title, as they are the productions of, or at least found in, the mineral kingdom. These acids unite with alkalis, earths, and metallic oxides so completely, that in the new compound, the properties of the component parts are wholly lost; while some of the weaker acids, particularly the carbonic acid gas, leave often an alkaline, urinous taste. These acids, like all others, owe their acidity to the oxygen they contain, united to separate bases, which give their peculiar properties; but this oxygen may be in excess, and the acid has then been styled oxygenated, or the particle (oxy) is added. The French chemists distinguish the acids with an excess of oxygen, by terminating their respective appellations with ic, as sulphureous and sulphuric. Their general properties have been already detailed.

The vitriolic acid is diffused so generally, as to be styled the universal acid; but many others can now dispute this honour, particularly the carbonic. In medicine it is of the highest importance, since, independent of its tonic powers when uncombined, its union with alkalis and metals furnishes a variety of useful remedies. Its basis is sulphur, and, from its apparent viscidity, it has been styled oleum sulphuris. When sulphur is inflamed, the oxygen is supplied by the atmosphere; and for this purpose it was formerly burnt under a bell, from whose sides it streamed into a receptacle below. It is now usually prepared in close vessels; and the oxygen is supplied by the addition of nitre, whose acid is in this operation decomposed.

Vitriolic acid will crystallize at 3° below 0 of Reaumur. With ice it produces an increased cold; with water considerable heat. With alkalis it forms the sul-phat of potash, formerly the vitriolated tartar, the sul-phat of soda (Glauber's salt), and the sulphat of ammonia (Glauber's secret ammoniacal salt).

The sulphat of potash is soluble in about sixteen parts of water, at 60° of Fahrenheit, which we shall afterwards call cold water, and five parts of boiling water. It is the least soluble salt, and on this account inconvenient in its exhibition, as it requires so much fluid. One hundred grains contain 30.21 of acid; 64.61 of alkali; and 5.18 of water. The sulphat of potash, of commerce, is the residuum after distilling nitrous acid, separated from nitre by means of the vitriolic. In extemporaneous prescription, it is only decomposed by barytes or any of its preparations. In some circumstances the nitric acid will displace the vitriolic; but this is of little importance to our subject, which is not strictly chemical. Mr. Chaptal found this salt in the analysis of tobacco.

The sulphat of soda is soluble in three parts of cold water, and in an equal weight of boiling water. It contains a large quantity of water entangled in its crystals, which occasions it to swell and effervesce when heated: 100 grains contain 14 of acid, 22 of alkali, and 64 of water. It is found in the ashes of the tamarix gallica; but the salt of commerce is the residuum, after preparing the muriatic acid. It is decomposed by potash, baritic salts, the muriat of potash, the acetite of soda, muriated lime, lime water, nitrated silver, acetat-ed and nitrated mercury, and acetite of lead.

The sulphat of ammonia is seldom, if ever, used in medicine. It is produced chiefly by decomposing earthy or metallic salts with volatile alkali, and by the decomposition of nitric, muriatic, or carbonated ammo-niacal salts by sulphuric acid. In extemporaneous prescription, fixed alkalis, barytes, and lime, the nitric and muriatic acids must be avoided in the same formula.

The basis of the nitrous acid is the nitrogen, which, with different portions of oxygen, forms nitrous oxide, nitrous gas, nitrous and nitric acids. The nitrous acid is brown, volatile, and suffocating. It assumes successively a blue, green, and yellow colour, by the addition of water; and in a state of vapour is absorbed by water, oil, and sulphuric acid. It consists of about three parts of oxygen, and one of nitrogen. The nitric acid consists of four parts of oxygen to one of nitrogen; is liquid, colourless, and transparent, but corrosive, tinging the skin of a yellow colour. It produces heat when mixed with water, and is deprived of a part of its oxygen by light; while the suffocating fumes of the nitrous acid are destroyed or suppressed by keeping it in the dark. When concentrated, it inflames oils, sulphurated hydrogen, and iron filings. The same effect is produced on zink, bismuth, and tin, when poured on them in a state of fusion. It oxygenizes all metals, mucilages, etc. and is itself decomposed. This facility in yielding its oxygen, which is seemingly separated by the animal fluids, and is certainly so by the phosphoric salts with which they abound, has rendered it an useful remedy where oxygenation is required. Its utility in curing syphilis is not indeed established by extensive experience; but it evidently prevents the disease from growing worse, when from weakness, inclement weather, or accidental circumstances, mercurials cannot be persisted in.

From the chemical composition of nitrous acid, its resemblance to atmospheric air is obvious, and by the air it is evidently formed. In many places, particularly in India, South America, and Spain, nitre requires only lixiviation and purification. In France and other countries, its formation is assisted by animal putrefaction, vegetable fermentation, and the rubbish of old walls, containing a calcareous substance. It is not easy to say what particular office either part of the process serves. It seems designed chiefly to adapt the mass for the absorption of the ingredients of the atmosphere, for these are only requisite to form the acid. The alkali is supposed to be the product of the vegetable fermentation. This is, however, improbable; and we strongly suspect that it arises from the calcareous matter, as we shall soon consider.