Acids, (from acesco, to sharpen). Acids form a species of salts, exciting upon the organ of taste the sensation called sour; which maybe regarded as synonymous with acid. Every substance is called acid which gives the impression above specified to the taste, will change certain blue vegetable colours into red, as the-juice of turnsole, syrup of violets, etc. and mill, usually, effervesce with alkalies; we say usually, because this property is not general; for the carbonic acid, and almost all weak acids, cannot be distinguished by this property; and the purest alkali, or what is called caustic or deaerated, combines with acids without effervescence. By a variety of experiments in modern chemistry, acids are found to consist of different substances: to the name of one they give the term oxygen; and to the other radical; the former considered to be the acidifying principle, the latter the acidifiable base. They further prove, that the oxygenous principle, in all the variety of acids, is universally the same; and that acids themselves only vary on account of the different radicals with which that principle is combined. Chemists have also altered the terms, in order to express the degrees of power acids possess; the weaker they express by the termination ous, the stronger by ie, added to the base or radical; as sulphurous, sulphuric; carbonous, carbonic, etc.; except the muriatic and nitrous acid; for the lower order of the former, they say muriatic; for the higher, oxygenated muriatic; taking the appellation from the acid, and not from the base.

Acids are animal, vegetable, and mineral. The vegetable are the native, as the juice of lemons, etc.; or the product of fermentation, as vinegar and tartar. The mineral are those of sulphur or vitriol, nitre, and common salt. The animal acid is obtained from ants, and some other insects, in considerable quantities; it is also contained in human fat, and in the suet of animals that ruminate; and an acetous fermentation is sometimes excited in some of the animal secretory organs, forming a kind of animal vinegar; in this way the urea of the urine is produced. See Adeps.

A vague, volatile, and liquid acid is in all parts of the earth: uniting with various substances, it forms different fossils. Except in the essential salts of vegetables or in tartar, acids are rarely found in a solid form.

There is great analogy betwixt acid and cold. The spirit of nitre increases the cold of ice. Acid and cold alike preserve from putrefaction, by increasing the cohesion of the component parts of the respective bodies. Strong acids, and excessive cold, it is true, when applied to the flesh of living animals, mortify them; but this mortification differs greatly in its nature from that produced by fire, and by alkaline., salts. South winds favour, but north winds check, the progress of putrid disorders.

Acids differ in their specific gravity when compared with water.

The acid of vitriol, as 18 to 10

Nitre 14 to 10, some say 15 to 10

Sea salt 12 to 10

Vegetables 10 plus to 10.

This difference shews that some acids are more tenacious of water than others. If the weaker acids arc used, you must pour on more of them to the same quantity of alkaline salt to saturate it; yet the salt will have only attracted the same weight of acid from each.

Acids differ in their colour: for the vitriolic is quite pale; the nitrous a dark yellow, frequently fuming, and sometimes of an orange red; the marine a pale golden colour. If bottles containing these three acids are stopped with cork, the cork is soon tinged, by the vitriolic acid, with a black colour; by the nitrous, with a yellow; and by the marine, with a whitish one. The vitriolic acid emits no visible vapours in the heat of the atmosphere, but imbibes moisture from it; the nitrous and muriatic emit copious corrosive fumes; the nitrous, yellowish red; and the muriatic, white fumes.

For the virtues of the vegetable acid, see Acetum.

The mineral acids, when intimately joined with vinous spirits, produce effect so similar to those of the vegetable class, that their properties, as medicines, are almost the same. In other respects, the effects of all the kinds of acids are similar.

Acids gently irritate and contract our fibres when taken in a dilute state, and thus corroborate; they resist a putrid tendency, and powerfully oppose putrescence when actually existing: by the irritation they promote various secretions; they excite an appetite, and aid digestion; their efficacy in fevers of every kind is not exceeded by any thing in use, nor equalled for their general safety, where causes so widely opposite produce such similar complaints: in some instances of coughs and asthmas, in consequence of irritation, their efficacy is singular: if the vegetable acid is made use of, the breathing is never disordered by it, though in some instances the mineral acids may offend. In dysenteries, and in diarrhoeas, produced by unripe fruits, the fossil acids allay the fermentation in the bowels; and when a putrid col-luvies in the primae viae is the cause, they will be a proper remedy. By their sedative quality, haemorrhages are restrained; and as bitters are neutralised by vegetable acids, so the excess and acrimony of the bile are allayed by their use.

Acids, astringents, and bitters, have a great affinity with each other. By a mixture with each other they lose their properties. Vegetable acids lessen the astric-tive power of galls on leather, etc.. The mineral acids have a contrary effect. Bitters, both animal and vegetable, are neutralised by vegetable acids, less perfectly by those from the mineral kingdom. See Lavoisier and Chaptal's Elements of Chemistry. Dictionary of Chem. Neumann's Chem. Works. Per-cival's Med. Essays.

Vegetable acids correct the deleterious effect of most, if not all, narcotic plants; but injure the phlegmatic habit, where the circulation is languid, the bile defective, or the digestion naturally weak.