THOSE substances which, in a liquid state, have a sour taste; and are capable of combining with alkalies, earths, and metallic oxides, while at the same time they lose their-acidity, and form compounds named neutral salts, in which the properties of the acid, the alkali, the earth, or the oxide employed, are lost, were formerly regarded as acids; but the discovery of the hydracids proved that oxygen is not essential to acidity. Many of them change to red, the blue, purple, and green colours of vegetables; and unite with water in almost every proportion: but others are insoluble, have no taste, and do not affect litmus: others do not fully neutralize potassa; thence the old definition of acids must be set aside. Chemists now regard all bodies as acids which unite with potassa or ammonia, and form salts.

On the supposition that all acids are compounds of oxygen with certain bases, the name of each was derived from the base of which it is formed; from sulphur, for instance, comes sulphuric acid; but the same base being supposed capable of uniting with different proportions of oxygen, the terminations ous and ic were added to indicate the degree of acidification: thus, when sulphur is united with the smaller proportion of oxygen, the acid produced is named sulphurous acid; when with the full proportion, sulphuric acid. One or two acids were, moreover, supposed to combine with a still larger proportion of oxygen, to denote which the syllable oxy (for oxygenized) was prefixed: thus hydrochloric acid, which was supposed to be muriatic acid combined with an excess of oxygen, became oxymuriatic acid: but farther experience demonstrated this to be an erroneous mode of expression.

The stronger acids require to be kept in glass bottles, furnished with well-ground glass stoppers, and having the name of the acid engraved on the glass. They should be dispensed also in glass-stopped phials. The acids known to chemists are very numerous: but of these a small proportion only is employed for medical and pharmaceutical purposes. In the London Pharmacopoeia, they are placed in alphabetical order; but in this place it may be proper first to exhibit them, in a table, according to the nature of their radicals or base.

III. Acids composed of a simple Radical and Hydrogen. Chlorine 1. Hydrochloric Acid.

IV. Acids composed of a compound Radical and Hydrogen. Cyanogen, Carbon, Nitrogen.

2. Hydrocyanic Acid.