in Chemistry, a class of substances which are distinguished by the following properties:

1. When applied to the tongue, they excite that sensation which is called sour or acid.

2. They change the blue colours of vegetables to a red. The vegetable blues employed for this purpose are generally tincture of litmus and syrup of violets or radishes, which have obtained the name of reagents or tests. If these colours have been previously converted to a green by alkalis, the acids restore them again.

3. They unite with water in almost any proportion.

4. They combine with all the alkalis, and most of the metallic oxides and earths, and form with them those compounds which are called salts.

It must be remarked, however, that every acid does not possess all these properties; but all of them possess a sufficient number of them to distinguish them from other substances. And this is the only purpose which artificial definition is meant to answer.

The acids are by far the most important class of bodies in chemistry. It was by their means indeed, by studying their properties, and by employing them as instruments in the examination of other bodies, that men of science laid the foundation of chemistry, and brought it to that state in which we find it at present. The nature and composition of acids, therefore, became a very important point of discussion, and occupied the attention of the most eminent cultivators of the science.

Paracelsus believed that there was only one acid principle in nature, which communicated taste and solubility to the bodies in which it was combined. Beccher embraced the same opinion; and added to it, that this acid principle was a compound of earth and water, which he considered as two elements. Stahl adopted the theory of Beccher, and endeavoured to prove that his acid principle is sulphuric acid; of which, according to him, all the other acids are mere compounds. But his proofs were only conjectures or vague experiments, from which nothing could be deduced. Nevertheless, his opinion, like every other which he advanced in chemistry, continued to have supporters for a long time, and was even countenanced by Macquer. At last its defects began to be perceived; Bergman and Scheele declared openly against it; and their discoveries, together with those of Lavoisier, demonstrated the fallacy of both parts of the theory, by shewing that sulphuric acid does not exist in the other acids, and that it is not composed of water and earth, but of sulphur and oxygen.

The opinion, however, that acidity is owing to some principle common to all the salts, was not abandoned. Wallerius, Meyer, and Sage, had advanced different theories in succession about the nature of this principle; but as they were founded rather on conjecture and analogy than direct proof, they obtained but few advocates. At last M. Lavoisier, by a number of ingenious and accurate experiments, proved that several combustible Acids combustible substances, when united with oxygen, form acids; that a great number of acids contain oxygen; and that when this principle is separated from them, they lose their acid properties. He concluded, therefore, that the acidifying principle is oxygen, and that acids are nothing else but combustible substances combined with oxygen, and differing from one another according to the nature of the combustible base.

This conclusion, as far as regards the greater number of acids, is certainly true. All the simple combustibles, except hydrogen, are convertible into acids; and these acids are composed of oxygen and the combustible body combined: this is the case also with four of the metals. It must not, however be admitted without some limitation.

1. When it is said that oxygen is the acidifying principle, it is not meant purely to affirm that oxygen possesses the properties of an acid, which would be contrary to truth; all that can be meant is, that it enters as a component part into acids, or that acids contain it as an essential ingredient.

2. But, even in this sense, the assertion cannot be admitted: for it is not true that oxygen is an essential ingredient in all acids, or that no body possesses the property of an acid unless it contains oxygen. Sulphurated hydrogen, for instance, possesses all the characters of an acid, yet it contains no oxygen.

3. When it is said that oxygen is the acidifying principle, it cannot be meant purely to affirm that the combination of oxygen with bodies produces in all cases an acid, or that whenever a body is combined with oxygen, the product is an acid; for the contrary is known to every chemist. Hydrogen, for instance, when combined with oxygen, forms not an acid, but water, and the greater number of metallic bodies form only oxides.

All that can be meant, then, when it is said that oxygen is the acidifying principle, is merely that it exists as a component part in the greater number of acids; and that many acids are formed by combustion, or by some equivalent process. The truth is, that the class of acids is altogether arbitrary; formed when the greater number of the bodies arranged under it were unknown, and before any precise notion of what ought to constitute the characteristic marks of an acid had been thought of. New bodies, when they were discovered, if they possessed any properties analogous to the known acids, were referred without scruple to the same class, how muchsoever they differed from them in other particulars. Hence we find, under the head of acids, bodies which have scarcely a single property in common, except that of combining with alkalies and earths. What substances, for instance, can be more dissimilar than sulphuric, prussic and uric acids? Hence the difficulty of affixing the general characters of the class of acids, and the disputes which have arisen about the propriety of classing certain bodies among acids. If we lay it down as an axiom that oxygen is the acidifying principle, we must either include among acids a great number of bodies which have not the smallest resemblance to those substances which are at present reckoned acids, or exclude from the class several bodies which have the properties of acids in perfection. The class of acids being perfectly arbitrary, there cannot be such a thing as an acidifying principle in the most extensive sense of the word.

The acids at present known amount to about 30; and all of them, eight excepted, have been discovered within these last 40 years. They may be arranged under two general heads: 1. Acids composed of two ingredients. 2. Acids composed of more than two component parts. (Thomson's Chemistry). See Chemistry.

ACIDULOUS, denotes a thing that is slightly acid: it is synonymous with the word sub-acid.

ACIDULÆ. Mineral waters that are brisk and sparkling without the action of heat are thus named; but if they are hot also, they are called Thermes.