Acid, a.
Etymology: [L. acidus sour, fr. the root ak to be sharp: cf. F. acide. Cf. Acute.]
1. Sour, sharp, or biting to the taste; tart; having the taste of vinegar: as, acid fruits or liquors. Also fig.: Sour-tempered. He was stern and his face as acid as ever. A. Trollope.
2. Of or pertaining to an acid; as, acid reaction.
1 other Webster entry under this spelling
Acid, n.
1. A sour substance.
2. (Chem.)
One of a class of compounds, generally but not always distinguished by their sour taste, solubility in water, and reddening of vegetable blue or violet colors. They are also characterized by the power of destroying the distinctive properties of alkalies or bases, combining with them to form salts, at the same time losing their own peculiar properties. They all contain hydrogen, united with a more negative element or radical, either alone, or more generally with oxygen, and take their names from this negative element or radical. Those which contain no oxygen are sometimes called hydracids in distinction from the others which are called oxygen acids or oxacids.
Note: In certain cases, sulphur, selenium, or tellurium may take the place of oxygen, and the corresponding compounds are called respectively sulphur acids or sulphacids, selenium acids, or tellurium acids. When the hydrogen of an acid is replaced by a positive element or radical, a salt is formed, and hence acids are sometimes named as salts of hydrogen; as hydrogen nitrate for nitric acid, hydrogen sulphate for sulphuric acid, etc. In the old chemistry the name acid was applied to the oxides of the negative or nonmetallic elements, now sometimes called anhydrides.
Shades of meaning
Fernald has no entry of its own for acid; he lists it under bitter.
Where he writes about it:
Acid, sour, and bitter agree in being contrasted with sweet, but the two former are sharply distinguished from the latter. — from bitter
Acid or sour is the taste of vinegar or lemon-juice; bitter that of quassia, quinine, or strychnine. — from bitter