[L. acidus, sour.] A general term used in chemistry to designate a special group of sub-stances, mostly, but not always, oxygen compounds. The chief distinguishing property of acids, and one which is common to all of them, is that of combining with bases to form salts. They are also distinguished by their sour or acid taste, and by the power of turning blue vegetable colors into red. These blue colors are litmus, syrup of violets or of radishes. When these blues have been changed into green by an alkali, their color is restored by an acid.