Acids. A vague hint from Nature gave mankind the first indication of the existence of acids. The juice pressed from ripe grapes is a sweetish liquid. If it is kept for some time, the sweetness goes, and the liquid acquires a burning taste. If kept still longer, the burning taste is lost, and in its place a sharp acid ?avour, not entirely displeasing to the palate, is devel oped. The liquid obtained in this way is now called wine vinegar; the particular substance which gives it its characteristic taste is acetic acid.