Emily Dickinson Lexicon

SES'QUI, n. [L. one and a half.]

In chimistry, this is much used as a prefix to the names of compounds of an acidifying and basifying principle with another element, to form a saldiable base; or of an acid with a salifiable base, to form a salt. It always denotes that the elementary or proximate principles of the compound are in the proportions of one and a half of the acidifying and basifying principle, or of the acid, to one of the other element or proximate principle; but as there can be no such thing as half of an equivalent, such compounds are always to be understood as consisting of three equivalents of the acidifying and basifying principle, or of the acid, to two equivalents of the other element or proximate principle.