One of the mysteries of the English language finally explained.
An element or radical whose valency is represented by an even number. Contrasted with "perissad". Now historical and rare.
Mid 19th century (in an earlier sense). From ancient Greek ἄρτιος (of numbers) even (from ἄρτι just, exactly from the same Indo-European base as classical Latin ars, Armenian ard recently, Lithuanian artus (adjective), arti (adverb) near) + -ad.
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