a celebrated Greek philosopher, respecting the exact period of whose birth and death we are ignorant. Neither is his birth-place known; but Diogenes Laertius, who furnishes us with a few facts respecting him, states that he was a native either of Velia in Italy, Abdera, or Miletus. According to Jamblichus, he had in his youth attended Pythagoras; he was the disciple of Melissus, and of Zeno of Velia, which confirms the supposition that he must have flourished about B.C. 430. He was the first who invented the system of atoms, which was afterwards more fully explained by Democritus and Epicurus. Posidonius (Strab. xvi. 512) ascribed the honour of this invention to Moschus, a Phoenician philosopher, who lived, it is said, before the siege of Troy; but there seems little reason to believe that this statement is correct. Several other philosophers had, before the time of Leucippus, considered matter as divisible into indefinitely small particles; but Leucippus and Democritus were the first who taught that these particles were originally destitute of all qualities except figure and motion. According to this theory, the universe, which is infinite, is in part a plenum and in part a vacuum. The plenum contained innumerable corpuscles or atoms of various figures, which, falling into the vacuum, struck against each other; and hence arose a variety of curvilinear motions, which continued, till at length atoms of similar forms met together, and bodies were produced. The primary atoms being specifically of equal weight, and not being able, on account of their multitude, to move in circles, the smaller rose to the exterior parts of the vacuum, whilst the larger, entangling themselves, formed a spherical shell, which revolved about its centre, and which included within itself all kinds of bodies. The central mass was gradually increased by a perpetual accession of particles from the surrounding shell, till at last the earth was formed. Lactantius has ably refuted the hypothesis of Leucippus (Inst. Divin. iii. 17.) (See a Memoir of Batteux on the Active Principle of the Universe, Mem. de l'Acad. des Inscript. xxix.; also Montucla, Hist. des Math. i. 147; and Bayle, Dictionnaire.)