Irish physicist who studied the conduction of heat by gases. He is best known, however, for discovering the Tyndall effect , in which a beam of light becomes visible when passing through a colloid. Tyndall popularized the molecular vibration theory of heat which had been developed by Maxwell in Heat as a Mode of Motion (1863). He also defended Julius Mayer's claim to be the originator of the Law of energy conservation because of his priority in discovering the mechanical equivalent of heat.