FORBES, JAMES DAVID (1809-1868), Scottish physi cist, was born at Edinburgh on April 20, 1809. He studied at Edinburgh, became F.R.S.E. (1828) and F.R.S. (1832), pro fessor at Edinburgh (1833-59), and principal of the United college at St. Andrews (1859-68). He died on Dec. 31, 1868.
As a scientific investigator he is best known for his researches on heat and on glaciers. Between 1836 and 1844 he published in the Trans. Roy. Soc. Ed. four series of "Researches on Heat," in the course of which he described the polarization of heat by tourmaline, by transmission through a bundle of thin mica plates inclined to the transmitted ray, and by reflection from the sur faces of a pile of mica plates placed at the polarizing angle, and also its circular polarization by two internal reflections in rhombs of rock-salt. In 1846 he began experiments on the temperature of the earth at different depths and in different soils near Edin burgh ; he determined the thermal conductivity of trap-tufa, sand stone and pure loose sand. Later he investigated the laws of the conduction of heat in bars, and his last piece of work was to show that the thermal conductivity of iron diminishes with increase of temperature. The Royal Society awarded him the Rumford Medal in 1838 and the Gold Medal in 1843 for his work on heat.
He made several visits to Switzerland and also to Norway to study the flow of glaciers and developed a theory which involved him in some controversy with Tyndall and others both as to priority and to scientific principle.
See Principal Shairp, Professor P. G. Tait and A. Adams-Reilly, Forbes's Life and Letters (1873) ; J. Tyndall, Professor Forbes and his Biographers (1873).