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Hales, Stephen,1677–1761, English physiologist and clergyman. From 1709 he was perpetual curate of Teddington. His experimental studies in animal and plant physiology contributed greatly to the progress of science. In his investigations of circulation he made the first measurements of blood pressure by inserting a tube in a horse's artery. Plant physiology was given impetus by his work on transpiration, root pressure, circulation of sap, and the relationship between green plants and air. His inventions included apparatus for ventilating buildings. Some of his studies are described in his Vegetable Staticks (1727), Haemostaticks (1733), and A Description of Ventilation (1743).
Born Sept. 7, 1677, in Bekesbourne, Kent; died Jan. 4, 1761, in Teddington. British botanist and chemist, member of the Royal Society (1718) and of the French Academy of Sciences (1735).
Hales studied the flow of sap in plants (using the quantitative method) and the phenomenon of spring transpiration in plants, and he discovered the existence of root force and established its significance for the flow of sap. On the basis of his research, he concluded that the blood circulation in animals was different from the movement of sap in plants, which always went from the roots to the leaves, where evaporation occurred. This view was opposed to the one generally held at the time. Hales believed that plants received part of their nourishment from the air, at the same time absorbing “light matter.” In his experiments with animals he showed that various chemical substances affected the contraction and expansion of capillaries.
WORKSVegetable staticks …. London, 1727.
Haemastaticks, 4th ed., vols. 1-2. London, 1769.