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Hales, Stephen

pressure blood biology water

(1677–1761) English chemist and physiologist: developed gas-handling methods; classic experimenter on plant physiology and on blood pressure.

Hales studied theology at Cambridge and in 1709 became perpetual curate of Teddington. There he stayed, refusing preferment, so that he could maintain his work in chemistry and biology. He seems to have been much influenced by work, and his own is marked by careful measurement and the early use of physics in biology.

His book Vegetable Staticks (1727) describes 124 experiments on gases, which he made in several ways and collected using a pneumatic trough. This device was a major advance in gas manipulation. Oddly, he assumed all the gases he made were air; it was , using and improving Hales’s methods, who examined their different properties. In the same book, Hales describes his experiments showing that plants take in a part of the air (actually CO2 ) and that this is used in their nutrition. He measured growth rates and showed that light is needed, and that water loss (by transpiration) is through the leaves and causes an upward flow of sap, whose pressure he measured. Later he examined blood pressure, inserting and tying a vertical tube 11 feet long into an artery of a horse to measure the height to which the blood rose, and calculating also the output from the heart and the flow-rate in arteries, veins and capillaries. He showed that capillaries are liable to constriction and dilation, which later was seen to be of great significance.

His inventiveness was wide-ranging; he worked on the preservation of foods, water purification, the ventilation of buildings and ships, and the best way to support pie crusts. His theme was usually the application of physics to problems in biology.

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