Stephen Hales, FRS, DD (17 September 1677 – 4 January 1761) was an English clergyman who made major contributions to a range of scientific fields including botany, pneumatic chemistry and physiology. He invented several devices, including a ventilator, a pneumatic trough and a surgical forceps for the removal of bladder stones. He was also a philanthropist and wrote a popular tract on alcoholic intemperance.
Stephen Hales was born in 1677 in Bekesbourne, Kent, England. He was son of Thomas Hales, heir to Baronetcy of Beakesbourne and Brymore, and his wife, Mary (née Marsham), and was one of twelve or possibly thirteen children.
Hales was educated in Kensington and then at Orpington before attending Corpus Christi College, Cambridge (or St Benedict's as it was then known) in 1696. Although he was an ordinand studying divinity, Hales would have received tuition in the Classics, mathematics, natural sciences and philosophy while in Cambridge. Hales was admitted as a Fellow of Corpus Christi in 1703, the same year as he took the degree of Master of Arts, and was ordained as Deacon at Bugden, Cambridgeshire.
In 1709 he was ordained Priest at Fulham and on 10 August 1709 he was appointed 'Perpetual Curate' of the parish of Teddington, Middlesex and left Cambridge, although he retained his Fellowship until 1718. He became a Bachelor of Divinity in 1711. He was an assiduous minister – in addition to parish duties he enlarged and repaired the church and commissioned a new water supply for the village – and well regarded although there is some evidence that his experimental work on animal physiology was viewed with misgivings.
Hales is best known for his Statical Essays. The first volume, Vegetable Staticks (1727), contains an account of experiments in plant physiology and chemistry. The second volume, Haemastaticks (1733), describes experiments on animal physiology including the measurement of the "force of the blood", i.e. blood pressure.
In Vegetable Staticks, Hales studied transpiration – the loss of water from the leaves of plants. He estimated the surface area of the leaves of the plant and the length and surface area of the roots. This allowed Hales to compare the calculated influx of water into the plant with the amount of water leaving the plant by transpiration through the leaves. He also measured 'the force of the sap' or root pressure. Hales commented that "plants very probably draw through their leaves some part of their nourishment from the air".
He also speculated that plants might use light as a source of energy for growth, based on Isaac Newton’s suggestion that "gross bodies and light" might be interconvertible. In Vegetable Staticks Hales also described experiments that showed that "… air freely enters plants, not only with the principal fund of nourishment by the roots, but also thro’ the surface of their trunks and leaves". While Hales’ work on the chemistry of air appears primitive by contemporary standards, its importance was acknowledged by the Lavoisier, and Hales’ invention of the pneumatic trough to collect gases over water was a major technical advance. Modified forms of the pneumatic trough were later used by William Brownrigg, Henry Cavendish and Joseph Priestley.
Hales began his work on animal physiology with William Stuckeley while in Cambridge, although much of it was published only after Vegetable Staticks appeared. Hales and Stuckeley performed a wide range of studies including making casts of the trachea and bronchial trees of dogs using molten lead and measuring the water lost due to breathing. Most famously, Hales made measurements of blood pressure in several animal species by inserting fine tubes into arteries and measuring the height to which the column of blood rose. In addition, Hales took wax casts of the ventricle of the heart and estimated how much blood was pumped by the heart; correctly described the roles of the mitral valve and aortic valve during systole and diastole; explained the pulsations of arteries in terms of their elasticity and attributed the resistance to blood flow to friction due to the passage of blood through small blood vessels.
Hales also described a diverse range of work in Haemastaticks including his attempts to find substances that could be used to dissolve bladder stones or calculi. This aim was unsuccessful but as part of this work he developed a double lumen bladder catheter and devised special forceps to enable the removal of urinary stones. Hales' work on the growth pattern of long bones, demonstrating epiphyseal growth; his demonstration of spinal reflexes in the frog and his suggestion that electricity played a role in allowing nerves to control muscle function are also noteworthy.
Bad or stale air was thought to be a cause of ill-health and death in the 18th century. Death and disease were common in overcrowded ships and prisons. Hales was one of several people in the early 18th century (other notable inventors being John Theophilus Desaguliers, Mårten Triewald and Samuel Sutton) who developed forms of ventilators to improve air quality. Hales’ ventilators were large bellows, usually worked by hand, although larger versions were powered by windmills.
They were widely installed in ships, prisons and mines and were successful in reducing disease; versions of Hales' ventilators were also used in preserving foods and drying grain. Hales also experimented with ways of distilling fresh water from sea water; preserving water and meat on sea-voyages; measuring depths at sea; measuring high temperatures; and wrote on a range of subjects including earthquakes; methods of preventing the spread of fires; and comparative mortality rates in relationship to rural and urban parishes.
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