Stephen Hales 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).

Stephen Hales

The English scientist and clergyman Stephen Hales (1677-1761) pioneered the study of plant physiology, contributed the first major account of blood pressure, and invented a machine for ventilating buildings.

Stephen Hales was born in Bekesbourne, Kent, on Sept. 17, 1677. He entered Corpus Christi College, Cambridge, in 1697, where he studied theology and took a degree in arts in 1703. He received his doctorate from Cambridge in 1733. At Cambridge, Hales was caught up in the backwash of Isaac Newton's great work at the university, and he acquired an interest in astronomy, physics, and chemistry as well as in biology. In 1708 Hales became perpetual curate of Teddington in Middlesex, and here he remained until his death. In 1719 he married Mary Newce, who died 2 years later without issue.

In 1711 Hales began his studies on blood pressure. True to his mechanistic views, he carefully measured the blood pressure of three horses and produced the first recorded estimates of blood pressure. Furthermore, he studied the pulse rates of various-sized animals and measured the heart's capacity to pump blood through the pulmonary veins. Hales also studied the effects of heat, cold, and various drugs on the blood vessels and experimented with animal reflexes.

Even though some research had been carried out by Jan van Helmont and Marcello Malpighi, Hales rightfully merits the title of father of plant physiology. Certainly in a century given over almost exclusively to the taxonomy of Carl Linnaeus, Hales's work was unique. In 1718, the year he became a member of the Royal Society, he read a paper entitled "Upon the Effect of ye Sun's warmth in raising ye Sap in trees." He then carefully measured sap pressure, velocity, and circulation in plants. Until this time sap circulation in plants was believed to parallel blood circulation in animals. Hales, however, clearly demonstrated that the transpiration of leaves draws the sap toward them at the same rate that the roots push sap upward. Furthermore, he found that plants draw some of their food from the gases in the air. He invented the pneumatic trough for collecting gases and developed gages and techniques to measure sap pressure.

Hales published his findings in Vegetable Staticks (1727), reissued in 1733 as volume 1 of his Statical Essays. Volume 2 was Haemastatics, primarily a summary of his earlier work on blood circulation. For his work he received the Copley Prize in 1739.

Hales thought his invention of a ventilator was his greatest contribution to the well-being of mankind. As early as 1741 Hales presented to the Royal Society a description of a ventilator to rid mines, prisons, hospitals, and shops of noxious airs. He published A Description of Ventilators (1743) and A Treatise of Ventilators (1758). Hales also sought ways to distill pure water from seawater, preserve meat and water for long ocean voyages, preserve foods in tropical climates, measure earthquakes, and prevent forest fires.

In 1751 Hales became clerk of the closet of the princess dowager, and subsequently he was made chaplain to her and to her son, the future George III. Though offered the canonry of Windsor by the royal family, Hales maintained an active ministry at Teddington until his death on Jan. 4, 1761.

Further Reading

The best work on Hales is Archibald E. Clark-Kennedy, Stephen Hales:An Eighteenth Century Biography (1929). General background studies include Charles Singer and E. Ashworth Underwood, A Short History of Medicine (1928; 2d ed. 1962), and Abraham Wolf, A History of Science, Technology, and Philosophy in the Eighteenth Century (1938; 2d rev. ed. 1952). □

Hales, Stephen

English Physiologist 1677-1761

Stephen Hales was a preeminent scientist of the late eighteenth century and the founder of plant physiology . Born in Kent, England, in 1677, Hales grew up in an upper-class Kent family and was educated at Cambridge University. Though he received no formal training in botany during college, Hales obtained a solid background in science, including physics and mechanics. Upon graduation from Cambridge, Hales moved to Teddington, a town on the Thames River in England, where he lived the rest of his life.

Hales has been called the first fully deductive and quantitative plant scientist. He made many significant discoveries concerning both animal and plant circulation. Crucially, Hales measured plant growth and devised innovative methods for the analysis and interpretation of these measurements.

Hales's most original contribution was his transfer of application of the so-called statical method he and others had used on animals to plant specimens . The basis behind the statical method was the belief that the comprehension of living organisms was possible only through the precise measurements of their inputs and outputs. Thus the way to understand a human being would be to measure the fluids and other materials that had entered and left it. In the case of a tree, a statistician would measure changes in the amount and quality of the water it consumed and the sap it contained.

In 1706, under the influence of Isaac Newton's new mechanics, Hales tried to figure out the mechanism that controlled animal blood pressure by experimenting on dog specimens. At the same time, Hales had the idea that the circulation of sap in plants might well be similar to the circulation of the blood in humans and other animals. As he was exploring animal circulation, Hales grew increasingly interested in plant circulation. He wrote later in his book Vegetable Staticks of his first circulation experiments: "I wished I could have made the like experiments to discover the force of the sap in vegetables."

After a decade of quiet research and study, Hales did indeed devise such experiments on plants. He attached glass tubes to the cut ends of vine plants. He then watched sap rise through these tubes, and he monitored how the sap flow varied with changing climate and light conditions. In 1724, Hales completed Vegetable Staticks (quoted above), wherein he distinguished three different aspects of water movement in plants. These he called imbibition, root pressure, and leaf suction.

The prevalent notion among Hales's contemporaries was that the movement of plant sap was similar to the circulation of human blood, which was discovered by William Harvey in 1628. Crucially, Hales demonstrated that this theory was false. Instead, he demonstrated the constant uptake (absorption) of water by plants and water's constant loss through transpiration (evaporation into the air). Drawing on this principle, Hales made many exact and careful experiments using weights and measures. All of these he repeated using different types of plants (willows and creepers, for example) in order to verify his conclusions. Thus, from his beginnings as a physiologist, Hales went on to create a mechanics of water movement.

see also de Saussure, Nicholas; Physiologist; Physiology, History of; Water Movement.

Hanna Rose Shell

Bibliography

Isley, Duane. "Hales." One Hundred and One Botanists. Ames, IA: Iowa State University Press, 1994.

Morton, Alan G. "Hales." History of Botanical Science. London: Academic Press, 1981.