STEPHEN, D.D., a distinguished physiologist, was born in 1667 at Beckeshour in Kent. He was educated at Bennet College, Cambridge, of which, in 1702, he became a fellow. In 1710 he was made perpetual curate of Teddington, near Twickenham, then rector of Porlock in Somersetshire, and afterwards of Faringdon in Hampshire. Despite these preferments, however, he continued to reside at Teddington. In 1718 he became a member of the Royal Society, to whose Transactions he contributed a large number of ingenious and valuable papers. In 1753, on the death of Sir Hans Sloane, he was chosen foreign associate of the French Académie des Sciences. He died at Teddington in 1761.

Soon after his election into the Royal Society, Hales published his Vegetable Statics, or an Account of some Statistical Experiments on the Sop in Vegetables, and has thus the honour of having made the first discoveries in what has since become the great science of vegetable physiology. The work was soon translated into French, Italian, German, and other continental languages, and is still an authority on the subject which it treats. In 1733 he published, under the title of Hemastatics, a similar essay on the circulation of the blood. But the most important services which he rendered to his country were rather those of a practical philanthropist than of a merely scientific observer. One of the most useful of his inventions was his "ventilator," with which he solved the difficult problem of introducing abundant supplies of pure and fresh air into jails, hospitals, mines, and the holds of ships. In 1749 one of these ventilators was fairly tested in the Savoy prison, where the annual average of mortality had been from fifty to a hundred victims. From 1749 till 1752 inclusive, however, only four persons died within the jail, though in 1750 the number of prisoners was 240. Of these four, one died of small-pox and another of delirium tremens. The advantages of Hales' ventilators were thus so fully established that they were immediately adopted by the French government, and applied with success in a far greater number of cases than the inventor had ever contemplated. In 1752 another of these machines was fitted up in Newgate, and the mortality immediately fell in the proportion of 7 to 16. Hales' other inventions were all of an equally practical and useful character. They had reference chiefly to such objects as preserving meats for long sea-voyages, distilling sea-water, cleaning and preserving corn, &c. He also gained the Copley gold medal for his essay on the medicines for dissolving the stone in the bladder.