Hales, Stephen (1677–1761)

English scientist who studied the role of water and air in the maintenance of life. He gave accurate accounts of water movement in plants. He demonstrated that plants absorb air, and that some part of that air is involved in their nutrition. His work laid emphasis on measurement and experimentation.

Hales's work on air revealed to him the dangers of breathing ‘spent’ air in enclosed places, and he invented a ventilator which improved survival rates when introduced on naval, merchant, and slave ships, in hospitals, and in prisons.

Hales was born in Kent and studied at Cambridge. A cleric, he was curate at Teddington, Middlesex, from 1709. His experiments on plants took place mainly between 1719 and 1725.

He measured the pressure of sap in growing vines, calculated its velocity, and found that the rate of flow varies in different plants. He measured plant growth and water loss, relating this to the upward movement of water from plants to leaves (transpiration). He also measured blood pressure and the rate of blood flow in animals.

Hales examined stones taken from the bladder and kidney, and suggested possible chemical solvents for their nonsurgical treatment. He also invented the surgical forceps.

Hales's findings were published in his book Vegetable Staticks 1727, enlarged in 1733 and retitled Statical Essays, Containing Haemastaticks, etc.