Stephen Hales

Stephen Hales (1677–1761) was an English clergyman and natural philosopher whose experiments advanced plant physiology, gas chemistry, and the measurement of blood pressure.

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William Stukeley: William Stukeley was an English antiquarian and physician who studied ancient monuments and corresponded with natural philosophers. Stukeley was Hales’s close friend and biographer, preserving details of his life and work.
Blood pressure: Blood pressure is the force exerted by circulating blood against the walls of blood vessels. Hales measured arterial pressure directly by connecting a horse’s artery to a glass tube.
Experimental philosophy: Experimental philosophy is an approach to knowledge that relies on observation and controlled experiments to investigate nature. Hales used measured interventions rather than speculation to study biological and chemical processes.
Air pump: An air pump is a device that removes air from a sealed vessel to create reduced pressure. Air-pump experiments helped Hales investigate gases released by materials and living processes.
Statistical analysis: Statistical analysis is the use of mathematical methods to summarize data and assess patterns or uncertainty. Hales used numerical tables and repeated measurements to compare plant growth and bodily fluids.
Joseph Priestley: Joseph Priestley was an English chemist and dissenting clergyman known for experiments on gases, including the isolation of oxygen. Priestley’s later gas experiments developed a field Hales helped establish.
Manometer: A manometer is an instrument that measures fluid pressure by balancing it against a column of liquid or another reference pressure. The vertical tube in Hales’s arterial experiment converted pressure into a measurable column of blood.
Plant physiology: Plant physiology is the study of the functions and processes of living plants. Hales’s measurements of water, growth, and plant fluids helped establish this field.
Ship ventilation: Ship ventilation is the circulation of fresh air through a vessel’s enclosed spaces. Hales devised mechanical ventilators to reduce foul air aboard ships.
Joseph Black: Joseph Black was a Scottish chemist who identified carbon dioxide as a distinct gas, which he called fixed air. Black’s gas research followed the experimental path opened by Hales’s pneumatic investigations.