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Hales' pneumatic trough. Diagram showing the concept of the pneumatic trough, as used by Stephen Hales (1677-1761). Hales used this device to collect gases for his research in plant physiology. Although Hales thought he was collecting different kinds of air (rather than different compounds), he understood many important principles. He showed that plants use part of the air (actually carbon dioxide) for nutrition. He showed that light is needed for plant growth, and that transpiration (water loss from the leaves) causes sap to rise. He was also a prodigious inventor, working on food preservation, water purification and ventilation systems.
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