Jan Ingenhousz

Jan Ingenhousz or Ingen-Housz (December 8, 1730, Breda - September 7, 1799) was Dutch physiologist, botanist and physicist. He is best remembered for showing that light is essential to plant respiration, a vital step in the discovery of photosynthesis. He was a physician to the Austrian Empress Maria Theresa.

Ingenhousz discovered that, in the presence of light, plants give off bubbbles from their green parts while, in the shade, the bubbles eventually stop. He identified the gas as oxygen (for this discovery he is known affectionately as "the Oxygen man"). He also discovered that, in the dark, plants give off carbon dioxide. He realized as well that the amount of oxygen given off in the light is more than the amount of carbon dioxide given off in the dark. This demonstrated that some of the mass of plants comes from the air, and not only the soil.

In 1785, he described the irregular movement of coal dust on the surface of alcohol and therefore has a claim as discoverer of what came to be known as Brownian motion.