Jan Ingenhousz (1730-1799) was a Dutch physiologist, biologist and chemist. In his lifetime he was best known for successfully inoculating the members of the Habsburg family in Vienna against smallpox in 1768 and subsequently being the private counsellor and personal physician to the Austrian Empress Maria Theresa. But most importantly, in 1779, Ingenhousz discovered that, in the presence of light, plants give off bubbles from their green parts while, in the shade, the bubbles eventually stop. He identified the gas as oxygen. 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, thus discovering what we have come to call photosynthesis and cellular respiration. He lived to be 68 and died of natural causes.