Jan Ingenhousz. Jan Ingenhousz (1730–1799) Jan Ingenhousz was born in Breda, Netherlands, and was a physician by trade. He was a proponent of using live smallpox virus in inoculations before Edward Jenner developed his safer vaccine based on cowpox virus. Having become famous promoting vaccination, he was appointed court physician to Austrian Empress Maria Theresa, and this stable income allowed him to pursue his interest in chemistry and physics. As we know, a candle can only burn in a closed jar for a few moments before going out, and animals can only breathe for a short while in an enclosed space before suffocating; but we did not always know why this was the case. Ingenhousz investigated this phenomenon. Joseph Priestley had earlier shown that plants could restore the ability of the air in the jar to support a flame or keep animals alive, but it was Ingenhousz who observed that plants only had this effect when exposed to light. He further concluded that plants were converting carbon dioxide into oxygen, which animals converted back into carbon dioxide. For elucidating this cycle, Ingenhousz is considered the discoverer of photosynthesis. He also showed that only the green parts of plants carry out photosynthesis. Ingenhousz’s other work included early observations of Brownian motion and investigations into electricity and magnetism. He lived and worked in England, the Netherlands, France, and Austria, and communicated with many of the great minds of his time, including Benjamin Franklin. He considered moving to the Philadelphia area, but changed his plans when Franklin died in 1790.