Jan Baptist van Helmont (January 12, 1580 - December 30, 1644) was a Flemish chemist, physiologist, and physician. He married well and retired early from his medical practice and occupied himself with chemical experiments. He is sometimes considered to be "the founder of pneumatic chemistry" as he was the first to understand that there are gases distinct in kind from atmospheric air. The very word "gas" he claimed as his own invention, and he perceived that his "gas sylvestre" (carbon dioxide) given off by burning charcoal, was the same as that produced by fermenting must, which sometimes renders the air of caves unbreathable. For Van Helmont, air and water were the two primitive elements. To determine where plants get their mass he grew a willow tree in a pot and measured the amount of soil, the weight of the tree and the water he added. After five years the plant had gained about 164 pounds. He argued believed that water was the source of the extra mass and the plant's source of life. He also wrote extensively on the subject of digestion and believed that digestion was aided by a chemical reagent, or "ferment", within the body, such as inside the stomach. He died in 1644 at the age of 65.