Johann Baptista Von Helmont
Johann Baptista Van Helmont (1579-1644) of Brussels studied several subjects before finally choosing to follow a career in medicine. He became a medical chemist as well as an ardent follower of Paracelsus. The chemical side of medicine finally took over from the medical side and he devoted much of his life to chemical experiments. He has been described as the last alchemist and the first chemist, because although he believed in alchemy, including the production of gold from lead which he claimed to have performed, his emphasis upon experiment rather than argument is a great advance. 

Van Helmont is an important figure in the development of chemical concepts because it is impossible to separate an understanding of the nature of air from an understanding of the nature of combustion, or burning in air. Air had been considered by Aristotle and the Greek philosophers as one of the four elements, with real "airs" or what we call gases all being more or less contaminated ideal air. 

Johann Baptista Van Helmont
The concept of different gases was not clearly understood. All known gases or vapors were considered as different mixtures of air and earth or air and fire. This Greek understanding of the nature of air persisted through the Middle Ages and through the period of alchemy. 

Van Helmont was probably the first to recognize, and was the first to state in print, that there existed or could be created several specific different kinds of gas each with different properties. Indeed, the word gas was first used by Van Helmont. Many of the 15 or so "different" gases mentioned by Van Helmont we now know to be mixtures of gases, or were carbon dioxide obtained in different ways. The major advance of recognition of the existence, and some of the properties, of different gases we owe to him. We also owe other advances in quantitative measurements, including the use of the balance for precise weighings, to him, as well as some advances in medicine. 
Van Helmont is also known for an experiment in which he planted a tree in a pot of earth and weighed both the earth and the tree after five years. Since the weight of the earth had decreased by at most a few ounces while the weight of the tree was about 170 pounds, Van Helmont concluded that the tree arose from water only, since he had added nothing else to the pot!   The period of Van Helmont's chemical contributions began in the seventeenth century, probably around 1609, but his work was not generally published until 1648.