Gases (ca. 1620–1670): To the ancient Greeks, air was one of the four fundamental elements from which all matter was derived. As such, there existed very little scientific interest in breaking down air into individual gases. However, in the 17th century the development of the scientific method challenged the historical basis of Greek science. In the early part of the century a number of scientists were redefining the Greek definition of an element. During this time there were a number of discoveries and inventions that revolved around air and the atmosphere. The invention of the vacuum pump and the barometer, coupled with explorations in air pressure and combustion and the development of chemistry as a scientific discipline, all heralded a renewed scientific interest in the study of air (see BAROMETER; BOYLE’S LAW; CHEMISTRY; COMBUSTION; VACUUM PUMP). Although the majority of the work in the study of individual gases would be accomplished in the next century, a number of important contributions were made during the 17th century.
The term gas originated in the 17th century with the work of the Belgian chemist Jan Baptista van Helmont. Owing to religious beliefs, Helmont rejected the Greek four-element theory of Aristotle. He contended that since fire is not mentioned in Genesis, it should not be considered an element. Furthermore, he believed that air was inert and did not contribute to chemical reactions. Thus, all matter must be formed from some form of water. To test his hypothesis he repeated an experiment first proposed by Nicolas of Cusa (1401–1464) in the 15th century. He placed a willow tree in a pot containing 200 pounds of soil. For the next several years Helmont provided water to the tree but did not make any additions to the soil. At the end of the experi-