text: In the eighteenth century, as the field of chemistry was evolving from alchemy, a field of the natural philosophy was created around the idea of air as a reagent. Before this, air was primarily considered a static substance that would not react and simply existed. However, as Lavoisier and several other pneumatic chemists would insist, the air was indeed dynamic, and would not only be influenced by combusted material, but would also influence the properties of different substances.
  The initial concern of pneumatic chemistry was combustion reactions, beginning with Stephen Hales. These reactions would give off different "airs" as chemists would call them, and these different airs contained more simple substances. Until Lavoisier, these airs were considered separate entities with different properties; Lavoisier was responsible largely for changing the idea of air as being constituted by these different airs that his contemporaries and earlier chemists had discovered.
  This study of gases was brought about by Hales with the invention of the pneumatic trough, an instrument capable of collecting the gas given off by reactions with reproducible results. The term gas was coined by J. B. van Helmont, in the early seventeenth century. This term was derived from the Ancient Greek word χάος, chaos, as a result of his inability to collect properly the substances given off by reactions, as he was the first natural philosopher to make an attempt at carefully studying the third type of matter. However, it was not until Lavoisier performed his research in the eighteenth century that the word was used universally by scientists as a replacement for airs.
  Van Helmont (1579 &ndash; 1644) is sometimes considered the founder of pneumatic chemistry, as he was the first natural philosopher to take an interest in air as a reagent. Alessandro Volta began investigating pneumatic chemistry in 1776 and argued that there were different types of inflammable air based on experiments on marsh gases. Pneumatic chemists credited with discovering chemical elements include Joseph Priestley, Henry Cavendish, Joseph Black, Daniel Rutherford, and Carl Scheele.  Other individuals who investigated gases during this period include Robert Boyle, Stephen Hales, William Brownrigg, Antoine Lavoisier, Joseph Louis Gay-Lussac, and John Dalton.