may be said to have its beginning with the work of Stephen Hales (1677-1761),
who early in the eighteenth century began his important study of the elasticity
of air. Departing from the point of view of most of the scientists of the
time, be considered air to be "a fine elastic fluid, with particles of
very different nature floating in it" ; and he showed that these "particles"
could be separated. He pointed out, also, that various gases, or "airs,"
as he called them, were contained in many solid substances. The importance
of his work, however, lies in the fact that his general studies were along
lines leading away from the accepted doctrines of the time, and that they
gave the impetus to the investigation of the properties of gases by such
chemists as Black, Priestley, Cavendish, and Lavoisier, whose specific
discoveries are the foundation-stones of modern chemistry.