uch an impetus, were showing in the attitude
of the experimenters. The works of some of the older writers, such
as Boyle and Hooke, were again sought out in their dusty corners and
consulted, and their surmises as to the possible mixture of various
gases in the air were more carefully considered. Still the phlogiston
theory was firmly grounded in the minds of the philosophers, who can
hardly be censured for adhering to it, at least until some satisfactory
substitute was offered. The foundation for such a theory was finally
laid, as we shall see presently, by the work of Black, Priestley,
Cavendish, and Lavoisier, in the eighteenth century, but the phlogiston
theory cannot be said to have finally succumbed until the opening years
of the nineteenth century.

II. THE BEGINNINGS OF MODERN CHEMISTRY

THE "PNEUMATIC" CHEMISTS

Modern chemistry 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, he 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.

JOSEPH BLACK

The careful studies of Hales were continued by his younger confrere, Dr.
Joseph Black (1728-1799), whose experiments in the weights of gases and
other chemicals were first steps in quantitative chemistry. But even
more important than his discoveries of chemical properties in general
was his discovery of the properties of carbonic-acid gas.

Black had been educated for the medical profession in the University of
Glasgow, being a friend and pupil of the famous Dr. William Cullen. But
his liking was for the chemical laboratory rather than for the practice
of medicine. Within three years after completing his medical course,
and when only twenty-three years of age, he made the discovery of the
proper