ties of carbonic acid, which he called by the name of "fixed air."
After discovering this gas, Black made a long series of experiments,
by which he was able to show how widely it was distributed throughout
nature. Thus, in 1757, he discovered that the bubbles given off in
the process of brewing, where there was vegetable fermentation, were
composed of it. To prove this, he collected the contents of these
bubbles in a bottle containing lime-water. When this bottle was
shaken violently, so that the lime-water and the carbonic acid became
thoroughly mixed, an insoluble white powder was precipitated from the
solution, the carbonic acid having combined chemically with the lime
to form the insoluble calcium carbonate, or chalk. This experiment
suggested another. Fixing a piece of burning charcoal in the end of a
bellows, he arranged a tube so that the gas coming from the charcoal
would pass through the lime-water, and, as in the case of the bubbles
from the brewer's vat, he found that the white precipitate was thrown
down; in short, that carbonic acid was given off in combustion. Shortly
after, Black discovered that by blowing through a glass tube inserted
into lime-water, chalk was precipitated, thus proving that carbonic acid
was being constantly thrown off in respiration.

The effect of Black's discoveries was revolutionary, and the attitude
of mind of the chemists towards gases, or "airs," was changed from that
time forward. Most of the chemists, however, attempted to harmonize the
new facts with the older theories--to explain all the phenomena on the
basis of the phlogiston theory, which was still dominant. But while many
of Black's discoveries could not be made to harmonize with that
theory, they did not directly overthrow it. It required the additional
discoveries of some of Black's fellow-scientists to complete its
downfall, as we shall see.

HENRY CAVENDISH

This work of Black's was followed by the equally important work of
his former pupil, Henry Cavendish (1731-1810), whose discovery of the
composition of many substances, notably of nitric acid and of water,
was of great importance, adding another link to the important chain of
evidence against the phlogiston theory. Cavendish is one of the most
eccentric figures in the history of science, being widely known in his
own time for his immense wealth and brilliant intellect, and also for
his peculiarities and his morbid sensibility, which made him dread
society, a