t and physicist, was born in
1728 at Bordeaux, where his father--a native of Belfast but of Scottish
descent--was engaged in the wine trade. At the age of twelve he was sent
to a grammar school in Belfast, whence he removed in 1746 to study
medicine in Glasgow. There he had William Cullen for his instructor in
chemistry, and the relation between the two soon became that of
professor and assistant rather than of master and pupil. The action of
lithontriptic medicines, especially lime-water, was one of the questions
of the day, and through his investigations of this subject Black was led
to the chemical discoveries associated with his name. The causticity of
alkaline bodies was explained at that time as depending on the presence
in them of the principle of fire, "phlogiston"; quicklime, for instance,
was chalk which had taken up phlogiston, and when mild alkalis such as
sodium or potassium carbonate were causticized by its aid, the
phlogiston was supposed to pass from it to them. Black showed that on
the contrary causticization meant the loss of something, as proved by
loss of weight; and this something he found to be an "air," which,
because it was fixed in the substance before it was causticized, he
spoke of as "fixed air." Taking _magnesia alba_, which he distinguished
from limestone with which it had previously been confused, he showed
that on being heated it lost weight owing to the escape of this fixed
air (named carbonic acid by Lavoisier in 1781), and that the weight was
regained when the calcined product was made to reabsorb the fixed air
with which it had parted. These investigations, by which Black not only
gave a great impetus to the chemistry of gases by clearly indicating the
existence of a gas distinct from common air, but also anticipated
Lavoisier and modern chemistry by his appeal to the balance, were
described in the thesis _De humore acido a cibis orto, et magnesia
alba_, which he presented for his doctor's degree in 1754; and a fuller
account of them was read before the Medical Society of Edinburgh in June
1753, and published in the following year as _Experiments upon magnesia,
quicklime and some other alkaline substances_.

It is curious that Black left to others the detailed study of this
"fixed air" he had discovered. Probably the explanation is pressure of
other work. In 1756 he succeeded Cullen as lecturer in chemistry at
Glasgow, and was also appointed professor of anatomy, though that post
he w