BIOGRAPHICAL SKETCHES. 53
his celebrated discovery of jxed air, or carbonic acid gas. We are informed
by himself, that he was led to the examination of the absorbent earths, partly
by the hope of discovering a new sort of lime and limewater, which might possibly
be a more powerful solvent of the stone than that commonly used. The
attention of the public had been directed to this subject for some years. Sir
Robert, as well as his brother, Horace, afterwards Lord Walpole, were troubled
with the stone. They imagined that they had received benefit from a medicine
invented by a Mrs. Stephens; and, through their interest principally, she
received five thousand pounds sterling for revealing the secret. It was accordingly
published in the Londm Gazette on the 19th June 1739. This .had
directed the attention of medical men to the employment of lime-water in cure
of the stone. Upon the publication of the thesis, it immediately attracted the
attention of chemical- philosophers ; and Dr. Black is now universally acknowledged
to be the founder of pneumatic chemistry, and to have opened an iinmense
field for observation and experiment to the philosophical world, which
before his time had never been explored or even thought of.
Dr. Cullen removing to Edinburgh in 1756, Dr. Black was appointed Professor
of Anatomy and Lecturer on Chemistry ; but not conceiving himself so well
qualsed for filling the anatomical chair, he obtained the concurrence of the University
to accomplish an exchange with the Professor of Medicine. He brought
to maturity his theory of latent heat, some time between 1759 and 1763 ; and
he read, in April 1762, to a select society in Glasgow, the result of his experiments
on the subject. Much about the same year he read the essay on latent
heat before a society in Edinburgh, bearing the name of the Newtonian Society,
instituted in 1759. The delicate state of his health was the cause of his
never publishing an account of his doctrine, as the slightest exertion, if continued
for any length of time, always brought on a spitting of blood ; and the
excitement which a publication of this description would necessarily have produced,
and the controversy and criticism that would have followed, was much
more than his feeble frame could have borne.
In 1764, it was fortunate both for Dr. Black and science, that Mr. James
Watt, so justly celebrated for his improvements of the steam-engine, became his
pupil, he being at that time employed in repairing the model of a steam-engine
for the Natural Philosophy class in the University.
In the year 1766, Dr. Cullen, the Professor of Chemistry in the University of
Edinburgh, was appointed Professor of Medicine; and the chemical chair in
the University thus becoming vacant, Dr. Black was immediately appointed to
it, and he continued one of the chief ornaments of the University for a space of
about thirty years.
Dr. Black lived on very friendly terms with most of the many literary
characters then resident in the northern metropolis. Amongst these we may
mention his relative, Dr. Adam Ferguson, Mr. Home, author of the tragedy of
DozcgZus, Dr. Alexander Carlyle, Sir George Clerk of Penycuick, his brother
Mr. Clerk of Eldin, Dr. Roebuck, and Dr. Hutton.
