as glad to exchange for the chair of medicine. The preparation of
lectures thus took up much of his time, and he was also gaining an
extensive practice as a physician. Moreover, his attention was engaged
on studies which ultimately led to his doctrine of latent heat. He
noticed that when ice melts it takes up a quantity of heat without
undergoing any change of temperature, and he argued that this heat,
which as was usual in his time he looked upon as a subtle fluid, must
have combined with the particles of ice and thus become latent in its
substance. This hypothesis he verified quantitatively by experiments,
performed at the end of 1761. In 1764, with the aid of his assistant,
William Irvine (1743-1787), he further measured the latent heat of
steam, though not very accurately. This doctrine of latent heat he
taught in his lectures from 1761 onwards, and in April 1762 he described
his work to a literary society in Glasgow. But he never published any
detailed account of it, so that others, such as J.A. Deluc, were able to
claim the credit of his results. In the course of his inquiries he also
noticed that different bodies in equal masses require different amounts
of heat to raise them to the same temperature, and so founded the
doctrine of specific heats; he also showed that equal additions or
abstractions of heat produced equal variations of bulk in the liquid of
his thermometers. In 1766 he succeeded Cullen in the chair of chemistry
in Edinburgh, where he devoted practically all his time to the
preparation of his lectures. Never very robust, his health gradually
became weaker and ultimately he was reduced to the condition of a
valetudinarian. In 1795 he received the aid of a coadjutor in his
professorship, and two years later he lectured for the last time. He
died in Edinburgh on the 6th of December 1799 (not on the 26th of
November as stated in Robison's life).

As a scientific investigator, Black was conspicuous for the carefulness
of his work and his caution in drawing conclusions. Holding that
chemistry had not attained the rank of a science--his lectures dealt
with the "effects of heat and mixture"--he had an almost morbid horror
of hasty generalization or of anything that had the pretensions of a
fully fledged system. This mental attitude, combined with a certain lack
of initiative and the weakness of his health, probably prevented him
from doing full justice to his splendid powers of experimental research.
Apart