6.3 Heat research
Andrew Plummer (c. 1698–1756), the chemistry professor at Edinburgh, suffered a stroke in 1755, and the Town Council appointed Cullen as his conjoint professor without consulting the stricken Plummer. Black, who had covered for Plummer until Cullen arrived, was appointed to Cullen's position at the University of Glasgow. This move also marked a change in the direction of Black's research. He now began to investigate the nature of heat, a central topic in eighteenth-century chemistry.
It is important to realise that most chemists in this period regarded heat as a substance, if perhaps one without measurable weight, and the study of heat was therefore considered an appropriate field for chemists. Hermann Boerhaave devoted a long section to ‘fire’ in his famous Elementa chemise (1732). In his lectures, Cullen listed ‘fire’ as the second primary cause of chemical change, after the elective attraction (chemical affinity) – precisely the order of Black's research (Donovan, 1975, p. 131). Black doubtlessly believed that some form of chemical combination took place between heated materials, such as water, and heat. At the same time, however, he was even more reluctant to hypothesise than Cullen. His work on latent and specific heats was not based on any theoretical foundation, except for a belief that substances possessed a capacity to take up heat.
It is thus unwise to regard Black's research as constructing a theory of heat. Black simply sought to make clear the manner in which a given substance, most notably water, absorbed heat. This was in keeping with the Enlightenment philosophy that it was important to establish the causes of natural phenomena by examining the facts, without resorting to speculative assumptions or ‘hypotheses’. As he later explained to his former assistant John Robison (1739–1805), he considered every hypothetical explanation as a mere waste of time and ingenuity‘ (in Robison, 1803, vol. 1, p. vii).