Professional historians of science generally recognize the importance of Lavoisier's theory of heat. However, it commonly receives scant attention in the historical treatment of his chemical theories except perhaps as an example illustrating his conservatism and giving the impression that the caloric theory, although perhaps important in the development of ideas on the nature of heat, is independent of and bears little relationship to his general chemistry or is incidental to an understanding of that chemistry.1 An examination of Lavoisier's writings suggests that the caloric theory is not merely a milestone in the development of physics; and rather than an omittable appendage, his concept of heat forms an integral part of his chemical system and plays a central, necessary role in his oxidation theory in particular. The purpose of this paper is to give a general description of Lavoisier's ideas on the nature and action of heat, the origin of these ideas, their development, and their relation to his general chemistry, pointing out his conservatism as well as his innovations.

In the late eighteenth century, which was for Scotland in many ways an ‘Age of Improvement’, the University of Edinburgh enjoyed a golden age. Under the enlightened principalship of the Reverend William Robertson, the University offered wide, flexible, and mainly secular courses of study which were taught by conspicuously able professors. If we restrict ourselves to scientific chairs, a roll-call of their occupants is distinctly impressive: John Robison (natural philosophy, 1774–1805); John Playfair (mathematics, 1785–1805); John Walker (natural history, 1779–1804); Daniel Rutherford (botany, 1786–1819); James Gregory (theory of medicine, 1776–89); William Cullen (practice of medicine, 1773–90); Alexander Monro secundus (anatomy, 1758–98); and their doyen Joseph Black (chemistry and medicine, 1766–99), ‘so pale, so gentle, so elegant, and so illustrious’. The scientific eminence of the University at that time is, of course, widely acknowledged.

Acceptance of Augustin Fresnel's wave theory of light posed numerous questions for early nineteenth-century physicists. Among the most pressing was the problem of the properties of the luminiferous ether. Fresnel had shown that light waves were transverse. Therefore, since, among ordinary materials, only solids support transverse vibrations, there existed striking likenesses between highly tangible solids and the highly intangible ether. Accordingly, such men as Augustin-Louis Cauchy, James MacCullagh, Franz Neumann, and George Green constructed various theories of an elastic-solid ether.1 At the same time, however, the disconcerting implausibilities of an all-pervasive solid provoked considerable apprehension in regard to the elastic-solid tradition. Thomas Young found the concept ‘perfectly appalling’ and argued that ‘the hypothesis [that fluids can support transverse vibrations] remains completely open for discussion, notwithstanding the apparent difficulties attending it.’2 John Herschel, probably the most important English advocate of the wave theory, regarded the concept of a solid ether as only a temporary device, useful ‘till the real truth shall be discovered.’3 Consequently, despite the accomplishments which helped to make the elastic-solid theory ‘the most celebrated special form of the wave theory’,4 there were important voices of reservation.