volume of ‘Essays and Observations’ (1756), with the title ‘Experiments upon Magnesia alba, Quicklime, and some other Alkaline Substances,' and subsequently twice reprinted (1777 and 1782).
As a model for philosophical investigation this essay was, by Brougham and Robison, placed second only to the ‘Optics‘ of Newton. Its importance in chemical history is two-fold. By setting an example of the successful use of the balance, it laid the foundation of quantitative analysis; and by the distinction of qualities conveyed in it between ‘fixed’ and common air, it opened the door to pneumatic chemistry. Up to that time the causticity of alkalis after exposure to strong heat had been universally attributed to an acrid principle derived from fire. Black showed that they lost instead of gained in weight by calcination; and that what they lost was a kind of ‘air' previously ‘fixed’ in them, and neutralising, by its acid qualities, their native causticity. The effervescence of ‘mild’ and non-effervescence of ‘caustic’ alkalis when dissolved in acids were alleged in countenance of the new theory, which, nevertheless, encountered a vigorous, though futile, opposition in Germany. It was pointed out in the same remarkable treatise that magnesia, until then generally held to he a variety of lime, formed, with the same acids, wholly different salts, and was consequently to he regarded as a distinct substance.
Black was fully aware of the vastness of the field of research thrown wide by the discovery (or rather individualisation) of fixed air, named by Lavoisier in 1784 ‘carbonic acid’ (Mém. de l'Acad. 1781, p. 455). In 1757 he ascertained its effects upon animals, and its production by respiration, fermentation, and the burning of charcoal (Lectures, ii. 87–8). He also inferred its invariable presence, in small quantities, in the atmosphere. Here, however, he stopped, leaving the path which he had struck out to be pursued by Cavendish, Priestley, and Lavoisier.
On the removal of Cullen to Edinburgh, Black was appointed in 1756 to replace him in the chair of anatomy and chemistry in the university of Glasgow; but dissatisfied with his qualifications for the former post, he exchanged duties with the professor of medicine, and lectured during the ensuing ten years with much care and success on the institutes of medicine, He was at the same time in large practice as a physician, and devoted the most anxious care to the welfare of his patients. Nevertheless he found time to complete the second achievement in science with which his name remains associated. This is the discovery of what is termed ‘latent heat.' In 1756 he began to meditate on the perplexing slowness with which ice melts, and water is dissipated in boiling. He divined the cause in 1757, and ascertained it in 1761. A large quantity of heat, he found, is consumed in bringing about these changes in the state of aggregation, and is thus rendered insensible to the thermometer. The cause of this disappearance, according to modern theory, is the employment of the absorbed heat in doing work—that is, conferring ‘potential energy' on material particles; in Black’s view it was the formation of a quasi-chemical combination between those particles and the subtle fluid of heat, But this erroneous conception in no way detracted from the importance of his discovery. The decisive experiment of obtaining from water during congelation an amount of heat equal to that expended or rendered ‘latent’ in its liquefaction was performed in December 1761. This quantity he measured at rather more than would have sufficed to raise the temperature of the same weight of water 140° Fahrenheit (accurately 143°). He, however, considerably underestimated the latent heat of steam, fixing it, with his pupil Irvine’s assistance, 9 Oct. 1764, at 750° (later at 810°) instead of 967°. The results of this brilliant investigation not only formed the basis of modern thermal science, but gave the first impulse to Watt's improvements in the steam-engine, and thereby to modern industrial developments. Black read an account of his successful experiments before a literary society in Glasgow, 23 April 1762, and from 1761 downwards carefully taught the doctrine of latent heat in his lectures, dwelling with sedate eloquence on the beneficent effects of the arrangement in checking and regulating the processes of nature. But he published nothing on the subject, and was thus scarcely entitled to complain if his ideas were appropriated with little or no acknowledgment. To the same society he detailed, 28 March 1760, a series of experiments instituted with the object of testing the validity of thermometrical indications. He originated, moreover, the theory of ‘specific heat,’ or of the various thermal ‘capacities’ of different bodies, but committed it to Irvine to work out. Still treading in his master's footsteps, Black became, on Cullen's advancement to a higher post in 1766, professor of medicine and chemistry in the university of Edinburgh. His career thenceforward was exclusively that of a teacher. Restricting his medical practice to a narrow circle of friends, and abandoning all thought of original research, he concentrated his powers upon the effective discharge of his official duties. His success was con-
