[758] He was appointed professor in 1756; and ‘it was during his residence in Glasgow, between the years 1759 and 1763, that he brought to maturity those speculations concerning the combination of heat with matter, which had frequently occupied a portion of his thoughts.’ Thomson's History of Chemistry, vol. i. pp. 319, 320.
[759] Black's Lectures on Chemistry, vol. i. pp. 116, 117; and in various places. Dr. Robison, the editor of these Lectures, says, p. 513, ‘Nothing could be more simple than his doctrines of latent heat. The experience of more than a century had made us consider the thermometer as a sure and accurate indicator of heat, and of all its variations. We had learned to distrust all others. Yet, in the liquefaction and vaporization of bodies, we had proofs uncontrovertible of the entrance of heat into the bodies. And we could, by suitable processes, get it out of them again. Dr. Black said that it was concealed in them,—latent,—it was as much concealed as carbonic acid is in marble, or water in zeolite,—it was concealed till Dr. Black detected it. He called it Latent Heat. He did not mean by this term that it was a different kind of heat from the heat which expanded bodies, but merely that it was concealed from our sense of heat, and from the thermometer.’ See also p. xxxvii.: ‘Philosophers had long been accustomed to consider the thermometer as the surest means for detecting the presence of heat or fire in bodies, and they distrusted all others.’
[760] ‘Fluidity is the consequence of a certain combination of calorific matter with the substance of solid bodies,’ &c. Black's Lectures, vol. i. p. 133. Compare p. 192, and the remarks in Turner's Chemistry, 1847, vol. i. p. 31, on Black's views of the ‘chemical combination’ of heat. Among the backward chemists, we still find traces of the idea of heat obeying chemical laws.
[761] ‘So much was he convinced of this, that he taught the doctrine in his lectures in 1761, before he had made a single experiment on the subject.’ … ‘The requisite experiments were first attempted by Dr. Black in 1764.’ Thomson's History of Chemistry, vol. i. p. 324. See also pp. 319, 320; and on the history of the idea in Black's mind as early as the year 1754, see the interesting extracts from his note-books in Robison's appendix to Black's Lectures, vol. i pp. 525, 526.
The statement of Dr. Thomson refers to the completion, or last stage, of the discovery, namely the vaporific combination of heat. But from a letter which Black wrote to Watt in 1780 (Muirhead's Life of Watt, London, 1859, p. 303), it appears that Thomson has even understated the question, and that Black, instead of first teaching his theory in 1761, taught it three years earlier, that is, six years before the decisive experiments were made. ‘I began,’ writes Black, ‘to give the doctrine of latent heat in my lectures at Glasgow in the winter 1757–58, which, I believe, was the first winter of my lecturing there; or if I did not give it that winter, I certainly gave it in the 1758–59; and I have delivered it every year since that time in my winter lectures, which I continued to give at Glasgow until winter 1766–67, when I began to lecture in Edinburgh.’
[762] And he distinctly states that, even in other matters, when he did make experiments, their object was to confirm theory, and not to suggest it. Thus, to give one of many instances, in his Lectures, vol. i. p. 354, he says, respecting salts, ‘When we examine the solidity of this reasoning by an experiment, we have the pleasure to find facts agree exactly with the theory.’
[763] See a good summary of this idea in Black's Lectures on Chemistry, vol. i. p. 118. Contrasting his theory of heat with that previously received, he says, ‘But, were the ice and snow to melt as suddenly as they must necessarily do, were the former opinion of the action of heat in melting them well founded, the torrents and inundations would be incomparably more irresistible and dreadful. They would tear up and sweep away every thing, and that so suddenly, that mankind should have great difficulty to escape from their ravages.’
[764] ‘Dr. Black quickly perceived the vast importance of this discovery; and took a pleasure in laying before his students a view of the extensive and beneficial effects of this habitude of heat in the economy of nature. He made them remark how, by this means, there was accumulated, during the summer season, a vast magazine of heat, which, by gradually emerging, during congelation, from the water which covers the face of the earth, serves to temper the deadly cold of winter. Were it not for this quantity of heat, amounting to 145 degrees, which emerges from every particle of water as it freezes, and which diffuses itself through the atmosphere, the sun would no sooner go a few degrees to the south of the equator, than we should feel all the horrors of winter.’ Robison's Preface to Black's Lectures, vol. i. p. xxxviii.
[765] As I am writing an account of Black's views, and not a criticism of them, I shall give them, without comment, in his own words, and in the words of one of his pupils. ‘Here we can also trace another magnificent train of changes, which are nicely accommodated to the wants of the inhabitants of this globe. In the equatorial regions, the oppressive heat of the sun is prevented from a destructive accumulation by copious evaporation. The waters, stored with their vaporific heat, are thus carried aloft into the atmosphere, till the rarest of the vapour reaches the very cold regions of the air, which immediately forms a small portion of it into a fleecy cloud. This also further tempers the scorching heat by its opacity, performing the acceptable office of a screen. From thence the clouds are carried to the inland countries, to form the sources in the mountains, which are to supply the numberless streams that water the fields. And, by the steady operation of causes, which are tolerably uniform, the greater part of the vapours pass on to the circumpolar regions, there to descend in rains and dews; and in this beneficent conversion into rain, by the cold of those regions, each particle of steam gives up the 700 or 800 degrees of heat which were latent in it. These are immediately diffused, and soften the rigour of those less comfortable climates.’ … ‘I am persuaded that the heat absorbed in spontaneous evaporation greatly contributes to enable animals to bear the heat of the tropical climates, where the thermometer frequently continues to show the temperature of the human body. Such heats, indeed, are barely supportable, and enervate the animal, making it lazy and indolent, indulging in the most relaxed postures, and avoiding every exertion of body or mind. The inhabitants are induced to drink large draughts of diluting liquors, which transude through their pores most copiously, carrying off with them a vast deal of this troublesome and exhausting heat. There is in the body itself a continual laboratory, or manufacture of heat, and, were the surrounding air of such a temperature as not to carry it off, it would soon accumulate so as to destroy life. The excessive perspiration, supplied by diluting draughts, performs the same office as the cold air without the tropics, in guarding us from this fatal accumulation.’ Black's Lectures, vol. i. pp. xlvi. 214.
[766] See his strong protest against the notion that heat is ever destroyed, in his Lectures, vol. i. pp. 125, 126, 164, 165.