[767] They were published after his death from such scanty materials, that their editor, Dr. Robison, says (Preface to Black's Lectures, vol. i. p. x.): ‘When I then entered seriously on the task, I found that the notes were (with the exception of perhaps a score of lectures) in the same imperfect condition that they had been in from the beginning, consisting entirely of single leaves of paper, in octavo, full of erasions, interlinings, and alterations of every kind; so that, in many places, it was not very certain which of several notes was to be chosen.’
[768] ‘On the other hand, were the heat which at present cherishes and enlivens this globe, allowed to increase beyond the bounds at present prescribed to it; beside the destruction of all animal and vegetable life, which would be the immediate and inevitable consequence, the water would lose its present form, and assume that of an elastic vapour like air; the solid parts of the globe would be melted and confounded together, or mixed with the air and water in smoke and vapour; and nature would return to the original chaos.’ Black's Lectures, vol. i. pp. 246, 247.
[769] Mr. Napier, in his Memoirs of Leslie, pp. 16, 17 (prefixed to Leslie's Treatises on Philosophy, Edinb. 1838), says, that he ‘composed the bulk of his celebrated work on Heat in the years 1801 and 1802;’ but that, in 1793, he propounded ‘some of its theoretical opinions, as well as the germs of its discoveries.’ It appears, however, from his own statement, that he was making experiments on heat, at all events, as early as 1791. See Leslie's Experimental Inquiry into the Nature and Propagation of Heat, London, 1804, p. 409.
[770] For specimens of some of his most indefensible speculations, see Leslie's Treatises on Philosophy, pp. 38, 43.
[771] Though he clearly distinguishes between the two. ‘It is almost superfluous to remark, that the term heat is of ambiguous import, denoting either a certain sensation, or the external cause which excites it.’ Leslie on Heat, p. 137.
[772] ‘Heat is an elastic fluid extremely subtle and active.’ Leslie on Heat, p. 150. At p. 31, ‘calorific and frigorific fluid.’ See also pp. 143, 144; and the attempt to measure its elasticity, in pp. 177, 178.
[773] ‘Heat is only light in the state of combination.’ Leslie on Heat, p. 162. ‘Heat in the state of emission constitutes light.’ p. 174. ‘It is, therefore, the same subtle matter, that, according to its different modes of existence, constitutes either heat or light. Projected with rapid celerity, it forms light; in the state of combination with bodies it acts as heat.’ p. 188. See also p. 403, ‘different states of the same identical substance.’
[774] In 1814, that is ten years after his great work was published, and about twenty years after it was begun, he writes from Paris: ‘My book on heat is better known’ here ‘than in England. I was even reminded of some passages in it which in England were considered as fanciful, but which the recent discoveries on the polarity of light have confirmed.’ Napier's Memoirs of Leslie, p. 28, prefixed to Leslie's Philosophical Treatises, edit. Edinb. 1838. Leslie died in 1832 (p. 40); and the decisive experiments of Forbes and Melloni were made between 1834 and 1836.
[775] ‘The easiest mode of conceiving the subject, is to consider the heat that permeates all bodies, and unites with them in various proportions, as merely the subtle fluid of light in a state of combination. When forcibly discharged, or suddenly elicited from any substance, it again resumes its radiant splendour.’ … ‘The same notion was embraced by the poets, and gives sublimity to their finest odes.’ … ‘Those poetical images which have descended to our own times, were hence founded on a close observation of nature. Modern philosophy need not disdain to adopt them, and has only to expand and reduce to precision the original conceptions.’ Leslie's Treatises on Philosophy, pp. 308, 309. Again, at p. 416: ‘This is not the first occasion in which we have to admire, through the veil of poetical imagery, the sagacity and penetration of those early sages. It would be weakness to expect nice conclusions in the infancy of science; but it is arrogant presumption to regard all the efforts of unaided genius with disdain.’
[776] ‘We should recollect that, in all her productions, Nature exhibits a chain of perpetual gradation, and that the systematic divisions and limitations are entirely artificial, and designed merely to assist the memory and facilitate our conceptions.’ Leslie on Heat, p. 506.