Sir John Leslie, natural philosopher, born at Largo, 16th April 1766, studied at St. Andrews and Edinburgh, and travelled as tutor in America and on the Continent, meanwhile engaging in experimental research. The fruits of his labors were a translation of Buffon’s “Birds” (1793), the invention of a differential thermometer, a hygrometer, and a photometer, and “Inquiry into Heat” (1804). In 1805 he obtained the chair of Mathematics at Edinburgh, though keenly opposed by the ministers as a follower of Hume. In 1810 he invented artificial refrigeration. Transferred to the chair of Natural Philosophy (1819), he invented the pyroscope, atmometer, and æthrioscope. Knighted in 1832, on 3rd November he died. See Memoir by Macvey Napier (1838).

Personal

  In 1819 the death of Playfair was followed by Leslie’s election to the chair of natural philosophy at Edinburgh without opposition. He devoted himself to improving the experimental equipment of the physical laboratory, and to the work of teaching his favourite science, but he is said to have been wanting, like so many original workers, in the power of lucid exposition. Of all his “great and varied gifts, none was more remarkable than the delicacy and success with which he performed the most delicate experiments, excepting perhaps his intuitive sagacity in instantly detecting the cause of an accidental failure.”

General

  Mr. Leslie is well known to the scientific world, by the ingenuity he has displayed in the contrivance of his methods and instruments, in those experimental investigations to which he has directed his attention…. Mr. Leslie’s experimental results are sometimes too briefly stated, and the grounds on which his conclusions rest, are not always brought sufficiently forward: the evidence for them, therefore, frequently appears not equal to the confidence with which they are delivered; and objections occur, which a more ample statement or illustration might perhaps have obviated. We need scarcely add, that the whole work is marked by that ingenuity of invention, and that minute discrimination, which have always distinguished Mr. Leslie’s investigations.

  It would be impossible, we think, for any intelligent and well-constituted mind, thoroughly acquainted with the powers and attainments of Sir John Leslie, to view them without a strong feeling of admiration for his vigorous and inventive genius, and of respect for that extensive and varied knowledge which his active curiosity, his excursive reading, and his happy memory had enabled him to amass and digest…. His theoretical notions may be thrown aside or condemned; but his exquisite instruments, and his experimental combinations, will ever attest the utility no less than the originality of his labours, and continue to act as helps to farther discovery.

  His real merit was, that, notwithstanding the difficulties which beset his path, he firmly seized the great truth, that there is no fundamental difference between light and heat. As he puts it, each is merely a metamorphosis of the other. Heat is light in complete repose. Light is heat in rapid motion. Directly light is combined with a body, it becomes heat; but when it is thrown off from that body, it again becomes light. Whether this is true or false, we cannot tell; and many years, perhaps many generations, will have to elapse before we shall be able to tell. But the service rendered by Leslie is quite independent of the accuracy of his opinion, as to the manner in which light and heat are interchanged. That they are interchanged, is the essential and paramount idea. And we must remember, that he made this idea the basis of his researches, at a period when some very important facts, or, I should rather say, some very conspicuous facts, were opposed to it; while the main facts which favoured it were still unknown. When he composed his work, the analogies between light and heat, with which we are now acquainted, had not been discovered; no one being aware, that double refraction, polarization, and other curious properties, are common to both. To grasp so wide a truth in the face of such obstacles, was a rare stroke of sagacity. But, on account of the obstacles, the inductive mind of England refused to receive the truth, as it was not generalized from a survey of all the facts. And Leslie, unfortunately for himself, died too soon to enjoy the exquisite pleasure of witnessing the empirical corroboration of his doctrine by direct experiment, although he clearly perceived that the march of discovery, in reference to polarization, was leading the scientific world to a point, of which his keen eye had discerned the nature, when, to others, it was an almost invisible speck, dim in the distant offing.