the ‘Edinburgh Review’ (vol. xx.) In 1813 appeared his ‘Geometry of Curve Lines.’ A volume on ‘Descriptive Geometry and the Theory of Solids,’ which was to have completed this part of the course, was never published. In 1817 he produced a treatise on the ‘Philosophy of Arithmetic,’ containing an elaborate discussion of fundamental principles and much interesting information on the history of the subject.
Meanwhile he continued his researches on heat. In 1810 he successfully applied the absorbent powers of sulphuric acid to freeze water under the receiver of the air-pump. This is the first recorded achievement of artificial congelation. The fact that the principle on which it is based had been stated by Nairne in the ‘Philosophical Transactions’ as far back as 1777 does not deprive Leslie of the honour of the discovery. ‘A Short Account of Experiments and Instruments depending on the Relations of Air to Heat and Moisture,’ published at Edinburgh in 1813, contains a description of this experiment, and is full of important and original work. In 1814 he published, under the pseudonym of ‘Philotechnus,’ ‘Remarks for a Series of Years on Barometrical Scales.’ As a physicist he appears to least advantage in his communication to the Royal Society of Edinburgh, dated 1818, ‘On Certain Impressions of Cold transmitted from the Higher Atmosphere.’ The true theory of the formation of dew had already been accurately worked out by Dr. Wells. Leslie expressly dissents from his conclusions, rejecting Wells's idea of heat radiations from the earth's surface in favour of his own notion of a ‘continual darting of cold pulsations by day and night from an azure sky.’ Leslie clung with curious tenacity to his theory that cold had an objective existence distinct from heat. In 1809 he began to write for the ‘Edinburgh Review,’ to which he contributed articles on ‘The Physical and Chemical Memoirs of the Society of Arcueil’ (vol. xv.); on ‘The History of the Barometer’ (vol. xx.); on Delambre's work on ‘The Arithmetic of the Greeks’ (vol. xviii.); on ‘Von Buch's Travels’ (vol. xxii.); on Humboldt's ‘Physical View of the Equatorial Regions,’ and on his ‘Travels’ (vols. xvi. xxv.); and on ‘The Attempts to Discover a North-West Passage’ (vol. xxx.). To the supplement to the ‘Encyclopædia Britannica,’ begun in 1815 and finished in 1824, he contributed articles on Achromatic Glasses, Acoustics, Aeronautics, Andes, Angle, Trisection of Angle, Arithmetic (Palpable and Figurate), Atmometer, Barometer, Barometrical Measurements, Climate, Cold and Congelation, Dew, Interpolation, Meteorology.
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.’
In 1820 he was made a corresponding member of the Institute of France, and published a ‘Description of Instruments for Extending and Improving Meteorological Observations.’ In 1823 he published ‘Elements of Natural Philosophy,’ vol. i. including mechanics and hydrostatics. In reference to hydrostatics Leslie had in 1802 (Phil. Mag. xiv. 193) given ‘the first correct explanation of the rise of a liquid in a tube by considering the effect of the attraction of the solid on the very thin stratum of liquid in contact with it’ (Maxwell, art. ‘Capillary Action,’ Encycl. Brit., 9th edition).
In the preface to the second edition of his work on ‘Natural Philosophy’ (1828) he says: ‘I had designed the second volume of this work to appear at the same time, but have since thought it better to wait for the results of a series of experiments projected on the constitution and power of steam.’ He appears to have been unacquainted with Carnot's work on this subject, published in 1824. This second volume never appeared.
On 22 July 1822 Leslie instituted proceedings against the proprietor of ‘Blackwood's Magazine’ for certain libels accusing him of having claimed as his own other men's discoveries, and he obtained a verdict for 100l. damages on two out of the four counts. A report of the trial was published.
To the first volume of the seventh edition of the ‘Encyclopædia Britannica’ he contributed a ‘Discourse on the Progress of Mathematical and Physical Science during the 18th Century,’ a work for which the extent of his knowledge and the ripeness of his judgment peculiarly fitted him. This was his last important work. He was knighted early in 1832, on the recommendation of Lord Brougham, and died unmarried on 3 Nov. in the same year, at Coates in Fifeshire, where he had purchased a small estate.
Other writings by Leslie not mentioned above are: ‘Tracts, Historical and Philosophical,’ 2 vols. Edinb. 1806; ‘Rudiments of Plane Geometry, including Geometrical Analysis
