Notwithstanding the labours which these duties entailed upon him, Leslie continued his experimental inquiries, and in June, 1810, discovered his beautiful process of artificial congelation, by which he was enabled to produce ice, and even to freeze mercury at pleasure. The process consists of a combination of the powers of rarefaction and absorption, effected by placing a very strong absorbent under the receiver of an air-pump. This experiment was performed in London in 1811, before a meeting of some members of the Royal Society; and the discovery was announced in the same year in the 'Memoirs' of the French Institute. He explained his experiments and views on this subject in 1813, in a volume published at Edinburgh, entitled, 'A short Account of Experiments and Instruments depending on the Relations of Air to Heat and Moisture.' Closely connected with the subject of this treatise was an ingenious paper, published in 1818, in the 'Transactions' of the Royal Society of Edinburgh, under the title, 'On certain Impressions of Cold transmitted from the Higher Atmosphere; with a Description of an Instrument to Measure them.' The æthrioscope was the instrument here alluded to. In 1819, upon the death of Playfair, Leslie was called to the chair of Natural Philosophy, when his first care was directed to the extension of the apparatus required in the more enlarged series of experiments which he thought necessary for the illustration of the course. "This, indeed," says his biographer, Mr. Napier, "was an object of which he never lost sight; and it is due to him to state, that, through his exertions, the means of experimental illustration in the Natural Philosophy class were for the first time made worthy of the place." In 1823 he published, chiefly for the use of this class, his 'Elements of Natural Philosophy,' a second edition of which was published in 1829, with corrections and additions. Besides the above-mentioned works, Leslie wrote the following:--'Elements of Geometry, Geometrical Analysis and Plane Trigonometry,' in 1809; 'Observations on Electrical Theories,' published in 1824, in the 'Edinburgh Philosophical Journal;' also many articles in the 'Edinburgh Review;' and the articles on Achromatic Glasses; Acoustics; Aeronautics; Andes; Angle and Trisection of Angle; Arithmetic; Atmometer; Barometer; Barometrical Measurements; Climate; Cold and Congelation; Dew; Interpolation; and Meteorology, in the seventh edition of the 'Encyclopædia Britannica.'