Sir John Leslie, FRSE KH (10 April 1766 – 3 November 1832) was a Scottish mathematician and physicist best remembered for his research into heat.

[1] Leslie gave the first modern account of capillary action in 1802[2] and froze water using an air-pump in 1810, the first artificial production of ice.

In his thirteenth year, encouraged by friends who had even then remarked his aptitude for mathematical and physical science, he entered the University of St Andrews.

[4] For the next twelve years (passed chiefly in London or at Largo, with an occasional visit to the continent of Europe) he continued his physical studies, which resulted in numerous papers contributed by him to Nicholson's Philosophical Journal, and in the publication (1804) of the Experimental Inquiry into the Nature and Properties of Heat, a work which gained him the Rumford Medal of the Royal Society of London.

By adapting to this instrument various ingenious devices, Leslie was able to employ it in a great variety of investigations, connected especially with photometry, hygroscopy and the temperature of space.

[10] Leslie died of typhus[11] in November 1832 (during the epidemic of that year) at Coates, a small property he had acquired near Largo in Fife, at the age of 66.