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134 ton and Burnet, which, without a sufficient number of ascertained facts for the analysis of the component parts of any portion of the earth's surfaces, showed in detail the method of its abstraction from the rest of the universe, and the minutiae of its formation. But Playfair never went beyond rational deduction on the facts which were known to him, limiting the extent of his theories to reasonings on what he knew; and it shows the accuracy of his logic, that, while the experiments of Sir James Hall and others (which were in progress but not complete while he wrote,) have tended to support his explication, especially in justifying his opinion that the reason of calcination in bodies subjected to heat was the necessity of the escape of the gases contained in them, we are aware of none which have contradicted him.

The period between 1797 and 1802 was occupied by Mr Playfair in preparing his Illustrations, and in 1803 his biographical sketch of Hutton was published in the Society Transactions. In 1805 he quitted the mathematical chair, and succeeded professor John Robison in that of natural philosophy; during the same year his mother died at the age of eighty-five, and he retired along with a younger brother, his youngest sister, and two nephews, to Burntisland, that he might devote the summer to uninterrupted preparation for the duties of his new class. In the controversy with the clergymen of Edinburgh, regarding his successor to the chair of mathematics, he took an active part A letter which he addressed to the provost of Edinburgh, in favour of the election of a scientific man, as opposed to a clergyman, was answered by Dr Inglis, and from the nature of the remarks directed against himself, he considered it necessary to reply. The pamphlet produced under these circumstances, showed that his calm temper might be made dangerous by interference: it is written in considerable asperity of spirit, but without vulgar raillery or much personality, and the serious reproof, mixed with occasional sarcasm which it contains, shows great power to wield the weapons of literary warfare. He next occupied himself in preparing papers on the solids of greatest attraction, and on the progress of heat in spherical bodies, which appeared in the Transactions of the Royal Society of Edinburgh. He also presented to the London Royal Society, of which he was admitted a member in 1807, an Account of the Survey of Schehallien. In 1814, he published for the use of his students his well known Outlines of Natural Philosophy, in two volumes octavo. The first volume of this work treats of Dynamics, Mechanics, Hydrostatics, Hydraulics, Aerostatics, and Pneumatics. The second is devoted to Astronomy. A third volume was intended to have embraced Optics, Electricity, and Magnetism; but the work was never completed. In the following year he presented to the Royal Society of Edinburgh a life of his predecessor, professor Robison. His labours for this institution will be perceived to have been very extensive, and they show him not to have been a mercenary man. He was long its chief support, arranging and publishing the Transactions, and gratuitously acting as secretary. In 1816, he published, in the Supplement to the Encyclopaedia Britannica, a "Dissertation on the Progress of Mathematical and Physical Science since the Revival of Letters in Europe," a work of great erudition and research. This work interrupted a new and much altered edition of his Illustrations of the Huttonian Theory, which he had previously designed, but which unfortunately he was never enabled to complete. "It was intended," says his biographer, "to commence with a description of all the well authenticated facts in geology collected during his extensive reading and personal observation, without any mixture of hypothesis whatever. To this followed the general inferences which may be deduced from the facts, an examination of the various geological systems hitherto offered to the world, and the exclusion of those which involved any