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The primitive play instinct or play impulse in man has been much discussed in recent years by psychologists in connexion with child study (see ), and with the expression of the emotions (see J. Sully, On Laughter, 1902, &c; also ). See generally Carl Groos, The Play of Animals (1898) and The Play of Man (1901); and Baldwin's ''Dict. of Philosophy, s.v.''

 PLAYA (a Spanish word meaning “shore”), the name applied in America to a level plain formed of the deposits of a river which has no outlet to the sea or a lake. If at seasons of high water a river floods any area and temporarily converts it into a lake, which subsequently dries up in hot weather, the tract thus left dry is called a playa. The barren Black Rock Desert in north-western Nevada, about 100 m. in length by 15 in breadth, is typical.

 PLAYFAIR, JOHN (1748–1819), Scottish mathematician and physicist, was born at Benvie, Forfarshire, where his father was parish minister, on the 10th of March 1748. He was educated at home until the age of fourteen, when he entered the university of St Andrews. In 1766, when only eighteen, he was candidate for the chair of mathematics in Marischal College, Aberdeen, and, although he was unsuccessful, his claims were admitted to be high. Six years later he made application for the chair of natural philosophy in his own university, but again without success, and in 1773 he was offered and accepted the living of the united parishes of Liff and Benvie, vacant by the death of his father. He continued, however, to carry on his mathematical and physical studies, and in 1782 he resigned his charge in order to become the tutor of Ferguson of Raith. By this arrangement he was able to be frequently in Edinburgh, and to cultivate the literary and scientific society for which it was at that time specially distinguished; and through Maskelyne, whose acquaintance he had first made in the course of the celebrated Schiehallion experiments in 1774, he also gained access to the scientific circles of London. In 1785 when Dugald Stewart succeeded Ferguson in the Edinburgh chair of moral philosophy, Playfair succeeded the former in that of mathematics. In 1802 he published his celebrated volume entitled Illustrations of the Huttonian Theory of the Earth. To its publication the influence exerted by James Hutton on the progress of geological knowledge is largely due. In 1805 he exchanged the chair of mathematics for that of natural philosophy in succession to Dr John Robison, whom also he succeeded as general secretary to the Royal Society of Edinburgh. He took a prominent part, on the Liberal side, in the ecclesiastical controversy which arose in connexion with Leslie's appointment to the post he had vacated, and published a satirical Letter (1806) which was greatly admired by his friends. He was elected F.R.S. in 1807. He died in Edinburgh on the 20th of July 1819.

A collected edition of Playfair's works, with a memoir by James G. Playfair, appeared at Edinburgh in 4 vols. 8vo. His writings include a number of essays contributed to the Edinburgh Review from 1804 onwards, various papers in the ''Phil. Trans. (including his earliest publication, “On the Arithmetic of Impossible Quantities,” 1779, and an “Account of the Lithological Survey of Schiehallion,” 1811) and in the Transactions of the Royal Society of Edinburgh'' (“On the Causes which affect the Accuracy of Barometrical Measurements,” &c.), also the articles “Aepinus” and “Physical Astronomy,” and a “Dissertation on the Progress of Mathematical and Physical Science since the Revival of Learning in Europe,” in the Encyclopaedia Britannica (Supplement to fourth, fifth and sixth editions). His Elements of Geometry first appeared in 1795 and have passed through many editions; his Outlines of Natural Philosophy (2 vols., 1812–1816) consist of the propositions and formulae which were the basis of his class lectures. Playfair's contributions to pure mathematics were not considerable, his paper “On the Arithmetic of Impossible Quantities,” that “On the Causes which affect the Accuracy of Barometrical Measurements,” and his Elements of Geometry, all already referred to, being the most important. His lives of Matthew Stewart, Hutton, Robison, many of his reviews, and above all his “Dissertation” are of the utmost value.

 PLAYFAIR, LYON PLAYFAIR, 1st (1818–1898), was born at Chunar, Bengal province, on the 21st of May 1818. He was sent to Europe by his father at an early age, and received his first education at St Andrews. Subsequently he studied medicine at Glasgow and Edinburgh. A short visit to India (in 1837–1838) was followed by his return to Europe to study

chemistry, which had always attracted him. This he did at University College, London, and afterwards under Liebig at Giessen, where he took his doctor's degree. At Liebig's request, Playfair translated into English the former's work on the Chemistry of Agriculture, and represented Liebig at a meeting of the British Association at Glasgow. The outcome of his studies was his engagement in 1841 as chemical manager of the Primrose print-works at Clitheroe, a post which he held for rather more than a year. In 1843 he was elected honorary professor of chemistry to the Royal Institution of Manchester, and soon afterwards was appointed a member of the Royal Commission on the Health of Towns, a body whose investigations may be said to have laid the foundations of modern sanitation. In 1846 he was appointed chemist to the geological survey, and thenceforward was constantly employed by the public departments in matters of sanitary and chemical inspection. The opportunity of his life came with the 1851 Exhibition, of which he was one of the special commissioners. For his services in this connexion he was made C.B., and his work had the additional advantage of bringing him into close personal relations with the Prince Consort, who appointed him gentleman usher in his household. From 1856 to 1869 he was professor of chemistry at Edinburgh University. In 1868 he was elected to represent the universities of Edinburgh and St Andrews in parliament, and retained his seat till 1885, from which date until 1892 he sat as member for Leeds. In 1873 he was made postmaster-general, and in the following year, after the dissolution of parliament, was applied to by the incoming Tory government to preside over a commission to inquire into the working of the civil service. Its report established a completely new system, which has ever since been officially known as the “Playfair scheme.” The return of Mr Gladstone to power in 1880 afforded opportunity for Playfair to resume his interrupted parliamentary career, and from that time until 1883 he acted as chairman of committees during a period when the obstructive tactics of the Irish party were at their height. On his retirement from the post he was made K.C.B. In 1892 he was created Baron Playfair of St Andrews, and a little later was appointed lord-in-waiting to the queen. In 1895 he was given the G.C.B. In spite of failing health the last years of his life were full of activity, one of his latest public acts being his suggestion that Queen Victoria's Diamond Jubilee of 1897 should be commemorated by the completion of the South Kensington Museum. He died in London, after a short illness, on the 29th of May 1898, and was buried at St Andrews. He was three times married. He was the author of a number of papers on scientific and social topics, a selection from which he published in 1889 under the title of Subjects of Social Welfare.

 PLEADING (Fr. plaider, plaidoyer), the term applied in English law to the preparation of the statement of the facts on which either party to a criminal prosecution or a civil action founds his claim to a decision in his favour on the questions involved in the proceeding; and also to the document in which these statements are embodied. The term “pleadings” is used for the collected whole of the statements of both parties; the term “pleading” for each separate part of the pleadings. The term “plea” (placitum, plaid) is now applied in England oftenest to the defence made by an accused person. To “plead” is to make a pleading or plea.

All systems of law agree in making it necessary to bring the grounds of a claim or defence before the court in a more or less definite and technical form.

Roman System.-In Roman law the action passed through three stages (see ), and the manner of pleading changed with the action. In the earliest historical period, that of the legis actiones, the pleadings were verbal, and made in court by the parties themselves, the proceedings imitating as far as possible the natural 