John Playfair

(Text modified from [http://www.1911encyclopedia.org/John_Playfair Britannica 1911)

John Playfair (1748-1819), mathematician and physicist, was born on the 10th of March 1748 at Benvie, Forfarshire, in Scotland where his father was parish minister. He was educated at home until he was fourteen, when he entered the University of St Andrews. In 1766, when only eighteen, he applied for the chair of mathematics in Marischal College, Aberdeen, but was unsuccessful. Six years later he applied for the chair of natural philosophy in St Andrews, but again without success, and in 1773 he 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 to become the tutor of Ferguson of Raith. This enabled him to be frequently in Edinburgh, and to cultivate the literary and scientific society for which it was then famous; and through Maskelyne, whose acquaintance he had first made in the course of the Schiehallion experiments in 1774, he gained access to the scientific circles of London. In 1785, when Dugald Stewart succeeded Ferguson in the Edinburgh chair of moral philosophy, Playfair succeeded Dugald Stewart in the chair of mathematics. Playfair however is remembered most not for his contributions to mathematics, but for the formidable role that he played in bringing the work of his friend, the geologist James Hutton, to prominence.

In 1802, Playfair published Illustrations of the Huttonian Theory of the Earth. To its publication the influence exerted by 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). He was elected a Fellow of the Royal Society in 1807. He died in Edinburgh on 20th July 1819.

A collected edition of Playfair's works, with a memoir by James G. Playfair, appeared at Edinburgh in 4 volumes. 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 Schehallion," 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.

Illustrations of the Huttonian Theory of the Earth

Playfair's "Illustrations of the Huttonian Theory of the Earth" expressed Hutton's evidence and conclusions in a crisp, tightly organised logical form, making it easy to follow the flow of the argument and very hard to refute it lightly. A critical step in Hutton's case was his argument that granite was a rock formed under great heat and pressure which in places had been forced upwards to displace overlying strata, and occasionally to infiltrate those strata

"... in Dr. Hutton’s theory, granite is regarded as a stone of more recent formation that the strata incumbent on it; as a substance which has been melted by heat, and which, when forced up from the mineral regions, has elevated the strata at the same time."

Playfair laid out the observational facts simply and clearly

"...It has been observed in many instances, that where granite and stratified rocks, such as primary schistus, are in contact, the latter are penetrated by veins of the former, which traverse them in various directions."

He systematically dismantled the conventional view, that granite was formed by mixtures of minerals dissolved in water (and produced at the time of the supposed Flood)

"These phenomena, which were first distinctly observed by Dr. Hutton, are of great importance in geology, and afford a clear solution of the two chief questions concerning the relation between granite and schistus. As every vein must be of a date posterior to the body in which it is contained, it follows, that the schistus was not superimposed on the granite, after the formation of this last. If it be argued, that these veins, though posterior to the schisti, are also posterior to the granite, and were formed by the infiltration of water in which the granite was dissolved or suspended; it may be replied, [first], That the power of water to dissolve granite, is a postulatum of the same kind that we have so often, and for such good reason, refused to concede; and [second], That in many instances the veins proceed from the main body of the granite upwards into the schistus; so that they are in planes much elevated in respect of the horizon, and have a direction quite opposite to that which the hypothesis of infiltration requires. It remains certain, therefore, that the whole mass of granite, and the veins proceeding from it, are coeval, and both of later formation than the strata."

Thus his conclusions were direct and forensic.

"the fluidity which preceded the consolidation of mineral substances was simple, that is, it did not arise from the combination of these substances with any solvent; and next, that after consolidation, these bodies have been raised up by an expansive force acting from below, and have by that means been brought into their present situation."