ophy. Through the works and the personal influence of Hamilton he was led to the study of quaternions, to which he gave much attention.
In 1860 he was elected to the chair of natural philosophy in Edinburgh, resigning it in February, 1901, on account of a lingering illness, which resulted in his death four months later. It is estimated that about ten thousand students passed through his class-room during those forty years, and few could do so without carrying away some impress from this notable teacher. Among the first of his 'researchers' were a remarkable trio—Robert Louis Stevenson, Wm. Robertson Smith, the distinguished Scottish Biblical scholar and orientalist, and Sir John Murray, the well-known publisher. Great must have been the attraction of Tait's personality to bring together three men so highly distinguished, yet so utterly different.
About the time of his appointment to the Edinburgh chair, Tait became acquainted personally with Lord Kelvin, who, though also a Peterhouse man, had left Cambridge before Tait came up, "and was already independently and in conjunction with Joule, and concurrently with Rankine and Clausius, writing his classical memoirs on the theory of energy. The first edition of Tait and Steele's 'Dynamics of a Particle' published in 1856, does not contain either of the words work or energy. In its original form it was founded on Pratt's 'Mechanical Philosophy' and written on the old-fashioned Cambridge lines, which knew not of Lagrange and Hamilton. Six years later it is on record that in his introductory lecture Tait handled the notions of the energetic school with freedom and laid down the foundations of a thoroughly modern course in physics. Probably, therefore, he had come under the influence of Joule and Kelvin before he met the latter personally." The conjunction with Kelvin produced the famous treatise on 'Natural Philosophy' by Thomson and Tait in 1867, which began a new era in mathematical physics. Dozens of men nourished by the strong meat of its pages have written treatises in continuation of the lines there laid down.
Tait's contributions to text-book literature include, besides the two works just mentioned, 'Elements of Quaternions' 1867; 'Introduction to Quaternions' by Kelland and Tait, 1868; 'Recent Advances in Physical Science' 1876; 'Thermodynamics' 1868; 'Light' 'Heat' 1884; 'Properties of Matter' 1885 (revised to 1899), and 'Dynamics' 1895.
Although Tait rarely spoke on religious topics, and in general avoided theological controversy, his friends were aware that he held decided views on such matters. He was the joint author with Balfour Stewart of the 'Unseen Universe' (first printed privately in 1875), a book showing, to use Tait's own words, how baseless is the common belief that science is incompatible with religion. "It calls attention