64 



NATURE 



[March 17, 1910 



The object of the memoir is first clearly announced, 

 then the process and its results are unfolded, with 

 a reference at every principal step to the physical 

 meaning of the operation performed and the result 

 obtained ; and in every part the process adopted is 

 preceded by a carefully worded statement of the as- 

 sumptions made, and the presuppositions involved. 

 Clausius, on the other hand, and with him almost 

 every Continental writer, begins by referring to an un- 

 defined substance called a "perfect gas," and by means 

 of that substance absolute temperature is defined as 

 R/pv, where R is a constant and p and v are respec- 

 tively the pressure and volume of a given mass of the 

 gas. Thomson obtained his scale of temperature by 

 means of an ideal engine ; and then came the com- 

 parison of his scale with that of the air thermometer, 

 by means of his porous plug experiment, which he 

 carried out in collaboration with Joule, in the famous 

 series of experiments on the thermal effects of fluids 

 in motion. This experiment falls at once into clear 

 relation to the whole theory in Thomson's sequence 

 of ideas ; this is hardly the case in the other mode of 

 treating the subject. 



The earlier papers on heat were all communicated 

 to the Royal Society of Edinburgh, of which Thomson 

 became a fellow in 1847. Of this society he was 

 Keith medallist in 1864, and one of the chief grounds 

 for the presentation of the medal was the discovery 

 of the theorem of minimum energy of a system of 

 connected particles, started by impulses applied at 

 specified points and subject to the condition that the 

 velocities of these points have specified values. This 

 theorem is stated by Prof. Thompson on p. 1141, with 

 rather less than sufficient caution ; for if the condition 

 be that the impulses applied at the specified points are 

 also specified the motion is one of maximum energy. 

 The far-reachipg scope of this theorem is now much 

 better understood than it used to be ; for example, by 

 analogy, certain theorems of electricity may be re- 

 garded as particular cases of it. The whole subject 

 of these general dynamical theorems has been dis- 

 cussed by Lord Rayleigh in his "Theory of Sound." 



The appointment of Tait to the chair of natural 

 philosophy in Edinburgh led to the literary partner- 

 ship which had as its result the publication in 1867 

 of the first volume of the " Treatise on Natural Philo- 

 sophy," and in 1873 <^f the companion volume, "The 

 Elements of Natural Philosophy." It is matter 

 of keen regret that the second volume at least 

 of the treatise was not forthcoming. For continually 

 in his lectures in 1874, and for some years later, 

 Thomson referred to the discussion of properties 

 of matter which would be contained in that volume, 

 and references to it are frequent in vol. i. If that 

 chapter had been given to the world the treatises on 

 electricity, sound, and hydrodynamics, which we owe 

 to Maxwell, Lord Rayleigh, and Lamb, might well 

 console us for the abandonment of the original scheme. 

 But, as it is, all the old Glasgow students of natural 

 philosophy, who have kept up the study of the subject, 

 will ever regret the loss of the promised chapter, of 

 which they obtained now and then glimpses, when 

 Thomson referred, for example, to the difficulties of 

 the elastic solid theory of the aether, and showed that 1 

 NO. 2107, VOL. 83] 



similar difficulties arose when we attempted to ex- 

 plain the properties of cobblers' wax! 



A good idea of Tail's breezy and energetic style of 

 work will be obtained from the letters printed at 

 p. 453, and elsewhere. He was a man of the most 

 kindly feeling and disposition, though the native force 

 of his character and intellect made him a formidable 

 opponent and a severe critic and controversialist. His 

 famous lecture on force will never be forgotten by 

 those who heard it in the Kibble Palace, in Glasgow, 

 in 1870; only a faint idea of it can be obtained by 

 reading his " Recent Advances in Physical Science.*^ 

 He was orderly and methodical ; his statements, 

 whether oral or written, were brief and precise, and his 

 lectures were commended by all his students for their 

 unfailing experimental illustrations and the clear- 

 ness of their expositions. 



Thomson could also be clear and precise, nobody 

 more so; but in his popular discourses he was 

 always so preoccupied, and every thought so inevit- 

 ably suggested new and interesting relations of 

 things, that all his hearers, except a very few, 

 quickly gave up the attempt to follow his lecture, 

 and settled down to listen in admiration and amaze- 

 ment. The writer will never forget the discourse on 

 " Isoperimetrical Problems" which Lord Kelvin 

 delivered at the Royal Institution in May, 1893. The 

 half-humorous picture of the wounded Horatius Coclesj 

 limping after the plough, and drawing his furrow soi 

 as to get the greatest possible area of land within 

 the given length of boundary, brought the problem 

 home to even the most unmathematical dweller in 

 Mavfair who was present ; but when the lecturer went 

 on to make Horatius take account at every step of the 

 quality of the soil, so as to place the maximum value 

 of corn land within his boundary, wonder melted into 

 sympathy for the crippled warrior confronted with 

 such a terrible task ! It is well known that the Friday 

 evening discourses must begin at nine o'clock and ter- 

 minate precisely at ten ; but this law, which no one 

 else dared to break, Lord Kelvin disregarded, for when 

 ten o'clock came he had just got into his subject, and 

 he went on— with apologies, of course — until nearly 

 eleven ! Such were his fire and enthusiasm ; and to the 

 few who remained to the end the tour de force was 

 amazing. This would never have happened with Tait ; 

 the whole matter would have been thought out from 

 beginning to end ; all ideas that might have led him 

 from his straight path would have been ruthlessly put 

 aside, and a model of polished and clear exposition 

 presented. As a rule, Thomson's sentences, both 

 written and spoken, were too heavily loaded with 

 saving clauses ; as if he considered himself too absc 

 lutely committed to a conditional statement, if ii 

 limitations were not all given with it in one wor^ 

 formula. 



It is little wonder in some ways that the Hterar 

 partnership broke down. But the book was, as Pro 

 Thompson has called it, emphatically an epoch-makin 

 one. It called for and brought about a return ti 

 Newton in dynamical method ; and it pointed out hov 

 the neglected scholium on Newton's third law con 

 tained in substance the theory of energy. Othe ^ 

 text-books are more popular; even the "Elements'— | 



