760 ANNUAL EEPORT SMITHSONIAN INSTITUTION, 1908. 



broke the neck of his thigh. For several months he had to lie on his 

 back ; and it was at this time that he adopted the famous green note- 

 books, which ever afterwards Avere the companions of his days. The 

 accident left him with a slight limp for the rest of his life. 



An admirable picture of Lord Kelvin as he was in the sixties, moving 

 among his students and incessant in his researches, has been given 

 in The Times of January 8, 1908, by Professor Ayrton, who was then 

 working at Glasgow. In these years Thomson was also writing on 

 the secular cooling of the earth and investigating the changes 'of form 

 during rotation of elastic spherical shells. And as if this were not 

 enough to have had in hand, he embarked with his friend. Professor 

 Tait, on the preparation of a text-book of natural philosophy. There 

 was at that date no satisfactory work to put into the hands of stu- 

 dents, and he must supply the need. At first a short pamphlet of 

 propositions on statics and dynamics, culled by Prof. John Ferguson 

 from mere lecture notes, was printed for the use of students. Thom- 

 son had told Helmholtz of his purpose, and in 1862 Helmholtz wrote 

 him: 



Your undertaking to write a text-book of natural philosophy is very praise- 

 worthy, but will be exceedingly tedious. At the same time I hope it will sug- 

 gest ideas to you for much valuable work. It is in writing a book like that 

 that one best appreciates the gaps still left in science. 



The first volume of Thomson and Tait's Treatise on Natural Phi- 

 losophy was published in 1867, the second only in 1874; when it ap- 

 peared that Helmholtz's hopes were just. For in approaching the 

 subject of elasticity the gaps still left were found to be such that 

 Avhole new mathematical researches were necessary before Volume I 

 could be finished. Thomson's contributions to the theory of elastic- 

 ity are no less important than those he made to other branches of 

 physics. In 1867 he communicated to the Royal Societj^ of Edin- 

 burgh his famous paper " On vortex atoms." Helmholtz had pub- 

 lished a mathematical paper on the hydrodynamic equations of vor- 

 tex motion, proving that closed vortices could not be produced in a 

 liquid perfectly devoid of internal friction. Thomson seized on this 

 idea. If no such vortex could be artificially produced, then if such 

 existed it could not be destroyed. But being in motion and having 

 the inertia of rotation, it would have elastic and other properties. He 

 showed that vortex rings (like smoke rings in air) in a perfect me- 

 dium are stable, and that in many respects they possess the qualities 

 essential to the properties of material atoms — permanence, elasticity, 

 and power to act on one another through the medium at a distance. 

 The different kinds of atoms known to the chemist as elements were 

 to be regarded as vortices of different degrees of complexity. Though 

 he seemed at the end of his life to doubt whether the vortex-atom 



