SOME VIEWS ON LORD KELVIN'S WORK 437 



the maximum ordinate in the initial pulse. In this we are virtu- 

 ally connecting temperature with some definite characteristic of 

 the group of molecules concerned in the emission of the sequence 

 of pulses. Certainly the artificial procedure of referring to tem- 

 perature as something belonging to a space full of radiant energy 

 of a definite constitution per wave-length does not arise in con- 

 nection with the pulse view of radiation. The applications of 

 thermodynamics to an enclosure full of radiant energy are of 

 course in reality applications to the matter within the enclosure. 

 They of necessity depend on the existence of a pressure on the 

 enclosure and on the radiating body applied by the radiation, 

 and this has been fully established by experiment. 



The above pages give in outline the developments in the direct 

 line of Lord Kelvin's later work in both its aspects. His attitude 

 with regard to other lines of investigation on the law of Radia- 

 tion may be understood from his opinions regarding the Boltz- 

 mann-Maxwell Law of partition of energy and regarding the 

 pressure due to radiation. With regard to the former, he says, 

 in Appendix B of the Baltimore Lectures : " I have never seen 

 validity in the demonstration on which Maxwell founds this 

 statement, and it has always seemed to me exceedingly im- 

 probable that it can be true." With regard to the latter, in a 

 letter to Prof. Larmor of date May 8, 1907, already published in 

 Proc. Roy. Soc. 1908, Obituary Notice, we have the statement : 

 " There are certainly very wonderful ' push and pull ' forces in 

 the action of light on movable bodies in high vacuum (and also 

 in not very high vacuum, as shown in Varley's communication 

 to Royal Society 'Proceedings' of about 1871, demonstrating 

 cathode torrent of ' negatively ' electrified particles). I do not, 

 however, think that there is any foundation for push and pull in 

 Maxwell's (a, /3, 7) formulas, or in the (a, /3, 7), (P, Q, R) of your 

 leaves." This latter subject has been cleared of uncertainties in 

 the interval since Lord Kelvin's death. The difficulties of the 

 former still remain. Lord Kelvin preferred the direct line of 

 attack on the difficulties of the subject of Radiation on the 

 ground clear of fundamental uncertainties presented by problems 

 of Wave Motion. The developments outlined in the preceding 

 pages, mixed with some speculation on the role of the pulse in 

 the genesis of radiation, serve to show how directly the main 

 sections of his later work bear on the foundations of the modern 

 theory of radiation. 



