Radiation effective on smallest as icell as larger Particles. 539 



correct. The determining factor is the extinction of light, 

 whether by scattering or by absorption, as indeed appears if 

 we take the view adopted in Prof. Poynting's work on the 

 subject, that a propagation of momentum accompanies the 

 transmission of light. The momentum is destroyed equally 

 whether the molecules act as scattering or absorbing- 

 centres. " 



This conclusion is in accordance with TyndalPs experi- 

 ments* on the colours of precipitated clouds of small 

 particles, and on the blue colour and polarization of the light 

 from the sky, and Lord Rayleigh's theoretical investigations 

 suggested by themf; and with the experimental evidence, 

 adduced by Lord Rayleigh in the last paper referred to, 

 tending to the conclusion that the molecules of air are 

 responsible for nearlj' a third of the atmospheric scattering 

 observed. 



The cases of plane waves normally incident upon a per- 

 fectly black body and a perfect reflector are capable of 

 simple treatment by elementary mathematical methods J, and 

 since the work done across a small surface of a wave-front 

 cannot depend on the question whether the wave is plane or 

 not, the relation so arrived at must hold good for any simply 

 periodic electromagnetic disturbance. The subject is treated 

 on these lines in a recent work by the present writer §, and 

 a more general analytical investigation is given by Sir 

 Joseph Larmor on p. lol of ' vEther and Matter.' It is 

 shown in chapters vii. and viii. of the latter work that 

 Maxwell's equations of electric force are not applicable to 

 the investigation of problems in which radiation is important. 

 These equations are derived from an electrodynamic stress- 

 formula in which the function of a uniform dielectric is 

 regarded as merely to transmit the forces without adding 

 anything to them. In the light of present knowledge, which 

 is most completely formulated in terms of the electron theory, 

 any material dielectric must be regarded as susceptible of 

 polarization analogous to that of a magnet. Now, while in 

 metallic conduction the current arising from this polarization 

 is usually negligible in comparison with the total, in radiation 

 it forms an important part of the total current. 



* Phil. Mag. vol. xxxvii. 1869, p. 384; and Pliil. Trans, vol. clx. 

 1870, p. 383. 



f Scientific Papers, vol. i. pp. 87, 104, 518 ; and vol. iv. pp. 305, 397. 



X See Drude's Lehrbuch der Optik, p. 447, or English edition, p. 488, 

 and Sir Joseph Larmor's article " Radiation," Supplement to Bncyclopcedxa 

 Briiannica. 



§ Treatise on Electrical Theory and the Problem of the Universe, 

 pp. 270-i ) 74. 



