782 Sir Oliver Lodge on the Transmission 



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up of some highly ionized layer by convection currents, — 

 would, amply account for serious obstruction to any but the 

 longest waves. 



Other Causes of Atmospheric Opacity. 



Apart from what is caused by regular electric conductivity 

 in the ordinary way, there is another possible kind of opacity,, 

 due to a curious and unexpected effect of radiation on 

 electrons, discovered mathematically as far as I know by 

 Sir J. J. Thomson in 1902, and contained in his paper " On 

 the Emission of Negatively Electrified Corpuscles by Hot 

 Bodies," in the Philosophical Magazine for August 1902 

 (ser. 6, vol. iv. p. 258). This paper appears to have attracted 

 very little attention, though it is referred to by Professor 

 Fleming in his comprehensive treatise on Wireless Tele- 

 graphy, 1910 edition, page 824. 



The foundation of it is the action of etherial waves in 

 propelling or sweeping up electrified particles in the direc- 

 tion of wave propagation, thereby communicating to them 

 energy which presumably must be taken from the available 

 energy of the waves ; though the author of the theory does 

 not seem to attach importance to this process as an absorber 

 of energy, nor to contemplate it from that point of view.. 

 He directs attention to another and more ordinary kind of 

 opacity, due to the scattering of radiation by corpuscles over 

 which the waves are passing. So far as light is concerned, 

 this process is indeed likely to be more effective than the 

 other as a disperser of radiant energy, though it leaves the 

 energy in the ether and does not transfer it to particles of 

 matter. The radiation scattered by the particles should be 

 polarized, as all light reflected from small particles is, and 

 Sir J. J. Thomson calculates the coefficient of absorption 

 due to this kind of scattering, as 



4:7rn 2 Ne± 

 3m 2 

 This therefore is independent of wave-length, but must 

 always be very small, even when the ions are electrons,, 

 because the fourth power of ionic charge occurs in the 

 numerator. The ionization might be great, say a million 

 electrons to the cubic centimetre, and yet the above absorp- 

 tion-coefficient would be only of the order 10~ 20 per centi- 

 metre. This is insignificant of course for all terrestrial 

 distances, but in the depths of space it seems likely to have 

 an effect of some importance, not indeed as destroying 

 energy in the ether, but as ultimately converting transparency 

 into translucencv. 



What the ionization of interstellar space is I have no 



