THE peesshee due to eadiation. 135 



2. Occluded gases or volatile materials upon the surface of the par- 

 ticles would be driven off by the sun's heat on the illuminated sides, 

 and the particles would thus receive a thrust in the direction away 

 from the sim, 



3. If the particles were porous or loosely put together, containing 

 cavities filled with more easily vaporizable substances, the resulting 

 \apors would be shot out upon the hotter sides and the particles 

 driven back by a kind of rocket action. 



That these combined gas forces are still large, even in high vacua, 

 will be seen from an actual experiment described later. 



If we accept Ari:lienius"'s theory that the solar activity produces 

 numberless negative electrons which serve as condensation points for 

 the vapors surrounding them in the solar atmosphere, and thus form 

 small, negatively charged nuclei, which are driven from the sun by 

 radiation pressure,'* these nuclei would exert a battering action upon 

 the particles of the tail. In the last case a strange meeting point is 

 found between the oldest, or Keplerian, and the latest explanation of 

 the solar repulsion of comets' tails. 



Finally Prof. J. J, Thomson,'' in investigating the action of electric 

 waves upon charged bodies immersed in the medium, has found that 

 a small repulsive effect may arise from this cause. This repulsive 

 force is entirely distinct from the radiation pressure so far considered, 

 but on the electro-magnetic theory of light it ma}^ be competent to 

 drive awa}^ electrons formed above the photosphere of the sun, inde- 

 pendenth^ of the sign of the charge and of whether they have formed 

 nuclei by condensation or not. 



These last two causes of repulsion are in all probability of very 

 minor importance when compared with radiation pressure, or even 

 with gas action. 



Experiment with a Jahoratory coinefs tall. — Some of the above con- 

 siderations led the writers to try to reproduce, as nearly as possible, 

 in a vacuum tube some of the conditions believed to exist in comets' 

 tails. The result of a hasty computation of the magnitude of the 

 effect which might be expected from radiation pressure provided a 

 suitable dust could be found was most encouraging. 



At the outset it was apparent that it would be very difficult to man- 

 ufacture a powder the grains of which would be sufficiently small, 

 light, and uniform for the purpose; so the spores of a great ^•ariet3^ of 

 degraded vegetable forms were examined. Finally a puffball of the 

 genus L[ic()p)erdon was discovered, the spores of which averaged 2 

 microns in diameter, and were as nearly spherical and uniform in size 



« The supposed electrical discharges in the tail of a comet which give rise to its 

 gaseous emission spectrum are attributed by Arriienius to the electrical disturbances 

 caused by the influx of these negative nuclei. 



&Phil. Mag., 4, 253, 1902. 



SM 1903 10 



