2 78 POPULAR SCIENCE MONTHLY. 



particles which, plunging into their outermost regions, give rise to 

 electric discharges and make their gases shine as the gases in a vacuum 

 tube. To this the intense cold is no bar, for Stark has shown that the in- 

 tensity of light excited in a vacuum tube is greater the lower the tem- 

 perature at which the experiment is tried. And this process should 

 take place at the surface of the nebula, where the lighter gases would 

 be found, the heavier settling inwards. Hence the few lines found 

 in the spectrum of a nebula, and the comparative brightness of the 

 outlying parts, especially to be observed in the planetary and the ring 

 uebulfe. 



Such is Arrhenius' theory. It is too early, as yet, to pronounce any 

 judgment upon it, but glancing back over the array of hitherto unex- 

 plained facts which fall into order, without forcing, at its touch, we 

 must admit that it is at least plausible. It springs from a single 

 principle, itself a necessary theoretical consequence of the accepted 

 Electromagnetic Theory of light, viz., that light must exert a pressure 

 which, in the case of small particles, may very greatly exceed their 

 weight. By means of this principle in conjunction with recent views 

 about the nature and properties of ions, which can all be experimentally 

 verified, this theory gives a rational explanation of the astounding 

 behavior of comets' tails; accounts for the 'hairy' structure of the 

 corona ; shows us how the prominences can float where the existence of 

 a supporting atmosphere is inadmissible; what is. the origin of the 

 zodiacal light and the Gegenschein; of 'the certain connection' between 

 sunspots and magnetic storms; of the aurora, and why it is subject to 

 such complicated periodical variations; why meteorites are porous and 

 limited in size; how the nebulae shine in the absolute cold of inter- 

 stellar space, and yet hang together; and why their constituents 

 appear to be so restricted, while the suns among which they are strewn 

 give evidence of most of the elements known on earth. 



A theory which sweeps the astronomical horizon of so many mys- 

 teries must not only arouse our profound interest, but claim the re- 

 spectful consideration of men of science. 



