192 comets' tails, the cokona, and the aurora. 



Ever since the spectroscope showed that man}' nebuke are gaseous, and 

 3^et shine by their own light, two problems have vexed the astronomers. 

 How can they be hot enough to send light to us. and yot be hold 

 together against the expansive force of the hi^ated gas ))\ the feelile 

 gravitation which such inconceivably diffuse masses can exert at their 

 borders? If they arc really at a temperature of not less than 500° C, 

 so as to shine, or, indeed, if the}" are much above absolute zero, their 

 own gravitation should not be able to prevent their speed}^ dissipation 

 into space. 



Again, why do they show the spectroscopic lines of so few gases, and 

 those the lighter ones, such as hydrogen and helium;' 



According to Arrhenius, the nebnhe are cold, with the cold of empty 

 space. Their light is due to the rain of negatively charged 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 inten- 

 sity of light excited in a vacuum tiil)e is greater the lower the tem- 

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

 take place at the surface of the ne])ula, where the lighter gases would 

 be found, the heavier settling inward. Hence the few lines found in 

 the spectrum of a nebula, and the c<)mparati\o brightness of the out- 

 lying parts, especialh' to be observed in the planetary and the ring 

 nebula?. 



Such is Arrhenius's theory. It is too early, as 3'et, to pronounce 

 any judgment upon it, but glancing l)ack over the array of hitherto 

 unexplained facts which fall into order, without forcing, at its touch, 

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

 principle, itself a necessary theoretical conseciuence of the accepted 

 electromagnetic theory of light — viz. that light must exei't a pressure 

 which, in the case of small })articles, ma\' very greatly exceed their 

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

 a])out the nature and properties of ions, which can all be exjierimen- 

 tally veriiied, this theory gives a rational cx})laiiation of the astounding 

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

 corona; shows us how the prominences can float whei'e the existence 

 of a supjxn-ting atmosphere is inadmissible; Avhat is the origin of the 

 zodiacal light and the (xegenschein; of '"the certain connection" 

 between sun spots and magnetic storms; of the aurora, and why it is 

 subject to such complicated periodical variations; why nu>teoi'ites are 

 porous and limited in size; how the nebuhe shine in the absolute cold 

 of interstellar space, and yet hang togethei-; and why their constitu- 

 ents appear to be so restricted, while the suns among which Ihcy are 

 strewn give evidence of most of the (demiMits known on ( arlh. 



A theory which swee])s the asti'onomicai horizon of so many mys 

 terics nuist not oidy arouse our profound interest, but claim the 

 respectful consideration of men of science. 



