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 intensity of light excited in a vacuum tube is greater the lower the temperature 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 nebuæ.
Such is Arrhenius' theory. It is too early, as yet, to pronounce any judgment upon it, but glancing back over the array of hitherto unexplained 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 nebulæ shine in the absolute cold of interstellar 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 mysteries must not only arouse our profound interest, but claim the respectful consideration of men of science.