'The sudden blazing forth of this star,' says Mr. Huggins, 'and then the rapid fading away of its light, suggest the rather bold speculation that in consequence of some great internal convulsion, a large volume of hydrogen and other gases was evolved from it, the hydrogen, by its combination with some other element,' in other words, by _burning_, 'giving out the light represented by the bright lines, and at the same time heating to the point of vivid incandescence the solid matter of the star's surface.' 'As the liberated hydrogen gas became exhausted' (I now quote not Huggins's own words, but words describing his theory in a book which he has edited) 'the flame gradually abated, and, with the consequent cooling, the star's surface became less vivid, and the star returned to its original condition.' On the other hand, the German physicists, Meyer and Klein, consider the sudden development of hydrogen, in quantities sufficient to explain such an outburst, exceedingly unlikely. They have therefore adopted the opinion, that the sudden blazing out of the star was occasioned by the violent precipitation of some mighty mass, perhaps a planet, upon the globe of that remote sun, 'by which the momentum of the falling mass would be changed into molecular motion, or in other words into heat and light.' It might even be supposed, they urge, that the star in the Crown, by its swift motion, may have come in contact with one of the star clouds which exist in large numbers in the realms of space. 'Such a collision would necessarily set the star in a blaze and occasion the most vehement ignition of its hydrogen.' Fortunately, our sun is safe for many millions of years to come from contact from any one of its planets. The reader must not, however, run away with the idea that the danger consists only in the gradual contraction of planetary orbits sometimes spoken of. That contraction, if it is taking place at all, of which we have not a particle of evidence, would not draw Mercury to the sun's surface for at least ten million millions of years. The real danger would be in the effects which the perturbing action of the larger planets might produce on the orbit of Mercury. That orbit is even now very eccentric, and must at times become still more so. It might, but for the actual adjustment of the planetary system, become so eccentric that Mercury could not keep clear of the sun; and a blow from even small Mercury (only weighing, in fact, 390 millions of millions of millions of tons), with a velocity of some 300 miles per second, would warm our sun considerably. But there is no risk of this happening in Mercury's case--though the unseen and much more shifty Vulcan (in which planet I beg to express here my utter disbelief) might, perchance, work mischief if he really existed.