Here, again, the test must be applied which is decisive of such pretensions. It may certainly be the case that chemical actions of one kind or another are going on in the sun, and among them are doubtless some of such a character that they evolve heat. But we happen to know exactly how much heat can be evolved by the action of specified quantities of elementary bodies by whose union heat is generated. It appears clear from the figures that chemical action is a wholly inadequate method of accounting for solar radiation. To take one instance, we may mention that if the sun had been a globe of white-hot carbon, and if there had been a sufficient supply of oxygen to effect its combustion, the total heat generated by the entire mass would not supply the solar radiation for the period that has elapsed since the building of the pyramids. It is, therefore, clear that the supposition that the sun is a burning globe, like the supposition of the sun as a cooling solid globe, is quite inadequate to explain the marvellous persistence with which, for countless ages, the orb of day has distributed its beams.
There is another supposition which, though not itself providing the explanation that we are searching for, still points so far in that direction that I have kept it till the last. It has been sometimes suggested that the dashing of meteoric matter into the sun from outside may afford the requisite supply of energy. There can be no doubt that the plunge of a meteor into the sun's atmosphere with the terrific velocity which it will necessarily acquire in consequence of the attraction of the sun, is accompanied by the transformation of the energy of the meteor's movement into light and heat. The quantity of energy that a meteor thus carries with it is so vast that
