The third hypothesis relates to currents of electricity. We have no knowledge of currents of electricity which could produce, however multiplied or intensified, such light and heat as are constantly poured forth from the sun into all space. That electricity is the intermediate cause of our sun's energy, and of all solar energy, it is the purpose of this work to demonstrate, but not electric currents, which find their attractiveness to theorists in the vague suggestion of which Professor Proctor speaks, referring to comets, in his article on "Cometic Mysteries," "that perhaps this is an electrical phenomenon; perhaps that other feature is electrical, too; perhaps all or most of the phenomena of comets depend on electricity." But he adds, "It is so easy to make such suggestions, so difficult to obtain evidence in their favor having the slightest scientific value. Still, I hold the electrical idea to be well worth careful study. Whatever credit may hereafter be given to any electrical theory of comets will be solely and entirely due to those who may help to establish it upon a basis of sound evidence,--none whatever to the mere suggestion, which has been made time and again since it was first advanced by Fontanelle." It will be seen that the present work, in demonstrating the true source and mode of solar energy, in itself presents a full and sufficient explanation of all the cometic mysteries referred to, as well as all those pertaining to other solar systems in space, and the multifarious phenomena which they present. Indeed, the philosophic mind will not be satisfied with the sufficiency of any hypothesis which will not unlock the mysteries and clearly explain the phenomena of other systems,--of comets, variable and temporary stars, double stars, and all the complicated celestial economy which to the eye of the mere observer presents a bewildering scene of the operation of independent and inscrutable forces. The fifth hypothesis cited, that of meteoric impact, doubtless plays a part, as we know from the generation of light and heat by the constant passage of similar bodies through our own atmosphere. And we know, of course, that the sun, by its vastly-increased attraction, must be subjected to the constant impact of such meteoric bodies in enormous numbers. But the fatal defect in the theory is that such impacts, to produce the radiant energy of the sun, must constantly add to its mass in like proportion, and as the motions and distances of the planets in their orbits are regulated and preserved by virtue of the substantially constant mass of the sun, any progressive and considerable increase in its mass must constantly bring the planets nearer and nearer, and thus increase their orbital velocity. Helmholtz quotes from Sir William Thomson's investigation, that, "assuming it to hold, the mass of the sun should increase so rapidly that the consequences would have shown themselves in the accelerated motion of the planets.