Another evidence that the solar system is temporal, and that the present cosmical order must come to an end, is found in the fact that the sun is radiating heat into space in quantities incomparably greater than it receives. If it were not so, we should receive, on the average, as much heat from every other quarter of the heavens as from the sun, and no vicissitudes of temperature would ever occur on the earth. Now, from what we know of the nature of heat, it is impossible that the supply contained in the sun should be inexhaustible. There is no apparent reason why the sun should form an exception to the fate of all fires, its only difference being one of size and time. It is larger and hotter than ordinary lamps, but is nevertheless a lamp in which invisible molecular energy is consumed, and consumed, too, at a rate which baffles all conception. From every square foot of its surface the sun gives out energy equal in amount to seven thousand horse-power. The total amount of heat sent off from the sun in _one minute_ is "five thousand millions of millions of units": a unit of heat being the quantity of heat required to raise one kilogramme--or about one quart--of water one Centigrade degree.[162] This enormous consumption of energy must finally exhaust the original stock. Were the sun a solid block of coal, and were it allowed a sufficient quantity of oxygen to enable it to burn at the rate necessary to produce the observed emission of heat, it would be utterly consumed in five thousand years. Or if we suppose, with Thomson, that the initial form of the energy of the universe is the potential energy of gravitation in matter diffused through space, and if this potential energy (energy of position) is transformed into heat (molecular kinetic energy) by condensation or contraction of the sun, and this energy of molecular motion (heat) is again transformed into radiant energy and diffused through infinite space, it is obvious that this condensation can not be continued forever, and Thomson has shown in his article on the "Age of the Sun's Heat" that its power of radiation must come to an end. Various theories have been suggested for replenishing the solar heat, one of the most plausible of which is the falling of meteoric and cometary bodies into the sun. Prof. Thomson, who was one of the first to adopt this view, has now abandoned it, or at least has denied its adequacy to account for the maintenance of solar heat. Even were the hypothesis accepted as valid, the supply of fuel is still finite. Time will drain the entire space inclosed by the orbit of the planet Neptune of all the meteors and comets. Even the planets must at length be ensepulchred in the sun. "As surely," writes Sir William Thomson, "as the weights of the clock run down to the lowest position, from which they can never rise again unless fresh energy is communicated to them from some source not yet exhausted, so surely must every planet