=Lifeless Planets.= The larger a planet the longer is the time that must elapse before the hot vapors of rock and metal, which largely compose its early atmosphere, cool and congeal into a crust, leaving as a residual an atmosphere of such heat, density, and composition as to permit of the beginnings of the forms of life that have inhabited the world. Before the sun can reach this condition, an indescribable period will have elapsed, its light will have gone out, its heat will have ceased to reach the earth and the other planets in quantities sufficient to maintain life, the earth will have been dead millions of years, and the sun itself will only receive heat and light from the feeble rays of the stars that, unlike itself, have not yet ceased to shine. But even then the sun ever must remain dead, for there is no external source whence it may receive heat. No vegetation can adorn it, no water flow upon its surface, neither can the foot of any man press its soil. Jupiter, and perhaps Neptune, Uranus, and Saturn, have hot atmospheres still in violent agitation,—molten surfaces composed of all kinds of matter, from which bubble and boil off hot clouds of vapor that surge about in huge eddies or cyclonic storms, and that here and there are shot outward in tongues of fire. The earth millions of years ago had a similar atmosphere. But when the heat energy of these vaporous planets wanes, and they cool down, as the earth did many years ago, the simplest forms of life cannot be evolved upon them, for they are too far away from the sun to receive life-giving heat. Mars receives less than half the intensity of the solar rays that come to the earth, Jupiter only 0.037, Saturn 0.011, Uranus 0.003, and Neptune 0.001. In due time—some hundreds of millions of years—the cooling of the sun will leave the earth to freeze and all life to become extinct, unless, perchance, the oxygen of the air is so far absorbed by its rocks, or filtered away into space, as to destroy life before that time. No matter what may be the achievements of the human mind, what wonderful civilizations may be developed, what powerful empires created, or what wonderful secrets of creation discovered, it seems certain that these all will pass away, and finally the surface of the earth be as if man never lived. The dust of ages will wipe out and obliterate every trace and vestige of the operations of life. Silence, cold, and darkness will then reign supreme. But the time of this is indescribably far off in the future, and man will have ample opportunity to develop to the highest mental and spiritual estates of which he has inherent possibilities.