Almost alone among all these results, his theory of the “Conservation of Solar Energy” dealt with a question not affecting, or at least not immediately affecting, human welfare. A great authority has characterised this as “one of the highest and most brilliant flights that the scientific imagination has ever made.” While astronomers quietly accepted the conclusion that the sun is cooling down, and will become at some distant but calculable epoch a mere cinder hung in space, he endeavored to show that energy can no more be lost in the solar system than it is in the laboratory or the factory. Sir William Siemens’s theory assumed that the interplanetary spaces are filled with an exceedingly thin or rare atmosphere of the compounds of carbon, hydrogen, and oxygen, such for example as aqueous vapor and hydro-carbons. In this atmosphere the sun is revolving with a velocity four times that of the earth, and hence the solar atmosphere at his equator is thrown out to an enormous distance from his surface. One consequence of this is a perpetual indraught, at the poles of the sun, of the surrounding atmosphere. Thus the sun is everlastingly being fed, and everlastingly sending out its light and heat, which thus recuperate themselves: in this way the solar energy, which is sometimes assumed to be lost in the empty void of interstellar space, really acts upon the rare vapors therein, and converts the universe into a kind of vast regenerative furnace! Had the author of this ingenious theory lived but a few years longer, he would doubtless have labored to strengthen it with further observations and arguments. As it is, it must remain as a daring and original suggestion, the effort of a keen and sagacious mind to bring to fresh subjects the experience and the knowledge accumulated by work of quite a different kind. It is more scientific to believe, with him, that there is some restorative and conservative agency at work, than to suppose that the universe is gradually cooling down into a ball of slag, were it only because his theory does not require an effort of creation at once tremendous and futile. It leaves us free to avoid contemplating a time when the solar system was not, and another when it will cease to be.