From the problem of heating a room, the mind of Siemens also passed to the maintenance of solar fires, and occupied itself with the supply of fuel to the sun. Some physicists have attributed the continuance of solar heat to the contraction of the solar mass, and others to the impact of cometary matter. Imbued with the idea of regeneration, and seeking in nature for that thrift of power which he, as an inventor, had always aimed at, Siemens suggested a hypothesis on which the sun conserves its heat by a circulation of its fuel in space. The elements dissociated in the intense heat of the glowing orb rush into the cooler regions of space, and recombine to stream again towards the sun, where the self-same process is renewed. The hypothesis was a daring one, and evoked a great deal of discussion, to which the author replied with interest, afterwards reprinting the controversy in a volume, ON THE CONSERVATION OF SOLAR ENERGY. Whether true or not--and time will probably decide--the solar hypothesis of Siemens revealed its author in a new light. Hitherto he had been the ingenious inventor, the enterprising man of business, the successful engineer; but now he took a prominent place in the ranks of pure science and speculative philosophy. The remarkable breadth of his mind and the abundance of his energies were also illustrated by the active part he played in public matters connected with the progress of science. His munificent gifts in the cause of education, as much as his achievements in science, had brought him a popular reputation of the best kind; and his public utterances in connection with smoke abatement, the electric light. Electric railways, and other topics of current interest, had rapidly brought him into a foremost place among English scientific men. During the last years of his life, Siemens advanced from the shade of mere professional celebrity into the strong light of public fame.