The scientists of to-day may well look after the soundness of their favorite theories of the great physical forces; for the uncertain tenure of old theories, by reason of recent discoveries, is becoming but too manifest. New phenomena are now observed which require solutions not met by present hypotheses. The nebular hypothesis which has so long possessed the scientific mind has, by the discovery of the moons of Mars, become a thing of the past. According to M. MAICHE, water is found to be no longer the old-fashioned conventional oxygen and hydrogen, but essentially a new element must be considered in estimating its composition.[6] Light is ascertained to be as veritable a substance as water. The sun is recognized to be dark, cool, and habitable. Messages go through the air from kite to kite ten miles apart without visible agency. Telephonic sounds leap from wire to wire through quite ten feet of space. _Present theories of Supply of Sun-heat._ The present theories of the production and dissemination of sun-heat, are simply accepted for want of better, and not because they account satisfactorily for the phenomena. The first and most prominent is the combustion theory, which, though bearing the seal of ages, is obnoxious both to common and philosophic reasoning. This theory presupposes a consumption of material beyond all conception, and the supply of which has been no small tax upon the scientific imagination. The source of this supply has been claimed to be the subsidence of useless worlds, and of asteroids, and meteors, showered down upon its surface. Estimates have been carefully made, and we are gravely informed of the probable amount of combustive material required to supply the sun's demands for given periods. It is said that the coal-fields of Pennsylvania, which would supply the world's consumption for centuries, would keep the sun's rate of emission for considerably less than 1/1,000 part of a second. POUILLET estimated the quantity of heat emitted by the sun per hour to be equal to the supply of a layer of anthracite coal ten feet thick, spread over the whole surface of the sun. The theory advocated by HELMHOLTZ, and by many other scientists, of "the gradual contraction of the solar orb," and that of SECCHI, "the dissociation of compound bodies in the sun's substance," are attempts after a more consistent philosophy. The foregoing theories pre-suppose the sun to be a glowing fiery mass, from which, in all directions, issue radiations of heat and light into space. Of this enormous quantity of radiated heat, the earth is supposed to receive but 1/2,000,000,000 part. MEYER observes: "_A general law of nature which knows no exception_ is the following: _In order to obtain heat, something must be expended._"