4. _Would this increase of the sun's mass derange the motions of the solar system?_--To this question Prof. Ennis gives an affirmative answer; his first objection to the theory under consideration being stated as follows: "The constant accumulation of such materials, during hundreds of millions of years, would increase the body of the sun and its consequent gravity so greatly as to derange the entire solar system, by destroying the balance between the centripetal and centrifugal forces now acting on the planets."[24] This, it must be confessed, would be a valid objection, if the meteoric matter were supposed to be derived from the extra-planetary spaces. As their source, however,--the zodiacal light--is interior to the earth's orbit, it can have no application to any planet exterior to Venus. Most probably the greater portion of the meteoric mass is even within the orbit of Mercury, so that the effect of its convergence could scarcely be noticed even in the motion of the interior planets. In pre-historic time the zodiacal light may have extended far beyond the earth's orbit. If so, its convergence to its present dimensions was undoubtedly attended by an acceleration of the earth's mean motion. We can of course have no evidence that such a shortening of the year has never occurred. The second objection urged against the meteoric theory by the author of "The Origin of the Stars" is thus expressed: "As we must believe that all stars were lighted up by the same means, so we must believe, according to this theory, that the present interior heat of the earth and its former melted condition in both exterior and interior, was caused by the fall of meteorites. But if so, they must have gradually ceased to fall, as space became cleared of their presence, and we would now find a thick covering of meteorites on the earth's cooled surface. Instead of this, we find them very rarely, and in accordance with their present very rare falls." To this it may be replied that the primitive igneous fluidity of the earth and planets was a necessary consequence of their condensation--a fact which has no inconsistency with the theory in question. A different _mechanical_ theory of the origin of solar heat is advocated by Professor Helmholtz in his interesting work _On the Interaction of Natural Forces_. In regard to the sun he says: "If we adopt the very probable view, that the remarkably small density of so large a body is caused by its high temperature, and may become greater in time, it may be calculated that if the diameter of the sun were diminished only the ten-thousandth part of its present length, by this act a sufficient quantity of heat would be generated to cover the total emission for 2100 years. Such a small change besides it would be difficult to detect by the finest astronomical observations."[25] The same view is adopted by Dr. Joel E. Hendricks, of Des Moines, Iowa.[26] CHAPTER X.