CHAPTER IX.

THE METEORIC THEORY OF SOLAR HEAT.

Of the various theories proposed by astronomers to account for the origin of the sun's light and heat, only two have at present any considerable number of advocates. These are--

1. _The Chemical Theory_; according to which the light and heat of the sun are produced by the chemical combination of its elements; in other words, by an intense combustion.

2. _The Meteoric Theory_, which ascribes the heat of our central luminary to the fall of meteors upon its surface. The former is advocated with great ingenuity by Professor Ennis in a recent work on "_The Origin of the Stars, and the Causes of their Motions and their Light_." It has, on the other hand, been ably opposed by Dr. Mayer, Professor William Thomson, and other eminent physicists. A brief examination of its claims may not be destitute of interest.

If the sun's heat is produced by chemical action, whence comes the necessary supply of fuel to support the combustion? The quantity of solar heat radiated into space has been determined with at least an approximation to mathematical precision. We know also the amount produced by the combustion of a given quantity of coal. Now it has been found by calculation that if the sun were a solid globe of coal, and a sufficient supply of oxygen were furnished to support its combustion, the amount of heat resulting from its consumption would be less than that actually emitted during the last 6000 years. In short, _no known_ elements would meet the demands of the case. But it is highly probable that the different bodies of the solar system are composed of the same elements. This view is sustained by the well-known fact that meteoric stones, which have reached us from different and distant regions of space, have brought us no new elementary substances. The _chemical_ theory of solar heat seems thus encumbered with difficulties well-nigh insuperable.

Professor Ennis' mode of obviating this objection, though highly ingenious, is by no means conclusive. The latest analyses of the solar spectrum indicate, he affirms, the presence of numerous elements besides those with which we are acquainted. Some of these may yield by their combustion a much greater amount of heat than the same quantity of any known elements in the earth's crust. "Every star," he remarks, "as far as yet known, has a different set of fixed lines, although there are certain resemblances between them. They lead to the conclusion that each star has, in part at least, its peculiar modifications of matter, called simple elements; but the number of stars is infinite, and therefore the number of elements must be infinite."[21] He argues, moreover, that in a globe so vast as the sun there may be forces in operation with whose nature we are wholly unacquainted. This leaving of the _known_ elements as well as the _known_ laws of nature for _unknown possibilities_ will hardly be satisfactory to unbiased minds.

Again: that the different bodies of the universe are composed of different elements is inferred by our author from the following among other considerations: "In our solar system Mercury is sixty or eighty times more dense than one of the satellites of Jupiter, and probably in a much greater proportion denser than the satellites of Saturn. This indicates a wide difference between the nature of their elements." This statement is again repeated in a subsequent page.[22] "The densities of the planets and their satellites prove that they are composed of very different elements. Mercury is more than sixty times, and our earth about fifty times, more dense than the inner moon of Jupiter. Saturn is only about one-ninth as dense as the earth; it would float buoyantly on water. There is a high probability that the satellites of Saturn and Uranus are far lighter than those of Jupiter. Between the two extremes of the attendants of the sun, there is probably a greater difference in density than one hundred to one; and from one extreme to the other there are regular gradations of small amount.

"The difference in constitution between the earth and the moon is seen in their densities: that of the moon being about half that of the earth. The nitrogen of our globe is found only in the atmosphere, and such substances as derive it from the atmosphere. The moon has no appreciable atmosphere, and therefore, in a high probability, no nitrogen."

The statements here quoted were designed to show that the physical constitution of the sun and planets is widely different from that of the earth, and that the combustion of _some_ of the elements in this indefinite variety may account for the origin of solar heat. Let us examine the facts.

According to Laplace the mass of Jupiter's first satellite is 0·000017328, that of Jupiter being 1. The diameter is 2436 miles. Hence the corresponding density is a little more than _one-fifth_ of the mean density of the earth. In other words, it is somewhat greater than the density of water, and very nearly equal to that of Jupiter himself. Professor Ennis' value is therefore erroneous.[23] In regard to the densities of the Saturnian and Uranian satellites nothing is known, and conjecture is useless. In short, Saturn has the least mean density of all the planets, primary or secondary, so far as known. This may be owing to the great extent of his atmospheric envelope. The density of the moon is but three-fifths that of the earth: it is to be borne in mind, however, that the _mass_ and _pressure_ are also much less.

With respect to meteorites the same author remarks that "like the moon, they are probably satellites of the earth; but being very small, they are liable to extraordinary perturbations, and hence strike the earth in many directions." Here, again, his _facts_ are at fault; for (1) the observed velocities of these bodies are inconsistent with the supposition of their being satellites of the earth; and (2) the amount of perturbation of such bodies does not vary with their masses: a _small_ meteorite would fall toward the earth or any other planet with no greater velocity than a _large_ one.

THE METEORIC THEORY.

It has been shown in a previous chapter that immense numbers of meteoric asteroids are constantly traversing the planetary spaces--that many millions, in fact, daily enter the earth's atmosphere. Reasons are not wanting for supposing the numbers of these bodies to increase with great rapidity as we approach the center of the system. Moreover, on account of the greater force of gravity at the sun's surface the heat produced by their fall must be much greater than at the surface of the earth. It has been calculated that if one of these asteroids be arrested in perihelion by the solar atmosphere, the quantity of heat thus developed will be 9000 times greater than that produced by the combustion of an equal mass of coal. There can, therefore, be no reasonable doubt that a _portion_ of the sun's heat is produced by the impact of meteoric matter. In considering the probability that it is _chiefly_ so generated, the following questions naturally present themselves:

1. _What amount of matter precipitated upon the sun would develop the quantity of heat actually emitted?_--This question has been satisfactorily discussed by eminent physicists, and it will be sufficient for our purpose to give the result. According to Professor William Thomson, of Glasgow, the present rate of emission would be kept up by a meteoric deposit which would form an annual stratum 60 feet in thickness over the sun's surface.

2. _Could such an increase of the sun's magnitude be detected by micrometrical measurement?_--This inquiry is readily answered in the negative. The apparent diameter would be augmented only one second in 17,600 years.

3. _Is there any known or visible source from which this amount of meteoric matter may be supplied?_--Thomson, Mayer, and other distinguished writers regard the zodiacal light as the source of such meteorites. The inner portions of this immense "tornado" must be resisted in their motions by the solar atmosphere, and hence precipitated upon the sun's surface.

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]