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]