THE UNIFORMITY OF MATTER AND ITS LAWS THROUGHOUT THE STELLAR UNIVERSE
I have shown in the second chapter of this work that none of the previous
writers on the question of the habitability of the other planets have
really dealt with the subject in any adequate manner, since not only do
they appear to be quite unaware of the delicate balance of conditions which
alone renders organic life possible on any planet, but they have altogether
omitted any reference to the fact that not only must the conditions be such
as to render life possible _now_, but these conditions must have persisted
during the long geological epochs needed for the slow development of life
from its most rudimentary forms. It will therefore be necessary to enter
into some details both as to the physical and chemical essentials for a
continuous development of organic life, and also into the combination of
mechanical and physical conditions which are required on any planet to
render such life possible.
THE UNIFORMITY OF MATTER
One of the most important and far-reaching of the discoveries due to the
spectroscope is that of the wonderful identity of the elements and
material compounds in earth and sun, stars and nebulæ, and also of the
identity of the physical and chemical laws that determine the states and
forms assumed by matter. More than half the total number of the known
elements have been already detected in the sun, including all those which
compose the bulk of the earth's solid material, with the one exception of
oxygen. This is a very large proportion when we consider the very peculiar
conditions which enable us to detect them. For we can only recognise an
element in the sun when it exists at its surface in an incandescent state,
and also above its surface in the form of a somewhat cooler gas. Many of
the elements may rarely or never be brought to the surface of so vast a
body, or if they do sometimes appear there, it may not be in sufficient
quantity or in sufficient purity to produce any bands in the spectroscope,
while the cooler gas or vapour may either not be present, or be so
dispersed as not to produce sufficient absorption to render its spectral
lines visible. Again, it is believed that many elements are dissociated by
the intense heat of the sun, and may not be recognisable by us, or they may
only exist at its surface in a compound form unknown on the earth; and in
some such way those lines of the solar spectrum which remain still
unrecognised may have been produced. One of these unknown lines was that of
Helium, a gas found soon afterwards in the rare mineral 'Cleveite,' and
since detected frequently in many stars. Some of the stars have spectra
very closely resembling that of the sun. The dark lines are almost as
numerous, and most of them correspond accurately with solar lines, so that
we cannot doubt their having almost exactly the same chemical constitution,
and being also in the same condition as regards heat and stage of
development. Other stars, as we have already stated, exhibit mainly lines
of hydrogen, sometimes combined with fine metallic lines. Of the spectra of
the nebulæ comparatively little is known, but many are decidedly gaseous,
while others show a continuous spectrum indicating a more complex
constitution.
But we also obtain considerable knowledge of the matter of non-terrestrial
bodies by the analysis of the numerous meteorites which fall upon the
earth. Most of these belong to some of the many meteoric streams which
circulate round the sun, and which may be supposed to give us samples of
planetary matter. But as it is now believed that many of them are produced
by the debris of comets, and the orbits of some of these indicate that they
have come from stellar space and have been drawn into our system by the
attractive power of the larger planets, it is almost certain that the
meteoric stones not infrequently bring us matter from the remoter regions
of space, and probably afford us samples of the solid constituents of
nebula; or the cooler stars. It is, therefore, a most suggestive fact that
none of these meteorites have been found to contain a single
non-terrestrial element, although no less than twenty-four elements have
been found in them, and it will be of interest to give the list of these,
as follows:--_Oxygen_, Hydrogen, _Chlorine_, _Sulphur_, _Phosphorus_,
Carbon, Silicon, Iron, Nickel, Cobalt, Magnesium, Chromium, Manganese,
Copper, Tin, _Antimony_, Aluminium, Calcium, Potassium, Sodium, _Lithium_,
Titanium, _Arsenic_, and Vanadium. Seven of the above, printed in italics,
have not yet been found in the sun, such as Oxygen, Chlorine, Sulphur, and
Phosphorus, which form the constituents of many widespread minerals, and
they supply important gaps in the series of solar and stellar elements. It
may be noted that although meteorites have supplied no new elements, they
have furnished examples of some new combinations of these elements forming
minerals distinct from any found in our rocks.
The fact of the occurrence in meteorites not only of minerals which are
peculiar to them or are found on the earth, but also of structures
resembling our breccias, veins, and even slicken-side surfaces, has been
held to be opposed to the meteoritic theory of the origin of suns and
planets, because meteorites seem to be thus proved to be the fragments of
suns or worlds, not their primary constituents. But these cases are
exceptional, and Mr. Sorby, who made a special study of meteorites,
concluded that their materials have usually been in a state of fusion or
even of vapour, as they now exist in the sun, and that they became
condensed into minute globular particles, which afterwards collected into
larger masses, and may have been broken up by mutual impact, and again and
again become aggregated together--thus presenting features which are
completely in accordance with the meteoritic theory.
But, quite recently, Mr. T.C. Chamberlin has applied the theory of tidal
distortion to showing how solid bodies in space, without ever coming into
actual contact, must sometimes be torn apart or disrupted into numerous
fragments by passing near to each other. Especially when a small body
passes near a much larger one, there is a certain distance of approach
(termed the Roche limit) when the increasing differential force of gravity
will be sufficient to tear asunder the smaller body and cause the fragments
either to circulate around it or to be dispersed in space.[10] In this way,
therefore, those larger meteorites which exhibit planetary structure may
have been produced. Of course they would rarely have been true planets
attached to a sun, but more frequently some of the smaller dark suns, which
may possess many of the physical characteristics of planets, and of which
there may be myriads in the stellar spaces.
On the whole, then, we have positive knowledge of the existence, in the
sun, stars, and planetary and stellar spaces, of such a large proportion of
the elements of our globe, and so few indications of any not forming part
of it, that we are justified in the statement, that the whole stellar
universe is, broadly speaking, constructed of the same series of elementary
substances as those we can study upon our earth, and of which the whole
realm of nature, animal, vegetable, and mineral, is composed. The evidence
of this identity of substance is really far more complete than we could
expect, considering the very limited means of inquiry that we possess; and
we shall, therefore, not be justified in assuming that any important
difference exists.
When we pass from the elements of matter to the laws which govern it, we
also find the clearest proofs of identity. That the fundamental law of
gravitation extends to the whole physical universe is rendered almost
certain by the fact that double stars move round their common centre of
gravity in elliptical orbits which correspond well with both observation
and calculation. That the laws of light are the same both here and in
inter-planetary space is indicated by the fact that the actual measurement
of the velocity of light on the earth's surface gives a result so
completely identical with that prevailing to the limits of the solar
system, that the measurement of the sun's distance, by means of the
eclipses of Jupiter's satellites combined with the measured velocity of
light, agrees almost exactly with that obtained by means of the transits of
Venus, or through our nearest approach to the planets Mars or Eros.
Again, the more recondite laws of light are found to be identical in sun
and stars with those observed within the narrow bounds of laboratory
experiments. The minute change of position of spectral lines caused by the
source of light moving towards or away from us enables us to determine this
kind of motion in the most distant stars, in the planets, or in the moon,
and these results can be tested by the motion of the earth either in its
orbit or in its rotation; and these latter tests agree with the theoretical
determination of what must occur, dependent on the wave-lengths of the
different dark lines of the solar spectrum determined by measurements in
the laboratory.
In like manner, minute changes in the widening or narrowing of spectral
lines, their splitting up, their increase or decrease in number, and their
arrangement so as to form flutings, can all be interpreted by experiments
in the laboratory, showing that such phenomena are due to alterations of
temperature, of pressure, or of the magnetic field, thus proving that the
very same physical and chemical laws act in the same way here and in the
remotest depths of space.
These various discoveries give us the certain conviction that the whole
material universe is essentially one, both as regards the action of
physical and chemical laws, and also in its mechanical relations of form
and structure. It consists throughout of the very same elements with which
we are so familiar on our earth; the same ether whose vibrations bring us
light and heat, electricity and magnetism, and a whole host of other
mysterious and as yet imperfectly known forces; gravitation acts throughout
its vast extent; and in whatever direction and by whatever means we obtain
a knowledge of the stellar universe, we find the same mechanical, physical,
and chemical laws prevailing as upon our earth, so that we have in some
cases been actually enabled to reproduce in our laboratories phenomena with
which we had first become acquainted in the sun or among the stars.
We may therefore feel it to be an almost certain conclusion that--the
elements being the same, the laws which act upon, and combine, and modify
those elements being the same--organised living beings wherever they may
exist in this universe must be, fundamentally, and in essential nature, the
same also. The outward forms of life, if they exist elsewhere, may vary
almost infinitely, as they do vary on the earth; but, throughout all this
variety of form--from fungus or moss to rose-bush, palm or oak; from
mollusc, worm, or butterfly to humming-bird, elephant, or man--the
biologist recognises a fundamental unity of substance and of structure,
dependent on the absolute requirements of the growing, moving, developing,
living organism, built up of the same elements, combined in the same
proportions, and subject to the same laws. We do not say that organic life
_could_ not exist under altogether diverse conditions from those which we
know or can conceive, conditions which may prevail in other universes
constructed quite differently from ours, where other substances replace the
matter and ether of our universe, and where other laws prevail. But,
_within_ the universe we know, there is not the slightest reason to suppose
organic life to be possible, except under the same general conditions and
laws which prevail here. We will, therefore, now proceed to describe, very
generally, what are the conditions essential to the existence and the
continuous development of vegetable and animal life.
FOOTNOTE:
[10] _The Astrophysical Journal_, vol. xiv., July 1901, p. 17.