_Lecture delivered in Heidelberg and in Cologne, in 1871._
It is my intention to bring a subject before you to-day which has been
much discussed--that is, the hypothesis of Kant and Laplace as to the
formation of the celestial bodies, and more especially of our planetary
system. The choice of the subject needs no apology. In popular
lectures, like the present, the hearers may reasonably expect from the
lecturer, that he shall bring before them well-ascertained facts, and
the complete results of investigation, and not unripe suppositions,
hypotheses, or dreams.
Of all the subjects to which the thought and imagination of man could
turn, the question as to the origin of the world has, since remote
antiquity, been the favourite arena of the wildest speculation.
Beneficent and malignant deities, giants, Kronos who devours his
children, Niflheim, with the ice-giant Ymir, who is killed by the
celestial Asas,[23] that out of him the world may be constructed--these
are all figures which fill the cosmogonic systems of the more
cultivated of the peoples. But the universality of the fact, that each
people develops its own cosmogonies, and sometimes in great detail,
is an expression of the interest, felt by all, in knowing what is our
own origin, what is the ultimate beginning of the things about us. And
with the question of the beginning is closely connected that of the
end of all things; for that which may be formed, may also pass away.
The question about the end of things is perhaps of greater practical
interest than that of the beginning.
[Footnote 23: Cox’s _Aryan Mythology_, vol. i. 372. Longmans.]
Now, I must premise that the theory which I intend to discuss to-day
was first put forth by a man who is known as the most abstract of
philosophical thinkers; the originator of transcendental idealism
and of the Categorical Imperative, Immanuel Kant. The work in which
he developed this, the _General Natural Philosophy and Theory of the
Heavens_, is one of his first publications, having appeared in his
thirty-first year. Looking at the writings of this first period of
his scientific activity, which lasted to about his fortieth year,
we find that they belong mostly to Natural Philosophy, and are far
in advance of their times with a number of the happiest ideas. His
philosophical writings at this period are but few, and partly like
his introductory lecture, directly originating in some adventitious
circumstance; at the same time the matter they contain is comparatively
without originality, and they are only important from a destructive and
partially sarcastic criticism. It cannot be denied that the Kant of
early life was a natural philosopher by instinct and by inclination;
and that probably only the power of external circumstances, the want of
the means necessary for independent scientific research, and the tone
of thought prevalent at the time, kept him to philosophy, in which it
was only much later that he produced anything original and important;
for the _Kritik der reinen Vernunft_ appeared in his fifty-seventh
year. Even in the later periods of his life, between his great
philosophical works, he wrote occasional memoirs on natural philosophy,
and regularly delivered a course of lectures on physical geography.
He was restricted in this to the scanty measure of knowledge and of
appliances of his time, and of the out-of-the-way place where he lived;
but with a large and intelligent mind he strove after such more general
points of view as Alexander von Humboldt afterwards worked out. It is
exactly an inversion of the historical connection, when Kant’s name is
occasionally misused, to recommend that natural philosophy shall leave
the inductive method, by which it has become great, to revert to the
windy speculations of a so-called ‘deductive method.’ No one would have
attacked such a misuse, more energetically and more incisively, than
Kant himself if he were still among us.
The same hypothesis as to the origin of our planetary system was
advanced a second time, but apparently quite independently of Kant,
by the most celebrated of French astronomers, Simon, Marquis de
Laplace. It formed, as it were, the final conclusion of his work on the
mechanism of our system, executed with such gigantic industry and great
mathematical acuteness. You see from the names of these two men, whom
we meet as experienced and tried leaders in our course, that in a view
in which they both agree, we have not to deal with a mere random guess,
but with a careful and well-considered attempt to deduce conclusions as
to the unknown past from known conditions of the present time.
It is in the nature of the case, that a hypothesis as to the origin
of the world which we inhabit, and which deals with things in the
most distant past, cannot be verified by direct observation. It may,
however, receive direct confirmation, if, in the progress of scientific
knowledge, new facts accrue to those already known, and like them
are explained on the hypothesis; and particularly if survivals of the
processes, assumed to have taken place in the formation of the heavenly
bodies, can be proved to exist in the present.
Such direct confirmations of various kinds have, in fact, been formed
for the view we are about to discuss, and have materially increased its
probability.
Partly this fact, and partly the fact that the hypothesis in question
has recently been mentioned in popular and scientific books, in
connection with philosophical, ethical, and theological questions, have
emboldened me to speak of it here. I intend not so much to tell you
anything substantially new in reference to it, as to endeavour to give,
as connectedly as possible, the reasons which have led to, and have
confirmed it.
These apologies which I must premise, only apply to the fact that I
treat a theme of this kind as a popular lecture. Science is not only
entitled, but is indeed beholden, to make such an investigation. For
her it is a definite and important question--the question, namely,
as to the existence of limits to the validity of the laws of nature,
which rule all that now surrounds us; the question whether they have
always held in the past, and whether they will always hold in the
future; or whether, on the supposition of an everlasting uniformity of
natural laws, our conclusions from present circumstances as to the
past, and as to the future, imperatively lead to an impossible state
of things; that is, to the necessity of an infraction of natural laws,
of a beginning which could not have been due to processes known to us.
Hence, to begin such an investigation as to the possible or probable
primeval history of our present world, is, considered as a question of
science, no idle speculation, but a question as to the limits of its
methods, and as to the extent to which existing laws are valid.
It may perhaps appear rash that we, restricted as we are, in the circle
of our observations in space, by our position on this little earth,
which is but as a grain of dust in our milky way; and limited in time
by the short duration of the human race; that we should attempt to
apply the laws which we have deduced from the confined circle of facts
open to us, to the whole range of infinite space, and of time from
everlasting to everlasting. But all our thought and our action, in the
greatest as well as in the least, is based on our confidence in the
unchangeable order of nature, and this confidence has hitherto been the
more justified, the deeper we have penetrated into the interconnections
of natural phenomena. And that the general laws, which we have found,
also hold for the most distant vistas of space, has acquired strong
actual confirmation during the past half-century.
In the front rank of all, then, is the law of gravitation. The
celestial bodies, as you all know, float and move in infinite space.
Compared with the enormous distances between them, each of us is but as
a grain of dust. The nearest fixed stars, viewed even under the most
powerful magnification, have no visible diameter; and we may be sure
that even our sun, looked at from the nearest fixed stars, would only
appear as a single luminous point; seeing that the masses of those
stars, in so far as they have been determined, have not been found to
be materially different from that of the sun. But, notwithstanding
these enormous distances, there is an invisible tie between them which
connects them together, and brings them in mutual interdependence. This
is the force of gravitation, with which all heavy masses attract each
other. We know this force as gravity, when it is operative between an
earthly body and the mass of our earth. The force which causes a body
to fall to the ground is none other than that which continually compels
the moon to accompany the earth in its path round the sun, and which
keeps the earth itself from fleeing off into space, away from the sun.
You may realise, by means of a simple mechanical model, the course
of planetary motion. Fasten to the branch of a tree, at a sufficient
height, or to a rigid bar, fixed horizontally in the wall, a silk
cord, and at its end a small heavy body--for instance, a lead ball.
If you allow this to hang at rest, it stretches the thread. This is
the position of equilibrium of the ball. To indicate this, and keep
it visible, put in the place of the ball any other solid body--for
instance, a large terrestrial globe on a stand. For this purpose the
ball must be pushed aside, but it presses against the globe, and,
if taken away, it still tends to come back to it, because gravity
impels it towards its position of equilibrium, which is in the centre
of the sphere. And upon whatever side it is drawn, the same thing
always happens. This force, which drives the ball towards the globe,
represents in our model the attraction which the earth exerts on the
moon, or the sun on the planets. After you have convinced yourselves
of the accuracy of these facts, try to give the ball, when it is a
little away from the globe, a slight throw in a lateral direction.
If you have accurately hit the strength of the throw, the small ball
will move round the large one in a circular path, and may retain this
motion for some time; just as the moon persists in its course round the
earth, or the planets about the sun. Now, in our model, the circles
described by the lead ball will be continually narrower, because the
opposing forces, the resistance of the air, the rigidity of the thread,
friction, cannot be eliminated, in this case, as they are excluded in
the planetary system.
[Illustration: FIG. 5.]
If the path about the attracting centre is exactly circular, the
attracting force always acts on the planets, or on the lead sphere,
with equal strength. In this case, it is immaterial according to what
law the force would increase or diminish at other distances from the
centre in which the moving body does not come. If the original impulse
has not been of the right strength in both cases, the paths will not be
circular but elliptical, of the form of the curved line in Fig. 5. But
these ellipses lie in both cases differently as regards the attracting
centre. In our model, the attracting force is stronger, the further
the lead sphere is removed from its position of equilibrium. Under
these circumstances, the ellipse of the path has such a position in
reference to the attracting centre, that this is in the centre, _c_,
of the ellipse. For planets, on the contrary, the attracting force
is feebler the further it is removed from the attracting body, and
this is the reason that an ellipse is described, one of whose foci
lies in the centre of attraction. The two foci, _a_ and _b_, are two
points which lie symmetrically towards the ends of the ellipse, and are
characterised by the property that the sum of their distances, _am_ +
_bm_, is the same from any given points.
Kepler had found that the paths of the planets are ellipses of this
kind; and since, as the above example shows, the form and position
of the orbit depend on the law according to which the magnitude of
the attracting force alters, Newton could deduce from the form of
the planetary orbits the well-known law of the force of gravitation,
which attracts the planets to the sun, according to which this force
decreases with increase of distance as the square of that distance.
Terrestrial gravity must obey this law, and Newton had the wonderful
self-denial to refrain from publishing his important discovery
until it had acquired a direct confirmation; this followed from the
observations, that the force which attracts the moon towards the earth,
bears towards the gravity of a terrestrial body the ratio required by
the above law.
In the course of the eighteenth century the power of mathematical
analysis, and the methods of astronomical observation, increased
so far that all the complicated actions, which take place between
all the planets, and all their satellites, in consequence of
the mutual action of each upon each, and which astronomers call
disturbances--disturbance, that is to say, of the simpler elliptical
motions about the sun, which each one would produce if the others were
absent--that all these could be theoretically predicted from Newton’s
law, and be accurately compared with what actually takes place in the
heavens. The development of this theory of planetary motion in detail
was, as has been said, the merit of Laplace. The agreement between this
theory, which was developed from the simple law of gravitation, and the
extremely complicated and manifold phenomena which follow therefrom,
was so complete and so accurate, as had never previously been attained
in any other branch of human knowledge. Emboldened by this agreement,
the next step was to conclude that where slight defects were still
constantly found, unknown causes must be at work. Thus, from Bessel’s
calculation of the discrepancy between the actual and the calculated
motion of Uranus, it was inferred that there must be another planet.
The position of this planet was calculated by Leverrier and Adams,
and thus Neptune, the most distant of all known at that time, was
discovered.
But it was not merely in the region of the attraction of our sun that
the law of gravitation was found to hold. With regard to the fixed
stars, it was found that double stars moved about each other in
elliptical paths, and that therefore the same law of gravitation must
hold for them as for our planetary system. The distance of some of
them could be calculated. The nearest of them, α, in the constellation
of the Centaur, is 270,000 times further from the sun than the earth.
Light, which has a velocity of 186,000 miles a second, which traverses
the distance from the sun to the earth in eight minutes, would take
four years to travel from α Centauri to us. The more delicate methods
of modern astronomy have made it possible to determine distances which
light would take thirty-five years to traverse; as, for instance, the
Pole Star; but the law of gravitation is seen to hold, ruling the
motion of the double stars, at distances in the heavens, which all the
means we possess have hitherto utterly failed to measure.
The knowledge of the law of gravitation has here also led to the
discovery of new bodies, as in the case of Neptune. Peters of Altona
found, confirming therein a conjecture of Bessel, that Sirius, the
most brilliant of the fixed stars, moves in an elliptical path about
an invisible centre. This must have been due to an unseen companion,
and when the excellent and powerful telescope of the University of
Cambridge, in the United States, had been set up, this was discovered.
It is not quite dark, but its light is so feeble that it can only be
seen by the most perfect instruments. The mass of Sirius is found to
be 13·76, and that of its satellite 6·71, times the mass of the sun;
their mutual distance is equal to thirty-seven times the radius of the
earth’s orbit, and is therefore somewhat larger than the distance of
Neptune from the sun.
Another fixed star, Procyon, is in the same case as Sirius, but its
satellite has not yet been discovered.
You thus see that in gravitation we have discovered a property common
to all matter, which is not confined to bodies in our system, but
extends, as far in the celestial space, as our means of observation
have hitherto been able to penetrate.
But not merely is this universal property of all mass shared by the
most distant celestial bodies, as well as by terrestrial ones; but
spectrum analysis has taught us that a number of well-known terrestrial
elements are met with in the atmospheres of the fixed stars, and even
of the nebulæ.
You all know that a fine bright line of light, seen through a glass
prism, appears as a coloured band, red and yellow at one edge, blue
and violet at the other, and green in the middle. Such a coloured
image is called a spectrum--the rainbow is such a one, produced by the
refraction of light, though not exactly by a prism; and it exhibits
therefore the series of colours into which white sunlight can thus be
decomposed. The formation of the prismatic spectrum depends on the
foot that the sun’s light, and that of most ignited bodies, is made
up of various kinds of light, which appear of different colours to
our eyes, and the rays of which are separated from each other when
refracted by a prism.
Now if a solid or a liquid is heated to such an extent that it becomes
incandescent, the spectrum which its light gives is, like the rainbow,
a broad coloured band without any breaks, with the well-known series
of colours, red, yellow, green, blue, and violet, and in no wise
characteristic of the nature of the body which emits the light.
The case is different if the light is emitted by an ignited gas, or by
an ignited vapour--that is, a substance vaporised by heat. The spectrum
of such a body consists, then, of one or more, and sometimes even a
great number, of entirely distinct bright lines, whose position and
arrangement in the spectrum is characteristic for the substances of
which the gas or vapour consists, so that it can be ascertained, by
means of spectrum analysis, what is the chemical constitution of the
ignited gaseous body. Gaseous spectra of this kind are shown in the
heavenly space by many nebulæ; for the most part they are spectra which
show the bright line of ignited hydrogen and oxygen, and along with
it a line which, as yet, has never been again found in the spectrum
of any terrestrial element. Apart from the proof of two well-known
terrestrial elements, this discovery was of the utmost importance,
since it furnished the first unmistakable proof that the cosmical
nebulæ are not, for the most part, small heaps of fine stars, but that
the greater part of the light which they emit is really due to gaseous
bodies.
The gaseous spectra present a different appearance when the gas is in
front of an ignited solid whose temperature is far higher than that of
the gas. The observer sees then a continuous spectrum of a solid, but
traversed by fine dark lines, which are just visible in the places in
which the gas alone, seen in front of a dark background, would show
bright lines. The solar spectrum is of this kind, and also that of a
great number of fixed stars. The dark lines of the solar spectrum,
originally discovered by Wollaston, were first investigated and
measured by Fraunhofer, and are hence known as Fraunhofer’s lines.
[Illustration: FIG. 6.]
[Illustration: FIG. 7.]
[Illustration: FIG. 8.]
Far more powerful apparatus was afterwards used by Kirchhoff, and then
by Angström, to push the decomposition of light as far as possible.
Fig. 6 represents an apparatus with four prisms, constructed by
Steinheil for Kirchhoff. At the further end of the telescope B is a
screen with a fine slit, representing a fine slice of light, which
can be narrowed or widened by the small screw, and by which the
light under investigation can be allowed to enter. It then passes
through the telescope B, afterwards through the four prisms, and
finally through the telescope A, from which it reaches the eye of the
observer. Figs. 7, 8, and 9 represent small portions of the solar
spectrum as mapped by Kirchhoff, taken from the green, yellow, and
golden-yellow, in which the chemical symbols below--Fe (iron), Ca
(calcium), Na (sodium), Pb (lead)--and the affixed lines, indicate
the positions in which the vapours of these metals, when made
incandescent, either in the flames or in the electrical spark, would
show bright lines. The numbers above them show how far these fractions
of Kirchhoff’s map of the whole system are apart from each other.
Here, also, we see a predominance of iron lines. In the whole spectrum
Kirchhoff found not less than 450.
[Illustration: FIG. 9.]
It follows from this, that the solar atmosphere contains an abundance
of the vapours of iron, which, by the way, justifies us in concluding
what an enormously high temperature must prevail there. It shows,
moreover, how our figs. 7, 8, and 9 indicate iron, calcium, and
sodium, and also the presence of hydrogen, of zinc, of copper, and
of the metals of magnesia, alumina, baryta, and other terrestrial
elements. Lead, on the other hand, is wanting, as well as gold, silver,
mercury, antimony, arsenic, and some others.
The spectra of several fixed stars are similarly constituted; they show
systems of fine lines which can be identified with those of terrestrial
elements. In the atmosphere of Aldebaran in Taurus there is, again,
hydrogen, iron, magnesium, calcium, sodium, and also mercury, antimony,
and bismuth; and, according to H. C. Vogel, there is in α Orionis the
rare metal thallium; and so on.
We cannot, indeed, say that we have explained all spectra; many fixed
stars exhibit peculiarly banded spectra, probably belonging to gases
whose molecules have not been completely resolved into their atoms by
the high temperature. In the spectrum of the sun, also, are many lines
which we cannot identify with those of terrestrial elements. It is
possible that they may be due to substances unknown to us, it is also
possible that they are produced by the excessively high temperature of
the sun, far transcending anything we can produce. But this is certain,
that the known terrestrial substances are widely diffused in space,
and especially nitrogen, which constitutes the greater part of our
atmosphere, and hydrogen, an element in water, which indeed is formed
by its combustion. Both have been found in the irresolvable nebulæ,
and, from the inalterability of their shape, these must be masses of
enormous dimensions and at an enormous distance. For this reason Sir W.
Herschel considered that they did not belong to the system of our fixed
stars, but were representatives of the manner in which other systems
manifested themselves.
Spectrum analysis has further taught us more about the sun, by which
he is brought nearer to us, as it were, than could formerly have
seemed possible. You know that the sun is an enormous sphere, whose
diameter is 112 times as great as that of the earth. We may consider
what we see on its surface as a layer of incandescent vapour, which,
to judge from the appearances of the sun-spots, has a depth of about
500 miles. This layer of vapour, which is continually radiating heat
on the outside, and is certainly cooler than the inner masses of the
sun, is, however, hotter than all our terrestrial flames--hotter even
than the incandescent carbon points of the electrical arc, which
represent the highest temperature attainable by terrestrial means. This
can be deduced with certainty from Kirchhoff’s law of the radiation
of opaque bodies, from the greater luminous intensity of the sun. The
older assumption, that the sun is a dark cool body, surrounded by a
photosphere which only radiates heat and light externally, contains a
physical impossibility.
Outside the opaque photosphere, the sun appears surrounded by a layer
of transparent gases, which are hot enough to show in the spectrum
bright coloured lines, and are hence called the _Chromosphere_. They
show the bright lines of hydrogen, of sodium, of magnesium, and iron.
In these layers of gas and of vapour about the sun enormous storms
occur, which are as much greater than those of our earth in extent and
in velocity as the sun is greater than the earth. Currents of ignited
hydrogen burst out several thousands of miles high, like gigantic
jets or tongues of flame, with clouds of smoke above them.[24] These
structures could formerly only be viewed at the time of a total eclipse
of the sun, forming what were called the rose-red protuberances. We now
possess a method, devised by MM. Jansen and Lockyer, by which they may
at any time be seen by the aid of the spectroscope.
[Footnote 24: According to H. C. Vogel’s observations in Bothkamp to
a height of 70,000 miles. The spectroscopic displacement of the lines
showed velocities of 18 to 23 miles in a second; and, according to
Lockyer, of even 37 to 42 miles.]
[Illustration: FIG. 10.]
On the other hand, there are individual darker parts on the sun’s
surface, what are called _sun-spots_, which were seen as long ago as
by Galileo. They are funnel-shaped, the sides of the funnel are not
so dark as the deepest part, the core. Fig. 10 represents such a spot
according to Padre Secchi, as seen under powerful magnification. Their
diameter is often more than many tens of thousands of miles, so that
two or three earths could lie in one of them. These spots may stand
for weeks or months, slowly changing, before they are again resolved,
and meanwhile several rotations of the sun may take place. Sometimes,
however, there are very rapid changes in them. That the core is deeper
than the edge of the surrounding penumbra follows from their respective
displacements as they come near the edge, and are therefore seen in a
very oblique direction. Fig. 11 represents in A to E the different
aspects of such a spot as it comes near the edge of the sun.
[Illustration: FIG. 11.]
Just on the edge of these spots there are spectroscopic indications of
the most violent motion, and in their vicinity there are often large
protuberances; they show comparatively often a rotatory motion. They
may be considered to be places where the cooler gases from the outer
layers of the sun’s atmosphere sink down, and perhaps produce local
superficial coolings of the sun’s mass. To understand the origin of
these phenomena, it must be remembered that the gases, as they rise
from the hot body of the sun, are charged with vapours of difficultly
volatile metals, which expand as they ascend, and partly by their
expansion, and partly by radiation into space, must become cooled.
At the same time, they deposit their more difficultly volatile
constituents as fog or cloud. This cooling can only, of course, be
regarded as comparative; their temperature is probably, even then,
higher than any temperature attainable on the earth. If now the upper
layers, freed from the heavier vapours, sink down, there will be a
space over the sun’s body which is free from cloud. They appear then as
depressions, because about them are layers of ignited vapours as much
as 500 miles in height.
Violent storms cannot fail to occur in the sun’s atmosphere, because
it is cooled on the outside, and the coolest and comparatively densest
and heaviest parts come to lie over the hotter and lighter ones. This
is the reason why we have frequent, and at times sudden and violent,
movements in the earth’s atmosphere, because this is heated from the
ground made hot by the sun and is cooled above. With the far more
colossal magnitude and temperature of the sun, its meteorological
processes are on a far larger scale, and are far more violent.
We will now pass to the question of the permanence of the present
condition of our system. For a long time the view was pretty generally
held that, in its chief features at any rate, it was unchangeable.
This opinion was based mainly on the conclusions at which Laplace had
arrived as the final results of his long and laborious investigations,
of the influence of planetary disturbances. By disturbances of the
planetary motion astronomers understand, as I have already mentioned,
those deviations from the purely elliptical motion which are due to
the attraction of various planets and satellites upon each other. The
attraction of the sun, as by far the largest body of our system, is
indeed the chief and preponderating force which produces the motion
of the planets. If it alone were operative, each of the planets would
move continuously in a constant ellipse whose axes would retain the
same direction and the same magnitude, making the revolutions always
in the same length of time. But, in point of fact, in addition to the
attraction of the sun there are the attractions of all other planets,
which, though small, yet, in long periods of time, do effect slow
changes in the plane, the direction, and the magnitude of the axes
of its elliptical orbit. It has been asked whether these attractions
in the orbit of the planet could go so far as to cause two adjacent
planets to encounter each other, so that individual ones fall into
the sun. Laplace was able to reply that this could not be the case;
that all alterations in the planetary orbits produced by this kind
of disturbance must periodically increase and decrease, and again
revert to a mean condition. But it must not be forgotten that this
result of Laplace’s investigations only applies to disturbances due
to the reciprocal attraction of planets upon each other, and on the
assumption that no forces of other kinds have any influence on their
motions.
On our earth we cannot produce such an everlasting motion as that of
the planets seems to be; for resisting forces are continually being
opposed to all movements of terrestrial bodies. The best known of these
are what we call friction, resistance of the air, and inelastic impact.
Hence the fundamental law of mechanics, according to which every motion
of a body on which no force acts goes on in a straight line for ever
with unchanged velocity, never holds fully.
Even if we eliminate the influence of gravity in a ball, for example,
which rolls on a plane surface, we see it go on for a while, and the
further the smoother is the path; but at the same time we hear the
rolling ball make a clattering sound--that is, it produces waves
of sound in the surrounding bodies; there is friction even on the
smoothest surface; this sets the surrounding air in vibration, and
imparts to it some of its own motion. Thus it happens that its velocity
is continually less and less until it finally ceases. In like manner,
even the most carefully constructed wheel which plays upon fine points,
once made to turn, goes on for a quarter of an hour, or even more,
but then stops. For there is always some friction on the axles, and
in addition there is the resistance of the air, which resistance is
mainly due to that of the particles of air against each other, due to
their friction against the wheel.
If we could once set a body in rotation, and keep it from falling,
without its being supported by another body, and if we could transfer
the whole arrangement to an absolute vacuum, it would continue to
move for ever with undiminished velocity. This case, which cannot be
realised on terrestrial bodies, is apparently met with in the planets
with their satellites. They appear to move in the perfectly vacuous
cosmical space, without contact with any body which could produce
friction, and hence their motion seems to be one which never diminishes.
You see, however, that the justification of this conclusion depends on
the question whether cosmical space is really quite vacuous. Is there
nowhere any friction in the motion of the planets?
From the progress which the knowledge of nature has made since the time
of Laplace, we must now answer both questions in the negative.
Celestial space is not absolutely vacuous. In the first place, it is
filled by that continuous medium the agitation of which constitutes
light and radiant heat, and which physicists know as the luminiferous
ether. In the second place, large and small fragments of heavy matter,
from the size of huge stones to that of dust, are still everywhere
scattered; at any rate, in those parts of space which our earth
traverses.
The existence of the luminiferous ether cannot be considered doubtful.
That light and radiant heat are due to a motion which spreads in
all directions has been sufficiently proved. For the transference
of such a motion through space there must be something which can be
moved. Indeed, from the magnitude of the action of this motion, or
from that which the science of mechanics calls its _vis viva_, we may
indeed assign certain limits for the density of this medium. Such a
calculation has been made by Sir W. Thomson, the celebrated Glasgow
physicist. He has found that the density may possibly be far less than
that of the air in the most perfect exhaustion obtainable by a good
air-pump; but that the mass of the ether cannot be absolutely equal
to zero. A volume equal to that of the earth cannot contain less than
2,775 pounds of luminiferous ether.[25]
[Footnote 25: This calculation would, however, lose its bases if
Maxwell’s hypothesis were confirmed, according to which light depends
on electrical and magnetical oscillations.]
The phenomena in celestial space are in conformity with this. Just
as a heavy stone flung through the air shows scarcely any influence
of the resistance of the air, while a light feather is appreciably
hindered; in like manner the medium which fills space is far too
attenuated for any diminution to have been perceived in the motion
of the planets since the time in which we possess astronomical
observations of their path. It is different with the smaller bodies
of our system. Encke in particular has shown, with reference to the
well-known small comet which bears his name, that it circulates round
the sun in ever-diminishing orbits and in ever shorter periods of
revolution. Its motion is similar to that of the circular pendulum
which we have mentioned, and which, having its velocity gradually
delayed by the resistance of the air, describes circles about its
centre of attraction, which continually become smaller and smaller. The
reason for this phenomenon is the following: The force which offers
a resistance to the attraction of the sun on all comets and planets,
and which prevents them from getting continually nearer to the sun, is
what is called the centrifugal force--that is, the tendency to continue
their motion in a straight line in the direction of their path. As
the force of their motion diminishes, they yield by a corresponding
amount to the attraction of the sun, and get nearer to it. If the
resistance continues, they will continue to get nearer the sun until
they fall into it. Encke’s comet is no doubt in this condition. But the
resistance whose presence in space is hereby indicated, must act, and
has long continued to act, in the same manner on the far larger masses
of the planets.
The presence of partly fine and partly coarse heavy masses diffused
in cosmical space is more distinctly revealed by the phenomena of
asteroids and of meteorites. We know now that these are bodies which
ranged about in cosmical space, before they came within the region
of our terrestrial atmosphere. In the more strongly resisting medium
which this atmosphere offers they are delayed in their motion, and at
the same time are heated by the corresponding friction. Many of them
may still find an escape from the terrestrial atmosphere, and continue
their path through space with an altered and retarded motion. Others
fall to the earth; the larger ones as meteorites, while the smaller
ones are probably resolved into dust by the heat, and as such fall
without being seen. According to Alexander Herschel’s estimate, we
may figure shooting-stars as being on an average of the same size as
paving-stones. Their incandescence mostly occurs in the higher and most
attenuated regions of the atmosphere, eighteen miles and more above the
surface of the earth. As they move in space under the influence of the
same laws as the planets and comets, they possess a planetary velocity
of from eighteen to forty miles in a second. By this, also, we observe
that they are in fact _stelle cadente_, falling stars, as they have
long been called by poets.
This enormous velocity with which they enter our atmosphere is
undoubtedly the cause of their becoming heated. You all know that
friction heats the bodies rubbed. Every match that we ignite, every
badly greased coach-wheel, every auger which we work in hard wood,
teaches this. The air, like solid bodies, not only becomes heated by
friction, but also by the work consumed in its compression. One of the
most important results of modern physics, the actual proof of which is
mainly due to the Englishman Joule, is that, in such a case, the heat
developed is exactly proportional to the work expended. If, like the
mechanicians, we measure the work done by the weight which would be
necessary to produce it, multiplied by the height from which it must
fall, Joule has shown that the work, produced by a given weight of
water falling through a height of 425 metres, would be just sufficient
to raise the same weight of water through one degree Centigrade. The
equivalent in work of a velocity of eighteen to twenty-four miles in a
second may be easily calculated from known mechanical laws; and this,
transformed into heat, would be sufficient to raise the temperature of
a piece of meteoric iron to 900,000 to 2,500,000 degrees Centigrade,
provided that all the heat were retained by the iron, and did not,
as it undoubtedly does, mainly pass into the air. This calculation
shows, at any rate, that the velocity of the shooting-stars is
perfectly adequate to raise them to the most violent incandescence. The
temperatures attainable by terrestrial means scarcely exceed 2,000
degrees. In fact, the outer crusts of meteoric stones generally show
traces of incipient fusion; and in cases in which observers examined
with sufficient promptitude the stones which had fallen they found them
hot on the surface, while the interior of detached pieces seemed to
show the intense cold of cosmical space.
To the individual observer who casually looks towards the starry
sky the meteorites appear as a rare and exceptional phenomenon. If,
however, they are continuously observed, they are seen with tolerable
regularity, especially towards morning, when they usually fall. But
a single observer only views but a small part of the atmosphere; and
if they are calculated for the entire surface of the earth it results
that about seven and a half millions fall every day. In our regions of
space, they are somewhat sparse and distant from each other. According
to Alexander Herschel’s estimates, each stone is, on an average, at a
distance of 450 miles from its neighbours. But the earth moves through
18 miles every second, and has a diameter of 7,820 miles, and therefore
sweeps through 876 millions of cubic miles of space every second, and
carries with it whatever stones are contained therein.
Many groups are irregularly distributed in space, being probably
those which have already undergone disturbances by planets. There are
also denser swarms which move in regular elliptical orbits, cutting
the earth’s orbit in definite places, and therefore always occur on
particular days of the year. Thus the 10th of August of each year is
remarkable, and every thirty-three years the splendid fireworks of the
12th to the 14th of November repeats itself for a few years. It is
remarkable that certain comets accompany the paths of these swarms, and
give rise to the supposition that the comets gradually split up into
meteoric swarms.
This is an important process. What the earth does is done by the other
planets, and in a far higher degree by the sun, towards which all the
smaller bodies of our system must fall; those, therefore, that are
more subject to the influence of the resisting medium, and which must
fall the more rapidly, the smaller they are. The earth and the planets
have for millions of years been sweeping together the loose masses in
space, and they hold fast what they have once attracted. But it follows
from this that the earth and the planets were once smaller than they
are now, and that more mass was diffused in space; and if we follow
out this consideration it takes us back to a state of things in which,
perhaps, all the mass now accumulated in the sun and in the planets,
wandered loosely diffused in space. If we consider, further, that the
small masses of meteorites as they now fall, have perhaps been formed
by the gradual aggregation of fine dust, we see ourselves led to a
primitive condition of fine nebulous masses.
From this point of view, that the fall of shooting-stars and of
meteorites is perhaps only a small survival of a process which once
built up worlds, it assumes far greater significance.
This would be a supposition of which we might admit the possibility,
but which could not perhaps claim any great degree of probability, if
we did not find that our predecessors, starting from quite different
considerations, had arrived at the same hypothesis.
You know that a considerable number of planets rotate around the
sun besides the eight larger ones, Mercury, Venus, the Earth, Mars,
Jupiter, Saturn, Uranus, and Neptune; in the interval between Mars
and Jupiter there circulate, as far as we know, 156 small planets or
planetoids. Moons also rotate about the larger planets--that is, about
the Earth and the four most distant ones, Jupiter, Saturn, Uranus, and
Neptune; and lastly the Sun, and at any rate the larger planets, rotate
about their own axes. Now, in the first place, it is remarkable that
all the planes of rotation of the planets and of their satellites,
as well as the equatorial planes of these planets, do not vary much
from each other, and that in these planes all the rotation is in the
same direction. The only considerable exceptions known are the moons
of Uranus, whose plane is almost at right angles to the planes of
the larger planets. It must at the same time be remarked that the
coincidence, in the direction of these planes, is on the whole greater,
the longer are the bodies and the larger the paths in question; while
in the smaller bodies, and for the smaller paths, especially for the
rotations of the planets about their own axes, considerable divergences
occur. Thus the planes of all the planets, with the exception of
Mercury and of the small ones between Mars and Jupiter, differ at most
by three degrees from the path of the Earth. The equatorial plane of
the Sun deviates by only seven and a half degrees, that of Jupiter
only half as much. The equatorial plane of the Earth deviates, it is
true, to the extent of twenty-three and a half degrees, and that of
Mars by twenty-eight and a half degrees, and the separate paths of the
small planet’s satellites differ still more. But in these paths they
all move direct, all in the same direction about the sun, and, as far
as can be ascertained, also about their own axes, like the earth--that
is, from west to east. If they had originated independently of each
other, and had come together, any direction of the planes for each
individual one would have been equally probable; a reverse direction of
the orbit would have been just as probable as a direct one; decidedly
elliptical paths would have been as probable as the almost circular
ones which we meet with in all the bodies we have named. There is, in
fact, a complete irregularity in the comets and meteoric swarms, which
we have much reason for considering to be formations which have only
accidentally come within the sphere of the sun’s attraction.
The number of coincidences in the orbits of the planets and their
satellites is too great to be ascribed to accident. We must inquire
for the reason of this coincidence, and this can only be sought in a
primitive connection of the entire mass. Now, we are acquainted with
forces and processes which condense an originally diffused mass, but
none which could drive into space such large masses, as the planets, in
the condition we now find them. Moreover, if they had become detached
from the common mass, at a place much nearer the sun, they ought to
have a markedly elliptical orbit. We must assume, accordingly, that
this mass in its primitive condition extended at least to the orbit of
the outermost planets.
These were the essential features of the considerations which led
Kant and Laplace to their hypothesis. In their view our system was
originally a chaotic ball of nebulous matter, of which originally, when
it extended to the path of the most distant planet, many billions of
cubic miles could contain scarcely a gramme of mass. This ball, when
it had become detached from the nebulous balls of the adjacent fixed
stars, possessed a slow movement of rotation. It became condensed under
the influence of the reciprocal attraction of its parts; and, in the
degree in which it condensed, the rotatory motion increased, and formed
it into a flat disk. From time to time masses at the circumference
of this disk became detached under the influence of the increasing
centrifugal force; that which became detached formed again into a
rotating nebulous mass, which either simply condensed and formed a
planet, or during this condensation again repelled masses from the
periphery, which became satellites, or in one case, that of Saturn,
remained as a coherent ring. In another case, the mass which separated
from the outside of the chief ball, divided into many parts, detached
from each other, and furnished the swarms of small planets between Mars
and Jupiter.
Our more recent experience as to the nature of star showers teaches us
that this process of the condensation of loosely diffused masses to
form larger bodies is by no means complete, but still goes on, though
the traces are slight. The form in which it now appears is altered by
the fact that meanwhile the gaseous or dust-like mass diffused in space
had united under the influence of the force of attraction, and of the
force of crystallisation of their constituents, to larger pieces than
originally existed.
The showers of stars, as examples now taking place of the process which
formed the heavenly bodies, are important from another point of view.
They develop light and heat; and that directs us to a third series of
considerations, which leads again to the same goal.
All life and all motion on our earth is, with few exceptions, kept up
by a single force, that of the sun’s rays, which bring to us light
and heat. They warm the air of the hot zones, this becomes lighter
and ascends, while the colder air flows towards the poles. Thus is
formed the great circulation of the passage-winds. Local differences
of temperature over land and sea, plains and mountains, disturb the
uniformity of this great motion, and produce for us the capricious
change of winds. Warm aqueous vapours ascend with the warm air, become
condensed into clouds, and fall in the cooler zones, and upon the
snowy tops of the mountains, as rain and as snow. The water collects
in brooks, in rivers, moistens the plains, and makes life possible;
crumbles the stones, carries their fragments along, and thus works at
the geological transformation of the earth’s surface. It is only under
the influence of the sun’s rays that the variegated covering of plants
of the earth grows; and while they grow, they accumulate in their
structure organic matter, which partly serves the whole animal kingdom
as food, and serves man more particularly as fuel. Coals and lignites,
the sources of power of our steam engines, are remains of primitive
plants--the ancient production of the sun’s rays.
Need we wonder if, to our forefathers of the Aryan race in India and
Persia, the sun appeared as the fittest symbol of the Deity? They were
right in regarding it as the giver of all life--as the ultimate source
of almost all that has happened on earth.
But whence does the sun acquire this force? It radiates forth a more
intense light than can be attained with any terrestrial means. It
yields as much heat as if 1,500 pounds of coal were burned every hour
upon each square foot of its surface. Of the heat which thus issues
from it, the small fraction which enters our atmosphere furnishes
a great mechanical force. Every steam-engine teaches us that heat
can produce such force. The sun, in fact, drives on earth a kind
of steam-engine whose performances are far greater than those of
artificially constructed machines. The circulation of water in the
atmosphere raises, as has been said, the water evaporated from the
warm tropical seas to the mountain heights; it is, as it were, a
water-raising engine of the most magnificent kind, with whose power no
artificial machine can be even distantly compared. I have previously
explained the mechanical equivalent of heat. Calculated by that
standard, the work which the sun produces by its radiation is equal to
the constant exertion of 7,000 horse-power for each square foot of the
sun’s surface.
For a long time experience had impressed on our mechanicians that
a working force cannot be produced from nothing; that it can only
be taken from the stores which nature possesses; which are strictly
limited and which cannot be increased at pleasure--whether it be taken
from the rushing water or from the wind; whether from the layers
of coal, or from men and from animals, which cannot work without
the consumption of food. Modern physics has attempted to prove the
universality of this experience, to show that it applies to the great
whole of all natural processes, and is independent of the special
interests of man. These have been generalised and comprehended in the
all-ruling natural law of the _Conservation of Force_. No natural
process, and no series of natural processes, can be found, however
manifold may be the changes which take place among them, by which a
motive force can be continuously produced without a corresponding
consumption. Just as the human race finds on earth but a limited supply
of motive forces, capable of producing work, which it can utilise but
not increase, so also must this be the case in the great whole of
nature. The universe has its definite store of force, which works in
it under ever varying forms; is indestructible, not to be increased,
everlasting and unchangeable like matter itself. It seems as if Goethe
had an idea of this when he makes the earth-spirit speak of himself as
the representative of natural force.
In the currents of life, in the tempests of motion,
In the fervour of art, in the fire, in the storm,
Hither and thither,
Over and under,
Wend I and wander.
Birth and the grave,
Limitless ocean,
Where the restless wave
Undulates ever
Under and over,
Their seething strife
Heaving and weaving
The changes of life.
At the whirling loom of time unawed,
I work the living mantle of God.
Let us return to the special question which concerns us here: Whence
does the sun derive this enormous store of force which it sends out?
On earth the processes of combustion are the most abundant source of
heat. Does the sun’s heat originate in a process of this kind? To this
question we can reply with a complete and decided negative, for we
now know that the sun contains the terrestrial elements with which we
are acquainted. Let us select from among them the two, which, for the
smallest mass, produce the greatest amount of heat when they combine;
let us assume that the sun consists of hydrogen and oxygen, mixed in
the proportion in which they would unite to form water. The mass of
the sun is known, and also the quantity of heat produced by the union
of known weights of oxygen and hydrogen. Calculation shows that under
the above supposition, the heat resulting from their combustion would
be sufficient to keep up the radiation of heat from the sun for 3,021
years. That, it is true, is a long time, but even profane history
teaches that the sun has lighted and warmed us for 3,000 years, and
geology puts it beyond doubt that this period must be extended to
millions of years.
Known chemical forces are thus so completely inadequate, even on the
most favourable assumption, to explain the production of heat which
takes place in the sun, that we must quite drop this hypothesis.
We must seek for forces of far greater magnitude, and these we can
only find in cosmical attraction. We have already seen that the
comparatively small masses of shooting-stars and meteorites can produce
extraordinarily large amounts of heat when their cosmical velocities
are arrested by our atmosphere. Now the force which has produced these
great velocities is gravitation. We know of this force as one acting
on the surface of our planet when it appears as terrestrial gravity.
We know that a weight _raised from the earth_ can drive our clocks,
and that in like manner the gravity of the water rushing down from the
mountains works our mills.
If a weight falls from a height and strikes the ground its mass loses,
indeed, the visible motion which it had as a whole--in fact, however,
this motion is not lost; it is transferred to the smallest elementary
particles of the mass, and this invisible vibration of the molecules is
the motion of heat. Visible motion is transformed by impact, into the
motion of heat.
That which holds in this respect for gravity, holds also for
gravitation. A heavy mass, of whatever kind, which is suspended in
space separated from another heavy mass, represents a force capable
of work. For both masses attract each other, and, if unrestrained by
centrifugal force, they move towards each other under the influence of
this attraction; this takes place with ever-increasing velocity; and
if this velocity is finally destroyed, whether this be suddenly, by
collision, or gradually, by the friction of movable parts, it develops
the corresponding quantity of the motion of heat, the amount of which
can be calculated from the equivalence, previously established, between
heat and mechanical work.
Now we may assume with great probability that very many more meteors
fall upon the sun than upon the earth, and with greater velocity, too,
and therefore give more heat. Yet the hypothesis, that the entire
amount of the sun’s heat which is continually lost by radiation, is
made up by the fall of meteors, a hypothesis which was propounded by
Mayer, and has been favourably adopted by several other physicists, is
open, according to Sir W. Thomson’s investigations, to objection; for,
assuming it to hold, the mass of the sun should increase so rapidly
that the consequences would have shown themselves in the accelerated
motion of the planets. The entire loss of heat from the sun cannot
at all events be produced in this way; at the most a portion, which,
however, may not be inconsiderable.
If, now, there is no present manifestation of force sufficient to cover
the expenditure of the sun’s heat, the sun must originally have had a
store of heat which it gradually gives out. But whence this store? We
know that the cosmical forces alone could have produced it. And here
the hypothesis, previously discussed as to the origin of the sun, comes
to our aid. If the mass of the sun had been once diffused in cosmical
space, and had then been condensed--that is, had fallen together under
the influence of celestial gravity--if then the resultant motion had
been destroyed by friction and impact, with the production of heat, the
new world produced by such condensation must have acquired a store of
heat not only of considerable, but even of colossal, magnitude.
Calculation shows that, assuming the thermal capacity of the sun to
be the same as that of water, the temperature might be raised to
28,000,000 of degrees, if this quantity of heat could ever have been
present in the sun at one time. This cannot be assumed, for such
an increase of temperature would offer the greatest hindrance to
condensation. It is probable rather that a great part of this heat,
which was produced by condensation, began to radiate into space before
this condensation was complete. But the heat which the sun could have
previously developed by its condensation, would have been sufficient to
cover its present expenditure for not less than 22,000,000 of years of
the past.
And the sun is by no means so dense as it may become. Spectrum analysis
demonstrates the presence of large masses of iron and of other known
constituents of the rocks. The pressure which endeavours to condense
the interior is about 800 times as great as that in the centre of the
earth; and yet the density of the sun, owing probably to its enormous
temperature, is less than a quarter of the mean density of the earth.
We may therefore assume with great probability that the sun will still
continue in its condensation, even if it only attained the density of
the earth--though it will probably become far denser in the interior
owing to the enormous pressure--this would develop fresh quantities
of heat, which would be sufficient to maintain for an additional
17,000,000 of years the same intensity of sunshine as that which is
now the source of all terrestrial life.
The smaller bodies of our system might become less hot than the sun,
because the attraction of the fresh masses would be feebler. A body
like the earth might, if even we put its thermal capacity as high as
that of water, become heated to even 9,000 degrees, to more than our
flames can produce. The smaller bodies must cool more rapidly as long
as they are still liquid. The increase in temperature, with the depth,
is shown in bore-holes and in mines. The existence of hot wells and of
volcanic eruptions shows that in the interior of the earth there is a
very high temperature, which can scarcely be anything than a remnant
of the high temperature which prevailed at the time of its production.
At any rate, the attempts to discover for the internal heat of the
earth a more recent origin in chemical processes, have hitherto rested
on very arbitrary assumptions; and, compared with the general uniform
distribution of the internal heat, are somewhat insufficient.
On the other hand, considering the huge masses of Jupiter, of Saturn,
of Uranus, and of Neptune, their small density, as well as that of the
sun, is surprising, while the smaller planets and the moon approximate
to the density of the earth. We are here reminded of the higher
initial temperature, and the slower cooling, which characterises
larger masses.[26] The moon, on the contrary, exhibits formations on
its surface which are strikingly suggestive of volcanic craters, and
point to a former state of ignition of our satellite. The mode of its
rotation, moreover, that it always turns the same side towards the
earth, is a peculiarity which might have been produced by the friction
of a fluid. At present no trace of such a one can be perceived.
[Footnote 26: Mr. Zoellner concludes from photometric measurements,
which, however, need confirmation, that Jupiter still possesses a light
of its own.]
[Illustration: FIG. 12.]
You see, thus, by what various paths we are constantly led to the same
primitive conditions. The hypothesis of Kant and Laplace is seen to be
one of the happiest ideas in science, which at first astounds us, and
then connects us in all directions with other discoveries, by which the
conclusions are confirmed until we have confidence in them. In this
case another circumstance has contributed--that is, the observation
that this process of transformation, which the theory in question
presupposes, goes on still, though on a smaller scale, seeing that all
stages of that process can still be found to exist.
[Illustration: FIG. 13.]
For as we have already seen, the larger bodies which are already formed
go on increasing with the development of heat, by the attraction of the
meteoric masses already diffused in space. Even now the smaller bodies
are slowly drawn towards the sun by the resistance in space. We still
find in the firmament of fixed stars, according to Sir J. Herschel’s
newest catalogue, over 5,000 nebulous spots, of which those whose
light is sufficiently strong give for the most part a coloured spectrum
of fine bright lines, as they appear in the spectra of the ignited
gases. The nebulæ are partly rounded structures, which are called
_planetary nebulæ_ (fig. 12); sometimes wholly irregular in form, as
the large nebula in Orion, represented in fig. 13; they are partly
annular, as in the figures in fig. 14, from the Canes Venatici. They
are for the most part feebly luminous over their whole surface, while
the fixed stars only appear as luminous points.
[Illustration: FIG. 14.]
[Illustration: FIG. 15.]
[Illustration: FIG. 16.]
In many nebulæ small stars can be seen, as in figs. 15 and 16, from
Sagittarius and Aurigo. More stars are continually being discovered
in them, the better are the telescopes used in their analysis. Thus,
before the discovery of spectrum analysis, Sir W. Herschel’s former
view might be regarded as the most probable, that that which we see
to be nebulæ are only heaps of very fine stars, of other Milky Ways.
Now, however, spectrum analysis has shown a gas spectrum in many
nebulæ which contains stars, while actual heaps of stars show the
continuous spectrum of ignited solid bodies. Nebulæ have in general
three distinctly recognisable lines, one of which, in the blue, belongs
to hydrogen, a second in bluish-green to nitrogen,[27] while the third,
between the two, is of unknown origin. Fig. 17 shows such a spectrum
of a small but bright nebula in the Dragon. Traces of other bright
lines are seen along with them, and sometimes also, as in fig. 17,
traces of a continuous spectrum; all of which, however, are too feeble
to admit of accurate investigation. It must be observed here that the
light of very feeble objects which give a continuous spectrum are
distributed by the spectroscope over a large surface, and are therefore
greatly enfeebled or even extinguished, while the undecomposable light
of bright gas lines remains undecomposed, and hence can still be seen.
In any case, the decomposition of the light of the nebulæ shows that by
far the greater part of their luminous surface is due to ignited gases,
of which hydrogen forms a prominent constituent. In the planetary
masses, the spherical or discoidal, it might be supposed that the
gaseous mass had attained a condition of equilibrium; but most other
nebulæ exhibit highly irregular forms, which by no means correspond
to such a condition. As, however, their shape has either not at all
altered, or not appreciably, since they have been known and observed,
they must either have very little mass, or they must be of colossal
size and distance. The former does not appear very probable, because
small masses very soon give out their heat, and hence we are left to
the second alternative, that they are of huge dimensions and distances.
The same conclusion had been originally drawn by Sir W. Herschel, on
the assumption that the nebulæ were heaps of stars.
[Footnote 27: Or perhaps also to oxygen. The line occurs in the
spectrum of atmospheric air, and according to H. C. Vogel’s
observations was wanting in the spectrum of pure oxygen.]
[Illustration: FIG. 17.]
With those nebulæ which, besides the lines of gases, also show the
continuous spectrum of ignited denser bodies, are connected spots which
are partly irresolvable and partly resolvable into heaps of stars,
which only show the light of the latter kind.
The countless luminous stars of the heavenly firmament, whose number
increases with each newer and more perfect telescope, associate
themselves with this primitive condition of the worlds as they are
formed. They are like our sun in magnitude, in luminosity, and on the
whole also in the chemical condition of their surface, although there
may be differences in the quantity of individual elements.
But we find also in space a third stadium, that of extinct suns;
and for this also there are actual evidences. In the first place,
there are, in the course of history, pretty frequent examples of the
appearance of new stars. In 1572 Tycho Brahe observed such a one,
which, though gradually burning paler, was visible for two years, stood
still like a fixed star, and finally reverted to the darkness from
which it had so suddenly emerged. The largest of them all seems to have
been that observed by Kepler in the year 1604, which was brighter than
a star of the first magnitude, and was observed from September 27,
1604, until March 1606. The reason of its luminosity was probably the
collision with a smaller world. In a more recent case, in which on May
12, 1866, a small star of the tenth magnitude in the Corona suddenly
burst out to one of the second magnitude, spectrum analysis showed that
it was an outburst of ignited hydrogen which produced the light. This
was only luminous for twelve days.
In other cases obscure heavenly bodies have discovered themselves by
their attraction on adjacent bright stars, and the motions of the
latter thereby produced. Such an influence is observed in Sirius and
Procyon. By means of a new refracting telescope Messrs. Alvan Clarke
and Pond, of Cambridge, U.S., have discovered in the case of Sirius a
scarcely visible star, which has but little luminosity, but is almost
seven times as heavy as the sun, has about half the mass of Sirius, and
whose distance from Sirius is about equal to that of Neptune from the
sun. The satellite of Procyon has not yet been seen; it appears to be
quite dark.
Thus there are extinct suns. The fact that there are such lends new
weight to the reasons which permit us to conclude that our sun also
is a body which slowly gives out its store of heat, and thus will some
time become extinct.
The term of 17,000,000 years which I have given may perhaps become
considerably prolonged by the gradual abatement of radiation, by the
new accretion of falling meteors, and by still greater condensation
than that which I have assumed in that calculation. But we know of no
natural process which could spare our sun the fate which has manifestly
fallen upon other suns. This is a thought which we only reluctantly
admit; it seems to us an insult to the beneficent Creative Power which
we otherwise find at work in organisms and especially in living ones.
But we must reconcile ourselves to the thought that, however we may
consider ourselves to be the centre and final object of Creation, we
are but as dust on the earth; which again is but a speck of dust in
the immensity of space; and the previous duration of our race, even
if we follow it far beyond our written history, into the era of the
lake dwellings or of the mammoth, is but an instant compared with
the primeval times of our planet; when living beings existed upon
it, whose strange and unearthly remains still gaze at us from their
ancient tombs; and far more does the duration of our race sink into
insignificance compared with the enormous periods during which worlds
have been in process of formation, and will still continue to form
when our sun is extinguished, and our earth is either solidified in
cold or is united with the ignited central body of our system.
But who knows whether the first living inhabitants of the warm sea on
the young world, whom we ought perhaps to honour as our ancestors,
would not have regarded our present cooler condition with as much
horror as we look on a world without a sun? Considering the wonderful
adaptability to the conditions of life which all organisms possess,
who knows to what degree of perfection our posterity will have been
developed in 17,000,000 of years, and whether our fossilised bones will
not perhaps seem to them as monstrous as those of the Ichthyosaurus
now do; and whether they, adjusted for a more sensitive state of
equilibrium, will not consider the extremes of temperature, within
which we now exist, to be just as violent and destructive as those of
the older geological times appear to us? Yea, even if sun and earth
should solidify and become motionless, who could say what new worlds
would not be ready to develop life? Meteoric stones sometimes contain
hydrocarbons; the light of the heads of comets exhibits a spectrum
which is most like that of the electrical light in gases containing
hydrogen and carbon. But carbon is the element, which is characteristic
of organic compounds, from which living bodies are built up. Who knows
whether these bodies, which everywhere swarm through space, do not
scatter germs of life wherever there is a new world, which has become
capable of giving a dwelling-place to organic bodies? And this life we
might perhaps consider as allied to ours in its primitive germ, however
different might be the form which it would assume in adapting itself to
its new dwelling-place.
However this may be, that which most arouses our moral feelings at
the thought of a future, though possibly very remote, cessation of
all living creation on the earth, is more particularly the question
whether all this life is not an aimless sport, which will ultimately
fall a prey to destruction by brute force? Under the light of Darwin’s
great thought we begin to see that not only pleasure and joy, but also
pain, struggle, and death, are the powerful means by which nature has
built up her finer and more perfect forms of life. And we men know
more particularly that in our intelligence, our civic order, and our
morality we are living on the inheritance which our forefathers have
gained for us, and that which we acquire in the same way, will in like
manner ennoble the life of our posterity. Thus the individual, who
works for the ideal objects of humanity, even if in a modest position,
and in a limited sphere of activity, may bear without fear the thought
that the thread of his own consciousness will one day break. But even
men of such free and large order of minds as Lessing and David Strauss
could not reconcile themselves to the thought of a final destruction of
the living race, and with it of all the fruits of all past generations.
As yet we know of no fact, which can be established by scientific
observation, which would show that the finer and complex forms of vital
motion could exist otherwise than in the dense material of organic
life; that it can propagate itself as the sound-movement of a string
can leave its originally narrow and fixed home and diffuse itself in
the air, keeping all the time its pitch, and the most delicate shade
of its colour-tint; and that, when it meets another string attuned to
it, starts this again or excites a flame ready to sing to the same
tone. The flame even, which, of all processes in inanimate nature,
is the closest type of life, may become extinct, but the heat which
it produces continues to exist--indestructible, imperishable, as an
invisible motion, now agitating the molecules of ponderable matter,
and then radiating into boundless space as the vibration of an ether.
Even there it retains the characteristic peculiarities of its origin,
and it reveals its history to the inquirer who questions it by the
spectroscope. United afresh, these rays may ignite a new flame, and
thus, as it were, acquire a new bodily existence.
Just as the flame remains the same in appearance, and continues to
exist with the same form and structure, although it draws every
minute fresh combustible vapour, and fresh oxygen from the air, into
the vortex of its ascending current; and just as the wave goes on
in unaltered form, and is yet being reconstructed every moment from
fresh particles of water, so also in the living being, it is not
the definite mass of substance, which now constitutes the body, to
which the continuance of the individual is attached. For the material
of the body, like that of the flame, is subject to continuous and
comparatively rapid change--a change the more rapid, the livelier the
activity of the organs in question. Some constituents are renewed from
day to day, some from month to month, and others only after years. That
which continues to exist as a particular individual is like the flame
and the wave--only the form of motion which continually attracts fresh
matter into its vortex and expels the old. The observer with a deaf ear
only recognises the vibration of sound as long as it is visible and can
be felt, bound up with heavy matter. Are our senses, in reference to
life, like the deaf ear in this respect?