In this Chapter we propose to treat briefly of the probable formation of
the various members of the solar system from matter which previously
existed in space in a condition different from that in which we at
present find it—_i.e._, in the form of planets and satellites.

It is almost impossible to conceive that our world with its satellite,
and its fellow worlds with their satellites, and also the great centre
of them all, have always, from the commencement of time, possessed their
present form: all our experiences of the working of natural laws rebel
against such a supposition. In every phenomenon of nature upon this
earth—the great field from which we must glean our experiences and form
our analogies—we see a constant succession of changes going on, a
constant progression from one stage of development to another taking
place, a perpetual mutation of form and nature of the same material
substance occurring: we see the seed transformed into the plant, the
flower into the fruit, and the ovum into the animal. In the inorganic
world we witness the operation of the same principle; but, by reason of
their slower rate of progression, the changes there are manifested to us
rather by their resulting effects than by their visible course of
operation. And when we consider, as we are obliged to do, that the same
laws work in the greatest as well as the smallest processes of nature,
we are compelled to believe in an antecedent state of existence of the
matter that composes the host of heavenly bodies, and amongst them the
earth and its attendant moon.

In the pursuit of this course of argument we are led to inquire whether
there exists in the universe any matter from which planetary bodies
could be formed, and how far their formation from such matter can be
explained by the operation of known material laws.

Before the telescope revealed the hidden wonders of the skies, and
brought its rich fruits into our garner of knowledge concerning the
nature of the universe, the philosophic minds of some early astronomers,
Kepler and Tycho Brahe to wit, entertained the idea that the sun and the
stars—the suns of distant systems—were formed by the condensation of
celestial vapours into spherical bodies; Kepler basing his opinion on
the phenomena of the sudden shining forth of new stars on the margin of
the Milky Way. But it was when the telescope pierced into the depths of
celestial space, and brought to light the host of those marvellous
objects, the nebulæ, that the strongest evidence was afforded of the
probable validity of these suppositions. The mention of “nebulous stars”
made by the earlier astronomers refers only to clusters of telescopic
stars which the naked eye perceives as small patches of nebulous light;
and it does not appear that even the nebula in Andromeda, although so
plainly discernible as to be often now-a-days mistaken by the
uninitiated for a comet, was known, until it was discovered by means of
a telescope, in 1612, by Simon Marius, who described it as resembling a
candle shining through semi-transparent horn, as in a lantern, and
without any appearance of stars. Forty years after this date Huygens
discovered the splendid nebula in the sword handle of Orion, and in 1665
another was detected by Hevelius. In 1667 Halley (afterwards Astronomer
Royal) discovered a fourth; a fifth was found by Kirsch in 1681, and a
sixth by Halley again in 1714. Half a century after this the labours of
Messier expanded the list of known nebulæ and clusters to 103, a
catalogue of which appeared in the “Connaissance du Temps” (the French
“Nautical Almanac”) for the years 1783-1784. But this branch of
celestial discovery achieved its most brilliant results when the rare
penetration, the indomitable perseverance, and the powerful instruments
of the elder Herschel were brought to bear upon it. In the year 1779
this great astronomer began to search after nebulæ with a seven-inch
reflector, which he subsequently superseded by the great one of forty
feet focus and four feet aperture. In 1786 he published his first
catalogue of 1000 nebulæ; three years later he astonished the learned
world by a second catalogue containing 1000 more, and in 1802 a third
came forth comprising other 500, making 2500 in all! This number has
been so far increased by the labours of more recent astronomers that the
last complete catalogue, that of Sir John Herschel, published a few
years ago, contains the places of 5063 nebulæ and clusters.

At the earlier periods of Herschel’s observations, that illustrious
observer appears to have inclined to the belief that all nebulæ were but
remote clusters of stars, so distant, so faint, and so thickly
agglomerated as to affect the eye only by their combined luminosity, and
at this period of the nebular history it was supposed that increased
telescopic power would resolve them into their component stars. But the
familiarity which Herschel gained with the phases of the multitudinous
nebulæ that passed in review before his eyes, led him ultimately to
adopt the opinion, advanced by previous philosophers, that they were
composed of some vapoury or elementary matter out of which, by the
process of condensation, the heavenly bodies were formed; and this led
him to attempt a classification of the known nebulæ into a cosmical
arrangement, in which, regarding a chaotic mass of vapoury matter as the
primordial state of existence, he arranged them into a series of stages
of progressive development, the individuals of one class being so nearly
allied to those in the next that, to use his own expression, not so much
difference existed between them “as there would be in an annual
description of the human figure were it given from the birth of a child
till he comes to be a man in his prime.” (_Philosophical Transactions,
Vol. CI., pp. 271_, _et seq._)

His category comprises upwards of thirty classes or stages of
progression, the titles of a few of which we insert here to illustrate
the completeness of his scheme.

Class 1. Of extensive diffused nebulosity. (A table of 52
patches of such nebulosity actually observed is given,
some of which extend over an area of five or six square
degrees, and one of which occupies nine square degrees.)
” 6. Of milky nebulosity with condensation.
” 15. Of nebulæ that are of an irregular figure.
” 17. Of round nebulæ.
” 20. Of nebulæ that are gradually brighter in the middle.
” 25. Of nebulæ that have a nucleus.
” 29. Of nebulæ that draw progressively towards a period of
final condensation.
” 30. Of planetary nebulæ.
” 33. Of stellar nebulæ nearly approaching the appearance of

In a walk through a forest we see trees in every stage of growth, from
the tiny sapling to the giant of the woods, and no doubt can exist in
our minds that the latter has sprung from the former. We cannot at a
passing glance discern the process of development actually going on; to
satisfy ourselves of this, we must record the appearance of some single
tree from time to time through a long series of years. And what a walk
through a forest is to an observer of the growth of a tree, a lifetime
is to the observer of changes in such objects as the nebulæ. The
transition from one state to another of the nebulous development is so
slow that a lifetime is hardly sufficient to detect it. Nor can any
precise evidence of change be obtained by the comparison of drawings or
descriptions of nebulæ at various epochs, with whatever care or skill
such drawings be made, for it will be admitted that no two draughtsmen
will produce each a drawing of the most simple object from the same
point of view, in which every detail in the one will coincide exactly
with every detail in the other. There is abundant evidence of this in
the existing representations of the great nebula in Orion; a comparison
of the drawings that have been lately made of this object, with the most
perfect instruments and by the most skilful of astronomical draughtsmen,
reveals varieties of detail and even of general appearance such as could
hardly be imagined to occur in similar delineations of one and the same
subject; and any one who himself makes a perfectly unbiassed drawing at
the telescope will find upon comparison of it with others that it will
offer many points of difference. The fact is that the drawing of a man,
like his penmanship, is a personal characteristic, peculiar to himself,
and the drawings of two persons cannot be expected to coincide any more
than their handwritings. The appearance of a nebula varies also to a
great extent with the power of the telescope used to observe it and the
conditions under which it is observed; the drawings of nebulæ made with
the inferior telescopes of a century or two centuries ago, the only ones
that, by comparison with those made in modern times, could give
satisfactory evidence of changes of form or detail, are so rude and
imperfect as to be useless for the purpose, and it is reasonable to
suppose that those made in the present day will be similarly useless a
century or two hence. Since then we can obtain no evidence of the
changes we must assume these mysterious objects to be undergoing, _ipso
facto_, by observation of _one nebula_ at _various periods_, we must for
the present accept the _primâ facie_ evidence offered (as in the case of
the trees in a forest) by the observation of _various nebulæ_ at _one

“The total dissimilitude,” says Herschel at the close of the
observations we have alluded to, “between the appearance of a diffusion
of the nebulous matter and of a star, is so striking, that an idea of
the conversion of the one into the other can hardly occur to any one who
has not before him the result of the critical examination of the
nebulous system which has been displayed in this [his] paper. The end I
have had in view, by arranging my observations in the order in which
they have been placed, has been to show that the above mentioned
extremes may be connected by such nearly allied intermediate steps, as
will make it highly probable that every succeeding state of the nebulous
matter is the result of the action of gravitation upon it while in a
foregoing one, and by such steps the successive condensation of it has
been brought up to the planetary condition. From this the transit to the
stellar form, it has been shown, requires but a very small additional
compression of the nebulous matter.”

Where the researches of Herschel terminated those of Laplace commenced.
Herschel showed how a mass of nebulous matter so diffused as to be
scarcely discernible might be and probably was, by the mere action of
gravitation, condensed into a mass of comparatively small dimensions
when viewed in relation to the immensity of its primordial condition.
Laplace demonstrated how the known laws of gravitation could and
probably did from such a partially condensed mass of matter produce an
entire planetary system with all its subordinate satellites.

The first physicist who ventured to account for the formation of the
various bodies of our solar system was Buffon, the celebrated French
naturalist. His theory, which is fully detailed in his renowned work on
natural history, supposed that at some period of remote antiquity the
sun existed without any attendant planets, and that a comet having
dashed obliquely against it, ploughed up and drove off a portion of its
body sufficient in bulk to form the various planets of our system. He
suggests that the matter thus carried off “at first formed a torrent the
grosser and less dense parts of which were driven the farthest, and the
densest parts, having received only the like impulsion, were not so
remotely removed, the force of the sun’s attraction having retained
them:” that “the earth and planets therefore at the time of their
quitting the sun were burning and in a state of liquefaction;” that “by
degrees they cooled, and in this state of fluidity they took their
form.” He goes on to say that the obliquity of the stroke of the comet
might have been such as to separate from the bodies of the principal
planets small portions of matter, which would preserve the same
direction of motion as the principal planets, and thus would form their
attendant satellites.

The hypothesis of Buffon, however, is not sufficient to explain all the
phenomena of the planetary system; and it is imperfect, inasmuch as it
begins by assuming the sun to be already existing, whereas any theory
accounting for the primary formation of the solar system ought
necessarily to include the origination of the most important body
thereof, the sun itself. Nevertheless, it is but due to Buffon to
mention his ideas, for the errors of one philosophy serve a most useful
end by opening out fields of inquiry for subsequent and more fortunate

Laplace, dissatisfied with Buffon’s theory, sought one more probable,
and thus was led to the proposition of the celebrated _nebular
hypothesis_ which bears his name, and which, in spite of its
disbelievers, has never been overthrown, but remains the only probable,
and, with our present knowledge, the only possible explanation of the
cosmical origin of the planets of our system. Although Laplace puts
forth his conjectures, to use his own words, “with the deference which
ought to inspire everything that is not a result of observation and
calculation,” yet the striking coincidence of all the planetary
phenomena with the conditions of his system gives to those conjectures,
again to use his modest language, “a probability strongly approaching

Laplace conceived the sun to have been at one period the nucleus of a
vast nebula, the attenuated surrounding matter of which extended beyond
what is now the orbit of the remotest planet of the system. He supposed
that this mass of matter in process of condensation possessed a rotatory
motion round its centre of gravity, and that the parts of it that were
situated at the limits where centrifugal force exactly counterbalanced
the attractive force of the nucleus were abandoned by the contracting
mass, and thus were formed successively a number of rings of matter
concentric with and circulating around the central nucleus. As it would
be improbable that all the conditions necessary to preserve the
stability of such rings of matter in their annular form could in all
cases exist, they would break up into masses which would be endued with
a motion of rotation, and would in consequence assume a spheroidal form.
These masses, which hence constituted the various planets, in their turn
condensing, after the manner of the parent mass, and abandoning their
outlying matter, would become surrounded by similarly concentric rings,
which would break up and form the satellites surrounding the various
planetary masses; and, as a remarkable exception to the rule of the
instability of the rings and their consequent breakage, Laplace cited
the case of Saturn surrounded by his rings as the only instances of
unbroken rings that the whole system offers us; unless indeed we include
the zodiacal light, that cone of hazy luminosity that is frequently seen
streaming from our luminary shortly before and after sunset, and which
Laplace supposed to be formed of molecules of matter, too volatile to
unite either with themselves or with the planets, and which must hence
circulate about the sun in the form of a nebulous ring, and with such an
appearance as the zodiacal actually presents.

This hypothesis, although it could not well be refuted, has been by many
hesitatingly received, and for a reason which was at one time cogent. In
the earlier stages of nebular research it was clearly seen, as we have
previously remarked, that many of the so-called nebulæ, which appeared
at first to consist of masses of vapoury matter, became, when
scrutinised with telescopes of higher power, resolved into clusters
containing countless numbers of stars, so small and so closely
agglomerated, that their united lustre only impressed the more feeble
eye as a faint nebulosity; and as it was found that each accession of
telescopic power increased the numbers of nebulæ that were thus
resolved, it was thought that every nebula would at some period succumb
to the greater penetration of more powerful instruments; and if this
were the case, and if no real nebulæ were hence found to exist, how, it
was argued, could the nebular hypothesis be maintained? One of the most
important nebulæ bearing upon this question was the great one in the
sword handle of Orion, one of the grandest and most conspicuous in the
whole heavens. On account of the brightness of some portions of this
object, it seemed as though it ought to be readily resolvable, supposing
all nebulæ to consist of stars, but all attempts to resolve it were in
vain, even with the powerful telescopes of Sir John Herschel and the
clear zenithal sky of the Cape of Good Hope. At length the question was
thought to be settled, for upon the completion of Lord Rosse’s giant
reflector, and upon examination of the nebula with it, his lordship
stated that there could be little, if any, doubt as to its
resolvability, and then it was maintained, by the disbelievers in the
nebular theory, that the last stronghold of that theory had been broken

But the truths of nature are for ever playing at hide and seek with
those who follow them:—the dogmas of one era are the exploded errors of
the next. Within the past few years a new science has arisen that
furnishes us with fresh powers of penetration into the vast and secret
laboratories of the universe; a new eye, so to speak, has been given us
by which we may discern, by the mere light that emanates from a
celestial body, something of the chemical elements of which it is
composed. When Newton two hundred years ago toyed with the prism he
bought at Stourbridge fair, and projected its pretty rainbow tints upon
the wall, his great mind little suspected that that phantom riband of
gorgeous colours would one day be called upon to give evidence upon the
probable cosmical origin of worlds. Yet such in truth has been the case.
Every substance when rendered luminous gives off light of some colour or
degree of refrangibility peculiar to itself, and although the eye cannot
detect any difference between one character of light and another, the
prism gives the means of ascertaining the quality and degree of
refrangibility of the light emanating from any source however distant,
and hence of gaining some knowledge of the nature of that source. If,
for instance, a ray of light from a solid body in combustion is passed
through a prism, a spectrum is produced which exhibits light of all
colours or all degrees of refrangibility; if the light from such a body,
before passing through the prism, be made to pass through gases or
certain metallic vapours, the resulting spectrum is found to be crossed
transversely by numbers of fine dark lines, apparently separating the
various colours, or cutting the spectrum into bands. The solar spectrum
is of this class; the once mysterious lines first observed by Wollaston,
and subsequently by Fraunhoffer, and known as “Fraunhoffer’s lines,”
have now been interpreted, chiefly by the sagacious German chemist
Kirchoff, and identified as the effects of absorption of certain of the
sun’s rays by chemical vapours contained in his atmosphere. The fixed
stars yield spectra of the same character, but varying considerably in
feature, the lines crossing the stellar spectra differing in position
and number from those of the sun, and one star from another, proving the
stars to possess varied chemical constitutions. But there is another
class of spectra, exhibited when light from other sources is passed
through the prism. These consist, not of a luminous riband of light like
the solar spectrum, but of bright isolated lines of coloured light with
comparatively wide dark spaces separating them. Such spectra are yielded
only by the light emitted from luminous gases and metals or chemical
elements in the condition of incandescent vapour. Every gas or element
in the state of luminous vapour yields a spectrum peculiar to itself,
and no two elements when vapourized before the prism show the same
combinations of luminous lines.

Now in the course of some observations upon the spectra of the fixed
stars by Dr. Huggins, it occurred to that gentleman to turn his
telescope, armed with a spectroscope, upon some of the brighter of the
nebulæ, and great was his surprise to find that instead of yielding
continuous spectra, as they must have done had their light been made up
of that of a multitude of stars, they gave spectra containing only two
or three isolated bright lines; such a spectrum could only be produced
by some luminous gas or vapour, and of this form of matter we are now
justified in declaring, upon the strength of numerous modern
observations, these remarkable bodies are composed; and it is a curious
and interesting fact that some of the nebulæ styled resolvable, from the
fact of their exhibiting points of light like stars, yield these gaseous
spectra, whence Dr. Huggins concludes that the brighter points taken for
stars are in reality nuclei of greater condensation of the nebular
matter: and so the fact of the apparent resolvability of a nebula
affords no positive proof of its non-nebulous character.

These observations—which have been fully confirmed by Father Secchi of
the Roman College—by destroying the evidence in favour of nebulæ being
remote clusters, add another attestation to the probability of the truth
of the nebular hypothesis, and we have now the confutation of the
luminologist to add to that of the astronomers who, in the person of the
illustrious Arago, asserted that the ideas of the great author of the
“Mécanique Céleste” “were those only which by their grandeur, their
coherence, and their mathematical character could be truly considered as
forming a physical cosmogony.”

Confining, then, our attention to the single object of the universe it
is our task to treat of—the Moon—and without asserting as an
indisputable fact that which we can never hope to know otherwise than by
inference and analogy, we may assume that that body once existed in the
form of a vast mass of diffused or attenuated matter, and that, by the
action of gravitation upon the particles of that matter, it was
condensed into a comparatively small and compact planetary body.

But while the process of condensation or compaction was going on,
another important law of nature—but recently unfolded to our
knowledge—was in powerful operation, the discussion of which law we
reserve for a separate Chapter.

In the preceding Chapter we endeavoured to show how the action of
gravitation upon the particles of diffused primordial matter would
result in the formation, by condensation and aggregation, of a spherical
planetary body. We have now to consider another result of the
gravitating action, and for this we must call to our aid a branch of
scientific enquiry and investigation unrecognized as such at the period
of Laplace’s speculations, and which has been developed almost entirely
within the past quarter of a century.

The “great philosophical doctrine of the present era of science,” as the
subject about to engage our attention has been justly termed, bears the
title of the “Conservation of Force,” or—as some ambiguity is likely to
attend the definition of the term “Force”—the “Conservation of Energy.”
The basis of the doctrine is the broad and comprehensive natural law
which teaches us that the quantity of force comprised by the universe,
like the quantity of matter contained in it, is a fixed and invariable
amount, which can be neither added to nor taken from, but which is for
ever undergoing change and transformation from one form to another. That
we cannot create force ought to be as obvious a fact as that we cannot
create matter; and what we cannot create we cannot destroy. As in the
universe we see no new matter created, but the same matter constantly
disappearing from one form and reappearing in another, so we can find no
new force ever coming into action—no description of force that is not to
be referred to some previous manner of existence.

Without entering upon a metaphysical discussion of the term “force,” it
will be sufficient for our purpose to consider it as something which
produces or resists motion, and hence we may argue that the ultimate
effect of force is motion. The force of gravity on the earth results in
the motion or tendency of all bodies towards its centre, and, similarly,
the action of gravitation upon the atoms or particles of a primeval
planet resulted in the motion of those particles towards each other. We
cannot conceive force otherwise than by its effects, or the motion it

And force we are taught is indestructible; therefore motion must be
indestructible also. But when a falling body strikes the earth, or a
gunshot strikes its target, or a hammer delivers a blow upon an anvil,
or a brake is pressed against a rotating wheel, motion is arrested, and
it would seem natural to infer that it is destroyed. But if we say it is
indestructible, what becomes of it? The philosophical answer to the
question is this—that the motion of the mass becomes transferred to the
particles or molecules composing it, and transformed to molecular
motion, and this molecular motion manifests itself to us as heat. The
particles or atoms of matter are held together by cohesion, or, in other
words, by the action of molecular attraction. When heat is applied to
these particles, motion is set up among them, they are set in vibration,
and thus, requiring and making wider room, they urge each other apart,
and the well-known _expansion by heat_ is the result. If the heat be
further continued a more violent molecular motion ensues, every increase
of heat tending to urge the atoms further apart, till at length they
overcome their cohesive attraction and move about each other, and a
_liquid or molten condition_ results. If the heat be still further
increased, the atoms break away from their cohesive fetters altogether
and leap off the mass in the form of vapour, and the matter thus assumes
the _gaseous or vaporous form_. Thus we see that the phenomena of heat
are phenomena of motion, and of motion only.

This mutual relation between heat and work presented itself as an embryo
idea to the minds of several of the earlier philosophers, by whom it was
maintained in opposition to the _material theory_ which held heat to be
a kind of matter or subtle fluid stored up in the inter-atomic spaces of
all bodies, capable of being separated and procured from them by rubbing
them together, but not generated thereby. Bacon, in his “Novum Organum,”
says that “heat itself, its essence and quiddity, is motion and nothing
else.” Locke defines heat as “a very brisk agitation of the insensible
parts of an object, which produces in us that sensation from whence we
denominate the object hot; so what in our sensation is _heat_, in the
object is nothing but _motion_.” Descartes and his followers upheld a
similar opinion. Richard Boyle, two hundred years ago, actually wrote a
treatise entitled “The Mechanical Theory of Heat and Cold,” and the
ingenious Count Rumford made some highly interesting and significant
experiments on the subject, which are described in a paper read before
the Royal Society in 1798, entitled “An Inquiry concerning the Source of
Heat excited by Friction.” But the conceptions of these authors remained
isolated and unfruitful for more than a century, and might have passed,
meantime, into the oblivion of barren speculation, but for the impulse
which this branch of inquiry has lately received. Now, however, they
stand forth as notable instances of truth trying to force itself into
recognition while yet men’s minds were unprepared or disinclined to
receive it. The key to the beautiful mechanical theory of heat was found
by these searching minds, but the unclasping of the lock that should
disclose its beauty and value was reserved for the philosophers of the
present age.

Simultaneously and independently, and without even the knowledge of each
other, three men, far removed from probable intercourse, conceived the
same ideas and worked out nearly similar results concerning the
mechanical theory of heat. Seeing that motion was convertible into heat,
and heat into motion, it became of the utmost importance to determine
the exact relation that existed between the two elements. The first who
raised the idea to philosophic clearness was Dr. Julius Robert Mayer, a
physician of Heilbronn in Germany. In certain observations connected
with his medical practice it occurred to him that there must be a
necessary equivalent between work and heat, a necessary numerical
relation between them. “The variations of the difference of colour of
arterial and venous blood directed his attention to the theory of
respiration. He soon saw in the respiration of animals the origin of
their motive powers, and the comparison of animals to thermic machines
afterwards suggested to him the important principle with which his name
will remain for ever connected.”

Next in order of publication of his results stands the name of Colding,
a Danish engineer, who about the year 1843 presented a series of memoirs
on the steam engine to the Royal Society of Copenhagen, in which he put
forth views almost identical with those of Mayer.

Last in publication order, but foremost in the importance of his
experimental treatment of the subject, was our own countryman, Dr. Joule
of Manchester. “Entirely independent of Mayer, with his mind firmly
fixed upon a principle, and undismayed by the coolness with which his
first labours appear to have been received, he persisted for years in
his attempts to prove the invariability of the relation which subsists
between heat and ordinary mechanical power.” (We are quoting from
Professor Tyndall’s valuable work on “Heat considered as a Mode of
Motion.”) “He placed water in a suitable vessel, agitated the water by
paddles, and determined both the amount of heat developed by the
stirring of the liquid and the amount of labour expended in its
production. He did the same with mercury and sperm oil. He also caused
discs of cast iron to rub against each other, and measured the heat
produced by their friction, and the force expended in overcoming it. He
urged water through capillary tubes, and determined the amount of heat
generated by the friction of the liquid against the sides of the tubes.
And the results of his experiments leave no shadow of doubt upon the
mind that, under all circumstances, the quantity of heat generated by
the same amount of force is fixed and invariable. A given amount of
force, in causing the iron discs to rotate against each other, produced
precisely the same amount of heat as when it was applied to agitate
water, mercury, or sperm oil. * * * * _The absolute amount of heat_
generated by the same expenditure of power, was in all cases the same.”

“In this way it was found that the quantity of heat which would raise
one pound of water one degree Fahrenheit in temperature, is exactly
equal to what would be generated if a pound weight, after having fallen
through a height of 772 feet, had its moving force destroyed by
collision with the earth. Conversely, the amount of heat necessary to
raise a pound of water one degree in temperature, would, if all applied
mechanically, be competent to raise a pound weight 772 feet high, or it
would raise 772 pounds one foot high. The term ‘foot pounds’ has been
introduced to express in a convenient way the lifting of one pound to
the height of a foot. Thus the quantity of heat necessary to raise the
temperature of a pound of water one degree Fahrenheit being taken as a
standard, 772 foot-pounds constitute what is called the _mechanical
equivalent_ of heat.”

By a process entirely different, and by an independent course of
reasoning, Mayer had, a few months previous to Joule, determined this
equivalent to be 771·4 foot-pounds. Such a remarkable coincidence
arrived at by pursuing different routes gives this value a strong claim
to accuracy, and raises the Mechanical Theory of Heat to the dignity of
an exact science, and its enunciators to the foremost place in the ranks
of physical philosophers.

In linking together the labours of the two remarkable men above alluded
to, Prof. Tyndall remarks, that “Mayer’s labours have in some measure
the stamp of profound intuition, which rose however to the energy of
undoubting conviction in the author’s mind. Joule’s labours, on the
contrary, are an experimental demonstration. Mayer _thought_ his theory
out, and rose to its grandest applications. Joule _worked_ his theory
out, and gave it the solidity of natural truth. True to the speculative
instinct of his country, Mayer drew large and mighty conclusions from
slender premises; while the Englishman aimed above all things at the
firm establishment of facts…. To each belongs a reputation which will
not quickly fade, for the share he has had, not only in establishing the
dynamical theory of heat, but also in leading the way towards a right
appreciation of the general energies of the universe.”

But from these generalities we must pass to the application of the
mechanical theory of heat to our special subject. We have learnt that
every form of motion is convertible into heat. We know that the falling
meteor or shooting star, whose motion is impeded by friction against the
earth’s atmosphere, is heated thereby to a temperature of incandescence.
Let us then suppose that myriads of such cosmical particles came into
collision from the effect of their mutual attraction, or that the
component atoms of a vast nebulous mass violently converged under the
like influence. What would follow? Obviously the generation of an
intense heat by the arrest of converging motion, such a heat as would
result in the fusion of the whole into one mass. Mayer, in one of his
most remarkable papers (“Celestial Dynamics”) remarks that the
“Newtonian theory of gravitation, whilst it enables us to determine,
from its present form, the earth’s state of aggregation in ages past, at
the same time points out to us a source of heat powerful enough to
produce such a state of aggregation—powerful enough to melt worlds: it
teaches us to consider the molten state of a planet as the result of the
mechanical union of cosmical masses, and to derive the radiation of the
sun and the heat in the bowels of the earth from a common origin.”

And the same laws that governed the formation of the earth, governed
also the formation of the moon: the variations of Nature’s operations
are _quantitative_ only and not _qualitative_. The Divine Will that made
the earth made the moon also, and the means and mode of working were the
same for both. The geological phenomena of the earth afford
unmistakeable evidence of its original fluid or molten condition, and
the appearance of the moon is as unmistakeably that of a body once in an
igneous or molten state. The enigma of the earth’s primary formation is
solved by the application of the dynamical theory of heat. By this
theory the generation of cosmical heat is removed from the quicksands of
conjecture and established upon the firm ground of direct calculation:
for the absolute amount of heat generated by the collision of a given
amount of matter is (of course, with some little uncertainty) deducible
from a mathematical formula. Mayer has computed the amount of heat that
the matter of the earth would have generated, if it had been formed
originally of only two parts drawn into collision by their mutual
attraction, and has found that it would be from 0 to 32,000 or 47,000[1]
Centigrade degrees, according as one part was infinitely small as
compared with the other, or as the two parts were of equal size.
Professor Helmholtz, another labourer in the same field of science, has
computed the amount of heat generated by the condensation of the whole
of the matter composing the solar system: this he finds would be
equivalent to the heat that would be required to raise the temperature
of a mass of water equal to the sum of the masses of all the bodies of
the system to 28,000,000 (twenty-eight million) degrees of the
Centigrade scale.

These examples afford abundant evidence of sufficient heat having been
generated by the aggregation of the matter of the moon to reduce it to a
state of fusion, and so to produce, from a nebulous chaos of diffused
cosmical matter, a molten body of definite outline and size.

It is requisite here to remark that fusion does not necessarily imply
combustion. It has been frequently asked, How can a volcanic theory of
the lunar phenomena be upheld consistently with the condition that it
possesses no atmosphere to support Fire? To this we would reply that to
produce a state of incandescence or a molten condition it is _not_
necessary that the body be surrounded by an atmosphere. The intensely
rapid motion of the particles of matter of bodies, which the dynamical
theory shows to be the origin of the molten state, exists quite
independently of such external matter as an atmosphere. The complex
mixture of gases and vapours which we term “air,” has nothing whatever
to do with the fusion of substances, whatever it may have to do with
their combustion. Combustion is a chemical phenomenon, due to the
combination of the oxygen of that air with the heated particles of the
combustible matter: oxygen is the sole supporter of combustion, and
hence combustion is to be regarded rather as a phenomenon of oxygen than
as a phenomenon of the matter with which that oxygen combines. The
greatest intensity of heat may exist without oxygen, and consequently
without combustion. In support of this argument it will be sufficient to
adduce, upon the authority of Dr. Tyndall, the fact that a platinum wire
can be raised to a luminous temperature and actually _fused_ in a
perfect vacuum.

But while the mass of condensing cosmical matter was thus accumulating
and forming the globe of the moon, the heat consequent upon the
aggregation of its particles was suffering some diminution from the
effect of radiation. So long as the radiated heat lost fell short of the
dynamical heat generated, no effect of cooling would be manifest; but
when the _vis viva_ of the condensing matter was all converted into its
equivalent of heat, or when the accession of heat fell short of that
radiated, a necessary cooling must ensue, and this cooling would be
accompanied by a solidification of that part of the mass which was most
free to radiate its heat into surrounding space: that part would
obviously be the outer surface.

With the solidification of this external crust began the “year one” of
selenological history.

The phenomena attendant upon the cooling of the mass we will consider in
the next Chapter.

In the foregoing Chapters we have endeavoured to show, by the light of
modern science, first, how diffused cosmical matter was probably
condensed into a planetary mass by the mutual gravitation of its
particles, and secondly, how, the after destruction of the gravitative
force, by the collision of the converging particles of matter, resulted
in the generation of such sufficient heat as to reduce the whole mass to
a molten condition. Our present task is to consider the subsequent
cooling of the mass, and the phenomena attendant upon or resulting
therefrom. This brief Chapter is important to our subject, as we shall
have frequent occasion to refer to the leading principle we shall
endeavour to illustrate in it, in subsequently treating of the causes to
which the special selenological features are to be attributed.

First, then, as regards the cooling of the igneous mass that constituted
the moon at the inconceivably remote period when possibly that body was
really a “lesser light” shining with a luminosity of its own, due to its
then incandescent state, and not simply a reflector, as it is now, of
light which it receives from the sun. If we could conceive it possible
that the igneous mass in the act of cooling parted with its heat from
the central part first and so began to solidify from its centre, or if
it had been possible for the mass to have cooled uniformly and
simultaneously throughout its whole depth, or that each substratum had
cooled before its superstratum, we should have had a moon whose surface
would have been smooth and without any such remarkable asperities and
excresences as are now presented to our view. But these suppositions are
inadmissible: on the contrary we are compelled to consider that the
portion of the igneous or molten body that first cooled was its exterior
surface, which, radiating its heat into surrounding space, became solid
and comparatively cool while the interior retained its hot and molten
condition. So that at this early stage of the moon’s history it existed
in the form of a solid shell inclosing a molten interior.

Now at this period of its formation, the moon’s mass, partly cooled and
solidified and partly molten, would be subject to the influence of two
powerful molecular forces: the first of these would consist in the
diminution of bulk or contraction of volume which accompanies the
cooling of solidified masses of previously molten substances; the second
would arise from a phenomenon which we may here observe is by no means
so generally known as from its importance it deserves to be: and as we
shall have frequent occasion to refer to it as one of the chief agencies
in producing the peculiar structural characteristics of the moon’s
surface, it may be well here to give a few examples of its action, that
our reference to it hereafter may be more clearly understood.

The broad general principle of the phenomenon here referred to is
this:—that fusible substances are (with a few exceptions) specifically
heavier while in their molten condition than in the solidified state, or
in other words, that molten matter occupies less space, weight for
weight, than the same matter after it has passed from the melted to the
solid condition. It follows as an obvious corollary that such substances
contract in bulk in fusing or melting, and expand in becoming solid. It
is this expansion upon solidification that now concerns us.

Water, as is well known, increases in density as it cools, till it
reaches the temperature of 39° Fahrenheit, after which, upon a further
decrease of temperature, its density begins to decrease, or in other
words its bulk expands, and hence the well-known fact of ice floating in
water, and the inconvenient fact of water-pipes bursting in a frost.
This action in water is of the utmost importance in the grand economy of
nature, and it has been accepted as a marvellous exception to the
general law of substances increasing in density (or shrinking) as they
decrease in temperature. Water is, however, by no means the exceptional
substance that it has been so generally considered. It is a fact
perfectly familiar to iron-founders, that when a mass of solid cast-iron
is dropped into a pot of molten iron of identical quality, the solid is
found to float persistently upon the molten metal—so persistently that
when it is intentionally thrust to the bottom of the pot, it rises again
the moment the submerging agency is withdrawn. As regards the amount of
buoyancy we believe it may be stated in round numbers to be at least two
or three per cent. It has been suggested by some who are familiar with
this phenomenon that the solid mass may be kept up by a spurious
buoyancy imparted to it by a film of adhering air, or that surface
impurities upon the solid metal may tend to reduce the specific gravity
of the mass and thereby prevent it sinking, and that the fact of
floatation is not absolutely a proof of greater specific lightness. But
in controversion of these suggestions, we can state as the result of
experiment that pieces of cast-iron which have had their surface
roughness entirely removed, leaving the bright metal exposed, still
float on the molten metal, and further that when, under the influence of
the great heat of the molten mass, the solid is gradually melted away,
and consequently any possible surface impurities or adhering air must
necessarily have been removed, the remaining portion continues to float
to the last. The inevitable inference from this is that in the case of
cast-iron the solid is specifically lighter than the molten, and,
therefore, that in passing from the molten to the solid condition this
substance undergoes expansion in bulk.

We are able to offer a confirmation of this inference in the case of
cast-iron by a remarkable phenomenon well known to iron-founders, but of
which we have never met with special notice. When a ladle or pot of
molten iron is drawn from the melting furnace and allowed to stand at
rest, the surface presents a most remarkable and suggestive appearance.
Instead of remaining calm and smooth it is the scene of a lively
commotion: the thin coat of scoria or molten oxide which forms on the
otherwise bright surface of the metal is seen, as fast as it forms at
the circumference of the pot, to be swept by active convergent currents
towards the centre, where it accumulates in a patch. While this action
is proceeding, the entire upper surface of the metal appears as if it
were covered with animated vermicules of scoria, springing into
existence at the circumference of the pot, and from thence rapidly
streaming and wriggling themselves towards the centre.

[Illustration: Fig. 1.]

Our illustration (Fig. 1) is intended, so far as such means can do so,
to convey some idea of this remarkable appearance at one instant of its
continued occurrence. To interpret our illustration rightly it is
necessary to imagine this vermicular freckling to be constantly and
rapidly streaming from all points of the periphery of the pot towards
the centre, where, as we have said, it accumulates in the form of a
floating island. We may observe that the motion is most rapid when the
hot metal is first put into the cool ladle: as the fluid metal parts
with some of its heat and the ladle gets hot by absorbing it, this
remarkable surface disturbance becomes less energetic.

Now if we carefully consider this peculiar action and seek a cause for
the phenomenon, we shall be led to the conclusion that it arises from
the expansion of that portion of the molten mass which is in contact
with or close proximity to the comparatively cool sides of the ladle,
which sides act as the chief agent in dispersing the heat of the melted
metal. The motion of the scoria betrays that of the fluid metal beneath,
and careful observation will show that the motion in question is the
result of an upward current of the metal around the circumference of the
ladle, as indicated by the arrows A, B, C in the accompanying sectional
drawing of the ladle (Fig. 2). The upward current of the metal can
actually be seen when specially looked for, at the rim of the pot, where
it is deflected into the convergent horizontal direction and where it
presents an elevatory appearance as shown in the figure. It is difficult
to assign to this effect any other cause than that of an expansion and
consequent reduction of the specific gravity of the fluid metal in
contact with or in close proximity to the cooler sides of the pot, as,
according to the generally entertained idea that contraction universally
accompanies cooling, it would be impossible for the cooler to float on
the hotter metal, and the curious surface-currents above referred to
would be in contrary direction to that which they invariably take,
_i.e._, they would diverge from the centre instead of converging to it.
The external arrows in the figure represent the radiation of the heat
from the outer sides of the pot, which is the chief cause of cooling.

[Illustration: Fig. 2.]

Turning from cast-iron to other metals we find further manifestations of
this expansive solidification. Bismuth is a notable example. In his
lectures on Heat, Dr. Tyndall exhibited an experiment in which a stout
iron bottle was filled with molten bismuth, and the stopper tightly
closed. The whole was set aside to cool, and as the metal within
approached consolidation the bottle was rent open by its expansion, just
as would have been the case had the bottle been filled with water and
exposed to freezing temperature. Mercury affords another example.
Thermometers which have to be exposed to Arctic temperatures are
generally filled with spirit instead of quicksilver, because the latter
has been found to burst the bulbs when the cold reached the congealing
point of the metal, the bursting being a consequence of the expansion
which accompanies the act of congelation. Silver also expands in passing
from the fluid to the solid state, for we are informed by a practical
refiner that solid floats on molten silver as ice floats on water; it
also, as likewise do gold and copper, exhibits surface converging
currents in the melting-pot like those depicted above for molten iron.

It may, however, be objected that metals are too distantly related to
volcanic substances to justify inferences being drawn from their
behaviour in explanation of volcanic phenomena. With a view therefore of
testing the question at issue with a substance admitted as closely
allied to volcanic material, we appealed to the furnace slag of
iron-works. The following are extracts from the letters of an iron
manufacturer of great experience[2] to whom we referred the question:—

“I beg to inform you that cold slag floats in molten slag in the same
way cold iron floats in molten iron.

“I filled a box with hot molten slag run quickly from a blast furnace;
the box was about 5½ feet square by 2 feet deep, and I dropped into the
slag a piece of cold slag weighing 16 lbs., when it came to the top in a
second. I pushed it down to the bottom several times and it always made
its appearance at the top: indeed a small portion of it remained above
the molten slag.”

[Illustration: Fig. 3.]

Here then we have a substance closely allied to volcanic material which
manifests the expansile principle in question; but we may go still
further and give evidence from the very fountain-head by instancing what
appears to be a most cogent example of its operation which we observed
on the occasion of a visit to the crater of Vesuvius in 1865 while a
modified eruption was in progress. On this occasion we observed
white-hot lava streaming down from apertures in the sides of a central
cone within the crater and forming a lake of molten lava on the plateau
or bottom of the crater; on the surface of this molten lake vast cakes
of the same lava which had become solidified were floating, exactly in
the same manner as ice floats in water. The solidified lava had cracked,
and divided into cakes, in consequence of its contraction and also of
the uprising of the accumulating fluid lava on which it floated, more
and more space being thus afforded for it to separate, on account of the
crater widening upwards, while through the joints or fissures the fluid
lava could be seen beneath. But for the decrease in density and
consequent expansion in volume which accompanied solidification, this
floating of the solidified lava on the molten could not have occurred.
Reference to Fig. 3, which represents a section of the crater of
Vesuvius on the occasion above referred to, will perhaps assist the
reader to a more clear idea of what we have endeavoured to describe. A A
are the streams of white-hot lava issuing from openings in the sides of
the central cone, and accumulating beneath the solidified crust B B in a
lake of molten lava at C C; the solidified crust B B as it was floated
upwards dividing into separate cakes as represented in Fig. 4. (See also
Plate I.)

[Illustration: Fig. 4.]

[Illustration: PLATE I.

Let us now consider what would be the effect produced upon a spherical
mass of molten matter in progress of cooling, first under the action of
the above described expansion which precedes solidification, and then by
the contraction which accompanies the cooling of a solidified body. The
first portion of such a mass to part with its heat being its external
surface, this portion would expand, but there being no obstacle to
resist the expansion there would be no other result than a temporary
slight enlargement of the sphere. This external portion would on cooling
form a solid shell encompassing a more or less fluid molten nucleus, but
as this interior has in its turn, on approaching the point of
solidification, to expand also, and there being, so to speak, no room
for its expansion, by reason of its confinement within its solid casing,
what would be the consequence?—the shell would be rent or burst open,
and a portion of the molten interior ejected with more or less violence
according to circumstances, and many of the characteristic features of
volcanic action would be thus produced: the thickness of the outer
shell, the size of the vent made by the expanding matter for its escape,
and other conditions conspiring to modify the nature and extent of the
eruption. Thus there would result vast floodings of the exterior surface
of the shell by the so extruded molten matter, volcanoes, extruded
mountains, and other manifestations of eruptive phenomena. The sectional
diagram (Fig. 5) will help to convey a clear idea of this action. Basing
our reasoning on the principle we have thus enunciated, namely, that
molten telluric matter expands on nearing the point of solidification,
and which we have endeavoured to illustrate by reference to actual
examples of its operation, we consider we are justified in assuming that
such a course of volcanic phenomena has very probably occurred again and
again upon the moon; that this expansion of volume which accompanies the
solidification of molten matter furnishes a key to the solution of the
enigma of volcanic action; and that such theories as depend upon the
agency of gases, vapour, or water are at all events untenable with
regard to the moon, where no gases, vapour, or water, appear to exist.

[Illustration: Fig. 5. A A. The solidified crust cooling,
contracting, and cracking; the cracking action enhanced by the
expansion of the substratum of molten matter, B B B, which,
expanding as it approaches the point of solidification, injects
portions of the molten matter up through the contractile cracks, and
results in producing craters, mountains of exudation, and districts
flooded with extruded lava, C C C. The nucleus of intensely hot
molten matter.]

That an upheaving and ejective force has been in action with varying
intensity beneath the whole of the lunar surface is manifest from the
aspect of its structural details, and we are impressed with the
conviction that the principle we have set forth, namely the paroxysms of
expansion which successively occurred as portions of its molten interior
approached solidification, supply us with a rational cause to which such
vast ejective and upheaving phenomena may be assigned. Many features of
terrestrial geology likewise require such an expansive force whereby to
explain them; we therefore venture to recommend this source and cause of
ejective action to the careful consideration of geologists.

[Illustration: Fig. 6.]

[Illustration: Fig. 7.]

When the molten substratum had burst its confines, ejected its
superfluous matter, and produced the resulting volcanic features, it
would, after final solidification, resume the normal process of
contraction upon cooling, and so retreat or shrink away from the
external shell. Let us now consider what would be the result of this.
Evidently the external shell or crust would become relatively too large
to remain at all points in close contact with the subjacent matter. The
consequence of too large a solid shell having to accommodate itself to a
shrunken body underneath, is that the skin, so to term the outer stratum
of solid matter, becomes shrivelled up into alternate ridges and
depressions, or wrinkles. In its attempt to crush down and follow the
contracting substratum, it would have to displace the superabundant or
superfluous material of its former larger surface by thrusting it (by
the action of tangential force) into undulating ridges as in Fig. 6, or
broken elevated ridges as in Fig. 7, or overlappings of the outer crust
as in Fig. 8, or ridges capped by more or less fluid molten matter
extruded from beneath, as indicated in Fig. 9, a class of action which
might occur contemporaneously with the elevation of the ridge or
subsequently to its formation.

[Illustration: Fig. 8.]

[Illustration: Fig. 9.]

A long-kept shrivelled apple affords an apt illustration of this wrinkle
theory; another example may be observed in the human face and hand, when
age has caused the flesh to shrink and so leave the comparatively
unshrinking skin relatively too large as a covering for it. We
illustrate both of these examples by actual photographs of the
respective objects, which are reproduced on Plate II. Whenever an outer
covering has to accommodate and apply itself to an interior body that
has become too small for it, wrinkles are inevitably produced. The same
action that shrivels the human skin into creases and wrinkles, has also
shrivelled certain regions of the igneous crust of the earth. A map of a
mountainous part of our globe affords abundant evidence of such a cause
having been in action; such maps are pictures of wrinkles. Several parts
of the lunar surface, as we shall by-and-by see, present us with the
same appearances in a modified degree.

To the few primary causes we have set forth in this chapter—to the
alternate expansion and contraction of successive strata of the lunar
sphere, when in a state of transition from an igneous and molten to a
cooled and solidified condition, we believe we shall be able to refer
well nigh all the remarkable and characteristic features of the lunar
surface which will come under our notice in the course of our survey.

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