THE UNITY OF MATERIAL IN THE HEAVENS AND THE EARTH.
Clouds—Fire-Mist—Vapour of Platinum—Components of Chalk—Constituents of
the Primæval Fire-Mist—Objections—Origin of the Mist—Remarkable
Discovery of the Century—Analysis of the Sun—Spectroscopic
Analysis—Simplicity of Solar Chemistry—Potassium—A Drop of Water—The
Solar Elements—Calcium—The Most Important Lines in the Solar
Spectrum—Photograph of the Sun—Carbon in the Solar Clouds—Function
of Carbon—Bunsen’s Burner Illustrates Carbon in the Sun—Carbon
Vapours in the Sun—The Supposed Limit to our Knowledge of the
Heavens—Characteristics of Spectroscopic Work—Bearing on the Nebular
Theory.
IN considering how the formation of our solar system was brought about,
we naturally first enquire as to the material of which this superb
scheme is constructed. What were the materials already to hand from
which, in pursuance of the laws of Nature, the solar system was evolved?
See the robust and solid nature of this earth of ours, and the robust
and solid nature of the moon and the planets. It might at first sight be
concluded that the primitive materials of our earth had also been in the
solid state. But such is not the case. The primitive material of the
solar system was not solid, it was not even liquid. What we may describe
as the mother-substance of the universe must have been of quite a
different nature; we can give an illustration of the physical character
of that substance.
The lover of Nature delights to look at the mountains and the trees, the
lakes and the rivers. But he will not confine his regard merely to the
objects on the earth’s surface. He, no less than the artist and the
poet, delights to gaze at that enchanting scenery which, day by day, is
displayed in infinite beauty overhead; that scenery which is not wholly
withheld even from observers whose lives may be passed amid the busy
haunts of men, that scenery which is so often displayed on fine days at
all seasons. We are alluding to those clouds which add the charm of
infinite variety to the sky above us.
It is necessary for us now to think of matter when it possesses neither
the density of a solid, nor the qualities of a liquid, but rather when
it has that delicate texture which the clouds exhibit. The primæval
material from which the solar system has been evolved is of a texture
somewhat similar to that of the clouds. This primæval material is
neither solid nor liquid; it is what we may describe as vapour.
But having pointed to the clouds in our own sky as illustrating, in a
sense, the texture of this original mother-substance of the solar
system, we can carry the analogy no further. Those dark and threatening
masses which forbode the thunderstorm, or those beautiful fleecy clouds
which enhance the loveliness of a summer’s day, are, of course, merely
the vapours of water. But the vapours in the mother-substance from which
systems have been evolved were by no means the vapours of water. They
were vapours of a very different character—vapours that suggest the
abodes of Pluto rather than the gentle rain that blesses the earth. In
the mother-substance of the solar system vapours of a great variety of
substances were blended. For in the potent laboratory of Nature every
substance, be it a metal or any other element, or any compound, no
matter how refractory, will, under suitable circumstances, be dissolved
into vapour.
Take, for instance, such a material as platinum. Could anything be less
like a vapour than this silvery metal? We know that platinum is the
densest of all the elements. We know that platinum, more effectually
than other metals, resists liquefaction from the application of heat. No
ordinary furnace can fuse platinum; yet in another way we can overcome
the resistance of this metal. The electric arc, when suitably managed,
yields a temperature higher than that of any furnace. Let the electric
current spring from one pole of platinum to another, and a brilliant arc
of light is produced by the glowing gas, which is characteristic of
platinum. The light dispensed from that arc is different from the light
that would be radiated if the poles were of any material other than
platinum. Some of the platinum has not alone been melted, it has
actually been turned into vapour by the overpowering heat to which it
has been subjected. Thus the solidity of this substance, which resists
so stubbornly the action of lower temperatures, can be overcome, and the
very densest of all metals is dissolved into wisps of vapour.
We choose the case of platinum as an illustration because it is a
substance exceptionally dense and exceptionally refractory. If platinum
can be vaporised, there is not much difficulty in seeing that other
elements must be capable of being vaporised also. In fact, given such
heat as is found abundantly in natural sources, there is no known
element, or combination of elements, which will not assume the form of
gas or vapour or cloud.
At the temperature of the sun a drop of water would be forthwith
resolved into its component gases of oxygen and hydrogen. In like manner
a piece of chalk, if exposed to the sun, would be speedily transformed;
it would first be heated red-hot and then white-hot; it is, indeed,
white-hot chalk that gives us that limelight which we know so well. But
the heat of the sun is far greater than the temperature of the
incandescent lime. The lime would not only be heated white-hot by
contact with solar heat, but still further stages would be reached. It
would suffer decomposition. It would break up into three different
elements: there would be the metal which we call calcium, there would be
oxygen, and there would be carbon. Owing to the tremendous temperature
of the sun the metal would not remain in the metallic form; it would not
be even in a liquid form; it would become a gas. The elements which
unite to form this chalk would be not only decomposed, but they would be
vaporised. What is thus stated about the drop of water and the chalk
may, so far as we know, be stated equally with regard to any other
compounds. It matters not how close may be the chemical association in
which the elements are joined: no matter how successfully those
compounds may resist the decomposition under the conditions ordinarily
prevailing on earth, they have to yield under the overwhelming trial to
which the sun would subject them. Though there are many elements in the
solar chemistry, there are no compounds. At the exalted temperature to
which they are exposed in the sun the elements are indisposed for union
with the other elements there met with, and which are at the same
temperature. In these circumstances, they successfully resist all
alliances.
Until the last few years no elements were known in our terrestrial
experience which possessed at ordinary temperatures the same qualities
of resolute isolation which all elements seem to display at extreme
temperatures. The famous discovery of argon, and of other strange gases
associated with argon in the atmosphere and elsewhere, has revealed, to
the astonishment of chemists and to the great extension of knowledge,
that we have with us here elements which resist all solicitations to
enter into chemical union with other substances. It is doubtless in
consequence of this absolute refusal to unite that, in spite of their
abundance and their wide distribution, these elements have eluded
detection for centuries. To the astronomer argon is both interesting and
instructive. It shows us an element which possesses, at the ordinary
temperatures of the surface of the earth, a property which is true of
all elements when subjected to such temperatures as are found in the
sun.
Think of the rocks which form the earth’s crust and of the minerals
which lie far below. Think of the soil which lies on its surface, of the
forests which that soil supports, and the crops which it brings forth.
Think of the waters of the ocean, and the ice of the Poles. Think of the
objects of every kind on this globe. Think of the stone walls of a great
building, of the iron used to give it strength, of the slates which
cover it, and of the timber which forms its floors; think of the
innumerable other materials which have gone towards its construction;
think even of the elementary substances which go to form the bodies of
animals, of the lime in their bones, and of the carbon which is so
intimately associated with life itself. The nebular theory declares that
those materials have not always been in the condition in which we now
see them; that there was a time in which they were so hot that they were
not in the solid state; they were not even in the fluid state, but were
all in rolling volumes of glowing vapour which formed the great primæval
fire-cloud.
We must understand the composite nature of the primitive fire-mist from
which our solar system originated. Let me illustrate the matter thus: We
shall suppose that a heterogeneous collection of substances is brought
together, the items of which may be somewhat as follows: let there be
many tons of iron and barrels of lime, some pieces of timber, and
cargoes of flint; let there be lead and tin and zinc, and many other
metals, from which copper and silver and several of the rarest metals
must not be excluded; let there be innumerable loads of clay, which
shall represent aluminium and silicon, and hogsheads of sea-water to
supply oxygen, hydrogen, and sodium. There should be also, I need hardly
add, many other elements; but there is no occasion to mention more;
indeed, it would be impossible to give a list which would be complete.
Suppose that this diverse material is submitted to a heat as intense as
the most perfect furnace can make it. Let the heat be indeed as great as
that which we can get from the electric arc, or even greater still. Let
us suppose this heat to be raised to such a point that, not only have
the most refractory metals been transformed into vapour, but the
elements which were closely in combination have also been rent asunder.
This we know will happen when compound substances are raised to a very
high temperature. We shall suppose that the heat has been sufficient to
separate each particle of water into its constituent atoms of oxygen and
hydrogen; we shall suppose that the heat has been sufficient to
decompose even lime itself into its constituent parts, and exhibit them
in the form of vapour. The heat is to be so great that even carbon
itself, the most refractory of substances, has had to yield, so that
after passing through a stage of dazzling incandescence it has melted
and ultimately dissolved into vapour. Next let us suppose that these
several vapours are blended, though we need not assume that the separate
elements are diffused uniformly throughout all parts of the cloud. Let
us suppose that these bodies, which contributed to form the nebula, have
been employed in amounts, not to be measured in tons, or in hundreds of
tons, but in a thousand millions of millions of millions of millions of
tons. Let the mass of vapour thus arising be expanded freely through
open space. Let it extend over a region which is to measure hundreds of
thousands of millions of miles in length and breadth and depth. Then the
doctrine of the earth’s beginning, which we are striving to unfold in
these lectures, declares that in a fire-mist such as is here outlined
the solar system had its origin.
Various objections may occur to the thoughtful reader when asked to
accept such statements. We must do our best to meet these objections.
The evidence we submit must be of an indirect or circumstantial kind.
Direct testimony on such a subject is from the nature of the case
impossible. The actual fire-mist in which our system had its origin is a
mist no longer. The material that forms the solid earth beneath our feet
did once, we verily believe, float in the great primæval fire-mist. Of
course we cannot show you that mist. Darwin could not show the original
monkeys from which it would seem the human race has descended; none the
less do most of us believe that our descent has really taken the line
that Darwin’s theory indicates.
In connection with this subject, as with most others, it is easy to ask
questions which, I think we may say, no one can answer with any
confidence. It may, for instance, be asked how this vast fire-mist came
into existence. If it arose from heat, how did that heat happen to be
present? Why was all the material in the state of vapour? What, in
short, was the origin of that great primæval nebula? Here we must admit
that we have proposed questions to which it is impossible for us to do
more than suggest answers. As to what brought the mist into existence,
as to whence the materials came, and as to whence the energy was derived
which has been gradually expended ever since, we do not know anything,
and, so far as I can see, we have no means of knowing. Conjectures on
the subject are not wanting, of course, and in a later chapter we shall
discuss what may be said on this matter.
I have shown you to some extent our reasons for believing that our solar
system did originate in a fire-mist And even if we are not able to
explain how the mist itself arose, yet we do not admit that our argument
as to the origin of our system is thereby invalidated. That such a
fire-mist as the solar system required did once exist, must surely be
regarded as not at all improbable so long as we can point to the
analogous nebulæ or fire-mists which exist at the present moment, and
which we see with our telescopes. Many of these are millions of times as
great as the comparatively small fire-mist that would have evolved into
our solar system.
A question has sometimes been asked as to the most important discovery
in astronomy which has been made in the century that has just closed.
If, by the most important discovery, we mean that which has most widely
extended our knowledge of the Universe, I do not think there need be
much hesitation in stating the answer. It seems to me beyond doubt that
the most astonishing discovery of the last century in regard to the
heavenly bodies is that which has revealed the elementary substances of
which the orbs of heaven are composed. This discovery is the more
interesting and instructive because it has taught us that the materials
of the sun, of the stars, and of the nebulæ are essentially the elements
of which our own earth is formed, and with which chemists had already
become well acquainted.
We know, of course, that this earth, no matter how various may be the
rocks and minerals which form its crust, and how infinite the variety of
objects, organic and inorganic, which diversify its surface, is really
formed from different combinations of about eighty different elements.
There are gases like oxygen and hydrogen, there are other substances
like carbon and sulphur, and there are metals like iron and copper.
These elements are sometimes met with in their free or uncombined state,
like oxygen in the atmosphere, or like gold in Klondike. More frequently
they are found in combination, and in such combinations the characters
of the constituent elements are sometimes completely transformed. A
deadly gas and a curious metal, which burns as it floats on water, most
certainly renounce their special characters when they unite to form the
salt on our breakfast-table. Who would have guessed, if the chemist had
not told him, that in every wheelbarrowful of ordinary earth there are
pounds of silvery aluminium, and that marble is largely composed of an
extremely rare metal, which but few people have ever seen?
Until the middle of the century just completed it seemed utterly
impossible to form any notion as to the substances actually present in
the sun. How could anyone possibly discern them by the resources of the
older chemists? It might well have been doubted whether the elements of
which the sun was made were the elements of which our earth was formed,
and with which ordinary chemistry had made us familiar. Just as the
animals and plants which met the gaze of the discoverers when they
landed in the New World were essentially different from those in the Old
World, so it might have been supposed, with good share of reason, that
this great solar orb, ninety-three million miles distant, would be
composed of elements totally different from those with which dwellers on
the earth had been permitted to become acquainted.
This great discovery of the last century revealed to us the character of
the elements which constitute the sun. It also added the astonishing
information that they are essentially the same elements as those of
which our earth itself and all which it contains are formed.
If any one had asked in the early years of the century what those
elements were which entered into the composition of the sun, the
question would have been deemed a silly one; it would have been regarded
as hopelessly beyond the possibility of solution, and it would have been
as little likely to receive an answer as the questions people sometimes
ask now as to the possible inhabitants on Mars.
But about the middle of the century a new era dawned; the wonderful
method of spectroscopic analysis was discovered, and it became possible
to examine the chemistry of the sun. The most important result was to
show that the elements which enter into the composition of the sun are
the same elements which enter into the composition of the earth. The
student of the solar chemistry enjoys, however, one advantage over the
terrestrial chemist, if it be an advantage to have his science
simplified to the utmost extent. Chemistry would, however, lose its
chief interest if all the elements remained as obstinately neutral as
argon, and disdained alliance with all other elements. It would seem
that those elements which most eagerly enter into combination here, and
which resist with such vehemence our efforts to divorce them, must
renounce all chemical union when exposed to the tremendous temperature
of the sun.
Those elements which unite with the utmost eagerness at ordinary
temperatures, seem to become indifferent to each other when subjected to
the extremes of heat and cold. Potassium unites fiercely with oxygen in
the most familiar of all chemical experiments. Potassium is indeed a
strange metal, for it is of such small density that a piece cast on a
basin of water will float like a chip of wood. It has such avidity for
oxygen that it will decompose the water to wrench the molecules of
oxygen from those of hydrogen. The union of the metal with the gas
generates such heat that the strange substance bursts into flame. This
is what takes place at the ordinary temperatures in the well-known
experiment of the chemical lecture-table. But at extreme temperatures
the greed of potassium for oxygen abates, if it does not vanish
altogether. In those excessively low temperatures at which Professor
Dewar experiments chemical affinities languish. He has reduced oxygen to
a liquid, and he tells us that “a berg of silvery potassium might float
for ever untarnished on an ocean of liquid oxygen.” At the excessively
high temperature of the electric arc the oxygen and the potassium, whose
union has been accomplished with such vehemence, cease to possess
affinity, and they separate again.
The solar chemistry seems to know no combination. If a drop of water
were transferred to the sun and subjected to the heat of the solar
surface, it must immediately undergo decomposition. That which was a
drop of water here would not remain a drop of water there; it would be
at once resolved into its component elements of oxygen and hydrogen. The
considerations just given greatly simplify the search for the particular
bodies which are at present in the sun. We have only to test for the
presence of each of eighty elements. We have not to take account of the
thousands of chemical combinations of which these elements are
susceptible under terrestrial conditions.
We are specially indebted to the late Professor Henry Rowland, of
Baltimore, for a profound study of the solar spectrum. In his great work
he enumerates thirty-six elements present in the sun, and the number may
be increased now by at least two. Eight elements he classes as doubtful,
fifteen are set down as absent from the solar spectrum, and several had
not been tried. Iron stands foremost among all the solar elements, so
far as the number of its lines are concerned. No fewer than 2,000 lines
in the spectrum of the sun are attributed to this element. At the other
end of the list lead is found. There is only one line apparently due to
this metal. Carbon is represented by about 200 lines, and calcium by
about 75. If, however, we test the significance of lines not by their
number, but by their intensity, then iron no longer heads the list, its
place being taken by calcium (Fig. 42). Among the elements which Rowland
sets down as not contributing any recognisable lines to the solar
spectrum we may mention arsenic and sulphur, phosphorus, mercury, and
gold.
Of the more prominent solar elements there are two or three of such
special importance that we pause to give them a little consideration.
Who does not remember the delight of the first occasion in childhood
when he was permitted to peep into a bird’s-nest and there see a group
of eggs, often so exquisitely marked or so delicately tinted? How
beautiful they seemed as they lay in their cosy receptacle concealed
with so much cunning! Among other delightful recollections of early
youth many will recall a ramble by the sea-shore. We may suppose the
tide had retreated, and with other objects left by the sea on the
gleaming sand a little cowrie shell is found. How enchanted we were with
our prize! How we looked at the curious marks on its lips, and the
inimitable beauty of its tints!
The shell of the hedge-sparrow and the shell cast up by the sea have
another quality in common besides their beauty. They have both been
fabricated from the same material. Lime is of course the substance from
which the bird, by some subtle art of physiology, forms those exquisite
walls by which the vital part of the egg is protected. The soft organism
that once dwelt in the cowrie was endowed with some power by which it
extracted from the waters of the ocean the lime with which it gradually
built an inimitable shell. Is it an exaggeration to say that this
particular element calcium, this element so excessively abundant and so
rarely seen, seems to enjoy some peculiar distinction by association
with exquisite grace and beauty? The white marble wrought to an
unparalleled loveliness by the genius of a Phidias or a Canova is but a
form of lime. So is the ivory on which the Japanese artist works with
such delicacy and refinement. Whether as coral in a Pacific island, as a
pearl in a necklace or as a stone in the Parthenon, lime seems often
privileged to form the material basis of beauty in nature and beauty in
art.
Though lime in its different forms, in the rocks of the earth or the
waters of the ocean, is one of the most ordinary substances met with on
our globe, yet calcium, the essential element which goes to the
composition of lime, is, as we have already said, not by any means a
familiar body, and not many of us, I imagine, can ever have seen it.
Chemistry teaches that lime is the result of a union in definite
proportions between oxygen gas and the very shy metal, calcium. This
metal is never found in nature unless in such intimate chemical union
with some other element like oxygen or chlorine, that its characteristic
features are altogether obscured, and would indeed never be suspected
from the mere appearance of the results of the union. To see the metal
calcium you must visit a chemical laboratory where, by electrical
decomposition or other ingenious process, this elusive element can be
induced to part temporarily from its union with the oxygen or other body
for which it has so eager an affinity, and to which it returns with such
alacrity. Though calcium is certainly a metal, it is very unlike the
more familiar metals such as gold or silver, copper or iron. A coin
might conceivably be formed out of calcium, but it would have no
stability like the coins of the well-known metals. Calcium has such an
unconquerable desire to unite with oxygen that the unstable metal will
speedily grasp from the surrounding air the vital element. Unless
special precautions are taken to withhold from the calcium the air, or
other source from whence it could obtain oxygen, the union will most
certainly take place, and the calcium will resume the stable form of
lime. Thus it happens that though this earth contains incalculable
billions of tons of calcium in its various combinations, yet calcium
itself is almost unknown except to the chemist.
It is plain that calcium plays a part of tremendous significance on this
earth. I do not say that it is the most important of all the elements.
It would indeed seem impossible to assign that distinction to any
particular element. Many are, of course, of vital importance, though
there are, no doubt, certain of the rarer elements with which this earth
could perhaps dispense without being to any appreciable extent different
from what it is at present. I do not know that we should be specially
inconvenienced or feel any appreciable want unsatisfied, if, let us say,
the element lanthanum were to be struck out of existence; and there are
perhaps certain other rare bodies among the known eighty elements, about
which the same remark might be made.
[Illustration: Fig. 42.—THE H. AND K. LINES IN THE PHOTOGRAPHIC SOLAR
SPECTRUM (Higgs).]
But without calcium there would neither be fertile soil for plants nor
bones for animals, and consequently a world, inhabited in the same
manner as our present globe, would be clearly impossible. There may be
lowly organisms on this earth to which calcium is of no appreciable
consequence, and it is of course conceivable that a world of living
types could be constructed without the aid of that particular element
which is to us so indispensable. But a world without calcium would be
radically different from that world which we know, so that we are
disposed to feel special interest in the important modern discovery that
this same element, calcium, is abundantly distributed throughout the
universe. The boldest and most striking features in the photograph of
the solar spectrum are those due to calcium (Figs. 42 and 44).
In the solar spectrum are two very broad, very dark, and very
conspicuous lines, known as H and K. In every photograph of that portion
of the solar spectrum which, lying beyond the extreme violet, is
invisible to our eyes, though intensely active on the photographic
plate, these lines stand forth so boldly as to arrest the attention more
than any other features of the spectrum. It had been known that these
lines were due to calcium, but there were certain difficulties connected
with their interpretation. Some recent beautiful researches by Sir
William and Lady Huggins have cleared away all doubt. It is now certain
that the presence of these lines in the spectrum demonstrates that that
remarkable element which is the essential feature of lime on this earth
is also found in the sun. We have also to note that these same lines
have been detected in the photographic spectra of many other bodies in
widely different regions of space. Thus we establish the interesting
result that this particular element which plays a part so remarkable on
our earth is not restricted to our globe, but is diffused far and wide
throughout the universe.
Perhaps the most astonishing discovery made in modern times about the
sun is connected with the wonderful element, helium. So long ago as 1868
Sir Norman Lockyer discovered, during an eclipse, that the light of the
sun contained evidence of the presence in that orb of some element which
was then totally unknown to chemists. This new body was not unnaturally
named the sun-element, or helium. But more than a quarter of a century
had to elapse before any chemist could enjoy the opportunity of
experimenting directly upon helium. No labour could prepare the smallest
particle of this substance, no money could purchase it, for at that time
no specimen of the element was known to exist nearer than the sun,
ninety-three million miles distant. But in 1895 an astonishing discovery
was made by Professor Ramsay. He was examining a rare piece of mineral
from Norway. From this mineral, clevite, the Professor extracted a
little gas which was to him and to all other chemists quite unknown. But
on applying the spectroscope to examine the character of the light which
this gas emitted when submitted to the electric current, it yielded, to
their amazement, the characteristic light of helium. Thus was the
sun-element at last shown to be a terrestrial body, though no doubt a
rare one. The circumstances that I have mentioned make helium for ever
famous among the constituents of the universe. It will never be
forgotten that though from henceforth it may be regarded as a
terrestrial body, yet it was first discovered, not in the earth beneath
our feet, but in the far-distant sun.
In a previous picture (Fig. 14) we showed a photograph of a part of the
sun’s surface; this striking view displays those glowing clouds from
which the sun dispenses its light and heat. These clouds form a
comparatively thin stratum around the sun, the interior of which is very
much darker. The layer of clouds is so thin that it may perhaps be
likened to the delicate skin of a peach in comparison with the luscious
interior. It is in these dazzling white clouds that we find the source
of the sun’s brightness. Were those clouds removed, though the sun’s
diameter would not be appreciably reduced, yet its unparalleled lustre
would be at once lessened. We use the expression “clouds” in speaking of
these objects, for clouds they certainly are, in the sense of being
aggregates of innumerable myriads of minute beads of some substance; but
those solar clouds are very unlike the clouds of our own sky, in so far
as the material of which they are made is concerned. The solar clouds
are not little beads of water; they are little beads of white-hot
material so dazzlingly bright as to radiate forth the characteristic
brilliance and splendour of the sun. The solar clouds drift to and fro;
they are occasionally the sport of terrific hurricanes; they are
sometimes driven away from limited areas, and in their absence we see
merely the black interior of the solar globe, which we call a sun-spot.
Now comes the important question as to the material present in these
clouds which confers on the sun its ability to radiate forth such
abundant light and heat.
The profound truth already stated, that the solar elements are the same
as the terrestrial elements, greatly simplifies the search for that
particular element which forms those solar clouds. As the sun is made of
substances already known to us by terrestrial chemistry, and as there
are no chemical compounds to embarrass us, the choice of the possible
constituents of those solar clouds becomes narrowed to the list of
elements experimented on in our laboratories.
We owe to Dr. G. Johnstone Stoney, F.R.S., the discovery of the
particular element which forms those fire-clouds in the sun, and confers
on the presiding body of the solar system the power of being so useful
to the planets which owe it allegiance. Carbon is the element in
question. I need hardly add that carbon is well known as one of the most
commonplace and one of the most remarkable substances in Nature. A piece
of coke differs from a piece of pure carbon only by the ash which the
coke leaves behind when burned. Timber is principally composed of this
same element, and when the timber is transformed into charcoal but
little more than the carbon remains. Carbon is indeed everywhere
present. It is, as we have mentioned, one of the elements which enter
into the composition of a piece of chalk. Carbon is in the earth beneath
our feet; it is in the air above us. Carbon is one of the chief
ingredients in our food, and it is by carbon that the heat of the body
is sustained. Indeed, this remarkable element is intimately connected
with life in every phase. Every organic substance contains carbon, and
it courses with the blood in our veins. It assumes the widest variety of
forms, renders the greatest diversity of services, and appears in the
most widely different places. Carbon is indeed of a protean character,
and there is a beautiful symbol of the unique position which it occupies
in the scheme of Nature (Fig. 43). Carbon is associated not alone with
articles of daily utility and of plenteous abundance, but it is carbon
which forms the most exquisite gems “of purest ray serene.” The diamond
is, of course, merely a specimen of carbon of absolute purity and in
crystalline form. Great as is the importance of carbon on this earth, it
is spread far more widely; it is not confined merely to the earth, for
carbon abounds on other bodies in space. The most important functions of
carbon in the universe are not those it renders on this earth. It was
shown by Dr. Stoney that this same wonderful substance is indeed a solar
element of vast utility. It is carbon which forms the glowing solar
clouds to which our very life owes its origin.
In the incandescent lamp the brilliant light is produced by a glowing
filament of carbon, and one reason why we employ this element in the
electric lamp, instead of any other, may be easily stated. If we tried
to make one of these lamps with an iron wire, we should find that when
the electric current is turned on and begins to flow through the wire,
the wire will, in accordance with a well-known law, become warm, then
hot, red-hot, and white-hot; but even when white-hot the wire will not
glow with the brightness that we expect from one of these lamps. Ere a
sufficient temperature can be reached the iron will have yielded, it
will have melted into drops of liquid, continuity will be broken, the
circuit will be interrupted, and the lamp destroyed. We should not have
been much more successful if instead of iron we had tried any other
metal. Even a platinum wire, though it will admit of being raised to a
much higher temperature than a wire of iron or a wire of steel, cannot
remain in the solid condition at the temperature which would be
necessary if the requisite incandescence is to be produced.
There is no known metal, and perhaps no substance whatever, which has so
high a temperature of fusion as carbon. A filament of carbon, alone
among the available elements, will remain continuous and unfused while
transmitting a current intense enough to produce that dazzling
brilliance which is expected from the incandescent lamp. This is the
reason why this particular element carbon is an indispensable material
for the electrician.
Modern research has now demonstrated that just as we employ carbon as
the immediate agent for producing our beautiful artificial light, so the
sun uses precisely the same element as the agent of its light and
heat-giving power. In the extraordinary fervour which prevails in the
interior of the sun all substances of every description must submit to
be melted, nay, even to be driven into vapour. An iron poker, for
instance, would vanish into iron vapour if submitted to this appalling
solar furnace. Even carbon itself is unable to remain solid when
subjected to the intense heat prevailing in the inner parts of the sun.
At that heat carbon must assume the form of gas or vapour, just as iron
or the other substances which yield more readily to the application of
heat.
By the help of a simple experiment we may illustrate the significance of
the carbon vapours in the solar economy. Let us take a Bunsen burner, in
which the air and gas are freely mingled before they enter into
combustion. If the air and the gas be properly proportioned, the
combustion is so perfect that though a great deal of heat is produced
there is but little light. The gas burned in this experiment ought to be
the ordinary gas of our mains, which depends for its illuminating power
on the circumstance that the hydrogen, of which the gas is chiefly
composed, is largely charged with carbon. The illuminating power of the
gas may indeed be measured by its available richness in carbon. As it
enters the burner the carbon is itself in a gaseous form. This is not,
of course, on account of a high temperature. The carbon of the coal-gas
is in chemical union with hydrogen, and the result is in the form of
invisible gases. It is these composite gases, blended with large volumes
of ordinary hydrogen, which form the illuminating gas of our mains.
In the Bunsen burner the admission of a proper proportion of air, which
becomes thoroughly mixed with the coal gas, produces perfect combustion.
In the act of burning, the oxygen of the air unites immediately with the
gas; it combines with the hydrogen to form watery vapour, and it
combines with the carbon to form gases which are the well-understood
products of combustion.
Suppose, now, we cut off the supply of air from the Bunsen burner, which
can be done in a moment by placing the hand over the ring of holes at
the bottom at which the air is admitted. Immediately a change takes
place in the combustion. In place of the steady, hardly visible, but
intensely hot flame which we had before, we have now a very much larger
flame which makes a bright and flickering flare that lights up the room.
If we re-admit the air at the bottom of the burner the light goes down
instantly; the small, pale flame replaces it, and again the perfect
combustion gives out intense heat at the expense of the light.
The remarkable change in the character of a gas-flame produced by
admitting air to mix with the gas before combustion is, of course,
easily explained. The chemical action takes place with much greater
facility under these circumstances. The union of the carbon in the coal
gas with the oxygen then takes place so thoroughly and instantaneously
that the carbon never seems to have abandoned the gaseous form even for
a moment in the course of the transformation. But in the case where air
is not permitted to mingle with the gas, the supply of oxygen to unite
with the incandescent gases can only be obtained from the exterior of
the flame. The consequence is that the glowing gas charged with carbon
vapour is chilled to some extent by contact with the cold air. It
therefore seems as if the union of the hydrogen with the oxygen
permitted the particles of carbon in the flame to resume their solid
form for a moment. But in that solid form these particles, being at a
high temperature, have a wonderful efficiency for radiation, and
consequently brilliance is conferred upon the light. Most of the
particles of carbon speedily unite with the surrounding oxygen, and
re-enter the gaseous state in a different combination. Some of them,
however, may escape this fate, in which case they assume the undesirable
form of smoke. The Bunsen lamp can thus be made to give an illustration
of the fact that when carbon vapours receive a chill, the immediate
effect of the chill is to transform the carbon from the gaseous form to
myriads of particles in the liquid, or more probably in the solid form.
In the latter state the carbon possesses a power of radiation greatly in
excess of that which it possessed in the gaseous state, even though the
gas may have been at a much higher temperature than the white-hot solid
particles.
We can now apply these principles to the explanation of the marvellous
radiation of light and heat from the great orb of day. The buoyancy of
the carbon vapours is one of their most remarkable characteristics; they
tend to soar upwards through the solar atmosphere until they attain an
elevation considerably over that of many of the other materials in the
heated vapours surrounding the great luminary. We may illustrate what
happens to these carbon vapours by considering the analogous case
presented in the formation of ordinary clouds in our own skins. It is
true, no doubt, that terrestrial clouds are composed of material very
different from that which enters into the solar clouds. Terrestrial
clouds of course arise in this way; the generous warmth of the sun
evaporates water from the great oceans, and transforms it into vapour.
This vapour ascends through our atmosphere, not at first as a visible
cloud, but in the form of an invisible vapour. It is gradually diffused
throughout the upper air, until at last particles of water, but recently
withdrawn from the oceans, attain an altitude of a mile or more above
the surface of the earth. A transformation then awaits this aqueous
vapour. In the coldness of those elevated regions the water can no
longer remain in the form of vapour. The laws of heat require that it
shall revert to the liquid state. In obedience to this law the vapour
collects into liquid beads, and it is these liquid beads, associated in
countless myriads, which form the clouds we know so well. The same
phenomenon of cloud-production is witnessed on a smaller scale in the
formation of the visible puffs which issue from the funnel of a
locomotive. We generally describe these rolling white volumes as steam;
but this language is hardly correct. Steam, properly so called, is truly
as invisible as the air itself; it is only after the steam has done its
work and is discharged into the atmosphere, and there receives a chill,
that it becomes suddenly transformed from the purely gaseous state into
clustering masses of microscopic spheres of water, and thus becomes
visible.
We can now understand the transformation of these buoyant carbon vapours
which soar upwards in the sun. They attain an elevation at which the
fearful intensity of the solar heat has been so far abated by the cold
of outer space that the carbon gas is not permitted to remain any longer
in the form of gas; it must return to the liquid or to the solid state.
In the first stage on this return the carbon gas becomes transformed,
just in the same way as watery vapour ascending from the earth becomes
transformed into the fleecy cloud. Under the influence of its fall in
temperature the carbon vapour collects into a clustering host of little
beads of carbon. This is the origin of the glorious solar clouds. Each
particle of carbon in that magnificent radiant surface has a
temperature, and consequently a power of radiation, probably exceeding
that with which the filament of carbon glows in the incandescent
electric arc. When we consider that millions of millions of square miles
on our luminary are covered with clouds, of which every particle is so
intensely bright, we shall perhaps be able to form some idea of that
inimitable splendour which even across the awful gulf of ninety-three
million miles transmits the indescribable glory of daylight.
We are perhaps at present living rather too close to the period itself
to be able to appreciate to its full extent the greatness of that
characteristic discovery made in astronomy during the century just
closed, to which the present chapter relates. In the early part of the
last century it might have been said—indeed, by a certain very
distinguished philosopher it actually was said—that a limit could be
laid down bounding the possibilities of our knowledge of the heavenly
bodies. It was admitted that we might study the movements of the
different orbs in vastly greater detail than had been hitherto
attempted, and that we might calculate the forces to which those orbs
were submitted. With the help of mathematical analysis we might pursue
the consequences of these forces to their remote ramifications; we might
determine where the various orbs were situated at inimitably remote
periods in the past. We might calculate the positions which they shall
attain at epochs to be reached in the illimitably remote future; we
might discover innumerable new stars and worlds; and we might map down
and survey the distant parts of the universe. We might even sound the
depths of space and determine the distances of the more remote celestial
bodies, much more distant than any of those which have already yielded
their secrets; we might measure the dimensions of those bodies and
determine their weights; we might add scores or hundreds to the list of
the known planets; we might multiply many times the number of known
nebulæ and star-clusters; we might make measurements of many thousands
of double stars; we might essay the sublime task of forming an inventory
of the stars of the universe and compiling a catalogue in which the
stars and their positions would be recorded in their millions; but, said
the philosopher to whom I have referred, though you might accomplish all
this, and much more in the same direction, yet there is a well-marked
limit to your possible achievements; you can, he said, never expect to
discover the actual chemical elements of which the heavenly bodies are
composed. Nobody could dispute the reasonableness of this statement at
the time he made it; indeed, it seemed to be a necessary deduction from
our knowledge of the arts of chemistry, as those arts were understood
before the middle of the last century.
In the prosecution of his researches by the older method, the chemist
could no doubt discover the different elements of which the body was
formed. That is to say, his art enabled him to accomplish this task,
provided one very essential and fundamental condition could be complied
with. However accomplished the chemist of fifty years ago might have
been, he would assuredly have thought that he was being mocked if asked
to determine the composition of a body which was 93,000,000 miles away
from him. The very idea of forming an analysis under such conditions
would have been scouted as preposterous. He would naturally ask that a
specimen of the body should be delivered into his hands, a specimen
which he could take into his laboratory, pulverise in his mortars, place
in his test-tubes, treat with his re-agents, or examine with his
blowpipe. Only by such methods was it then thought possible to obtain an
analysis and discover the elements from which any given substance was
formed.
For in the early part of this century the splendid method of spectrum
analysis, that method which has revealed to us so many of the secrets of
Nature, had not yet come into being. When that memorable event took
place it was at once perceived that the spectroscope required no actual
contact with the object to be tested, but only asked to receive some of
the rays of light which that object dispersed when sufficiently heated.
It was obvious that this new method must be capable of an enormously
enlarged application. The flame producing the vapour might be at one end
of the room, while the spectroscope testing the elements in that vapour
might be at the other end. This new and beautiful optical instrument
could analyse an object at a distance of a hundred feet. But if
applicable at a distance of a hundred feet, why not at a hundred yards,
or a hundred miles, or a hundred million miles? Why might the method not
be used if the source of light were as far as the sun, or as far as a
star, or even as far as the remotest nebula, whose faint gleam on the
sky is all that the mightiest telescope can show.
Presently another great advance was recorded. As the study of this
subject progressed, it was soon found that a spectrum visible to the
human eye was not always indispensable for the success of the analysis.
The photographic plate, which so frequently replaces the eye in other
classes of observation, has also been used to replace the eye in the use
of the spectroscope. A picture has thus been obtained showing the
characteristic lines in the spectrum of a celestial object. That object
may have been sunk in space to a distance so tremendous that even though
the light travelled at a pace sufficient to complete seven circuits of
our earth in each second of time, yet the rays from the object in
question may have been travelling for centuries before they reached our
instrument.
However the rays of light may have become weakened in the course of that
journey, they still faithfully preserve the credentials of their origin.
At last the light is decomposed in the spectroscope, and the several
rays, which have been so closely commingled in their long voyage of
myriads of miles, are now for the first time forced to pursue different
tracks; they thus reach their different destinations on the photographic
plate, and they there engrave their characteristic inscriptions. Nature
in this operation imparts for our instruction a message which it is our
business to interpret. It is true that these inscriptions are not always
easily deciphered; many of them have not yet been understood. A portion
of the solar spectrum showing many of the lines in the visible region is
represented in the accompanying plate.
[Illustration: Fig. 43.—SPECTRUM OF COMET SHOWING CARBON LINES.
(_Sir W. Huggins, K.C.B._)]
Considering the insignificance of our earth when viewed in comparison
with the millions of other orbs in the universe, considering also the
stupendous distances by which the earth is separated from innumerable
globes which are very much greater, it is certainly not a little
astonishing to learn that the elements from which the various bodies in
the universe have been composed are practically the same elements as
those of which our earth is built. Is not this a weighty piece of
evidence in favour of the theory that earth, sun, and planets are all
portions of the same primæval nebula in which these elements were
blended?
[Illustration: THE SOLAR SPECTRUM.]
We do not, of course, mean to affirm that the great primæval nebula was
homogeneous throughout its vast extent. The waters of ocean are not
strictly the same in all places; even the atmosphere is not absolutely
uniform. Nature does not like homogeneity. The original nebula, we may
well believe, was irregular in form, and denser in some places than in
others. We do not suppose that if we could procure a sample of nebula in
one place and another sample from the same nebula, but in a different
place, say a hundred million miles distant, the two would show an
identity of chemical composition; two samples of rock from different
parts of the same quarry will not always be identical. But we may be
assured that, in general, whatever elements are present in the nebula
will be widely dispersed through its extent. If from different parts of
the nebula two globes are formed by condensation, though we should not
affirm, and though in fact we could not believe, that those globes would
be of identical composition, yet we should reasonably expect that the
elementary bodies which entered into their composition would be in
substantial agreement. If one element, say iron, was abundant in one
globe, we should expect that iron would not be absent from the other.
Thus the elements represented in one body should be essentially those
which were represented in the other.
[Illustration: Fig. 44.—SPECTRUM OF SUN DURING ECLIPSE.
THE TWO CHIEF LINES ARE DUE TO CALCIUM.
(_Evershed._)]
It is obvious that if the sun and the earth—to confine our attention
solely to those two bodies—had originated from the primæval nebula, they
would bear with them, as a mark of their common origin, a resemblance in
the elementary bodies of which they were composed. When Laplace framed
his theory, he had not, he could not have had, the slightest notion as
to the particular elements in the sun. For anything he could tell, those
elements might be absolutely different from the elements in the earth.
Yet, even without information on this critical point, the evidence for
the nebular theory appeared to him so cogent that he gave it the
sanction of his name.
It cannot be denied that if spectroscopic analysis had demonstrated that
the elements in the sun were totally different from the elements in the
earth a serious blow would have been dealt to the nebular theory. The
collateral evidence, strong as it undoubtedly is, might hardly have
withstood so damaging an admission. If, on the other hand, we find, as
we actually have found, that the elements in the sun and the elements in
the earth are practically identical, we obtain the most striking
corroboration of the truth of the nebular theory. Had Kant and Laplace
been aware of this most significant fact, they would probably have cited
it as most important testimony. They would have pointed out that the
iron so abundant in the earth beneath our feet is also abundant in the
sun overhead. They would, I doubt not, if they had known it, have dwelt
upon the circumstance that with that element, carbon, which enters into
every organic body on this earth, our sun is also richly supplied, and
they would have hardly failed to allude to the wide distribution in
space of calcium, hydrogen, and many other well-known elements.
Laplace mainly based his belief in the nebular theory on some remarkable
deductions from the theory of probabilities. To the consideration of
these we proceed in the next three chapters. We may, however, remark at
the outset that if the evidence derived from probabilities seemed
satisfactory to Laplace one hundred years ago, this same line of
evidence, strengthened as it has been by recent discoveries, is
enormously more weighty, at the present day.
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