THE DISTRIBUTION AND CONSERVATION OF SOLAR ENERGY.
What, then, becomes of the light and heat flashed forth with
eternal energy from the fiery waves of the sun's incandescent
atmosphere? Professor Ball ("In the High Heavens") says, "Much
of what has been said with regard to light may be repeated with
regard to heat. We know that radiant heat consists of ethereal
undulations of the same character as the waves of light. Hence we
see that the heat or the light radiated from a glowing gas is mainly
provided at the expense of the energy possessed by the molecules
in virtue of their internal oscillations." Conversely, of course,
the ethereal undulations thus induced by high molecular motion in
the heated gas or vapor must disappear in so-called absorption or
transference by contact with other molecules, themselves devoid of
such specific internal oscillations. The heat motion then disappears
as heat by its conversion into work, just as the motion of a belt
in a mill disappears in the work of the machine which it drives. One
two-hundred-and-thirty-two-millionth part of the radiant solar energy,
we know, is caught by the flying planets of our system in the forms of
heat and light, adapted to sustain life and its continued potentiality,
and we know that this solar energy is the sole source of all the
development and maintenance of the planets as the possible abodes of
organic life, past, present or future.
But what of the vast total, of which we consume so minute a
fraction? It is true that, in addition to the planets, space is
occupied by many small meteoric bodies, which manifest themselves to us
as shooting stars and meteorites, but the mass of these is too trifling
to be estimated. Professor Helmholtz, in his "Popular Scientific
Lectures," says, "According to Alexander Herschel's estimates, each
stone is, on an average, at a distance of four hundred and fifty miles
from its neighbors." When these bodies enter our atmosphere by force of
the earth's attraction they are heated by its atmospheric friction to
incandescence, and in most cases are even volatilized before reaching
the earth's surface. The vast volumes of solar heat and light, however,
are poured forth from the sun indiscriminately in all directions into
illimitable space, wherein all the masses of concrete matter, including
the stars, are relatively far less in volume than the flying motes of
the purest morning air which sparkle in the flood of light sent forth
by the rising sun. Is all the rest wasted? Professor Balfour Stewart,
in his work "The Conservation of Energy," says, "If this be the fate of
the high-temperature energy of the universe, let us think for a moment
what will happen to its visible energy. We have spoken already about
a medium pervading space, the office of which appears to be to degrade
and ultimately extinguish all differential motion, just as it tends to
reduce and ultimately equalize all difference in temperature. Thus,
the universe would ultimately become an equally heated mass, utterly
worthless as far as the production of work is concerned, since such
production depends upon difference of temperature."
It is obvious that the starting-point taken by the author last
quoted, but which, nevertheless, is in accordance with the views
now generally prevalent, is diametrically opposed to that sought
to be established in this work. Professor Stewart takes the sun's
inherent energy as the initial point of departure, and reasons from
that as to the final consequence when all its light and heat shall
have been distributed or dissipated into the attenuated medium which
occupies space, and which will be thus slowly heated until all space
has been raised in temperature to that of the last dying sun, when
all will thenceforth remain unchanged and unchangeable, silent, dark,
and dead, to all eternity. On the contrary, the purpose of the present
work is to establish a directly opposite principle, based, however, on
demonstrated scientific facts and not on theory, that the medium which
pervades all space was originally in the same equally and universally
potential state (with its molecules raised to a tension constituting
an unstable equilibrium) in which, practically, Professor Stewart's
argument leaves it finally, and that this universal molecular energy
of position was permanently maintained by the employment of the forces
which afterwards, transformed into light and heat, were shed abroad
by the sun in the work of again overcoming the intermolecular tension
of cohesion, and that the light and heat of the sun are merely caught
up again by these same or other molecules and successively employed in
the same manner, while the planetary electrospheres utilize these same
forces of internal tension in the generation of electricity, which,
sent to the sun, is converted into light and heat, and these are again
transferred to their original source. The rotation of the planets is
the grand exciting cause, and the process, in its complete cycle of
development, has live stages: first, planetary generation; second,
transference by currents of electricity to the sun; third, conversion
into light and heat; fourth, emission; and, fifth, reabsorption and
conversion again into molecular energy of position. All space is thus
found to be pervaded by extremely attenuated vapors, which contain
the elemental constituents out of which suns and planets are evolved
under favorable circumstances of development, and, among other vapors,
aqueous vapor, and that these are the agency upon which the planetary
electrospheres operate in their generation of electrical currents, and
which vapors, in turn, by absorption of the solar energy of radiation,
again transform this energy into mutually balanced electric potential,
until it is once more disengaged as electricity by the rotating
planetary electrospheres, and so on in a constant circuit forever
repeated. It differs from perpetual motion, however, in that the
planetary rotation is the external and not the internal generative
cause, since the electrical forces neither cause nor control these
motions; they belong to the realm of gravity. The disassociation,
moreover, is electrical and not chemical disassociation. The tensions
are against cohesion and not against chemical affinity; are, in fact,
similar to those which constitute our atmosphere a vast electrical
reservoir; and the aqueous vapors, through all their changes,
permanently remain as aqueous vapors, except those condensed portions
disassociated by electrolytic action at the electrospheric poles, and
which have no relation to the attenuated vapors of space, except in
that the latter are their sources of supply. The process is analogous
to what we see around us at all times in the atmosphere. While the
process described by Professor Stewart resembles the emptying of the
inherent water of a cloud, in the form of rain, into an ocean which
never yields up its water again, so that, when the cloud has rained
itself out, it is gone forever, the processes here sketched are like
the vapors which are caught up by the heated air, carried over the
thirsty lands, distributed in rain to fertilize and vivify them,
then gathered in a thousand tiny rills from countless fountains,
again descending to the sea and again carried up in vapor, and so on
over and over in unceasing round. It is the difference between an
old-fashioned flintlock musket and a modern magazine rifle, except
that the magazine is always full.
This great ocean of space was primordially charged with these
potential vapors; it is the constitution of space itself. We are so
accustomed to consider space as empty, and that it is nothingness,
the antithesis of something or anything, that it is a negation or
a blank, that it requires an effort to even think of it as a fully
stocked establishment with all the goods necessary for use or ornament,
in the latest styles and of prime quality, only not made up, and that
all our suns and worlds are merely tailoring establishments where
the operatives cut and fit and make them up to order. When more goods
are wanted they have to go to the store.
Is space, then, eternal, and is this constant round of energies to
be eternal? If one is eternal, so is the other, and surely nothing
can be more eternal than space, and we cannot conceive of any
other space than this space. Out of it came all created things,
and so long as the orbs rotate without retardation, so long will
these interchanges go on without impairment, and that they do so
rotate is the necessary corollary of the fact that they ever began
to rotate. If rotation, on the contrary, was imparted by special
creative power, then the same power established the laws by which
they rotate, and took cognizance of resistance as well. Whatever the
impulse was, it still remains; whatever caused the rotation to begin
maintains it; if the cause is eternal the rotation may be eternal;
and, in any case, its period must be measured by cycles of æons,
to which the allotted lifetime of a dying sun--a few million years,
perhaps--is but as the sunburst of a morning-glory flower to the
hoary age of a mighty planet. Compared with the popular view of the
sun's life-period, we may formulate the terms of an equation in which
the sun's mass, compared with the realms of infinite space, is as
the sun's lifetime--on a basis of contraction of his volume--to the
lifetime which actually is to be. As one of the terms is practically
infinite, so must be the answer to the problem. Professor Stewart says,
"We cannot help believing that there is a material medium of some
kind between the sun and the earth; indeed, the undulatory theory
of light requires this belief." It has already been shown that the
transmission of electricity also requires it, but that there must be
a medium quite different from the undulatory ether. Professor Proctor
("Mysteries of Time and Space") says, "We may admit the possibility
that the aqueous vapor and carbon compounds are present in stellar
or interplanetary space." Again he says, "Assuming, as we well may,
that space is really occupied by attenuated vapors." The same writer
says further, "To this end all thoughtful study of the mechanism
seems to tend (associating, perhaps, our visible universe with others,
permeating it as the ether of space permeates the densest solids, and
in turn with others so permeated by it); there may be that constant
interchange, that perpetual harmony, of which Goethe sung:
'Balanced worlds from change defending,
While everywhere diffused is harmony unending.'"
The light and heat poured forth from the sun are, as stated, in
the form of radiated energy. They penetrate the attenuated vapors
as far as vision extends, and doubtless farther, but they cannot
reach the boundaries of space, for even the mind of man cannot reach
those limits. Aqueous vapor absorbs heat; we know this without any
demonstration, for the radiated heat of the earth is arrested by
a veil of clouds, so that on cloudy nights frost will not form. So
also the sun shining into water will raise its temperature, as in
a glass globe, and such absorption of heat by aqueous vapors or
water would be much more manifest were not a large part employed in
loosening the tension of the constituent molecules, since, when thus
employed, it is not manifest as sensible heat. Professor Tyndall, in
"The Forms of Water," states that "The quantity of heat which would
raise the temperature of a pound of water one degree would raise the
temperature of a pound of iron ten degrees." Professor Stewart, in
"The Conservation of Energy," says, "That peculiar motion which is
imparted by heat when absorbed into a body is, therefore, one variety
of molecular energy.... Part of the energy of absorbed heat is spent in
pulling asunder the molecules of the body under the attractive force
which binds them together, and thus a store of energy of position is
laid up, which disappears again after the body is cooled.
"Heat will only be changed into work while it passes from a body of
high temperature to one of low.... At very high temperatures it is
possible that most compounds are decomposed, and the temperature
at which this takes place, for any compound, has been termed its
temperature of disassociation. Heat energy is changed into electrical
separation when tourmalines and certain other crystals are heated." It
may be added that it is also changed into electrical energy by the
operation of all electrical machines, as molecular motions are all
mutually interconvertible, and heat itself is only a mode of such
motion. Of radiant energy, the same writer says, "This form of energy
[radiant heat] is converted into absorbed heat whenever it falls upon
an opaque substance ... and heats it. It is a curious question to ask
what becomes of the radiant light from the sun that is not absorbed
either by the planets of our system or by any of the stars. We can
only reply to such a question that, as far as we can judge from
our present knowledge, the radiant energy that is not absorbed must
be conceived to be traversing space at the rate of one hundred and
eighty-eight thousand miles a second."
We know, of course, that aqueous vapors are partially opaque to heat
rays, as the radiated heat of the earth is partially arrested by
such vapors in the atmosphere, but they are apparently transparent to
the rays of light. But we know that this cannot be entirely true in
fact, for light rays only differ from heat rays in the comparative
length of their waves or impulses, while rays of light are always
accompanied--when emitted by a thermally incandescent body--by a much
larger number of those of heat. As a body is raised in temperature
radiant dark rays first appear; these being raised higher, become
visible as light, and new dark rays are radiated behind them, and this
continues till after the state of highest incandescence is reached
and the invisible chemical rays beyond the spectrum appear. It is
like a crowd surging forth in flight from the doors of a building;
as the speed of those in front increases to a run, others follow more
slowly in the mass, and as these gain speed others continue to follow,
while the great mass of laggards still trails along in a lengthening
line to the rear. The perception of light is itself merely due to the
constitution of the optic apparatus of the observer, which only takes
cognizance of vibrations radiated from the middle portion of the scale,
just as the ear does with sounds, and not to any actual difference in
their mode of production. That heat rays and light rays are identical
in constitution can be readily shown by the experiment described by
Professor Tyndall in his "Forms of Water," in which an opaque screen
of iodine solution in bisulphide of carbon was employed to arrest, in
a beam of light, all the light waves (to which it is entirely opaque),
while transmitting the dark rays. These non-luminous rays are then
converged by a lens: "Let us, then, by means of our opaque solution,
isolate our dark waves and converge them on the cotton. It explodes
as before.... At the same dark focus sheets of platinum are raised
to vivid redness; ... a diamond is caused to glow like a star, being
afterwards gradually dissipated." Sir William Herschel (see article
"Spectrum," Appleton's Cyclopædia) says, "If we call light those rays
which illuminate objects, and radiant heat those which heat bodies,
it may be inquired whether light be essentially different from radiant
heat. In answer to which I would suggest that we are not allowed by the
rules of philosophizing to admit of two different causes to explain
certain effects, if they may be accounted for by one."... "Tyndall,
by similar experiments, found that the thermal energy of the invisible
radiation of a very powerful electric light is eight times that of
the visible.... Seebeck showed that the position of maximum heat
in the spectrum changes with the nature of the prism and sometimes
occurs in the red." Melconi, with prisms of alcohol and water, found
it in the yellow. Athermic bands are also found in the heat-spectrum,
corresponding to the Fraunhofer lines seen in the visible spectrum.
We may illustrate this successive development of more and more rapid
light-waves by conceiving of a harp having musical strings of various
length and thickness, but not strung up, so that, when swept by the
hand, the vibrations are felt, but no musical tones are produced. If,
now, all the strings are simultaneously and gradually stretched
while under continuous vibration, we will first hear the hum of
the lighter strings, but deep down in the scale; and as the tension
gradually increases the pitch of these will rise higher and higher
and be succeeded by other new tones below, until the whole register
is simultaneously sounded. And if the tension be further increased,
the vibrations of the upper strings will gradually grow so rapid
that the ear can take no cognizance of them, corresponding to the
invisible chemical rays of the spectrum, while the middle strings
will be sounding loudly, and others will be slowly vibrating below the
musical scale, but without sound, corresponding to the invisible heat
rays. In addition to this gradual ascent of pitch along the scale,
however, there is reason to believe that sympathetic vibrations are
induced in the spectrum of thermal and chemical light corresponding to
the over-tones in music and to those hidden rhythms which differentiate
the "timbre" of one kind of musical instrument from that of another,
so that a definite wave-length will not only repeat itself among
adjacent molecules, but will give rise to harmonious vibrations
quite different in amplitude and velocity. An example of this is
found in some of the phenomena of phosphorescence and fluorescence,
in which chemical rays totally invisible are able, under suitable
conditions, to excite molecular movements corresponding to parts
of the visible spectrum, and quite different in wave-lengths and
in rapidity. This process is precisely the converse of what we
perceive in thermal light; in the latter case the colors ascend,
loaded with invisible heat rays; in the former they descend, loaded
with invisible chemical rays, only noted, perhaps, by their actinic
action on the photographic plate. Others, as the sulphide of calcium
paints and the like, repeat their own vibrations for many hours, and
we find in certain chemical salts of some rare metals, as lanthanum
and cerium, the curious property of suddenly raising the whole scale,
as in a recently introduced gas-lamp, in which a skeleton mantle of
these oxides glows with a wondrously beautiful white light under the
relatively low temperature of a small Bunsen burner; similar phenomena
are manifested in the behavior of electric discharges in attenuated
gases, as well as in what is known to children as "fox-fire," wood
undergoing slow decomposition in damp places, or in the self-luminous
secretions (corresponding, perhaps, to ptomaines or like products)
of glow-worms and other animals. If we ever--as we probably soon
shall--reach that point where we can illuminate our dwellings with
"cold candles," as the inhabitants of tropical countries carry about a
few fire-flies in a paper box for a lantern on dark nights, it must be
by the study of these phenomena. But meantime "Old Sol" will continue
to discharge his accumulating stores of both heat and light, for both
these are essential, not only for use upon the planets, but throughout
all the realms of space. In the transformation into and emission of
his radiant energy the sun is not a chemical engine, but a mill,--one
of those which "grind slowly, but they grind exceeding small."
The difference between radiated thermal light and heat is obviously
one of degree only and not of kind. The undulations of light may
be compared to the thrust of a rapier, and the more massive waves
of radiant heat to the blow of a bludgeon, but the same resistance
which arrests the advance of the one must retard and finally arrest
that of the other, if sufficiently extended. Within the limits of
a space in which Professor Stewart conceives that the first rays of
light which ever flashed forth at the dawn of creation, in the primal
æons of the universe, are still to this day, along their original
lines of radiation, "traversing space at the rate of one hundred
and eighty-eight thousand miles per second," there must certainly
be room enough and absorption enough (which even a few yards of mist
will supply) to curb these runaway steeds somewhere along their lines
of flaming passage. At that very point they are at work acting upon
the molecules of the attenuated vapors of space, and assisting to
re-establish the potential energy which has there been converted, into
another form of force by the planetary rotations of the solar systems
of those distant regions. By the law of the diffusion of gases, and
that of the diffusion or transference of heat-energy from molecule
to molecule, the vast realms of interstellar space must tend to be
all brought into approximate uniformity of tensions, and the force
abstracted at those points of space occupied by the relatively few
and insignificant solar systems will be returned, not directly at
the identical places where such solar systems may exist, but at every
part of space to which their radiant energy extends. As we give from
our own supplies to other systems for their support, so they, in turn,
give back again to us. It is said that in the earliest days of creation
the stars sang together; they still sing together, planets and suns, as
"Jura answers from her misty shroud
Back to the joyous Alps, who call to her aloud."
When old Earth lifts his brimming beaker from the great crystal sea
and drains it to the good health of all the stars of heaven, they each
respond with fiery energy, and by their merry twinkle we may know
how highly they appreciate the toast. We are all one family,--but
what a family! Comets, planets, double stars, variable stars, stars
of complementary colors, blue, yellow, orange, and red stars, stars
which blaze up in sudden conflagration, apparently new stars, nebulæ
half star and half vapor, nebulæ all vapor and others all stars, the
vast milky-way like a wondrous river of hundreds of millions of solar
systems, the insulated stars scattered through space like watchmen
on the distant hills beyond the city walls, streams of stars, stars
which are parting from each other in space like scattering families,
and those which travel together in groups like pioneers in a strange
country,--all these and doubtless other unknown types and forms
compose this sidereal family. Will they fall into their categories as
lawful subjects, so as to be properly classified in a single scheme
of the visible order of creation, or shall we fail to interpret their
apparent mysteries when we apply the same principles which have been
successfully applied to the phenomena of our own solar system? Let
us see.
In examining the sun, we find that a beam of its light passed
through a prism is thrown upon the wall in a wedge-shaped streak of
rainbow-tinted colors. Fraunhofer, many years ago, found that this
spectrum was crossed at irregular intervals by a series of dark lines,
of variable width and distance apart, of which he catalogued more
than five hundred. These lines were subsequently found to correspond
in the aggregate, in their position in the spectrum, with a series of
bright lines of different colors which formed the separate spectra
of various metals when burned, in vapor or powder, in the flame of
an alcohol lamp. Each of these transverse lines was found to have a
fixed and invariable position in the extended scale of the spectrum,
and scarcely any lines of the different elements are alike; so that,
when the spectrum is properly magnified under telescopic observation
and the lines identified, we have the means of determining the
presence or absence of such elements in the vaporous constitution
of any incandescent body by examination of its spectrum. In this way
many of our terrestrial elements are found to exist in the sun,--so
many, in fact, that we know that the sun's nucleus, or core, must
be composed substantially of the same elements, the same sort of
matter, as exists on earth,--that we are, in fact, "a chip of the
old block." But it was found--and this is the real basis of spectrum
analysis--that if a certain metal or other element be burned in the
flame of an alcohol lamp, and a more brilliant flame of the same metal
or element burned in another lamp be observed through the first flame,
it will be seen that, "while the general illumination of the spectrum
is increased, the previous bright lines characterizing the element
are now replaced by dark lines or lines relatively very faint; in
a word, the spectrum characteristic of the given element is exactly
reversed" (Appleton's Cyclopædia, article "Spectrum Analysis"). We have
referred to this fact above in considering the origin of sun-spots,
showing that they are due to increased heat acting upon the core of
the sun so as to volatilize an abnormally large proportion of the
elements usually in a more condensed state upon the surface of the
solar body beneath its hydrogen envelope. These vapors, thus raised
in temperature, are driven upward by their volatilization into the
incandescent atmosphere of hydrogen, and the vaporous matters in the
higher strata thus produce the characteristic absorption bands of
these elements, while the overheated vapors, by a vast uprush from
beneath, hurl aside the more highly heated hydrogen above to appear
as faculæ around the sun-spot, the cooler upper layers of hydrogen
following downward the subsiding vaporous metallic uprush as it sinks
back beneath the photospheric level.
It is obvious that by similar spectrum analysis we may determine
to a large extent the constitution of the fixed stars and other
self-luminous bodies of space and interpret the phenomena which they
exhibit. We quote the following from the previously cited article
in Appleton's Cyclopædia, by Professor Proctor: "Spectroscopic
analysis applied to the stars has shown that they resemble the sun in
general constitution and condition. But characteristic differences
exist, insomuch that the stars have been divided into four orders
distinguished by their spectra. These are thus presented by Secchi,
who examined more than five hundred star spectra: The first type is
represented by Alpha Lyræ, Sirius, etc., and includes most of the
stars shining with a white light, as Altair, Regulus, Rigel, the
stars Beta, Gamma, Epsilon, Zeta, and Eta of Ursa Major, etc. These
give a spectrum showing all the seven colors, and crossed usually
by many lines, but always by the four lines of hydrogen, very dark
and strong. The breadth of these four lines indicates a very deep,
absorptive stratum at a high temperature and at great pressure. Nearly
half the stars observed by Secchi [more than two hundred out of five
hundred] showed this spectrum. The second type includes most of the
yellow stars, as Capella, Pollux, Arcturus, Aldebaran, Alpha of Ursa
Major, Procyon, etc. The Fraunhofer lines are well seen in the red
and blue, but not so well in the yellow. The resemblance of this
spectrum to the sun suggests that stars of this type resemble the
sun closely in physical constitution and condition. About one-third
of the stars observed by Secchi [more than one hundred and fifty
out of five hundred] showed this spectrum. The third type includes
Antares, Alpha of Orion, and Alpha of Hercules, Beta of Pegasus,
Mira, and most of the stars shining with a red light. The spectra
show bands of lines; according to Secchi, there are shaded bands,
but a more powerful spectroscope shows multitudes of fine lines. The
spectra resemble somewhat the spectrum of a sun-spot, and Secchi has
advanced the theory that these stars are covered in great part by
spots like those of the sun. About one hundred [out of five hundred]
of the observed stars belong to this type." (It should be noted that
the presence of sun-spots is no evidence of diminished heat in a sun;
see Professor Proctor in his "Myths and Marvels of Astronomy," article
"Suns in Flames:" "It may be noticed, in passing, that it is by no
means certain that the time when the sun is most spotted is the time
when he gives out least light.... All the evidence we have tends to
show that when the sun is most spotted his energies are most active. It
is then that the colored flames leap to their greatest height and show
their greatest brilliancy, then also that they show the most rapid and
remarkable changes of shape.") ... "The fourth type differs from the
preceding in the arrangement and appearance of the bands. It includes
only faint stars. A few stars, as Gamma of Cassiopeia, Eta of Argus,
Beta of Lyra, etc., show the lines of hydrogen bright instead of dark,
as though surrounded by hydrogen glowing with a heat more intense
than that of the central orb itself around which the hydrogen exists."
All the above five hundred stars reveal the presence of hydrogen under
precisely such conditions as conform to the general principle involved
in the source and mode of solar energy as herein stated. But a single
star (Betelgeuse) was observed by Huggins and Miller in England which
showed the lines of sodium, magnesium, iron, bismuth, and calcium,
"but found those of hydrogen wanting." Of the spectrum of this gas,
Professor Ball says, "The hydrogen spectrum appears to present a
simplicity not found in the spectrum of any other gas, and therefore
it is with great interest that we examine the spectra of the white
stars, in which the dark lines of hydrogen are unusually strong and
broad." Referring to the new star in the Northern Crown, which burst
forth in 1866, the same writer says, "The feature which made the
spectrum of the new star essentially distinct from that of any other
star that had been previously observed was the presence of certain
bright lines superposed on a spectrum with dark lines of one of the
ordinary types. The position of certain of these lines showed that
one of the luminous gases must be hydrogen." Of this particular star
(Betelgeuse) it is said (Proctor's "Familiar Essays"), "Red stars
and variable stars affect the neighborhood of the Milky Way or of
well-marked star-streams. The constellation Orion is singularly rich
in objects of this class. It is here that the strange 'variable'
Betelgeuse lies. At present this star shows no sign of variation,
but a few years ago it exhibited remarkable changes." We thus see
that Betelgeuse is a variable star, and it must have passed in its
different variations between the limits of extreme brilliancy, in which
the lines of hydrogen appear bright, and that of a less brilliant
stage, in which they appear dark,--that is, as absorption bands. It
has thus, in fact, run the gamut, so to speak, of color changes, and
now occupies an intermediate position in the scale. In his article
"Star unto Star," the same writer says, "On this view we may fairly
assume that the darkness of the hydrogen lines is a characteristic
of stars at a much higher temperature than our sun and suns of the
same class." We have already seen that the spectra of stars of the
fourth type--Appleton's Cyclopædia, "Spectrum Analysis"--"show the
lines of hydrogen bright instead of dark, as though surrounded by
hydrogen glowing with a heat more intense than that of the central
orb itself." Professor Dunkin says, in his work "The Midnight Sky,"
"One of the conclusions drawn by Kirchhoff from these experiments
is that each incandescent gas weakens, by absorption, rays of the
same degree of refrangibility as those it emits; or, in other words,
that the spectrum of each incandescent gas is reversed when this gas
is traversed by rays of the same refrangibility emanating from an
intensely luminous source which gives of itself a continuous spectrum
like that of the sun." ... "The third division, including Betelgeuse,
Antares, Alpha Herculis, and others of like color, seems to be
affected by something peculiar in their physical composition, as if
their photospheres contained a quantity of gas at a lower temperature
than usual. The stars in this class have generally a ruddy tint,
probably owing to their light having undergone some modification
while passing through an absorbing atmosphere.... A great number of
the stars in the third division are variable in their lustre." We may
therefore readily conclude that midway between the inverted lines which
constitute the dark absorption bands and the faint spectra which show
the bright lines of hydrogen direct there must be an atmosphere of
glowing hydrogen superposed upon a deeper one in such proportion that
it will not reveal its presence in the spectroscope at all; for when
the dark and light bands, which occupy precisely the same position in
the spectrum, are of approximately equal intensity the result will
obviously be the neutralization of both. That among a myriad suns,
some with dark hydrogen lines and some with bright, there should occur
occasionally an example corresponding to this point of divergence,
and especially among variable stars, is not only to be expected, but
is, in fact, confirmatory of the general hypothesis itself. It is an
exception which emphatically proves the rule, when we can trace the
operative cause which has produced it.