PLEASANT WAYS IN SCIENCE.
WORKS BY RICHARD A. PROCTOR.
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PLEASANT WAYS
IN SCIENCE
BY
RICHARD A. PROCTOR
AUTHOR OF
“ROUGH WAYS MADE SMOOTH,” “THE EXPANSE OF HEAVEN,” “OUR PLACE
AMONG INFINITIES,” “MYTHS AND MARVELS OF ASTRONOMY,”
ETC. ETC.
_NEW IMPRESSION_
LONGMANS, GREEN, AND CO.
39 PATERNOSTER ROW, LONDON
NEW YORK AND BOMBAY
1905
CONTENTS.
PAGE
OXYGEN IN THE SUN 1
SUN-SPOT, STORM, AND FAMINE 28
NEW WAYS OF MEASURING THE SUN’S DISTANCE 56
DRIFTING LIGHT WAVES 77
THE NEW STAR WHICH FADED INTO STAR-MIST 106
STAR-GROUPING, STAR-DRIFT, AND STAR-MIST 136
MALLET’S THEORY OF VOLCANOES 151
TOWARDS THE NORTH POLE 156
A MIGHTY SEA-WAVE 178
STRANGE SEA CREATURES 199
ON SOME MARVELS IN TELEGRAPHY 232
THE PHONOGRAPH, OR VOICE-RECORDER 274
THE GORILLA AND OTHER APES 296
THE USE AND ABUSE OF FOOD 330
OZONE 347
DEW 357
THE LEVELLING POWER OF RAIN 367
ANCIENT BABYLONIAN ASTROGONY 388
PREFACE.
It is very necessary that all who desire to become really proficient in
any department of science should follow the beaten track, toiling more
or less painfully over the difficult parts of the high road which is
their only trustworthy approach to the learning they desire to attain.
But there are many who wish to learn about scientific discoveries
without this special labour, for which some have, perhaps, little
taste, while many have scant leisure. My purpose in the present work,
as in my “Light Science for Leisure Hours,” the “Myths and Marvels of
Astronomy,” the “Borderland of Science,” and “Science Byways,” has been
to provide paths of easy access to the knowledge of some of the more
interesting discoveries, researches, or inquiries of the science of
the day. I wish it to be distinctly understood that my purpose is to
interest rather than to instruct, in the strict sense of the word. But
I may add that it seems to me even more necessary to be cautious, and
accurate in such a work as the present than in advanced treatises. For
in a scientific work the reasoning which accompanies the statements
of fact affords the means of testing and sometimes of correcting
such statements. In a work like the present, where explanation and
description take the place of reasoning, there is no such check. For
this reason I have been very careful in the accounts which I have given
of the subjects here dealt with. I have been particularly careful not
to present, as established truths, such views as are at present only
matters of opinion.
The essays in the present volume are taken chiefly from the
_Contemporary Review_, the _Gentleman’s Magazine_, the _Cornhill
Magazine_, _Belgravia_, and _Chambers’ Journal_. The sixth, however,
presents the substance (and official report) of a lecture which I
delivered at the Royal Institution in May, 1870. It was then that I
first publicly enunciated the views respecting the stellar universe
which I afterwards more fully stated in my “Universe of Stars.” The
same views have also been submitted to the Paris Academy of Science, as
the results of his own investigations, by M. Flammarion, in words which
read almost like translations of passages in the above-mentioned essay.
RICHARD A. PROCTOR.
PLEASANT WAYS IN SCIENCE.
_OXYGEN IN THE SUN._
The most promising result of solar research since Kirchhoff in 1859
interpreted the dark lines of the sun’s spectrum has recently been
announced from America. Interesting in itself, the discovery just made
is doubly interesting in what it seems to promise in the future. Just
as Kirchhoff’s great discovery, that a certain double dark line in the
solar spectrum is due to the vapour of sodium in the sun’s atmosphere,
was but the first of a long series of results which the spectroscopic
analysis of the sun was to reveal, so the discovery just announced that
a certain important gas—the oxygen present in our air and the chief
chemical constituent of water—shows its presence in the sun by bright
lines instead of dark, will in all probability turn out to be but the
firstfruits of a new method of examining the solar spectrum. As its
author, Dr. Henry Draper, of New York, remarks, further investigation
in the direction he has pursued will lead to the discovery of other
elements in the sun, but it was not “proper to conceal, for the sake
of personal advantage, the principle on which such researches are to
be conducted.” It may well happen, though I anticipate otherwise, that
by thus at once describing his method of observation, Dr. Draper may
enable others to add to the list of known solar elements some which
yet remain to be detected; but if Dr. Draper should thus have added but
one element to that list, he will ever be regarded as the physicist
to whose acumen the method was due by which all were detected, and to
whom, therefore, the chief credit of their discovery must certainly be
attributed.
I propose briefly to consider the circumstances which preceded the
great discovery which it is now my pleasing duty to describe, in order
that the reader may the more readily follow the remarks by which I
shall endeavour to indicate some of the results which seem to follow
from the discovery, as well as the line along which, in my opinion, the
new method may most hopefully be followed.
It is generally known that what is called the spectroscopic method
of analyzing the sun’s substance had its origin in Kirchhoff’s
interpretation of the dark lines in the solar spectrum. Until
1859 these dark lines had not been supposed to have any special
significance, or rather it had not been supposed that their
significance, whatever it might be, could be interpreted. A physicist
of some eminence spoke of these phenomena in 1858 in a tone which ought
by the way seldom to be adopted by the man of science. “The phenomena
defy, as we have seen,” he said, “all attempts hitherto to reduce them
within empirical laws, and no complete explanation or theory of them is
possible. All that theory can be expected to do is this—it may explain
how dark lines of any sort may arise within the spectrum.” Kirchhoff,
in 1859, showed not only how dark lines of any sort may appear, but how
and why they do appear, and precisely what they mean. He found that
the dark lines of the solar spectrum are due to the vapours of various
elements in the sun’s atmosphere, and that the nature of such elements
may be determined from the observed position of the dark lines. Thus
when iron is raised by the passage of the electric spark to so intense
a degree of heat that it is vaporized, the light of the glowing
vapour of iron is found to give a multitude of bright lines along the
whole length of the spectrum—that is, some red, some orange, some
yellow, and so on. In the solar spectrum corresponding dark lines are
found along the whole length of the spectrum—that is, some in the
red, some in the orange, yellow, etc., and precisely in those parts
of these various spectral regions which the bright lines of glowing
iron would occupy. Multitudes of other dark lines exist of course in
the solar spectrum. But those corresponding to the bright lines of
glowing iron are unquestionably there. They are by no means lost in
the multitude, as might be expected; but, owing to the peculiarity of
their arrangement, strength, etc., they are perfectly recognizable as
the iron lines reversed, that is, dark instead of bright. Kirchhoff’s
researches showed how this is to be interpreted. It means that the
vapour of iron exists in the atmosphere of the sun, glowing necessarily
with an intensely bright light; _but_, being cooler (however intensely
hot) than the general mass of the sun within, the iron vapour absorbs
more light than it emits, and the result is that the iron lines,
instead of appearing bright, as they would if the iron vapour alone
were shining, appear relatively dark on the bright rainbow-tinted
background of the solar spectrum.
Thus was it shown that in the atmosphere of the sun there is the
glowing vapour of the familiar metal, iron; and in like manner other
metals, and one element (hydrogen) which is not ordinarily regarded as
a metal, were shown to be present in the sun’s atmosphere. In saying
that they are present in the sun’s atmosphere, I am, in point of fact,
saying that they are present in the sun; for the solar atmosphere is,
in fact, the outer part of the sun himself, since a very large part,
if not by far the greater part, of the sun’s mass must be vaporous.
But no other elements, except the metals iron, sodium, barium,
calcium, magnesium, aluminium, manganese, chromium, cobalt, nickel,
zinc, copper, and titanium, and the element hydrogen, were shown
to be present in the sun, by this method of observing directly the
solar dark lines. In passing, I may note that there are reasons for
regarding hydrogen as a metallic element, strange though the idea may
seem to those who regard hardness, brightness, malleability, ductility,
plasticity, and the like, as the characteristic properties of metals,
and necessarily fail to comprehend how a gas far rarer, under the
same conditions, than the air we breathe, and which cannot possibly
be malleable, ductile, or the like, can conceivably be regarded as a
metal. But there is in reality no necessary connection between any one
of the above properties and the metallic nature; many of the fifty-five
metals are wanting in all of these properties; nor is there any reason
why, as we have in mercury a metal which at ordinary temperatures is a
liquid, so we might have in hydrogen a metal which, at all obtainable
temperatures, and under all obtainable conditions of pressure, is
gaseous. It was shown by the late Professor Graham (aided in his
researches most effectively by Dr. Chandler Roberts) that hydrogen will
enter into such combination with the metal palladium that it may be
regarded as forming, for the time, with the palladium, an alloy; and
as alloys can only be regarded as compounds of two or more metals, the
inference is that hydrogen is in reality a metallic element.
Fourteen only of the elements known to us, or less than a quarter
of the total number, were thus found to be present in the sun’s
constitution; and of these all were metals, if we regard hydrogen as
metallic. Neither gold nor silver shows any trace of its presence,
nor can any sign be seen of platinum, lead, and mercury. But, most
remarkable of all, and most perplexing, was the absence of all trace
of oxygen and nitrogen, two gases which could not be supposed wanting
in the substance of the great ruling centre of the planetary system.
It might well be believed, indeed, that none of the five metals just
named are absent from the sun, and indeed that every one of the
forty metals not recognized by the spectroscopic method nevertheless
exists in the sun. For according to the nebular hypothesis of the
origin of our solar system, the sun might be expected to contain all
the elements which exist in our earth. Some of these elements might
indeed escape discovery, because existing only in small quantities;
and others (as platinum, gold, and lead, for example), because but a
small portion of their vaporous substance rose above the level of that
glowing surface which is called the photosphere. But that oxygen, which
constitutes so large a portion of the solid, liquid, and vaporous mass
of our earth, should not exist in enormous quantities, and its presence
be very readily discernable, seemed amazing indeed. Nitrogen, also,
might well be expected to be recognizable in the sun. Carbon, again,
is so important a constituent of the earth, that we should expect to
discover clear traces of its existence in the sun. In less degree,
similar considerations apply to sulphur, boron, silicon, and the other
non-metallic elements.
It was not supposed, however, by any one at all competent to form an
opinion on the subject, that oxygen, nitrogen, and carbon are absent
from the sun. It was perceived that an element might exist in enormous
quantities in the substance of the sun, and yet fail to give any
evidence of its presence, or only give such evidence as might readily
escape recognition. If we remember how the dark lines are really
caused, we shall perceive that this is so. A glowing vapour in the
atmosphere of the sun absorbs rays of the same colour as it emits. If
then, it is cooler than the glowing mass of the sun which it enwraps,
and if, notwithstanding the heat received from this mass, it remains
cooler, then it suffers none of those rays to pass earthwards.[1] It
emits rays of the same kind (that is, of the same _colour_) itself,
but, being cooler, the rays thus coming from it are feebler; or, to
speak more correctly, the ethereal waves thus originated are feebler
than those of the same order which _would_ have travelled earthwards
from the sun but for the interposed screen of vapour. Hence the
corresponding parts of the solar spectrum are less brilliant, and
contrasted with the rainbow-tinted streak of light, on which they lie
as on a background, they appear dark.
In order, then, that any element may be detected by its dark lines, it
is necessary that it should lie as a vaporous screen between the more
intensely heated mass of the sun and the eye of the observer on earth.
It must then form an enclosing envelope cooler than the sun within it.
Or rather, some part of the vapour must be thus situated. For enormous
masses of the vapour might be within the photospheric surface of the
sun at a much higher temperature, which yet, being enclosed in the
cooler vaporous shell of the same substance, would not be able to send
its light rays earthwards. One may compare the state of things, so
far as that particular element is concerned, to what is presented in
the case of a metallic globe cooled on the outside but intensely hot
within. The cool outside of such a globe is what determines the light
and heat received from it, so long as the more heated mass within has
not yet (by conduction) warmed the exterior shell. So in the case of
a vapour permeating the entire mass, perhaps, of the sun, and at as
high a temperature as the sun everywhere except on the outside: it is
the temperature of the outermost part of such a vaporous mass which
determines the intensity of the rays received from it—or in other
words, determines whether the corresponding parts of the spectrum shall
be darker or not than the rest of the spectrum. If the vapour does
not rise above the photosphere of the sun in sufficient quantity to
exercise a recognizable absorptive effect, its presence in the sun will
not be indicated by any dark lines.
I dwell here on the question of quantity, which is sometimes overlooked
in considering the spectroscopic evidence of the sun’s condition, but
is in reality a very important factor in determining the nature of the
evidence relating to each element in the solar mass. In some cases,
the quantity of a material necessary to give unmistakable spectroscopic
evidence is singularly small; insomuch that new elements, as thallium,
cæsium, rubidium, and gallium, have been actually first recognized
by their spectral lines when existing in such minute quantities in
the substances examined as to give no other trace whatever of their
existence. But it would be altogether a mistake to suppose that some
element existing in exceedingly small quantities, or, more correctly,
existing in the form of an exceedingly rare vapour in the sun’s
atmosphere, would be detected by means of its dark lines, or _by any
other method depending on the study of the solar spectrum_. When we
place a small portion of some substance in the space between the carbon
points of an electric lamp, and volatilize that substance in the
voltaic arc, we obtain a spectrum including all the bright lines of the
various elements contained in the substance; and if some element is
contained in it in exceedingly small quantity, we may yet perceive its
distinctive bright lines among the others (many of them far brighter)
belonging to the elements present in greater quantities. But if we have
(for example) a great mass of molten iron, the rainbow-tinted spectrum
of whose light we examine from a great distance, and if a small
quantity of sodium, or other substance which vaporizes at moderate
temperatures, be cast into the molten iron so that the vapour of the
added element presently rises above the glowing surface of the iron,
no trace of the presence of this vapour would be shown in the spectrum
observed from a distance. The part of the spectrum where the dark lines
of sodium usually appear would, undoubtedly, be less brilliant than
before, in the same sense that the sun may be said to be less brilliant
when the air is in the least degree moist than when it is perfectly
dry; but the loss of brilliancy is as utterly imperceptible in the one
case as it is in the other. In like manner, a vapour might exist in
the atmosphere of the sun (above the photosphere, that is), of whose
presence not a trace would be afforded in the spectroscope, for the
simple reason that the absorptive action of the vapour, though exerted
to reduce the brightness of particular solar rays or tints, would not
affect those rays sufficiently for the spectroscopist to recognize any
diminution of their lustre.
There is another consideration, which, so far as I know, has not
hitherto received much attention, but should certainly be taken into
account in the attempt to interpret the real meaning of the solar
spectrum. Some of the metals which are vaporized by the sun’s heat
below the photosphere may become liquid or even solid at or near the
level of the photosphere. Even though the heat at the level of the
photosphere may be such that, under ordinary conditions of pressure and
so forth, such metals would be vaporous, the enormous pressure which
must exist not far below the level of the photosphere may make the
heat necessary for complete vaporization far greater than the actual
heat at that level. In that case the vapour will in part condense into
liquid globules, or, if the heat is considerably less than is necessary
to keep the substance in the form of vapour, then it may in part be
solidified, the tiny globules of liquid metal becoming tiny crystals
of solid metal. We see both conditions fulfilled within the limits of
our own air in the case of the vapour of water. Low down the water
is present in the air (ordinarily) in the form of pure vapour; at a
higher level the vapour is condensed by cold into liquid drops forming
visible clouds (cumulus clouds), and yet higher, where the cold is
still greater, the minute water-drops turn into ice-crystals, forming
those light fleecy clouds called cirrus clouds by the meteorologist.
Now true clouds of either sort may exist in the solar atmosphere even
above that photospheric level which forms the boundary of the sun we
see. It may be said that the spectroscope, applied to examine matter
outside the photosphere, has given evidence only of vaporous cloud
masses. The ruddy prominences which tower tens of thousands of miles
above the surface of the sun, and the sierra (or as it is sometimes
unclassically called, the chromosphere) which covers usually the
whole of the photosphere to a depth of about eight thousand miles,
show only, under spectroscopic scrutiny, the bright lines indicating
gaseity. But though this is perfectly true, it is also true that we
have not here a particle of evidence to show that clouds of liquid
particles, and of tiny crystals, may not float over the sun’s surface,
or even that the ruddy clouds shown by the spectroscope to shine with
light indicative of gaseity may not also contain liquid and crystalline
particles. For in point of fact, the very principle on which our
recognition of the bright lines depends involves the inference that
matter whose light would _not_ be resolved into bright lines would
not be recognizable at all. The bright lines are seen, because by
means of a spectroscope we can throw them far apart, without reducing
their lustre, while the background of rainbow-tinted spectrum has its
various portions similarly thrown further apart and correspondingly
weakened. One may compare the process (the comparison, I believe, has
not hitherto been employed) to the dilution of a dense liquid in which
solid masses have been floating: the more we increase the quantity of
the liquid in diluting it with water, the more transparent it becomes,
but the solid masses in it are not changed, so that we only have to
dilute the liquid sufficiently to see these masses. _But_ if there were
in the interstices of the solid masses particles of some substance
which dissolved in the water, we should not recognize the presence of
this substance by any increase in its visibility; for the very same
process which thinned the liquid would thin this soluble substance in
the same degree. In like manner, by dispersing and correspondingly
weakening the sun’s light more and more, we can recognize the light of
the gaseous matter in the prominences, for this is not weakened; but if
the prominences also contain matter in the solid or liquid form (that
is, drops or crystals), the spectroscopic method will not indicate the
presence of such matter, for the spectrum of matter of this sort will
be weakened by dispersion in precisely the same degree that the solar
spectrum itself is weakened.
It is easy to see how the evidence of the presence of any element
which behaved in this way would be weakened, if we consider what would
happen in the case of our own earth, according as the air were simply
moist but without clouds, or loaded with cumulus masses but without
cirrus clouds, or loaded with cirrus clouds. For although there is not
in the case of the earth a central glowing mass like the sun’s, on
whose rainbow-tinted spectrum the dark lines caused by the absorptive
action of our atmosphere could be seen by the inhabitant of some
distant planet studying the earth from without, yet the sun’s light
reflected from the surface of the earth plays in reality a similar
part. It does not give a simple rainbow-tinted spectrum; for, being
sunlight, it shows all the dark lines of the solar spectrum: but the
addition of new dark lines to these, in consequence of the absorptive
action of the earth’s atmosphere, could very readily be determined. In
fact, we do thus recognize in the spectra of Mars, Venus, and other
planets, the presence of aqueous vapour in their atmosphere, despite
the fact that our own air, containing also aqueous vapour, naturally
renders so much the more difficult the detection of that vapour in the
atmosphere of remote planets necessarily seen through our own air.
Now, a distant observer examining the light of our own earth on a day
when, though the air was moist, there were no clouds, would have ample
evidence of the presence of the vapour of water; for the light which he
examined would have gone twice through our earth’s atmosphere, from its
outermost thinnest parts to the densest layers close to the surface,
then back again through the entire thickness of the air. But if the
air were heavily laden with cumulus clouds (without any cirrus clouds
at a higher layer), although _we_ should know that there was abundant
moisture in the air, and indeed much more moisture then there had been
when there had been no clouds, our imagined observer would either
perceive no traces at all of this moisture, or he would perceive traces
so much fainter than when the air was clear that he would be apt to
infer that the air was either quite dry, or at least very much drier
than it had been in that case. For the light which he would receive
from the earth would not in this case have passed through the entire
depth of moisture-laden air twice, but twice only through that portion
of the air which lay above the clouds, at whose surface the sun’s light
would be reflected. The whole of the moisture-laden layer of the air
would be snugly concealed under the cloud-layer, and would exercise
no absorptive action whatever on the light which the remote observer
would examine. If from the upper surface of the layer of cumulus clouds
aqueous vapour rose still higher, and were converted in the cold upper
regions of the atmosphere into clouds of ice-crystals, the distant
observer would have still less chance of recognizing the presence of
moisture in our atmosphere. For the layer of air between the cumulus
clouds and the cirrus clouds would be unable to exert any absorptive
action on the light which reached the observer. All such light would
come to him after reflection from the layer of cirrus clouds. He would
be apt to infer that there was no moisture at all in the air of our
planet, at the very time when in fact there was so much moisture that
not one layer only, but two layers of clouds enveloped the earth, the
innermost layer consisting of particles of liquid water, the outermost
of particles of frozen water. Using the words ice, water, and steam,
to represent the solid, liquid, and vaporous states of water, we may
fairly say that ice and water, by hiding steam, would persuade the
remote observer that there was no water at all on the earth—at least
if he trusted solely to the spectroscopic evidence then obtained.[2]
We might in like manner fail to obtain any spectroscopic evidence of
the presence of particular elements in the sun, because they do not
exist in sufficient quantity in the vaporous form in those outer layers
which the spectroscope can alone deal with.
In passing, I must note a circumstance in which some of those who have
dealt with this special part of the spectroscopic evidence have erred.
It is true in one sense that some elements may be of such a nature that
their vapours cannot rise so high in the solar atmosphere as those of
other elements. But it must not be supposed that the denser vapours
seek a lower level, the lighter vapours rising higher. According to
the known laws of gaseous diffusion, a gas or vapour diffuses itself
throughout a space occupied by another gas or several other gases,
in the same way as though the space were not occupied at all. If we
introduce into a vessel full of common air a quantity of carbonic
acid gas (I follow the older and more familiar nomenclature), this
gas, although of much higher specific gravity than either oxygen or
nitrogen, does not take its place at the bottom of the vessel, but so
diffuses itself that the air of the upper part of the vessel contains
exactly the same quantity of carbonic acid gas as the air of the
lower part. Similarly, if hydrogen is introduced, it does not seek
the upper part of the vessel, but diffuses itself uniformly throughout
the vessel. If we enclose the carbonic acid gas in a light silken
covering, and the hydrogen in another (at the same pressure as the air
in the vessel) one little balloon will sink and the other will rise;
but this is simply because diffusion is prevented. It may be asked
how this agrees with what I have said above, that some elements may
not exist in sufficient quantity or in suitable condition above the
sun’s photospheric level to give any spectroscope evidence of their
nature. As to quantity, indeed, the answer is obvious: if there is only
a small quantity of any given element in the entire mass of the sun,
only a very small quantity can under any circumstances exist outside
the photosphere. As regards condition, it must be remembered that the
vessel of my illustrative case was supposed to contain air at a given
temperature and pressure throughout. If the vessel was so large that
in different parts of it the temperature and pressure were different,
the diffusion would, indeed, still be perfect, because at all ordinary
temperatures and pressures hydrogen and carbonic acid gas remain
gaseous. But if the vapour introduced is of such a nature that at
moderate temperatures and pressures it condenses, wholly or in part, or
liquefies, the diffusion will not take place with the same uniformity.
We need not go further for illustration than to the case of our own
atmosphere as it actually exists. The vapour of water spreads uniformly
through each layer of the atmosphere which is at such a temperature
and pressure as to permit of such diffusion; but where the temperature
is too low for complete diffusion (at the actual pressure) the aqueous
vapour is condensed into visible cloud, diffusion being checked at
this point as at an impassable boundary. In the case of the sun, as
in the case of our own earth, it is not the density of an element
when in a vaporous form which limits its diffusion, but the value
of the temperature at which its vapour at given pressure condenses
into liquid particles. It is in this way only that any separation can
be effected between the various elements which exist in the sun’s
substance. A separation of this sort is unquestionably competent to
modify the spectroscopic evidence respecting different elements. But
it would be a mistake to suppose that any such separation could occur
as has been imagined by some—a separation causing in remote times
the planets supposed to have been thrown off by the sun to be rarest
on the outskirts of the solar system and densest close to the sun.
The small densities of the outer family of planets, as compared with
the densities of the so-called terrestrial planets, must certainly be
otherwise explained.
But undoubtedly the chief circumstance likely to operate in veiling
the existence of important constituents of the solar mass must be that
which has so long prevented spectroscopists from detecting the presence
of oxygen in the sun. An element may exist in such a condition, either
over particular parts of the photosphere, or over the entire surface
of the sun, that instead of causing dark lines in the solar spectrum
it may produce bright lines. Such lines may be conspicuous, or they
may be so little brighter than the background of the spectrum as to be
scarcely perceptible or quite imperceptible.
In passing, I would notice that this interpretation of the want of all
spectroscopic evidence of the presence of oxygen, carbon, and other
elements in the sun, is not an _ex post facto_ explanation. As will
presently appear, it is now absolutely certain that oxygen, though
really existing, and doubtless, in enormous quantities, in the sun, has
been concealed from recognition in this way. But that this might be so
was perceived long ago. I myself, in the first edition of my treatise
on “The Sun,” pointed out, in 1870, with special reference to nitrogen
and oxygen, that an element “may be in a condition enabling it to
radiate as much light as it absorbs, or else very little more or very
little less; so that it either obliterates all signs of its existence,
or else gives lines so little brighter or darker than the surrounding
parts of the spectrum that we can detect no trace of its existence.”
I had still earlier given a similar explanation of the absence of all
spectroscopic evidence of hydrogen in the case of the bright star
Betelgeux.[3]
Let us more closely consider the significance of what we learn from
the spectral evidence respecting the gas hydrogen. We know that when
the total light of the sun is dealt with, the presence of hydrogen is
constantly indicated by dark lines. In other words, regarding the sun
as a whole, hydrogen constantly reduces the emission of rays of those
special tints which correspond to the light of this element. When we
examine the light of other suns than ours, we find that in many cases,
probably in by far the greater number of cases, hydrogen acts a similar
part. But not in every case. In the spectra of some stars, notably in
those of Betelgeux and Alpha Herculis, the lines of hydrogen are not
visible at all; while in yet others, as Gamma Cassiopeiæ, the middle
star of the five which form the straggling W of this constellation,
the lines of hydrogen show bright upon the relatively dark background
of the spectrum. When we examine closely the sun himself, we find that
although his light as a whole gives a spectrum in which the lines of
hydrogen appear dark, the light of particular parts of his surface,
if separately examined, occasionally shows the hydrogen lines bright
as in the spectrum of Gamma Cassiopeiæ, while sometimes the light of
particular parts gives, like the light of Betelgeux, no spectroscopic
evidence whatever of the presence of hydrogen. Manifestly, if the whole
surface of the sun were in the condition of the portions which give
bright hydrogen lines, the spectrum of the sun would resemble that of
Gamma Cassiopeiæ; while if the whole surface were in the condition of
those parts which show no lines of hydrogen, the spectrum of the sun
would resemble that of Betelgeux. Now if there were any reason for
supposing that the parts of the sun which give no lines of hydrogen
are those from which the hydrogen has been temporarily removed in some
way, we might reasonably infer that in the stars whose spectra show no
hydrogen lines there is no hydrogen. But the fact that the hydrogen
lines are sometimes seen bright renders this supposition untenable.
For we cannot suppose that the lines of hydrogen change from dark
to bright or from bright to dark (both which changes certainly take
place) without passing through a stage in which they are neither bright
nor dark; in other words, we are compelled to assume that there is
an intermediate condition in which the hydrogen lines, though really
existent, are invisible because they are of precisely the same lustre
as the adjacent parts of the spectrum. Hence the evanescence of the
hydrogen lines affords no reason for supposing that hydrogen has become
even reduced in quantity where the lines are not seen. And therefore
it follows that the invisibility of the hydrogen lines in the spectrum
of Betelgeux is no proof that hydrogen does not exist in that star in
quantities resembling those in which it is present in the sun. And
this, being demonstrated in the case of one gas, must be regarded as
at least probable in the case of other gases. Wherefore the absence of
the lines of oxygen from the spectrum of any star affords no sufficient
reason for believing that oxygen is not present in that star, or that
it may not be as plentifully present as hydrogen, or even far more
plentifully present.
There are other considerations which have to be taken into account, as
well in dealing with the difficulty arising from the absence of the
lines of particular elements from the solar spectrum as in weighing the
extremely important discovery which has just been effected by Dr. H.
Draper.
I would specially call attention now to a point which I thus presented
seven years ago:—“The great difficulty of interpreting the results
of the spectroscopic analysis of the sun arises from the circumstance
that we have no means of learning whence that part of the light comes
which gives the continuous spectrum. When we recognize certain dark
lines, we know certainly that the corresponding element exists in the
gaseous form at a lower temperature than the substance which gives the
continuous spectrum. But as regards that continuous spectrum itself
we can form no such exact opinion.” It might, for instance, have
its origin in glowing liquid or solid matter; but it might also be
compounded of many spectra, each containing a large number of bands,
the bands of one spectrum filling up the spaces which would be left
dark between the bands of another spectrum, and so on until the entire
range from the extreme visible red to the extreme visible violet were
occupied by what appeared as a continuous rainbow-tinted streak. “We
have, in fact, in the sun,” as I pointed out, “a vast agglomeration
of elements, subject to two giant influences, producing in some sort
opposing effects—viz., a temperature far surpassing any we can form
any conception of, and a pressure (throughout nearly the whole of
the sun’s globe) which is perhaps even more disproportionate to the
phenomena of our experience. Each known element would be vaporized by
the solar temperature at known pressures; each (there can be little
question) would be solidified by the vast pressures, did these arise
at known temperatures. Now whether, under these circumstances, the
laws of gaseous diffusion prevail where the elements _are_ gaseous in
the solar globe; whether, where liquid matter exists it is in general
bounded in a definite manner from the neighbouring gaseous matter;
whether any elements at all are solid, and if so under what conditions
their solidity is maintained and the limits of the solid matter
defined—all these are questions which _must_ be answered before we
can form a satisfactory idea of the solar constitution; yet they are
questions which we have at present no means of answering.” Again, we
require to know whether any process resembling combustion can under
any circumstances take place in the sun’s globe. If we could assume
that some general resemblance exists between the processes at work upon
the sun and those we are acquainted with, we might imagine that the
various elements ordinarily exist in the sun’s globe in the gaseous
form (chiefly) to certain levels, to others chiefly in the liquid form,
and to yet others chiefly in the solid form. But even then that part
of each element which is gaseous may exist in two forms, having widely
different spectra (in reality in five, but I consider only the extreme
forms). The light of one part is capable of giving characteristic
spectra of lines or bands (which will be different according to
pressure and may appear either dark or bright); that of the other is
capable of giving a spectrum nearly or quite continuous.
It will be seen that Dr. H. Draper’s discovery supplies an answer to
one of the questions, or rather to one of the sets of questions, thus
indicated. I give his discovery as far as possible in his own words.
“_Oxygen discloses itself_,” he says, “_by bright lines or bands in
the solar spectrum_, and does not give dark absorption-lines like the
metals. We must therefore change our theory of the solar spectrum,
and no longer regard it merely as a continuous spectrum with certain
rays absorbed by a layer of ignited metallic vapours, but as having
also bright lines and bands superposed on the background of continuous
spectrum. Such a conception not only opens the way to the discovery
of others of the non-metals, sulphur, phosphorus, selenium, chlorine,
bromine, iodine, fluorine, carbon, etc., but also may account for some
of the so-called dark lines, by regarding them as intervals between
bright lines. It must be distinctly understood that in speaking of the
solar spectrum here, I do not mean the spectrum of any limited area
upon the disc or margin of the sun, but the spectrum of light from the
whole disc.”
In support of the important statement here advanced, Dr. Draper submits
a photograph of part of the solar spectrum with a comparison spectrum
of air, and also with some of the lines of iron and aluminium. The
photograph itself, a copy of which, kindly sent to me by Dr. Draper,
lies before me as I write, fully bears out Dr. Draper’s statement. It
is absolutely free from handwork or retouching, except that reference
letters have been added in the negative. It shows the part of the solar
spectrum between the well-known Fraunhofer lines G and H, of which G
(an iron line) lies in the indigo, and H (a line of hydrogen) in the
violet, so that the portion photographed belongs to that region of
the spectrum whose chemical or actinic energy is strongest. Adjacent
to this lies the photograph of the air lines, showing nine or ten
well-defined oxygen lines or groups of lines, and two nitrogen bands.
The exact agreement of the two spectra in position is indicated by
the coincidence of bright lines of iron and aluminium included in the
air spectrum with the dark lines of the same elements in the solar
spectrum. “No close observation,” as Dr. Draper truly remarks, “is
needed to demonstrate to even the most casual observer” (of this
photograph) “that the oxygen lines are found in the sun as bright
lines.” There is in particular one quadruple group of oxygen lines in
the air spectrum, the coincidence of which with a group of bright lines
in the solar spectrum is unmistakable.
“This oxygen group alone is almost sufficient,” says Dr. Draper, “to
prove the presence of oxygen in the sun, for not only does each of
the four components have a representative in the solar group, but the
relative strength and the general aspect of the lines in each case is
similar.[4] I shall not attempt at this time,” he proceeds, “to give
a complete list of the oxygen lines, ... and it will be noticed that
some lines in the air spectrum which have bright anologues in the sun
are not marked with the symbol of oxygen. This is because there has not
yet been an opportunity to make the necessary detailed comparisons. In
order to be certain that a line belongs to oxygen, I have compared,
under various pressures, the spectra of air, oxygen, nitrogen, carbonic
acid, carburetted hydrogen, hydrogen, and cyanogen.
“As to the spectrum of nitrogen and the existence of this element in
the sun there is not yet certainty. Nevertheless, even by comparing
the diffused nitrogen lines of this particular photograph, in which
nitrogen has been sacrificed to get the best effect for oxygen, the
character of the evidence appears. There is a triple band somewhat
diffused in the photograph belonging to nitrogen, which has its
appropriate representative in the solar spectrum, and another band of
nitrogen is similarly represented.” Dr. Draper states that “in another
photograph a heavy nitrogen line which in the present one lies opposite
an insufficiently exposed part of the solar spectrum, corresponds to a
comparison band in the sun.”
But one of the most remarkable points in Dr. Draper’s paper is what
he tells us respecting the visibility of these lines in the spectrum
itself. They fall, as I have mentioned, in a part of the spectrum where
the actinic energy is great but the luminosity small; in fact, while
this part of the spectrum is the very strongest for photography, it is
close to the region of the visible spectrum,
“Where the last gleamings of refracted light
Die in the fainting violet away.”
It is therefore to be expected that those, if any, of the bright lines
of oxygen, will be least favourably shown for direct vision, and most
favourably for what might almost be called photographic vision, where
we see what photography records for us. Yet Dr. Draper states that
these bright lines of oxygen can be readily seen. “The bright lines
of oxygen in the spectrum of the solar disc have not been hitherto
perceived, probably from the fact that in eye-observation bright lines
on a less bright background do not make the impression on the mind that
dark lines do. When attention is called to their presence they are
readily enough seen, even without the aid of a reference spectrum. The
photograph, however, brings them into greater prominence.” As the lines
of oxygen are not confined to the indigo and violet, we may fairly hope
that the bright lines in other parts of the spectrum of oxygen may be
detected in the spectrum of the sun, now that spectroscopists know that
bright lines and not dark lines are to be looked for.