_TEMPERATURE OF THE SUN_
Newton was the first who attempted to measure the quantity of heat
received by the earth from the sun. His object in making the experiment
was to ascertain the temperature encountered by the comet of 1680 at its
passage through perihelion. He found it, by multiplying the observed
heating effects of direct sunshine according to the familiar rule of the
"inverse squares of the distances," to be about 2,000 times that of
red-hot iron.[698]
Determinations of the sun's thermal power, made with some scientific
exactness, date, however, from 1837. A few days previous to the
beginning of that year, Herschel began observing at the Cape of Good
Hope with an "actinometer," and obtained results agreeing quite
satisfactorily with those derived by Pouillet from experiments made in
France some months later with a "pyrheliometer."[699] Pouillet found
that the vertical rays of the sun falling on each square centimetre of
the earth's surface are competent (apart from atmospheric absorption) to
raise the temperature of 1·7633 grammes of water one degree Centigrade
per minute. This number (1·7633) he called the "solar constant"; and the
unit of heat chosen is known as the "calorie." Hence it was computed
that the total amount of solar heat received during a year would suffice
to melt a layer of ice covering the entire earth to a depth of 30·89
metres, or 100 feet; while the heat emitted would melt, at the sun's
surface, a stratum 11·80 metres thick each minute. A careful series of
observations showed that nearly half the heat incident upon our
atmosphere is stopped in its passage through it.
Herschel got somewhat larger figures, though he assigned only a third as
the spoil of the air. Taking a mean between his own and Pouillet's, he
calculated that the ordinary expenditure of the sun per minute would
have power to melt a cylinder of ice 184 feet in diameter, reaching from
his surface to that of Alpha Centauri; or, putting it otherwise,
that an ice-rod 45·3 miles across, continually darted into the sun with
the velocity of light, would scarcely consume, in dissolving, the
thermal supplies now poured abroad into space.[700] It is nearly certain
that this estimate should be increased by about two-thirds in order to
bring it up to the truth.
Nothing would, at first sight, appear simpler than to pass from a
knowledge of solar emission--a strictly measurable quantity--to a
knowledge of the solar temperature; this being defined as the
temperature to which a surface thickly coated with lamp-black (that is,
of standard radiating power) should be raised to enable it to send us,
from the sun's distance, the amount of heat actually received from the
sun. Sir John Herschel showed that heat-rays at the sun's surface must
be 92,000 times as dense as when they reach the earth; but it by no
means follows that either the surface emitting, or a body absorbing
those heat-rays must be 92,000 times hotter than a body exposed here to
the full power of the sun. The reason is, that the rate of
emission--consequently the rate of absorption, which is its
correlative--increases very much faster than the temperature. In other
words, a body radiates or cools at a continually accelerated pace as it
becomes more and more intensely heated above its surroundings.
Newton, however, took it for granted that radiation and temperature
advance _pari passu_--that you have only to ascertain the quantity of
heat received from, and the distance of a remote body in order to know
how hot it is.[701] And the validity of this principle, known as
"Newton's Law" of cooling, was never questioned until De la Roche
pointed out, in 1812,[702] that it was approximately true only over a
low range of temperature; while five years later, Dulong and Petit
generalised experimental results into the rule, that while temperature
grows by arithmetical, radiation increases by geometrical
progression.[703] Adopting this formula, Pouillet derived from his
observations on solar heat a solar temperature of somewhere between
1,461° and 1,761° C. Now, the higher of these points--which is nearly
that of melting platinum--is undoubtedly surpassed at the focus of
certain burning-glasses which have been constructed of such power as
virtually to bring objects placed there within a quarter of a million of
miles of the photosphere. In the rays thus concentrated, platinum and
diamond become rapidly vaporised, notwithstanding the great loss of heat
by absorption, first in passing through the air, and again in traversing
the lens. Pouillet's maximum is then manifestly too low, since it
involves the absurdity of supposing a radiating mass capable of heating
a distant body more than it is itself heated.
Less demonstrably, but scarcely less surely, Mr. J. J. Waterston, who
attacked the problem in 1860, erred in the opposite direction. Working
up, on Newton's principle, data collected by himself in India and at
Edinburgh, he got for the "potential temperature" of the sun 12,880,000°
Fahr.,[704] equivalent to 7,156,000° C. The phrase _potential
temperature_ (for which Violle substituted, in 1876, _effective
temperature_) was designed to express the accumulation in a single
surface, postulated for the sake of simplicity, of the radiations not
improbably received from a multitude of separate solar layers
reinforcing each other; and might thus (it was explained) be
considerably higher than the _actual_ temperature of any one stratum.
At Rome, in 1861, Father Secchi repeated Waterston's experiments, and
reaffirmed his conclusion;[705] while Soret's observations, made on the
summit of Mont Blanc in 1867,[706] furnished him with materials for a
fresh and even higher estimate of ten million degrees Centigrade.[707]
Yet from the very same data, substituting Dulong and Petit's for
Newton's law, Vicaire deduced in 1872 a _provisional_ solar temperature
of 1,398°.[708] This is below that at which iron melts, and we know that
iron-vapour exists high up in the sun's atmosphere. The matter was taken
into consideration on the other side of the Atlantic by Ericsson in
1871. He attempted to re-establish the shaken credit of Newton's
principle, and arrived, by its means, at a temperature of 4,000,000°
Fahrenheit.[709] Subsequently, an "underrated computation," based upon
observation of the quantity of heat received by his "sun motor," gave
him 3,000,000°. And the result, as he insisted, followed inevitably from
the principle that the temperature produced by radiant heat is
proportional to its density, or inversely as its diffusion.[710] The
principle, however, is demonstrably unsound.
In 1876 the sun's temperature was proposed as the subject of a prize by
the Paris Academy of Sciences; but although the essay of M. Jules Violle
was crowned, the problem was declared to remain unsolved. Violle (who
adhered to Dulong and Petit's formula) arrived at an _effective_
temperature of 1,500° C., but considered that it might _actually_ reach
2,500° C., if the emissive power of the photospheric clouds fell far
short (as seemed probable) of the lamp-black standard.[711] Experiments
made in April and May, 1881, giving a somewhat higher result, he raised
this figure to 3,000° C.[712]
Appraisements so outrageously discordant as those of Waterston, Secchi,
and Ericsson on the one hand, and those of the French _savants_ on the
other, served only to show that all were based upon a vicious principle.
Professor F. Rosetti,[713] accordingly, of the Paduan University, at
last perceived the necessity for getting out of the groove of "laws"
plainly in contradiction with facts. The temperature, for instance, of
the oxy-hydrogen flame was fixed by Bunsen at 2,800° C.--an estimate
certainly not very far from the truth. But if the two systems of
measurement applied to the sun be used to determine the heat of a solid
body rendered incandescent in this flame, it comes out, by Newton's mode
of calculation, 45,000° C.; by Dulong and Petit's, 870° C.[714] Both,
then, are justly discarded, the first as convicted of exaggeration, the
second of undervaluation. The formula substituted by Rosetti in 1878 was
tested successfully up to 2,000° C.; but since, like its predecessors,
it was a purely empirical rule, guaranteed by no principle, and hence
not to be trusted out of sight, it was, like them, liable to break down
at still higher elevations. Radiation by this new prescription increases
as the _square_ of the _absolute_ temperature--that is, of the number of
degrees counted from the "absolute zero" of -273° C. Its employment gave
for the sun's radiating surface an effective temperature of 20,380° C.
(including a supposed loss of one-half in the solar atmosphere); and
setting a probable deficiency in emission (as compared with lamp-black)
against a probable mutual reinforcement of superposed strata, Professor
Rosetti considered "effective" as nearly equivalent to "actual"
temperature. A "law of cooling," proposed by M. Stefan at Vienna in
1879,[715] was shown by Boltzmann, many years later, to have a certain
theoretical validity.[716] It is that emission grows as the fourth power
of absolute temperature. Hence the temperature of the photosphere would
be proportional to the square root of the square root of its heating
effects at a distance, and appeared, by Stefan's calculations from
Violle's measures of solar radiative intensity, to be just 6,000° C.;
while M. H. Le Chatelier[717] derived 7,600° from a formula, conveying
an intricate and unaccountable relation between the temperature of an
incandescent body and the intensity of its red radiations.
From a series of experiments carefully conducted at Daramona, Ireland,
with a delicate thermal balance, of the kind invented by Boys and
designated a "radio-micrometer," Messrs. Wilson and Gray arrived in
1893, with the aid of Stefan's Law, at a photospheric temperature of
7,400° C.,[718] reduced by the first-named investigator in 1901 to
6,590°.[719] Dr. Paschen, of Hanover, on the other hand, ascribed to the
sun a temperature of 5,000° from comparisons between solar radiative
intensity and that of glowing platinum;[720] while F. W. Very showed in
1895[721] that a minimum value of 20,000° C. for the same datum resulted
from Paschen's formula connecting temperature with the position of
maximum spectral energy.
A new line of inquiry was struck out by Zöllner in 1870. Instead of
tracking the solar radiations backward with the dubious guide of
empirical formulæ, he investigated their intensity at their source. He
showed[722] that, taking prominences to be simple effects of the escape
of powerfully compressed gases, it was possible, from the known
mechanical laws of heat and gaseous constitution, to deduce minimum
values for the temperatures prevailing in the area of their development.
These came out 27,700° C. for the strata lying immediately above, and
68,400° C. for the strata lying immediately below the photosphere, the
former being regarded as the region _into_ which, and the latter as the
region _from_ which the eruptions took place. In this calculation, no
prominences exceeding 40,000 miles (1·5') in height were included. But
in 1884, G. A. Hirn of Colmar, having regard to the enormous velocities
of projection observed in the interim, fixed two million degrees
Centigrade as the lowest _internal_ temperature by which they could be
accounted for; although admitting the photospheric condensations to be
incompatible with a higher _external_ temperature than 50,000° to
100,000° C.[723]
This method of going straight to the sun itself, observing what goes on
there, and inferring conditions, has much to recommend it; but its
profitable use demands knowledge we are still very far from possessing.
We are quite ignorant, for instance, of the actual circumstances
attending the birth of the solar flames. The assumption that they are
nothing but phenomena of elasticity is a purely gratuitous one.
Spectroscopic indications, again, give hope of eventually affording a
fixed point of comparison with terrestrial heat sources; but their
interpretation is still beset with uncertainties; nor can, indeed, the
expression of transcendental temperatures in degrees of impossible
thermometers be, at the best, other than a futile attempt to convey
notions respecting a state of things altogether outside the range of our
experience.
A more tangible, as well as a less disputable proof of solar radiative
intensity than any mere estimates of temperature, was provided in some
experiments made by Professor Langley in 1878.[724] Using means of
unquestioned validity, he found the sun's disc to radiate 87 times as
much heat, and 5,300 times as much light as an equal area of metal in a
Bessemer converter after the air-blast had continued about twenty
minutes. The brilliancy of the incandescent steel, nevertheless, was so
blinding, that melted iron, flowing in a dazzling white-hot stream into
the crucible, showed "deep brown by comparison, presenting a contrast
like that of dark coffee poured into a white cup." Its temperature was
estimated (not quite securely)[725] at about 2,000° C.; and no
allowances were made, in computing relative intensities, for atmospheric
ravages on sunlight, for the extra impediments to its passage presented
by the smoke-laden air of Pittsburgh, or for the obliquity of its
incidence. Thus, a very large balance of advantage lay on the side of
the metal.
A further element of uncertainty in estimating the intrinsic strength of
the sun's rays has still to be considered. From the time that his disc
first began to be studied with the telescope, it was perceived to be
less brilliant near the edges. Lucas Valerius, of the Lyncean Academy,
seems to have been the first to note this fact, which, strangely enough,
was denied by Galileo in a letter to Prince Cesi of January 25,
1613.[726] Father Scheiner, however, fully admitted it, and devoted some
columns of his bulky tome to the attempt to find its appropriate
explanation.[727] In 1729 Bouguer measured, with much accuracy, the
amount of this darkening; and from his data, Laplace, adopting a
principle of emission now known to be erroneous, concluded that the sun
loses eleven-twelfths of his light through absorption in his own
atmosphere.[728] The real existence of this atmosphere, which is totally
distinct from the beds of ignited vapours producing the Fraunhofer
lines, is not open to doubt, although its nature is still a matter of
conjecture. The separate effects of its action on luminous, thermal, and
chemical rays were carefully studied by Father Secchi, who in 1870[729]
inferred the total absorption to be 88/100 of all radiations taken
together, and added the important observation that the light from the
limb is no longer white, but reddish-brown. Absorptive effects were thus
seen to be unequally distributed; and they could evidently be studied to
advantage only by taking the various rays of the spectrum separately,
and finding out how much each had suffered in transmission.
This was done by H. C. Vogel in 1877.[730] Using a polarising
photometer, he found that only 13 per cent. of the violet rays escape at
the edge of the solar disc, 16 of the blue and green, 25 of the yellow,
and 30 per cent. of the red. Midway between centre and limb, 88·7 of
violet light and 96·7 of red penetrate the absorbing envelope, the
abolition of which would increase the intensity of the sun's visible
spectrum above two and a half times in the most, and once and a half
times in the least refrangible parts. The nucleus of a small spot was
ascertained to be of the same luminous intensity as a portion of the
unbroken surface about two and a half minutes from the limb. These
experiments having been made during a spot-minimum when there is reason
to think that absorption is below its average strength, Vogel suggested
their repetition at a time of greater activity. They were extended to
the heat-rays by Edwin B. Frost. Detailed inquiries made at Potsdam in
1892[731] went to show that, were the sun's atmosphere removed, his
thermal power, as regards ourselves, would be increased 1·7 times. They
established, too, the practical uniformity in radiation of all parts of
his disc. A confirmatory result was obtained about the same time by
Wilson and Rambaut, who found that the unveiled sun would be once and a
half times hotter than the actual sun.[732]
Professor Langley, now of Washington, gave to measures of the kind a
refinement previously undreamt of. Reliable determinations of the
"energy" of the individual spectral rays were, for the first time,
rendered possible by his invention of the "bolometer" in 1880.[733] This
exquisitely sensitive instrument affords the means of measuring heat,
not directly, like the thermopile, but in its effects upon the
conduction of electricity. It represents, in the phrase of the inventor,
the finger laid upon the throttle-valve of a steam-engine. A minute
force becomes the modulator of a much greater force, and thus from
imperceptible becomes conspicuous. By locally raising the temperature of
an inconceivably fine strip of platinum serving as the conducting-wire
in a circuit, the flow of electricity is impeded at that point, and the
included galvanometer records a disturbance of the electrical flow.
Amounts of heat were thus detected in less than ten seconds, which,
expended during a thousand years on the melting of a kilogramme of ice,
would leave a part of the work still undone; and further improvements
rendered this marvellous instrument capable of thrilling to changes of
temperature falling short of one ten-millionth of a degree
Centigrade.[734]
The heat contained in the diffraction spectrum is, with equal
dispersions, barely one-tenth of that in the prismatic spectrum. It had,
accordingly, never previously been found possible to measure it in
detail--that is, ray by ray. But it is only from the diffraction, or
normal spectrum that any true idea can be gained as to the real
distribution of energy among the various constituents, visible and
invisible, of a sunbeam. The effect of passage through a prism is to
crowd together the red rays very much more than the blue. To this
prismatic distortion was owing the establishment of a pseudo-maximum of
heat in the infra-red, which disappeared when the natural arrangement by
wave-length was allowed free play. Langley's bolometer has shown that
the hottest part of the normal spectrum virtually coincides with its
most luminous part, both lying in the orange, close to the D-line.[735]
Thus the last shred of evidence in favour of the threefold division of
solar radiations vanished, and it became obvious that the varying
effects--thermal, luminous, or chemical--produced by them are due, not
to any distinction of quality in themselves, but to the different
properties of the substances they impinge upon. They are simply bearers
of energy, conveyed in shorter or longer vibrations; the result in each
separate case depending upon the capacity of the material particles
meeting them for taking up those shorter or longer vibrations, and
turning them variously to account in their inner economy.
A long series of experiments at Allegheny was completed in the summer of
1881 on the crest of Mount Whitney in the Sierra Nevada. Here, at an
elevation of 14,887 feet, in the driest and purest air, perhaps, in the
world, atmospheric absorptive inroads become less sensible, and the
indications of the bolometer, consequently, surer and stronger. An
enormous expansion was at once given to the invisible region in the
solar spectrum below the red. Captain Abney had got chemical effects
from undulations twelve ten-thousandths of a millimetre in length. These
were the longest recognised as, or indeed believed, on theoretical
grounds, to be capable of existing. Professor Langley now got heating
effects from rays of above twice that wave-length, his delicate thread
of platinum groping its way down nearly to thirty ten-thousandths of a
millimetre, or three "microns." The known extent of the solar spectrum
was thus at once more than doubled. Its visible portion covers a range
of about one octave; bolometric indications already in 1884 comprised
between three and four. The great importance of the newly explored
region appears from the fact that three-fourths of the entire energy of
sunlight reside in the infra-red, while scarcely more than one-hundredth
part of that amount is found in the better known ultra-violet
space.[736] These curious facts were reinforced, in 1886,[737] by
further particulars learned with the help of rock-salt lenses and
prisms, glass being impervious to very slow, as to very rapid
vibrations. Traces were thus detected of solar heat distributed into
bands of transmission alternating with bands of atmospheric absorption,
far beyond the measurable limit of 5·3 microns.
In 1894, Langley described at the Oxford Meeting of the British
Association[738] his new "bolographic" researches, in which the
sensitive plate was substituted for the eye in recording deflections of
the galvanometer responding to variations of invisible heat. Finally, in
1901,[739] he embodied in a splendid map of the infra-red spectrum 740
absorption-lines of determinate wave-lengths, ranging from 0·76 to 5·3
microns. Their chemical origin, indeed, remains almost entirely unknown,
no extensive investigations having yet been undertaken of the slower
vibrations distinctive of particular substances; but there is evidence
that seven of the nine great bands crossing the "new spectrum" (as
Langley calls it)[740] are telluric, and subject to seasonal change.
Here, then, he thought, might eventually be found a sure standing-ground
for vitally important previsions of famines, droughts, and
bonanza-crops.
Atmospheric absorption had never before been studied with such precision
as it was by Langley on Mount Whitney. Aided by simultaneous
observations from Lone Pine, at the foot of the Sierra, he was able to
calculate the intensity belonging to each ray before entering the
earth's gaseous envelope--in other words, to construct an
extra-atmospheric curve of energy in the spectrum. The result showed
that the blue end suffered far more than the red, absorption varying
inversely as wave-length. This property of stopping predominantly the
quicker vibrations is shared, as both Vogel and Langley[741] have
conclusively shown, by the solar atmosphere. The effect of this double
absorption is as if two plates of reddish glass were interposed between
us and the sun, the withdrawal of which would leave his orb, not only
three or four times more brilliant, but in colour distinctly
greenish-blue.[742]
The fact of the uncovered sun being _blue_ has an important bearing upon
the question of his temperature, to afford a somewhat more secure answer
to which was the ultimate object of Professor Langley's persevering
researches; for it is well known that as bodies grow hotter, the
proportionate representation in their spectra of the more refrangible
rays becomes greater. The lowest stage of incandescence is the familiar
one of _red_ heat. As it gains intensity, the quicker vibrations come
in, and an optical balance of sensation is established at _white_ heat.
The final term of _blue_ heat, as we now know, is attained by the
photosphere. On this ground alone, then, of the large original
preponderance of blue light, we must raise our estimate of solar heat;
and actual measurements show the same upward tendency. Until quite
lately, Pouillet's figure of 1.7 calories per minute per square
centimetre of terrestrial surface, was the received value for the "solar
constant." Forbes had, it is true, got 2.85 from observations on the
Faulhorn in 1842;[743] but they failed to obtain the confidence they
merited. Pouillet's result was not definitely superseded until Violle,
from actinometrical measures at the summit and base of Mont Blanc in
1875, computed the intensity of solar radiation at 2.54,[744] and Crova,
about the same time, at Montpellier, showed it to be above two
calories.[745] Langley went higher still. Working out the results of the
Mount Whitney expedition, he was led to conclude atmospheric absorption
to be fully twice as effective as had hitherto been supposed. Scarcely
60 per cent., in fact, of those solar radiations which strike
perpendicularly through a seemingly translucent sky, were estimated to
attain the sea-level. The rest are reflected, dispersed, or absorbed.
This discovery involved a large addition to the original supply so
mercilessly cut down in transmission, and the solar constant rose at
once to three calories. Nor did the rise stop there. M. Savélieff
deduced for it a value of 3.47 from actinometrical observations made at
Kieff in 1890;[746] and Knut Ångström, taking account of the arrestive
power of carbonic acid, inferred enormous atmospheric absorption, and a
solar constant of four calories.[747] In other words, the sun's heat
reaching the outskirts of our atmosphere is capable of doing without
cessation the work of an engine of four-horse power for each square yard
of the earth's surface. Thus, modern inquiries tend to render more and
more evident the vastness of the thermal stores contained in the great
central reservoir of our system, while bringing into fair agreement the
estimates of its probable temperature. This is in great measure due to
the acquisition of a workable formula by which to connect temperature
with radiation. Stefan's rule of a fourth-power relation, if not
actually a law of nature, is a colourable imitation of one; and its
employment has afforded a practical certainty that the sun's
temperature, so far as it is definable, neither exceeds 12,000° C., nor
falls short of 6,500° C.
FOOTNOTES:
[Footnote 698: _Principia_, p. 498 (1st ed.).]
[Footnote 699: _Comptes Rendus_, t. vii., p. 24.]
[Footnote 700: _Results of Astr. Observations_, p. 446.]
[Footnote 701: "Est enim calor solis ut radiorum densitas, hoc est,
reciproce ut quadratum distantiæ locorum a sole."--_Principia_, p. 508
(3d ed., 1726).]
[Footnote 702: _Jour. de Physique_, t. lxxv., p. 215.]
[Footnote 703: _Ann. de Chimie_, t. vii., 1817, p. 365.]
[Footnote 704: _Phil. Mag._, vol. xxiii. (4th ser.), p. 505.]
[Footnote 705: _Nuovo Cimento_, t. xvi., p. 294.]
[Footnote 706: _Comptes Rendus_, t. lxv., p. 526.]
[Footnote 707: The direct result of 5-1/3 million degrees was doubled in
allowance for absorption in the sun's own atmosphere. _Comptes Rendus_,
t. lxxiv., p. 26.]
[Footnote 708: _Ibid._, p. 31.]
[Footnote 709: _Nature_, vols. iv., p. 204; v., p. 505.]
[Footnote 710: _Ibid._, vol. xxx., p. 467.]
[Footnote 711: _Ann. de Chim._, t. x. (5th ser.), p. 361.]
[Footnote 712: _Comptes Rendus_, t. xcvi., p. 254.]
[Footnote 713: _Phil. Mag._, vol. viii., p. 324, 1879.]
[Footnote 714: _Ibid._, p. 325.]
[Footnote 715: _Sitzungsberichte_, Wien, Bd. lxxix., ii., p. 391.]
[Footnote 716: _Wiedemann's Annalen_, Bd. xxii., p. 291; _Scheiner,
Strahlung und Temperatur der Sonne_, p. 27.]
[Footnote 717: _Comptes Rendus_, March 28, 1892; _Astr. and
Astrophysics_, vol. xi., p. 517.]
[Footnote 718: _Phil. Trans._, vol. clxxxv., p. 361.]
[Footnote 719: _Proc. Roy. Society_, December 12, 1901.]
[Footnote 720: Scheiner, _Temp. der Sonne_, p. 36.]
[Footnote 721: _Astroph. Jour._, vol. ii., p. 318.]
[Footnote 722: _Astr. Nach._, Nos. 1,815-16.]
[Footnote 723: _L'Astronomie_, September, 1884, p. 334.]
[Footnote 724: _Amer. Jour. of Science_, vol. i. (3rd ser.), p. 653.]
[Footnote 725: Young, _The Sun_, p. 310.]
[Footnote 726: _Op._, t. vi., p. 198.]
[Footnote 727: _Rosa Ursina_, lib. iv., p. 618.]
[Footnote 728: _Méc. Cél._, liv. x., p. 323.]
[Footnote 729: _Le Soleil_ (1st ed.), p. 136.]
[Footnote 730: _Monatsber._, Berlin, 1877, p. 104.]
[Footnote 731: _Astr. Nach._, Nos. 3,105-6; _Astr. and Astrophysics_,
vol. xi., p. 720.]
[Footnote 732: _Proc. Roy. Irish Acad._, vol. ii., No. 2, 1892.]
[Footnote 733: _Am. Jour. of Sc._, vol. xxi., p. 187.]
[Footnote 734: _Amer. Jour. of Science_, vol. v., p. 245, 1898.]
[Footnote 735: For J. W. Draper's partial anticipation of this result,
see _Ibid_. vol. iv., 1872, p. 174.]
[Footnote 736: _Phil. Mag._, vol. xiv., p. 179, 1883.]
[Footnote 737: "The Solar and the Lunar Spectrum," _Memoirs National
Acad. of Science_, vol. xxxii.; "On hitherto Unrecognised Wave-lengths,"
_Amer. Jour. of Science_, vol. xxxii., August, 1886.]
[Footnote 738: _Astroph. Jour._, vol. i., p. 162.]
[Footnote 739: _Annals of the Smithsonian Astroph. Observatory_, vol.
i.; _Comptes Rendus_, t. cxxxi., p. 734; _Astroph. Jour._, vol. iii., p.
63.]
[Footnote 740: _Phil. Mag._, July, 1901.]
[Footnote 741: _Comptes Rendus_, t. xcii., p. 701.]
[Footnote 742: _Nature_, vol. xxvi., p. 589.]
[Footnote 743: _Phil. Trans._, vol. cxxxii., p. 273.]
[Footnote 744: _Ann. de Chim._, t. x., p. 321.]
[Footnote 745: _Ibid._, t. xi., p. 505.]
[Footnote 746: _Comptes Rendus_, t. cxii., p. 1200.]
[Footnote 747: _Wied. Ann._, Bd. xxxix., p. 294; Scheiner, _Temperatur
der Sonne_, pp. 36, 38.]