
268 

another in the Sun’s atmosphere, and his interpretation of these 
observations according to the laboratory results of Frankland and 
himself, go far towards confirming the conviction that in a few 
years all the marvels of the Sun will be dynamically explained 
according to known properties of matter. 
During six or eight precious minutes of time, spectroscopes 
have been applied to the solar atmosphere and to the corona 
seen round the dark disc of the moon eclipsing the sun. Some 
of the wonderful results of such observations, made in India on 
the occasion of the eclipse of August 1868, were described by 
Prof. Stokes in a previous address. Valuable results have, 
through the liberal assistance given by the British and American 
Governments, been obtained also from the total eclipse of last 
December, notwithstanding a generally unfavourable condition 
of weather. It seems to have been proved that at least some 
sensible part of the light of the ‘‘corona” is a terrestrial at- 
mospheric halo or dispersive reflection of the light of the glow- 
ing hydrogen and ‘‘helium ” * round the sun. I believe I may 
say on the present occasion when preparation mast again be made 
to utilise a Total Eclipse of the sun, that the British Association 
confidently trusts to our Government exercising the same wise 
liberality as heretofore in the interests of science. 
The old nebular hypothesis supposes the solar system and 
other similar systems through the universe which we see at a dis- 
tance as stars, to have originated in the condensation of fiery 
nebulous matter. This hypothesis was invented before the dis- 
covery of thermo-dynamics, or the nebulz would not have been 
supposed to be fiery ; and the idea seems never to have occurred 
to any of its inventors or early supporters that the matter, the 
condensation of which they supposed to constitute the Sun and 
stars, could have been other than fiery in the beginning. Mayer 
first suggested that the heat of the Sun may be due to gravita- 
tion ; but he supposed meteors falling in to keep always gene- 
rating the heat which is radiated year by year from the Sun. 
Helmholtz, on the other hand, adopting the nebular hypothesis, 
showed in 1854 that it was not necessary to suppose the nebulous 
matter to have been originally fiery, but that mutual gravitation 
between its parts may have generated the heat to which the pre- 
sent high temperature of the Sun is due. Further he made the 
important observations that the potential energy of gravitation in 
the Sun is even now far from exhausted ; but that with further 
and further shrinking more and more heat is to be generated, and 
that thus we can conceive the Sun even now to possess a sufficient 
store of energy to produce heat and light, almost as at present, 
for several million years of time future. It ought, however, to 
be added that this condensation can only follow from cooling, and 
therefore that Helmholtz’s gravitational explanation of future Sun- 
heat amounts really to showing that the Sun’s thermal capacity is 
enormously greater, in virtue of the mutual gravitation between 
the parts of so enormous a mass, than the sum of the thermal 
capacities of separate and smaller bodies of the same material 
and the same total mass. Reasons for adopting this theory, and 
the consequences which follow from it, are discussed in an article 
“*On the Age of the Sun’s Heat,” published in AZacmillan’s 
Magazine for March, 1862. 
For a few years Mayer’s theory of solar heat had seemed to me 
probable ; but I had been led to regard it as no longer tenable, 
because I had been in the first place driven, by consideration of 
the very approximate constancy of the Earth’s period of revolu- 
tion round the Sun for the last 2000 years, to conclude that ‘‘ The 
principal source, perhaps the sole appreciably effective ‘‘source 
of Sun-heat, is in bodies circulating round the Sun at present 
inside ‘‘ the Earth’s orbit” +; and because Le Verrier’s researches 
on the motion of the planet Mercury, though giving evidence of 
a sensible influence attributable to matter circulating as a great 
number of small planets within his orbit round the Sun, showed 
that the amount of matter that could possibly be assumed to cir- 
culate at any considerable distance from the Sun must be very 
small ; and therefore ‘‘if the meteoric influx taking place at pre- 
sent is enough to produce any appreciable portion of the heat 
radiated away, it must be supposed to be from matter circulating 
round the Sun, within very short distances of his surface. The 
density of this meteoric cloud would have to be supposed so great 
that comets could scarcely have escaped as comets actually have 
escaped, showing no discoverable effects of resistance, after pass- 
* Frankland and Lockyer find the yellow prominences to give a very 
decided bright line not far from D, but hitherto notidentified with any terres- 
trial flame. It seems to indicate a new substance, which they propose to call 
Helium. 
+ On the Mechanical Energies of the Solar System. Transactions of the 
Royal Society of Edinburgh, 1854 ; and Phil. Mag. 1854, second half year. 
NATURE 




ing his surface within a distance equal to one-eighth of his radius. 
All things considered, there seems little probability in the hypo- 
thesis that solar radiation is compensated to any appreciable 
degree, by heat generated by meteors falling in, at present ; and, 
as it can be shown that no chemical theory is tenable,* it must 
be concluded as most probable that the Sun is at present more an 
incandescent liquid mass cooling.’”*+ ’ 
Thus on pwely astronomical grounds was I long ago led to 
abandon as very improbable the hypothesis that the Sun’s heat 
is supplied dynamically from year to year by the influx of 
meteors. Put now spectrum analysis gives proof finally con<lu- 
sive against it. 
Each meteor circulating round the Sun must fall in along a 
very gradual spiral path, and before reaching the Sun must have 
been for a long time exposed to an enormous heating effect from 
his radiation when very near, and must thus have been driven 
into vapour before actually falling intothe Sun. Thus, if Mayer’s 
hypothesis is correct, friction between vortices of meteoric vapours 
and the Sun’s atmosphere must be the immediate cause of solar 
heat ; and the velocity with which these vapours circulate round 
equaterial parts of the Sun must amount to 435 kilometres per 
second. ‘The spectrum test of velocity applied by Lockyer 
showed but a twentieth part of this amount as the greatest ob- 
served relative velocity between different vapours in the Sun’s 
atmosphere. 
At the first Liverpool meeting of the British Association 
(1854), in advancing a gravitational theory to account for all the 
heat, light, and motions of the universe, I urged that the imme- 
diately antecedent condition of the matter of which the Sun and 
Planets were formed, not being fiery. could not have been ga- 
seous ; but that it probably was solid, and may have been like 
the meteoric stones which we still so frequently meet with through 
space. The discovery of Huggins, that the light of the Nebulz, 
so far as hitherto sensible to us, proceeds from incandescent 
hydrogen and nitrogen gases, and that the heads of comets also 
give us light of incandescent gas, seems at first sight literally to 
fulfil that part of the Nebular hypothesis to which I had ob- 
jected. But a solution, which seems to me io the highest 
degree probable, has been suggested by Tait. He supposes 
that it may be by ignited gaseous exhalations proceeding 
from the collisions of meteoric stones that Nebule and 
the heads of Comets show themselves to us, and he sug- 
gested, at a former meeting of the Association, that experi- 
ments should be made for the purpose of applying spectrum 
analysis to the lizht which has been observed in gunnery trials. 
such as those at Shoeburyness, when iron strikes against iron 
at a great velocity, but varied by substituting for the iron various 
solid materials, metallic or stony. Hitherto this suggestion has 
not been acted upon; but surely it is one the carrying out of 
which ought to be promoted by the British Association. 
Most important steps have been recently made towards the 
discovery of the nature of comets ; establishing with nothing short 
of certainty the truth of a hypothesis which had long appeared 
to me probable,—that they consist of groups of meteoric stones ; 
—accounting satisfactorily for the light of the nucleus ; and giy- 
ing a simple and rational explanation of phenomena presented by 
the tails of comets which had been regarded by the greatest 
astronomers as almost preternaturally marvellous. The meteoric 
hypothesis to which I have referred remained a mere hypothesis 
(I do not know that it was ever even published) until, in 1866, 
Schiaparelli calculated, from observations on the August meteors, 
an orbit for these bodies which he found to agree almost per- 
fectly with the orbit of the great comet of 1862 as calculated by 
Oppolzer ; and so discovered and demonstrated that a comet 
consists of a group of meteoric stones. Professor Newton, of 
Yale College, United States, by examining ancient records, 
ascertained that in periods of about thirty-three years, since the 
year 902, there have been exceptionally brilliant displays of the 
November meteors. It had long been believed that these in- 
teresting visitants came from a train of small detached planets 
circulating round the Sun all in nearly the same orbit, and con- 
stituting a belt analogous to Saturn’s ring, and that the reason for 
the comparatively large number of meteors which we observe 
annually about the 14th of November is, that at that time the 
earth’s orbit cuts through the supposed meteoric belt. Professor 
Newton concluded from his investigation that there is a denser 
part of the group of meteors which extends oyera portion of the 
orbit so great as to occupy about one-tenth or one-fifteenth of the 
* “ Mechanical Energies,” &c. 
t “Age of the Sun's Heat ” ( Macmillan's Magazine, March 1862). 
