90 
so utterly powerless compared with the light of the sun, 
that they actually appeared as black lines, and coinciden 
with the two lines D in the solar spectrum. : 
We have seen that the bright line due to the radiation 
from sodium vapour can be very easily obtained by 
placing some sodium in a colourless gas flame, but if we 
now pass the continuous light coming from the carbon 
points of an electric light, or from the oxyhydrogen lime- 
light, through this same sodium flame, the result will be 
that we obtain a black absorption line on a continuous 
spectrum, in precisely the same position as the yellow 
line was originally. This is Kirchhofi’s crucial experi- 
ment, which at once determined not only that the dark 
line in the sun was absolutely coincident with the bright 
line of sodium vapour, but that, under certain conditions, 
bright, incandescent sodium vapour could actually be 
made to absorb the light coming through it, and reverse 
its own spectrum. Kirchhoff goes on;—‘‘I then ex- 
changed the sunlight for the Drummond or oxyhydrogen 
lime-light, which, like that of all incandescent solid and 
liquid bodies, gives a spectrum containing no dark lines.” 
When this light was allowed to fall through a suitable 
flame, coloured by common salt (or chloride of sodium), 
dark lines were seen in the spectrum in the position of 
the sodium lines.” You may imagine that this conclusive 
experiment—perhaps the most wonderful experiment that 
has been made during the century—gave Kirchhoff food 
for thought, and at once his genius travelled toa possible 
explanation of this strange fact he had observed ; a fact, 
as you know, entirely in accordance with the previsions 
of Prof. Stokes, Dr. Balfour Stewart, and Foucalt. 
Kirchhoff said to himself, “I have now got the bright 
lines in the spectrum of the vapour of sodium coin- 
cident with the two dark lines in the solar spectrum, 
What does it mean?” And again the philosopher was 
not at fault. He said to himself—it is almost possible te 
see the train of his reasoning in his memoirs—“ Sodium 
has a most simple spectrum ; suppose I take the most 
complicated spectrum I can find.” He took for this pur- 
pose the spectrum of iron, which I think you will acknow- 
ledge to be one of sufficient complication, for the 
spectrum is traversed by lines throughout its whole 
length, and I may tell you at once that no less than 460 
lines have been already mapped, and their positions are 
now thoroughly well known to us—as well known as the 
position of any star in the heavens. Kirchhoff tried the 
iron spectrum, and he found, absolutely corresponding in 
position in the spectrum and in width and darkness to the 
bright iron lines which he saw, black lines in the solar 
spectrum. He waited no longer ; he instantly convinced 
himself, and soon convinced the world, that he had dis- 
covered this very remarkable fact, that gases and vapours 
have the power of absorbing those very rays which they 
themselves give out when in a state of incandescence. 
So that, if you take sodium, and get its bright lines, and 
mark their positions on the screen, and then observe a 
continuous spectrum, and interpose sodium vapour in the 
path of the beam, you will find black lines absolutely. cor- 
responding with the bright ones ; that is to say, that the 
sodium vapour has the faculty of entirely eating up, 
absorbing, or stopping that light which would otherwise 
go onto the screen. In the case of iron, it is worthy of 
notice that when Kirchhoff made his discovery, he was 
only able to obtain a spectrum of iron consisting of some- 
thing like go lines, but since then the spectrum of iron 
has been mapped to the extent of 460 lines, and sure 
enough there are solar lines corresponding to nearly all 
the 460 bright lines which we are able to get in our labo- 
ratories. Not only was the bright line of sodium reversed 
or changed into a dark one, but it was soon found that 
the lines of other metals, such as lithium, potassium, 
strontium, calcium, and barium, could be reversed in a 
similar manner. This grand discovery of Kirchhoff’s 
met with immediate acceptance, and with it you sce at 
NATURE 
[May 29, 1873 — 
once the explanation of the wonderful black lines disco- 
vered by Wollaston, about which I said something in my 
first lecture, The riddle of the sun was read to a certain 
extent, and Kirchhoff read it in this way. He said :— 
“There is a solid or a liquid something in the sun, 
giving a continuous spectrum, and around this there are 
vapours of sodium, of iron, of calcium, of chromium, of 
barium, of magnesium, of nickel, of copper, of cobalt, and 
aluminium ; all those are existing in an atmosphere, and 
are stopping out the sun’s light. If the sun were not 
there, and if these things were observed in an incan- 
descent state, we should get exactly these bright lines 
from them.” Later researches by many distinguished 
physicists have shown that the following terrestrial 
elements are present in the vaporous condition round the 
sun :— : 
1. Sodium, 6, Chromium, 11. Cobalt. 
2, Calcium, 4. Nickel. 12, Hydrogen. 
3. Barium, 8. Copper. 13. Manganese. 
4. Magnesium, 9, Zine, 14, Aluminium, 
5. Iron, to, Cadmium, 15. Titanium, 
Kirchhoff further imagined that he had reason to 
believe that the visible sun, the sun which we see—and 
we may take the sun as an example of every star in the 
heavens—was liquid. 
In the sun we have, first, a bright, shining orb, dimmed 
to a certain degree at the edge; and here and there, over 
the sun, we see what are called spots. Kirchhoff wished, 
not only to connect his discoveries with the solar atmo- 
sphere, but was anxious to connect them with this 
dimming near the limb and the spots. He said that the 
solar atmosphere, to which all the absorption lines were 
due, extended far outside the sun, and formed the corona; 
and that this dimming of the limb was really due to the 
greater absorption of this atmosphere, owing, of course, 
to the light of the sun travelling through a much greater 
length at the limb than at the centre of the disc. Further- 
more, he said that the sun-spots, which astronomers, from 
the time of Wilson, had asserted to be cavities, were 
nothing but clouds floating in this atmosphere of vapour. 
Such was the very bold hypothesis put forward by Kirchhoff 
—an hypothesis which you see at once explains these 
strange observations from Wollaston upwards, includin: 
Fraunhofer’s observation of the spectrum of the sun an 
stars, and the brilliant ideas. of Prof. Stokes, Dr. Balfour 
Stewart, and others in other lands. A little simple ex- 
periment, made by means of a little sodium vapour and a 
beam of sunlight, with the powerful aid of a little prism, 
gave us this tremendous knowledge about distant worlds 
so immeasurably remote that it seemed absurd for men to 
try and grapple with any of the difficulties that are pre- 
sented to us. Such, then, is Kirchhoff’s theory of the 
sun, which I hope I have been able to make clear to you. 
There is a something—Kirchhoff said it was a liquid— 
which gives us a continuous spectrum, and between our 
eye and that incandescent liquid surface there is an 
enormous atmosphere, built up of vapours of sodium, 
iron, and so on; and the reason that we get these dark 
lines is, that the molecules of the substances named 
absorb certain rays, because when they are in an incan- 
descent state they produce them. ‘Bhis brilliant idea of 
Kirchhoff's was soon carried, as you know, to the stars by 
Mr. Huggins in our own country. In Fig. 34 will be seen 
the spectra of two stars, Aldebaran and a Orionis (Betel- 
geux), which are so distant that it is absolutely impossible 
to measure their distance from us. We know a great deal 
about our own sun, but these suns are so lost in the 
depths of space that it is quite impossible that we can get 
anything like a correct knowledge of their size, or know 
much of their belongings. By means of the prism, how- 
ever, we learn in a momenta great deal. In the first star 
we get three lines, due to the absorption of magnesium 
vapour, as we get them in the sun. We know, therefore, 
that magnesium vapour is present in the atmosphere 
