
250 
NATURE 



and lastly green. And M. Midler remarks on this very admir- 
ably, ‘ The violet band is the link between the prominences and 
the corona.” 
Before going further, I will show you the difference in the 
appearance of what we may term hot hydrogen and cold hydro- 
gen, that is, hydrogen which we drive into different degrees of 
incandesceace by means of the spark. After Dr. Frankland and 
myself were able to determine that the pressure in these solar 
regions was small, we came to the conclusion that outside the 
hot hydrogen there must be some cooler hydrogen, in order that 
the phenomena we observed, both in the laboratory and in the 
observatory, should agree. 
I have in this tube hydrogen ata certain pressure, and here we 
have a coil which will enable us to send a spark through it ; you 
see we get a certain amount of redness in that tube, and if you 
look on one side or above you will see a sort of bluish-greenish 
light. Now that redness represents the condition of the hydro- 
gen in the region of the sun where Dr. Schmidt gave us that ex- 
tremely thin red ring, and the combination of the blue and red 
would give you something very like violet. 
But here [ have hydrogen under adifferent condition. In the 
tube its rareness is not excessive ; but in this globe, of which I am 
about to speak, you have the nearest approach to a vacuum ever 
obtained through which a spark will pass ; and I beg to call your 
attention to what will now happen. This globe contains the 
same chemical element prepared at the same time as the 
chemical element you have in the tube, but you see that, so 
far as colour goes, we have something perfectly different in this 
case. Now we send the spark through it. I would beg Prof. 
Tyndall, if he will be good enough, to observe the spectrum 
of this hydrogen in this globe. [Prof. Tyndall did so.] You 
will see that there is one line? [Prof Tyndall: Yes.| Anda 
continuous spectrum? [Prof Tyndall: And a continuous 
spectrum, ] Cool hydrogen gives us only the bright line I’, plus 
A continuous spectrum, and many of you will know the extreme 
importance of that observation. It accounts for the F line being 


observed without the C line in 1868 and last year, and also for 
he continuous spectrum observed in the Indian eclipse. 
¢.—The American Lclipse 
When we come from the Indian to the American eclipse with 
the considerations to which I have drawn your attention, namely, 
the existence of these different layers due to the different ele- 
ments and conditions of the same element thinning out, we shall 
see the extreme importance of the American observations, for 
they establish the fact that outside the hydrogen layer there was 
a layer giving only a line in the green, the line which Rayet and 
Pogson had observed associated with the hydrogen spectrum and 
the spectrum of the yellow substance. Here obviously we have, 
I think, merely an indication of another substance thinning out, 
in spite of the extraordinary suggestion which was put forward 
that the corona was nothing but a Aermanent solar aurora. 
I need hardly tell you that the idea of a permanent aurora any- 
where was startling, and that of a permanent solar aurora more 
startling still ; but what I claim is, that during last year’s obser- 
vations we made this very startling idea into a most beautiful 
fact, namely, that this outer layer of the chromosphere is in all 
eed nothing more nor less than an indication of an element 
ighter than hydrogen, although this is not yet absolutely estab- 
lished, for the line is coincident with one of the lines in the 
spec rum of iron, 
U—The layers increase very rapidly in Density. 
Reproduction 
of the Coloured Phenomena 
Dr. Frankland and myself were early drawn to consider the 
solar nature of the large coronas, to which I have called your 
attention, as extremely questionable, even on the supposition of 
cool hydrogen, because we did not see how, with its temperature 
and pressure, it could extend very far: and an experiment which 
I have to make here will probably make that clearer. 
We have in these glass vessels hydrogen a little more brilliant 
now the spark passes through it than that you saw in the globe, 
because I have been compelled to mix with it a certain amount of 
mercury vapour. Below, we have at the present moment sodium 
vapour being generated from metallic sodium in one tube, and 
mereury vapour in the other. I hope, if the experiment suc- 
ceeds, you will see that a good many of the coloured phenomena 
seen in the chromosphere during eclipses may be easily repro- 
duced by such experiments as this; and not only the coloured 

phenomena but ‘he increase of brilliancy accompanied by changes 
of colour recorded. You can now all see the yellow tinge at the 

bottom of one tube, and the green tinge at the bottom of the other; 
and if there were time to continue this experiment by increasin: 
the densily of the vapours now associated with the hydrogen, L 
could make the bottom portion of each tube where the vapours 
are densest shine outalmost like the sun, while the cool hydrogen 
at the top would remain not more brilliant than it is at present, 
We should have as it were a section of the chromosphere. 
V.—CONCLUSION 
I will proceed now, if you will allow me, to some of the 
general results obtained during the last eclipse. 
I think that, although the work has been very unfortunately 
interrupted, still the result has been most satisfactory. By putting 
together observations here and observations there, I consider our 
knowledge of the sun is enormously greater than it was a few 
months ago, Tor instance, we are enabled to understand the 
long-neglected observation of Rayet, and the equally long- 
neglected observation of Pogson ; and we know that outside the — 
hydrogen there is, in all probability, a new element existing in a 
state of almost infinite tenuity. And we are sure of the existence 
of cool hydrogen above the hot hydrogen, a fact which seemed — 
to be negatived by the eclipse of 1869. 
I think if we had merely determined that there was this cool - 
hydrogen, all our labour would not have been in vain, as it shows: 
the rapid reduction of temperature. But there is more behind. 
I told you that M. Midler, in summing up the observations 
made up to 1860, came to the conclusion that part of the corona 
was certainly solar, and that whether the outer portions were or 
were not solar, was a matter of doubt. I do not say that we 
have settled that absolutely, but we have firm evidence that 
some of the light of the corona is due to reflexion between 
the earth and the moon, The outer corona was observed 
to have a rosy tinge over the prominences, and the spectrum 
of the prominences was detected many minutes above them, 
as well as on the dark moon. It could not have got this 
colour at éie sur, for its intrinsic colour is green, and the red 
light of the hydrogen supplied at the sun is abolished altogether, 
is absorbed, and can only reach the corona af ¢/e sur, so to speak, 
as dark light. 
It isa great fact that we are sure, as far as observation can 
make us sure, that there isa glare round the hydrogen which 
gives us the spectrum of hot hydrogen on the corona, where we 
know that hot hydrogen does not exist, Assume the hot hydrogen 
which gives us the red light to be only two minutes high, the 
spectroscope has picked it up eight minutes from the sun! The 
region of cool hydrogen is exaggerated in the same way. We 
get it where there is no indication of the cool hydrogen existing. 
And then with regard to the element which gives us the line of 
the green, we get that twenty minutes or twenty-five minutes 
away from the sun, Well, no man who knows anything about 
the matter will affirm that it is certain that the element exists at 
that distance from the sun. 
Therefore I think we have absolutely established the fact that 
as the sun—the uneclipsed sun—gives us a glare round it, so each 
layer of the chromosphere gives us a glare round it. That is 
exactly what was to be expected, and that it is true is proved by 
the observation—a most important observation made in Spain— 
that the air, the cloud, ever between us and the dark moon, gives 
us the same spectrum that we get from the prominences 
themselves. 
Given, however, the layers and elements in the chromosphere 
extended as far as you will, and apparently increased or not by 
reflection ot at the sun, we have still to account for rays, rifts, 
and the like. If anyone will explain either Mr. Brothers’s photo- 
graph or Mr. Gilman’s picture of the eclipse of 1869, containing 
those dark bands starting from the moon and fading away into 
space, and the bright variously-coloured rays between them, on 
any solar theory, he will render great service to science. But in 
the meantime I must fall back upon M. Miadler’s opinion of 
1860, with the addition to it that I have stated that we have 
found, at all events, that some of the doubtful light is non-solar ; 
we have turned the opinion into a fact. 
Bear in mind that close to the sun you have a white layer com- 
posed of vapours of many substances, including all the outer 
ones ; outside this isa yellow region; above that a region of 
hydrogen, incandescent and red at the base, cooler, and therefore 
blue, higher up, the red and blue commingling and giving us 
violet ; and then another element thinning out and giving us 
green. Take these colours in connection with those which are 
thrown on our landscapes or on the sea during eclipses, each 
region being lit up in turns with varying, more or less mono- 

