Dec. 22, 1881] 
SOLAR PHYSICS * 
I. 
\ E now have to consider what is the best method by which we 
can obtain, not a reversed image of the infra-red region, but 
a direct image of that portion of the spectrum ; the problem had 
to be attacked in an experimental manner. It was really a 
matter of physics, and nothing more; the chemical question was 
hors de combat. Every silver salt which I have already shown 
you, you saw absorbed in the blue end of the spectrum, and not 
in the red ; and therefore from what I had previously told you, 
you were prepared to hear that those salts would not be photo- 
graphically effective for the red end of the spectrum, although 
they would be eminently so for the blue end. The question 
then we asked ourselves was this: Isit possible to obtain a silver 
salt which shall absorb in the red eid of the spectrum? Is it 
possible, for instance, to obtain a salt of silver which will exhibit 
two molecular states—one absorbing ia the blue, and the other 
in the red? If we turn to other bodies I think I can show you 
that there are bodies which exist in two or more molecular states. 
The very example of obtaining a reversed negative in the red of 
the spectrum by Draper’s plan is an example of it. I have here 
another very good example of the oxidation proce.s. This is 
chloride of silver paper which has been darkened by white light, 
and you will see that it has a tint which naturally would absorb 
to a certain extent the red rays. You will further see by the 
oxidising action we are able to produce a coloured oxide of 
silver. In other words, we have a coloured spectrum produced 
by the action of light itself, owing to the oxidising process. 
Alongside of this is the spectrum, taken on similar paper, without 
any preliminary exposure to light. You see where we get a 
darkened salt we have an impression of the spectrum in various 
colours, bezinning with the blue, green, and then the red of the 
spectrum. Where the red end of the spectrum is you have a red 
oxide of silver formed. 
Now let me show you that there are two different molecular 
states of elementary matter with which we are well acquainted. 
First of ali I will throw a spectrum on the screen, and try to show 
you that there are two forms of iodine which absorb in different 
parts of the spectrum, telling us that they are molecularly different, 
when in solution at all events. The spectrum is on the screen, 
and I place a solution of iodine which has been dissolved in 
water in front of the slit, and you see that it cuts off the whole 
of the blue end of the spectrum, leaving only a red band. If you 
look at the white screen on the wall on the right you willsee an 
inage of the slit reflected from the back of the prism, which gives 
the real colour of the iodine in solution. In this form, then, we 
have one molecular state of iodine. We will take another mole- 
cular state obtained by dissolving it in bisulphile of carbon, You 
see that we have a totally differentabsorption, The yellow is cut 
out, and the blue and also the red are left behind. Here, then, is 
one proof that we can have two molecular states of an element. 
But there is another interesting example in gold. If you will 
allow me to read from a book written by my colleague, Mr. 
Lockyer, he refers to the two molecular states of gold; and if 
possible, I should like to show you those different molecular 
states as far as wecan onthe screen. After talking about dif- 
ferent kinds of spectra he goes on to say: ‘‘ Gold is sometimes 
yellow, as you know, but gold is sometimes blue and sometimes 
red. It must be perfectly clear to all that if particles vibrate, 
the colours of substances must have something to do with the 
vibrations. If the colours have anything to do with the particles 
it must be with their vibrations. Now as the spectrum in the 
main consists of red, yellow, and blue, the red and the blue rays 
are owing to something in a substance which only transmits or 
reflects the yellow light: if we put gold leaf in front of the 
limelight, we can see whether the yellow light does or does not 
suffer any change. The yellow disappears; we have a green 
colour; the red and blue are absent. The gold leaf is of ex- 
cessive thickness. What would happen could I make it thicker ? 
Its colour would become more violet. This I have proved by 
using aqua regia, But we can obtain a solution of fine gold 
which lets the red light through. Its particles are doing some- 
thing with the blue vibrations. We can obtain another solution 
which only transnits the blue. Now what is the difference— 
the ‘particular’ difference—between the gold in these solutions 
and that which is yellow by reflected light, and green or violet 
+ Lecture delivered on May 25, 1881, at the Lecture Theatre, South Ken- 
sington Museum, by Capt. Abney, R.E., F.R.S. Continued from p. 166. 
NATURE 
187 
by transmitted light ? It is a question worthy of much study.” ? 
I will now throw on the screen an image of a thin film of 
gold kindly lent me by Mr. Lockyer, and you will see the 
colour of gold as it really is. It is not yellow, as we ordi- 
narily know it, but is green when of that particular thickness, 
and it cuts off the red of the spectrum, I have here a solu- 
tion of gold, which however does let red light through. It 
is purely metallic gold precipitated in water, and you will see 
what a beautiful red colour this has. This ruby colour of gold 
was first obtained by Dr. Hugo Miiller, and experimented upon 
by Faraday. You can obtain also another solution of gold 
which is a greenish blue, It is rather a ticklish thing to show on 
the screen, but I daresay we shall be able to show it to you. 
Thus, then, we have gold in three states: the red molecular 
state, the blue molecular state, and the green molecular state ; 
or perhaps the green may be referred to the difference between 
those two, or a combination of those two. Evidently, then, it 
is possible to obtain matter in two or three molecular states at 
the very least. 
Now to apply this to our silver salts. Experience seems to 
show that the green molecules will be much more likely to 
absorb in the red than the blue molecules. I will just try to 
explain this by passing one or two green bodies before the slit of 
the spectrum apparatus (Fig. 6). Iu this green glass, for instance, 
Fic 6.—Absorption spectra of different coloured matters. 
you see that the red is cut off markedly from the green. Now 
if we take a solution of a salt of copper—chloride of copper—you 
will remark that the same phenomenon presents itself ; we have 
the red cut off as well as the blue. You may ask the question 
whether a blue colour may not be equally as effective in absorb- 
ing the red as the green. I think I can answer this question 
experimentally. Here we have a piece of ordinary blue glass ; 
you will see that althouzh the red of the spectrum is dimmed to 
a certain extent, still a streak of red appears, and the principal 
absorption takes place in the yellow. One would naturally infer 
that as the red was not entirely cut off, those rays which lie below 
the red would also not be cut off. That, practically speaking, is 
found to be the case. We will take an ordinary blue dye, and 
you will find that we get the same phenomenon occurring. You 
will notice that the image of the slit on the side screen is a most 
beautiful Oxford blue, and you notice in the spectrum that it is 
gradually cutting out the yellow. Such experiments might be 
multiplied, but from what you have seen it is evident that a 
green is more likely to be effective as a red absorber than is 
blue, and this would apply also to the silver salt as regards the 
molecular state which we wish to produce. 
You may ask me—why cannot we use a green dye accord- 
ing to Vogel’s method, which I mentioned last time? I can 
show you on the screen what would have happened with a 
green dye. There are greens and greens: some greens absorb 
in the red, others do not. In the ordinary green dye, which is 
a very complex body, part of the blue and part of the yellow is 
cut off, but not the red or the green, and consequently, as the 
red appeared it was perfectly useless to attempt to dye a film in 
order to produce a photograph of that end of the spectrum, What 
remained then to do? It simply remained to take some simple 
silver salt itself, and then to convert it into the molecular state, 
which would absorb the red. After four years of labour we 
succeeded in effecting this. In this test-tabe we have some 
precipitated bromide of silver, which, as you saw on the screen 
last time, is of a yellow tint, or rather of an orange tint. Now 
bromide of silver is to a very small extent soluble in nitrate of 
silver, more particularly when acidified with nitric acid ; and if 
such bromide of silver as we have here be boiled in a solution of 
meen 120, 
“Studies in Spectrum Analysis.”” by J. “Norman Lockyer, 
\ F.R.S. ; “International Scientific Series.”” (Kegan Paul and Co.) 
