4, 
Dec. 15, 1881] 
NATURE 
163 
in other words, to the comparative heating effect of different 
parts of the solar spectrum. Thus you see that Sir William 
Herschel, by the use of thermometers, was able to discover that 
there were rays existing below the red, and this he did by the 
second method, viz. by noting the ri-e of temperature in the 
absorbent body, lampblack, placed in the path of the rays. There 
are other modes of showing a rise in temperature in lampblack, 
amongst others by the thermopile, and to that I shall have to 
refer more at length in a subsequent lecture, and therefore I 
pass on at once to the chemical effect of the different rays of the 
spectrum, 
In order to show you how we can arrive at a knowledge of 
the existence of energy in the different parts of the spectrum by 
their doing chemical work, 1 must take you through one or two 
very simple experiments, experiments which I dare say you may 
have heard of before, but the sight of which perhaps may 
impress your minds more than if you merely read of them. 
The first experiment will show the effect upon chloride of silver. 
1 have in this frame a piece of paper impregnated with chloride 
of silver, and behind this screen, which I have had erected to save 
your eyes, isa Siemens electric lamp, which gives out a very intense 
light indeed. A lens placed at a proper distance from the points 
will cause an image of them to fall on this paper. After a few 
seconds’ exposure you see that where the image fell we shall 
have a darkening effect; in fact we have an image of the 
are printed on the chloride of silver paper. Now this paper 
apparently to all casual observation is white ; what rays of the 
spectrum are they then that cause this blackening effect? By 
and by we shall have to find out to what this blackening is due. 
I have here some chloride of silver ia collodion, and with this I 
think we shall be able to see what rays they are which affect the 
chloride of silver. In front of the slit of this spectroscopic 
arrangement we place this chloride of silver, and you will remark 
that the image of the slit cast by the prism on yonder white 
wall is of a lemon colour, and that the violet of the spectrum is 
much subdued in intensity. The true colour of chloride of 
silver in that particular state in which it exists in the paper then is 
that which absorbs the violet, and from what has been said on the 
principle of work being done where absorption takes place, we 
should expect to find that it is the violet rays which discolour that 
chloride of silver, and not the yellow rays, which, as we saw, 
passed freely through it. We will prove that. I have here a 
blue glass, and we will see whether we can print through the 
blue glass in the same satisfactory manner that we can when the 
light is unshaded. Placing a yeilow glass in front of the light, 
first you see that there is no action whatever—the paper remains 
perfectly white. Now, taking the blue glass, and trying to 
print through it, you see we have a print of the arc—not perhaps 
quite so deep as where the unshaded light acted on the paper, 
but sti'l sufficiently so to show that tbe blue and violet light 
are effective. The proposition enunciated then in this case is 
correct, that the rays active in the dissociation of chloride of 
silver are the rays it absorbs. But we may consult the 
spectium still further, and, by placing such a piece of paper 
directly in it, and allowing it to print, we shall find that the 
chloride of silver is only attacked by the blue and violet rays, 
and not at all by the yellow. Such, you see, isthe case in the 
photograph before you. There is an invisible part of the spec- 
tram beyond the violet by which the silver chloride is even more 
affected, but that is a region of the spectrum with which I will not 
trouble you, as it is beyond the scope of my lectures. It will be 
seen then that we have a specimen of the chemical decomposi- 
tion of a solid body. Now I should like to show you what is 
the cause of that darkening ; namely, how the chloride of silver 
is changed. I am obliged to use chemical symbols, because 
they are short, but I will try to explain them. In ordinary 
chemistry the chloride of silver is designated AgC], Ag meaning 
one atom of silver, and Cl meaning one atom of chlorine, which 
are joined together. But in ord:r to explain phenomena which 
are met with in photography, silver really requires two atoms of 
chlorine to be combined with it to form chloride of silver; that 
is silver is a diad element. ‘This 1 have expressed by this 
symbol which I have here, Ag’Cl,; that is two equivaleats of 
chlorine are obliged to be combined with one equivalent of 
silver, I will give the reason why the old formula is not per- 
fectly correct: whea you have light acting on chloride of silver, 
work of some description is done amongst the atoms of the 
molecules forming it, and we have one atom of chlorine thrown 
off by the vibrations of the blue part of the spectrum, and the 
new molecule Ag”Cl is what is formed, which will grasp any 
other unsatisfied atom or molecule which may come in its way. 
In chloride of silver, then, we have an example of the decom- 
position of a solid body by the action of light. 1 have here two 
bottles, both containing eihyl iodide, a body which we will say 
roughly is composed of ethyl and iodine combined together ; the 
action of light on this is to cause the iodine to separate from the 
ethyl, and the iodine liberated colours the liquid, as we have it in 
one bottle. The other bottle is ethyl iodide unaltered by light. 
So that you see we are obliged to shield this liquid from the 
light in order to prevent it from decomposing into ethyl and 
iodine. Here we have decomposition of a liquid by light. 
TI will now endeavour to show you the decomposition, or 
dissociation, which is perhaps a better term, of the molecules of 
a gas, and its combination with something else. I have here a 
jar of chlorine, and I think you will see by holding it in the rays 
of the spectrum that we have certainly the violet cut off, and a 
good deal of the blue. Therefore, if we find any work can 
take place within this chlorme it must be by those rays which are 
absorbed or cut off. I have here a jar of hydrogen which is 
perfectly colourless, and were I to put that in the spectrum I 
should teach you nothing, because the whole of the rays would 
pass through it. If then I have a mixture of hydrogen and 
chlcrine together, and allow light to act upon them, it is quite 
evident that the only matter which can be acted upon is the 
chlorine. Now, in these small glass bulbs which are covered with 
yellow paper, are equal volumes of chlorine and hydrogen, When 
chlorine and hydrogen are combined together we have what is 
known as hydrochloric acid, a gas of the same tenuity as a 
mixture of the two. It has been found by practical experiment 
that if you have an intense source of light acting upon chlorine 
and hydrogen a combination between these two at once takes 
place, and we have the hydrochloric acid formed with a violent 
explosion—not enough to do any harm, but one which will 
make the room echo. Now our conception of the matter is 
this, that no hydrogen atom can exist by itself ; that there must be 
two atoms to form hydrogen molecules; and that there must be 
at least two atoms of chlorine to form a chlorine molecule, per- 
haps more. Anyhow, you cannot have less than two atoms to 
form onc molecule of cblorine, or less than two of hydrogen to 
form a molecule of hydrogen. If then we have these two 
mixed togetoer, and cause light to act upon them, what is the 
physical result? The physical result is that the atoms of 
chlorine will swing violently apart from one another, they will 
be dissociated, and in their swing will catch up one of the atoms 
of hydrogen, avd hydrochloric gas will be formed. Now you 
saw that the mixture of chlorine and hydrogen, or rather that the 
chlorine itself cut off the blue ; in other words, it was the blue 
light which would have any chemical effect upon the mixture. 
T will now get Mr. Greening to allow the Siemens light to act 
through red glass, and you will see, I think, that there will be 
no effect whatever. Now, directly he takes it away there isa 
violent combination of the two with an explosion. To show that 
it is the blue light that is the effective light we will cause those 
two gases to explode by means of white light filtered through 
blue glass. I may say that the arrangement is very simple. We 
have the Siemens light; a lens brings the rays from that 
Siemens light to a focus on the centre of the bulb, and the 
vibration of the ether proceeding from those points causes those 
two molecules to combine in the way that you saw just now. 
We will put the blue and yellow glasses together first, and start 
the Siemens machine; when we draw away the yellow glass 
we have the same result as before. 
For photographic purposes silver salts are the most convenient ; 
if, however, we had to wait until the silver salts visibly darkened 
before we obtained a photograph, we should have to wait much 
longer than it isin the experience of all that we have to do. 
I will try and explain what happens when a very short exposure 
is given toa silver salt. For certain reasons silver chloride is 
not used, but we have recourse to silver iodide or silver bromide, 
and in some cases where both are u-ed together a better result is 
obtained, What happens to these salts by a short exposure? 
What happens to silver chloride when acted upon by light ? 
You will remember I told you that (with our notation for silver) 
after light had acted on silver chloride we had one atom of silver 
combined with one atom of chlorine. In the same way, if we 
replace the chlorine by iodine we have sub-iodide of silver 
formed. The visible image aud the image impressed by a very 
short exposure are identical except in the quantity of matter 
altered, We will suppose, for instance, according to the modern 
theory that each of these molecules are charged with electricity, 
