August 27, 1885] 
the wondrous power of the eye can aid us to an extent far 
surpassing that of the most delicate pile and galvanometer 
for the dark rays. 
Wollaston and Fraunhofer were the first to show that 
in the solar spectrum numerous dark bands occur, which 
indicate the absence of certain definite kinds of light. 
Sir David Brewster afterwards showed that similar 
bands make their appearance when the spectrum is 
made to pass through nitrous acid gas, and it was thus 
rendered probable that the bands which appear in the 
solar spectrum were due to absorption likewise. 
Brewster, J. Herschel, Talbot, Wheatstone, and W. A. 
Miller were amongst the first to make observations upon 
the luminous spectrum obtained by heating various sub- 
stances, and it was soon perceived that such spectra 
consist of bright lines on a dark background, and thus 
appear to be a reversal of the solar spectrum, which con- 
sists of dark lines on a bright background. Fraunhofer 
was the first to notice a coincidence in spectral position 
between the dark double line D occurring in the solar 
spectrum and the bright yellow flame produced by in- 
candescent sodium. Swan afterwards showed that the 
correspondence between the two black lines and the two 
bright lines is very exact, and that a very small quantity 
of salt is sufficient to call forth the bright lines. Ang- 
stroém (PAi/. Mag., May, 1855), referring to a conjecture 
of Euler that a body absorbs all the series of oscillations 
which it can itself assume, expresses his conviction that 
the same body, when heated so as to become luminous, 
must emit the very rays which at ordinary temperatures 
are absorbed, and that the explanation of the dark lines 
in the solar spectrum embraces that of Juminous lines in 
the electric spectrum. Probably, however, the first to 
give definite expression to this conception was Prof. 
Stokes, who, about the year 1850, commented on an ex- 
periment recently made by Foucault. This observer had 
found that, when a voltaic arc formed between charcoal 
poles was placed in the path of a beam of solar light, the 
double line D is thereby rendered considerably darker. 
If, on the other hand, the sun and the arc jut out the one 
beyond the other, the line D appears darker than usual 
in the solar light, and stands out bright in the electric 
spectrum. Thus the arc, remarks Foucault, presents us 
with a medium which emits the rays D on its own ac- 
count, and which at the same time absorbs them when 
they come from another quarter. 
The explanation given by Stokes of this experiment 
assumes that the vapour of sodium must possess, by its 
molecular structure, a tendency to vibrate in periods 
corresponding to the degrees of refrangibility of the 
double line D. 
Hence the presence of sodium in a source of light must 
tend to originate light of that quality. On the other 
hand, vapour of sodium in an atmosphere around a source 
must have a great tendency to absorb light from the 
source of the precise quality in question. 
In the atmosphere around the sun, therefore, there must 
be present vapour of sodium, which, according to the 
mechanical explanation thus suggested, being particularly 
opaque for light of that quality, prevents such of it as is 
emitted from the sun from penetrating to any consider- 
able distance through the surrounding atmosphere. 
It appears, from the historical sketch here given, that 
two independent lines of research were progressing to- 
wards the same conclusion. The one of these had for its 
basis the theory of exchanges, and endeavoured theoretic- 
ally and experimentally to render this theory complete. 
The other was founded upon spectroscopic investigation, 
and endeavoured to apply to light an analogy deduced 
from sound, believing that, just as a string or tuning-fork 
when at rest takes up that note it gives out when struck, 
so a molecule when co/d adsords that ray which it gives 
out when fot. 
In October, 1859, Prof. Kirchhoff of Heidelberg made 
NATURE 
397 
a communication to the Berlin Academy on the subject of 
Fraunhofer’s lines. His observations were made on this 
occasion by an examination of the spectrum of coloured 
flames made by Bunsen and himself, and he derived from 
them the following conclusions:—He concluded that 
coloured flames in the spectrum ‘of which bright sharp 
lines present themselves so weaken rays of the colour 
of these lines, when such rays pass through the 
flames, that, in place of the bright lines, dark ones 
appear as soon as there is brought behind the flame a 
source of light of sufficient intensity in the spectrum of 
which these lines are otherwise wanting. He concluded 
further that the dark lines of the solar spectrum which 
are not evoked by the atmosphere of the earth exist in 
consequence of the presence in the incandescent atmo- 
sphere of the sun of those substances which in the 
spectrum of a flame produce bright lines in the same 
place. 
Carrying out this train of thought, Kirchhoff, about the 
end of 1859, shows that as a mathematical consequence 
of the theory of exchanges, a definite relation must sub- 
sist between the radiating and absorbing power of bodies 
for individual descriptions of light and heat. 
It will be noticed in this historical statement that I 
made my first experiments on dark heat; afterwards I 
proceeded to the subject of light. Meanwhile, however, 
Kirchhoff had independently been led to experiment in 
this direction, and, although his memoir slightly preceded 
mine in publication, I shall now give the experiments 
which I was led to make, more especially as they are 
very similar to those of Kirchhoff. In February, 1860, I 
communicated to the Royal Society of London a paper in 
which I showed that the light radiated by coloured 
glasses is intense, in proportion to their depth of colour, 
transparent glass giving out very little light. I also showed 
that the radiation from red glass has a greenish tint, 
while that from green glass has a reddish tint. It was 
likewise shown that polished metal gives out less light 
than tarnished metal and that when a piece of black and 
white porcelain is heated in the fire the black parts give 
out much more light than the white, thereby producing a 
curious reversal of the pattern. 
Finally, in a paper communicated in May of the same 
year, it was shown that tourmaline, which absorbs in 
excess the rays of light polarised in a plane parallel to 
the axis of the crystal, also radiates, when heated, this 
kind of light in excess, but that when it is viewed against 
an illuminated background of the same temperature as 
itself, this peculiarity disappears. All these facts are a 
natural consequence of a movable equilibrium of tempera- 
ture holding separately for every variety of heat, the word 
“variety ” embracing any difference either in wave-length 
or polarisation which is the cause of unequal absorption. 
The theory of exchanges, as here exhibited, has been 
founded upon the fact that in an enclosure of constant 
temperature all bodies will ultimately attain the tempera- 
ture of the walls of the enclosure. This is the experimental 
foundation upon which our structure has been built, and 
we have not attempted to work under it or to find whether 
in its turn it be not founded upon some principle of a still 
deeper and more fundamental nature. We shall now 
briefly indicate that such is the case, and that this law of 
ultimate equality of temperature is a consequence of the 
theory of energy in which we are told that no work can 
possibly be got out of heat which is all at the same tem- 
perature. For if the ultimate result in our enclosure 
should be a variety of temperatures, then it would be 
possible to utilise this temperature-difference and convert 
heat into work, so that there would practically result a 
case of perpetual motion. Now, it is one of the most 
fundamental axioms of physical science that such a motion 
is impossible. 
I have endeavoured to make use of this method of 
viewing the problem, in order to point out what forms 
