468 
tion. What I have to dwell on now is, that the absorption 
or sifting of light by different bodies is very like radiation 
in its results—that is to say, in some cases we have an 
absorption which deals equally with every part of the 
spectrum, and in other cases we have absorption which 
only picks out a particular part of the spectrum here and 
there to act upon. But there is one important point to be 
borne in mind; when dealing with absorption we must 
always have a continuous spectrum toact upon. Ifwe had 
a discontinuous spectrum to act upon, the thing would not 
be at all soclear. Having this continuous spectrum, the 
problem is, what the action of the different substances on 
the light will be. Let me give you an instance of general 
absorption. If we take the continuous spectrum above re- 
ferred to, and interpose a piece of smoked glass, or better, a 
piece of neutral-tint glass, you will find that the substance 
will cut off the light and deaden the spectrum, so to speak, 
throughout its whole length. This neutral-tinted glass, 
then, has the faculty 
evidently of keeping 
back the light, red, yel- 
low, blue, green, violet, 
and so on; and is an 
instance of general ab- 
sorption. A very dense 
vapour would furnish 
us with another similar 
instance. Now, instead 
of using the neutral-tint 
glass, we will introduce 
a piece of coloured 
glass, the action of 
which, instead of being 
general throughout the 
spectrum, will be limit- 
ed to a particular part 
of it. I have now in- 
terposed a piece of red 
glass, which cuts off 
nearly all the light ex- 
cept the red; and now 
I interpose a piece of 
blue glass, which cuts 
off everything except 
the extreme violet. By 
introducing both these 
pieces in the beam, 
the spectrum is entirely 
obliterated. 
In these latter cases 
we have instances, not 
of general, but of selec- 
tive absorption, one 
substance cutting off 
everything but the red, 
and the other cutting 
off everything but the 
violet. Now the fact that we can absorb any definite 
part of the spectrum by properly tinted glasses provides 
us with a practical application of spectrum analysis in the 
manufacture of the coloured glass used for lighthouses or 
signals. Further, if astronomers could find a glass of a 
certain red, or a glass of a certain green colour, we should 
be able to see the solar prominences every day without a 
spectroscope. 
The first practical application which springs out of 
these phenomena of absorption is this, that as different 
substances are known by the effects which they produce 
on radiation, so also chemists find it perfectly easy to 
detect different substances by means of their absorption ; 
for instance, the absorption spectrum of nitrous fumes 
can be shown by taking first our continuous spectrum, 
which we must always have to start with, and intro- 
ducing some nitric peroxide between the source of light 
Fic. 45.—Absorption spectra of iodine and 
nitrous fumes. 
NATURE 
| April 17, 1873 
and the prism. The nitric oxide, immediately it comes in 
contact with the air, produces dense red fumes, and num- 
bers of fine black lines will be seen immediately crossing _ 
the spectrum at right angles to its length, and to a certain 
extent resembling the solar spectrum with its Fraunhofer 
lines. Iodine is another substance which gives a coloured 
vapour, the absorption spectrumbof which is very definite 
and well defined. Fig. 45, Spectrum No. 1, shows the 
absorption spectrum of iodine vapour, and No. 2 that of 
nitrous fumes. We are not limited to these substances ; 
we will try something else—blood, for instance, about 
which I shall have something more to say presently. 
We shall find that the action of blood upon the light is 
perfectly distinct from the action of those fumes which 
we have spoken of; and instead of having typical 
lines in the green and blue parts of the spectrum, we have 
two very obvious lines in the more luminous part of the 
spectrum. The colour of a solution of blood is not un- 
like the colour of a solution of magenta ; but if, instead of 
using a solution of blood, we use a solution of magenta, 
we should have only a single black band. The absorp- 
tion spectrum of potassic permanganate solution is an- 
other beautiful instance. We have here something totally 
unlike anything we had before. Instead of the two dark 
bands which we saw in the case of the blood, or the single 
band in the case of magenta, we have four very definite 
absorption bands in the green part of the spectrum. So 
that you see the means of research spectrum analysis 
affords as far as regards radiation, is entirely reproduced 
in the case of absorption, and it is perfectly easy, by 
means of the absorption of different vapours and different 
substances held in solution, to determine not only what 
the absorbers really are, but to determine the presence of 
an extremely small quantity. Further, by allowing the 
light to pass through a greater thickness of the absorbing 
substance, the absorption lines are thickened and new 
regions of absorption are observed. This fact was dis- 
covered by Dr. Gladstone, who used hollow prisms con- 
taining the substance. 
J. N. LOCKYER 
(To be continued.) 
PROFESSOR ZOLLNER ON THE CONNECTION 
BETWEEN COMETS AND METEORS 
PROEESSOR F, ZOLLNER alludes in the com- 
mencement of his-paper read before the members of 
the “‘Kon; Sachs., Gesellschaft der Wissenschaften” to the 
epoch which Schiaparelli’s discovery of the concordance 
of the orbits of some small comets with those of peri- 
odically returning showers of shooting stars has made in 
the astronomical world. He quotes an instance in proof 
of this, namely, Biela’s Comet. On November 27, last 
year, the earth was crossing the exact spot in her orbit, 
which had been cut by Biela’s Comet two and a half 
months before. Observers aware of the coming event 
were on the alert with their instruments, but no good 
results were obtained owing to the unfavourableness. of 
the weather. 
From these facts, he says, we must naturally conclude 
that the physical constitution of these bodies is the same, 
and we are strengthened in our conclusions by Schia- 
parelli’s discovery of the identity of the envelopes and 
tails of comets with clouds of meteors seen by reflected 
sunlight, the separate elements of which only become 
visible at a shorter distance. 
Observations, however, with the spectroscope, con- 
Te 
tradict this assumption; the light given out by comets. - 
is found not to correspond with that of the Sun; itisa 
light peculiar to them, like that of a glowing gas. 
Further on he quotes Schiaparelli’s own words to some 
length, with respect to the attraction exercised by other 
bodies on the matter composing the nuclei of comets, 
