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D. S. (David Samuel) Margoliouth.

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I have hitherto left unmentioned one important method of investi-
gating the periods of molecular vibrations, a method which is applica-



•^.f T



THE TEACHINGS OF MODERN SPECTROSCOPY. 473

ble to low temperatures. If I have a transparent body and allow light
sent out by a body giving a continuous S2)ectrum to fall through it, I
often observe that the transparent body sifts out of the light falling
through it certain kind of rays. Spectra are thus produced which are
called absorption spectra, because the body which is under examination
does not send out any light, but absorbs some vibrations which are
made to pass through it. It is an important fact that a molecule
absorbs just the rays which it is capable itself of sending out. I can
therefore investigate the spectrum of a body just as well by means of
the absorption it produces as by means of the light which it sends out.

Vapors like bromine or iodine examined in this way give us a spec-
trum of fluted bands. A powerful spark in these gases gives, how-
ever, a line-spectrum. Here, then, a change of spectrum has taken
place. The same body at different temperatures gives us a different
spectrum, and the change which takes place is the same as that ob-
served in the spectrum of a compound body the moment the tempera-
ture has risen sufliciently to decompose that body. I conclude from
spectroscopic observations, therefore, that the molecules of bromine
and iodine just above their boiling-point are complex molecules, which
are broken up at the temperature of the electric spark. At high tem-
peratures the molecules of these bodies contain a smaller number of
atoms, and it follows from this that the gases must be lighter or that
their density must be smaller. These conclusions, which on spectro-
scopic grounds have been definite and clear for some years, have re-
cently, by independent methods, been confirmed by Victor Meyer and
others. It has been directly proved that at high temperatures the
molecules of iodine and bromine contain a smaller number of atoms
than they do just above their boiling-point. In other cases the change
of density has not been directly proved, only because these necessary
measurements are difficult or even impossible at very high tempera-
tures, but we may be perfectly sure that chlorine, as well as the metal-
lic vapors of silver, sodium, potassium, etc., which sho\Y an analogous
change of their spectra, will ultimately be proved to undergo a change
of density at high temperatures.

As we can trace the change from a line-spectrum to a band-spec-
trum taking place simultaneously with an increase of density, so may
we follow the change from a band-spectrum to a continuous spectrum
indicating the formation of a molecule still more complex.

Sulphur-vapor, at a temperature just above its boiling-point, con-
tains three times the number of atoms in one molecule that it does at
a temperature of 1,000° Centigrade. The spectrum of sulphur-vapor
observed by absorption is continuous when the heavier molecule only
is present. At the higher temperatures, when each molecule is decom-
posed into three, the spectrum belongs to the type of fluted baud-
spectra. From the cases in which we can thus prove the change in
the spectra and in the densities to go on simultaneously, we are justi-



^"f^y^Miy*^



474 THE POPULAR SCIENCE MONTHLY.



fied in concluding that also in other cases, where no such change of
density has yet been observed, it yet takes place ; and it is not a very
daring generalization to believe that a change in spectra is always due
to a change in molecular arrangement, and generally, perhaps always,
accompanied by a change in the number of atoms which are bound
together into one molecule.

With regard to the well-known statement that solids and liquids
give continuous spectra, while gases give line-spectra, it must be
remarked that metallic vapors show in nearly all cases a continuous
spectrum before they condense. Oxygen gives a continuous spectrum
at the lowest temperature at which it is luminous. Examining liquids
and solids by the method of absorption, we find that many of them
show discontinuous spectra, presenting fairly narrow bands. It is not
denied that the nearness of molecules does not affect the spectrum. It
may render the bands more wide and indistinct at their edges, but its
influence is more of a nature which in gas spectra is sometimes ob-
served at high pressures when the lines widen, and does not consist of
an alteration in type. Though in a solid or liquid body the molecules
are much nearer together, they are less mobile ; and hence the number
of actual collisions need not be necessarily much increased. The fact
that a crystal may sliow a difference in the absorption spectrum ac-
cording as the vibrations of the transmitted light take place along or
across the axis, shows, I think, that mutual impacts can not much
affect the vibrations, but that each molecule, at least in a crystal, must
be kept pretty well in its place.

"We have divided spectra into three types, but in all attempts at
classification we are met by the same difiiculty. The boundaries be-
tween the different types are not in all cases very well marked. Every
one will be able to distinguish a well-defined band-spectrum from a
line-spectrum, but there are spectra taking up intermediate positions
both between the line- and band-spectra and between band-spectra
and continuous spectra. With regard to these it may be difficult to
tell to which type the spectrum really belongs. It may happen that a
change of spectrum takes place, the spectrum retaining its type ; but
in these cases, as a rule, the more complex molecule will have a spec-
trum approaching the lower type, although it may not actually belong
to that lower type. To be perfectly general, we may say that a com-
bination of atoms always produces an alteration in the spectrum in
the direction of the change from the line-spectrum, through the band-
spectrum to the discontinuous spectrum.

If we accept the now generally received opinion as to the cause of
the different types of spectra, we may obtain information on molecular
arrangement and complexity where our ordinary methods fail. At
high temperatures, or under much diminished pressure, measures of
density become difiicult or impossible ; and it is just in these cases
that the spectroscope furnishes us with the most valuable information.



THE TEACHINGS OF MODERN SPECTROSCOPY.



475



If -we find three spectra of nitrogen and the same number for oxygen,
we must accept the verdict, and conclude that these gases can exist in
three different allotropic states.

Among the remarkable phenomena observed in vacuum-tubes, per-
haps not the least curious is the spectrum observed at the negative
pole, which in several cases is only observed there, and under ordinary
circumstances in no other part of the tube. Both oxygen and nitro-
gen have a spectrum which is generally confined to the negative glow.
Some years ago I tried to prove that also in these cases we have only
to deal with a special modification of the gases which, curiously enough,
only exists near the negative pole, and is broken up and decomposed in
every other part of the tube. The experiments I then made seem to
me to prove the point conclusively. After a current of electricity had
passed through the tube for some time in one direction, the current
was suddenly reversed ; the negative pole now became positive, but
the spectrum still was visible for some time in its neighborhood, and
only gradually disappeared. This experiment shows that the spectrum
may exist in other parts of the tube, and that it is therefore due to a
peculiar kind of molecule, and not to anything specially related to
electric phenomena taking place in the neighborhood of the negative
pole. Other experiments supported this view.

The classification of spectra, according to the complexity of the
vibrating molecule, is of great theoretical importance ; for by its
means we may hope to obtain some information on the nature of the
forces which bind together the atoms into one molecule. Our whole
life is a chemical process, and a great part of the mysteries of Nature
would be cleared up if we could gain a deeper insight into the nature
of chemical forces. I believe no other line of investigation to be as
hopeful in this respect as the one which examines directly the vibra-
tions of the molecules which take place under the influence of these
chemical forces. If we could find a connection between the vibrations
of a compound molecule and the vibrations of the simpler elements
which it contains, we should have made a very decided step in the de-
sired direction. I need not say that various attempts have been made
to clear up so important a point ; but we have to deal with compli-
cated forces, and the attempts have as a rule not been crowned with
much success.

There are, however, a few exceptions, a few cases of greater sim-
plicity than the rest, where we are able to trace to their mechanical
causes the spectroscopic changes which take place on .chemical com-
bination. These few and simple cases may serve as the finger-posts
which show us the way to further research, and, we may hope, to
further success. To make the spectroscopic changes of which I am
speaking clear to you, I must have recourse to the analogy between
sound and light, and remind you of the fact that when the prongs of
a tuning-fork are weighted its tone is lowered, which means that the



476



THE POPULAR SCIENCE MONTHLY.






period of vibration is increased, and consequently that the length of
the wave of sound sent out is lengthened. Now, suppose a molecule
or atom, the spectrum of which I am acquainted with, enters into com-
bination with another ; and suppose that the vibrations of the second

molecule are weak, or lie outside
the visible range of the spectrum :
then the most simple assumption
which I could make would be that
the addition of the new molecule
is equivalent to an increase of the
mass of the other. An increase
of mass without alteration of the
force of the molecule will, as in the
case of the tuning-fork, lengthen
the period of vibration, and in-
crease the wave-length. If a case
of that kind were actually to hap-
pen, I should observe the whole
spectrum shifting toward the red ;
and this is what is observed in the
few simple cases to which I have
referred. The first observation to
that effect is due to Professor Bun-
sen, of Heidelberg. Examining
the absorption spectra of different
didymium salts, he found such a
close resemblance between them,
that no difference could be detect-
ed with instruments of small pow-
ers ; but with larger instruments it
was found that the bands varied
slightly in position, that in the
chloride they were placed most
toward the blue end of the spec-
trum, that when the sulphate was
substituted for the chloride a slight
shift toward the less refrangible
end took place, and that a greater
shift in the same direction occurred
on examining the acetate. Prof.
Bunsen remarks that the molecular weight of the acetate is larger
than that of the sulphate, and that the molecule of the sulphate,
again, is heavier than that of the chloride. He adds : " These differ-
ences in the absorption spectra of different didymium compounds can
not, in our present complete state of ignorance of any general theory
for the absorption of light in absorptive media, be connected with



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THE TEACHINGS OF MOBERy SPECTROSCOPY. 477

other phenomena. They remind one of the slight gradual alterations
in pitch which the notes from a vibrating elastic rod undergo when
the rod is weighted, or of the change of tone which an organ-pipe ex-
hibits when the tube is lengthened." The accompanying woodcut
(Fig. 1), copied from Professor Bunsen's paper, may serve to illustrate
the shift observed in one of the absorption bands.

Similar changes take place when some substances like cyanin and
chlorophyl are dissolved in different liquids. Absorption bands char-
acteristic of these various substances appear, but they slightly vary in
position. Professor Kundt, who has carefully examined this displace-
ment of absorption bands, has come to the conclusion that as a rule
the liquids of high dispersive powers were those which shifted the
bands most toward the red end of the spectrum. But, though there is
an apparent tendency in this direction, no rule can be given which
shall be absolutely true whatever the substance which is dissolved.
Fig. 2 shows the absorption spectrum of cyanin when dissolved in dif-




A -= Ab.-orpiioii of Cyaniu in Bisulphide of Carbon.
B = '• " Nitrobenzene.

C = '* " Benzene.

D= " •' Ether.

E =: " " Alcohol.

ferent liquids. The measurements made by Claes* are employed.
"We have here an interesting proof that a solution is sometimes much
more of a chemical compound than is generally supposed. The sol-
vent and the substance must, indeed, be closely connected in order to
produce a shifting of the absorption band. On the other hand, it is
not astonishing that no general law can be given which connects the
displacement with the physical properties of the solvent, for the close-
ness of connection depending on the special chemical affinity for each
* "Wiod. Annalen," iii, p. 388, 1878.



478 THE POPULAR SCIENCE MONTHLY.

solvent has as mucli to do with the amount of shifting observed as
the molecular weight or the dispersion or refractive power may have.
The shifting of the absorption bands in different solutions of the same
substances is only one of many applications of spectroscopes to the
examination of molecular phenomena in liquids. Into the interesting
researches of Professor Russell, who has greatly extended this field of
inquiry, we have no time to enter.

The changes of spectra due to molecular combiiiations and rear-
rangements have in addition to their theoretical iAiportance a great
practical interest, for they will afford us some day a means of answer-
ing approximately a great many questions relating to the temperature
of sun and stars. The gases and vapors in the solar atmosphere are
for the greater part in the molecular condition in which they give a
line-spectrum, and we know of stars the spectra of which resemble our
solar spectrum very nearly. We shall not be far wrong in ascribing
to such stars a temperature similar to that of our sun. Other stars
have absorbing envelopes showing spectra of fluted bands. We know
that fluted bands belong to a more complex molecular condition, which
only can exist at lower temperatures. These stars, therefore, must
have a lower temperature than our sun. Dr. Huggins, who has suc-
ceeded in obtaining most valuable photographs of star-spectra, has
been able to classify and arrange star-spectra ; and it is more than
likely that, in the series of stars arranged in order by means of their
spectra, we have at one end those of the highest, at the other those of
the lowest, temperature. We are as yet far from being able to assign
any particular temperature to a star, but the question by means of the
spectroscope has been reduced to one which can be decided in our
laboratories, and, however difiicult it may be, we may rest assured that
it will ultimately be solved. As to our sun, its temperature has been
the subject of many investigations. Attempts have been made to
deduce it (at least approximately) from the amount of heat it sends
out. Different experimental laws have been proposed to connect to-
gether the heat radiation of a body, and the temperature of that body.
The first law which was thus proposed gives 10,000,000° Centigi'ade
as a lower limit ; the second law reduces that lower limit to a little
over 1,500°. Both these laws we now know to be wrong. More ac-
curate laws give something like 10,000° or 20,000°, but the whole
method employed is one which is open to a great many objections.

We measure the combined heat radiation of different layers on the
solar surfaces, all of w^hich are at different temperatures, and we ob-
serve only an average effect which is much influenced by the absorp-
tion in the outer layers of the solar atmosphere and in the corona, and
does not admit of easy interpretation. The spectroscopic method,
which is yet in its infancy, has the advantage that we can observe
separately each layer of the sun ; and we thus examine the tempera-
ture not as an average, but for every part of the solar body. Our way



THE TEACHINGS OF MODERN SPECTROSCOPY. 479

to proceed would consist in carefully observing the spectra in different
layers of the sun. Supposing we observe a change at one point, we
may investigate at what temperature that change takes place, and we
may then ascribe the same temperature to that particular place at the
solar surface, if no other cause has interfered which may have affected
our result. This last conditional limitation leads us to the discussion
of the important but difficult question, whether we can determine any
such interfering cause, which, not being temperature, yet produces the
same change in a spectrum which we have hitherto only ascribed to
changes of temperature.

I must here remark that a change in type is not the only spectro-
scopic change in the spectrum which is observed to take place on vary-
ing the temperature. Line-spectra especially are subject to curious
variations in the relative intensities of their lines. These variations
follow no general rule, and must be investigated separately for each
element. The cause of this variation is a subject on which there exists
a great difference of opinion ; but, whatever this cause may be, if the
chaises always take place at one fixed temperature, we can turn them
into account in measuring that temperature. However strong our
wish that such a spectroscopic measurement of temperature may ulti-
mately be obtained, a remarkable complication of facts has delayed
the realization of this hope for at least a considerable period of time.

We have to enter partly into a theoretical question, and I must
necessarily allude to some of the facts recognized by all who believe
in the molecular theory of gases. Each molecule, which, as we have
seen, sends out rays of light and heat on account of its internal mo-
tion, is surrounded by other molecules. These are, indeed, very closely
packed, and continually moving about with enormous velocities. Gen-
erally they move in straight lines, but it must necessarily happen that
often they come very near, and then affect and deflect each other.
Perhaps they come into actual contact, perhaps they repel each other
so strongly when near, that contact never takes place. The time
elapsing between two such collisions is very small. If you can imagine
one second of time to be magnified to the length of a hundred years,
it would only take about a second, on the average, from the time a
molecule has encountered one other molecule until it encounters the
second. During the greatest part of this very short time, it moves in
a straight line, for the forces between molecules are so small that they
do not affect each other unless their distance is exceedingly small. It
is, therefore, only during a very small fraction of time that one mole-
cule is under the influence of another, and it is one of the greatest
problems of molecular physics to find out what happens during that
short element of time. I should like to explain to you how I believe
the spectroscope may contribute its share to the settlement of that
question. In his first great paper on the molecular theory of gases,
the late Professor Clerk Maxwell assumed that two molecules may



48o THE POPULAR SCIEXCE MONTHLY.

actually come into contact, that tliey may strike each other, as two
billiard-balls do, and then separate, according to the laws of elastic
bodies. This theory is difficult of application when a molecule con-
tains more than one atom, and, especially as it did not in the case of
conduction of heat give i-esults ratified by the experimental test, Max-
well abandoned it in favor of the idea that molecules repel each other
according to the inverse fifth power of the distance. This second
theory not only gave what at the time was believed to be the correct
law for the dependence of the coefficient of conduction on tempera-
ture, but it also helped its author over a considerable mathematical
difficulty. Further experiments have shaken our faith in the first of
these two reasons, and the second is not sufficient to induce us to adopt
without further inquiry the new law of action between two molecules.

It is exceedingly likely that the forces acting between two mole-
cules when they are in close proximity to each other are partly due
to, or at least modified by, the vibrations of the molecules themselves.
Such vibrations must, as in the case of sound, produce attractive and
repulsive forces, and vibrating molecules will affect each other in a simi-
lar way as two tuning-forks would. Now, if the forces due to vibra-
tions play any important part in a molecular encounter, the spectroscope
will, I fancy, give us some information. If two molecules of the same
kind encounter, the periods of vibration are the same, and the forces
due to vibration will remain the same during, perhaps, the whole en-
counter. If two dissimilar molecules encounter, the relative phase of
the vibrations, and hence the forces, wall constantly change. Attraction
will rapidly follow repulsion, and the whole average effect will be much
smaller than in the case of two atoms of the same kind. We have no
clear notion how such differences may act, and we must have recourse
to experiment to decide whether any change in the effect of an encoun-
ter is observed when a molecule of a different kind is substituted for a
molecule having the same periods of vibration.

"When a body loses energy by radiation, that energy is restored
during an encounter ; the way in which this energy is restored will
profoundly affect the vibrations of the molecule, and hence the ob-
served spectrum. I have endeavored by means of theoretical consid-
erations, or speculations, as you may perhaps feel inclined to call them,
to lead you on to an experimental law w^hich I believe to be of very
great importance. The spectrum of a molecule is in fact variable at
any given temperature, and changes if the molecule is surrounded by
others of different nature.

Placing a molecule in an atmosphere of different nature xoithout
change of temperature produces the same effect as loould be observed
on lowering of temperature.

Let me give you one example. Lithium at the temperature of the
Bunsen flame has almost exclusively one red line in its spectrum. At
the high temperature of the arc or spark the red line becomes weak.



TUE TEACHIXGS OF MODERN SPECTROSCOPY. 481

and almost entirely disappears. It is replaced by a strong orange line,
which is already slightly visible, though weak, at low temperatures,
and by additional green and blue lines.

But even at the high temperature of the spark we may obtain again
a spectrum containing the red line only if we mix a small quantity of
lithium with a large quantity of other material. The same spark, for
instance, will give us the low-temperature spectrum of lithium when
taken from a dilute solution of a lithium salt, and the high-temperature
spectrum when that solution is concentrated.

The spectra of zinc and tin furnish us other examples in the same
direction, but the spectra of nearly all bodies show the same law in a
more or less striking way.

If this law which I have given you is a true one,* and I believe it
will stand any test to which no doubt it will be subjected, we shall be'
able to draw some important conclusions from it. In the first place, it
will be proved that the forces between atoms do depend on their vibra-
tions. If this is true, any change in the vibrations of the spectrum,
however small, will entail a corresponding change in all the other prop-
erties of the body. On the other hand, any change in the affinities of
the element observed by other means will be represented by a change
in the spectrum.

It is also possible that the introduction of forces due to vibratory
motion will help us over a considerable difficulty in the molecular
theory of gases. Some of the conclusions of that theory are at pres-
ent absolutely contrary to fact, A spectroscopist, for instance, who is
acquainted with the mercury spectrum and all the changes in that
spectrum which can take place, feels more than skeptical when he is



Online LibraryD. S. (David Samuel) MargoliouthThe Popular science monthly (Volume 19) → online text (page 59 of 110)