THE
ABSORPTION SPECTRA OF SOLUTIONS

OF COMPARATIVELY RARE SALTS

INCLUDING THOSE OF GADOLINIUM, DYSPROSIUM, AND SAMARIUM

THE SPECTROPHOTOGRAPHY OF CERTAIN CHEMICAL REACTIONS

AND THE EFFECT OF HIGH TEMPERATURE ON THE ABSORPTION SPECTRA

OF NON-AQUEOUS SOLUTIONS

BY
HARRY C. JONES and W. W. STRONG

WASHINGTON, D. C.
Published by the Carnegie Institution of Washington

1911

CARNEGIE INSTITUTION OF WASHINGTON

Publication No. 160

ID y ro

PRESS OF J. B. LIPPINCOTT COMPANY
PHILADELPHIA

THE ABSORPTION SPECTRA OF SOLUTIONS

OF COMPARATIVELY RARE SALTS

INCLUDING THOSE OF GADOLINIUM, DYSPROSIUM, AND SAMARIUM

THE SPECTROPHOTOGRAPHY OF CERTAIN CHEMICAL REACTIONS

AND THE EFFECT OF HIGH TEMPERATURE ON THE ABSORPTION SPECTRA

OF NON-AQUEOUS SOLUTIONS

BY

HARRY C. JOXES and W. W. STRONG

PREFACE.

The work recorded in this monograph on the absorption spectra of solu-
tions is a continuation of that already published by Jones and Uhler (Publi-
cation No. 60, Carnegie Institution of Washington), Jones and Anderson
(Publication No. 110, Carnegie Institution of Washington), and Jones and
Strong (Publication No. 130, Carnegie Institution of Washington). This
work, like that recorded in the preceding publications, has been made possible
by grants awarded by the Carnegie Institution of Washington.

The attempt has been made in this work to solve three problems:

First, to map out the absorption spectra of certain comparatively rare
substances. Urbain has generously forwarded Dr. Strong ample quantities of
the oxides of gadolinium, dysprosium, arid samarium with which to prepare
their salts and study their spectra. We find that the salts of dysprosium and
samarium have spectra that are almost as interesting as those of neodymium,
with very sharp, characteristic bands.

Second. Under the spectrophotography of chemical reactions we have
studied especially the effect of oxidizing agents on uranous salts. We have
used milder oxidizing agents and stronger oxidizing agents, and have dissolved
the uranous salt in single solvents and in mixed solvents. When uranous
chloride is dissolved in a mixture of alcohol and water, both the "alcohol" and
the "water" bands come out simultaneously on the plate. A mild oxidizing
agent was found to oxidize the "hydrated" salts and to leave unaffected
the " alcoholated " salts. A strong oxidizing agent, on the other hand, oxi-
dized both the "hydrated" and the "alcoholated" uranous salt to the uranyl
condition. This is, therefore, an example of "selective oxidation."

Third. By means of the closed cell we have been able to study the absorp-
tion spectra of solutions in methyl and ethyl alcohols up to temperatures as
high as 195°. The absorption bands widen with rise in temperature up to the
highest temperatures employed. Colored solutions, therefore, become more
and more nearly opaque as the temperature to which the solutions are sub-
jected is raised.

The effect of rise in temperature on the absorption spectra of neodym-
ium salts in mixtures of alcohol and water, where both the "alcohol" and
the "water" bands appear simultaneously, has been studied. The "water"
bands are more affected by rise in temperature than the "alcohol" bands,
showing that the "hydrates" are less stable with rise in temperature than the
"alcoholates."

The problems now under investigation include the effect of ions as com-
pared with molecules on the absorption spectra of solutions, the effect of
high temperature on the absorption spectra of aqueous solutions, and the
measurement of the intensity of the various absorption bands by means of
the radiomicrometer and thermoelectric junctions.

It gives me special pleasure to express my indebtedness and thanks to
the Carnegie Institution of Washington for the generous support which they

VI PREFACE.

have given this work in the past and for the aid which they have tendered for
the future.

I wish to express my thanks to Professor J. S. Ames for ample space in
the Physical Laboratory for carrying out the physical part of this work; to
Professor J. A. Anderson for a large number of valuable suggestions in con-
nection with the work; to Professor A. H. Pfund for advice in connection with
the construction of the radiomicrometer; and to Dr. J. Sam Guy, who will
continue this work, for aid especially in preparing the various salts and solu-
tions of the salts of dysprosium and samarium.

Harry C. Jones.
June 1911.

CONTENTS.

PA.OE.

Chapter I. The Absorption and Emission Centers of Light and Heat:

Atomic Structure and Spectra 1

The Ionization Theory and Absorption and Emission Centers 2

Sulphur Bands 3

The Carriers of Canal-Ray Spectra 4

The Carriers of Spark Spectra 5

The Emission Centers of Flames and Arcs t>

Schumann Waves G

The Possible Catalytic Action of Light ('»

Emission Spectra of Organic Compounds 7

The Absorption Spectra of Organic Compounds 8

The Unit of this Absorption 8

The Theory of Chromophores 10

Influence of the Solvent 15

The Absorption Spectra of Benzene and Its Derivatives 16

Theory of Dynamic Isomerism ' 18

Theory of Stark 23

Other Benzene Theories 24

Chapter II. Experimental Methods and Apparatus:

Chapter III. Mapping the Absorption Spectra of Various Salts in Solution:

The Absorption of Certain Cyanides and Chromates 31

Calcium Ferrocyanide and Calcium Ferricyanide 32

The Chromate and Bichromate of Lithium 33

Aluminium and Calcium Chromates 33

Potassium Nickel Chromate and Copper Bichromate 33

The Absorption of Solutions of Certain Erbium Salts 33

The Absorption of Solutions of Certain Neodymium Salts 34

Neodymium Chloride in Water 35

Neodymium Chloride as a Methyl Alcoholate 35

Neodymium Chloride in Propyl Alcohol 30

Neodymium Chloride in Isopropyl Alcohol 30

Neodymium Chloride in Butyl Alcohol 36

Neodymium Chloride in Isobutyl Alcohol 37

Neodymium Chloride in Ether 37

Neodymium Nitrate in Propyl Alcohol 38

Neodymium Nitrate in Isopropyl Alcohol 38

Neodymium Nitrate in Butyl Alcohol 38

Neodymium Nitrate in Isobutyl Alcohol 39

Neodymium Nitrate in Acetone 39

Neodymium Nitrate in Ethyl Ester 39

Neodymium Acetate in Acetone 40

Neodymium Acetate in Formamide 40

Summary of Neodymium Spectra 40

The Absorption Spectrum of Solutions of Certain Salts of Uranium 43

Uranyl Chloride in Propyl Alcohol 43

Uranyl Chloride in Isopropyl Alcohol 43

Uranyl Chloride in Butyl Alcohol 43

Uranyl Chloride in Isobutyl Alcohol 43

Uranyl Chloride in Ether 43

Uranyl Chloride in Methyl Ester 44

Uranyl Chloride in Ethyl Ester 44

Uranyl Chloride in Formamide 44

Uranyl Nitrate in Propyl Alcohol 44

Uranyl Nitrate in Acetone 44

Uranyl Nitrate in Methyl Ester 44

Uranous Chloride in Propyl Alcohol 44

Uranous Chloride in Isobutyl Alcohol 45

Uranous Chloride in Methyl Ester 45

The Absorption Centers of Uranium Spectra 45

The Absorption Spectrum of Gadolinium 46

Gadolinium Chloride in Water 46

Gadolinium Chloride in Ethyl Alcohol 46

vii

viii CONTENTS.

Chapter 111 continued. page.

The Absorption Spectrum of Dysprosium 4b

Dysprosium Chloride in Water 47

Dysprosium Chloride in Methyl Alcohol 47

Dysprosium Chloride in Ethyl Alcohol 47

Dysprosium Acetate in Water 48

The Absorption Spectrum of Samarium 48

Samarium Chloride in Water 48

Samarium Nitrate in Water 49

Samarium Chloride in Methyl and Ethyl Alcohols 49

Samarium Chloride in Water and Ethyl Alcohol 50

Chapter IV. The Spectrophotography of Chemical Reactions:

Introduction ■ • 51

Phosphorescence of Praseodymium in Calcium Oxide 54

Phosphorescence of Neodymium in Calcium Oxide 54

Phosphorescence of Erbium in Calcium Oxide 54

Phosphorescence of Praseodymium in Calcium Sulphate 54

Phosphorescence of Erbium in Calcium Sulphate 54

Phosphorescence of Erbium in Calcium Fluoride 54

Neodymium Chloride in Ethyl Acetate and Anthracene 56

The Uranyl and Uranous Bands 57

The Oxidization of Uranous to Uranyl Salts in Solution 58

The ( bridization of Uranous Chloride by Hydrogen Peroxide 58

Oxidization of Uranous Chloride in Hydrochloric Acid by Hydrogen Peroxide 60

The Oxidization of Uranous Sulphate 61

The Oxidization of Uranous Bromide by Hydrogen Peroxide 61

A Possible Method of Measuring the Strengths of Acids 62

Are the Ions Factors in the Absorption of Light ? 62

The Oxidization of Uranous Salts by Nitric Acid 64

Uranous Acetate and the Effect of the Addition of Nitric Acid 65

The Selective Action of Chemical Reagents on Solvates 66

The Reduction of Uranyl Salts in Solution 69

Reduction of Uranyl Chloride in Methyl Ester 69

Selective Reduction of Uranyl Aggregates 70

Direct Spectroscopic Evidence for the Effect of Mass 71

Chapter V. The Effect of Temperature on Absorption Spectra:

Solvates and the Effect of Change in Temperature on the Relative Intensity of

Solvate Bands 7.4

Neodymium Chloride in Water and Ethyl Alcohol 77

Neodymium Bromide in Water and Methyl Alcohol 77

Neodymium Chloride, Bromide, and Nitrate in Water 77

Neodymium Chloride in Methyl Alcohol 78

Neodymium Bromide in Methyl Alcohol 79

Neodymium Nitrate in Isobutyl Alcohol 79

Neodymium Nitrate in Acetone 80

Erbium Chloride in Water 80

I franyl Chloride in Acetone 81

Uranyl Nitrate in Propyl Alcohol 81

I Iranyl Chloride and Nitrate in Isobutyl Alcohol 81

I i-anyl Chloride and Nitrate in Methyl Ester 81

Uranous Chloride in Water and Methyl Alcohol 82

I ranous Chloride in Acetone 82

The Existence of Aggregates and Their Properties, and the Effect of Rise in Tem-
perature on the Aggregates 83

Neodymium Salts in Acid Solutions 84

Uranous Sulphate in Sulphuric Acid 85

Uranyl Nitrate in Nitric Acid, Uranyl Chloride in Hydrochloric Acid, and

iranyl Sulphate in Sulphuric Acid 86

Chapter VI. Summary and General Discussion:

Mapping of Spectra 87

A Theory of Absorption Spectra 89

Solvation 92

The Uranyl and I'ranous Bands 94

Aggregates and Their Properties 95

The Effect of Temperature on Absorption Spectra 98

Description of Plates 101

Index 102

CHAPTER I.

THE ABSORPTION AND EMISSION CENTERS OF LIGHT

AND HEAT.

During the last decade a considerable number of physicists have directed
their efforts towards solving some of the many problems of spectroscopy. The
following chapter will contain a discussion of some of this work, with the view
of recording part of our experimental knowledge concerning the nature of the
emission and absorption centers of light; the connection between these centers
and molecular and atomic structures; the effect of ionization and recombina-
tion on these centers, and the effects and changes that can be produced by
physical and chemical agents such as temperature, the presence of a mag-
netic field, etc., upon the constitution of the emission and absorption centers.

Emission and absorption centers will be denned as the smallest particles
from which we can obtain characteristic absorption or emission spectra. A
further division of the centers of any characteristic spectrum would make it
impossible to obtain that spectrum, though the resultant particles may possess
characteristic absorption or emission spectra of their own. When emission
or absorption centers move with reference to an observer, the frequencies of
the spectral lines and bands will show the Doppler effect.

ATOMIC STRUCTURE AND SPECTRA.

A very important problem is that of the relation between chemical con-
stitution and absorption or emission spectra, although the relation between
flame, spark, and arc spectra, and the chemistry of the absorption and emission
centers may not be known. Even the source of spectra like that from the blue
cone of a bunsen burner, the Swan spectra, is at present a much mooted ques-
tion. It is probable that chemical reactions play an important role in emis-
sion and absorption spectra, and especially in band spectra. We usually think
of most spectrum lines, like Di and D2 of sodium, as coming from the metallic
atoms. Fredenhagen points out that under most conditions oxygen is present.
In chlorine, hydrogen, or fluorine flames, calcium, strontium, thallium, sodium,
barium, and copper emit spectra that are very different from those obtained
when oxygen is present. Under these conditions thallium does not emit the
characteristic green line, and the lines Di and D2 are completely absent. Work
on the absorption of sodium, mercury, potassium, and various other vapors
shows that the presence of foreign gases modifies the character of the absorp-
tion very much.

Chemical reactions and processes of ionization and recombination are
believed to place the atom or molecule in a peculiar condition, in which it
can emit energy to the ether or absorb energy from it. Under ordinary con-
ditions the atom does not seem capable of doing this. In sodium vapor, for
instance, according to present theories only one atom in thousands is taking
part in absorption at any one time. The problem as to how energy is trans-
1 1

2 THE ABSORPTION SPECTRA OF SOLUTIONS.

ferred to and from matter is one of the most fundamental problems of science.
A striking example of the fact that a few atoms under peculiar conditions have
the power of absorbing an enormous amount of energy is exhibited by the iron
absorption lines in the solar spectrum. An arc of carbon electrodes containing
iron as an impurity emits enough iron vapor to absorb as much as the iron
vapor in the sun. It is thus seen that an infinitesimal amount of iron in the
very great atmosphere of the sun is sufficient to absorb a large part of the
energy emitted by the photosphere.

Our present theory of the mechanism of the absorption and emission of
radiations is very simple. Light and heat are electromagnetic radiations, and
hence the emission and absorption centers must contain either or both electric
charges and magnetic poles. As free magnetic poles are unknown to us, while
free electric charges are known, this theory makes the electric charge the origin
of electromagnetic phenomena. At present no positive electric charges are
known to be associated with portions of matter smaller than the hydrogen
atom. On the other hand, negative electrons are known to be associated with
masses only about one two-thousandth that of the hydrogen atom. As far as
experiment shows, these electrons always have the same properties and the
same charge (the charge is invariably considered as constant when e/m varies),
no matter from what element they may come. It is for these reasons that the
electron is made the fundamental unit in the electromagnetic theory.1

Electromagnetic radiations, then, have their origin in electric charges.
Continuous spectra (as from hot metals) are due to free electrons, and these
apparently have very little connection with the chemical constitution of the
metal molecules. Fine line and band spectra are apparently due to different
systems of electrons within the atom, and are greatly affected in intensity
by the presence of neighboring atoms. The electrons of this type vibrate in
definite frequencies that can be changed onty very slightly by changing the
external conditions.

THE IONIZATION THEORY AND ABSORPTION AND EMISSION CENTERS.

Several investigators have considered that band spectra are due to vibra-
tions in some way peculiar to a condition existing during the dissociation of mole-
cules or the recombination of the dissociated parts. According to Koenigs-
berger and Kiipferer2 the band spectra of iodine, bromine, nitrogen dioxide,
sulphur, iodine trichloride, and probably nitrogen and the other gases are due
to a dissociation of this kind. Iodine is a typical example. At 60° C. and
about 4 mm. pressure the reaction, as expressed by them, is as follows:

I2<=±I+I

At about 800° C. this reaction is practically complete. According to them,
iodine possesses a continuous absorption which has a maximum in the green,
and this continuous absorption decreases with rising temperature. At 600°
and above, the absorption is almost entirely in the violet and ultra-violet and
is due to the iodine atom. The banded absorption, consisting of thousands of
fine lines, results whenever chemical reactions represented by the above equa-

'Phys. Zeit., 8, 72!) (1907). 2 Ibid., 11, 568 (1910).

ABSORPTION AND EMISSION CENTERS. 3

tion take place. During these reactions it is possible that the atoms may exist
in the so-called "nascent " condition, and their electrons may then be subjected
to very little, if any, damping. In such reactions it might be expected that
the gas would be greatly ionized. This, however, is not the case, as was shown
by Henry1 and Whiddington.2

Several workers, including Galitzin and Wilip,3 have studied the absorp-
tion spectra of bromine at different temperatures and pressures. As the tem-
perature is raised the fine band absorption spectrum changes, becomes more and
more indistinct, and finally disappears at high temperatures. Evans4 has
shown that the temperature of the disappearance of the absorption bands is
higher the greater the pressure. The disappearance of the absorption lines
is closely connected with the dissociation of the diatomic bromine molecules,
and the disappearance of absorption at high temperatures can be explained
by assuming that the monatomic molecules have no absorption between X 3500
and X 6800.

Heated bromine and iodine vapors give an emission spectrum that coin-
cides with the absorption banded spectrum. Evans believes that the absorp-
tion spectra disappear as soon as the emission due to the dissociation and
recombination of the diatomic molecules is equal to the absorption of the
undissociated state. Evans gives the following table:

Pressure of
bromine

Temperature
of disappear-

Fraction of

Fraction of
dissociation

ance of

dissociation.

observed by

spectrum.

spectroscope.

mm.

°c.

24

950

0.22

0.25

35

1030

.37

.39

49

1090

.49

.51

63

1136

.58

.58

88

1220

.74

.68

160

1320

.84

.79

246

0)

o

1 Spectrum still present at 1320° C.

- Tube becomes opaque, collapsing at 1400° C.

Sulphur Bands.

Graham5 has investigated the absorption spectra of sulphur vapor between
530° and 900° C, the pressure being in general about 10 mm. of mercury.
Above 580° C. the dissociation is considered to be from S8 to S2. At or below
520° C, Graham believes that there are intermediate compounds formed
between S8 and S2. The following are the wave-lengths of some of the sul-
phur bands:

1 Proc. Camb. Phil. Soc, 9, 319 (1897).

2 Ibid., 15, 189 (1909).

3 Memoires de 1'Academie des Sciences de St.-Petersbourg, 17, 1-112 (1906).
« Astrophys. Journ., 32, 2S1 (1910).

6 Proc. Roy. Soc, A, 84, 311 (1910).

THE ABSORPTION SPECTRA OF SOLUTIONS.

e

S2

4775

4245

3415

3060

2805

4705

4195

3365

3025

2770

4645

4150

3330

2990

2745

4580

4100

3290

2960

2715

4530

4050

3255

2930

2690

4465

4005

3215

2900

2665

4405

3985

3170

2860

2640

4350

3130

2835

2620

4290

3095

The question as to whether the fluorescence of a gas or liquid influences
its conductivity has in general been answered in the negative. Nichols and
Merritt1 reported that the conductivity of an alcoholic solution of eosin was
increased when the solution was caused to fluoresce. Carmichel2 and Regner3
found no such effect, while Hodge4 and Goldman5 have shown that the effect
found by Nichols and Merritt was due to an electromotive force produced
by the light at the electrodes of the cell. Howe6 finds that if the fluorescence
of anthracene increases the conductivity, the increase is too small to measure.
Wood could detect no increase in the conductivity of sodium vapor when it
was caused to fluoresce.

On the other hand, Nichols and Merritt7 believe that in the case of the
fluorescence of solutions of fluorescein, the emission center may be the ion,
although they have examined the effect of change of concentration upon the
absorption spectrum of eosin and find very little effect. The absorption curve
is steep towards the red, and gradual towards the violet. The fluorescent
curve slopes in the opposite manner. Their work indicates that the molecules
and ions seem to behave in much the same way in absorption phenomena.
The same writers* have investigated the problem as to whether the fluorescence
excited per unit of absorbed energy is constant for all wave-lengths. Confin-
ing the range of absorption to that of a single band, they conclude that the
light near the red side of the band is more effective in producing fluorescence
than that lying on the violet side; and the change in specific exciting power
in passing along the band is continuous, without any indication of anything
selective in the neighborhood of the region of maximum absorption.

THE CARRIERS OF CANAL-RAY SPECTRA.

A large number of investigations have been made on the spectra produced
by canal rays. In the earlier work Stark was of the opinion that the renewal
of energy to a vibrating atom took place at the moment of collision between
the radiating atom and some other moving part; and the smaller the mass of
the particle collided with, the more efficient it would be in exciting vibrations
inside the atom, since the velocity of the small particle is greater and the time
occupied by a collision would be shorter. Continuous radiation of energy by
an atom moving with a comparatively small velocity would only be made

'Phys. Rev., 19,296 (1904).

2 Jour, de Phys.; 4, 873 (1905).

3 Phys. Zeit., 4,862 (1903).

4 Ibid., 28, 25 (1908).

5 Ann. d. Phys., 27, 332 (1908).

6 Phys. Rev., 30, 453 (1910).

7 Ibid., 31, 376 (1910).

8 Ibid., 381 (1910).

ABSORPTION1 AND EMISSION CENTERS. 5

possible, then, by its frequent collisions with free electrons. On the other
hand, band spectra, which are not in general affected by electric fields, Stark
considers as being due to the combining of a positive ion and a negative
electron. During this recombination there is a considerable decrease in the
potential energy of the system, and it is from this energy that the band spec-
trum is produced.

In their early work Stark and Riecke1 describe an arc-like discharge
between copper electrodes of about 3600 volts difference in potential. The
vapors of sodium or lithium salts placed in this discharge spread towards
the negative wire, thus indicating that the carriers are positive; this being
a direct contradiction of Lenard's results.

Most of Stark's later work has been done with canal rays. They move
towards the cathode and pass through any openings in it, with a velocity of
about 10s cm. per second. If there is hydrogen in the tube both the line and
band spectra of hydrogen may be seen. In nitrogen the line spectra are diffi-
cult to obtain. The lines of hydrogen, nitrogen, mercury, sodium, potassium,
etc., are found to show the Doppler effect when viewed in the direction in
which the canal-ray particles are moving.

The substances showing the Doppler effect also give spectra that do not
show the Doppler effect, i.e., there are rest lines and shifted lines. The rest
line is usually much the sharper of the two, and the space between the two
lines is usually dark. The line shifted towards the violet usually has its sharp-
est side on the red. The rest line comes largely from the gas through which
the canal rays are passing. The displaced line is composed of light radiated
by the canal-ray particles themselves, and since these lines are hazy, the canal
rays are moving with different velocities, according to the part of the "cathode
fall" region from which the particles originate. The particles that traverse
the whole region of the cathode fall will accordingly have the maximum veloc-
ity. Having passed through a field of from 300 to 500 volts, the velocity of
the canal-ray particles becomes sufficient to produce ionization by collision.

Knowing the cathode fall, the mass of the canal particle, and the maxi-
mum displacement, Stark calculates the charge of the canal particle. For the
hydrogen series, the principal and subordinate series of sodium and potas-
sium, and for certain lines of mercury, the value of the charge, according to
the earlier papers of Stark, is the same as that of the elementary charge of
electricity. Stark also considered the mercury triplets to be due to particles
carrying a double charge, and the mercury line X 4078.1 to be due to a carrier
having more than two charges.

THE CARRIERS OF SPARK SPECTRA.

Among investigators who have taken up the problem as to what is the
nature and velocity of the particles in sparks that are acting as absorption and
emission centers of line spectra, may be included Schuster and Hemsalech,"
Schenck,3 Royds,4 Milner,5 Miss Schaeffer,6 and others. These workers have

1 Phys. Zeit., 5, 537 (1904).

2 Phil. Trans., 193, 209 (1900); Compt. Rend., 142, 1511 (1906).

3 Astrophys. Journ., 14, 110 (1901).

4 Phil. Trans., 208. A. 333 (1908).
5 Ibid., 209, 71 (1908).
"Astrophys. Journ., 28, 121 (1908).

6 THE ABSORPTION SPECTRA OF SOLUTIONS.

studied the velocity of the light streamers, and in some instances have found
indications that the emitting centers carried a negative charge. The subject is
such a complicated one, however, that it must be stated that our knowledge at
present concerning the constitution of these emission centers is extremely

meager.

THE EMISSION CENTERS OF FLAMES AND ARCS.

The study of the Zeeman effect has shown beyond a doubt that the vibra-
tions of the emitting centers of many arc, spark, and flame lines are affected
by an outside magnetic field, in the same way as negative electrons would be
affected; and it is remarkable how closely the value of e/m, as calculated from
the Zeeman effect, agrees with the value as found for the cathode and jS-ray
particles. The Zeeman effect does not, however, give us much knowledge of
the radiating centers, since the electrons form only a small part of these centers.

A very important series of experiments has been carried out by Lenard1
and others. The main conclusion from this work was that the carriers of the
first subordinate series in the Kayser and Runge classification of spectrum
lines are positively charged ions. The reason that ions in liquids do not radi-
ate, Lenard explains as due to their being loaded down by neutral molecules.
The work of other investigators has made this conclusion very doubtful, so
that, as in the case of spark and canal-ray spectra, it must be concluded that
at present nothing is known with certainty as to what the constitution of the
radiating centers may be.

Schumann Waves.

In 1893 Schumann studied the extremely short ultra-violet wave-lengths
to about X 1200. Lyman2, by using a concave grating, has studied the spectra of
hydrogen from X 2000 to X 1030, there being no radiations apparently between
X 3000 and X 1700. Argon possesses a rich emission spectrum in this region.
The spectra of carbon monoxide and dioxide seem to be very much alike, and
produce radiations to X 1300. Oxygen and nitrogen do not seem to emit in
this region. Fluorite is the only substance transparent to X 1225. Quartz
0.5 mm. thick absorbs everything beyond X 1500. Argon, helium, hydrogen,
and nitrogen in a layer of 1 cm. are perfectly transparent. The absorption of
air is due to the presence of oxygen. Oxygen of 9 mm. depth and 380 mm.
of mercury pressure has an absorption band extending from X 1750 to X 1275.

THE POSSIBLE CATALYTIC ACTION OF LIGHT.

Weigert3 has worked upon the equilibrium of the gas COCl2 and its
products, CO+Cb. The reaction

C0 + C12-C0C12

is accelerated by the action of light, but the position of the final equilibrium is
unaffected by the action of the light, and is a function only of the temperature.
The light thus acts as a catalyzer and not as a source of energy.

'Ann. d. Phys., 9, 642 (1902); 11, 6-19 (1903); 12, 475 (1903); 12, 737 (1903); 17,
197 (1905).

2 Ibid., 77, 777 (1907).

3Le Radium, 4, 373 (1907): Ann. .1. Phys., 24, 24:; (1907).

ABSORPTION AND EMISSION CENTERS. 7

Weigert believes that there are formed molecular complexes or "reaction
nuclei" by the light, analogous to the ions formed by ultra-violet light in air;
and these "reaction nuclei" play the role of a catalyzer, the reaction being
produced with very great velocity on the surfaces of these nuclei, and the speed
of the reaction will then be a function of the rate of diffusion of these nuclei.

Weigert finds that these "reaction nuclei" act as nuclei for the conden-
sation, and thus supports the views of Burgess and Chapman.1 When the
light has produced a number of the "reaction nuclei," it is found that the
number of these decay like ordinary ions. The "reaction nuclei" also accel-
erate the following reactions:

(1) Dissociation of oxychloride of carbon.

(2) Oxidization of hydrogen.

(3) Oxidization of sulphurous acid.

(4) Decomposition of ozone.

(5) Oxidization of hydrochloric acid gas.

(6) Formation of ammonia.

EMISSION SPECTRA OF ORGANIC COMPOUNDS.

Goldstein2 has shown that bright, fluorescent, and phosphorescent light
is emitted by solid aromatic compounds. The stimulation is best produced by
cathode-ray bombardment. To prevent the evaporation of the compounds,
they are kept cooled by liquid air. Goldstein has investigated a large number
of organic compounds such as benzene, the three xylenes, benzonitrile, the
quinolines, acetophenone, etc. In many cases three spectra appear which
Goldstein calls the initial, the chief, and the solution spectra.

During the first moments of excitation the initial emission spectrum is
quite strong, but it soon becomes very weak without disappearing. At the
same time that the initial spectrum begins to diminish in intensity the chief
spectrum appears. This spectrum is a very characteristic one, even for iso-
meric compounds. The third type of spectra only appears when an aromatic
compound is dissolved in a liquid and the solidified solution is exposed to
cathode rays.

The chief spectrum consists largely of narrow channeled bands, which
usually have their sharper edges on the short wave-length side. These spectra
never have bands of shorter wave-length than X 4600. The initial spectra
extend much farther than this into the region of short waves.

Chief Spectrum of Naphthalene.

/ 5390 (very bright) 5890 (very bright)

5550 6150 (probably double)

5600 6300

5730 6480

Spectrum of Naphthalene in Monochlorbenzene.

3. 4730 bright 5050 5170 faint 5400 faint 5570 faint 5820 faint

4830 " 5100 5230 " 5450 " 5650 "

The solution spectrum varies greatly for different solvents, even for
isomeric compounds.

1 Trans. Chem. Soc, 89, 1423 (1906).

2 Verhandl. d. deutsch. Ges., vi, 156, 185 (1904); Phil. Mag., 20, 619 (1910).

8 THE ABSORPTION SPECTRA OF SOLUTIONS.

The phosphorescence of several organic compounds has been studied by
Kowalski.1 Phosphorescence was found to appear in many cases at about
— 150° C, depending on the substance dissolved in the alcohol. When phos-
phorescence first appears it only lasts a few hundredths of a second, but as
the temperature is lowered the duration of the phosphorescent light increases
very appreciably. If the time of excitation is short, an "instantaneous" phos-
phorescence results which resembles the fluorescence of the solution. If the
time of excitation is increased, fine bands are superimposed upon the broad
fluorescent bands, and these increase in intensity as the time of excitation is
increased. The time required to reach a maximum intensity is different for
different bands. The duration of these bands is much longer than that of the
"instantaneous" phosphorescence, and is called by Kowalski "progressive
phosphorescence . ' '

Kowalski and Dzierzbicki give the following wave-lengths for the pro-
gressive phosphorescent bands in ethyl alcohol:

(1) Benzene, 0.05 normal solution in alcohol : XX 3390, 3460, 3520, 3570,
3650, 3710, 3800, 3850, 3970, 4020, 4130, 4190, 4290, 4350.

(2) Toluene, ethylbenzene, and propylbenzene, 0.05 normal solution
in alcohol. The introduction of the methyl group into the benzene nucleus
transforms the 14 benzene doublets into 7 broad bands, occupying almost the
same position in the spectrum. An introduction of a methyl group in a
side chain produces very little effect. The toluene bands are at XX 3460, 3580,
3650, 3800, 3890, 4060, and 4120. The ethylbenzene bands are at XX 3450,
3580, 3640, 3780; 3870, 4050, and 4120. The propylbenzene bands are at
XX 3440, 3580, 3650, 3790, 3890, 4050, and 4130.

(3) The bands of a 0.05 normal solution of o-xylene, C6H4(CH3)2, are at
XX 3480, 3560, 3610, 3670, 3780, 3830, 3900, 4000, 4070, and 4130; m-xylene,
C6H4(CH3)2, at 3540, 3610, 3670, 3730, 3820, 3880, 3970, 4090, 4160, and 4230;
and p-xylene, C6H4(CH3)2, at 3550, 3650, 3700, 3770, 3890, 3950. 4010, 4120,
4190, and 4270.

(4) Pseudocumene, mesitylene, and cymene all show bands at XX 3560,
3650, 3770, 3880, 4000, 4120, and 4270.

(5) Phenol shows bands at XX 3510, 3610, 3710, 3830, 3960, and 4080.

(6) o-cresole has bands at XX 3530, 3630, 3740, 3850, and 3970; ra-cresole
at 3540, 3620, 3730, 3850, 3970, and 4080; p-cresole at 3630, 3730. 3850, 3980,
and 4110.

THE ABSORPTION SPECTRA OF ORGANIC COMPOUNDS.

The Unit of this Absorption.
In discussions concerning the color of organic compounds it is customary
to speak of the selective absorption as being due to certain ions or molecules.
This is probably true in the infra-red, the electric charges absorbing these long
wave-length radiations being probably associated with masses of molecular
size. However, in the visible and ultra-violet portions of the spectrum the
value e/m of the absorbers is invariably of the same magnitude as that of the
electron. Drude2 has investigated a large number of organic compounds,

1 Compt. Rend.. 151, S10 (1910).

2 Ann. (1. Phys., 14, 677, 726, 936, 961 (1904).

ABSORPTION AND EMISSION CENTERS. 9

and shows that the absorber of all the shorter waves of the spectrum is the
negative electron.

Throughout this part of the present monograph the absorbers are con-
sidered as negative electrons. These electrons have certain free periods cor-
responding to the bands of selective absorption. These free periods are greatly
modified by the presence of certain chemical radicals, and seem to be elec-
trons that are situated either in the outer parts of the atom or between two
or more atoms. Stark and others call these the valency electrons, and con-
sider that chemical valency is due to them. Chemical bonds will then be
closely associated with the electric fields of these electrons. While the theory
in its present state is confronted with many difficulties, yet it seems a step
towards the explanation of the more or less vague chemical bond. As an aid
to our imagination, we shall consider atoms or ions as large spherical regions
throughout which a positive charge is uniformly distributed. These regions
are sometimes spoken of as "spheres of influence." Two atoms collide when
their "spheres of influence" touch. Groups of atoms composing ions, radicals,
or molecules will have "spheres of influence." No ion can penetrate the
sphere of influence of another atom or molecule. On the other hand, the elec-
trons are very small and bear much the same relations in size to the atom
that the sun bears to the solar system. The electric fields of the electrons,
however, occupy quite large volumes, although the energy of this field is for
the most part situated in a very small space. Electrons can, therefore, move
through ions and atoms if they have sufficient velocity. In most organic com-
pounds it is considered that the valency electrons move in the interatomic
spaces with considerable ease. In the metals a large number of the electrons
are free. In organic compounds that are transparent to certain wave-lengths,
the electrons in general will be held within certain regions by forces that are
supposed to be elastic in their nature.

A considerable amount of work has been done by Koenigsberger and
Kichling1 on the determination of the coefficients of absorption of organic
coloring matters, compounds of the metalloids, minerals, etc. Selective
absorption is classified under two heads. In the first class the absorbing and
reflecting powers are closely connected, and the absorption is probably due to
electrons or ions vibrating in the chemical molecule; in the second class the
absorption has no effect on the reflecting power, and the absorbers in this case
are rare and are not connected with the molecular structure.

Letting p be the number of electrons per molecule, it is found that absorp-
tion curves give the value of p e/m better than the dispersion curves. For
organic coloring matters p e/m = 1.3(H))7. For a temperature of absolute
zero, they consider that p e/m = 1.78(H))7. This quantity decreases as the
temperature rises to the probable limit l/2 1.78(10)7. For the metalloids there
is a single vibrating electron for the bromine family of elements, two for the
selenium family, and three for the phosphorus family.

For a large number of substances, the absorption curve is displaced towards
the red when the temperature is raised. It widens and becomes flatter at the
same time. From this it seems that the quasi-elastic force that acts on the
electron decreases with rise in temperature.

1 Ann. .1. Phys., 28, 889 (1909); 32, 843 (1910); PHys. Zeit., 12, 1 (1911).

10 THE ABSORPTION SPECTRA OF SOLUTIONS.

The Theory of Chromophores.

In considering absorption spectra it is often quite sufficient to speak qual-
itatively of the color of different compounds. The introduction of certain
groups into colorless compounds often results in a colored compound. Any-
such group is a chromophore. Sometimes the chromophore may be weak, and
it may require the addition of several chromophores to produce a colored com-
pound. Ultimately the color is due to absorbers existing in the chromophore,
probably the valency electrons. Among the better known chromophores are
the groups:

>C = C< =CO >C = NH, — CH = N— — N = N-

>-N~N- \ .N- ,Q X>

?C— Nf — N=0 — Nf — NC i

O V> V) 0

= N = 0 =C = S — C— S2— C— .

The ethylene group, >C = C<, is a weak chromophore, and several
chromophores must be present before the absorption is sufficient to produce
color. An example is that of benzene and its isomer :

CH=CH— CH
CH = CH— CH

CH=CHX

;c=ch

CH^CHK

Benzene.

Isomer.

Benzene is colorless and exhibits absorption only in the ultra-violet,
whereas the isomeric compound is orange yellow.

The carbonyl group, =C = O, is also weak, and several of the groups
must be present in a compound to produce color. The following examples
indicate how an increasing number of the carbonyl groups produces a greater
and greater absorption of the shorter wave-lengths:

R— CO— R, colorless.
CH3— CO— CO— CH3, yellow.
C6H5— CO— CO— C6H5, yellow.
CH3— CO— CO— CO— CH3, orange.
CflH5— CO— CO— CO— CO— C6H5, red.

The combination of two phenyl groups causes absorption only in the
ultra-violet. The introduction of the — CH = N — chromophore is sufficient
to produce color, as is shown in the yellow benzylidenaniline, C6H5CH =
N — C6H5. The azochromophore, — N = N — , produces the same effect, as
shown in the orange azobenzene, C6H5 — N = N — C6H5. The chromophores

-^C — N — N — and — C — Nf , on the other hand, are very weak.

/ \/ / X)

0
The nitroso group, — N = 0, is a very strong chromophore when it is
joined to a carbon atom directly. The effect of this chromophore is shown in

ABSORPTION AND EMISSION CENTERS. 11

nitrosobenzene, ON — C6H5, which is green. Wallach,1 and recently J. Schmidt,2
have investigated the following compounds, which are of a deep blue color:

CH — CH— C(CH3), CH3— CH— C(CH3)2

NO O— NO NO O— N02

Alkylenenitrosite. Alkylenenitrosate.

According to Wolff,3 the following nitroso compounds, the first of which
is green and the second blue, have the structures:

ON— C— C— CH, ON— C— C— CH,

\

N

N

CH3— C— N— C6H5 CH3— C— NH

l-phenyl-3.5-dimethyl-4- 3-5-dimethyl-4-

nitrosopyrazol. nitrosopyrazol.

The carrier of the color in these cases is the C — N = 0 group. Many of
the aliphatic nitroso compounds show polymerization and are then often col-
orless. The polymerization of R — N = 0 to (RNO)2 is probably accompanied
by a rearrangement as follows:

/°\

R— N< >N— R.

This would explain why the polymer is colorless.

An application of the theory of isodynamic isomerism has been made by
K. Schaefer4 to the absorption of the N03 group. An exhaustive study has
been made of the absorption of alkyl and metallic nitrates. In every nitrate
investigated, the characteristic ultra-violet absorption was detected, and in
general Beer's law was found to hold. It would be interesting to know whether
the presence of other salts, acids, or solvents would change the N03 absorption.
Schaefer finds for pure neutral solutions of inorganic salts that the absorption
is independent of the metal. The absorption band appears to be independ-
ent of the ionization, since in the case of potassium nitrate the absorption
is the same for the solid as for the concentrated and dilute aqueous solutions.
The absorption is suggested to be due to the following oscillation:

— 0— Nf <=± — 0— N< |
v0 x0

The character of the absorption is similar for metallic and many organic
salts. Methyl, ethyl, amyl, and allyl nitrates, however, show only general
absorption. There was no evidence here of selective absorption, either in
the liquid, vapor, or dissolved condition of the salts.

1 Lieb. Ann., 241, 288 (1887); 322, 305 (1902).

2 Ber. d. chem. Ges., 35, 2323, 3721 (1902); 36, 1768 (1903).

3 Lieb. Ann.. 325, 192 (1902).

*Zeit. wiss. Phot., 8, 212-234 and 257-287 (1910).

12

THE ABSORPTION SPECTRA OF SOLUTIONS.

The nitro group — N

\0or

-N<

,0
^0

is a very weak chromophore.

The aliphatic nitro compounds, CH3N02, C2H5N02, etc., are colorless. When
combined with other chromophores, colored compounds, such as nitroben-
zene or nitronaphthalene, can be obtained. Stobbe1 has investigated the
influence of the nitro group on the fulgides. In solution the p-nitrophenyl-
fulgide has a deep red color. The ortho and meta compounds are much less
deeply colored.

Hantzsch2 and Raschig3 have shown that the = N =0 group acts as a
chromophore in the sulphonate, O = N = (S03K)2, which is violet in solution
and orange in the solid state. The thiocarbonyl group, = C = S, is a rather
strong chromophore. Examples are the blue compounds thioacetophenone,
C6H5 — CS — CH;i, and thiobenzophenone, C6H5 — CS — C6H5. For a fuller
account of the color properties of various compounds, the reader is referred
to Ley's paper,4 from which the data here recorded have been largely taken.

There has been a great deal of discussion whether quinone has the for-
mula I or II:

0 = 0

-0

HO

\

hc \m

HO

\/

II

c = o

HC /CH

/
C

I.

II.

0

Willstatter and F. Miiller5 have found that o-benzoquinone can exist in
two forms, I and II, the former being colorless and the latter red:

/

\

//

-0
-0

0

0

/

II.

This fact may possibly explain several outstanding difficulties among
quinone derivatives. It would be expected that the substitution of an (NR)
group for oxygen in a quinone would give a deeply colored substance. As a
matter of fact, quinonediphenylimide,

C6H6N=<

NCJL

1 Ber. d. chem. Ges.. 38, 4082 (1905).

2 Ibid., 28,2744(1895).

8 Dammer: Handb. d. anorg. Chem.

4 Jahrb. d. Rad. u. Elek.. 6, 274-381 (1909).

5 Ber. d. chem. Ges., 41, 2580 (1908).

ABSORPTION AND EMISSION CENTERS.

13

is brownish red. But the compounds NH : CGH4 : NH and 0 : CaH4 : NH are
colorless. This can easily be explained if the latter compounds are assumed
to have the superoxide formula similar to that of o-benzoquinone. The qui-
none chromophore enters into the composition of quite a large number of
colored compounds.

The space relations of the chromophores seem to affect the color of
compounds. For instance, the ethylene group can have two isomeric config-
urations :

R — C— H R— C— H

or

R'— C— H

H— C— R'

This geometric isomerism may explain the following facts:

1. Diethoxynaphthostilbenes, C2H50— C10H6.CH : CH.C^He.OC.H..1
The form which has the highest melting-point is colorless, while the lower
melting form is yellow.

2. Dibenzoylethylenes, C6H5CO.CH : CH.COC6H5.2 The higher melt-
ing form is colorless, while the lower melting form possesses a deep yellow
color.

That the color-producing power of the chromophores is due to the double
bonds seems quite certain. When these bonds are saturated the resulting
compounds are colorless.

Colored.
C6H5. CO. CO. CO. C6H5, diphenyltriketone.
C6H5 — N = N — C6H5, azobenzene.
C6H6 — N = 0, nitrosobenzene.
0 = C6H4 = 0, quinone.

Colorless.

C„H»

H. CO. CH„. CO. C6H5, dibenzoylmethane.
C6HSNH— NHC6H5, hydrazobenzene.
C6H5 — N. H. OH, phenylhydroxylamine.
HO. C6H4. OH, hydroquinone.

The different di- and tri-substitution products usually give rise to dif-
ferent absorption bands when the absorption is selective. Ortho- and meta-
xylene have one band, whereas paraxylene has two. The following gives the
value of the limit of absorption when light is passed through a gram molecule
of the substance :

Cresol.

Dihydroxy-
benzene.

Hydroxy-
benzoic acid.

Meta

Ortho

Para

3433
3413
3359

3466
3399
3151

3359
3080
2986

When a chromophore is introduced into a compound the bands may be
shifted towards the red or towards the violet. The former effect is batho-
chromous, the latter hypsochromous. The effect of joining chromophores is
usually bathochromous. For example:

A

A

A

A

Benzene

2610
2^50

2540
2720
3600

2450
2630
3430

2440
2550
3280

Naphthalene

Anthracene

1 Elbs: Journ. prakt. Chem., 47, 72 (1893).

2 Paal and Schulze: Ber. d. chem. Ges., 33, 3795 (1900); 35, 168 (1902).

14

THE ABSORPTION SPECTRA OF SOLUTIONS.

In triphenylmethane the limit of the visible portion of the spectrum
is reached. It may be stated that anthracene and phenanthrene have
entirely different absorption spectra, although the fluorescent spectra are very
similar.1 An auxochrome is a radical that causes the absorption to be more
intense. An example is — CO — C = C — CO — , which appears in the indigos,

/COx /CO

C,H4<

;C = C

XH

XH

;C6H4

and also in the deeply colored compounds

/COx .CO. COx XOx

C6H4< >C = C< >06H4 C6H4 /C = C< >C6H4

0

0

s

s

The groups CH3, OCH3, C2H5, the halogens, etc., are bathochromes, while
the groups N02 and NH2 are hypsochromes.

The infra-red absorption spectra to about 15/* of a large number of organic
compounds have been investigated by Julius2 and by Coblentz.3 Isomeric
compounds are found to possess very different absorption, depending on the
combinations of the atoms in the molecule. Stereomeric compounds, on the
other hand, were found to possess the same absorption spectra. The replac-
ing of hydrogen by an NH2 or CH3 group usually results in the appearance
of new bands. In the spectrum of certain benzene derivatives, however, the
benzene spectrum is usually present. The carbohydrates investigated had
characteristic spectra with absorption bands at 0.83 to 0.86/*; 1.67 to 1.72/*;
3.25 to 3.43/i ; 6.75 to 6.86/*; and 13.6 to 14/*. The three isomeric xylenes have
banded spectra in which the most important line in each group lies farthest
toward the long wave-lengths in the order ortho, meta, and para.

Considerable work has recently been done by Weniger4 upon the absorp-
tion of organic compounds in the infra-red. He finds that the alcohols have
bands at 3.0 and 6.9/*; that changes from the primary to the iso-compounds
cause small shifts; and that the secondary alcohols have a band at 7.6/* and
the tertiary alcohols a corresponding one at 7.9/*. The band 9.6/* in the pri-
mary alcohols is shifted to 9.1/* in the secondary alcohols and 8.6/* in the ter-
tiary alcohols. There are two minor bands in the primary alcohols which
appear as follows:

Methyl.

Ethyl.

Propyl.

Butyl.

3.0/'

3.0

3.0

3.0

3.5

3.5

4.9

5.2

5.5

5 . 6

5.9

6.0

6.1

6.1

9.9

9.6

9.6

9.6

10.6

10.4

10.4

13.3

13.0

13.0

13.0

1 Elston: Astrophys. Journ., 25, 3 (1907).

I Verh. Konikl. Akad., Amsterdam, 1, No. 1 (1892).

3 Invest igal ions of Infra-red Spectra, Carnegie Institution of Washington Publication

No. 35, by W. W. Coblontz.

4 Phys. Rev., 31, 408 (1910)

ABSORPTION AND EMISSION CENTER*.

15

It is seen that the shifts are towards the 9m region of the spectrum. For
the secondary alcohols Weniger gives the following:

Propyl.

Butyl.

Capryl.

7.2
9.0
9.9

7.3
9.1
9.8

7.3
9.1
9.4

In the case of primary acids Coblentz and Weniger give the positions of
the following minima:

Acetic

3.5,"

5.9

7.2

8.9

9.9

10.6

11.0

13.9

Butyric

2.6

5.9

7.1

7.9

9.3

10.6

12.9

Valeric

3.6

5.9

7.1

7.9

9.1

10.7

12.2

13.3

Caproic

3.5

5.9

7.0

8.0

9.1

10.8

11.6

12.5

Stearic

3.5

5.9

7.0

7.8

9.2

10.6

12.2

13.3

Cerotic

3.5

5.9

6.9

7.7

10.6

10.9

12.9

Weniger considers the 3.0 and 6.9mm bands in the alcohols to be related to the
hydroxyl group; the 3.4m band in the alcohol, acids, and esters to be related to
methylene (CH2); the 7.3m band in the esters and higher alcohols to be also
related to methylene; and the bands 5. 9m and 8.2m in all substances for which
there are data to carbon monoxide.

The 9.6^ band in the primary alcohols shifts by 0.5m to the violet when the
linking of the hydroxyl is changed to secondary, and 0.5m further changes
when the tertiary alcohol is formed. The change from the primary to the
secondary linking of the carboxyl group (CH2COOH to CHCOOH) causes
the doubling of a band in the 8m region, this being true for acids and esters.
The band of the carbonyl group is independent of the way in which this
group is linked.

The authors1 have shown that the effect of the N03 group and of free
nitric acid is hypsochromous, causing the uranyl bands to shift towards the
violet. The effect of free hydrochloric acid or of zinc, aluminium, or calcium
chlorides on the uranyl chloride bands is bathochromous. Recently the
authors have found that the N03 group is hypsochromous with respect to the
neodymium and erbium bands.

Influence of the Solvent.

Theoretically it would be expected that the position of the absorption
bands would be different for different solvents. Kundt's rule that the bands
should be shifted to the red as the refractive index increases does not hold.
It is usually believed that the application of Kundt's rule is obscured by the
formation of compounds between the dissolved compound and the solvent.
The formation of such solvates seems to be quite definitely proved for some
inorganic compounds and organic solvents.2 Kauffman, Hantzsch and Glover,
Gorke, Koppe and Staiger, and others, have recently shown that the extinc-

1 Phys. Rev., 28, 143, 29, 555 (1909); 30, 279 (1910).

2 Anderson: Phys. Rev., 26, 520 (1908). Jones an. 1 Strong: Phys. Zeit., 10,499 (1909);
Amer. Chem. Journ., 43, 37, 97 (1910).

16

THE ABSORPTION SPECTRA OF SOLUTIONS.

tion coefficient varies somewhat for every solvent. In the same solvent the
extinction coefficient usually increases as radicals are added to the dissolved
substance, increasing its molecular weight. Hantzsch1 furnishes evidence
which shows the presence of solvates and molecular aggregates in the case of
nitrohydroquinone dimethyl ether.

An interesting investigation has been made by Kalandek.2 Resonators
will emit electromagnetic waves of different wave-lengths, depending on the
index of refraction of the liquid in which they are vibrating. Kalandek inves-
tigated the effect of different solvents on the period of various resonators,
and also on the positions of the absorption bands of a large number of organic
compounds. In general, the relations were not very close. It would be very
interesting, however, to carry these investigations into the infra-red.

The Absorption Spectra of Benzene and Its Derivatives.

Benzene and several of its derivatives show selective absorption in the
ultra-violet. Generally the absorption spectra of organic compounds consist
of very wide, diffuse bands. The absorption bands of gaseous benzene, on the
other hand, are very fine. Benzene in solution shows seven absorption bands
between X 2330 and X 2710, and in the gaseous state about 30 bands. Pauer,3
Friedrichs,4 Grebe,0 and Hartley6 have investigated several of the benzene
compounds. The bands in the gaseous state are much finer and usually more
numerous than in solution or in the solid state.

Both the vapor and solution bands are shifted to the red when CI, Br,
CH3, etc., are substituted for hydrogen. The shift is generally greater the
greater the molecular weight of the radical. The bands of benzene shifted in
this way are the ones that are common to benzene, toluene, ethylbenzene,
and hydroxy xylene, and are unaffected by temperature and pressure. Hartley
gives the following wave-lengths for benzene:

A

A A

A

A

A

A

In solution

As vapor

2682
2670

2657-2642 2614-2600
2630 2590

2539
2523

2480
2466

2126.5
2411

2376
2360

As far as investigated, the substitution products of benzene have much less
characteristic spectra than the spectrum of benzene itself. It would be very
interesting to know whether the shift of the bands is gradual as the state is
changed or as different radicals are added.

Anthracene has the following bands:

A

A

A

A

A

Solid

4250
4050
3900

4495
4275
4150

4745
4540
4320

4980
4820

5300

Solution

(Fluorescent) vapor

1 Ber. d. chem. Ges., 40, 1556 (1907).
2Phys. Zeit, 9, 128 (1908).

3 Wied. Ann., 61, 363 (1897).

4 Z. wiss. Phot., 3, 154 (1905).
6 Ibid., 3, 363 (1905).

8 Journ. Chem. Soc., 77, 839 (1900). Phil. Trans., 208, A, 475-528 (1908).

ABSORPTION AND EMISSION CENTERS.

17

The substitution of saturated groups in benzene has been found to change
the absorption spectra but little. Unsaturated radicals like NIL, C.'OOH, etc.,
change the spectra very greatly, so that there is hardly any relation to the
benzene spectra. In the various di-substitution products, the para com-
pounds retain the characteristics of the benzene absorption better than the
ortho or meta compounds. A considerable amount of work has been done by
Hartley,1 Baly and Desch,2 Ley and von Engelhardt,3 Baly and Collie,4 Ley,''
Hartley and Hedley,6 Baly and Tuck;7 and others have worked on the halogen,
amino, and nitro compounds of benzene, the phenols, pyridine, and its substi-
tution products. Below are a few of the results obtained.

Purvis8 has studied the absorption spectra of the vapors of pyridine and
some of its derivatives at different temperatures and pressures. Following are
some of the bands:

Pyridine.

Pyridine.

Pyridine.

<7-Picoline.

Piperidine.

Piperidine.

2930 n. wk.

2815 dif.

2747 wk.

2880 wk. sh.

2637 wk.

2535 st.

2918 n. wk.

2809 dif.

2743 wk.

2861 wk.

2633 wk.

2530 st.

2913 n. wk.

2806 wk.

2738 wk.

2859 wk.

2628 wk.

2528 wk.

2895 n. wk.

2798 wk. n.

2736 wk.

2856 wk.

2625 wk.

2526 wk.

2892 n. wk.

2796 wk.

2733 wk.

2846 wk.

2599 wk.

2521 wk.

2878 sh. wk.

2795 wk.

2730 st,

2834 wk.

2596 wk.

2519 st.

2869 sh. wk.

2789 st.

2726 st.

2821 wk.

2595 wk.

2513 st.

2866 sh. wk.

2784 wk. n.

2718 st,

2819 wk.

2591 wk.

2507 st .

2861 sh. wk.

2782 wk. n.

2713 st.

2814 wd.

2590 wk.

2503 st.

2859 sh. wk.

2778 wk. n.

2696 dif.

2809 wd.

2589 wk.

2502 wk.

2855 wk.

2762 wk. n.

2690 dif.

2790 n.

2586 wk.

2499 wk.

2849 wk.

2760 wk. n.

2685 dif.

2786 wd.

2579 sh.

2495 wk.

2843 wk.

2758 wk. n.

2678 wk.

2785 wd.

2565 wd.

2490 wk.

2832 sh. st.

2754 st.

2673 wk.

2781 wd.

2558 wd.

2472 wk.

2822 sh. wk.

2552 wk.
2550 wk.
2547 wk.
2543 wk.
2539 wk.

2467 wk.
2461 wd.
2455 wd.
2450 wd.

n. = narrow; wk. =weak; sh.=sharp; st.=strong; dif. = diffuse; wd. = wide.

In the pyridine vapor spectrum it was found that the longer wave-length
bands became wider and increased in intensity as the temperature and pres-
sure were increased. In fact, all bands showed an increase in width and inten-
sity under these conditions, and the general absorption in the ultra-violet was
also greatly increased. The piperidine bands are wider and more diffuse than
the pyridine bands. Some of the piperidine bands were coincident with the
benzene bands. Benzene bands, however, have most of the sharp edges of the
bands on the red side, whereas the piperidine bands are wider and are diffuse
on both sides.

Hartley has shown that the vapors of o-, m-, and p-xylenes, l-methyl-4-
propylbenzene, and 1:3: 5-trimethylbenzene have very few bands compared
with benzene vapor, which has 82 bands. The same is true of pyridine vapor,
in comparison with the much smaller number of bands of a-picoline and the

1 Handbuch der Spectroscopic, vol. in.

2 Journ. Chem. Soc, 93, 1345, 1747, 190:

3 Ber. d. chem. Ges., 41, 2990 (1908).

4 Journ. Chem. Soc, 87, 1344 (1905).

5 Ber.d. chem. Ges., 41, 1637 (1908).
(1908). 6 Journ. Chem. Soc, 91, 319 (1907).
'Ibid., 93, 1902 (1908).
8 Journ. Chem. Soc, 97, 692 (1910).

18

THE ABSORPTION SPECTRA OF SOLUTIONS.

disappearance of the bands in the lutidines and trimethylpyridine. The dis-
appearance of the bands as the number of methyl groups in the pyridine ring
increases is analogous to the lessened persistency of the absorption band as
the number of methyl groups is increased when the substances are examined
in alcoholic solution. No absorption bands are found for alcoholic solutions of
piperidine.

Vapors.

Furane.

Furfuraldehyde.

Thiophene.

2652 weak.

2726 weak.

2681

2590 weak.

2647 "

2725 "

2679

2530 "

2642 "

2720 "

2676 weak.

2500 weak, wide.

2637 "

2718 "

2674 weak.

2416 about 0.5 A. U.

wide.

2634 strong.

2712 strong.

2668 "

2406 about 0.7 A. U.

wide.

2630 weak.

2709 "

2666 "

2395 wide.

2627 wide.

2706 weak.

2660 "

2620 "

2704 "

2654 "

2601 weak.

2701 "

2652 "

2599 "

2699 "

2641 "

2593 "

2697 "

2639 "

2589 head of strong band.

2688 strong.

2541 weak.

2538 "

2530 head of strong band.

Alcoholic solutions of furane, thiophene, and pyrrol show no bands
according to Purvis.1 Neither do the liquids furfuraldehyde, thiophene, or
pj'rrol have any absorption bands.

Theory of Dynamic Isomerism.

Baly, Stewart, Desch, and others have recently supported the view that
the absorption of light by organic compounds does not take place under ordi-
nary conditions, but only when there is a change in the union of the atoms. In
some cases this change of union takes place when a chemical compound
changes into an isomeric form. Dynamic isomerism exists when there is
some third substance to act as an intermediary. Many substances are iso-
dynamic only at high temperatures or in the presence of a catalytic agent.
Sometimes the solvent may promote isomeric change. The point of equilib-
rium is determined by the velocities of the isomeric change, and these veloc-
ities are affected by the solvent, concentration, temperature, catalytic reagent,
or the presence of free alkalis or acids.

An example of the above change is the transfer of a labile hydrogen atom
from an oxygen or sulphur atom to a carbon or nitrogen atom. Sometimes
an OH or CN group is transferred in the same way. The atom or radical
transferred assumes a neutral condition compared with its condition as a
powerful negative radical in the inorganic acids.

The theory of dynamic isomerism is useful in explaining a large number
of phenomena in organic chemistry, and especially those connected with

1 Journ. ('hem. Soc, 98, 1648 (1910).

ABSORPTION AND EMISSION CENTERS. 19

light emission and absorption. Tschugaeff1 has examined some five hundred
compounds for triboluminescence (luminescence due to the crushing of the
compound) and has found that 25 per cent of the organic compounds inves-
tigated showed a more or less intense flash when crushed. Most of the lumin-
escent compounds could have existed in several isodynamic forms. Phos-
phorescence and fluorescence may be explained as being due to a dynamic
isomerism existing between two or more forms. For instance, fluorescein
may exist in two or more forms:

O O

V \oh ho \>:

c c

c6h/\o

C6H4COOH

CO

We shall now consider some recent work by Baly and Desch. They first
took up acetylacetone, CH3 — CO — CH2 — CO — CH3, and ethyl acetacetate,
CH3 — CO — CH2 — C02C2H5, and their metallic derivatives. Acetylacetone
and its metal derivatives were found to give similar spectra. Ethyl acetace-
tate was found to give only a very slight general absorption, whereas its alu-
minium derivative gave a spectrum almost exactly like that of acetylacetone.
The work of Perkin and others has shown that free acetylacetone is enolic
and ethyl acetacetate ketonic.

H OH
CH,— CO— C = C— CH,

0

H

0

CH3-

II

-c-

-c-

H

II

-c-

-C2H

Ethyl acetacetate (ketonic).

Acetylacetone (enolic).

These results would then indicate that the metallic derivatives of ethyl
acetacetate have the enolic structure, the metal taking the place of hydrogen
in the OH group. But by using various compounds it was shown that neither

H '

the ketonic group, — C — C — , nor the enolic group, — CH = C — , gives rise to a

I II I

HO OH

trace of banded absorption. This would also result from Hartley's work.

It would seem probable, then, that the absorption was not to be attrib-
uted to any definite molecular structure, but to a dynamic isomerism exist-
ing between the two modifications of the compound in solution. It was stated
before that alkalis and acids change the velocity of the transformation from

1 Ber. d. chem. Ges., 34, 1820 (1901).

20 THE ABSORPTIOX SPECTRA OF SOLUTIONS.

one form to the other, the final state being unaffected by the catalytic agent.
This latter condition means that the direct and reverse actions are equally
changed. If now intra-molecular change is the source of the absorption
bands, then a catalytic agent should affect the persistence of the band. Experi-
mental results verified these conclusions. Other compounds were tried with
similar results.

From these results it follows that there must exist, in connection with
the reversible transformation of one tautomeric form into the other, a system
that is synchronous with the light absorbed. This can not be a vibration of
the labile atom itself, since the oscillation frequency of the absorption band-
does not bear any direct relation to the mass of the labile atom, and the fre-
quency of atomic motions is never so high as these. The absorption must be
due, however, to the oscillation of linking of the keto-enol tautomers:

H

— CH = C— <=> — C— C— , or, symbolically, — CH— fi-
ll I
OH H O O*

where the asterisks indicate the points where the migrations of linkages occur.
From a summary of work previously done on ultra-violet absorption spectra,
it was seen that most of the compounds showing absorption also were tau-
tomeric. As was also seen, an increase in the mass of the molecule in general
decreased the oscillation frequency of the absorbed light, although by only a
small amount.

Assuming the saturnian form of atom similar to the arrangement assumed
by Sir J. J. Thomson, chemical bonds would simply consist of Faraday tubes
of force. By a rearrangement of Faraday tubes, it is quite probable that
a vibrational disturbance would be set up. If Hewitt's explanation of the
origin of fluorescence is correct, it would follow that disturbances set up by
isodynamic changes are of the same frequency as light waves. The lumin-
osity due to thermal or electric action is caused by rapid changes of stress
or of the electric action to which the atoms are subjected. Here the disturb-
ances of the electrons are due to the oscillation of linkages within the mole-
cule. The comparatively small displacement of the absorption band by a
change in the mass of the molecule is to be expected, since an increase in the
mass of matter near a vibrating electron has the effect of retarding its motion,
the oscillation frequency becoming less. For instance, the spectral series of
Kayser for various related elements show a displacement towards the red on
increasing the atomic mass. This is in general true for all emission spectra
and for the absorption spectra of the rare earths and organic dyes.

The sodium and aluminium derivatives of ethyl acetacetate show the
enolic and ketonic modifications in dynamical equilibrium. The sodium com-
pound is found to be easily decomposed into sodium hydroxide and ethyl
acetacetate, whereas the aluminium derivatives show dissociation and hydrol-
ysis to only a very slight extent. The absorption spectrum is not dependent
on ionization or hydrolysis. Hartley has found that the ultra-violet absorp-
tion spectra of metallic nitrates is exhibited even on very great dilution, show-

ABSORPTION AND EMISSION CENTERS.

21

ing a close connection between the anion and cation in such solutions. Baly
and Desch conclude that the Faraday tubes may be lengthened out on dilu-
tion, and that the force necessary for the separation is furnished by the attrac-
tion of the solvent. Solvents which thus exert a strong attracting force are
ionizing agents; and this attraction is exerted both on molecules and ions.
We have thus hydrated molecules and ions. When the Faraday tubes are
lengthened beyond a certain critical value, an interchange of ions between
the molecules becomes possible. A completely dissociated solution of a salt
is not one in which the ions are moving independently of one another, but
one in which the length of the Faraday tubes is greater than the critical
value. In tautomeric compounds the Faraday tubes connect the labile atom
with the rest of the molecule, being lengthened out to such an extent as to
allow these atoms to change their positions in the molecule, and there is a
sort of internal ionization within the molecule. To this making and breaking
of Faraday tubes may be attributed the absorption of the light.

In the tautomeric aliphatic compounds the substitution of an alkyl group
for the labile atom destroys the tautomerism. This would be expected, since
alkyl ions are unknown. Water and other solvents do not have sufficient
attractive force to lengthen out the Faraday tubes in this case. Whether
there is a banded absorption in these cases is not stated. One consequence of
the theory can be tested. Since the persistence of the absorption band is a
measure of the number of molecules undergoing transformation at any mo-
ment, this persistence should reach a limit for each tautomeric compound when
the length of the Faraday tubes has reached their critical length, so that free
interchange takes place. By the successive addition of an accelerating com-
pound a maximum should be found. Experiment shows this to be true. Take
the case of ethyl benzoylsuccinate, to which 1, 10, 20, and 100 equivalents of
sodium hydroxide have been added. The limits of persistence referred to a
0.0001 normal solution of the ester are as follows:

Absorption band begins at

Absorption band ends at

Change of dilution over which the
band persists

Free

ester.

1 eq.
NaOH.

10 eq.
NaOH.

20 eq.
NaOH.

100 eq.
NaOH.

mm.

120.0
83.2

mm.

63.0
34.7

mm.

40

20

mm.

31.7
15.2

mm.

21.9
10.4

Per cent.

Per cent.

Per cent.

Per cent.

Per cent.

30.7

44.9

50

52.0

52.5

At this point it may be well to refer to the work of Stewart and Baly.
Quite a number of chemical facts have been explained by the theory of steric
hindrance, although this theory also fails to explain a great many things.
For instance, acetic acid, CH:iCOOH, is esterified with ease. The methyl,
ethyl, etc., derivatives are much more difficult to esterify. This is explained
as due to the larger volumes occupied by these radicals, and the consequent
hindrance to the approach of the alcohol to the carboxyl radical. If, as is
probable, however, the intra-molecular mean free path is large compared with
the size of these radicals, this explanation in terms of steric hindrance breaks
down. The theory of isodynamic change will explain all these facts and also

22

THE ABSORPTION SPECTRA OF SOLUTIONS.

others where the theory of steric hindrance would lead to conclusions directly
contrary to the facts. According to this theory acetacetic ester exists as

CH3— C = CH— COOC2H5
OH

CH3— C— C— H— COOC2H5
O H

During this dynamic equilibrium there are periods during which a nas-
cent carbonyl group exists. From analogy to the action of other nascent
substances, this would occasion a very much greater reactivity.

Stewart and Baly, on further investigation, found that the absorption
band in this case has a persistence which decreases proportionately to the
decrease of reactivity of the ketone's carbonyl group. It has been noticed by
chemists that the velocity of tautomeric change depends on the solvent.
Stewart and Baly also found that the persistence of the absorption band was
very different in aqueous and alcoholic solutions. In the case of pyruvic
ester they show that the facts may be represented by the scheme

CH3— C— C— OC,H6

II II

o o

CH3— C = C— OC2H£
0—0

In tautomerism we have a wandering of the hydrogen atom, so Stewart and
Baly propose to call this process isorropesis (from the Greek word toopponela,
equipoise), or an oscillation in the carbonyl grouping. By the persistence of
absorption bands it is possible to measure the activity of chemical compounds.
The band produced by the isorropesis is also much nearer the red than that
produced by the process of enol-keto tautomerism. An example of isorropesis
would be the quinone band l/\ = 2480, which would be due to the following
change :

O

C-
\

-0

C

c

c

ic

cl

-0

c

II

o

In general, then, according to the theory of Baly, Desch, Stewart, and
Collie, any absorption by organic compounds is due to the conditions that
occur during isomeric changes. Benzene, for example, appears in two forms,
and the absorption spectrum is due to a condition of benzene while it is chang-
ing from one form to the other. This theory explains quite well the action of
chromophores, since, as we have seen, every chromophore contains at least
one double bond.

ABSORPTION AND EMISSION CENTERS.

23

Theory of Stark.

Stark1 considers that chemical valency can be explained as due to the
presence of negative electrons that hold the positive parts of atoms together.
In fig. 1 it is seen how this can take place, the dotted lines representing lines
of electric force.

Fig. 1.

The conditions represented in fig. 1 are such that there is very little stray
electric field beyond the atoms. Under such conditions the valency electrons
are saturated and Stark represents this by the symbol < — ». Under many
conditions, however, the valency electrons are not so closely united to the
atoms and are more or less unsaturated. Under certain conditions an electron
maybe thrown off from the atom, and Stark considers that it is under some con-
dition such as this that selective absorption of light takes place. When the elec-
tron returns to, the atom it will undergo certain accelerations along its path, and
during these accelerations it will emit radiations. Stark believes that under
some condition at least similar to this, the fluorescent radiation is emitted, and
that the period of this radiation will depend on the amount of energy set free
when an electron recombines with the positive part
of the molecule. From the heat changes that occur
in various chemical reactions, Stark calculates what
the approximate period of these radiations should
be, and in many cases obtains values which agree
with the positions of known bands in the spectrum.
Among these bands is the ultra-violet band of
benzene. Stark's formula for benzene would be
the following :

The symbol — - simply means that one elec-
tron is not as closely joined as the other three to
the carbon atom. It is possible that even this
unsaturated electron may have lines of force run-
ning to the other carbon atoms. In this way partial valency can easily
be explained.

H-<— ^-C

Fig. 2.

1 Phys. Zeit., 9, 85 (1908).

24

THE ABSORPTION SPECTRA OF SOLUTIONS.

In this consideration of atoms it would be expected in general that the
space containing the electric field of the electron would more or less envelop
the positive nuclei of the atoms, and that the volumes of atoms would depend
on their valency. This would agree very well with the assumptions of Barlow
and Pope.1 These assumptions are as follows: In any chemical body the
relative volumes of the atoms are proportional to their valencies; in a crystal-
lized body the atoms are close packed; in a compound the volumes of the
spheres of atomic influences of atoms of the same valency may differ slightly.
The latter variation of the sizes of the spheres of influence (interpreted here
as the space filled by the electric fields of the valency electrons) may be due
to the different amounts of saturation, or, as Stark calls it, the lack of the
valency electrons. It would be interesting to know whether the energy rela-
tions support this view, it being expected that the greater the lack of the
electrons, the less the potential energy of the compound.

Stark and Steubing2 have investigated the fluorescence of a large number
of organic compounds. Most spectra are banded, and a band never runs in
both directions. The band consists of a tail where the smaller bands are close
together. From the tail the small bands may run towards the red or towards
the ultra-violet, getting farther and farther apart all the time. But there are
never small bands on both sides of the tail. The absorption of light by bands
running "to the red" is not accompanied by fluorescence or by any photoelec-
tric effects. The absorption of light in bands running towards the violet
(benzene bands) is accompanied by a photo-electric effect, a fluorescence of
the bands themselves and by a fluorescence of bands due to the connection of
carbonyl, ethylene, or other chromophoric groups if these be present in the
molecule. The addition of more chromophores to the compound shoves
the fluorescent bands to the red.

Other Benzene Theories.

There are some color changes of benzene compounds for which the general
theory does not seem easily to account. For instance, nitronaphthalene is
yellow. By introducing two nitro groups a colorless compound is obtained.
In order to explain phenomena of this kind Kauffmann:i assumes that the
introduction of groups into a benzene nucleus may change the condition of the
benzene ring itself. He considers that benzene may have the diagonal, the
Kelule, or the Dewar4 formula.

/

\

\ /

/

/

/

II.

III.

1 Pope: Journ. Chem. Soc, 89, 1675 (1906); 91, 1150 (1907). Swartz: Amer. Chem.
Journ., 37, 638 (1907); 42, 158 (1909).

2 Phys. Zeit., 9, 481, 661 (1908).

3 Zeit. phys. Chem., 50, 530 (1905). Ber. d. chem. Ges., 37, 2941 (1904).

*Ber. d. chem. Ges., 33, 1725 (1900); 34, 682 (1901); 35, 366S (1902). Zeit. phys.
Chem., 55, 547 (1906.)

ABSORPTION AND EMISSION CENTERS. 25

In condition I benzene has an aliphatic character, as in nitrobenzoic acid,
C0H4(NO2)COOH. In this condition, having no double bonds, it does not
possess strong color properties. The second condition has an aromatic char-
acter and is exemplified in the phenols. Examples of the third condition are
found in aniline, p-phenylenediamine, naphthalene, or anthracene. Of the
three conditions, the second one is the best chromophore. According to KaulT-
mann, benzene vapor exists in the diagonal condition, and for that reason it
is not luminous when exposed to electric discharges of high frequency or to
the rays from radium. Auxochromes and chromophores cause the benzene to
become luminous, and for this and other reasons Kauffmann thinks that the
condition of the benzene grouping has been changed.

The different changes in color may be due to one or more of three con-
ditions :

(1) There may be no intramolecular changes of constitution, but the
whole change of color may be due to the change in the radicals.

(2) There may be an intramolecular rearrangement.

(3) There may be an association of the molecules or compounds formed
with the solvent. The above classification includes color changes that are not
explained by isomerism.

In many cases it is practically impossible to decide between the different
possibilities. Auwers1 and Tuck1' give evidence to show that the sodium salt
of hydroxy azobenzene owes its color simply to the introduction of the sodium.
Baly and Schaefer,3 Hantzsch,4 Vey, Gorke,5 and others, give some cases
coming under class 2. As an example, we may take dinitroethane :

.NO, N02 N02

CH,CH<: //O /yO

'.(

/ ran = N/

XN02 CH3C = Nf CH3C = N<

X)Na xOH

Pseudo acid, colorless. Sodium salt, yellow. Acid, yellow.

o2n/ V>ch3 "/N=</ N=o

Nitrophenol ether, faintly yellow. Chromonitrophenol ether, deep red.

Benzene derivative. Quinone derivative.

A very full discussion of the "Umlagerung" theory is given by Ley.0 A
short discussion is also given of the theory of indicators, of polymerization,
and of metallic derivatives of organic compounds, especially of cases where the
metal is supposed to be present in the inner part of the molecular complex.

^ieb. Ann., 360, 11 (1908).

2 Journ. Chem. 80c, 91, 454 (1907).

3 Ibid., 93, 1S0G (1908).

4 Hantzsch: Ber. d. chem. Ges., 32, 575 (1899). Hantzsch and Veit: Ibid., 33, 626
(1900). Ley and Hantzsch: Ibid., 39, 3149 (1906).

5 Hantzsch and Gorke: Ibid., 39, 1073 (1906).

6 Jahrb. d. Rad. u. Elek., 6, 341, 381 (1909).

CHAPTER II.

EXPERIMENTAL METHODS AND APPARATUS.

In this work the mapping of the spectra was done in the same way and
by the same methods as already described in Publication No. 130 of the
Carnegie Institution of Washington, pages 19-22. The general arrangement
of apparatus is given in fig. 4.

New difficulties that required special treatment quite frequently presented
themselves in the work. As an example, one might take that of mapping the
absorption spectrum of solutions of samarium, dysprosium, and gadolinium
salts, very kindly lent us by Professor Urbain. In order to bring out as
many bands as possible, the absorption cell was made wedge-shaped, the apex
of the wedge being about 2 mm. wide and about 15 mm. long. In this way a
much greater cell depth could be obtained with the same amount of solution
than with the Uhler cell. Another difficulty consisted in getting the strip of
the photographic film uniformly exposed. The appearances of bubbles, of
precipitates, etc., during the heating of the solutions are further examples.
In some cases these difficulties could be overcome. For instance, if the Uhler
cell was moved back and forth in the path of the beam of light, bubbles and
precipitates only decreased the amount of light passing through the solution
and did not cause an uneven exposure on the strip.

Part of the work consisted in extending the Beer's law tests to very dilute
solutions. Some work was done on uranyl solutions, using a trough with
plane parallel ends. Other cells consisted of glass tubes with quartz lenses at
the ends. One of these was 500 cm. long. With cells of this kind it is impos-
sible to obtain a uniform exposure, unless the cell is moved back and forth in
the path of the beam of light.

Salt solutions that have bands in the violet and ultra-violet were exposed
for much longer intervals of time than those which have bands only in the
visible part of the spectrum. In making an exposure of this kind (the uranyl
salts are typical examples) the length of exposure to the shorter wave-lengths
would be from 5 to 10 times longer than to the longer wave-lengths. First,
an exposure would be made to the whole spectrum of the Nernst glower ; then a
screen would be placed in front of the plate, cutting out all light of wave-length
greater than about \ 4500 or X 4800. A long exposure would then be made to
the short wave-length spectrum of the glower; and lastly, a short exposure
would be made directly to the spark.

For high-temperature work on acid solutions the fused silica cell was used,
while for room temperatures this cell and the Uhler cell were used for such
solutions.

Part of this investigation consisted in extending the work on absorption
spectra to high temperatures, by means of closed cells. Two cells, one 1.0 cm.
and the other 10 cm. in length, were used. Fig. 3 represents a longitudinal
section of the longer cell. Since both cells were exactly alike in all respects

27

28

THE ABSORPTION SPECTRA OF SOLUTIONS.

except in length and in the size of the side tube, only the longer cell will be
described here. The main part of the cell (T) was made of tool steel and was
heavily copper plated and gold plated on all the inner surfaces.

The side tube was very tightly fitted into the main part of the horizontal
tube. The open part of the tube was 1.0 cm. in diameter. The windows
of the cell (U, U) were 2.5 cm. in diameter and were either of quartz or glass.
One of the troubles with this form of cell is the formation of precipitates on
the inside surfaces of the windows. Every time a precipitate is formed, the
windows have to be taken out and cleaned. On being put back there is
always great danger of the quartz or glass ends being broken. During the
work a number of ends were broken in this way.

Quartz ends are much tougher and less easily broken than glass ends.
They are, however, quite expensive and in most of the work the solutions were
not transparent in the ultra-violet. For this work glass ends were used. Some
of these were cut out of ordinary plate glass and others were made from

Fig. 3.

"uviole" glass, which is tougher and much more transparent in the ultra-
violet than the ordinary plate glass. For cutting the ends a steel tube was
fastened to the axle of an ordinary fan motor. The steel tube was 2.5 cm. in
diameter and the motor was placed so that the tube was vertical, the free end
of the steel tube being at the bottom. An old glass end was then cemented to
a piece of plate glass with hot sealing wax and served as a guide for the steel
tube. The plate glass was then held against the end of the steel tube and the
motor started. Wet carborundum was fed constantly against the grinding
steel tube. Plates nearly 1.0 cm. thick could be cut in this way in 20 or 30
minutes.

The quartz windows rested on gold washers, and these rested directly
against the gold-plated shoulders of the tube T. PPP are plungers. Two
of these at the ends of the main tube have guide pins that prevent the plungers
from turning. Between the plungers and the windows were placed washers.
Various kinds of washers of hard leather, lead, zinc, etc., were used. The

EXPERIMENTAL METHODS AND APPARATUS.

29

leather washers, however, seemed to be the most satisfactory. Steel cap-,
EEE serve to tighten the plungers. MM are receptacles for thermometers
for measuring the temperature. C is an iron air-bath and protects the cell
from rapid changes in temperature.

In heating a cell of this kind it was found that the rise in temperature should
be very gradual. Very great difficulty was encountered in gel ting the ends to
hold liquid tight. The screw ends were tightened gradually for several days
and for several heatings. On one occasion, when the tube was filled with one
of the higher alcohols, a very effective closing was made. It is possible that
dried films of oils (like linseed oil) might be of use as washers.

Fig. 4.

In beginning the work no serious trouble from the formation of precipi-
tates was anticipated. This interference was encountered and a new form of
cell is being made which it is hoped will overcome some of the imperfections of
the form above described. In this form (fig. 4) the quartz ends are fastened in
the ends E' in the same way as in fig. 3. Instead of the plunger P having guide
pins it has guide grooves. Part of the plunger has screw threads, by means of
which it can be taken out. The whole cap can be removed from the tube T by
unscrewing E' , during which the quartz end is untouched. When the ends are
removed the quartz window can easily be cleaned. Gold washers are required
here between T and E' and between E' and U.

The remaining parts of fig. 4 represent a diagrammatic arrangement of
the apparatus. Only the cell is drawn to scale. The cell was kept in a hori-
zontal position so that all bubbles that form would rise in the side tube. As

30 THE ABSORPTION SPECTRA OF SOLUTIONS.

the spectroscope (containing the grating G, photographic plate holder C, and
slit S) was kept in a vertical position, a 45° quartz prism was used to change
the horizontal beam of light into a vertical beam, the beam being totally
reflected by the hypothenuse surface of Q. The source of light N.G. (Nernst
glower) or S.G. (spark gap) was focused by the concave speculum mirror M
on the slit S. A similar arrangement was used with the fused silica cell.
D.T.S. represents a double throw switch, by means of which either the Nernst
glower or the spark gap may be thrown in circuit. B is a ballast. R is a
variable resistance, by means of which the current (read by the ammeter A)
in the Nernst glower may be kept constant.

O.C. is an oil condenser. Some trouble was given by the condensers,
especially when a large spark gap (2 or more centimeters) was used for
several hours. Paraffin condensers often become heated so that the paraffin
melts. A condenser was made of transformer oil. Unfortunately, the box
part was made of wood and it is very difficult to prevent the leaking of the
oil from such a box, especially when it becomes heated. I.C. is an X-ray
induction coil and R2 is a resistance in the primary circuit of this coil.

Some preliminary tests were made with the cells at high pressure. The
Cailletet pump belonging to the University was used for this purpose , the cell
represented in fig. 3 being made so as to fit on this pump. It was not at all
difficult to obtain pressures of 200 atmospheres with water and alcohol solu-
tions. Spectrograms were made of the absorption spectra of neodymium solu-
tions under pressures as high as 275 atmospheres. No effect of pressure was
detected. The work at high pressures is easier than at high temperatures, on
account of the fact that there is no expansion of the cell due to heating.

CHAPTER III.

MAPPING THE ABSORPTION SPECTRA OF VARIOUS

SALTS IN SOLUTION.

An accurate knowledge of the absorbing power of solutions is the first
requisite in understanding the nature of absorption, and, accordingly, the
mapping of the spectrum is the first thing to be done. This has been accom-
plished for a large number of solutions, but there are quite a few salts that
have thus far been omitted, and it is the purpose of this chapter to give the
results with some of these solutions. This has been made possible largely
through the kindness of Professor Urbain, who has loaned us the oxides of
samarium, dysprosium, and gadolinium. Dr. Guy has converted these oxides
into the various salts such as the chloride, nitrate, etc., and has dissolved these
in various solvents. The other salts whose absorption spectra have been inves-
tigated have been obtained, for the most part, from Kahlbaum. The numbers
of plates described in this chapter run from 1 to 34 inclusive.

Jones and Anderson have shown that the absorption bands of a given
salt in any solvent were characteristic of that solvent. For this reason the
photographing of the absorption spectra of a given salt in different solvents
will be considered as the mapping of characteristic spectra. On the other
hand, when a salt is gradually changed to another salt by the addition of an
acid to the solution, or by the addition of a foreign salt, the absorption bands
show gradual changes. This has been interpreted as being due to changes in
the molecular aggregates of the salts, and to the formation of intermediate
compounds. These changes are considered as being of a chemical nature,
and will, therefore, be taken up in the chapter dealing with the spectrophotog-
raphy of chemical reactions; and the facts there described will be regarded as
furnishing strong evidence for the existence of molecular clustering in liquids,
and also for the theory that the absorption and emission centers of spectrum
bands consist of more or less complex atomic or molecular aggregates, probably
in a process of ionization. The full development of this view appears in the
summary.

The experimental methods are essentially those described in Publication
130 of the Carnegie Institution of Washington, and in the chapter on experi-
mental methods in this monograph. Nothing more need be said here, except
that it would probably be desirable in some cases to place the negative film
of an absorption spectrum just below the photographic film that is being
exposed. Then, by lengthening the time of exposure very greatly, it should
be possible to get a spectrogram containing a great many bands.

THE ABSORPTION OF CERTAIN CYANIDES AND CHROMATES.

The early workers on absorption spectra supposed that if two salts, dis-
solved in the same solvent, had absorption bands that were close together,
the wave-lengths of the bands would be modified by the salts being
together in the solution. Experiments of this kind have been made without

31

32 THE ABSORPTION SPECTRA OF SOLUTIONS.

much success. According to the theory of aggregates, however, it would not
be expected that any very marked effect would result, unless the two salts
formed parts of the same aggregate; the term aggregate being used in the
broader sense here to include also molecules of the solvent. Accordingly, the
list of colored chemical compounds was searched for salts that contained
colored anions and colored cations. An effort was also made to obtain solvents
and salts that had bands in the same part of the spectrum. So far no good
examples were found where the same spectral aggregates contained different
absorbing centers that possess bands in the same region of the spectrum. It
seems quite possible, however, that aggregates of this kind among organic
compounds could be found, or that in the infra-red region many examples of
this kind will be found. The region of the infra-red is especially inviting for
work of this kind, inasmuch as nearly all groups possess bands, and the
absorbers seem to be of molecular dimensions; whereas in the visible portion
of the spectrum it seems probable that, in most cases at least, the constitution
of the spectral aggregate only influences the period of vibrators that seem to
be of the nature of electrons.

It has been shown that the presence of calcium or aluminium chloride has
a considerable influence upon the wave-lengths of the uranyl chloride bands.
This has been explained1 as being due to the presence of chlorine, rather than
to the direct presence of the calcium or aluminium atoms. It is possible to
obtain aqueous solutions of calcium ferricyanide or of aluminium and calcium
chromate. If the change of wave-lengths of the uranyl bands was not due to
calcium and aluminium, then the absorption of the potassium, aluminium, and
calcium ferricyanides and chromates should be the same. These salts were
studied to test whether or not this was the case.

The dissociation2 of the ferricyanides, ferrocyanides, and chromates is
an interesting one, and the study of the absorption spectra of solutions of
these salts, especially in the infra-red, will probably throw much light upon
this subject, but in this work time did not permit us to take up this problem.

Calcium Ferrocyaxide and Calcium Ferricyanide.

The absorption spectrum of calcium ferricyanide is given in plate 2, A,
and of calcium ferrocyanide in plate 2, B. The concentrations are given in
the chapter on the description of plates. Starting with strip 1 of A, the edge
of the absorption band is at about X 4700; the concentration (c) being 0.031
normal and the depth of cell (d) 24 mm. (hereafter the product cd will be
given without definition, c being expressed in terms of normal and d in milli-
meters), the value of cd being 0.74. In the case of potassium ferricyanide3
for a value of cd of 0.69 the edge of the absorption band comes at about
X4690, considering the limit of absorption as at X4710 and the distance be-
tween this limit and the place where the absorption is 50 per cent as being 20
Angstrom units. It is thus seen that the absorption of calcium ferricyanide
is approximately the same as that of potassium ferricyanide. The conclusion
follows that calcium shows no bathochromous effect in this instance.

1 I'hys. Zeit., II, 668 (1910).

- Publication 130, Carnegie Institution of Washington, 28 ami 29.

:i Ibid., 29.

MAPPING ABSORPTION SPECTRA OF VARIOUS SALTS. 33

In strip 1, B, plate 2, cd is 6 and the middle of the edge of absorption is at
about X 4150, the edge of the band being about 300 Angstrom units wide. The
edge of the absorption is very wide and diffuse, resembling the absorption of
potassium ferrocyanide. No direct comparison of the absorption in the two
cases can be made, as the value of cd is much larger in the case of the calcium salt.

The Chromate and Bichromate of Lithium.

The absorption spectrum of an aqueous solution of lithium chromate is
given in A, plate 1, and of lithium bichromate in B, plate 1. In the case of
lithium chromate a changejn cd of 0.75 to 0.0 produces an increase in width of
the absorption of over 200 Angstrom units; the edge of the band being at about
X 4850 for cd = 0.75. This is approximately the position of the edge of the
absorption band of potassium chromate for a corresponding value of cd. For
lithium bichromate, the edge of the absorption for strip 1, cd = 0.75 is about
X 5350. The values of cd here are much larger, and the absorption is, therefore,
much greater than in the case of potassium bichromate previously investigated.

Aluminium and Calcium Chromates.

The absorption spectrum of an aqueous solution of aluminium chromate
is given in A, plate 3. The depths of cell are 3, 24, 24, and 24 mm. Starting
with the lowest strip the solution used in strip 1 was the same as in strip 2.

In B, plate 3, is given the absorption of an aqueous solution of calcium
chromate, the depths of cell being 24 mm. and the concentrations 0.025, 0.033,
0.046, 0.066, 0.1, 0.15, and 0.2 normal. For a value of cd of 0.6 the edge of the
absorption is at about X 4670. For corresponding values of cd it seems, there-
fore, that the absorption of calcium chromate is less than the absorption of
lithium or potassium chromate, and that the presence of calcium does not
increase the width of the absorption bands.

Potassium Nickel Chromate and Copper Bichromate.

The absorption of these two salts is an example of the absorption of salts
in which both the anion and the cation are colored. A, plate 4, represents the
absorption of an aqueous solution of copper bichromate, the depths of cell
being 3, 24, 24, 24, 24, 24, and 24 mm. and the concentrations 0.044, 0.044,
0.08, 0.117, 0.175, 0.26, and 0.35 normal. B gives the absorption of an aqueous
solution of potassium nickel chromate. A is seen to show the edge of the red
copper band. For a value of cd of 0.13, the edge of the absorption in the blue
is about X 4900, which is seen to be about the same as that for potassium
bichromate. The absorption of salts having both ions colored does not seem
to be at all different from what one would expect if the absorption was addi-
tive. Of course, these experiments are very largely qualitative, and it is
expected that a rigorous examination along quantitative lines will be made
with the radiomicrometer. For photographic methods the above salts are
not at all well suited, since the limits of absorption and transmission are very
large and ill defined.

The Absorption op Solutions op Certain Erbium Salts.
On account of the slight solubility of the erbium chloride in the higher
alcohols and other organic solvents, very few solutions could be made of
sufficient concentration to show the erbium absorption bands. Several of
3

34 THE ABSORPTION SPECTRA OF SOLUTIONS.

these will be taken up under the effect of change in temperature on absorption
spectra.

Hofmann and Kirmreuther1 have examined the absorption spectra of
certain salts of erbium, using the reflection method, and find a very marked
similarity between the spectra of all of the salts investigated, although these
may be as different as the chloride and sulphide of erbium. For this reason
they conclude that the erbium bands are due to locked electrons and not to
saturated valency electrons.

THE ABSORPTION OF SOLUTIONS OF CERTAIN NEODYMIUM SALTS.

Previous work has shown us that neodymium and uranous salts show
characteristic solvent spectra better than any other colored salts. On account
of the sharpness of the neodymium bands, neodymium salts have been made
the object of especial study for the existence of solvent spectra. In many
cases it is known how the neodymium bands break up in a magnetic field,
and in general it is probably true that the absorption of salts of this element
has been studied more than that of any of the others. It seems, therefore,
probable that ultimately some knowledge of the forces within the solvate may
be learned, when it is found how the various neodymium bands are broken
up for the different kinds of solvates. In the following pages considerable
emphasis will be laid upon the finer structure of the neodymium bands for
solutions in the various solvents. A large field of investigation is open for the
study of the difference in the structure of these bands in solids and especially
in crystals. Only a few examples of the latter kind will be described here.

In the description of the groups of neodymium absorption bands, the
following nomenclature will be used. This is done for the reason that each
one of these groups of bands possesses a characteristic structure for the various
solvents, and very often for different salts in the same solvent. While the
groups of bands do not change greatly in relative intensities, the finer bands
in each group show most extraordinary changes of this kind. The a group
includes bands .in the region X 3400 to X 3600; the /3 group the bands at
about X 4300; the y group from X 4600 to X 4800; the 8 group from X 5000 to
X 5400 ; the e group in the region X 5800 and the £ group at X 6300. In the
general discussion of results these groups will be compared under various
conditions of temperature, solvent, acid, etc.

In the measurements of the wave-lengths of the neodymium bands, the
standard spark lines were photographed only in the ultra-violet, so that the
measurements of the long wave-length bands here given are not claimed to
be very accurate and are made largely for comparison. On the other hand,
the difference in wave-length of bands in the same group is much more accurate.

In designating the groups of bands of the neodymium spectra, previous
workers started with the red end of the spectrum. This, however, is an un-
natural method of procedure when a grating is used, since the spectrograms
are all printed with the short wave-lengths on the left side, with the wave-
lengths increasing linearly as we pass towards the right. Moreover, it is very
doubtful if the ultra-violet absorption spectra of neodymium can be investi-
gated much farther in this region, so that this is the natural end of the spectrum

1 Zeit. phys. Chem., 71, 312 (1910).

MAPPING ABSORPTION SPECTRA OF VARIOUS SALTS. 35

from which to begin naming the groups of bands. On the other hand, it is
very probable that there are many more neodymium bands in the infra-red,
and if these occur in groups, these groups can then be named in their proper
order. The reason that the uranyl series of absorption bands was named in the
opposite direction was on account of the fact that the strongest of these bands
are at the red end of the series, while the weak bands are in the ultra-violet,
the weaker ones of the series in many cases still remaining to be photographed.

Neodymium Chloride in Water.

Some of the weak and fine bands of neodymium and uranium are extremely
difficult to obtain clearly on a photographic plate, and it has sometimes seemed
to us that the presence of absorption bands of some wave-lengths might be
due to some absorber that was not always present in the solution. For in-
stance, the presence or absence of foreign nuclei might be a determining factor
in the constitution of the aggregates. It might also be possible that aggregates
in solution are quite stable as far as their inner constitution is concerned, so
that an aqueous solution of neodymium chloride, for instance, might show
different absorption bands depending upon what the species of neodymium
chloride aggregates was before the salt was dissolved. It might be possible
to detect phenomena of this kind. It would also be very interesting to find
whether there is any relation between the kinds of spectral aggregates of a
salt in solution, and the ionic and nucleating centers produced by spraying
or bubbling the solution. Nuclei of various salts have been investigated in
this manner by Broglie and others. The spray from solutions of the same salt
in two or more solvents could be taken up. Samarium and uranous salts could
be studied in the same manner.

Plate 60, B, in the original film shows bands at XX 4000, 4180, 4271, 4280,
4295, 4330, 4650, 4695, 4805, 5330, etc. So far as we remember, this is the
first plate of ah aqueous solution on which the very narrow and faint band
at X 4280 has clearly appeared. The band X4805 is also a very weak one. In
the first strip of the original film, the band X 4271 possessed very sharp edges
and was about 8 Angstrom units in width. The band X 4280 was very weak
and was not more than 1.5 or 2 Angstrom units wide. The band X4295, on
the other hand, was about 8 Angstrom units wide and very weak. X 4650 is
very weak and is seldom seen.

An attempt will be made to compare as much in detail as possible the
bands at X4280, X5200, and X5800 for the various solvents, since it is these
bands that give evidences of the existence of solvates and the various aggregates.

Neodymium Chloride as a Methyl Alcoholate.

A solution of neodymium chloride in methyl alcohol, that had been
allowed to stand over the summer, was found to contain a gelatinous precipi-
tate. The absorption spectrum was found to show the neodymium bands quite
sharply, these bands having a somewhat different appearance from those in
an ordinary methyl alcohol solution.

A sharp band appears at X 4270, a weaker band about 8 Angstrom units
wide at X4295; a wide weak band at X3440; the triplets at XX 4700, 4760, and
4830; a very weak band at X5100; a wide hazy band at X5140; two strong,
sharp bands at X 5220 and X 5235 ; a wide and weak band at X 5260, and bands
more or less blurred together at XX 5704, 5780, 5815, and 5860. It will be noticed

36 THE ABSORPTION SPECTRA OF SOLUTIONS.

that the bands at X 4270 and X 4295 are relatively stronger than usual. The
appearance of these two bands suggests that the precipitate is largely the
hydrate of neodymium chloride, the band X4270 being a water band, and
the band X4295 being an alcohol band.

Neodymium Chloride in Propyl Alcohol.

The only spectrogram of the absorption spectra of neodymium chloride
in propyl alcohol that is reproduced is that of A, plate 7.

The a group is composed of the following bands : X 3445, a rather sharp
band about 4 Angstrom units in width; X 3460, about 10 Angstrom units in
width; X 3490 of about the same intensity and width as X 3460; X 3510 very
weak; X 3525 about 4 Angstrom units wide and quite strong; X 3540 quite
strong and narrow, this band almost running into the more diffuse band at
X 3560; a very weak band appears at about X 3580. The /3 group is quite
characteristic of the solvent, consisting of a very weak band at about X 4270,
a band at X 4285 about 10 Angstrom units wide, and a much wider and weak
band at about X 4330; a wide and weak band appears at X 4450. The y group
consists of a very weak and diffuse band at X 4600, and other bands at X 4700,
X4770, and X 4830. The <5 group consists of two wide and diffuse bands at
X 5130 and X 5180, the latter being very weak; and finer bands at X 5220,
X 5230, X 5250, X 5290 and a very weak band at X 5330. The e group consists
of the four bands X 5740, X 5780, X 5810, and X 5850, all being of about the
same width, the latter two being considerably the stronger. There are other
bands in the red region, but only X 6880 is at all strong.

Neodymium Chloride in Isopropyl Alcohol.

A, plate 6, gives the absorption spectrum of neodymium chloride in iso-
propyl alcohol. The finer structure of the different groups of bands is quite
different from that of the propyl alcohol spectrum.

The a group of this alcohol is very simple, showing only three hazy bands,
at X 3460, X 3510, and X 3535, the middle band being much the weaker. Only
a single band of the 13 group shows, X 4265. Other bands appear at X 4420,
X4600 very diffuse, X4690 and X4730.

The 8 group consists of X5100 and X5320. These bands are quite wide
and diffuse. On account of the plate not being properly developed the bands
do not show as much detail as they probably would under more favorable
conditions. Even this plate shows the finer bands slightly.

The e group consists of a broad diffuse band at X 5720 and two much
stronger bands at X 5780 and X 5810.

Neodymium Chloride in Butyl Alcohol.

The absorption spectrum of neodymium chloride in butyl alcohol is given
in plate 5, A . Starting with the ultra-violet, we have X 3450 very sharp and
narrow; X 3460 weak; X 3492 somewhat diffuse; X 3535 very sharp and narrow;
X 3545, X 3560 very diffuse. The bands XX 4265, 4285, and 4300 are weak and
of approximately the same intensity, the band X 4265 being slightly narrower
than the other two. These bands differ not only in relative intensity from
the water bands but also in wave-length, band X 4265 being of shorter wave-
length and the band X 4300 of greater wave-length than the water bands.

MAPPING ABSORPTION SPECTRA OF VARIOUS SALTS. 37

The other bands are quite diffuse and appear at XX 5085 (narrow), 5095
(narrow and very weak), 5130, 5200, 5215, 5240, 5270, 5300, 5710, 5750,
5780, 5820, 5860, 5900, and 5930.

This plate is described in detail in the chapter at the end of the mono-
graph. It may be noticed that for the solution of greatest length, the absorp-
tion in the ultra-violet is very strong, extending to X 4100. This is probably
due to the propyl alcohol. It is certainly not due to the absorption of the
neodymium chloride itself.

Neodymium Chloride in Isobutyl Alcohol.

A, plate 8, represents the absorption spectrum of neodymium chloride
in isobutyl alcohol. It will be seen from the wave-lengths given below that
there are quite marked differences between the absorption in this alcohol and
that in butyl alcohol. The band X 3455 is about 10 Angstrom units wide, and
is quite weak in the above photograph, X 3485 is of about the same width and
is much stronger, X 3515 is very weak, XX 3545 and 3570 are of about the same
intensity and each 10 Angstrom units wide. There seems to be a very weak
band at about X 4300. Wide and very diffuse bands appear at XX 4550, 5120,
5220, 5250, 5720, 5780, 5800, 5850, and 5890. The four bands last mentioned
are of about equal intensity.

The general characteristics of the absorption spectrum of neodymium
chloride in isobutyl alcohol are the weakness and diffuseness of the bands in
general, their different relative intensities compared with butyl alcohol bands,
and their slightly greater wave-lengths. The butyl alcohol bands are very
much finer and sharper than the isobutyl alcohol bands. In general, it seems
that the shorter the wave-lengths of the solvent bands the finer and sharper
are those bands.

Another spectrogram taken of a solution containing a much longer layer
contains the following bands: Theo|3 group X 4270, a very weak and narrow
band; X4290, a weak band about 6 Angstrom units wide; X4310, the strongest
band in the group, being a little stronger than X 4290; X 4330 weak and quite
diffuse, completing the bands of this group ; a very wide (50 Angstrom units)
diffuse band appears at X 4450; the y group consists of bands about 20 Ang-
strom units wide at X4700, X4730, X4780, X4830, and X4880, this band being
very weak; the 8 bands at X5150 and X5260 are about 80 Angstrom units in
width and consist of smaller bands that appear practically fused together;
some of the finer bands being at X 5215, X 5230, X 5250, and X5300; the e group
composed of the following bands: X 5740 rather weak, X5810 strong, X 5850
strong, X 5890, X 5920, X 5950 very weak, X 5995 very weak, and X 6020 very
weak.

The ultra-violet absorption of isobutyl alcohol is very considerable and
prevents the a bands from being shown very plainly. The smaller bands are
also more numerous, although weaker than in most other solvent spectra.

Neodymium Chloride in Ether.
Neodymium chloride is only very slightly soluble, if soluble at all, in ordi-
nary ether. By adding a small amount of a concentrated solution of neodym-
ium chloride in methyl alcohol to ether, a solution of about 0.01 normal was
obtained. At about io° C. this solution is transparent and its absorption was

38 THE ABSORPTION SPECTRA OF SOLUTIONS.

photographed. Heated to about 35° C. a white precipitate was formed, mak-
ing the solution quite opaque. When cooled again the solution became trans-
parent. A solution of this kind might serve to give us a better knowledge of
the constitution of aggregates. Suppose it could be shown, spectroscopically,
that the neodymium chloride aggregates existed in the ether as the methyl
alcoholate, and that when the cloud was formed the ether solution was com-
pletely freed from any neodymium salt. The number of cloud particles
could easily be counted with an ultra-violet microscope and the size of the
neodymium aggregates thus approximately determined. A study of the
Brownian movements and the growth of such a cloud would also be interest-
ing. It might be possible in some case to obtain a transparent solution of
colored solvates in a solvent having the same index of refraction and density
as the solvate, but not miscible with it, and by heating such a solution the
liquid of the solution and the solvate might mix at higher temperatures and
the resultant salt aggregate, if insoluble, would then be precipitated.

Neodymium Nitrate in Propyl Alcohol.

B, plate 7, represents the absorption spectrum of neodymium nitrate in
propyl alcohol. The absorption bands of this spectrum are quite diffuse and
present a sort of washed-out appearance.

The a group consists of three bands, the inner band being the wider and
weaker. These bands are nearly 20 Angstrom units wide and are located at
X3455, X3500, and X3585. The 13 group consists of but a single band about 12
Angstrom units in width at about X 4268. The y and other bands in that
region are very broad and weak. The 8 group is resolvable into the single bands
X 5100 and X 5220. For the greater concentrations, however, two bands can be
distinguished near the center of X 5220, being at about X 5220 and X 5235.
The e group consists of the hazy bands X 5700, X 5750, X 5780, and X 5810.

Neodymium Nitrate in Isopropyl Alcohol.

The absorption spectrum of neodymium nitrate in isopropyl alcohol
resembles quite closely the general diffuseness of the propyl alcohol spectrum
previously described.

The a group consists of three hazy bands at X 3460, X 3505, and X 3535.
Each of these bands is about 15 Angstrom units in width. The 13 band is
weak, being located at X 4270. Bands appear at X 4430, X 4690 and X 4730.
The 5 group consists of a band at X 5100 and one at X 5230. The latter con-
sists of a wide, diffuse, short wave-length component, and two finer bands
that resemble very closely the corresponding propyl alcohol bands. The e
group consists of a weak hazy band at X 5720, and two stronger bands at X 5790
and X 5810.

Neodymium Nitrate in Butyl Alcohol.

The absorption spectrum of neodymium nitrate in butyl alcohol is given
in the first two strips of A, plate 9. The general characteristics of the bands
is their general diffuseness.

The a group consists of three diffuse bands, the outer bands being much
the strongest at XX 3450, 3500, and 3540. The /3 group consists of a band at
about X 4265, and a very weak band at about X 4280. Weak and diffuse bands

MAPPING ABSORPTION SPECTRA OF VARIOUS SALTS. 39

appear at X 4420, X 4690, X 4730, and X 4820. The other bands are at X 5090,
X 5220, X 5710, X 5800, and X 5930.

The absorption spectrum of neodymium nitrate in butyl alcohol resembles
very closely the spectrum in isopropyl alcohol, but is quite different from
the spectrum in isobutyl alcohol.

Neodymium Nitrate in Isobutyl Alcohol.

The absorption spectrum of neodymium nitrate in isobutyl alcohol is
similar to that of the chloride in the same alcohol. The bands in some cases
are not so wide, but they are all quite weak when they first appear in the spec-
trum. The ultra-violet absorption was so great that in the plate taken the
a group does not appear at all. At X 4285 appears a weak double band about
15 Angstrom units wide. These bands are so close to one another that it
is difficult to be certain that they are separate bands. There are also very
weak bands at X 4305 and X 4325. The /5 group of the chloride in isobutyl
alcohol is very different in its structure from that of the /3 group of the
nitrate. A very weak band appears at X 4450 and X 4600, which is about 80
Angstrom units wide. The y group consists of weak bands, each about 20
Angstrom units in width at XX 4715, 4750, 4770, and 4850. The 8 group con-
sists of the wide and very hazy band at X 5120, two comparatively weak bands
at X5215 and X5230, and three strong bands that practically merge into each
other at X 5245, X 5255, and X 5275. The e group forms a single wide band
extending from X 5730 to X 5900. Very hazy and weak bands appear at
X 6000, X 6050, and about X 6800.

The composition of the various groups of the nitrate isobutyl bands is
quite different from the chloride bands. This is a general phenomenon, the
nitrate bands, in general, being quite different from the chloride and bromide
bands in the same solvent.

Neodymium Nitrate in Acetone.

The absorption spectrum of neodymium nitrate in acetone also consists

of wide, diffuse, and weak bands. The ultra-violet absorption is large.

The following bands appear: X 4285 very weak, X 5130, X 5250, X 5750

and X 5840.

Neodymium Nitrate in Ethyl Ester.

The absorption spectrum of neodymium nitrate in ethyl ester is given
in A and B, plate 12, and B, plate 10. The absorption bands of this spectrum
are in general quite diffuse. The a group consists of three hazy bands, the
middle one being much the weakest at XX 3455, 3500, and 3540. The /3 group
consists of a single band about 15 or 20 Angstrom units wide at about X 4270.
The bands at X 4440 and X 4600 are very broad and weak. The y group is
quite different from the y group of other solvents, in that the bands are not
even approximately of the same intensity or at equal distances from each
other, being at about X 4710, X 4730, and X 4830. The 8 group consisted of
X 5120, X 5210, X 5240, and X 5260. The e group consists of XX 5700, 5750,
5780, and 5810. These bands are very diffuse, about 20 Angstrom units wide,
and the latter two are much the stronger. A band also appears at X 5980.

The absorption spectrum of neodymium nitrate in methyl ester is identical
with that in ethyl ester.

40 THE ABSORPTION SPECTRA OF SOLUTIONS.

Neody.mil m Acetate in Acetone.

The absorption spectrum of neodymium chloride in acetone has bands
that are much more diffuse than the corresponding water bands. Similarly,
the acetone bands of the acetate are more diffuse than the water bands, and
both spectra are more diffuse than the chloride spectra.

The a group of the acetate in acetone apparently consists of but a single
band, quite weak, at X 4270 and about 15 Angstrom units wide. The region
X 4300 to X 5100 is practically continuous, the bands appearing there being
so weak and diffuse that they can hardly be detected. The <5 group consists
of a wide diffuse band at X 5110 and finer bands at X 5200 (weak), X 5235,
X 5250, and X 5260; the latter three practically blending into one. The e group
extended from X 5J720 to X 5860, this absorption showing a weak band at
X 5730, about 30 Angstrom units in width. From the above description it
will be noticed that there is a strong ultra-violet absorption (reaching to
X 3900), and that the neodymium bands themselves are very wide and diffuse.

Neodymium Acetate in Formamide.

A, plate 13, represents the absorption spectrum of neodymium acetate in
a formamide solution. The absorption bands of this solution are quite wide
and diffuse, but not as much so as the bands of the acetate in acetone. The
ultra-violet absorption is so strong as to prevent the appearance of the a group.
The /3 group consists of a band at X 4285, which is about 10 Angstrom units
wide. The bands XX 4440, 4690, 4750, 5110, and 5230 are wide, diffuse, and
with the exception of the latter, are all very weak. The e group consists of
four diffuse bands that run into each other at XX 5710, 5740, 5790, and 5830.

Summary of Neodymium Spectra.

a Group in Water. — Neodymium chloride in water gives X 3390 a very
weak band, X 3465 narrow and strong, X 3505, X 3540 narrow and strong, and
X 3560. The anhydrous chloride gives a rather strong and narrow band at
X 3500, a weaker band at X 3537, narrow and intense bands at X 3570 and
X 3595, and a rather hazy band at X 3612.

a Group in Methyl and Ethyl Alcohols. — The chloride shows the bands
XX 3475, 3505, and 3560. These bands are much hazier than the water bands.
The latter one is by far the most intense. The nitrate in methyl alcohol has
two bands, X 3465 and X 3545.

a Group in Acetone. — The nitrate has rather faint and wide bands at
X 3475 and X 3555.

a Group in Glycerol. — The chloride gives a weak band at X 3520, and
strong and sharp bands at X 3475 and X 3550.

/3 Group in Water. — Neodymium chloride in water gives a very sharp
band at X 4271, and a very narrow and weak band at X 4290. The anhydrous
salt gives narrow and intense bands at X 4308 and X 4313, a wider band at
X 4333 and a narrow band at X 4357. Neodymium nitrate in water has a band
at about X 4280, which is more hazy than the X 4271 chloride band, and which
breaks up into a band :it X 4271 and a sort of shading on the red side of this
band at about X 4280.

o/3 Group in Methyl and Ethyl Alcohols. — A band appears at X 4290 about
10 Angstrom units wide. This is wider and fainter than the water band at

MAPPING ABSORPTION SPECTRA OF VARIOUS SALTS. 41

X 4271 . X 4325 is still fainter than X 4290. The nitrate in methyl alcohol gives
a band at X 4280, which is not very intense, and which is about 10 Angstrom
units wide.

j8 Group in Acetone. — The nitrate has the band X 4280 quite weak and
about 15 Angstrom units wide.

j(3 Group in Glycerol. — This group in glycerol consists of a sharp and very
persistent band at X 4288, and two very fine bands of the same intensity at
X 4270 and X 4305.

7 Group in Water. — For the chloride there is a band at about X 4610 with
hazy edges; X 4645 a very weak band; a band at X 4685 with its red edge the
more sharply defined ; a rather sharp band at X 4755 and one at X 4820, which
is narrow and quite intense. The anhydrous chloride gives faint and hazy
bands at X 4640 and X 4680, narrow and intense bands at X 4717, X 4725, X 4735,
X 4785, X 4815, and X 4855. The nitrate shows bands at X 4737, X 4755, X 4772.

7 Group in Methyl and Ethyl Alcohols. — There are bands at X 4700, X 4780,
and X 4825. These bands are of about the same intensity, X 4780 being some-
what the narrower. X 4700 and X 4825 have faint companions to the violet.
The nitrate shows three faint bands at XX 4690, 4735, and 4825.

7 Group in Glycerol. — Glycerol bands of the chloride appear at XX 4620,
4710, 4730, 4760, 4790, and 4840.

8 Group in Water {Green). — The bands in this group form a rather com-
plicated series. The chloride has a deep narrow band at X 5090, a wide hazy
band at X 5125, a pair of very intense, narrow bands at X 5205 and X 5222, a
narrow band at X 5255, and a faint, hazy band at X 5315. The anhydrous
chloride gives a weak band at X 5088, X 5117, a narrow, intense band at X 5147,
X 5174, X 5183, X 5216, X 5254, X 5267, X 5282, etc. The nitrate band at X 5090
is much wider and hazier than the corresponding chloride band, whereas the
nitrate band at X 5125 seems narrower. There is a narrow band at X 5205, a
much more intense one at X 5225, and another narrow band at X 5235. There
seems to be a tendency for the chloride and nitrate spectra to become much
more alike as the dilution is increased.

8 Group in Methyl and Ethyl Alcohols. — These bands are X 5125 hazy and
moderately intense; X 5180 hazy and fainter; X 5220 intense and narrow;
X 5245 intense with faint companion on the red; X 5290 narrow and a faint
band at X 5315. The nitrate has two rather intense bands at X 5225 and X 5240.

8 Group in Acetone. — The nitrate has a hazy, but rather intense band at
X 5110, X 5215, and X 5255.

8 Group in Glycerol— The chloride gives bands at XX 5120 wide and hazy;
5170 narrow; 5190 narrow; 5230, 5240, 5250, and 5270 weak.

e Group in Water (Yellow). — For the chloride, this group consists of a
narrow and quite strong band at X 5725, a strong doublet at X 5745 and X 5765,
a band at X 5795 very similar to the one at X 5725, although more hazy. The
anhydrous chloride has bands at X 5768, X 5782, a narrow and intense band at
X 5807, X 5829, a narrow band at X 5858, etc.

e Group in Methyl and Ethyl Alcohols. — This group consists of X 5725
moderately intense with hazy edges; X 5765 narrower; X 5800 strong; X 5835
very intense; X 5860 hazy; X 5895 faint; X 5925 faint. The nitrate shows a
band at X 5720, probably double, a band from X 5755 to X 5845 and at X 5760,
X 5835, and a very intense band at X 5790.

42 THE ABSORPTION SPECTRA OF SOLUTIONS.

e Group in Glycerol. — The chloride gives the following bands: XX 5740
hazy, 5790, 5805, 5820, and 5850.

Although the absorption spectra of the dry neodymium salts are very
different from one another, those of the aqueous solutions, especially when
dilute, are very much the same for the different salts. For concentrated
solutions the chloride is very similar to that of the bromide, but these are
quite different from the nitrate.

The absorption spectra of neodymium chloride in methyl alcohol and in
ethyl alcohol are practically the same, and differ considerably from that of
the nitrate. The absorption bands of the nitrate in ethyl alcohol are, in
general, more diffuse than in methyl alcohol.

The chloride is but slightly soluble in acetone. The absorption spectrum
of the nitrate in acetone is especially characterized by the weak and hazy
appearance of the bands.

The absorption spectrum of neodymium chloride in glycerol is more like
that of the water spectrum, in that the bands are often very sharp and narrow.

The absorption spectra of the chloride and nitrate in isobutyl alcohol
are quite different. The structure and distribution of the intensity of the
absorption in each group is entirely different. The center of gravity of the
absorption of each group is considerably farther to the red in the case of the
chloride, but the composition of the groups is so different for the two salts
that no relationship can be seen between them when the individual bands are
compared. These solutions would afford a very good example for the spectro-
photography of chemical actions in isobutyl alcohol at low temperatures.

The above summary is made simply to give a few of the conclusions
reached in previous publications from this laboratory on absorption spectra,
and could be extended almost indefinitely. As the subject has not been studied
exhaustively, it is only given as typical of what should be done later. In the
present chapter the water, methyl and ethyl alcohols and glycerol solutions
were not taken up. For a further treatment the reader should refer to the
chapters that are to follow. On account of lack of time, the work on this
subject must be regarded as just begun. The following is the purpose of the
investigation. In the case of the uranyl bands it was found possible in many
instances to trace the bands by gradual changes from one salt to another. By
this means it was hoped to study chemical reactions in various solvents, to
find if a chemical reaction had the same effect on the uranyl bands under
different conditions. For a similar reason the neodymium spectrum has been
broken up into groups of bands, and it was proposed to study these minutely as
conditions were changed. The purpose here is to give a description of as many
characteristic groups as possible, and to correlate the groups that are alike.
It might be assumed, when the groups were constituted of the same bands with
the same relative intensity, sharpness, etc., that the physical and chemical
environment of the absorbing centers was the same, etc. Much more work
of the above kind remains to be done, especially with the rare earths already
described, erbium, holmium, etc. The absorption of the dry salts, the phos-
phorescent spectra, and the absorption spectra at low temperatures should
be obtained. This, naturally, brings us to the subject as it is presented in
the two chapters that follow.

MAPPING ABSORPTION SPECTRA OF VARIOUS SALTS. 43

THE ABSORPTION SPECTRUM OF SOLUTIONS OF CERTAIN SALTS

OF URANIUM.

For a full discussion of the absorption spectrum of a large number of
uranium solutions the reader should consult Publication No. 130 of the Car-
negie Institution of Washington. In the following pages only a few solutions
are discussed, these being for the most part solutions of the uranyl salts in some
of the organic solvents. This was taken up because corresponding neodymium
solutions were being investigated, and it was thought to be of some interest
to study a few uranyl solutions in the same way. In the description of the
bands of uranium solutions it must be remembered that the terms wide, fine,
sharp, narrow, broad, etc., are relative to other uranyl or uranous bands
and not to neodymium or samarium bands. A sharp uranyl band would,
therefore, be a very wide and hazy neodymium band.

Uranyl Chloride in Propyl Alcohol.

The absorption spectrum of uranyl chloride in propyl alcohol is given in
B, plate 18. The absorption bands of the propyl alcohol solution are quite
strong and appear at the following positions: XX 3980, 4100, 4230, about 4400
(this band is apparently double), 4580, 4750, and 4910. The upper strip shows
quite well the difference in intensity of the a and b bands. In this strip the
b band is very broad and strong, running from X 4690 to about X 4820. On
the other hand, the a band is quite weak, and is not more than about 40
Angstrom units in width.

Uranyl Chloride in Isopropyl Alcohol.

A, plate 14, gives the absorption spectrum of uranyl chloride in isopropyl
alcohol. The blue-violet band is seen to be quite prominent, its middle coming
at about X 4350, about 100 Angstrom units farther towards the red than for
the aqueous solutions. The uranyl bands are very weak and diffuse, being
much more so than for the propyl alcohol solution. The a band does not
appear on the spectrogram. The following bands show: X 4100, X 4250, X 4360
(these two bands practically merge into one another), X 4560, and X 4750.

Uranyl Chloride in Butyl Alcohol.

The absorption spectrum of uranyl chloride in butyl alcohol is given in
the three upper strips of A, plate 15. It will be seen that the uranyl bands are
very wide and diffuse. The following bands can be distinguished from the gen-
eral absorption: XX 3850, 3960, 4100, 4240, 4390, 4560, 4750, 4970. The bands
are all very diffuse, the last band being about 150 Angstrom units in width.

Uranyl Chloride in Isobtttyl Alcohol.

The absorption spectrum of uranyl chloride in isobutyl alcohol is given

in B, plate 19. The spectrogram gives the following uranyl bands: XX 4400,

4560, 4720, and 4900.

Uranyl Chloride in Ether.

The following uranyl bands have been photographed for a solution of
uranyl chloride in ether: XX 4040, 4160, 4300, 4444, and 4630. The solubility
of uranyl chloride in dry ether is very small.

44 THE ABSORPTION SPECTRA OF SOLUTIONS.

Uranyl Chloride in Methyl Ester.

The uranyl bands show quite strongly in the methyl ester solution of the
chloride. The bands are as follows: X 4030(gf),X4160(/),X 4280(e), X 4440(d)
(this band may be double and thus really represent de instead of simply d as
given above), X 4620(c), X 4790(6), and X 4920(a). The a band is very weak,
compared with the b band. Beyond the a band towards the red, there is a
very weak band which is so weak that it is very difficult to distinguish it from
the absorption of the blue-violet band itself.

Uranyl Chloride in Ethyl Ester.

B, plate 14, represents the absorption spectrum of a solution of uranyl
chloride in ethyl ester. The absorption bands are very strong and are quite
sharp. The wave-lengths and general appearance of the bands are practically
the same as that of the methyl ester solution previously described.

Uranyl Chloride in Forma.midk.

B, plate 15, represents the absorption of a solution of uranyl chloride in
formamide. The uranyl bands appear quite strong, especially the long wave-
length bands. The three bands in the second strip appear of about equal inten-
sity at X 4450, X 4650, and X 4840. The latter band is probably the 6 band.
The a band does not appear at all on the spectrogram.

Uranyl Nitrate in Propyl Alcohol.

B, plate 21, represents the absorption spectrum of uranyl nitrate in propyl

alcohol. The absorption bands are quite strong and sharp, and appear as

follows: XX 3640, 3750, 3850, 3970, 4080, 4190, 4320, 4470, 4640, and 4820. The

latter band is probably the a band, as it is very much narrower than the band

at X 4640.

Uranyl Nitrate in Acetone.

The photograph of the absorption spectrum of uranyl nitrate in acetone
showed only three bands. These bands were quite narrow, but were not as
sharp as the chloride bands. They are at X 4510, X 4660, and X 4830.

Uranyl Nitrate in Methyl Ester.

The absorption spectra of uranyl nitrate in methyl ester is given in A,

plate 15. The bands are rather weak, appearing at XX 3900, 4000, 4110, 4220,

4340, and 4480.

Uranous Chloride in Propyl Alcohol.

The absorption spectrum of uranous chloride in propyl alcohol is given in
A, plate 18. The absorption bands are all quite strong, especially the uranyl
bands. The uranyl bands shown most prominently are those at X 4590 and
X 4750, each about 80 Angstrom units wide, and a very wide band at X 4950
about 150 Angstrom units wide. Two fine bands appear at about X 5190 and
X 5210. These bands are about 10 Angstrom units in width. A band at

o

X 5500 is about 120 Angstrom units in width. A group of three narrow bands
appears at about X 5720, X 5750, and X 5770. The middle band is the strongest
one of the group, and is about 15 Angstrom units wide. Absorption maxima
appear at about X 6100, X 6270, and X 6520. This whole region is one of more
or less general absorption, due to the above very wide bands. The band at

MAPPING ABSORPTION SPECTRA OF VARIOUS SALTS. 45

X 6720 is very similar to the uranyl bands at X 4590 and X 4750. The general
appearance of these uranous bands in the red is very much like that of the
aqueous uranous bands.

Uranoits Chloride in Isobutyl Alcohol.

A, plate 20, represents the absorption spectrum of uranous chloride in
isobutyl alcohol. The uranous bands of this absorption spectrum appear
quite sharp, especially when they are broad, as is the case in the upper strips
of this spectrogram. The first strip shows a double band at about X 4300.
These bands practically merge, especially in the other strips, the shorter wave-
length band being much more intense. Quite a sharp band appears at X 4950,
with a broad region of absorption on the violet side. With a larger amount
of salt this absorption is greatly increased, resulting in a very great widening
of the band on the violet side, while the red side widens but little. A band
appears at X 5480, a weak band at X 6300, a strong band from X 6400 to X 6600,
and a strong band at X 6720. The spectrogram shows the total absence of
the uranyl bands, and very slight absorption in the region of the ultra-violet.
The upper 'strip shows the wide uranous bands very sharply indeed.

Uranous Chloride in Methyl Ester.

B, plate 24, represents the absorption spectrum of uranous chloride in
methyl ester. From the absorption spectrum it is apparent that the uranyl
salt has been largely reduced, since the uranyl bands do not show at all.
Uranous bands appear at X 4300 (about 120 Angstrom units in width). There
is a region of strong absorption extending from about X 4700 to X 5100, with
the strongest absorption in the longer wave-length portion of this region;
resulting in a much greater widening of the band towards the violet as the
amount of osalt in the beam of light is increased. There is a band at X 5500
about 150 Angstrom units wide and the red bands appear. The red bands are
both rather wide, the stronger and narrower one being at X 6730. In strip 3
the absorption runs from X 6400 to about X 6760. As the absorption increases
this red absorption region widens very unsymmetrically towards the region
of shorter wave-lengths. This solution, like most other clear uranous solutions,
shows very little general absorption in certain regions of the spectrum. Even
the general absorption in the ultra-violet is not so very great. When a large
amount of the solution is placed in the beam of light, considerable light still
passes through, and the edges of the absorption bands appear quite sharp.

THE ABSORPTION CENTERS OF URANIUM SPECTRA.

Important results will probably be obtained by a study of uranyl and
uranous compounds at low temperatures. It might be possible to obtain
aggregates of sufficient size to be seen by the ultra-violet microscope, or to be
observed in a manner somewhat similar to that of the scintillation method of
observing the a rays on a phosphorescent screen. The uranyl aggregates could
be illuminated by flashes of ultra-violet light and by proper sector arrange-
ments the aggregates could be viewed in the intervals between the illumina-
tion. These aggregates should then appear as centers of the green uranyl
phosphorescence. Neodymium compounds in phosphorogens could possibly
be treated in the same manner. These salts should also be studied when under

46 THE ABSORPTION SPECTRA OF SOLUTIONS.

bombardment by /3 rays, if they do not show any effect by a rays. Recent
studies of phosphorescent screens subjected to a-ray bombardment has led to
the belief that these substances are composed of aggregates.

THE ABSORPTION SPECTRUM OF GADOLINIUM.

Very little has been done on the absorption spectrum of gadolinium. A
dilute nitrate solution in a 10 mm. layer showed several weak bands at X 3500
to X 3390, with a maximum at X 3470. A weak band appeared at X 3300 to
X 2890 and continuous absorption commenced at about X 2200. According to
Soret there is a band from X 2800 to X 2450.

Gadolinium Chloride in Water.

B, plate 28, shows the absorption of a solution of gadolinium chloride,
1.407 normal; the depths of cell starting from the lowest strip being 2, 10, 15,
22, 22, and 100 mm. The spectrogram shows that there is quite a strong,
general absorption in the ultra-violet, amounting to several hundred Ang-
strom units in the case of the 2 mm. length of layer, transmission beginning
at about X 2700. For the 100 mm. length of layer this absorption extends
to about X 3700, the edge of the transmission being very broad.

The plate shows, besides this general absorption, two very sharp bands at
X 2925 and X 2980. These appear dearly for the 10 mm. layer. For the 100
mm. layer a weak band, about 25 Angstrom units in width, appears at X 3910.
A very weak band appears at X 3970. This band is so weak that it can hardly
be seen in the original film.

Gadolinium Chloride in Ethyl Alcohol.

Plate 28, A, gives the absorption of a 0.8 normal solution of gadolinium
chloride in ethyl alcohol; the depths of cell, starting from the lowest strip,
being 2, 4, 9, 18, 27, and 27 mm.

One of the most pronounced characteristics of the absorption of this
alcoholic solution is the enormous absorption in the ultra-violet, compared
Avith the aqueous solution. The edge of this absorption is very diffuse. It
extends to about X 3000 for the 2 mm. solution, and to about X 4400 for the
27 mm. solution.

The only characteristic part of the absorption spectrum is a very diffuse
band at X 4360. This band is gradually included in the region of general
absorption, as the depth of cell is increased.

It will be noticed that the absorption spectra of the alcohol and aqueous
solutions are very different indeed.

THE ABSORPTION SPECTRUM OF DYSPROSIUM.

Very little work has been published on the absorption spectrum of dyspro-
sium. Lecoq de Boisbaudran1 has described the following bands arranged
according to the intensity of the bands: Dya, X = 4515; Dy(3, X = 4750; Dyy,
X=7565; Dy8, X=4275.

Urbain2 has observed only X 4740 and a weak band at X 4530 to X 4500.

1 Compt. Rend., 102, 1005 (1886). 2 Ann. Chini. Phys., 19, 2-44 (1900).

MAPPING ABSORPTION SPECTRA OF VARIOUS SALTS. 47

Dysprosium Chloride in Water.

The absorption spectrum of aqueous solutions of dysprosium chloride is
given in B, plate 29, and A, plate 30. The absorption in the ultra-violet is
very small for aqueous solutions of dysprosium chloride, even when the
depth of cell is as great as 21 mm., and the concentration is 1.86 normal, as
it is for the last strip of A, plate 30.

Strip 2, B, plate 29, represents the absorption of a 6 mm. layer of 1.86
normal concentration. As this strip shows many of the bands, it will be
described in detail. The general appearance of the bands is much the same,
although they vary greatly in intensity. The edges are quite sharp, although
none of the bands are as sharp and clear as the neodymium water band,
X4271.

The first band observed in the ultra-violet is X 3130. As will be seen from
the spectrograms the bands in the ultra-violet are by far the strongest
in the spectrum. X 3130 is one of the strongest of these ultra-violet bands,
and is about 40 Angstrom units wide. The next band at X3260 is about 20
Angstrom units wide and is quite weak. It is very similar and of the same
order of intensity as the bands at XX 3470, 3670, 3690, 3800, 3850, 4170, 4340,
4400, and 4430. Of these bands, the latter two are the strongest. The band
X 3390 is the strongest Jband in the whole spectrum. It possesses sharp
edges and is over 50 Angstrom units in width, while the strong band
X 3535 has only a width of 30 Angstrom units. The band X 3760 is also quite
strong and has a width of about 25 Angstrom units. Other bands appear
faintly at XX 4650, 5300, and 6350, but these are so weak that their positions
can hardly be measured. The band at X 3970 is very broad and weak, and
shows best in the fifth strip.

In the last strip of 100 mm. depth, quite a number of new bands appear.
The ultra-violet absorption has become so strong that the whole region to
X4050 is absorbed. The band at X 4160 is apparently double, being much more
intense on the violet side. The bands at X4190 and X4220 are much weaker,
the former almost merging into X 4160. Absorption bands extend from X 4250
to X 4300, X 4450 to X 4580, X 4680 to X 4820. Very weak bands appear at
XX 4840, 4890, 4930, 4960, 5425, 5460, 5490, 5520, 6460, 6570, and 6600. The
band X 5380 is quite strong and almost merges into the other bands in this
region. The band X 6440 is also quite strong. X 6440 and X 6460 are very
diffuse and almost form a single band.

Dysprosium Chloride in Methyl Alcohol.

The absorption spectrum of a solution of dysprosium chloride in methyl
alcohol is given in A, plate 29. The absorption is greater for the alcohol
solution, especially the general ultra-violet absorption. Otherwise the relative
intensity, number, wave-length, and general appearance of the absorption
bands are exactly the same as the bands of the aqueous solution.

Dysprosium Chloride in Ethyl Alcohol.

With the exception of the very much greater ultra-violet absorption, the
spectra of dysprosium chloride in water, methyl alcohol, and ethyl alcohol (B,
plate 31) are very much the same; the ethyl alcohol band at X 5400 being wider
and of greater wave-length than the water and methyl alcohol bands at X 5380.

48 THE ABSORPTION SPECTRA OF SOLUTIONS.

The fact that this one band should appear of a different wave-length in
ethyl alcohol, as compared with water, while the other dysprosium bands appear
to be the same for both solvents, might be taken to indicate that this band
was not due to dj'sprosium. A difference of this kind might lead to a possible
method of ascertaining the presence of the salts of two different elements in
solution. In the case of neodymium salts it was found, in general, that if one
solvent had one characteristic band, all the solvent bands were more or less
characteristic. On the other hand, it must be remembered that in the case
of the uranyl bands the differences for different solvents were usually much
greater for the longer wave-length bands. So, in the above case of dysprosium
chloride in ethyl alcohol, we may assume that the band X 5380 may not be
due to dysprosium. It may be possible that the difference in the action of
acids on the bands of the spectra of a given salt, or the difference of the spectra
as obtained with the same salt in different solvents, may be useful in separating
the different elements.

Dysprosium Acetate in Water.

The absorption spectrum of dysprosium acetate (0.4 normal) in water is
given in A, plate 31. The depths of cell were 4, 16, 25 and 34 mm. The spec-
trum is very similar to that of the aqueous solution of the chloride. The bands
are, however, somewhat more diffuse. The wave-lengths of the bands, while
comparatively weak, appear to be the same for the chloride and the acetate.
The acetate bands widen more to the red, so that when the bands are
quite wide, the acetate bands appear to be relatively shifted towards the
red. The addition of- concentrated nitric acid produces little effect on the
wave-length, or the appearance of the dysprosium acetate bands in water.

THE ABSORPTION SPECTRUM OF SAMARIUM.

The absorption spectrum of samarium has been studied much more exten-
sively than that of gadolinium and dysprosium. On the following page is a
table giving the wave-lengths of some of the bands as determined by various
observers.

In general appearance the absorption spectra of samarium salts are very
similar to those of the dysprosium salts, the strong bands of the spectrum
being located in approximately the same violet and ultra-violet part of the
spectrum. The general appearance of the individual bands is very similar
to that of the dysprosium bands, the samarium bands being, however, some-
what narrower and having sharper edges.

Samarium Chloride in Water.

The absorption spectrum of samarium chloride in water is shown in A,
plate 32. The concentration is 1.31 normal and the depths of cell, starting
from the lowest strip, are 2, 6, 12, 16, 20, and 100 mm.

From the spectrogram it is seen that for the aqueous solution the ultra-
violet absorption is very small indeed. A weak and apparently quite wide
band appears at about X 3050. For the 6 mm. depth of cell, second strip, bands
appear at XX 3200, 3320, 3510, 3630, 3910, etc. The other bands are quite
weak and will be given for the 20 mm. layer. Of the above bands, the X 3910
is by far the strongest of all the samarium bands^ For the 20 mm. layer the
following bands appear : X 3420 (diffuse, about 20 Angstrom units wide), X 3470

MAPPING ABSORPTION SPECTRA OF VARIOUS SALTS.

49

to X 3540, X 3550 very weak, X 3570 very weak, X 3600 to X 3660, X 3720 very
weak and wide, X 3770 weak, X 3800 (about 20 Angstrom units wide), X 3860
to X 3945, X 3970, X 4050, X 4070, X 4320 very wide and weak, X 4540 (about
40 Angstrom units wide) ; X 4640 to X 4750 is a region consisting of two diffuse
bands that practically run together, X 4800 weak, X 5110 weak. Other bands
appear in the region of longer wave-lengths, but they are very weak.

A 100-mm. strip showed complete absorption of wave-lengths shorter
than X 4130. Bands appeared from X 4210 to X 4370; a band about 30 Ang-
strom units wide at X 4420; a band from X 4470 to X 4840, X 4890 to X 4950,
X 4200, X 4230 (these two bands are of about the same intensity, rather dif-
fuse and about 20 Angstrom units wide), X 5500, X 5525; these bands are very
similar to the preceding pair, being, however, considerably more intense.

Lecoq de
Boisbandran.1

Soret.

Thalen.2

Kriiss and
Nilson.

Bettendorf.

Forsling.3

Bohm.4

Demar-
cay.

For-
manek.5

5590

5590

5590-5560

5587

5590-5582

5600

5590
5290

5588
5282

5010-5000

5000

5015-4970

5004

5039*4995

5001

4980

5211

4890

4890

4891

....

5005

4860-4740

4800

4860-4720

4830-4750

4885-4736

4804-4783
4761-4727

4800

4760

4892
4880

4640-4630

4635

4660-4600
4450-4370

4633

4690-4619

4632
4443-4383

4625

4630
4530
4430

4750
4634
4530

4170

4190-4150

4185-4150

4174

4220-4151

4174
4157

4165

4170

4436
4175

4080-4060

4090

4083

....

....

4077

4070

....

4035-4030

4020

4090

4007.5

3760^-3720

4016-4007
3942-3932

3906
3752-3742
3738-3732

3750"-3730

3900
3750

3640-3600

3630-3615

3640-3600

3620

1 Spectres Lumineux, Paris bei Gauthier-Villars (1S74).

2 Journ. de Phys., 2, 446 (1883).

3Bih. K. Svensk.Vet.-Ak. Handl., 2S, n. Nr. 1 (1902).

4Zeit. angew. Chemie., 15, 1282 (1902).
5 Die qualitative Spectralanalyse anorganischer Korper,
Berlin bei Muckenberger (1900).

Samarium Nitrate in Water.
The absorption spectrum of samarium nitrate in water is given in A,
plate 33. The absorption bands have almost the same characteristics, rela-
tive intensity, wave-lengths, etc., that water-bands of samarium chloride have.

Samarium Chloride in Methyl and Ethyl Alcohols.

The absorption spectrum of samarium chloride in methyl alcohol, B,
plate 32, is so similar to the absorption of this salt in ethyl alcohol, A, plate
34, that only the former will be described in detail. It will be seen from the
spectrograms that the general absorption in the ultra-violet and violet in
the case of the ethyl alcohol solution is very much greater than in the case
of the methyl alcohol solution.

B, plate 32, represents the absorption of a normal solution of sama-
rium chloride in methyl alcohol, the depths of all being 2, 5, 9, 18, 27, and
27 mm. The last spark spectrum was taken with the solution removed simply

50 THE ABSORPTION SPECTRA OF SOLUTIONS.

for the purpose of making wave-length measurements. The first strip shows
the following bands: X 3200, X 3330, X 3500, X 3540, X 3640, X 3910, X 3925,
and some weaker bands farther towards the red. The third strip shows the
following bands: X 3200 quite weak; X 3330, X 3350, X 3430, and X 3450 very
weak and diffuse, appearing almost as a single band; X 3500, X 3540, X 3570,
X3640; X 3910, the strongest band in the spectrum, is about 35 Angstrom
units wide, X 3950 about 10 Angstrom units wide, X 3970 about 6 Angstrom
units wide, X 4060 and X 4100 are each about 30 Angstrom units wide and quite
weak. Other bands are easy to see and will be described in the last strip. In
this strip the ultra-violet absorption is complete to X 3350. In addition to
the bands mentioned above there were bands at XX 4290 weak, 4310 weak,
4350 weak, 4410 weak, 4520, 4545, 4570; this triple group is perfectly sym-
metrical, the middle band being much the strongest; 4650, 4705, and 4760
are each about 40 Angstrom units wide, are diffuse and weak with the middle
band having the greatest intensity; 4810 is about 10 Angstrom units in width,
4920 quite weak and diffuse, and 5200 is very weak. No other bands in the
longer wave-length region are visible in addition to those described.

Samarium Chloride in Water and Ethyl Alcohol.

The absorption spectra of samarium chloride in mixtures of water and
ethyl alcohol are given in B, plate 33, and B, plate 34. The change from the
alcohol to the water spectrum can easily be seen in the shift of the bands of
strip 1 compared with strip 2 in B, plate 34. The very great persistency of
the water bands is shown, since in this case (strip 2) only about 5 per cent
of water is present. The band X 3970 seems to be a characteristic water band.
In strip 1, B, plate 33, this band does not appear, but in strip 2 it can be
clearly seen and in the succeeding strips it becomes stronger. It seems, there-
fore, that the samarium bands behave in the same way as the neodymium
bands, although there is no single band that shows the effect as clearly as the
neodymium X 4271 water band, and the corresponding X 4290 alcohol band.
Since the water bands are so persistent it would be very interesting to carry
this work to temperatures almost as low as the freezing-point of alcohol. In
general, a lowering of temperature has increased the persistency of the water
bands.

CHAPTER IV.
SPECTROPHOTOGRAPHY OF CHEMICAL REACTIONS.

INTRODUCTION.

Prior to these investigations, very little work had been done on the effects
produced on the absorption spectrum of a salt in solution when acids and
various other reagents that produce chemical reactions were added. Indeed,
much of our knowledge of what is in solution is obtained indirectly rather than
by a direct knowledge of the composition of compounds while in solution. Con-
siderable work on the effect of the introduction of certain radicals, groups, etc.,
on the absorption spectra of organic compounds has been done, and a fairly
complete survey of this work is given in the first chapter. Especial efforts
have been made to discuss, as completely as possible, our knowledge of the
absorbing and emitting centers of light and heat.

It is true that a few observers had noticed that in some cases different
salts had bands of different wave-lengths, when dissolved in the same solvent.
A typical example of this kind is given in the following table. Then again,
observers had noted that the bands of one salt of a metal were finer and
sharper than the bands of another salt (for example, neodymium chloride as
compared with the nitrate in water), but, as far as we know, no effort has
been made to study what changes in the bands resulted as one salt of a metal
was changed into another salt.

The following table gives the absorption bands of some double acetates
of uranyl according to Morton and Bolton:2

NH4

4620

4455

4310 ! 4200 4075

Ba

4625

4465

4330 4200

4075

Co

4625

4465

4335 4200

40S0

K

4625

4460

4320 4200

4070

Li

4620

4455

4320 4205

4080

Mg

4627

4480

4337 4215

4085

Na

4620

4453

4310 4185

4055

Rb

4625

4465

4315 4202

4080

Sr

4627

4480

4335 ! 4205

4075

Tl

4640

4480

4320 4205

4085

Zn

4655

4490

4340 , 4325

4100

For the following double sulphates they give :

Ammonium uranyl sulphate 4935

Ammonium diuranyl sulphate .... 4875

Magnesium uranyl sulphate

Potassium uranyl sulphate i ....

Rubidium uranyl sulphate 4885

Thallium uranyl sulphate , 4905

Sodium uranyl sulphate 4875

4890 4620 4450

4720 4575 4480

4790 4540 4370

4790 4540 4370

4805 4645 4475

4630 4480 4295

4775 i 4580 4440

4330 4240

4330 4210

4230

4100

4230

4100

4325

4205

4335

4180

1 Publication 130, Carnegie Institution of Washington.

2 Chem. News, 28, 47, 113, 164, 233, 244, 257, 268 (1S73).

51

52 THE ABSORPTION SPECTRA OF SOLUTIOX>.

One of the purposes of the present research is to study one group of
bands throughout a long series of reactions. The first group of bands to be
studied in this way was the uranyl bands. Although not completely followed,
and not identified with certainty in every case, yet for aqueous solutions, at
least, it has been possible to designate these bands as a, b, c, d .... and to
follow each band as the salt is changed from acetate to nitrate, from nitrate
to chloride, from chloride to sulphate, etc. It is found in all these changes
that the individual uranyl bands undergo a gradual shifting of their 'position
in the spectrum. Various chemical changes have been accompanied by charac-
teristic changes in the absorption spectrum; e.g., a narrowing of the bands, a
shifting of the bands to the violet, etc. It is natural to try to correlate as
many of these characteristics as possible, but as yet not enough work has
been done to furnish many generalizations. It may be said, qualitatively, that
in many instances a shifting of the bands towards the violet is often accom-
panied by a narrowing and sharpening of the edges of the bands. This is a
general characteristic also of the temperature effect, since a lowering of the
temperature of an absorbing compound is nearly always accompanied by a
narrowing of the absorption bands and a slight shifting towards the violet.

Again, it is natural to inquire whether the same chemical reaction produces
the same changes in the absorption spectra when this reaction takes place
in different solvents. Unfortunately there are not many examples of this
kind that can be tested. In general, the absorption bands of different salts
in the same solvent are practically the same, so that a chemical reaction
can not be photographed spectroscopically. It was for this reason that a
considerable number of absorption spectra have been mapped. A number
of solvents showing a difference in the absorption spectra of different salts
are not suited very well for this kind of work. Some very good examples,
however, have been found, and these will soon be studied. Acetone was
found to give a greater difference between the absorption spectra of uranyl
nitrate and the other uranyl salts than water; and as the uranyl acetone
bands are very sharp and these solutions can be studied at very low temper-
atures, this solvent offers exceptional advantages for making a comparison
between the same chemical reactions in the two solvents.

The closely related problem as to whether apparently similar chemical
reactions for different salts in the same solvent are really similar has already
been answered in some cases in the negative. We have seen that the chem-
ical reaction that takes place when uranyl chloride is changed to uranyl
nitrate has resulted in a narrowing and sharpening of the uranyl bands,
and a gradual shifting of these bands towards the violet. The same reaction
of neodymium chloride to neodymium nitrate in water results in the bands
being made broader and more diffuse, and in some cases these bands are
slightly shifted towards the red.

It must be stated that the study of the changes that take place in the
neodymium bands has been too complicated for the present research. No
other element shows such an extremely wide diversity in the structure of its
various groups of bands, and at present no examples have been found where
any bands can be traced throughout chemical reactions. The changes seem
rather to be connected with a change in the relative intensity of bands in the

SPECTROPHOTOGRAPHY OF CHEMICAL REACTIONS. 53

same group, and, therefore, a shifting of the center of gravity of the absorp-
tion of the group itself. This may be true also of the uranyl bands, since it is
known that the uranyl bands break up into much finer bands at low tempera-
tures. At any rate, the changes in the uranyl bands at ordinary tempera-
tures, when chemical reactions take place, are comparatively simple compared
with the same changes in the case of the neodymium bands.

It is very important that a careful study be made of a series of chemical
transformations of neodymium salts (especially at low temperatures). For
instance, it has been shown that different anhydrous neodymium salts pos-
sess very different absorption spectra. Comparisons between the spectro-
photographs of dry chemical reactions should be made and these compared
with the same reactions in various solvents. As soon as we know what
physical and chemical conditions are connected with given spectroscopic
changes, then a study of the absorption spectra of crystals should be of aid
in giving us an insight into the physical and chemical conditions within
crystals. An example of this kind of study might be found in the different
varieties of ice. Tammann1 has shown that there are four distinct varieties,
ice i and iv being lighter than water, and ice n and in being denser.
According to Tammann the latter two varieties are composed of the simpler
molecules. These different forms of ice are produced at different tempera-
tures and pressures. The freezing-point of ice i is

t° -5.53° -10.42° -15.66°

p 675 1141 1597 kg./cm.2

Whereas ice in is formed by compressing to 3000 kg./cm.2 and cooling
to —80°, ice n is formed by cooling to —80° and then compressing to 2700
kg./cm.2 Ice iv is probably identical with the tetragonal ice observed by
E. Nordenskiold2 or with the regular ice crystals formed from 75 per cent alco-
holic solutions.3 The absorption spectra of colored salts like those of neodym-
ium, samarium, or uranium would probably aid in a study of these different
crystalline types.

Another subject that seems to be of interest and promise is the possibility
of breaking up absorption spectra into related groups. The work of Wood on
sodium vapor has been very successful in this respect, each monochromatic
stimulating wave-length exciting a certain resonating system within the atom
or molecule, the resultant resonance spectrum including light of the same
wave-length as that of the exciting light.

Among the rare earth elements praseodymium, neodymium, samarium,
europium, terbium, dysprosium, and erbium show characteristic absorption
spectra in the visible wave-lengths, and also are examples of excellent phos-
phorogens. Lanthanum, gadolinium, and yttrium do not possess absorption
spectra, and only act as diluents. Following are some tables given by Urbain
of the various phosphorescent spectra studied by him :

1 Zeit. phys. Chem., 72, 609 (1910).

2 Ann. d. Phys., 114, 612 (1861).

3 Barendrecht: Zeit, phys. Chem., 20, 240 (1896).

54

THE ABSORPTION SPECTRA OF SOLUTIONS.

Phosphorescence of Praseodymium, Xeodymium, and Erbium in Calcium Oxide.

Praseodymium.

Neodymium.

Erbium.

4875 vs.

3920 s.

4040 w.

4905 w.

3980 s.

4045 w.

4940 m.

3640 to 4130

4060 w.

4985 to 5030 vw.

4190 sn.

4085 s.

5090 to 5130 vw.

4220 sn.

4095 m.

5170 m. dif.

4230 sn.

4460 in.

5225 to 5280 vw.

4270

4520 m.

5335 to 5370 w.

4295 vs.

4550 s.

5527 w.

4400 w.

4590 s.

5560 to 5600 w.

4515 w.

4620 w.

5790 w.

4575 s.

4670 m.

6045 s.

4610 w.

4690 m.

6065 s.

4660

4730 w.

6130 to 6220

4690 w

4760 m.

6260 vs

4785 w.

6340 s.

5270 to 5355 m.

6670 to 0800 dif.

5495 to 5550 w.
5595 to 5630 m.

vs. = very strong; w. = weak; m.=medium; dif. = diffuse; v-w. = very
weak; s. = strong; sn. = strong and narrow.

The phosphorescence of compounds of the rare earths in other diluents
is much simpler than in calcium oxide :

Phosphorescence of Praseodymium in Calcium Sulphate.
XX 5876, 6000, and 6100.

Phosphorescence of Erbium in Calcium Sulphate.
XX 5220, 5435, 5535.

Phosphorescence of Erbium in Calcium Fluoride.
XX 5170. 5200, 5250, 5310, 5400, 5465 strong, 5510 strong, 5620.

By using monochromatic light or homogeneous cathode rays of different
velocities, it might be possible to excite only certain related bands and not
the whole absorption spectrum. This might apply especially to the uranium
salts. On the other hand, the presence of the diluent might preclude any
possibility of this kind. The recent work of Wood and Franck1 on the effect
of the presence of helium on the resonance of iodine vapor would lead to a
conclusion of this kind. A study of the fluorescence of sodium, potassium,
mercury, and iodine vapor shows that the intensity of the emitted light is
greatly reduced if air or some other inert gas is present. The effectiveness
of a gas in destroying fluorescence appears to increase with the molecular
weight. In the case of the fluorescence of anthracene, Elston found that hydro-
gen and nitrogen had very little effect, while oxygen and carbon dioxide had
a very great effect.

Warburg2 has shown that the current obtained from the negative point
discharge is much greater in nitrogen, helium, argon, and hydrogen when the
last traces of oxygen3 have been removed. To explain this Warburg assumed

1 Phil. Ma^., 21, 309, 314 (1910).

- Ann. d. Phys., 40, 1 (1896).

3 J. Franck: Yerh. der deutsch. phys. Ges., July (1910).

SPECTROPHOTOGRAPHY OF CHEMICAL REACTIONS. 55

that the mobility of the negative ions in the pure gases was much greater
than when traces of oxygen were present. In other words, the electrons
existed largely in a free condition, and as soon as oxygen was introduced, the
electrons and negative ions were supposed to condense on the oxygen mole-
cules. Small traces of the less electro-negative gases are found to act in the
same way. Similarly, Franck and Wood believe that the more electro-nega-
tive elements have a greater power to destroy fluorescence, and this agrees
with the relative destructive power as given by the series hydrogen, air, car-
bon dioxide, and ether vapor, the latter having the greatest effect. The con-
clusion reached is that the gases that interfere with the motions of the free
electrons are also the ones that modify the motion of the bound electrons
to the greatest extent. In this connection the influence of the presence of
helium, argon, nitrogen, oxygen, and chlorine on the fluorescence of mercury
and iodine has been studied.

According to Lorentz's hypothesis that the damping results from col-
lisions, the damping factor would be a function of the molecular weight. On
the other hand, the destructive effect of helium on the fluorescence of iodine
is much less than that of hydrogen, the effects due to hydrogen and argon
being about the same, although the molecular weight of the latter is forty
times as great as the former. The gas having the greatest effect is chlorine,
and next to it ether. It therefore seems that the power of a gas to combine
with electrons is a very important factor in determining the role which that
gas will play in the suppression of fluorescence.

The fact that the maximum intensity of fluorescence in a pure gas occurs
at different pressures for different gases may also be explained on the above
hypothesis. To obtain visible fluorescence there must be a sufficient number
of molecules present to give a certain intensity of the emitted light, but this
number of molecules must not be so great as to disturb each other. In a
strongly electro-negative gas the vibration of electrons in one molecule will
be influenced by the presence of other molecules in the neighborhood, so that
for a gas like bromine the fluorescence would be expected to take place at much
lower pressures than is the case for a much less electro-negative gas like
iodine ; while in the case of mercury the pressure would be very much greater.
This has been found to be the case.

In treating the following cases of chemical reactions, the theory of aggre-
gates, as fully developed in the general remarks at the end of this monograph,
will be assumed. It may be possible to obtain some knowledge of these aggre-
gates by other methods. In the study of a, /3, and 7 rays it is found that, in
general, the absorption of these radiations depends on the atomic structure
of the matter they traverse, rather than on the molecular structure. There
may possibly be some exceptions to this rule. For instance, uranium and
thorium salts emit a rays, the a rays probably coming from the uranium and
thorium atoms themselves, before they break down into the next radio-active
product. If one had a very thin layer of a uranium or thorium salt, it would
be expected that the range of most of the a particles would be less if the ura-
nium or thorium existed as an aggregate or in the simple molecular condi-
tion. It might, therefore, be found that the range of the a particles from
uranyl nitrate would be related to the wave-length of the uranyl or uranous

56 THE ABSORPTION SPECTRA OF SOLUTIONS.

absorption bands. Knowing something of the electromagnetic forces neces-
sary to alter the path of the a particles, it might be possible to find some of
the properties of the fields of force that exist in the neighborhood of the ura-
nium and thorium atoms, and the correlated effects upon the frequency of
the absorption bands. Similar effects might be obtained in the case of the
/3 and y rays. The particles of the active deposits might also be used as
nuclei about which aggregates would be formed.

Another possible method of studying aggregates is by the phosphores-
cence produced by a and /3 rays. The effect of changes in temperature could
also be taken up in this connection. Several workers have concluded that
the phosphorescence produced by the bombardment of certain kinds of screens
by a rays is not due to the breaking up of crystals along lines or planes of
cleavage, but that it is due to the breaking up of aggregates that are of
approximately molecular dimensions; and that unless such aggregates are pres-
ent, no phosphorescence is produced. For example, pure zinc sulphide is not
phosphorescent, but when there are impurities present phosphorescent scin-
tillations can be produced. It is probable that in most phosphorescent screens
there are a large number of active centers present. When the centers on the
surface are destroyed, those farther in the screen will become sources of scin-
tillations. When the screen is new, each bombarding a particle probably
excites a considerable number of centers. As the bombardment is continued
the number of centers excited by each a particle decreases, although each par-
ticle will excite one or more centers, so that only the intensity and not the
number of scintillations is decreased. The calculated diameters of the active
centers of zinc sulphide and willemite are of molecular magnitude, whereas,
in the case of barium platinocyanide, the diameter is about a hundred times
this size. Marsden1 has found that the number of scintillations is slightly
less at 100° than at 15°.

It would be interesting to prepare salts so that various solvates and aggre-
gates would be found, and then study the scintillations produced by them.
The above suggestions are made since they are along somewhat different
lines than those usually followed in the study of solutions.

NEODYMIUM CHLORIDE IN ETHYL ACETATE AND ANTHRACENE.

It was supposed by the older writers on the absorption spectra of solu-
tions, that if two colored salts were dissolved in the same solution the fre-
quencies of the absorbing centers of the one salt would be modified by the
presence of the other salt, if the frequencies of the absorbing centers of the
latter were almost the same as those of the former. Several attempts have
been made to test this theory by experiment, but thus far no case is known
where such an effect is shown, at least by the inorganic salts.

According to our theory of aggregates we would hardly expect any two
absorbing centers to have any mutual effects on each other, unless both centers
were part of the same aggregate. It is, however, quite difficult to find any
solution containing such aggregates; indeed, it is doubtful if there are any
that contain absorbing centers that have bands in the same region of the

1 Proc. Roy. Soc, 84, A, 548 (1910).

SPECTROPHOTOGRAPHY OF CHEMICAL REACTIONS. 57

visible part of the spectrum. This line of work seems to be much more promis-
ing in the infra-red.

It is known that, in the ultra-violet, anthracene and the neodymium
salts have absorption bands that are near together. It was, therefore, hoped
that a neodymium salt dissolved in anthracene would form aggregates con-
taining both of these compounds, and that the resultant absorption spec-
trum would not be simply the superposition of the two separate absorption
spectra of the given neodymium salt and anthracene. Unfortunately, the
neodymium salts that were tried were not soluble in anthracene, and it seems,
therefore, unlikely that we obtained aggregates containing both salts. Never-
theless, some spectrograms were taken with this idea in mind. Neodymium
nitrate and anthracene are both soluble in ethyl acetate, and several solutions
containing these three substances were prepared. It was found that only very
dilute solutions of anthracene allowed the ultra-violet part of the spectrum
to be transmitted, and no good photograph was obtained that showed the
anthracene and neodymium nitrate ultra-violet bands on the spectrogram.
It would seem very doubtful that any effect would be obtained, because of
the improbability of anthracene and neodymium nitrate being contained in
the same aggregate.

A and B, plate 11, give the absorption spectrum of neodymium nitrate
and anthracene dissolved in ethyl acetate. In these spectrograms the ultra-
violet absorption is so great that none of the anthracene bands are shown.
A and B, plate 12, represent neodymium nitrate and anthracene in ethyl
acetate. The lower strip of B shows several of the ultra-violet bands of
anthracene, but does not show any of the neodymium ultra-violet bands. The
upper strip of B shows the neodymium bands quite clearly, and also the very
great general ultra-violet absorption. It is hoped that this effect can be
satisfactorily tested with reference to the theory of aggregates, especially in
the infra-red.

THE URANYL AND URANOUS BANDS.

The uranyl bands are usually ten or twelve in number and, starting at
about X 5000, have been designated by the letters a, b, c, etc., and form a series,
the head band being the a band. The uranous bands appear throughout the
spectrum and do not form any series. The uranous bands are characteristic
of the uranous salts, and spectrograms of the absorption spectra of a uranous
salt gradually oxidized into a uranyl salt by the addition of hydrogen peroxide
show the complete independence of the two spectra.

A problem of considerable interest, and one that seems capable of solu-
tion, is the complete correlation of the a, b, c, . . . bands for the various
uranyl salts in each solvent. For most solvents the uranyl bands of the dif-
erent uranyl salts are very much alike. In aqueous solutions the differences
are very marked, but by taking spectrophotographs of the transformation of
one salt into another salt1 Jones and Strong have succeeded in obtaining many
relationships of this kind. By extending the work to low temperatures, and
to the partly overlapping phosphorescent band spectra, it ought to be possible
to connect continuously the various changes that these bands undergo.

1 Publication 130, Carnegie Institution of Washington.

58

THE ABSORPTION SPECTRA OF SOLUTION^.

OXIDIZATION OF URANOUS TO URANYL SALTS IN SOLUTION.

Some spectrophotographs have been made of the formation of a uranous
salt from a uranyl salt by the nascent hydrogen reduction method. These
spectrograms are somewhat unsatisfactory, since there is a considerable
amount of gas, opaque residues, etc., formed, and the reduction takes a con-
siderable length of time. It is hoped in the future that this reduction may be
produced by the action of occluded hydrogen (for example in palladium) that
will be liberated on being heated, or by some other method that will not
require the addition of other acids or salts.

The reverse process of the oxidization of uranous salts in solution is a
very satisfactory one, and the spectrograms can be photographed very well
indeed. This method simply consists in the addition of a small amount of
hydrogen peroxide to the uranous solution. The spectrophotography of
these reactions, as already mentioned, has furnished an admirable method
by means of which the uranyl and uranous bands can be completely separated.
Several oxidizing agents have been added to aqueous and alcohol solutions
of uranous salts that showed both the water and alcohol bands, in order to
see whether there is any selective oxidation.

OXIDIZATION OF URANOUS CHLORIDE BY HYDROGEN PEROXIDE.

The first solution of uranous chloride to be oxidized, in order to compare
the effect of the presence of acid and of other salts, was that of the very
intensely colored solution of uranous chloride formed by reducing an ether
solution of uranyl chloride. To one part of this solution were added four parts
of water, and this required for oxidization about a one-fourth part of a solu-
tion of hydrogen peroxide. A cloudy and semi-transparent precipitate was
formed during oxidization. When one part of the ether solution of uranous
chloride was added to six parts of concentrated hydrochloric acid, the result-
ant liquid contained a cloudy precipitate. The addition of a few drops of
hydrogen peroxide clarified the solution, but it required over two parts of
hydrogen peroxide before the uranous chloride was completely oxidized.
Following are the tabulated results of these experiments:

Parts
uranous
chloride
solution.

Parts of solvent added.

1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0

1.0
1.0

4.0 water

6.0 hydrochloric acid. Soluble

12.0 A1C13 in water. Small precipitate. . . .

6.0 Cone. HNO3. Only partly oxidized. .

10.0 acetic acid. Precipitate formed

12.0 ethyl alcohol

12.0 glycerol

12.0 methyl alcohol. Precipitate formed. . .
12.0 Ca(N03)2 in 2H20 and 3CH40. Pre-
cipitate

90.0 ethyl alcohol

12.0 water, 4.0 XaC104 in water, and HC1.

Parts

hydrogen

peroxide in

water
necessary for
oxidization.

0.6

2.0

0.5
0.7
0.6

0.4
0.6
1.5

A solution of uranous chloride in water was prepared in the usual manner,
and made strongly acid by the addition of hydrochloric acid, in order to

SPECTROPHOTOGRAPHY OF CHEMICAL REACTIONS.

59

increase the stability of the solution. Solutions of uranous salts sometimes
form precipitates on standing. Uranous chloride was dissolved in the follow-
ing solvents, and mixed with the following compounds:

Parts
uranous
chloride
solution.

1.0
1.0
1.0
1.0

Parts of solvei't added.

1.0
1.0
1.0
1.0
1.0
1.0
3.0

10

10

10

10

10.0

10.0

10.0

10

10

10

10

Parts of hydrogen peroxide
necessary for oxidization.

0 water

0 ethyl alcohol

0 glycerol

0 hot water

water +H2S04

acetic acid

acetone

nitric acid

calcium nitrate in 2H20 + 3CH40 .

0 hydrochloric acid 2.0

0 aluminium chloride in water 0.25

10.0 sodium perchloride in water 0.2

10 . 0 aluminium chloride in hot water .... 0.16

7 . 0 hot hydrochloric acid 0.8

S.O hot nitric acid 0.0 NO

0.2

0.2

0.35

0.2 precipitated.

0.1

.0
.0

0.2

0.2 precipitated.

2.0

0.2

formed.

A spectrogram was made showing the effect of the addition of hydrogen
peroxide to a solution of about 0.75 normal concentration of an aqueous solu-
tion of uranous chloride. The uranous chloride solution did not show any
uranyl bands at all, and even when the uranous bands appeared very strongly
there was very little absorption in the region from X 3500 to X 4200. The edges
of the uranous bands were well denned, there being four very strong bands
extending between the following limits : X 4250 to X 4400, X 4700 to about
X 5080, X 5400 to X 5600, and X 6100 to X 6800. The transmission between
these bands was quite strong and appeared to be uniformly distributed.

The addition of hydrogen peroxide caused the uranous bands to decrease
in intensity. As far as the quite sharp uranous bands are concerned it seems
that the individuality of these bands decrease at about the same rate; but
the X4340 band is replaced by the blue-violet uranyl band, and the X 5500
band is also apparently replaced by wide general absorption in this region,
extending 500 or 600 Angstrom units. On the other hand, the transmission
seems to be pretty complete in the red and, with the exception of the very
weak uranyl bands, in the region X 4500.

A, plate 44, represents the effect of the addition of hydrogen peroxide to
a solution of uranous chloride in water, acetone, and hydrochloric acid. The
solution did not contain the right proportion of the solvents to show the tine
structure of the uranous and uranyl bands brought out by some acid acetone
solutions. The spectrogram shows very clearly the various uranous bands,
and how they are replaced by the uranyl bands as hydrogen peroxide is added.

The band that appears at about X 4970, and is quite strong on several
of the strips, is a uranous band, and disappears when sufficient hydrogen
peroxide is added. In the upper strip, which represents the absorption of the
uranyl salt, there is a wide, very diffuse and weak region of absorption, running
from about X 5000 to X 5100.

60 THE ABSORPTION* SPECTRA OF SOLUTIONS.

In addition to the ordinary uranous and uranyl bands, there is a weak
band about 15 Angstrom units wide at about X 5080. This band appears
best in the central strips, and can hardly be detected at all in the lowest and
highest strips. It is, therefore, difficult to say whether it is a uranyl or a
uranous band. In fact it may not be due to either of the two uranium salts.
The band is very similar to the one described under the oxidization of uranous
chloride in hydrochloric acid, only with the exception that the bands have very
different wave-lengths.

OXIDIZATION OF URANOUS CHLORIDE IN HYDROCHLORIC ACID

BY HYDROGEN PEROXIDE.

It seems that the acid uranous aggregates are in general more stable
than the neutral aggregates. Hydrogen peroxide oxidizes the strongest acid
solutions. A spectrogram was, however, made of a neutral uranous chloride
solution, to which a large amount of concentrated hydrochloric acid had been
added, and to which increasing amounts of hydrogen peroxide were added.
The addition of the hydrochloric acid caused the uranous chloride bands to
be shifted about 100 Angstrom units towards the red. In other respects the
bands were not greatly changed, with the exception of the bands in the red.

The uranous bands, on the addition of hydrogen peroxide, gradually
decreased in intensity without having their wave-length changed. At the
same time the uranyl bands appeared and, as is the case for strongly acid
solutions, the uranyl bands appeared quite sharp and intense.

It may be supposed that in the case of the existence of acid aggregates
it would require a longer time for the oxidization to take place than in the case
where only neutral aggregates are present. It would be interesting to know
what the reaction velocities of these oxidization reactions are for the various
kinds of aggregates.

It is a difficult matter to preserve uranous nitrate in the uranous condi-
tion. When uranyl nitrate is mixed with nitric acid and zinc is added, some
uranous nitrate is formed, but it is soon re-transformed into the uranyl con-
dition. This would indicate that uranous nitrate in water containing nitric
acid is unstable and is quickly oxidized. On the other hand, uranous chlo-
ride can be mixed with concentrated nitric acid and the uranous absorption
spectrum will be found to be entirely different from that of uranous chloride.
In this case the uranous salt will remain as such for at least a day or two
when kept at room temperatures. This may be explained by supposing that
the nitric acid aggregate of uranous nitrate is much more stable than the
salt itself.

According to the law of mass action, if a nitrate is mixed with uranous
chloride, some uranous nitrate ought to be formed. To test the stability
of uranous nitrate, uranous chloride was dissolved in a solution of calcium
nitrate in water and methyl alcohol. Under these conditions both " water"
and "alcohol" bands appeared. This solution did not oxidize, but a black
precipitate was formed. This experiment could also be explained by suppos-
ing that the uranous nitrate in this solution existed in the condition of some
kind of an aggregate. The precipitate formed appeared to be similar to that
which is formed on heating a neutral uranous salt to the boiling-point.

SPECTROPHOTOGKAPIIY OF CHEMICAL REACTIONS. 61

Uranyl chloride, hydrochloric acid, and potassium nitrate were mixed in
water and methyl alcohol, and zinc was added. The reduction took place easily
and the resulting uranous salt remained stable. It is probable that in this
case, also, more stable aggregates than neutral uranous nitrate were formed.
As a reciprocal reaction, uranyl nitrate, aluminium chloride, and hydrochloric
acid (in small quantity) were dissolved in water and zinc added. A stable
solution of a uranous salt, probably largely chloride, was formed. In this
case very little uranyl nitrate existed in the solution after the aluminium
chloride and hydrochloric acid were added. These three experiments would
then indicate that there is probably but little oxidization of uranous chloride
to the uranyl salt under these conditions.

THE OXIDIZATION OF URANOUS SULPHATE.

The preparation of concentrated aqueous solutions of uranous chloride
and uranous bromide is not difficult, using the method of reduction of the
corresponding uranyl salt with nascent hydrogen. A concentrated aqueous
solution of uranous sulphate can be obtained in a similar manner, but on
standing almost all of the uranium is precipitated. The presence of sulphuric
acid, however, enabled us to prepare a far more concentrated solution of
uranous sulphate, and this was quite stable.

A solution of uranous sulphate was prepared by reducing uranyl sulphate
with nascent hydrogen. The precipitate formed was partly dissolved in water.
The aqueous solution of uranous sulphate thus prepared was approximately
0.1 normal, and was fairly stable when kept in ground-glass stoppered bottles.
When exposed to the air it oxidized slowly to the uranyl condition. The
addition of methyl alcohol, ethyl alcohol, or acetone to the aqueous solution
of uranyl sulphate precipitated some of the salt.

A spectrogram, B, plate 35, shows the oxidization of an aqueous solution
of uranous sulphate to uranyl sulphate, by the addition of an aqueous solution
of hydrogen peroxide.

The original film shows quite a large number of the uranyl bands. The
wave-lengths of the uranyl bands seem somewhat greater than those of uranyl
sulphate solutions made from the chemically pure salt. This is probably due
to the presence of a considerable amount of zinc sulphate. In addition to the
regular uranyl sulphate bands there is present quite a fine and weak band at
X 5100. This band does not seem to vary to any considerable extent from
strip to strip, and it is difficult to say whether it is a uranyl or a uranous band.

THE OXIDIZATION OF URANOUS BROMIDE BY HYDROGEN PEROXIDE.

An aqueous solution of uranous bromide is oxidized by hydrogen peroxide
in the same way that uranous chloride is oxidized. The absorption spectra of
the bromide and chloride are quite similar for the uranyl and the uranous salts.

A solution of uranous bromide in ethyl alcohol is oxidized by hydrogen
peroxide. In the case photographed the alcoholic solution had a very great
ultra-violet, violet, and blue absorption after the oxidization had taken place.

B, plate 44, represents the oxidization of a glycerol solution by the addition
of hydrogen peroxide. In addition to a few of the uranyl bands, the character-
istic uranous glycerol bands consist of a wide diffuse band atX 4400 and a strong

62 THE ABSORPTION SPECTRA OF SOLUTIONS.

and quite persistent band at X 4680. This is about the most persistent band
in the uranous spectrum, although in the lower strip it is only about 40
Angstrom units in width, while the red band is about ten times as wide. The

° ' o

bands XX 4400, 5000, and 5250 are very much alike, being about 100 Angstrom
units in width, very weak and diffuse. The band X 5000 is apparently double.
The red band is quite strong, running from X 6050 to X 6400. It is accompanied
by a very weak band at X 6600. This band is extremely weak, and probably
corresponds to the water band in the same region. The addition of hydrogen
peroxide simply causes each one of these bands to decrease in intensity, while
the many] bands increase in intensity.

A POSSIBLE METHOD OF MEASURING THE STRENGTHS OF ACIDS.

Several methods have been employed for measuring the relative strengths
of acids. Among these are : The power of acids to divide a base between them,
as determined by thermo-chemical and volume-chemical methods; their power
to invert cane-sugar; to saponify an ester; to convert acetamide into am-
monium acetate; their conductivity, etc. All of these methods measure the
relative concentrations of the hydrogen ions in the solutions of the various
acids, the degree of dissociation determining, of course, the relative strengths
of acids. All acids at infinite dilution, or when completely dissociated, are,
then, of the same strength. In more concentrated solutions the strengths
of acids vary directly as their dissociation. Our work on absorption spectra
has shown that the positions of the uranyl and the uranous bands for different
uranium salts are often quite different. We could add just enough of the acid
in question to transform completely the original salt into the salt of the acid
in question, and then express the concentration of the acids and salts in gram
molecules per liter, or by any other convenient method.

Some spectrograms of this kind have been made indicating the relatively
great strength of hydrochloric acid. As yet, however, the subject has only
been touched upon. It is important that an accurate quantitative method for
measuring the intensity and wave-length of the bands be first devised. There
will be one obstacle that may cause some difficulty in this method, and that
is the presence of the acid (above, the acetic acid) displaced from the colored
salt by the addition of the acid to be tested.

Experiments of this kind should be carried out with various uranyl,
uranous, and neodymium salts, etc., in different solvents, to find whether the
relative strengths of acids are the same for the various colored salts, for the
different solvents, for different concentrations, for different temperatures,
for salts when in the presence of colorless salts, etc. Spectrophotography
of this kind only includes a study of the spectra as they are changed from that
of one neutral salt to that of another. Such a method, however, could hardly
be made as accurate a measure of the strength of acids as the conductivity
method, and not nearly so general.

ARE THE IONS FACTORS IN THE ABSORPTION OF LIGHT?

It is one of the remarkable facts brought out in this investigation, that
ions as such do not seem to play any role in the absorption of light. If the
ions were the absorbers of light in solution, then the absorption spectrum of
a solution would vary with the dilution of the solution, since the dissociation
of a solution varies with the dilution.

SPECTROPHOTOGRAPHY OF CHEMICAL REACTIONS.

63

There are solutions of the different colored salts of a certain metal, for
which the wave-lengths of the absorption bands are different. It will be
remembered in the case of neodymium that the absorption spectra of the
acetate, chloride, and nitrate are quite different from one another. The
spectra of these salts should all be the same, in dilute solution, if the absorp-
tion of light were due solely to the ions.

Jones and Guy are now studying very dilute solutions of certain salts,
to see whether there is any detectable difference between the spectrograms
in very dilute and in more concentrated solutions.

There are not many cases known where the wave-lengths of the absorp-
tion bands of different salts of the same metal in the same solvent are very
different. Uranyl nitrate in water, however, gives bands that are of quite
different wave-lengths from those given by other uranyl salts in water. The
following will illustrate this point :

a

b ■ c

d

/

Uranyl nitrate

Uranyl sulphate ....

4870 4705 4550
4900 4740 4580

4460

4310
4330

4155
4200

While previous work of Jones and Strong1 indicated that there was little
or no change in the positions of the nitrate and sulphate bands with change
in concentration, recent experiments made with the concentrations 0.5 and
0.003 normal indicate that in the more dilute solutions the sulphate bands
have been slightly shifted towards the violet, while the nitrate bands have
been slightly shifted towards the red. It may be possible that, with sufficient
dilution, the sulphate and nitrate bands would occupy the same position, as
would be the case if the ions were the only absorbers in the solutions; yet the
shifts are so small with the dilution that this seems improbable.

Indeed, it has recently been shown by Jones and Guy, working in very
dilute solutions and with a depth of layer of 250 cm., that the uranyl nitrate
bands are only very slightly shifted towards the red, and the uranyl sulphate
bands are only very slightly shifted towards the violet. The evidence is there-
fore convincing that the presence of the N03 and the S04 groups influ-
ences the periods of the absorbing centers in the uranyl atoms or groups.
Another good example that can be tested in a similar way is that of uranyl
chloride and uranyl nitrate in acetone. These uranyl bands are quite strong,
and the difference in the wave-lengths of corresponding bands is at least from
50 to 60 Angstrom units.

Uranyl nitrate in acetone .
Uranyl chloride in acetone .

4840
[4960
\4935

double

4665

«<*} double

4515
4610

Such facts as the above can be readily explained in terms of the theory
of aggregates. A uranyl nitrate aggregate loses an NOa group, and becomes
an ionized aggregate or complex ion. The loss of the N03 group causes a
decrease in the hypsochromous effect, but this would be smaller if the aggre-
gate were large. There would be a similar result in the case of the uranyl

'Proc. Amer. Phil. Soc, 48, 192 (1909).

64 THE ABSORPTION SPECTRA OF SOLUTIONS.

sulphate aggregates. It is, however, surprising that these aggregates, if they
exist, are not broken up when the solution becomes very dilute.

It is important that acid aggregates should be studied in the same way,
It is possible that these would break up more easily with increase in dilution
than the neutral aggregates apparently do.

THE OXIDIZATION OF URANOUS SALTS BY NITRIC ACID.

On account of the very great instability of uranous nitrate, it was ex-
pected that the addition of concentrated nitric acid to aqueous solutions of
the other uranous salts would cause their rapid oxidization. In the case of
uranous bromide, bromine was given off. In general, the addition of nitric
acid to an aqueous solution of uranous chloride causes the oxidization of that
salt, but if a large amount of nitric acid is added, the solution changes
from a very deep green to a light greenish-yellow, and the absorption spec-
trum shows that there is still a very considerable amount of uranous salt in
the solution; and the bands are quite different from those of neutral uranous
chloride. This uranous solution will remain for days, and seems to be quite
stable.

A, plate 45, represents the absorption of a solution of uranous bromide
in water and methyl alcohol to which dilute nitric acid is added in increasing-
amounts. The addition of the nitric acid caused a slight increase in the trans-
mission in the ultra-violet, but this was quite small. The uranyl bands appear,
but are very weak. In strip 1 there is a band running from X 4200 to X 4400.
This is probably due to a water and alcohol band, the alcohol band being on
the red side. Apparently the alcohol band disappears, since the band nar-
rows rapidly on its red side, and then decreases slowly in intensity. The alco-
hol band at X 4650 has practically disappeared in the third strip. In strip 1
there is a weak, very diffuse band at X 4800, and a strong band at about X 4950.
As nitric acid is added these bands are replaced by a wide and strong region
of absorption, running from X 4600 to X 5000. The band X 5200 disappears.
The band X 5500 is shifted about 150 Angstrom units towards the violet. The
alcohol band running from about X 6000 to X 6300 narrows rapidly on its short
wave-length edge, disappears, and is replaced by a weak, diffuse band at about
X 6100. The red water bands increase and then gradually decrease in inten-
sity. When this decrease begins, the long wave-length band is much the
stronger of the two and indicates that the uranous bromide alcoholate and
the uranous bromide hydrate have both been changed to some uranous nitrate
hydrate, and that this compound is then oxidized to the uranyl salt.

The addition of nitric acid to the aqueous solution of uranous sulphate
previously described produces a slow oxidization of the uranous into the ura-
nyl salt. This oxidization requires several minutes and would be a very good
reaction with which to study reaction velocity, the intensity of the absorp-
tion bands being determined by a radiomicrometric method. B, plate 46, is
a spectrograph of the oxidization produced by adding nitric acid to an aqueous
solution of uranous sulphate. Strip 5 is the absorption of the same solution
as that shown by strip 4, the only difference being that the solution had
been allowed to stand for several minutes.

In the solution of uranous sulphate oxidized by nitric acid, the uranyl
bands are shifted slightly towards the violet, compared with the uranyl bands

SPECTROPHOTOGRAPHY OF CHEMICAL REACTIONS. 65

of the uranous sulphate solution oxidized by hydrogen peroxide, showing that
probably some uranyl aggregate is formed that contains some N03 groups.

The addition of the first amount of nitric acid causes the transmission
in the ultra-violet to be increased very greatly, with the appearance of a large
number of the uranyl bands. In the red the wide uranous sulphate band is
decreased in intensity. Strips 2 and 3 show the narrow red sulphate water
band, but in strip 4 this band has completely disappeared and is replaced
by a single weak band 300 Angstrom units wide.

B, plate 45, represents the change in the absorption spectrum of a solu-
tion of uranous chloride in methyl alcohol and acetone, to which nitric acid
is added. This spectrogram shows the rapid disappearance of the alcohol
bands. The various uranous bands seem to be replaced by the bands of
some hydrate of a uranous nitrate aggregate, and this aggregate is then grad-
ually oxidized. The ultra-violet transmission increases until the last strip,
which shows a very great increase in the ultra-violet absorption. The last
strip still shows the presence of a considerable amount of uranous salt, and
the absorption bands are entirely new.

A, plate 37, represents the absorption spectrum of uranous chloride as it
is gradually changed into uranous nitrate by the addition of nitric acid. B,
plate 37, represents the corresponding changes in the absorption spectra that
take place, when to a solution of uranous chloride in equal parts of water and
methyl alcohol is added nitric acid also dissolved in equal parts of water and
methyl alcohol. The upper strip represents the absorption of the resultant
uranyl salt after oxidization with hydrogen peroxide. As the bands are quite
wide and strong most of the changes can be seen on the plate itself. It will be
seen that the absorption spectrum of the nitric acid solution of uranous chlo-
ride is very different from that of the neutral chloride. This spectrogram also
seems to indicate that there is not a gradual shifting of the chloride bands
into the nitrate bands, but in the case of the X 6300 chloride band there is a
decrease in intensity without being shifted; and this is replaced by the uranous
nitrate band at about X 6100. B shows that the addition of nitric acid results
in a rapid and almost complete disappearance of the alcohol bands, while the
uranous nitrate water bands increase in intensity.

URANOUS ACETATE AND THE EFFECT OF THE ADDITION OF

NITRIC ACID.

The reduction of uranyl acetate to the uranous salt is not complete, and
a precipitate was formed, so that the resultant solution here used was quite
dilute. A spectrogram (B, plate 36) was made of this solution, and the first
four strips represent the absorption of layers of different thicknesses, for the
last layer this being about 20 cm. To this solution were added 10, 20, and 40
drops of concentrated nitric acid, the absorption being represented by strips
6, 7, 8. The first five strips represent quite well the absorption of uranous
acetate, and they also show very clearly the uranyl acetate bands. The
addition of nitric acid causes a great increase in the general transmission
throughout the spectrum, and a very rapid decrease in the intensity of the
uranous bands. The resultant uranous bands are, of course, those of uranous
nitrate. This spectrogram shows that uranous acetate is quickly oxidized
in the presence of nitric acid, whereas this is not the case with uranous chloride.

5

66

THE ABSORPTION SPECTRA OF SOLUTIONS.

THE SELECTIVE ACTION OF CHEMICAL REAGENTS ON SOLVATES.

After photographing the effect of oxidizing the uranous salts to the uranyl
condition by the addition of hydrogen peroxide, it was natural to study what
effect the addition of other oxidizating agents would have upon the various
solvate bands. It will be remembered that the addition of hydrogen peroxide
caused the complete oxidization of the various solvates in the solution. It was
found that the action of the other oxidizing agents used was, in general, of a
more or less selective nature.

A, plate 47, represents the absorption of a solution of uranous bromide
in 3.5 parts water and 6 parts methyl alcohol, to which was added calcium
nitrate in methyl alcohol, 1.2 normal.

Uranous bro-
mide (amount
constant).

Drops of cal-
cium nitrate
in methyl
alcohol.

Percentage
water in
solution.

Strip 1

Strip 2

Strip 3

Strip 4

Strip o

0
10
20
40
80

37
33
28
21
14

The addition of calcium nitrate causes a very great increase in the amount
of the general absorption of the short wave-lengths. This is to be especially
noticed in the last addition of the calcium nitrate. The most important thing
to be seen from this spectrogram is the rapid and almost total disappearance
of the water bands. The intensity of the alcohol bands changes very little,
if any, and it seems quite improbable that the uranous hydrate could have
been converted into uranous alcoholate without the alcohol bands increasing
very much in intensity. The natural conclusion is that the uranous hydrate
has been oxidized to the uranyl condition by the calcium nitrate. The original
film shows that in the third strip the two red uranous water bands have almost
the same intensity. This may mean that the uranous bromide has been
changed to some other salt.

B, plate 47, represents the absorption of a solution of uranous bromide in
2 parts of water and 3 parts of methyl alcohol, the resultant solution being
about 0.12 normal. To this were added small amounts of a solution of sodium
perchlorate in methyl alcohol. It will be seen that the water bands decrease
in intensity as the sodium perchlorate is added, and that the longest wave-
length water band becomes more diffuse. The reason why the fifth strip is
vacant is on account of the formation of a precipitate. The precipitate was
not analyzed, but the spectrogram shows that the uranous hydrate has almost
entirely disappeared from the solution, while there is still present quite a
considerable amount of uranous alcoholate. This spectrogram is, therefore,
a good example of a case of selective precipitation of solvates. B, like A,
plate 47, indicates that there is a change in the constitution of the uranous
hydrate before it was precipitated.

A, plate 46, represents the effect of the addition of potassium nitrate and
calcium nitrate to a solution of uranous bromide. In strip 1 we have the

SPECTROPHOTOGRAPHY OF CHEMICAL REACTIONS. 67

absorption due to a solution of uranous bromide in 3.5 parts water and 6 parts
methyl alcohol. To this is added a solution of potassium nitrate in 2 parts
water and 3 parts methyl alcohol, which results in a precipitate. The precipi-
tate was filtered off and the resultant solution gives the absorption as repre-
sented in strips 2 and 3. It will be seen that only the uranous hydrate has
been precipitated. Strip 4 represents another solution of uranous bromide in
3.5 parts of water and 6 parts of methyl alcohol, and to this solution is added
calcium nitrate dissolved in 2 parts of water and 3 parts of methyl alcohol.
The effect of the addition of this calcium nitrate is to cause the almost com-
plete disappearance of the water bands, while the intensity of the alcohol
bands remains practically the same. In the fifth strip it will be seen that the
long wave-length water band is nearly 150 Angstrom units in width, and is
very much wider than the other red water band. This spectrogram furnishes
striking evidence in support of the view of the selective oxidization of the
uranous hydrate in this solution.

A, plate 42, represents the absorption of uranous bromide in water and
methyl alcohol to which an aqueous solution of calcium nitrate is added. The
spectrogram shows the decrease in the intensity of the alcohol bands. This
is most likely due to the increase in the percentage of water present in the
solution. The second strip shows the very quick change in the relative inten-
sity of the two red uranous water bands. This effect seems to be a general
one for the addition of calcium nitrate to solutions of uranous chloride, bro-
mide, or sulphate. The spectrogram does not seem to indicate that any of
the uranous salt has been oxidized.

B, plate 41, shows that when hydrogen peroxide is dissolved in the same
proportion of water and methyl alcohol as the uranous bromide, the alcohol
and water bands both disappear at the same rate, indicating that hydrogen
peroxide has no selective action on these two solvates.

The solution of uranous chloride used was acidulated with hydrochloric
acid, and when mixed with an equal volume of methyl alcohol gave the water
and methyl alcohol bands of about the same intensity. Two solutions were
used: one (spectrogram A, plate 36) in which the alcohol bands were much the
stronger, and the other (spectrogram A, plate 41) in which the water bands
were slightly stronger. Calcium nitrate in 2 parts of water and 3 parts
alcohol was added, the amounts being 10, 20, 40, and about 120 drops. The
last strip of A, plate 41, represents the result of the addition of hydrogen
peroxide. Spectrogram A, plate 38, represents the effect produced by add-
ing sodium chlorate dissolved in equal parts of water and alcohol. In this
case no oxidization resulted. A, plate 36, shows very little change in either
the water or the alcohol bands. The red water bands have their relative
intensity greatly changed, however, on the addition of calcium nitrate. A,
plate 41, shows this same relative change in the intensity of the two red water
bands. The spectrogram shows a very considerable increase in the intensity
of the alcohol bands, and a decrease in the intensity of the water bands. A,
plate 38, shows very little if any change at all in the absorption spectra. Even
the red water bands remain of about the same relative intensity.

B, plate 38, represents the absorption of one part of uranous chloride in
water to which one part of methyl alcohol is added. Succeeding strips, with

68 THE ABSORPTION SPECTRA OF SOLUTIONS.

the exception of the last, represent the effect of adding potassium chlorate in
water and methyl alcohol. The last strip represents the effect of adding hydro-
gen peroxide. This spectrogram shows the almost complete disappearance of
the water bands, while the alcohol bands remain of about the same intensity.

Plate 43, A, represents the absorption of uranous chloride in one part of
water and one part of methyl alcohol. Hydrogen peroxide, in equal volumes
of water and methyl alcohol, was added to this solution, three drops at a time;
except to the solution whose spectrogram is shown in the last strip, twelve
drops of hydrogen dioxide were added. The uranous alcoholate and hydrate
are oxidized to the same extent.

Plate 43, B, represents the absorption due to uranous bromide 0.5 normal
in water, to which three parts of methyl alcohol were added. After a time a
precipitate was formed. This was filtered off, and strip 2 is the spectrogram
of the solution. Successive strips represent the effect of adding increasing
amounts of potassium chlorate in equal volumes of water and methyl alcohol.
Very little, if any, relative action manifests itself.

A, plate 27, represents the absorption of concentrated nitric acid to which
increasing amounts of uranous chloride were added.

B, plate 27, represents the absorption of the filtrate of a uranous bromide
solution. A solution of uranous bromide of sufficient strength when mixed
with alcohol to give the water and alcohol bands of the same intensity usualty
forms a precipitate if allowed to stand. Strip 1 shows the absorption of the
filtrate of such a solution, and indicates the selective precipitation of the
hydrate. The succeeding strips represent the absorption of uranous bromide
in water and alcohol, to which is added nitric acid in the same proportion of
water and alcohol. The last strip represents the solution containing the
greatest amount of nitric acid, to which was added a few drops of hydrogen
peroxide.

C, plate 27, represents the absorption of uranous chloride in propyl
alcohol, to which acetone is added. A gives what we shall call the uranous
nitrate spectrum, although it is probably quite different from that of a neutral
aqueous uranous nitrate absorption spectrum. B is a good example of
selective precipitation, and of the action of nitric acid in increasing the inten-
sity of the hydrate bands at the expense of the alcoholate bands. C shows
how the addition of acetone causes the almost complete disappearance of the
uranous bands, and how, at the same time, the uranyl acetone bands appear
in place of the uranous bands.

Plate 39, A, strip 1, represents the absorption of uranous chloride in
equal parts of water and methyl alcohol. Strip 2, the same, to which is added
a solution of sodium per chlorate in water and methyl alcohol. Strip 3 is a
corresponding strip when sodium perchlorate is replaced by potassium chlorate
in 4 parts of water and 3 of methyl alcohol. Strip 4 is the absorption
of a solution of uranous chloride in methyl alcohol and ether. Strips 2 and
3 show quite a strong selective action of these salts in increasing the relative
intensity of the alcohol bands. This action is especially marked in the case
of potassium chlorate.

Plate 39, B, strip 1 , represents the absorption of uranous chloride in equal
parts of water and methyl alcohol, to which calcium nitrate is added; the

SPECTROPHOTOGRAPHY OF CHEMICAL REACTIONS. 69

latter being dissolved in 2 parts of water and 3 parts of methyl alcohol in
strip 1 ; in 1 part water and 1 part methyl alcohol in strip 2, and in pure water
in strip 3. Strips 4, 5, 6 represent the absorption of uranous chloride in equal
parts of water and methyl alcohol, to which potassium nitrate is added; in
strip 4, potassium nitrate is in 2 parts water and 3 parts of methyl alcohol,
strip 5 in equal parts of water and methyl alcohol, and in strip 6 in pure water.
The last strip represents the absorption of uranous chloride in equal parts of
water and methyl alcohol. This spectrogram indicates that the action of the
calcium and potassium nitrates may be that of driving back the amount of
hydrate in equilibrium with the alcoholate, and that by increasing the pro-
portion of water present in the solution the amount of hydrate present is
increased at the expense of the alcoholate.

THE REDUCTION OF URANYL SALTS IN SOLUTION.

The reduction of uranyl to uranous salts deserves a thorough study, and
undoubtedly offers a fertile field for investigation, and this partly on account
of the light that quantitative spectrophotography will probably throw upon
the subject. In certain solvents some interesting results have already been
obtained. In the case of an ether solution of uranyl chloride it was found that,
on the addition of zinc and hydrochloric acid, three phases resulted — an oily
and extremely concentrated solution of uranous chloride formed at the bottom;
a much more dilute solution of uranous chloride which was above the oily
layer and which, on standing, gave up its uranous salt to the oily liquid
below; and the upper layer, which only contained uranyl chloride in solution.
These three liquid layers were completely separate from one another. When
the whole system was thoroughly shaken and allowed to stand, the three
strata soon separated again from one another.

Uranyl chloride dissolved in isobutyl alcohol, to which zinc and strong-
hydrochloric acid are added, also shows the formation of an oily and dense
liquid, which was almost opaque on account of the large amount of uranous
chloride dissolved in it. These very concentrated solutions of uranous salts
would serve admirably for low-temperature work, and for the detection of
the Zeeman effect.

When hydrochloric acid is added to a solution of uranyl chloride in propyl
alcohol, the two solutions mix. Zinc is then added and the reduction takes
place very slowly.

A, plate 26, represents the absorption spectrum of the uranous chloride
solution in ether, to which some hydrochloric acid had been added. The
absorption spectrum is quite characteristic, and shows some new bands.
Several of these bands are quite sharp and narrow. As an example, there is
a band near X 5000 that is quite strong and is only about 25 Angstrom units
in width.

REDUCTION OF URANYL CHLORIDE IN METHYL ESTER.

To a solution of uranyl chloride in methyl ester were added hydrochloric
acid and metallic zinc. The hydrochloric acid does not mix with the ester
solution. For a considerable time hydrogen is given off, the ester solution
remaining of a greenish-yellow color. Quite suddenly, however, the ester

70 THE ABSORPTION SPECTRA OF SOLUTIONS.

loses its greenish-yellow color and becomes slightly clouded, the solution being
of a pale-white color. At the same time an almost black solution is formed,
whose volume is about the same as that of the hydrochloric acid added.
Further addition of hydrochloric acid results in the ester solution becoming
of a darkish-green color, while a small amount of the very deeply colored,
oil-like solution still remained at the bottom of the solution.

SELECTIVE REDUCTION OF URANYL AGGREGATES.

The met hod employed for the preparation of uranous salts for work on their
absorption spectra has usually been to add an acid corresponding to that of the
uranyl salt,, and some zinc. There would be present in the solution of the
uranous salt the corresponding zinc salt. It is possible, under these conditions,
to obtain fairly concentrated solutions of uranous chloride, uranous bromide,
and uranous sulphate. It does not seem, by this method, to be possible to
obtain uranous nitrate in stable condition. A small amount of uranous
nitrate is formed, but it is very quickly oxidized again. Dilute aqueous
solutions of uranous acetate can be obtained by this method.

The most concentrated solution of any uranous salt thus far obtained
is that of uranous chloride. A considerable volume of a solution of uranyl
chloride in ether was prepared; to this was added a small amount of concen-
trated hydrochloric acid and zinc. The uranous chloride formed is nearly
insoluble in the ether, and collects in a dark, oily liquid at the bottom of the
vessel.

The action of certain acids on the uranyl salts is peculiar. As has been
stated, the addition of nitric acid and zinc to a uranyl chloride solution did
not result in the formation of any uranous salt, although the absorption
spectra would probably have shown very little effect on the uranyl chloride
bands produced by the addition of nitric acid. The sulphate would probably
have been a better uranyl salt to use in this connection, because the uranyl
sulphate bands are harder to change to the nitrate bands then the uranyl
chloride bands are. To a uranyl chloride solution containing some free hydro-
chloric acid were then added, as before, some nitric acid and zinc. In this case
some uranous salt was produced, but it was quite unstable and was quickly
oxidized again.

On the other hand, to an aqueous solution of uranyl chloride were added
some hydrochloric acid and zinc. A uranous salt was formed, but in a few
minutes this was transformed back into the uranyl condition again. This is
rather unexpected, since it would be thought that the uranous salt would be
in the condition of the chloride, and the chloride is quite stable. To an aqueous
uranyl nitrate solution were then added a large amount of concentrated hydro-
chloric acid and a little zinc. The solution turns green during the first few
minutes and then goes over to the uranyl state, although hydrogen gas i^
being rapidly evolved. Very little could be done with the bromides along this
line on account of the liberation of bromine. It would be very interesting
to find how much the uranyl bromide bands are changed before the bromine
gas is evolved, since this would give the condition of the aggregate when it is
broken up and bromine evolved.

SPECTROPHOTOGRAPHY OF CHEMICAL REACTIONS. 71

B, plate 25, is a spectrogram giving the result of the addition of a large
amount of acetic acid and a little zinc to uranyl chloride dissolved in acetone.
The chemical conditions in this solution are quite complex and, therefore,
little can be said about them. The composition of the red uranous band is
quite unique in this case. For a considerable amount of the solution the red
band resembles that of the red glycerol band of uranous chloride. On decreas-
ing the amount of salt the band breaks up into four very weak, diffuse bands,
resembling more or less the aqueous uranyl chloride bands. The extreme
red part of the absorption consists of a triplet with the central and strongest
band located at about X 6750. The other bands are approximately at X 6670
and X 6820. The uranyl bands are exceptionally strong, as will be seen from
the spectrogram.

DIRECT SPECTROSCOPIC EVIDENCE FOR THE EFFECT OF MASS.

According to the law of Mass Action if the salts, uranyl chloride and cal-
cium nitrate, were dissolved in water, uranyl chloride and uranyl nitrate would
both be present in the solution. According to the theory of aggregates the
addition of calcium nitrate should shift the uranyl chloride bands to the violet,
while the addition of calcium chloride to uranyl nitrate should shift the uranyl
nitrate bands to the red. This was found to take place.

Preliminary tests showed that the addition of calcium nitrate to uranyl
nitrate caused a slight shift towards the violet. It appears, therefore, that
calcium has very little effect, it being only the acid radicle that is effective.
The result, then, is what would be expected according to the law of Mass
Action — the effect of adding aluminium chloride to an aqueous solution of
uranyl nitrate causes the same shift of the uranyl bands that would be pro-
duced if uranyl chloride were added.

A spectrogram was made of the absorption of uranyl nitrate in water
that showed about eight of the uranyl bands, the blue-violet band being about
300 Angstrom units in width. The addition of a concentrated solution of
calcium nitrate increased the general absorption of the blue-violet band very
greatly, so that only the a, b, and c uranyl bands remained. The position
and general appearance of these bands did not seem to be changed. Further
work of this kind will be done.

To a solution of uranyl chloride in water a concentrated solution of
calcium nitrate was added. The uranyl chloride solution showed the a, b,
and c bands, and the blue-violet band was about 400 Angstrom units in width.
The addition of the calcium nitrate caused the blue-violet band to be shifted
towards tthe violet, the shift being greater on the long wave-length edge of
the band. The uranyl bands also appear to be shifted towards the violet,
although in this particular spectrogram they are so weak that they can hardly
be detected. Work of this kind will be done with uranyl sulphate and uranyl
nitrate, since the neutral uranyl sulphate bands are quite sharp. The addition
of the other salts should also be gradual.

A, plate 16, represents the spectrophotographs of the addition of increas-
ing amounts of calcium nitrate to a uranyl chloride solution in water. The
spectrogram shows the gradual shifting of the blue-violet band and the
decrease in intensity of the uranyl chloride bands.

72 THE ABSORPTION SPECTRA OF SOLUTIONS.

B, plate 16, shows a spectrophotograph of the changes^produced in a
nitric acid solution of uranyl chloride to which increasing amounts of a concen-
trated solution of aluminium chloride in water are added. The spectrogram
shows the very great increase in the amount of the ultra-violet absorption
on the addition of the aluminium chloride. The blue-violet band does not
appear at all. The uranyl bands are shifted slightly towards the red, and
apparently change but slightly in intensity. The change from the first to[the
second strip is brought about by the addition of three drops of the aluminium
chloride solution.

Plate 17 is to show the different effect produced by adding an acidjmd
a neutral salt. .4 represents the absorption of uranyl nitrate in nitric acid
to which hydrochloric acid is added, and this plate can be compared directly
with B, plate 16. B, plate 17, on the other hand, shows the effect of the
addition of an aqueous solution of aluminium chloride to an aqueous solution
of uranyl nitrate.

A, plate 17, shows the increase in the ultra-violet absorption. The great
increase is in the intensity of the middle uranyl bands, whereas in the case
of the first strip the intensity of the various uranyl bands was much more
evenly distributed. A very small amount of hydrochloric acid produced a
very great change in the uranyl bands. The addition of aluminium chloride
produces a much more gradual change in the position and character of the
uranyl bands. It produces a much greater change in the ultra-violet absorp-
tion, causing the uranyl bands to shift towards the red and to increase very
great!}- in intensity.

A, plate 22, shows the effect of the addition of calcium nitrate to an
aqueous solution of uranyl nitrate.

CHAPTER V.

EFFECT OF TEMPERATURE ON ABSORPTION SPECTRA.

The purpose of the work on absorption spectra at high temperatures
was to obtain some knowledge of the chemical changes that take place at
these temperatures, by means of changes produced in the absorption spectra.
The cells used have, in the main, been the quartz cell and the two steel
cells described in a previous chapter. Some qualitative observations were
made with a hand spectroscope on solutions showing water and alcohol
bands. The work done with the quartz cell was mainly with acid solutions,
and this was done with a view to testing the stability of acid aggregates, as
will be explained a little later. Some work was carried out on the change
in the relative intensities of various solvate bands with change in temperature.
This will also be taken up in detail.

The problem has been found to be a very difficult one, on account of
closing the cell tightly, and very largely because of the formation of precipi-
tates upon the windows of the cell.

Some work has been done dealing with the effects of pressure and high
temperature upon solutions. W. N. Ipatieff1 has shown that under a pressure
of about 125 atmospheres of hydrogen, anthracene and phenanthrene were
reduced, when heated in the presence of nickel oxide, to compounds such as
C14H12, C14H14, C14H2(1, etc. From this work and that of others it has been
shown that metals are precipitated from solutions of their salts by hydrogen
under pressure. There appears to be a critical temperature for the precip-
itation of each metal, silver and mercury being precipitated at ordinary
temperatures under a pressure of 200 atmospheres from a decinormal solution,
while copper and the more electro-positive metals could not be precipitated
even when the pressure exceeded 500 atmospheres. At 120° to 130°, however,
branched crystals of copper were formed at 100 atmospheres; and at lower
temperatures cuprous oxide was formed. Cobalt was precipitated at 180° to
200°, nickel at 200°, lead and bismuth at 240° to 250°, iron at 400° and 420
atmospheres. Ferric oxide was precipitated from an acetate solution at 350°
and 230 atmospheres.

Among some of the investigators who have studied the effect of pressure
and temperature on the conductivity of solutions are A. Bogojawlensky and
G. Tammann in 1898, and F. Korber2 in 1909. In most cases the resistance
decreased as the pressure increased, but in many cases a minimum resistance
is reached, after which a further increase in pressure will cause an increase
in the resistance. This is true of solutions of potassium chloride at all tem-
peratures between 0° and 100° C. At 0° the minimum resistance is 0.872 of its
original value, and is reached at 3000 kg. per sq. cm.; at 100° C. the minimum
resistance ratio is 0.998 and this is reached at 900 kg. per sq. cm. Among

1 Ber. d. chem. Ges., 41, 966 (190S); 42, 207S (1909).
Zeit. phys. Chem., 67, 212 (1909).

73

74 THE ABSORPTION SPECTRA OF SOLUTIONS.

halogen compounds the largest decrease of resistance is found for hydrogen
chloride (17 per cent for i> = 100, p = 3000 and t=l9° C.) and lithium chloride
(14 per cent); and then follow potassium chloride (9), sodium chloride (8),
potassium bromide (6), sodium bromide (5), potassium iodide (2), and sodium
iodide (1).

SOLVATES AND THE EFFECT OF CHANGE IN TEMPERATURE ON
THE RELATIVE INTENSITY OF SOLVATE BANDS.

The absorption spectra of colored salts, in general, in the visible region
are not very characteristic, but the absorption spectra of neodymium, erbium,
uranyl, uranous, and a few of the other rare-earth salts show fine characteristic
bands, more or less distributed throughout the spectrum. There are, however,
even among these spectra, very few cases where the absorption spectra of the
same salt in different solvents, or of different salts of the same metal in the
same solvent, are very different. There are, however, a few examples of well-
defined "solvent bands," and some of these will now be discussed.

The most striking examples of the effect of the solvent on absorption
spectra are: those of water and ethyl alcohol upon several of the neodymium
salts, water and alcohol upon uranous chloride or uranous bromide, and water
and glycerol upon several uranyl and uranous salts. The spectra may well
be described as consisting of the "water," the "methyl alcohol," the "glycerol,"
etc., bands of the particular salt. When the salt is dissolved in a mixture of
two of the solvents, both sets of solvate bands appear, and if the solvents are
mixed in a certain proportion, both sets of bands may be obtained of approxi-
mately the same intensity. The intensity of any set of solvate bands will be
proportional to the amount of that solvent present. As the amount of the
solvent present is changed, the solvate bands, in general, do not shift their
position, but only change in intensity, and this change in intensity seems to
be the same for all of the bands characteristic of that solvent. Solvent bands,
in this respect, differ from the phosphorescent bands described by Lenard1
and Klatt.

Just as a characteristic arc or spark spectrum has been regarded as
indicating the presence of a certain element, just so a characteristic absorption
spectrum will be regarded as evidence for the presence of a compound; and
these compounds will be considered as being composed of one or more mole-
cules or ions of the salt ("aggregate") and one or more molecules of the sol-
vent. On account of the definite spectra, we shall speak of the "methyl
alcoholate" of uranous chloride, the "hydrate" of neodymium bromide, etc.

Suppose, for instance, that the number of radicles in an aggregate of neo-
dymium salt molecules and ions can vary, then the compound in which absorp-
tion takes place might be represented symbolically in the following manner:

x { \J02}y\ U02C12 } 2 { CI } a { CH3OH }
and

+
+

;r';Nd;?/'{NdCl3}2'{Cl}a'{CH30H}

1 Ann. d. Phys., 31, 642 (1910).

EFFECT OF TEMPERATURE ON ABSORPTION SPECTRA. 75

In most cases x, x', z, and z' would probably be small; y and y' may also
be small, and if hydrochloric acid or some other chloride is present, it might
be possible to have systems of the following composition:

++ + t

./-,; U02}ar2{HorCa . . . }^{U02Cl2}t/2{HCl,CaCl2 . . . }z{Cl}«{CH3OH}

+

+ +

Zi'{Nd}s2'{H,Ca . . . Jy/JNdCL, }.(//{ HCl,CaCl2 . . . ]v{Cl}a'{CH3OH}

The absorption may be due to some condition, possibly one of internal
ionization of the aggregate, and as long as the number of aggregates remains
constant, Beer's law will hold. Free ions may split off from the aggregate
without the character of the absorption being greatly changed. It might be
that if the aggregates were completely broken up into their constituent ions
and molecules no characteristic absorption would be shown.

In this case, a and a' n\&y either be considered as being constant, or as
being so large that the atmosphere of the solvate around the absorber is so
extensive that it is immaterial whether the outer solvent molecules are present
or not. On account of the fact that solvent bands coexist, and do not shift
into each other as the proportion of the solvents is changed, it will be assumed
that at least the inner and effective solvent molecules are all of one kind in
any "spectral" compound.

As to the numerical value of the y's the absorption spectra of course do
not furnish evidence. The phenomena of the constancy of the wave-length
of the uranyl sulphate and uranyl nitrate bands, with great dilution of these
salts, leads to the conclusion that in this case at least the value of y is greater
than unity. Freezing-point, boiling-point, conductivity, and osmotic pressure
measurements should aid in determining the values of x, y, and z in the above
equations.

The existence of complex aggregates of the above kind is quite rare in
inorganic chemistry, although cases have been known. Our own work on the
reduction and oxidization of certain uranium salt and acid aggregates has
shown that these different aggregates have very different chemical properties.
Some cases of a series of a similar set of compounds might be cited, e.g., the
cobaltamines. A large number of cobaltamines are known, and these have
been divided into six series.

The diamine series [Co(NH3)2]X4M. In this series X = N02 and M is
one atomic proportion of a monovalent metal, or the equivalent quantity of
a divalent metal. These salts are prepared by the action of alkaline nitrites
on cobaltous salts in the presence of a large amount of ammonium chloride
or nitrate. The salts are yellow or brown crystalline solids, and are not very
soluble in water.

The triamine series [Co(NH3)3]X3. X may be CI, NO.,, N02, ^S04, etc.

The tetramine series, including,

Praseo salts [R,Co(NH,) JX. X = CI.
Croceo salts [(N02),Co(NH3)JX.
Purpureo salts [RCo(NH3)4,H,0]X2.
Roseo salts [Co(NH3)4(H,0),]X,.
Fuseo salts [Co(NH3)JOH. A\.

76 THE ABSORPTION SPECTRA OF SOLUTIONS.

For the methods of preparation see 0. Dammer's Handbuch der anor-
ganischen Chemie, vol. 3.

The pentamine purpureo salts [RCo(NH3)5]X>, where Ar may be CI,
Br, N03, N02, 3^S04, etc. The pentamine nitrito salts are known as the
xanthocobalt salts, and have the general formula [NOvCo(NH3).]X2. The
pentamine roseo salts can be obtained by the action of concentrated acids in
the cold on oxidized solutions of cobaltous salts. They are of a reddish
color. On heating with concentrated acids they become purpureo salts.

The hexamine or luteo series has the composition [Co(NH3)6]X,. These
salts form yellow or bronze-colored crystals which decompose on boiling. The
above examples are cited because the cobalt salts give an absorption band
spectrum in some cases similar to the uranyl bands. It is very probable that
uranium, neodymium, erbium, etc., give rise to much more complex com-
pounds than even cobalt.

It will be assumed that the absorption bands are due to compounds that
represent the average condition of the given dissolved salt. It may be that
only a few of the particles of the absorbing salt may be active at any one
instant. For instance, Becquerel1 considers that only a very small number
of the neodymium atoms are taking part in the absorption of light at any
moment. It may be that the absorption only takes place when the compound
is in a special condition. For example, in the case of the uranyl salts we may
think of the absorption as being due to the valency electrons in the UOo
group. The absorption may take place when one of these electrons leaves
the uranium or oxygen atoms, or when it returns. Whatever the mechanism
may be, it will be assumed that the frequency of the absorber is determined
by the nature of the compound in which it is located, and that this compound
is typical of the average of all the possible compounds in the solution. Accord-
ingly, if the "methyl alcohol" and "water" bands of uranous bromide dis-
solved in methyl alcohol and water are of approximately the same intensity,
it follows that there are equal amounts of the uranous bromide existing as
the "methyl alcoholate" and as the "hydrate."

In the case of neodymium chloride or neodymium bromide Jones and
Anderson2 have shown that the "water" and "alcohol" bands are of about
equal intensity when the solution consists of 8 per cent of water and 92 per
cent of alcohol. In the case of uranous bromide or uranous chloride in water
and methyl alcohol, it is found that the "water" and "methyl alcohol" bands
are of about equal intensity when there are approximately 2 parts of water
to 3 parts of methyl alcohol in the solution. In the case of the neodymium
salts it would seem that the "water" bands are the more "persistent." The
less "persistence" of the "hydrate" of uranous chloride or uranous bromide
may be due to the presence of the zinc salts formed during the reduction of
the uranyl salts. It is found that the presence of free hydrochloric acid in
a solution containing the "hydrate" and "methyl alcoholate" of uranous
chloride causes the "water" bands to decrease in intensity with reference to
the "methyl alcohol" bands. The action of any chloride would probably be
the same. In the case of neutral salts of neodymium and uranium the "water "

1 Becquerel: Le Radium, Sept. (1907). ; Phys. Rev., 26, 520 (1908).

EFFECT OF TEMPERATURE ON ABSORPTION SPECTRA. 77

bands are usually more persistent than the "glycerol" bands, and these in
turn are more persistent than the "alcohol" bands.

In the case of neodymium chloride in a mixture of 92 per cent alcohol
and 8 per cent water it has been found1 that Beer's law does not hold, the
"water" and "alcohol" bands being of about equal intensity for a 0.5 normal
solution, while for a 0.05 normal solution the "water" bands are much stronger
than the "alcohol" bands. Whether the "water" bands in general show
such a "persistence" is being investigated.

On the other hand, it has been found that the " alcoholates " are much
more "persistent" than the "hydrates" at the higher temperatures. This
is especially pronounced in the case of the "water" and "alcohol" bands of
uranous chloride and uranous bromide. If the two sets of bands are of equal
intensity at ordinary temperatures, at 80° C. the "water" bands have practi-
cally disappeared. This observation can easily be verified with a small pocket
spectroscope.

Several very characteristic spectra have been obtained for neodymium
salts dissolved in isomeric alcohols. The bands of neodymium chloride in iso-
butyl alcohol are of considerably greater wave-length than the corresponding
bands of neodymium chloride in butyl alcohol; whereas the "propyl alcohol"
bands are displaced to the red with reference to the "isopropyl alcohol" bands.
The "butyl" and "isobutyl" bands of neodymium nitrate are very much alike,
while the "propyl" bands are displaced to the red as compared with the "iso-
propyl" bands, being in the same direction as for the chloride.

Neodymium Chloride in Water and Ethyl Alcohol.

A, plate 52, represents the absorption of a 0.3 normal solution of neo-
dymium chloride in water and ethyl alcohol. The temperature range is from
40° to 80° C. .The original film shows that at the higher temperatures the
water bandX 4271 almost disappears, while the alcohol band becomes stronger.
The e group shows the same thing in a general way, in that the total absorp-
tion, especially on the violet side of the band, is much less at the highest
temperature than at the lowest temperature.

B, plate 52, shows the same effect for a more dilute solution. The finer

water bands of the a, 8, and e groups are shown by the original film practically

to disappear at the higher temperatures, while the alcohol bands become more

intense.

Neodymium Bromide in Water and Methyl Alcohol.

C, plate 53, represents the absorption of neodymium bromide in water
and methyl alcohol, 0.2 normal and 1.0 cm. length of cell. The variations
in temperature were from 30° to 80° C.

In the lower strip the alcohol band at about X 4285 hardly appears at all,
while the water band at X 4271 is quite strong. At 80° the water band is still
quite strong, and the alcohol band is possibly a third as strong as the water band.

Neodymium Chloride, Bromide, and Nitrate in Water.

B, plate 55, strip 1, shows the absorption of a 2.05 normal solution of
neodymium chloride in water at 20° ; strip 2 is the same at 50°, and strip 3 is
the same at 75°.

lPhys. Zeit., 11, 671 (1910), 12,269, (1911).

78 THE ABSORPTION SPECTRA OF SOLUTION*.

A, plate 55, represents the absorption of a 1.66 normal aqueous solution
of neodymium bromide at 20°, 40°, 60°, 80°, and 93°, the highest temperature
being that represented by the upper strip.

B, plate 55, strip 4, represents the absorption spectrum of a 2.05 normal
aqueous solution of neodymium nitrate at 20°, strip 5 at 75°, and strip 6 at 95°.

These spectrograms show the very great increase in the absorption of the
short wave-lengths as the temperature reaches about 90°. The spectrograms
show the widening of the individual neodymium bands. There is a slight shift
towards the red with rise in temperature, but this is very small. The absorp-
tion spectra of the three solutions are practically the same, that of the chloride
and bromide being nearly identical, while that of the nitrate differs a little
in some of the minute details of the individual bands.

Neodymium Chloride in Methyl Alcohol.

Plate 54, A, represents the absorption of a 0.1 normal solution of neo-
dymium chloride in methyl alcohol, 10 cm. depth of cell. This spectrogram
shows the methyl alcohol bands quite sharply. A weak band appears on the
first strip at X 4015, and at X 4200. X 4285 is quite strong and only about 10
Angstrom units wide. A weak and quite narrow band appears at X 4270.
The band X 4270 is rather hazy; X 4450 is about 50 Angstrom units in width
and is very diffuse, its red side not being as diffuse as the violet side. X 4620
is very weak; X 4700, X 4770, and X 4820 all have about the same intensity and
width, the first two being accompanied by very weak bands on their violet
sideos at X 4680 and X 4750. X 5040 is very weak and broad; X 5120 is about
20 Angstrom units wide and is very strong. X 5175 is very similar to X 5120,
except that it is only about one-half as intense. X5215, X5250, and X5290 are
all very intense and are quite sharp, the middle band being about 20 Angstrom
units in width, while the other two are only about 10 Angstrom units wide.
There is a wide absorption band from X 5710 to X 5940, with the sharper edge
on the violet side. The red band at X 6850 appears to be quite strong and
about 20 Angstrom units in width.

The other strips represent the same solution at different temperatures,
these being 15°, 26°, 40°, 55°, 78°, and 85°, starting with the lowest strip.

With rise in temperature the bands all become somewhat more intense
and wider. The general absorption over the whole spectrum region increases,
especially at the higher temperatures, and begins to encroach quite rapidly
in the violet and red regions. This violet absorption is probably a general
absorption, but the encroaching on the red side is probably due to the increase
in the intensity of the group of red bands.

In the upper strip transmission extends from about X 4200 to X 6100.
A very weak band appears at X 4200 and a very weak one at X 4305. The
bands in the blue and green have changed but little. Absorption is pretty
complete from X 5100 to X 5190, X5210 to X4350, and from X 5690 to X5990.
Weak bands appear at X 6230, X 6280, and X 6750. If any of the bands showed
any shift it was too small to measure. The band at 15°, extending from X 5710
to X 5940, widens approximately 20 units on its violet side and 50 units on
its red side. It is probably more or less general that wide absorption bands
usually broaden unsymmetrically towards the red, especially when this side is
the more diffuse.

EFFECT OF TEMPERATURE ON ABSORPTION SPECTRA. 79

A, plate 53, represents the absorption of a methyl alcohol solution 0.2

normal and with 1.0 cm. depth of cell. The temperatures are 25°, 40°, 55°,

and 70°. The only important change clue to temperature is that at 70°, the

intense absorption near the center of the e group of bands having practically

disappeared.

Neodtmium Bromide in Methyl Alcohol.

B, plate 54, represents the absorption spectrum of a 0.1 normal solution
of neodymium bromide in methyl alcohol. The length of cell was 10 cm.
The temperatures were 25°, 35°, 44°, 60°, 82°, 100°, and 120° C, beginning
with the lowest strip. At the highest temperature a slight precipitate was
formed, but still some light was transmitted. The neodymium solutions all
become much more deeply colored at the higher temperatures, and this can
easily be shown by heating such a solution in an ordinary test tube.

The absorption spectrum of neodymium bromide in methyl alcohol is
quite different, as far as minute detail goes, from that of the chloride. In
general, the bands of the chloride are from 5 to 15 Angstrom units farther
towards the red than the bromide bands.

For the lowest strip, very weak and diffuse bands appear at about X 4000,
X 4180, X 4600, X 4900, X 5040^ X 5320, X 6230, X 6260, X 6730, and X 6790. The
(3 group of the bromide is very different from that of the chloride. It consists
of a very sharp, narrow (3 units) band at X 4265, a very sharp and less intense
band at X 4275, a hazy band at about X 4280 which more or less overlaps
X 4275, and at higher temperatures X 4275 can not be noticed at all. A very
weak band appears at X 4300 and a broader band at X 4325, being about 20
Angstrom units in width.

The 7 group of the chloride is also quite different from that of the bromide,
which has four bands of almost equal intensity at XX 4690, 4745, 4765, and
4815. Weak bands appear at X 4670, X 4700, and X 4725. The 5 group consists
of a rather narrow band at X 5090 and a very strong band at about X 5115;
a lot of narrow and intense bands at X 5200, X 5220, X 5235; X 5250 and X 5275
practically merge into a single band. The e group consists of a single wide
band extending from X 5700 to X 5880.

As the temperature is raised the violet absorption increases quite rapidly,
and the 5 and e groups of bands become wider and stronger. The latter band
widens very greatly towards the red. All the bands become very much more
diffuse. This is particularly true of the /3 group, since at 120° only two very
hazy, indistinct bands appear, while at 25° some of the bands in this group
were almost as fine as spark lines.

No measurable shift of the bands towards the red could be observed.

That the absorption spectra of the chloride and the bromide in methyl
alcohol should be so different was quite unsuspected, since these two salts
have almost identically the same absorption spectra in aqueous solution.

Neodymium Nitrate in Isobutyl Alcohol.

The absorption spectrum of a solution of neodymium nitrate in isobutyl
alcohol was photographed at 20°, 45°, 75°, 95°, 110°, and 120° C. The con-
centration was 0.05 normal and the depth of cell 10 cm. The first strip of
this spectrogram is described under the chapter on the mapping of spectra.

80 THE ABSORPTION SPECTRA OF SOLUTIONS.

The general result of rise in temperature is to cause a large increase in the
general absorption in the ultra-violet. This is especially strong at 120°, where
the general absorption is complete to about X 4100, and extends as a partial
absorption to beyond X 5000. The neodymium bands are considerably
widened by rise in temperature, and are shifted in man}' cases some 15 or 20
Angstrom units towards the red (120° compared with 20°).

Some of the more pronounced changes may be noted if the following
wave-lengths are compared with those described under the chapter on map-
ping of spectra. At 1 20° a weak band appears at X 4260. It is about 10
Angstrom units wide. The other bands of the /3 group are at X 4295, X 4305,
X 4325, and X 4360. The lines of the y group are at X 4725, X 4775, and
X 4850. The 8 group consists of the wide band at X 5140 and the double band
at X 5250 and X 5275. The e group runs from X 5720 to X 5960 as a single band.

Neodymium Nitrate in Acetone.

A spectrogram was made of the absorption spectrum of a solution of
neodymium nitrate in acetone, in the long cell, at 16°, 40°, 70°, 100°, and 180°.
The solution became opaque at about 110°, and afterwards quite transparent
at about 160°.

The effect of the rise in temperature was to cause a slight shift of the

neodymium bands to the red, but as these bands were so very broad and hazy,

no measurements could be made. The strip taken at 180° shows a very great

absorption in all regions except the yellow and red, and the absorption in the

red is, indeed, almost complete. There is no indication of any neodymium

bands at all, so that it seems that practically all the neodymium nitrate had

been precipitated.

Erbium Chloride in Water.

A, plate 60, represents the spectrum of an aqueous solution of erbium
chloride at 20°, 30°, 85°, and 115° C.

The effect of rise in temperature on the individual erbium bands is very
small, their diffuseness being hardly increased with rise in temperature. The
increase in the general absorption throughout the region of smaller wave-
lengths is enormous, resulting in the solution being practically opaque at the
higher temperatures.

Strip 1 shows the following bands, the wave-lengths being only approxi-
mate: a band from X 3430 to X 3520; a strong band at X 3540 which is about
10 Angstrom units wide; weak bands at X 3555, X 3570, and X 3595; the previous
band is the beginning of a region of absorption that extends to X 3660; a
group of three bands that practically merge into a single band running from
X 3750 to X 3790; weak bands at X 3800 and X 3840; diffuse bands at X 3870;
strong X 3900, X 3950 weak, X 4000 weak; X 4045 sharp and narrow (10 units);
X 3070 sharp and narrow; X 4100 very weak; XX 4155, 4165, 4185, 4215— these
bands are much alike, the first being the most intense; X 4270 is wide and very
weak; X 4430 diffuse; X 4450 very weak; X 4480 weak; X 4500 strong; X 4530 is
apparently covered by a diffuse band on its red side; X 4685 weak; X 4750 and
X 4800 wide and weak; X 4855 narrow; X 4880 intense; X 4930 probably double;
X5200; X5225; X 5245 intense; X 5290 weak; X5380; X 5390 weak; X 5435;
X 5455; X 5520 weak; group at X 5770 very weak; X 6440, X 6460, X 6530, X 6560
strong. X 6600 and X 0710 all very diffuse and having a very "washed-out"
appearance.

EFFECT OF TEMPERATURE ON ABSORPTION SPECTRA. 81

Uranyl Chloride in Acetone.

A spectrogram was made of the absorption spectrum of a solution of
uranyl chloride at 22°, 40°, 60°, 170°, 185°, and 195°. At about 70° a precip-
itate was formed, but the solution became transparent again at 160°.

At 22° six of the uranyl bands appear, some being clearly double, as is
characteristic of the acetone bands. At 40° only two bands appear, the general
absorption throughout the violet and blue regions of the spectrum being so
great. At 60° no uranyl bands appear at all, the short wave-length absorption
extending to about X 5500. After the formation of the precipitate there is
quite a strong transmission in the violet and about five uranyl bands appear.
The uranyl bands are weaker and considerably narrower at the higher temper-
atures. Above 60° the bands appear to be single. From 170° to 195° the
uranyl bands rapidly decrease in intensity, and at 195° they are so weak that
one can not be certain that they can be seen at all. At the same time the gen-
eral absorption in the violet increases, but not to any great extent.

Uranyl Nitrate in Propyl Alcohol.

B, plate 64, represents the absorption of a 0.005 normal solution of uranyl
nitrate in propyl alcohol, the depth of cell being 10 cm. The temperatures,
starting at the lowest strip, are 20°, 40°, 65°, 85°, 105°, 115°, 130°, and 145°.
The sixth and seventh strips show a very weak transmission on the violet
side of the blue-violet uranyl band. At 145° this has become a broad region
of transmission. This spectrogram shows the enormous extension of the gen-
eral absorption as the temperature rises.

The first strip shows the uranyl bands X 4085, X 4200, X 4330, and X 4470.

Uranyl Chloride and Nitrate in Isobutyl Alcohol.

A, plate 62, represents the absorption of a 0.076 normal solution of uranyl
chloride in isobutyl alcohol at 20°, 60°, 85°, and 115° C.

B, plate 62, represents the absorption of uranyl nitrate of 0.033 normal
concentration in isobutyl alcohol, the temperatures being 20°, 50°, 80°, 100°,
and 105° C.

In the case of the chloride solution the uranyl bands are quite strong and
fairly sharp. The two bands that show are at X 4570 and X 4730. In the case
of the nitrate solution only a single band at about X 4630 appears. This band
is very diffuse and weak. For aqueous solutions, as will be remembered, the
reverse is the case, the chloride bands being very weak and diffuse and the
nitrate bands being quite strong and narrow.

Uranyl Chloride and Nitrate in Methyl Ester.

B, plate 66, represents 0.005 normal solution of uranyl nitrate in methyl
ester at 20°, 50°, 75°, and 100° C. The spectrogram shows thatjthe uranyl
bands are quite strong and clear. As the temperature rises the general violet
absorption increases, and the uranyl bands are slightly shifted towards the
red. The approximate wave-lengths of some of the bands are XX 3900, 4020,
4130, 4250, 4380, 4520, 4670, and about 4820, this last band being very weak.

B, plate 63, shows uranyl and calcium chlorides in methyl ester (first
three strips), and uranyl and calcium chlorides in methyl alcohol (last three

82 THE ABSORPTION SPECTRA OF SOLUTIONS.

strips). The temperatures are 20°, 45°, 85°, 20°, 50°, and 95° C. The methyl
ester at 20° showed very strong bands at XX 4600, 4760, and 4930.

It was thought that by adding a large amount of a salt like calcium
chloride to solutions of uranyl chloride in different solvents the absorption
spectra of the resultant solutions would be more alike. The spectrogram
shows that this is not the case, since it will be seen in the lower strips that the
uranyl bands are very strong and quite sharp, whereas in the case of the
methyl alcohol solution the bands are very wide and very weak. Their wave-
lengths are also considerably greater than those of the bands in the ester solu-
tion. These spectrograms show that although double salt aggregates may
be formed, the solvent part of the aggregate still plays a very important role
in influencing the wave-lengths of the absorption bands.

The effect of the addition of calcium chloride to the ester solution was
not tested by regular steps, but for the pure uranyl chloride solution the a
band appears as a double band with the components at about X 4930 and
X 4965. These bands practically merge into one another, and it is rather
difficult to see that the band is really double. Apparently the effect of the
addition of calcium chloride is to cause the component X 4965 to disappear,
and, at the same time, the other component widens. The methyl ester solu-
tions offer a promising field for studying the spectrophotography of chemical
reactions and also for the effect of dilution.

A, plate 64, represents the absorption of 0.005 normal solution of uranyl
chloride in methyl ester, the depth of cell being 10 cm. The temperatures,
starting with the lowest strip, are 20°, 45°, 70°, 90°, 110°, 135°, and 140° C.

At 20° the following uranyl bands appear: X 3930 fine and weak; X 4050
quite narrow; X 4170, X 4300, X 4450, X 4630, and X 4800 probably the b band.
At 140° C. only the b, c, and d bands appear at XX 4480, 4650, and 4820. At
the highest temperature the transparency of the solution in the violet has
apparently increased. The character of the uranyl bands is but slightly
affected by the above temperature changes.

Uranous Chloride in Water and Methyl Alcohol.

Strip 5, A, plate 67, represents the absorption of uranous chloride in a
mixture of water and methyl alcohol, in such proportions that the water and
alcohol bands were of about equal intensity at 10° C. Strip 6 represents the
same solution at about 70° C.

These strips show that at the higher temperature the uranous water
bands have almost disappeared. The uranyl bands have also become very
much weaker, and the uranous alcohol bands slightly weaker. No appreciable
shifting of the bands is to be noticed.

Uranous Chloride in Acetone.

A spectrogram of the absorption spectrum of uranous chloride in acetone
was taken at 20°, 40°, 65°, 80°, 95°, and 105°.

At about 60° a precipitate was formed, and above this temperature the
red absorption band hardly appears at all. The uranous bands in the green
and yellow seem to be considerably stronger after the formation of the pre-
cipitate.

EFFECT OF TEMPERATURE ON ABSORPTION SPECTRA. 83

THE EXISTENCE OF AGGREGATES AND THEIR PROPERTIES, AND
THE EFFECT OF RISE IN TEMPERATURE ON THE AGGREGATES.

The presence of free acid1 or of other salts has been found, in many cases,
to modify very considerably the uranyl, uranous, and neodymium bands of
a given salt in solution. Up to the present, work of this kind has been practi-
cally restricted to aqueous solutions, although similar changes take place in
other solvents. One of the most pronounced cases is that of the uranyl nitrate
and the uranyl chloride bands. Most of the nitrate bands have shorter wave-
lengths than the chloride bands. As increasing amounts of hydrochloric acid
are added to an aqueous solution of uranyl nitrate, the nitrate bands are found
to shift gradually into the position of the chloride bands. Furthermore, the
addition of nitric acid to an aqueous solution of uranyl nitrate causes most
of the uranyl bands to shift towards the violet, whereas the addition of hydro-
chloric acid to an aqueous solution of uranyl chloride causes most of the bands
to shift towards the red. These changes are quite different from those that
take place when the solvent is changed; and if it is supposed that a character-
istic absorption spectrum corresponds to a more or less stable compound,
then these changes indicate a series of compounds which will be referred to
as aggregates. We would have, then, nitric acid aggregates of uranyl nitrate,
or hydrochloric acid aggregates of neodymium chloride.

Whether there is an actual change in the frequency of vibration for a
series of uranyl or uranous aggregates, or whether there is simply a relative
change in the intensity of a number of finer bands which blend into the
rather broad and diffuse bands that are photographed, can not be decided at
present. In the case of neodymium salts2 the various spectrophotographs
indicate the latter effect. It is unlikely that the case can be settled very easily
for uranyl and uranous salts, since the greatest shifts take place in the absorp-
tion of aqueous "solutions, and these can not be studied at very low tempera-
tures. Subsequently, it will be assumed that a spectrogram of a chemical
reaction showing a gradual shifting of bands indicates the presence of a series
of closely related aggregates existing in the solution.

The mixture of varying proportions of two neutral salts in a solution may
result in a gradual change from the bands of one salt into the bands of the
other salt (mixtures of uranyl nitrate and uranyl chloride in water). On the
other hand, there are cases in which each salt seems to have its own definite
spectrum. In this case there will be no shifting of the bands but only a change
in intensity, and we may assume that, in this case, there are no double salts
formed. In the former case, however, it seems probable that there are aggre-
gates formed which contain one or more molecules of each salt.

The addition of salts3 containing the same cation as the corresponding
uranyl or uranous salt, has an effect similar to that of the addition of free
acid, and may be considered as indicating the presence of aggregates. In the
case of uranyl chloride the effect seems to be due largely to the chlorine present.

Acid aggregates of uranous salts are found to be much more stable than
the neutral aggregates. Uranous nitrate, for example, is very unstable, but

1 Phvs. Zeit., 11, 66S (1910), 12, 269 (1911).

2 Ibid., 11, 671 (1910), 12,269 (1911).

:i Publication No. 130, Carnegie Institution of Washington, p. 91.

84 THE ABSORPTION SPECTRA OF SOLUTIONS.

uranous chloride dissolved in concentrated nitric acid will stand for hours
before it is oxidized to the uranyl condition. Most of the neutral uranous
salts are precipitated when heated to 80° or 90° C; but if some free acid, or
if in the case of uranous chloride other chlorides are present, the uranous salts
are stable at these higher temperatures.

It might be expected that if the effect of rise in temperature is to break
up aggregates, then in the case of concentrated acid solutions of the uranyl
salts the shift towards the red should be very great for a concentrated nitric
acid solution of uranyl nitrate; while in the case of a hydrochloric acid solu-
tion of uranyl chloride, or of a sulphuric acid solution of uranyl sulphate, the
shift should be in the opposite direction. On the other hand, if the shift caused
by change in temperature is due to some other cause and the aggregates are
not broken up, no effect of this kind would be expected.

In a solution of uranous sulphate in sulphuric acid the uranyl bands are
slightly shifted to the red as the temperature is raised from 10° to 90°, while
the uranous bands are slightly shifted to the violet. The shift of the neutral
uranyl sulphate bands to the red is greater than that of the acid sulphate
bands. The shift of the uranyl nitrate bands in nitric acid is about 15 Ang-
strom units. All bands are shifted about the same amount.

Neodymium Salts in Acid Solutions.
Plate 59, A. This spectrogram shows the effect of hydrochloric acid,
sodium chlorate, and temperature on the neodymium chloride bands. Strip
1 is the absorption of neodymium chloride in water at about 10°, strip 2
the same at about 90°. Strip 3 represents the absorption of the same amount
of neodymium chloride in concentrated hydrochloric acid at 10°, strip 4 the
same at 90°. Strip 5 represents the absorption at 10° of neodymium chloride
in methyl alcohol, and strip 6 the same containing sodium chlorate.

Plate 59, B. This spectrogram shows the absorption of a 0.02 normal
solution of neodymium acetate at 10° in strip 1 and at about 90° in strip 2.
Strip 3 represents the absorption of a small amount of neodymium chloride
added to acetic acid so as to fill the quartz cell at 10°, and strip 4 the same at
90°. Strip 5 represents the absorption of neodymium acetate in acetic acid
at 10°, and strip 6 the same at about 90°. Strip 7 represents the absorp-
tion of a solution in methyl acetate and acetic acid at 10°, and strip 8 the
same at 90°.

The first two strips in A show that the effect of rise in temperature on
an aqueous solution of the chloride is to cause a very slight shift of the bands
towards the red, and to give them a much more diffuse and washed-out appear-
ance. The hydrochloric acid solution of neodymium chloride shown in strips
3 and 4 is but very slightly changed by rise in temperature, and there is no indi-
cation that the spectrum becomes more closely like that of the neutral aqueous
solution. The shift to the red is very small. It can easily be noticed for the
bands of the e group. At 10° the long wave-length bands of the 8 group are
very much like the two bands of the neutral aqueous solution. At 90° these
bands have both become very diffuse, blending into a single band, and the
whole center of the band is greatly shifted towards the red. This is the great-
est temperature change in the whole spectrum. The presence of sodium

-

EFFECT OF TEMPERATURE ON ABSORPTION SPECTRA. 85

perchlorate in a methyl alcohol solution of neodymium chloride causes the
neodymium bands to be shifted very slightly towards the violet.

A neutral acetate solution in water, as shown in strips 1 and 2 of B, is
affected like the neutral chloride solution, in that at the higher temperature
the bands have a much more diffuse washed-out appearance. This is especially
true of the a group, which appear very weak at 90°. Very little shifting of the
bands towards the red takes place. Strips 4 and 5 show about the same effect
and the same very great decrease in the intensity of the a group at the higher
temperature. In the next two strips the a group remain of about the same
intensity at the two temperatures. There is less increase in the diffuseness
of the acetate bands in acetic acid than in the case of a neutral acetate solution.

Plate 56, A. This spectrogram represents the absorption of neodymium
chloride in acetone at 10° (strip 1) and at 70° (strip 2); neodymium chloride
in ether at 10° (strip 3; a precipitate is formed at higher temperatures) ; neody-
mium chloride in ethyl alcohol 0.02 normal at 10° (strip 4) and about 70°
(strip 5) ; neodymium chloride in ethyl alcohol to which hydrochloric acid gas
has been added at 10° (strip 6) and at about 70° (strip 7).

Plate 56, B. This spectrogram represents the absorption of neodymium
nitrate in nitric acid at 10° (strip 1) and at about 50° (strip 2; if heated to
much higher temperatures nitrous oxide is formed) ; of neodymium chloride in
methyl alcohol and 8 per cent of water at 10° (strip 3) and at about 70° (strip
4) ; neodymium nitrate in acetone and 8 per cent water at 10° (strip 5) and
at about 60° (strip 6); neodymium chloride in 60 per cent ethyl alcohol and
40 per cent water at 10° (strip 7); and neodymium chloride in 40 per cent
water and 60 per cent glycerol at 10° (strip 8).

The acetone solution of neodymium chloride shows very little if any
change with rise in temperature. The ethyl alcohol solution shows a small
increase in the diffuseness, and a very great shift of the intensity of the band
groups with rise in temperature. When hydrochloric acid is present there is
very little change of this kind. The new and very strong bands appear at
about X 3590 and X 3750, the latter band being about 50 Angstrom units wide.

The nitric acid solution of neodymium nitrate shows very little change in
the absorption spectrum with rise in temperature. The bands become some-
what more diffuse and decrease in intensity. The red bands do not show in
the strips. Strips 3 and 4 show the very considerable increase of the alcohol
bands over the water bands. The a, j8, and 8 fine water bands have practically
disappeared at the higher temperature.

Uranous Sulphate in Sulphuric Acid.

Plate 67, A. The absorption of uranous sulphate in sulphuric acid at 10°
is represented in strip 1, and at about 90° in strip 2. The absorption of the
neutral uranous sulphate in water at 10° is shown in strip 3, and at about
90° in strip 4. The absorption of uranous chloride in water and in methyl
alcohol at 10° is shown in strip 5, and at about 70° in strip 6. The uranyl
bands of the sulphuric acid solution are shifted towards the red with rise in
temperature, whereas the uranous bands seem to be shifted slightly towards
the violet.

86 THE ABSORPTION SPECTRA OF SOLUTIONS.

Ukanyl Nitrate in Nitric Acid, Uranyl Chloride in Hydrochloric Acid,
and Uranyl Sulphate in Scjlphuric Acid.

Plate 67, B. The absorption of uranyl nitrate in nitric acid at 10° is rep-
resented in strip 1, and at about 70° in strip 2; of uranyl chloride in hydrochlo-
ric acid at 10° in strip 3, and at about 80° in strip 4; of uranyl sulphate in water
at 10° in strip 5, and at about 80° in strip 6; of uranyl sulphate in sulphuric acid
at 10° in strip 7, and at about 70° in strip 8.

Every one of the uranyl nitrate bands, due to the rise in temperature,
seems to be shifted about 10 Angstrom units towards the red, the uranyl
chloride bands in hydrochloric acid being shifted about 15 Angstrom units.

In the case of uranyl sulphate, only the uranyl bands in the region of
the blue-violet band were greatly shifted towards the red with rise in tem-
perature. The shift seemed to be considerably greater for the neutral than
for the acid solution.

According to the aggregate theory we might expect that if the uranyl
nitric acid aggregates break down at the higher temperatures, the uranyl
bands would be very greatly shifted towards the red on account of less nitric
acid molecules affecting the uranyl vibrations. But this does not seem to be
the case to any very great extent, since here the shift to the red is not any
greater than for the other uranyl bands, and seems to be about the same for
each one of the bands. It may be considered that the hydrate is more simple,
however, and that this allows more of the nitric acid molecules to come within
the range of the absorbing centers, and this might in a way counteract the
effect of any decomposition of aggregates. Whether the effect of an increase
in the number of molecular collisions brings a greater number of nitric acid
molecules within the effective range of the absorbing centers, and has an
effect on the frequency, can not be decided from the spectrograms.

The very considerable shift of each one of the uranyl chloride bands of
the hydrochloric acid solution towards the red with rise in temperature
probably indicates that there is very little decomposition of the uranyl acid
chloride aggregates.

Since apparently only the e, f, g, and h uranyl sulphate bands are greatly
shifted towards the red, we might assume that there is a slight decomposition
of the uranyl sulphate aggregates, and this counteracts what we might call
the normal temperature shift.

In the above strips temperature has very little effect on the uranyl bands,
except a shifting of their wave-lengths towards the red.

CHAPTER VI.

SUMMARY AND GENERAL DISCUSSION OF THE MOST

IMPORTANT RESULTS.

MAPPING OF SPECTRA.

The first problem to be solved in a study of absorption spectra is the
recording of the spectra themselves. The method employed in this series of
investigations is that of photographing the absorption spectrum of a solution
placed in a beam of light having a continuous spectrum. Such a photograph
may be called a spectrogram or a map of the given absorption spectrum.

The study of the spectrograms of colored solutions shows that this color
is due to a selective absorption of the solution in some part of the spectrum,
and the extent of this selective absorption may be over regions of the spectrum
hundreds of Angstrom units in width, or over regions only a fraction of an
Angstrom unit wide — a width that is not much greater than that of a spark
or arc line. In fact, bands of almost all widths and degrees of diffuseness are
found in addition to the absorption, which is one-sided. If the region of one*
sided absorption lies in the red, it is probable that it has another edge in the
infra-red. Practically all solutions show an encroaching general absorption
in the ultra-violet as the amount of the solution in the path of the light is
increased or as the temperature is raised.

The plates and the description of these plates in Publications Nos. 60,
110, 130, and the present monograph of the Carnegie Institution of Washing-
ton give a pretty thorough representation of the details of the absorption
spectra of most of the typical solutions of the colored inorganic salts.

It can be said, in general, that the absorption spectra of the various salts
of the same element are very much alike; indeed, so much alike that it is only
when considerable dispersion is used that any differences can be noted. It
can also be said that the absorption spectra of the same salt in various sol-
vents are very similar. For these reasons we are justified in assuming that
the color of solutions of colored salts dissolved in colorless solvents is due
primarily to the metal or metallic radicle of the colored salt. The various
spectra that are characteristic of a given kind of salts, say the uranous salts,
will, therefore, be called the uranous spectra.

In the mapping of the absorption spectra of solutions of increasing con-
centration or of increasing depth, it is always found that the absorption bands
widen and become more intense as the amount of salt in the path of the beam
of light is increased. It can also be stated, as an approximately general law,
that the more diffuse a band is the greater will be the widening under these
conditions. Examples of this kindoare shown by the uranous bands, which
may widen from fifty or a hundred Angstrom units to a width of thousands of
Angstrom units. The widening of single, very sharp bands with increasing
amounts of the salt in the path of the light is invariably small. As an example
we can take the X 4271 neodymium chloride water band.

When characteristic absorption spectra of salts of the rare earths, of
cobalt, chromium, etc., are studied with the use of a high-dispersion spectro-

87

88 THE ABSORPTION SPECTRA OF SOLUTIONS.

scope, it is found that the finer absorption bands are different for the same
salt in different solvents, and for different salts in the same solvent. This
difference may be a difference in the number of the bands present, a difference
in the intensity and width of the bands, or a difference in wave-length.

It can be said, in general, that the absolute differences of intensity,
diffuseness, and wave-length, under these conditions, is greater for the larger
and the more intense bands. Unfortunately the very large bands are usually
so situated in the spectrum that only small changes in the amount of salt
can be made before the band becomes one-sided on account of the compara-
tively small region of the spectrum that can be investigated by photographic
methods. But it can be said that the uranous bands show greater changes
with change of salt or solvent than the uranyl bands; that the uranyl bands
show greater changes than the neodymium bands, and these in turn seem to
show much greater changes than the dysprosium and samarium bands; and
these, in turn, probably greater changes than the erbium bands.

When we consider the minute structure of the bands and groups of bands
under high dispersion we find very great differences, especially in the case of
neodymium. The different salts of neodymium in the same solvent, especially
in some of the organic solvents, give entirely different absorption spectra.
The same is true of the same salt in different solvents. Isomeric solvents very
often show characteristic spectra. The absorption bands of neodymium have
been divided into groups a, (3, y, 8, e, etc. When there is a large amount of
salt in the path of the beam of light, each group usually forms a single broad
band. It is found that the relative intensities and characteristics of these
groups is very much the same for different salts in different solvents. The
minute structure of the bands in the same group is usually more widely
different than the absorption spectra of dysprosium and samarium; so that
it is quite probable that if the region of spectrum that we could study were wide
enough to include one of these groups and sufficient dispersion was at hand,
we would consider that we were dealing with different elements whenever the
salt and the solvent were changed.

The spectrograms and the descriptions in detail give a large number of
examples illustrating the above. Very much more work of this kind remains
to be done with solutions like those of neodymium at low temperatures, using
very high dispersion.

From the above description of absorption spectra it follows at once that
any law such as the supposed law of Kundt, connecting the wave-length of
the absorption band with the value of the dielectric constant of the solvent,
is impossible. It is probable that the so-called Melde effect is equally chi-
merical. Looked at from the point of view of the aggregate theory the Melde
effect has very little if any meaning.

Beer's law is found to hold approximately for nearly all solutions of a
single neutral salt in a single solvent. Exceptions are found when very con-
centrated solutions are used, and with only one known exception (solutions
of uranyl acetate give a Aery large negative deviation) the deviation is such
that the absorption is greater than would be given by Beer's law when the
concentration is increased. The fact that Beer's law holds indicates that, as
far as our knowledge of absorption spectra is concerned, there is no difference

SUMMARY AND GENERAL DISCUSSION OF RESULTS. 89

between ions and molecules. It therefore can not be said, in general, that for
solutions showing characteristic bands there is any part of the spectrum due
to ions or any part due to molecules. Yet the work of Jones and Anderson1
showed that certain bands are probabfy due to certain constituents of the
molecules, such as the atom, the hydrate, etc.

A THEORY OF ABSORPTION SPECTRA.

It is natural to try to correlate as many spectra as possible, and this has
been done in many cases. For organic compounds, benzene for example, it is
found that the ultra-violet absorption spectra in the gaseous, in the liquid,
and in the dissolved states are very similar. It is therefore natural to endeavor
to test colored salts in the same way. Unfortunately no metallic salts having
a characteristic absorption spectrum can be obtained in the gaseous state.
As pure salts, in solutions, and in transparent solids, however, the absorption
spectra of salts of the same element are very much alike as far as the grosser
structure is concerned. None of the vapors of the metals of these salts can
be obtained unaccompanied by the effects of high temperature and intense
ionization. It is for this reason that the branch of spectroscopy dealing with
the absorption spectra of inorganic solutions is quite completely separated
from the other branches of spectroscopy.

However, a study of other branches of spectroscopy will be of considerable
importance in helping to give us a working hypothesis concerning the mate-
rial centers that are absorbing or emitting light.

An emission or absorption center of light and heat will be denned as the
smallest particle existing by itself that is capable of emitting or absorbing a
given characteristic spectrum. Until proved to be an ion, an atom, a molecule
or an aggregate of these we can not assume that the division of matter into
emission and absorption centers is at all identical with the division of matter
into ions, atoms, and molecules. In a study of these light centers one of the
most difficult problems that confronts us is the complete separation of indi-
vidual light centers, and it is owing to this fact that our knowledge of these
light centers is so meager. An example of this kind might be taken in the
more or less independent series of lines as classified by Kayser and Runge,
Ritz, and others. It has never been proved that there are separate emitters
and absorbers for each one of these series of lines, neither can it be said that
these centers correspond to atomic, ionic, or molecular units. It may be that
each series of lines is due to a system, and that several systems are joined
together to form an absorption or emission center, but that these systems
can not exist as separate units and still act as light centers. The same con-
dition probably applies to the systems that give rise to the simple series of
fluorescent bands discovered by Wood, so that there would not be a different
absorption center for each one of these series of bands, but that there is one
type of absorbing centers that contains several absorbing systems that can be
separately excited, and that as soon as the absorbing center is broken into
parts it loses its characteristic absorption spectrum.

It seems quite probable that many of the systems in the light centers
contain electrons, and it is probable that it is these electrons that give the

1 Publication No. 110, Carnegie Institution of Washington.

90 THE ABSORPTION SPECTRA OF SOLUTIONS.

Zeeman effect. The electrons that are indicated by the Zeeman effect of arc
and spark lines are but slightly affected by external forces, so that it has been
very generally assumed that these form parts of systems that make up the
fundamental structure of the ultimate units of matter. Indeed, a character-
istic spectrum has been used as defining an atom; and, in general, it has been
found that a characteristic arc and spark spectrum accompanies matter
which, according to other physical and chemical methods, is believed to be
composed of exactly the same atomic units. It is for this reason that the
assumption is generally made that the absorption and emission centers of
these arc and spark spectra are either the same as the atoms of the element,
or contain one or more atoms of the element.

In making an assumption of this kind two of the greatest difficulties
encountered are the complexity of the spectra that are found to be character-
istic of an element and the fact that the light centers only seem to exist, or
at least are only active under certain conditions. The first problem is answered
by supposing that if the light center corresponds to the atomic unit of matter,
then there are subatomic systems that correspond to the various spectra.
Very little evidence of such subatomic systems is at hand, yet the present
theory of radioactivity gives very strong support to this view and indicates
that, whenever these subatomic systems are separated, entirely new atomic
units are formed, and that one atomic unit may consist of several smaller
atomic units. The second problem is illustrated by many examples, a good
one being that of sodium. The sodium atoms exist under conditions that
make them parts of entirely different light centers. The sodium atom may
form part of the absorbing or emitting centers of the arc or spark spectra,
the emitting centers of a certain vacuum discharge spectrum, the absorbing
centers of the fine-banded absorption spectrum, the emission centers of the
fluorescent spectrum, etc. The sodium atoms may also exist in various mole-
cules and be perfectly transparent in the visible part of the spectrum. Sug-
gestions have been made only as to what the constitution of the light centers
of these spectra may be, and these suggestions have usually been based on
the assumption that these light centers consisted of atomic or molecular
complexes.

Most light centers seem to be formed only during very exceptional
physical and chemical conditions, so that they have been often considered
as being very unstable; and that an atom or molecule only forms a part of a
light center during a small part of the time. This view is strengthened by the
theory of absorption and dispersion, according to which it often happens
that the number of electrons taking part in the absorption or emission of
light is much less than the total number of atoms or molecules present in the
matter that is either emitting or absorbing the light. Exactly what conditions
are necessary for producing light centers ? Some evidence has been accumu-
lated which indicates that these conditions may be furnished by the dissocia-
tion or recombination of parts that form complex molecules. These are
apparently the conditions under which the light centers of the fine iodine,
bromine, sulphur, chlorine, etc., bands exist.

The length of duration of the fluorescent and phosphorescent bands of
solids and liquids furnishes a method of analysis of spectra in that the duration

SUMMARY AND GENERAL DISCUSSION OF RESULTS. 91

of different bands after the excitation has ceased is very different. No relations
have, however, been found between fluorescence and phosphorescence and the
conductivity, so that the current theories are based on the assumption that
the light centers are very complex molecular aggregates (Lenard and Klatt)
and that the dissociation or recombination effects take place within these
aggregates.

The absorption spectra of organic compounds supports the view that the
absorbing centers in these cases are generally complex. These centers are
called chromophores, and as a rule they form only a part of the molecular
structure. Unfortunately the absorption spectra of chromophores are not
characteristic, so that the whole study is more or less a colorimetric one.
The peculiar conditions under which the absorption centers are active has
been assumed to be the same as those accompanying phenomena that are
explained by the theory of dynamic isomerism, and consist in a supposed
change of the interlinking of radicles in the organic compounds. If valency
is of an electromagnetic nature, a condition of dynamic isomerism would also
be a condition of intramolecular ionization.

The nature of the absorbing centers of inorganic salt solutions is probably
somewhat similar to the absorption centers found in the case of solutions of
organic compounds, the phosphorescent compounds of the rare earths studied
by Goldstein, Kowalski and others. Every characteristic absorption spectrum
will be considered as evidence for the existence of a compound, and these
compounds will be called aggregates and may be assumed to consist of one
or more molecules or ions of the dissolved salt, and one or more molecules of
the solvent. That the number of these aggregates seems to be quite large is
no evidence against the theory of aggregates, since there is no reason why
an element like uranium should not form a very large number of compounds.

As a typioal example of an aggregate, we will take a mixture of two kinds
of dissolved compounds in a mixture of two solvents. Assuming that the
UO2 group can act as an ion and that it carries a double charge, a general
formula for aggregates would be the following:

+

a;{U02S04}^{H2S04}w{U02}y{H}w;{S04}a{H20}

4-

x' { U02S04 [ y' \ H2S04 [ u' { UO, } v' { H } w' { S04 } 6 { CH,OH }

We may assume that at ordinary concentrations u, v, w, u' , v', and w'
have small values. Whenever a definite and characteristic absorption spectrum
is obtained it will be assumed that the absorption centers are aggregates of
a definite composition, and that all the coefficients in the above equation have
a definite value.

From the fact that absorption centers are found in solids such as the
various glasses and crystals, it must be assumed that the existence of active
absorption centers need not be connected with any conducting particles. On
the other hand, the work of Becquerel and others seems to indicate that
the number of absorbing centers is much smaller than the number of atoms
present. If the aggregates are very complex then the number of absorbing
centers would be much less than the number of molecules of the colored salt.

92 THE ABSORPTION SPECTRA OF SOLUTIONS.

This would probably require very large values for x and x' , and, therefore,
makes this view improbable. On the other hand, it is not necessary to assume
that the aggregate is an active light center all the time, but that the aggregate
is only active while it is in a peculiar condition. This peculiar condition may
be assumed to be a kind of internal ionization, somewhat similar to that which
is assumed in the theory of dynamic isomerism, or by the theory of Lenard
and Klatt. For instance, electrons may pass back and forth within the UOo
group, or between the UO2 and surrounding groups. The system whose vibra-
tions give rise to the uranyl group is probably located in the uranyl radicle,
and possibly in the uranium atom itself. When an electron is ejected, or
when it recombines with the U02 group, it may be considered that the system
absorbs light and is then an active center. When this absorption takes place,
the frequency of the absorption bands and their width and intensity will
depend on the structure of the aggregate — i.e., on the values of x, y, u, v, w,
and a.

In a spectroscopic study of the aggregates found in salt solutions, it may
be said that the absorption spectrum indicates only a condition of a small
part of the salt. In other words, the values of .r, y, u, v, w, a, etc., may be
functions of the time. In order to solve this problem it will be necessary to
study the nature of the aggregates by other methods; and if widely different
methods give the same values for x, y, u . . ., etc., it may then be assumed
that the value of these variables is not a function of the time.

In the present treatment it will be assumed that these spectroscopic
aggregates represent the statistical average condition of the dissolved salt
molecules, and although these aggregates may be active absorbing centers
for a small part of the time it will be assumed that the difference between an
absorbing and a non-absorbing aggregate, if such there be, is not due to any
change in the values of x, y, u, v, w, a, etc. The intensity of a given absorption
spectrum will be assumed to be a measure of the amount of the aggregate
present in the solution that corresponds to the given absorption spectrum.
In every known example it has been found that the changes in the values
of x, y, z, a, etc., produce changes that indicate that each characteristic spec-
trum is due to a system whose parts form one organic whole.

SOLVATION.

In the general formula for the structure of the absorbing centers, the
coefficient of the solvent part of the compound is represented as being con-
stant. The fact that in the examples studied each solvent is characterized
by a definite absorption spectrum, and that a salt dissolved in mixtures of
varying proportions of two solvents shows only two definite absorption spectra,
indicates that definite compounds of salt and solvent are formed. It is a very
remarkable fact that one solvent spectrum does not gradually change into
the other solvent spectrum, but that only the relative intensities of the two
spectra vary as the percentage of each solvent present is changed, and that
for a certain percentage of the two solvents the two sets of solvent spectra are
of approximately the same intensity.

Taking as an example uranous bromide in water and methyl alcohol, we
have an equilibrium of probably the following type:

SUMMARY AND GENERAL DISCUSSION OF RESULTS. 93

x { UBr4 } a { H20 } <=± y { UBr4 } b { CH3OH }

The variables x and y may be equal and both may be very small. These
quantities will be taken up later in the treatment of aggregates.

The fact that there is no gradual shifting of the "water" bands into the
"alcohol" bands indicates that the equation representing the equilibrium
is not of the type: —

a { UBr4 } x { H20 } y { CH3OH }

where x and y are variable and depend on the amount of water and methyl
alcohol present. The above constancy of a and b must not be interpreted
too literally, however, as it may be that in the "atmosphere" of solvent
molecules only the inner molecules are effective. In this case the constancy
of a and b would indicate that only the number of molecules in the outer
region of this atmosphere could change, or could consist of a mixture of water
and alcohol molecules. Accordingly, the inner solvent atmosphere consists
of but a single kind of molecules.

(1) The percentage of solvents for which each set of solvent bands has
the same intensity will be used as a measure of the "persistency" of the given
solvates. The "persistency" of any given solvate will vary inversely as the
proportion of that solvent that is necessary for the bands to appear of a given
intensity. Whether there is any relation between the persistency of solvate
bands and the absolute values of a and b is not known.

(2) The persistency of the same solvate bands for different salts is quite
different. In the case of neodymium chloride in water and alcohol the bands
are of the same intensity when 8 per cent of water and 92 per cent of alcohol
are present. In the case of samarium chloride a smaller percentage of water
is required, the water bands of samarium chloride having a greater persistency
than the water bands of neodymium chloride. The persistency of the water
bands of neodymium nitrate is different from those of the chloride. In the
case of the uranous salts the water bands are much less persistent, the water
and alcohol spectra being of about the same intensity when there is about
40 per cent of water and 60 per cent of alcohol present.

(3) The persistency of the bands of the same salt in different solvents is
very different. In general the "water" bcmds are the most persistent of all the
solvent bands at ordinary temperatures.

(4) The persistency of solvent bands depends on the concentration of the
solution. Keeping the percentage of water and alcohol constant, it is found
that the alcohol bands of neodymium chloride and of the uranous salts are
relatively more persistent for the greater dilutions, i.e., Beer's law does not
hold for solutions containing two solvents.

(5) Although it has not been shown that only two solvates exist in the
case of a neodymium salt in a mixture of two isomeric solvents, yet there are
several instances where the absorption spectra of the same salt in isomeric
solvents are very different. A marked example of this kind is that of neody-
mium chloride in isobutyl and butyl alcohols, the butyl alcohol bands having
the shorter wave-lengths. On the other hand, the corresponding propyl
alcohol bands are shifted to the red with reference to the isopropyl bands.
The absorption of neodymium nitrate in butyl and isobutyl alcohol is very
much the same.

94 THE ABSORPTION SPECTRA OF SOLUTIONS.

(6) From the above examples it must be remembered that because the
absorption spectra of a salt in two different solvents prove to be very much
the same, this fact is not an argument that no solvation exists in the two cases.

(7) The values of x and y probably have a very considerable effect on the
nature of the solvates, but even under these conditions the little work that has
been done indicates, for cases where y has a value, i.e., where there are other
salts, or the acid corresponding to the given salt is present, that we still have
the constants a, b, etc., although these may have a different value from what
they have when y = 0.

(8) Very little work has been done on the change in the persistency of
solvate bands when foreign salts are added to the solution. The addition of
aluminium and calcium chlorides to solutions of colored chlorides seems to
increase the persistency of the alcohol bands relative to the water bands.

(9) The selective action of foreign salts on solvates is shown very strik-
ingly by the addition of oxidizing agents to solutions of uranous salts in
water and alcohol. Substances like potassium and calcium nitrates and
sodium perchlorate cause the water bands to decrease very greatly in inten-
sity. The alcohol bands, on the other hand, seem to remain of about the
same intensity. Whether this is due to the oxidization of the hydrate or
simply to a decrease in its persistency, can not always be stated. Some cases
indicate that selective oxidization takes place. The oxidization of hydrogen
peroxide affects water and alcohol bands in the same way.

(10) In many cases the solubility of a salt like uranous bromide is much
less after alcohol has been added to the water than before. Sometimes pre-
cipitates form when a second solvent is added, and in some cases the filtrate
shows the presence of only one solvate, whereas before the precipitate was
formed both solvates were present. This may be denoted as selective solvate
precipitation.

(11) The effect of rise in temperature changes the relative intensity of
the solvate bands of a solution. A very marked example of this kind is a
solution of uranous chloride in a mixture of water and ethyl alcohol in such
proportions as to show the water and alcohol bands of the same intensity at
ordinary room temperatures. Heat the solution to about 80° C. and the
water bands practically disappear, leaving only the uranous alcohol bands in
the spectrum.

(12) Whether or not there is a dynamic equilibrium of solvates is not
certain, but the selective action of foreign salts, the effect of changing the
relative quantities of the solvents and of changing the temperature, would
lead us to believe that there is some interchange of solvates going on. The
velocity of reactions of this kind may, however, be comparatively slow.

THE URANYL AND URANOUS BANDS.

The uranyl spectrum consists of some twelve bands starting from X 5000
and runs into the ultra-violet. Starting with the long wave-length band
they have been designated by the letters a, b, c, etc. These bands form a series,
the distance between the bands decreasing with the wave-length. The uranous
bands do not form any series, and resemble the uranyl bands only in their
general appearance. The uranium bands are quite wide and diffuse as com-
pared with the erbium and neodymium bands. A considerable amount of

SUMMARY AND GENERAL DISCUSSION OF RESULTS. 95

confusion has been caused in the past by the fact that most of the uranous
salts contain uranyl salts mixed with them, and, therefore, their absorption
spectra include the uranyl bands. The complete independence of the two
sets of bands is very clearly shown on spectrograms of the absorption spectra
of a uranous salt as it is gradually oxidized to the uranyl salt by the addition
of hydrogen peroxide.

(1) A problem of considerable interest, and one for which an answer1 has
been partly obtained, is the complete correlation of the a, b, c, etc., bands
when one uranyl salt is transformed into another salt, or when the solvent
is changed. Thus, for instance, the a band could be traced from the nitrate
to the sulphate, acetate, etc., and also for these salts in various solvents. It
may be that the neodymium, erbium, and samarium bands can be studied
in the same manner, especially at low temperatures. The results of such a
study should extend very greatly our knowledge of chemical reactions.

(2) If uranyl chloride and calcium nitrate are dissolved in water, uranyl
chloride and uranyl nitrate should both be present in the solution. In terms
of the theory of aggregates, then, it would be expected that the addition of
calcium nitrate to an aqueous solution of uranyl chloride would cause the
uranyl chloride bands to shift towards the violet. On the other hand, the
addition of aluminium or calcium chloride to an aqueous solution of uranyl
nitrate should cause the uranyl nitrate bands to shift towards the red. The
experimental results verify these conclusions.

Preliminary spectrograms showed that the addition of calcium nitrate
to an aqueous solution of uranyl nitrate caused the uranyl nitrate bands to
shift slightly towards the violet, the amount of the shift, however, being very
small. This seems to show that calcium itself has very little if any effect upon
the absorption, and this is in agreement with the results of Becquerel and
others.

(3) When an acid is added to a neutral uranyl salt in sufficient quantity,
this salt is changed to a salt of the acid added. Up to the present no quanti-
tative study of the strengths of acids by the spectroscopic method has been
made, but by means of radiomicrometric measurements this method should
give a reasonably accurate means of measuring the relative strengths of vari-
ous acids, especially in aqueous solutions.

(4) The modus operandi by which uranyl salts are transformed into
uranous, and vice versa, is not known. This subject has, however, been dis-
cussed in the chapter dealing with the spectrophotography of chemical reac-
tions. It seems probable that there is not the usual dynamic equilibrium
between the uranyl and uranous salts

AGGREGATES AND THEIR PROPERTIES.

(1) The presence of free acid or of foreign salts2 has been found to change
the frequency of many of the uranyl, uranous, and the neodymium bands.
At present our knowledge of these effects is practically restricted to aqueous
solutions, although similar effects are known to occur in other solvents. An
example of the above effect is that of the uranyl chloride and nitrate bands.

1 Publication 130, Carnegie Institution of Washington.
2Phys. Zeit., II, 668 (1910), 12, 269 (1911).

96

THE ABSORPTION SPECTRA OF SOLUTIONS.

The wave-lengths of the uranyl nitrate bands are considerably smaller than
those of uranyl chloride. This has been shown to be due to the presence of
NO., and H20 groups in the absorbing centers of the nitrate in solution. By
adding hydrochloric acid to an aqueous solution of uranyl nitrate, the uranyl
nitrate bands shift gradually to the positions of the uranyl chloride bands.
The addition of more hydrochloric acid causes most of the uranyl bands to
continue to shift towards the red. On the other hand, the addition of nitric
acid to an aqueous solution of uranyl nitrate causes the uranyl bands to shift
towards the violet. We thus obtain a gradual shifting of the uranyl bands of
a nitric acid solution of uranyl nitrate into the position of the uranyl bands
of uranyl chloride dissolved in concentrated hydrochloric acid. The magni-
tude of this shift is shown by the following wave-lengths :

Uranyl bands.

a

b

c

d

/

4125

4280

0

h

Nitrate in nitric acid

Chloride in hydrochloric acid

4790
4950

4670
4800

4510
4635

4370
4450

4000
4050

3900
4015

These changes are quite different as compared with those that take place
when the solvent is changed, and if it is assumed that every characteristic
absorption spectrum corresponds to a more or less stable system, then the
above changes indicate the existence of a series of systems or compounds.
These compounds will be called aggregates. There will be, accordingly,
nitric acid aggregates of uranyl nitrate, uranyl nitrate and chloride aggregates,
and hydrochloric acid aggregates of uranyl chloride.

(2) Whether there is a gradual change in the frequency of vibration of
the absorbers for a series of uranyl or uranous aggregates, or whether there
is simply a relative change in the intensity of a number of finer bands which
blend so as to form the uranyl or uranous bands, can not be decided from the
data now in hand. In the case of the neodymium salts, the latter effect seems
to manifest itself in the spectrograms that have been made.

(3) It is probable that the. presence of free acid in solvents other than
water will lead to the discovery of many new bands of neodymium, erbium,
etc. A striking example of this kind is the effect of dissolving hydrochloric
acid gas in a neodymium chloride solution in ethyl alcohol. The effect of the
free acid is to bring out very strongly bands at X 3695 and X 3760. The pres-
ence of sodium perchlorate has a similar effect.

(4) Mixtures of varying amounts of two salts in the same solvent may
result in a gradual shift from the bands of one salt into the bands of the other,
the shift depending on the amount of each salt present. As an example of this,
uranyl sulphate and uranyl nitrate were studied. On the other hand, there
are examples where the two salts do not seem to form any intermediate aggre-
gates, and the bands of each simply vary in intensity without showing any
change in frequency.

(5) Acid aggregates of uranous salts are found to be much more stable
than the neutral aggregates. Uranous nitrate, for instance, is very unstable;
but uranous chloride dissolved in concentrated nitric acid (the absorption
spectrum is entirely different from that of uranous chloride) will stand for hours

SUMMARY AND GENERAL DISCUSSION OF RESULTS. 97

before it is oxidized to the uranyl condition. Neutral uranous salts in solu-
tion, when heated to 80° or 90°, form a black precipitate; but if some free
acid is present (or in the case of uranous chloride if other chlorides are present)
the uranous salts are much more stable.

(6) Aggregates form compounds with solvents in many cases (for example,
various uranous aggregates in water and methyl alcohol) that are as charac-
teristic as those formed by the pure neutral salt. As yet no work has been
done on the effect of temperature and concentration on solvate aggregates.

(7) In discussing the differences in the chemical properties of aggregates
mention will be made of the reduction and oxidization of uranium salts. It
is probable that many other chemical properties differ very much for the
various aggregates, but as this work has been mainly concerned with the
reduction and the oxidization of the salts, only these properties will be con-
sidered. As some of the methods used in the reduction of the uranyl salts
are in themselves of considerable interest, they will be taken up in more
detail than would otherwise be done.

The method employed for the preparation of the uranous salts for work
on absorption spectra has been that described by Jones and Strong.1 Under
these conditions it is possible to obtain quite concentrated solutions of the
chloride, the bromide, and the sulphate. No aqueous solution of uranous
nitrate could be obtained, although small amounts would be formed when the
hydrogen was first liberated from the acid, but in a very short time the uranous
nitrate was oxidized.

The most concentrated solution of any uranous salt thus far obtained
is that of the chloride. Uranyl chloride is dissolved in ether (the solution
forms three distinct, unmiscible layers, the concentration of uranyl chloride
in each being different). To this solution is added a small amount of zinc
and concentrated hydrochloric acid. The uranous chloride formed is insoluble
in ether and accumulates in the dark oily liquid at the bottom of the vessel.
Layers of this liquid 1 mm. thick are almost opaque. The reduction of
uranyl chloride in isobutyl alcohol is very similar to that in ether.

When hydrochloric acid and zinc are added to a solution of uranyl nitrate
a considerable amount of a uranous salt is formed, although it is quickly
oxidized again. This is true even when quite large amounts of hydrochloric
acid are present. A much more complete spectrographic study of the reduc-
tion of uranyl aggregates should be made, as it would probably lead to some
knowledge as to how the oxygen atoms are removed from the uranyl group
and the influence the other atoms and molecules of the aggregate have upon
this reduction.

(8) Oxidization reactions also indicate very clearly that different aggre-
gates apparently possess different properties, although the action of the other
salts or acids present in the solution may modify the action of the oxidizing
agent. However this may be, it is found that uranous acid aggregates require
a great deal more of the oxidizing agent, especially when this is hydrogen
peroxide, than do the neutral aggregates.

(9) Assuming the presence of aggregates, it is important to know what
effect dilution has upon their composition. It would be supposed that the
aggregates would break down at high dilution. To test this supposition the

1 Phil. Mag., 19, 566 (1910)

98 THE ABSORPTION SPECTRA OF SOLUTIONS.

absorption spectra of uranyl nitrate and uranyl sulphate were photographed
at very great dilution. If dissociation is sufficiently complete it would be
expected that the uranyl nitrate bands would shift towards the red with
decrease in concentration, and the uranyl sulphate bands would shift towards
the violet, so that for very great dilution the uranyl bands of both salts would
be identical. This was not found to be the case. There appeared a very
slight shift of the uranyl nitrate bands to the red, and possibly a slight shift
of the uranyl sulphate bands to the violet; but these shifts were extremely
small; so small, indeed, that some observers who looked at the spectrograms
did not detect it at all. If the shifts occur, it is exactly what would be expected
if the uranyl nitrate and uranyl sulphate aggregates break up into complex
ions. The loss of an N03 group would cause the uranyl nitrate bands to shift
only slightly to the red if the aggregate was of some size, and the same would
be true of the uranyl sulphate aggregate. If this theory is true it indicates
that the aggregates possess quite a high degree of complexity.

(10) The question has been asked as to whether the aggregates are definite
chemical compounds, whether they have a definite composition, and how
stable they are. Unfortunately the spectroscopic method itself can not solve
these problems, but by studying the composition of the acid aggregate precipi-
tates that are often formed, and the chemical composition and absorption
spectra of these, much light may be thrown on the subject. The other physical
and chemical properties of solutions containing aggregates should also be
studied.

THE EFFECT OF TEMPERATURE ON ABSORPTION SPECTRA.

(1) The general effect of rise in temperature is to give a solution of an
inorganic salt a deeper color. This deepening of the color signifies that the
absorption of light has become more selective, and spectroscopic work indi-
cates that this selective absorption is usually due to a widening of the absorp-
tion bands. As these bands are never so distributed over the spectrum as to
give a colorless solution, it follows that a widening of the absorption bands will
intensify the color of the solution. In many cases this widening appears to
be quite unsymmetrical, but this need not necessarily mean that the center of
gravity of the individual absorption band is shifted. Many examples of the
neodj'mium absorption bands show this phenomenon very clearly. For
instance, the absorption due to the y and 8 groups of bands may be sufficiently
intense to make these groups appear as single bands. If the absorption is
not so intense as this, in some solvents it is found that with rise in temperature
the shortest wave-length bands may decrease in intensity, and may even dis-
appear. The long wave-length bands increase in intensity, and in some cases
new bands appear. Knowing this, it is easy to understand that if the absorp-
tion is so strong that each of these groups of bands appears as a single band,
these broad bands will widen very unsymmetrically towards the red with rise
in temperature. It may be that some change like this takes place in the case
of the uranyl bands. It is for this reason that a formula calculated for the widen-
ing of a band with rise in temperature would not apply to many wide bands.

(2) In the case of all pure salts dissolved in a single solvent, the bands
have been found to widen with rise in temperature, and at the same time the

SUMMARY AND GENERAL DISCUSSION OF RESULTS. 99

bands become more diffuse, the edges becoming hazier. When there is a
mixture of salts in the same solvent, the bands may become much weaker
with rise in temperature. This is the case in a mixture of neodymium and
calcium chlorides in water. In a similar manner the absorption of a salt in
two solvents probably decreases in intensity with rise in temperature. An
example of this kind is that of urarous bromide in 40 per cent water and 60
per cent alcohol. At ordinary temperatures the bands are of about equal
intensity. At 80° the water-bands have practically disappeared, without the
alcohol bands having widened to any great extent.

(3) In general the center of intensity of single bands changes but little.
Whenever there is any change of wave-length, the shift is invariably towards
the red.. It seems that this shift is greater the wider the band, so that it is
difficult to say in most cases whether the shift is real or only apparent. In
the case of solutions of pure neodymium and erbium salts the shift is, in general,
too small to be observed.

(4) A study of gaseous aggregates such as N204 +± 2N02 indicates that
raising the temperature or lowering the pressure increases the relative number
of the simpler molecules. Many vapors like those of the fatty acids show
molecular clustering at low temperatures. In a similar maimer it would be
expected that aggregates would gradually break down at the higher tempera-
tures. In considering specific examples, it would be expected that acid uranyl
sulphate aggregates in breaking down would result in a shift of the uranyl
bands, the shift being towards the violet. On the other hand, if the nitric
acid uranyl nitrate aggregates are broken down, it would be expected that
the uranyl bands would be shifted towards the red. According to this view
the shift of the uranyl bands of uranyl nitrate in nitric acid and of uranyl
sulphate in sulphuric acid, with rise in temperature, should be quite different
if the only effect of rise in temperature is a breaking up of the aggregates.

In advancing an hypothesis of this kind it is assumed that it is only the
molecules in an aggregate that are effective in changing the frequency of
vibration of the absorbing systems of the light centers. This means that the
kinetic energy of the aggregate corresponds to that of a molecule at the same
temperature in the solution, the individual molecules in an aggregate all
moving together. Whether there is a constant interchange of these molecules
and the molecules of the solution, spectroscopic evidence does not as yet show.
Neither can it be said with certainty that molecules outside the aggregates
do not affect the frequencies of vibration of the absorbing system within the
aggregate. Assuming that this is not the case, then it follows of necessity
that with rise in temperature the acid aggregates are not broken up, because
the uranyl bands of acid solutions are then shifted to the red. This shift
seems to be about as great for uranyl sulphate in sulphuric acid as it is for
uranyl nitrate in nitric acid. In a similar manner, acid solutions of neodym-
ium salts do not have their absorption spectra changed so as to resemble more
closely that of the neutral salt, with rise in temperature, as one would expect
if the acid neodymium aggregates were broken down.

(5) When foreign salts like calcium chloride are added to solutions of
neodymium chloride, it is probable that aggregates containing the two salts
are formed. In the case of aqueous solutions of these salts it is found that

100 THE ABSORPTION SPECTRA OF SOLUTIONS.

the neodymium bands are shifted to the red with rise in temperature, whereas
aqueous solutions of pure neodymium chloride do not show this effect at all.
Exactly what takes place in this case is not evident.

(6) The effect of rise in temperature on solutions showing the solvate
bands of equal intensity has been to cause a change in the intensity of the
solvate bands; very little if any change, however, in the wave-lengths of the
bands takes place. In the case of water and alcohol, the alcohol bands increase
in persistency as the temperature is raised.

(7) In the work with the closed cell at high temperatures it was found
that precipitates were formed in practically every case, probably due to hy-
drolysis, alcoholysis, etc., precipitation taking place in dilute as well as in con-
centrated solutions. Several examples were tested of concentrated solutions
of colored salts mixed with calcium or aluminium chloride, and precipitation
was found to take place under these conditions at comparatively low tem-
peratures. It would be very interesting to learn whether acid aggregates of
uranyl, neodymium, erbium and such salts are less likely to form precipi-
tates than the neutral salt solutions.

In the case of neutral uranous solutions these precipitates form at 70°
or 80°. The presence of acid prevents this precipitation at temperatures
below 100°.

Whether the salt precipitation at high temperatures is complete or not,
can not be decided in general at present. In the case of several neodymium
and erbium solutions this seemed to be the case. In one of the uranyl solu-
tions, however, some uranyl salt remained in the solution after the precipitate
had settled.

(8) It may be said, in general, that there is a very great increase in the
absorption of all solutions in the short wave-length region of the spectrum
as the temperature is raised. How great this increase in absorption would
be if pure solutions were used has not yet been determined. The formation
of precipitates is usually preceded by a very great increase in the short wave-
length absorption.

(9) The problem as to whether a rise in temperature produces a perma-
nent change in the structure of the aggregates in solution has not been studied
to any great extent. In the case of the existence of two solvent spectra it is
found that the spectra on cooling the solution are exactly the same as before
heating. In the case of an acetone solution of uranyl chloride it was found
that the bands are apparently single after the precipitate was formed during
the heating. Whether these bands would have become double again when the
solution was cooled was not tested. It would be interesting to learn whether
selective solvate precipitation would take place on heating solutions.

(10) It seems worth while in conclusion to call attention to the promising
and important application of spectrophotography at low temperatures to
the study of the nature of chemical reactions. As soon as the changes in
breadth, intensity, and frequency of individual absorption bands and the
constitution of the groups of bands can be interpreted, our knowledge of the
relation between absorption centers or aggregates and the other physical and
chemical units of matter will be much more fully understood; and the changes
which these particles undergo will be much better comprehended.

DESCRIPTION OF PLATES.

With the exception of the plates representing changes in temperature of the solution,
the times of exposure and the width of the slit are the same for each strip of the plate. The
current through the Nernst glower was kept constant. In the case of the uranyl salts a
much longer exposure is often made in the ultra-violet and violet, in order to bring out the
uranyl bands as strongly as possible, and this will be noted in the description. In some
strips ultra-violet wave-length spark lines will be found, these being photographed without
the solution in the path of the light, and only for purposes of measurement. In exposures
to the spark of this kind, the film holder was never moved between the photographing
of the absorption spectra of the solution and that of the spark spectra. In some instances
the strips are not uniformly exposed. This was due in many cases to the formation of
precipitates. A great deal of trouble of this kind was encountered, especially with the
uranous salts, in the high temperature work, and in the spectrophotography of chemical
reactions. In referring to the strips, the first or lowest strip will be that at the bottom of
the plate — the one that is nearest the scale of wave-lengths. The scale of wave-lengths
is in Angstrom units, the whole number standing for so many hundred Angstrom units.

Plate 1. A. Lithium Chromate in Water. Depths of cell and concentrations, starting
with the lowest strip: 0.25 normal, 3mm.; 0.25 normal, 24 mm.; 0.46 nor-
mal, 24 mm.; 1.0 normal, 24 mm.; 1.5 normal, 24 mm.; and 2.0 normal
24 mm.
B. Lithium Bichromate in Water. Depths of cell and concentrations, start-
ing with the lowest strip: 0.25 normal, 3 mm.; 0.25 normal, 24 mm.; 0.46
normal, 24 mm.; 1 .0 normal, 24 mm.; and 2.0 normal, 24 mm.
Plate 2. .4. Calcium Ferricyanide in Water. Depth of cell kept constant, 24 mm. Con-
centrations, starting with the lowest strip, 0.031, 0.058, 0.125, 0.175,
and 0.25 normal.
B. Calcium Ferrocyanide in Water. Depth of cell kept constant, 24 mm. Con-
centrations, starting with the lowest strip, 0.25, 0.46, 0.66, 1.0, 1.5, and
2^0 normal.
Plate 3. A. Aluminium Chromate in Water. Depths of cell, 3, 24, 24. and 24 mm. Con-
centration could not be determined on account of hydrolysis.
B. Calcium Chromate in Water. Depth of cell constant, 24 mm. Concentra-
tion somewhat less than 0.01 normal.
Plate 4. A. Copper Bichromate in Water. Depths of cell and concentrations, starting
with the lowest strip: 3 mm., 0.044 normal; 24 mm., 0.044 normal; 24 mm.,
0.08 normal; 24 mm., 0.117 normal; 24 mm., 0.175 normal; 24 mm., 0.26
normal; and 24 mm., 0.35 normal.
B. Potassium Nickel Chromate in Water. The depth of cell was kept constant
at 24 mm., and the concentration could not be determined on account of
hydrolysis.
Plate 5. A. Neodymium Chloride in Butyl Alcohol. Depth of cell 10 em. and concen-
tration 0.024 normal, strip 1, and in Butyl Alcohol, depth of cell 3 cm.
and concentration 0.04 normal, strip 2.
B. Neodymium Chloride in Glycerol and Ethyl Alcohol, concentration con-
stant, 0.5 normal. Depth of cell constant, 18 mm.; starting with the
lowest strip the following numbers represent the percentage of the solvents :
10, 15, 20, 40, 60 glycerol.
90, 85, 80, 60, 40 ethyl alcohol.
Plate 6. A. Neodymium Chloride in Isopropyl Alcohol. Depth of cell 30 mm. Con-
centration, 0.0266 normal.

B. Neodymium Nitrate in Tertiary Butyl Alcohol. Concentration, 0.2 normal.

C. Neodymium Chloride in Water and Alcohol. The lower strip represents

about 8 per cent water and shows both sets of bands. Concentration,
0.5 normal. The other strips show the effect of adding hydrogen peroxide.

101

102

DESCRIPTION OF PLATES.

Plate 7. ^4. Neodymium Chloride in Propyl Alcohol. Depth of cell constant, 30 mm. Con-
centrations, starting with lowest strip, 0.04, 0.03, 0.02, and 0.0133 normal.
B. Neodymium Nitrate in Propyl Alcohol. Depth of cell constant, 16 mm.
Concentrations, starting with lowest strip, 0.05, 0.07, 040, 0.15, 0.225,
and 0.3 normal.
Plate 8. A. Neodymium Chloride in Isobutyl Alcohol. Depth of cell constant, 34 mm.
Concentrations, starting with lowest strip, 0.024, 0.018, 0.012, and 0.008
normal.
B. Neodymium Nitrate in Isobutyl Alcohol. Depth of cell, 16 mm. Concen-
trations, starting with lowest layer, 0.05, 0.07, 0.10, 0.15, 0.225, and 0.3
normal.
Plate 9. A. Neodymium Nitrate in Butyl and Isobutyl Alcohols. Strip 1 is a 3 mm.
and strip 2 a 13 mm. layer of a 0.3 normal solution in butyl alcohol;
strip 3 is a 3 mm. and strip 4 a 13 mm. layer of a 0.6 normal solution in
isopropyl alcohol.
B. Neodymium Nitrate in mixtures of Ethyl and Isobutyl Alcohols. Depth of
cell constant, 34 mm. Concentration constant, 0.1 normal. Starting
with lowest strip the percentages of the solvents were:
20, 40, 60, 80, 100 ethyl alcohol.
80, 60, 40, 20, 0 isobutyl alcohol.
Plate 10. A. Neodymium Nitrate in Methyl Ester. Depth of cell constant, 18 mm.
Concentrations, starting with lowest strip, 0.05, 0.07, 0.10, 0.15, 0.225,
and 0.3 normal.
B. Neodymium Nitrate in Ethyl Ester. Depth of cell constant, 18 mm. Con-
centrations, starting with lowest strip, 0.05, 0.07, 0.10, 0.15, 0.225, and
0.3 normal.
Plate 11. A. Neodymium Nitrate in Anthracene and Ethyl Acetate. The variable quantity
here is depth of cell. The early investigators made experiments to test
whether two colored salts in the same solvent having bands that were of
almost the same wave-lengths had the wave-lengths of these bands changed
with reference to the wave-lengths of the bands for the solutions of the
separate salts. At present this would hardly be expected to result, unless
double solvates were formed, i.e., compounds containing the two salts
and the solvent. This spectrogram was taken to find whether the
anthracene and neodymium bands had their wave-lengths affected by
both being dissolved in ethyl ester. This might be expected if compounds
were formed containing both anthracene and neodymium nitrate. The
' bands are in the ultra-violet. Unfortunately it is very difficult to obtain
the anthracene and neodymium bands together.
B. Neodymium Nitrate in Ethyl Acetate and Anthracene. To obtain this
spectrogram it was necessary to heat the solutions in order to keep the
anthracene in solution. The percentages of anthracene for the 6 strips,
starting with the lowest, were: 0.25, 0.5, 0.75, 1, 1.5, and 2.
Plate 12. A. Neodymium Nitrate in Ethyl Acetate and Anthracene. Concentration of
neodymium nitrate, 0.24 normal. Succeeding strips show the effect of
adding ethyl alcohol, methyl alcohol and acetic acid, respectively.
B. Neodymium Nitrate in Ethyl Acetate and Anthracene. The lowest strip
is the only one that shows the anthracene bands.
Plate 13. A. Neodymium Acetate in Formamide. The acetate slowly decomposes, form-
ing a white precipitate. Strips 1 and 2 represent different depths of cell
(strip 2 being 35 mm.). Strip 3 is the same as 2 after water has been
added. The original film shows quite a large shift of the neodymium
bands towards the violet in the upper strip, compared with the second strip.
B. Uranyl and Uranous Sulphates in Water, to which Acetic Acid is added.
Starting with strip 1 increasing amounts of acetic acid are added to an
aqueous solution of uranyl sulphate. This results in a precipitate (strip
5), and the solution is filtered and the absorption spectra again taken
(strip 6). Strips 7 and 8 represent the uranous salt formed from the solu-
tion used for strip 6 when zinc is added.

DESCRIPTION OF PLATES.

103

B

Plate 15. A.

B.

Plate 16.

B.

Plate 17. A.

B.

Plate 14. A. Uranyl Chloride in Isopropyl Alcohol. Concentration of the chloride,
0.005 normal. Depth of cell variable.

Uranyl Chloride in Ethyl Acetate. Depths of cell, starting with lowest
strip, 12, 24, 24, 24, 24, 24, 24, and 24 mm. Concentrations, 0.075,
0.075, 0.01, 0.014, 0.02, 0.03, 0.045, and 0.06 normal.

Strip 1 is the absorption of a solution 35 mm. deep of Uranyl Nitrate in
Methyl Ester. Strip 2 is the absorption of Erbium Chloride (35 mm.)
in Propyl Alcohol. The other strips represent the absorption of Uranyl
Chloride in Butyl Alcohol.

Uranyl Chloride in Formamide. Depths of cell, 3, 15, 15, 15, 15, and 15 m.
Respective concentrations, 0.073, 0.073, 0.1, 0.147, 0.219, and 0.293 normal.
A. Uranyl Chloride in Water, to which increasing amounts of an aqueous solu-
tion of Calcium Nitrate are added. This spectrogram was made to deter-
mine whether the addition of calcium nitrate resulted in a gradual shift
of the uranyl bands. The original film shows that the A and B bands
decrease in intensity, and the decrease seems to be considerably greater
on the red side of the bands.

Uranyl Nitrate in Nitric Acid, to which increasing amounts of a Concen-
trated aqueous solution of Aluminium Chloride are added. The first addi-
tion consisted of three drops, and shows a large shift of the uranyl bands
towards the red. A gradual shift towards the red is shown by the suc-
ceeding strips in the original film.

Uranyl Nitrate in Nitric Acid to which increasing amounts of Hydrochlo-
ric Acid are added.

Uranyl Nitrate in Water to which an aqueous solution of Aluminium
Chloride is added. This spectrogram shows very clearly the shift of the
uranyl bands towards the red.

Uranous Chloride in Propyl Alcohol. The variable here is the depth of cell.

Uranyl Chloride in Propyl Alcohol. Depths of cell, 3, 12, 24, 24, 24, 24.
and 24 mm. Respective concentrations, 0.025, 0.025, 0.033, 0.046, 0.06,
0.1, 0.15, and 0.2 normal.

Uranous Chloride in Isobutyl Alcohol.

Uranyl Chloride in Isobutyl Alcohol. Starting with strip 1 the depth of

cell was 10 mm.; succeeding depths were 24 mm. The corresponding

'concentrations were: 0.02, 0.027, 0.037, 0.053, 0.08, 0.12, and 0.16 normal.

Uranous Chloride in Isobutyl Alcohol. When uranyl chloride in isobutyl
alcohol is reduced by the addition of zinc and a small amount of con-
centrated hydrochloric acid, the uranous chloride formed appears as dark
green, oily looking drops, which collected at the bottom of the solution.
The remainder of the solution dissolves only a very small portion of the
uranous chloride. 19 A represents the absorption of the dilute portion;
whereas this plate represents the absorption of very thin layers of the
concentrated solution of uranous chloride.

Uranyl Chloride in Methyl Ester. Starting with strip 1 the depths of cell and
concentration were: 10 mm., 0.021; 24 mm., 0.028; 24 mm., 0.04; 24 mm.,
0.057; 24 mm., 0.085; 24 mm., 0.13; and 24 mm., 0.17 normal.
Plate 21. A. Uranous Chloride in Propyl Alcohol.

B. Uranyl Nitrate in Propyl Alcohol. Strip 1 has a depth of cell of 10 mm.
and the depth for the other strips is 24 mm. The concentrations were
0.08, 0.10, 0.15, 0.21, 0.32, 0.48, and 0.63 normal.
Plate 22. A. Uranyl Nitrate in Water to winch increasing amounts of Calcium Nitrate
were added. The addition of calcium chloride would cause the uranyl
bands to be shifted towards the red. If this is due to the presence of
calcium, then calcium nitrate should also cause the uranyl bands to
shift towards the red. This does not appear from the plate, the bands
being practically of the same wave-lengths after calcium nitrate had
been added. This spectrogram (the film at least) gives evidence that the
effective agent in causing the shifts of the uranyl bands is the acid radicle.
B. Uranous Chloride in Ether and Hydrochloric Acid to which Acetone is added.

Plate 18.

A.

B.

Plate 19.

A.

B.

Plate 20.

A.

B.

104

DESCRIPTION OF PLATES.

Plate 23. A

B.

Plate 24.

A.

B

Plate 25.

A,

B

Plate 26.

A,

B

Plate 27.

A

Plate 28.

B.

C.
A.

B

Plate 29. A

B.

Plate 30. A.
B.

Plate 31. A.

B.

Plate'32. A,

B.

Uranous Bromide in Water to which Methyl Alcohol is added. This spec-
trogram shows the great difference between the water and methyl alco-
hol bands of uranous bromide.

Uranous Chloride in Water to which Methyl Alcohol is added. The methyl
alcohol was added in smaller quantities than to the solutions whose
spectrograms are recorded in the above plate.

Uranous Chloride in Methyl Ester to which Water is added.

Uranous Chloride in Methyl Ester. Depth of cell is gradually increased.

Uranous Chloride in Ethyl Ester. Depth of cell variable.

Uranous Acetate in Acetone. Depth of cell variable.

Uranous Chloride in Ether. Depth of cell variable.

Uranous Chloride in Acetone. Depth of cell variable.

Uranous Chloride in concentrated Nitric Acid. The uranous chloride is
added to the acid which was placed in the Uhler cell. It is very remark-
able that the uranous salt is not oxidized under these conditions.

Uranous Bromide in Water and Methyl Alcohol after the precipitate has
been filtered off (strip 1). This shows a selective precipitation of the
hydrate. The succeeding strips show the absorption of uranous bromide
in water and methyl alcohol to which increasing amounts of nitric acid
(in the same proportion of water and alcohol) are added.

Uranous Chloride in Propyl Alcohol to which Acetone is gradually added.

Gadolinium Chloride in Ethyl Alcohol. Concentration constant, 0.8 normal.
Depths of cell, starting with the lowest strip, 2, 4, 9, 18, 27, and 27 mm.
In the upper strip an exposure was made directly to the ultra-violet spark
fines.

Gadolinium Chloride in Water. Concentration constant, 1.407 normal.
Depths of cell, starting with the lowest strip, 2, 10, 15, 22, 22, and 100 mm.
In the upper two strips an exposure was made directly to the ultra-violet
spark lines.

Dysprosium Chloride in Methyl Alcohol. Concentration about normal. In
all the spectrograms of gadolinium, dysprosium, and samarium, the slit
width was 0.10 mm., the current in the Nernst glower 0.9 ampere, and
the length of exposure was 1 minute to the visible portion of the Nernst
glower spectrum, and about 5 minutes to the ultra-violet part of the
Nernst glower spectrum. The depths of cell were 1, 5, 12, 20, 31, and
31 mm. The upper strip was exposed directly to the ultra-violet spark lines.

Dysprosium Chloride in Water. Concentration, 1.86 normal. Depths of
cell, starting with lowest strip, 2, 6, 10, 15, 21, and 100 mm. All the
strips except the lowest one were exposed directly to the ultra-violet spark
lines.

Dysprosium Chloride in Water. Concentration, 1.86 normal. Depths of
cell, starting with lowest strip, 2, 8, 16, and 21 mm.

Dysprosium Acetate in Water to which varying amounts of Nitric Acid
were added. Concentration of the neutral solution, 0.4 normal. The in-
creased depth of cell is due to the addition of concentrated nitric acid.
Starting with the lowest strip the depths of cell were: 15, 15.1, 15.3, 15.7,
16.7, and 32 mm.

Dysprosium Acetate in Water. Concentration, 0.4 normal. Depths of cell,
starting with lowest strip, 4, 16, 25, and 34 mm. Exposures were made
in strips 2, 3, 5 directly to the ultra-violet spark lines.

Dysprosium Chloride in Ethyl Alcohol. Concentration, 0.74 normal. Dept hs
of cell, 2, 10, 15, 24, 30, and 30 mm. The upper si rip was exposed directly
to the ultra-violet spark lines.

Samarium Chloride in Water. Concentration, 1.31 normal. Depths of
cell, starting with lowest strip, 2, 6, 12, 16, 20, and 100 mm. Strips 'A, 4,
5, <) were exposed directly to the ultra-violet spark lines.

Samarium Chloride in Methyl Alcohol. Concentration about normal.
Depths of cell, 2, 5, 9, 18, 27, and 27 mm. The upper strip was exposed
directly to the ultra-violet spark lines.

DESCRIPTION OF PLATES. 105

Plate 33. A. Samarium Nitrate in Water. Concentration, 1.06 normal. Depths of cell,
1, 4, 12, 22, and 22 mm. Strips 3 and 5 were exposed directly to the ultra-
violet spark lines.
B. Samarium Chloride in Ethyl Alcohol to which Water is gradually added.
The first strip represents the absorption of a 0.9 normal aqueous solution
of 5.5 mm. depth. To this solution were added small amounts of water,
so that the depths of cell were 5.5, 5.6, 5.7, 5.9, 6.0, 6.2, and 6.4 mm.
The change from the first to the second strip represents a change from
the alcohol to the water spectra. After the second strip there is very
little change in the spectrum.

Plate 34. A. Samarium Chloride in Ethyl Alcohol. Concentration, 0.9 normal. Depths
of cell, 1, 4, 9, 18, 25, and 25 mm. The upper strip was exposed directly
to the ultra-violet spark lines.
B. Samarium Chloride in Ethyl Alcohol to which Water is added. The first
strip represents the absorption of a 0.09 normal solution in ethyl alcohol.
Succeeding strips represent the absorption of the same solution to which
small quantities of water have been added. Starting with the lowest strip
the depths of cell were: 18, 19, 20, 21.5, 22.5, 25, and 28 mm.

Plate 35. A. Cobalt Bromide in Methyl Alcohol. Concentration, 0.2 normal. Depth
of cell, 1.0 cm. Current in the Nernst glower 0.8 amperes. Strip 1 was
exposed 4 minutes to the visible, and 4 minutes to the violet and ultra-
violet, the temperature being 29° C. Strip 2 was exposed 6 minutes to
the whole spectrum, at 44°. Strip 3 was exposed 6 minutes at 57°, and
strip 4, 15 minutes at 70°.
B. This spectrogram shows the gradual oxidization of an aqueous solution of
Uranous Sulphate to which small amounts of Hydrogen Peroxide have
been added. The plate shows the gradual decrease in intensity of the
uranous bands, and the corresponding increase in intensity of the uranyl
bands. A spectrogram of this kind makes it very easy to differentiate
between the uranous and the uranyl bands.

Plate 36. A. This plate represents the absorption of an acidified solution of Uranous
Chloride, dissolved in equal volumes of Water and Methyl Alcohol
(strip 1) to which was added a concentrated solution of Calcium Nitrate
dissolved in two parts of Water and three parts of Methyl Alcohol (suc-
ceeding strips). The original film shows very clearly the selective action
of the calcium nitrate on the water and alcohol bands, the water bands
becoming much weaker, whereas the alcohol bands remain of about con-
stant intensity.
B. Uranous Acetate in Water to which Nitric Acid is added. The first f out-
strips represent uranous acetate in water for different depths of cell,
the depth of cell in strip 4 being 2.0 cm. The succeeding strips show the
absorption of the same solution as that used in strip 4, increasing amounts
of nitric acid having been added. This spectrogram is then a spectro-
graph of the transformation of uranous acetate into uranous nitrate, and
then the oxidization of uranous nitrate to uranyl nitrate.

Plate 37. A. Uranous Chloride in Water to which increasing amounts of Nitric Acid are
added. The first strip was accidentally exposed a short time dhectly to
the Nernst glower. This is a spectre-photograph of the chemical reaction
represented by uranous chloride being converted into uranous nitrate.
B. Uranous Chloride in equal parts of Water and Methyl Alcohol (strip 1) to
which Nitric Acid is added (strips 2, 3, and 4), and to which Hydrogen
Peroxide is added (strip 5). This spectrogram shows how the water
and alcohol bands of uranous chloride are changed to the nitrate
bands, and how these in turn are replaced by the uranyl nitrate bands.
The uranous nitrate bands are seen to be very different from the chlo-
ride bands.

106

DESCRIPTION OF PLATES.

Plate 38. A. Uranous Chloride in equal parts by volume of Water and Methyl Alcohol
(strip 1) to which is added increasing amounts of Sodium Chlorate dis-
solved in equal volumes of Water and Methyl Alcohol (succeeding strips).
This spectrogram shows very little of any selective action on the uranous
chloride water and alcohol bands.
B. Uranous Chloride in equal volumes of Water and Methyl Alcohol (strip 1)
to which Potassium Chlorate in Water and Methyl Alcohol is added
in increasing amounts (strips 2, 3, 4, and 5), and to which Hydrogen
Peroxide is added (strip 6). This spectrogram shows the selective action
of potassium chlorate on the uranous chloride water and methyl alcohol
bands. The water bands are seen to decrease in intensity, while the
alcohol bands increase in intensity. In this example the addition of
hydrogen peroxide seems to have oxidized only the alcoholated uranous
chloride.

Plate 39. A. Uranous Chloride in equal volumes of Water and Methyl Alcohol (strip 1)
to which is added Sodium Chlorate in equal volumes of Water and Methyl
Alcohol (strip 2) in one case, and Potassium Chlorate in the other (strip
3). The last strip represents the absorption of uranous chloride in
methyl alcohol and ether. This plate shows the selective action of the
above salts on the water and alcohol bands.
B. Uranous Chloride in equal volumes of Water and Alcohol to which is added
Calcium Nitrate in 2 parts Water and 3 parts Methyl Alcohol (strip 1) ;
in 1 part Water and 1 part Methyl Alcohol (strip 2) ; and in pure Water
(strip 3). A corresponding addition of potassium nitrate was made,
the potassium nitrate being dissolved in 2 parts water and 3 parts
methyl alcohol (strip 4), 1 part water and 1 of alcohol (strip 5) and in
pure water (strip 6). The last strip represents the absorption of uranous
chloride itself in equal parts of water and methyl alcohol. This spec-
trogram shows the selective action of the salts on the uranous water and
alcohol bands. The selective action is particularly marked in the case of
potassium nitrate. The presence of this salt seems to weaken the alcohol
bands much less than the water bands, the proportion of water and alcohol
present being kept constant.

Plate 40. A. Uranous Bromide in Water to which Methyl Alcohol is added. The con-
centration of the aqueous solution was 0.5 normal, and the percentages
of alcohol in the solution, starting with the first strip, were: 0, 24, 39,
49, 56, and 62. This spectrogram shows that in the case of uranous
"bromide at this temperature and concentration (the solution also con-
tains zinc bromide) it is necessary that the amount of alcohol required
to make the water and alcohol bands have approximately the same inten-
sity is about 1 }4 times that of the water present.
B. Uranous Chloride in acidified (Hydrochloric Acid) Ethyl Ester to which
Ethyl Alcohol is added. The absorption of the ester solution is very
similar to that of an aqueous solution. The spectrogram shows the much
greater absorbing power of the ethyl alcohol solution compared with the
ester solution, the amount of uranous chloride being kept constant.

Plate 41. A. Uranous Chloride in Water and Methyl Alcohol (strip 1) to which are added
increasing amounts of Calcium Nitrate in 2 parts Water and 3 parts
Methyl Alcohol (strips 2, 3, 4), and finally Hydrogen Peroxide (strip 5).
The spectrogram shows that the original solution was almost entirely
free from uranyl chloride; that the addition of calcium nitrate had a
marked selective action on the uranous bands, the water bands being
greatly decreased in intensity while the alcohol bands became more
intense. The addition of hydrogen peroxide causes the complete disap -
pearance of the uranous bands and the appearance of the uranyl bands.
B. Uranous Bromide in 2 parts Water and 3 parts Methyl Alcohol (strip 1)
to which Hydrogen Peroxide (2 parts of a 3 per cent Hydrogen Peroxide
solution in Water with 3 parts of Methyl Alcohol) was added 5 drops at
a time (strips 2, 3, 4, 5, 6, and 7). Strip 1 shows that the oxidization of
the uranous chloride by the hydrogen peroxide is the same, as indicated
either by the intensity of the water bands or the alcohol bands.

DESCRIPTION OF PLATES.

107

Plate 42. A. Uranous Bromide in 7 parts Water and 12 parts Methyl Alcohol (strip 1)
to which are added 10 (strip 2), 20 (strip 3), 40 (strip 4), and 80 (strip 5)
drops of a concentrated solution of Calcium Nitrate in Water. This
spectrogram shows that in this case the water bands are intensified and
also changed in character, the two red water bands having their relative
intensities greatly changed. The percentages of water present, start-
ing with strip 1, were: 37, 43, 52, 64, and 75.
B. Uranous Chloride in Ether and Methyl Alcohol to which increasing amounts
of Hydrogen Peroxide were added.
Plate 43. A. Uranous Chloride in Water and Methyl Alcohol to which Hydrogen Per-
oxide is gradually added. This spectrogram shows the simultaneous
oxidization of the hydrate and alcoholate of uranous chloride.
B. Uranous Bromide in Water and Methyl Alcohol to which Potassium Chlo-
rate in Water and Methyl Alcohol was gradually added.
Plate 44. A. Uranous Chloride in Water and Acetone to which Hydrogen Peroxide is
gradually added.
B. Uranous Bromide in Glycerol to which Hydrogen Peroxide is gradually
added.
Plate 45. A. Uranous Bromide in Water and- Methyl Alcohol to which Nitric Acid is
added.
B. Uranous Chloride in Acetone and Methyl Alcohol to which Nitric Acid is
added.
Plate 46. A. Uranous Bromide in 7 parts Water and 12 parts Methyl Alcohol (strip 1)
to which is added Potassium Nitrate in 2 parts Water and 3 parts Methyl
Alcohol (strips 2 and 3), and corresponding strips (4, 5, and 6) where
Calcium Nitrate is added instead of Potassium Nitrate. This spectro-
gram shows the selective action of these salts on the water and alcohol
bands, the water bands having practically disappeared in strips 3 and 6.
B. Uranous Sulphate in Water to which Nitric Acid is added in increasing
amounts.
Plate 47. A. Uranous Bromide in 7 parts Water and 12 parts Methyl Alcohol to which
increasing amounts of Calcium Nitrate in Methyl Alcohol are added.
B. Uranous Bromide in 2 parts Water and 3 parts Methyl Alcohol to which
, increasing amounts of Sodium Perchlorate in Methyl Alcohol are added.
These strips of A and B show the selective action of the calcium nitrate
and sodium perchlorate on the water and alcohol bands, the water bands
practically disappearing. In this case part of the effect may be due to
the increased percentage of alcohol present.
Plate 48. A. Uranous Sulphate in Sulphuric Acid to which Nitric Acid is added. No
oxidization takes place.
B. Uranyl Bromide in Water to which Nitric Acid is added. Tbe two upper
strips represent the absorption of a solution of uranyl chloride to which
were added acetic acid and zinc, to find whether uranous acetate or
uranous chloride would be formed, and in what amounts.
Plate 49. A. Uranous Bromide in Water and Methyl Alcohol.

B. The first three strips represent the oxidization of an aqueous solution of
Uranous Sulphate by Hydrogen Peroxide. The other four strips rep-
resent the oxidization of a Sulphuric Acid solution of Uranous Sulphate
by Hydrogen Peroxide
Plate 50. A. Cobalt Chloride in Methyl Alcohol. Concentration, 0.01 normal. Depth
of cell, 10 cm. Starting with the lowest strip the temperatures are: 30°,
45°, 55°, 70°, and 80°.
B. Cobalt Bromide in Methyl Alcohol. Concentration, 0.01 normal. Depth
of cell. 10 cm. Starting with the lowest strip the temperatures are: 34°,
44°, 55°, 66°, 81°, and 100°.
Plate 51. A. Cobalt Bromide in Methyl Alcohol. Concentration, 0.1 normal. Depth
of cell, 1.0 cm. Starting with the lowest strip the temperatures are: 31°,
46°, 58°, 64°, 68°, and 72°.
B. Cobalt Chloride in Methyl Alcohol. Concentration, 0.1 normal. Depth
of cell, 1.0 cm. Starting with the lowest strip the temperatures are: 23°,
37°, 47°, 55°, 62°, 73°, and 93°.

108

DESCRIPTION OF PLATES.

Plate 52. .4.

B.

Plate

53.

A

B

C

Plate

54.

A
B

Plate

55.

A
B,

Plate 56. A.

B.

Plate

57.

A.
B.

Plate

58.

A.
B.

Plate

59.

A.

Plat j : 60.

B.

A.
B.

C.

10° and 70° C.

unci Hydrochloric Acid at 10c

and

Neodymium Chloride in Water (S per cent) and Ethyl Alcohol. Concentra-
tion, 0.3. Depth of cell, 1.0 cm. The exposures were started at 40° and
ended at 80° C, the temperature being gradually increased in the interim.

Neodymium Chloride in Water and Alcohol. Concentration, 0.1 normal.
Depth of cell, 1.0 cm. Range of temperature, starting from lowest
strip, 20° to 85° C.

Neodymium Chloride in Methyl Alcohol. Concentration, 0.2 normal.
Depth of cell, 1.0 cm. Temperatures, 25°, 40°, 55°, and 70° C.

Uranous Chloride in Water. Range of temperature from 30° to about 80° C.

Neodymium Bromide in Water (8 per cent) and Methyl Alcohol. Concentra-
tion, 0.2 normal. Range of temperature, 30° to 80° C.

Neodymium Chloride in Methyl Alcohol. Concentration, 0.1 normal.
Depth of cell, 10 cm. Temperatures, 26°, 40°, 55°, 78°, and 85° C.

Neodymium Bromide in Methyl Alcohol. Concentration, 0.1 normal.
Depth of cell, 10 cm. Temperatures, 25°, 35°, 44°, 60°, 82°, 100°, and
120° C. At the highest temperature a precipitate was formed.

Neodymium Bromide in Water. Concentration, 1.66 normal. Depth of
cell, 1.0 cm. Temperatures, 20°, 40°, 60°, 80°, and 93° C.

Neodymium Chloride in Water. Concentration, 2.05 normal. Depth of
cell, 1.0 cm. Temperatures, 20°, 50°, and 75° C.

Neodymium Nitrate in Water. Concentration, 2.15 normal. Depth of
cell, 1.0 cm. Temperatures, 20°, 75°, and 95° C.

Neodymium Chloride in Acetone, 10° and 70° C.

Neodymium Chloride in Ether, 10° C.

Neodymium Chloride in Ethyl Alcohol,

Neodymium Chloride in Ethyl Alcohol
70° C.

Neodymium Nitrate in Nitric Acid, 10°

Neodymium Chloride in Water (8 per
and 70° C.

Neodymium Nitrate in Water (8 per cent) and Acetone at 10° and 60° C.

Neodymium Chloride in Water (40 per cent) and Ethyl Alcohol at 10° C.

Neodymium Chloride in Water (40 per cent) and Glycerol at 10° C.

Neodymium Nitrate in Water and Methyl Alcohol. Concentration, 0.25
normal. The variable here is the percentage of water.

Neodymium Bromide in Water (8 per cent) and Methyl Alcohol. Concen-
tration, 0.2 normal. Depth of cell, 1.0 cm. Temperature range, 25° to
115° C.

Neodymium Nitrate in Water (14 per cent) and Methyl Alcohol. Concen-
tration, 0.3 normal. Depth of cell, 1.0 cm. Temperature range, 25°
to 115° C.

Neodymium Bromide in Water (8 per cent) and Ethyl Alcohol. Concentra-
tion, 0.2 normal. Depth of cell, 1.0 cm. Temperature range, 25° to
87° C.

Neodymium Chloride in Water at 10° and 90° C.

Neodymium Chloride in Hydrochloric Acid at 10° and 90° C.

Neodymium Chloride in Methyl Alcohol.

Neodymium Chloride and Sodium Chlorate in Methyl Alcohol.

Neodymium Acetate in Water at 10° and 90° C.

Neodymium Chloride in Acetic Acid at 10° and 90° C.

Neodymium Acetate in Acetic Acid at 10° and 90° C.

Neodymium Acetate in Methyl Ester and Acetic Acid at 10° and 90° C.

Erbium Chloride in Water at 20°, 30°, 85°, and 115° C.

Neodymium Chloride in Water. Concentration, 0.2 normal. Depth of
cell, 10 cm. Temperatures, 20°, 50°, So°, and 115° C.

Neodymium Nitrate in Tertiary Butyl Alcohol. Concentration, 0.2 normal.
Deptli of cell, 1.0 cm. Temperatures, 25°, 50°, and 65° C.

Neodymium Nitrate in Isobutyl Alcohol at 20°, 75°, and 100° C.

and 50° C.

cent) and Methyl Alcohol at 10°

DESCRIPTION OF PLATES.

109

Plate 61. A. Uranyl Chloride in Isobutyl Alcohol.

of cell, 10 cm. Temperatures, 18°
B. Erbium Chloride in Methyl Alcohol.

20°, 45°, 75°, 110°, 125°, and 165°
Plate 62. A. Uranyl Chloride in Isobutyl Alcohol.

of cell, 10 cm. Temperatures, 20°
B. Uranyl Nitrate in Isobutyl Alcohol.

Concentration, 0.005 normal. Depth
, 40°, 60°, 80°, 100°, and 90° C.
Depth of cell, 10 cm. Temperatures,
(after a precipitate had been formed).
Concentration, 0.076 normal. Depth
60°, 85°, and 115° C.
Concentration, 0.033 normal. Depth
of cell, 10 cm. Temperatures, 20°, 50°, 80°, 100°, and 115° C.
Plate 63. A. Uranyl Nitrate in Methyl Alcohol. Concentration, 0.2 normal. Depth
of cell, 1.0 cm. Temperatures, 20°, 100°, 135°, 145° (precipitate), and
160° C.
B. Uranyl and Calcium Chlorides in Methyl Ester at 20°, 45°, and 85° C.
Uranyl and Calcium Chlorides in Methyl Alcohol at 20°, 50°, and 95° C.
Plate 64. A. Uranyl Chloride in Methyl Ester. Concentration, 0.005 normal. Depth
of cell, 10 cm. Temperatures, 20°, 45°, 70°, 90°, 110°, 135°, and 140° C.
B. Uranyl Nitrate in Propyl Alcohol. Concentration, 0.005 normal. Depth
of cell, 10 cm. Temperatures, 20°, 40°, 65°, 85°, 105°, 115°, 130°, and
145° C.
Plate 65. A. Uranyl Chloride in Propyl Alcohol. Concentration, 0.01 normal. Depth
of cell, 10 cm. Temperatures, 22°, 45°, 70°, 90°, and 100° C.
B. Uranyl Chloride in Methyl Alcohol. Concentration, 0.005 normal. Depth
of cell, 10 cm. Temperatures, 20°, 45°, 65°, 85°, 110°, 120°, and 145° C.
Plate 66. A. Uranyl Chloride in Methyl Ester. Concentration, 0.034 normal. Uranous
Chloride in Methyl Ester (strips 2 and 3); Uranyl Nitrate in Methyl
Ester; Uranyl Chloride in Methyl Alcohol; and Uranyl Nitrate in Methyl
Alcohol.
B. Uranyl Nitrate in Methyl Ester. Concentration, 0.005 normal. Depth
of cell, 10 cm. Temperatures, 20°, 50°, 75°, 100°, 125°, and 145° C.
Plate 67. A. Uranous Sulphate in Sulphuric Acid at 10° and 90° C.
Uranous Sulphate in Water at 10° and 90° C.
Uranous Chloride in Water and Methyl Alcohol at 10° and 70° C.
B. Uranyl Nitrate in Nitric Acid at 10° and 70° C.

Uranyl Chloride in Hydrochloric Acid at 10° and 80° C.
Uranyl Sulphate in Water at 10° and 80° C.
Uranyl Sulphate in Sulphuric Acid at 10° and 70° C.

INDEX.

PAGE

Absorption and emission centers —

And the ionization theory 2

Of light and heat 1

Absorption spectra —

Of benzene and its derivatives 16

Of organic compounds 8

Of various salts in solution, map-
ping the • ■> 1

Acetone —

Neodymium acetate in 40

Xeodyniium nitrate in 39

Solution of neodymium nitrate 80

Solution of uranous chloride 82

Solution of uranyl chloride 81

Uranyl nitrate in 44

Acid solutions of neodymium salts 84

Acids, strengths of, possible method of

measuring 62

Aggregates and their properties 95

Effect of temperature on 83

Alcohols, samarium chloride in 49

Aluminium and calcium chromates 33

Anthracene, neodymium chloride in

ethyl acetate and 56

Apparatus and methods, experimental. . 27

Arcs and flames, emission centers of. ... 6

Bands, uranous and uranyl 94

Benzene —

And its derivatives, absorption

spectra of 16

Theories 25

Bichromate —

And chromate of lithium 33

Of copper 33

Butyl alcohol —

Neodymium chloride in 36

Neodymium nitrate in 3S

Uranyl chloride in 43

Calcium —

And aluminium chromates 33

Ferricyanide and calcium ferro-

cyanide 32

Ferrocyanide and calcium ferri-
cyanide 32

Canal-ray spectra, carriers of 4

Carriers —

Of canal-ray spectra 4

Of spark spectra 5

Catalytic action of light 6

Centers —

Absorption and emission of light

and heal 1

Absorption, of uranium spectra. ... 45
Emission and absorption, and the

ionization theory 2

Emission, of flames and arcs 6

110

PAGE

Chromate —

And bichromate of lithium 33

Potassium nickel 33

Chromates—

And cyanides, absorption of certain 31

Of calcium and aluminium 33

Chromophores, theory of 10

( 'upper bichromate 33

Cyanides and chromates. absorption of

certain 31

Description of plates 101

Discussion of results 87

Dynamic isomerism, theory of 18

Dysprosium —

Al (sorption spectrum of 46

Acetate in water 4s

Chloride in ethyl alcohol 47

Chloride in methyl alcohol 47

Chloride in water 47

Ether —

Neodymium chloride in 37

Uranyl chloride in 43

Ethyl alcohol —

Dysprosium chloride in 47

Solution of neodymium chloride.. . . 77

Ethyl ester, neodymium nitrate in.... :;'.>

Emission and absorption centers —

And the ionization theorv 2

Of light and heat 1

Emission —

Centers of flames and arcs 6

Spectra of organic compounds 7

Erbium —

( hloride in water 80

Salts, absorption of solutions of . . . . 33

Experimental methods and apparatus. . 27

Flames and arcs, emission centers of . . . 6
Ferrocyanide and ferricyanide of calcium 32
Formamide —

Neodymium acetate in 40

Uranyl chloride in 44

Gadolinium —

Absorption spectrum of 46

Chloride in ethyl alcohol 46

Chloride in water 46

Heat, absorption and emission centers of 1
Hydrochloric acid solution of uranyl

chloride 86

Ionization theory and absorption and

emission centers 2

Ions, are they factors in the absorption

of lisrht 62

INDEX.

Ill

PAGE

Isobutyl alcohol —

Neodymium chloride in 37

Neodymium nitrate in 39

Solution of neodymium nitrate 79

Solution of uranyl chloride and

nitrate 81

Uranous chloride in 45

Uranyl chloride in 43

Isomerism, dynamic, theory of IS

Isoprophyl alcohol —

Neodymium chloride in 36

Neodymium nitrate in 38

Uranyl chloride in 43

Isorropesis 22

Light-
Absorption and emission centers of,

and heat 1

Catalytic action of 6

Lithium chromate and bichromate 33

Mapping —

Of absorption spectra of various

salts in solution 31

Of spectra 87

Mass, spectroscopic evidence for the

effect of 71

Methods and apparatus, experimental. . 27

Methyl alcohol —

Dysprosium chloride in 47

Solution of neodymium bromide. . 77

Solution of neodymium chloride. . 78

Solution of uranous chloride 82

Uranous chloride in 45

Uranyl chloride in 44

Uranyl nitrate in 44

Naphthalene, spectrum of 7

Neodymium —

Acetate in acetone 40

Acetate in formamide 40

Bromide in methyl alcohol 79

Bromide in water and methyl alcohol 77

Chloride as a methyl alcoholate .... 35
Chloride, bromide and nitrate in

water. 77

Chloride in butyl alcohol 36

Chloride in ethyl acetate and

anthracene 56

Chloride in isobutyl alcohol 37

Chloride in isoprophyl alcohol 36

Chloride in methyl alcohol 78

Chloride in propyl alcohol 36

Chloride in water 35, 37

Chloride in water and ethyl alcohol 77

Nitrate in acetone 39

Nitrate in butyl alcohol 38

Nitrate in ethyl ester 39

Nitrate in isobutyl alcohol 39

Nitrate in isopropyl alcohol 38

Nitrate in propyl alcohol 38

Salts, absorption of solutions of

certain 34

Salts in acid solutions 84

Spectra, summary of 40

PAGE

Nitric acid —

Action on uranous acetate 65

Oxidation of uranous salts by 64

Nitric acid solution of uranyl nitrate. . . 86

Organic compound —

Absorption spectra 8

Emission spectra of 7

Oxidation —

Of uranous bromide by hydrogen

peroxide 61

Of uranous chloride by hydrogen

peroxide 58

Of uranous chloride in hydrochloric

acid by hydrogen peroxide 60

Of uranous salts by nitric acid. ... 64

Of uranous sulphate 61

Of uranous to uranyl salts in solution 58

Phosphorescence of salts 54

Plates, description of 101

Potassium nickel chromate 33

Properties of aggregates 95

Propyl alcohol —

Neodymium chloride in 36

Neodymium nitrate in 38

Solution of uranyl nitrate 81

Uranous chloride in 44

Uranyl chloride in 43

Uranyl nitrate in 44

Reactions, chemical, spectrophotogra-

phy of 52

Reduction —

Of uranyl chloride in methyl ester 69

Of uranyl salts in solution 69

Selective, of urany] aggregates .... 70
Results, summary and general discus-
sion of 87

Samarium —

Absorption spectrum of 48

Chloride in methyl and ethyl alcohols 49

Chloride in water 48

Chloride in water and ethyl alcohol 50
Nitrate in water 49

Schumann waves 6

Selective reduction of uranyl aggregates 70

Solvates —

Effect of temperature on the rela-
tive intensity of solvate bands. . 74
Selective action of chemical reagents
on 66

Solvation 92

Solvent, influence of 15

Spark spectra, carriers of 5

Spectra- —

Carriers of canal-ray 4

Carriers of spark 5

Emission, of organic compounds. . . 7
Mapping of 87

Spectrophotography of chemical reac-
tions 51

Spectroscopic evidence for the effect of

mass 71

Stark's theory 23

112

INDEX.

PAGE

Strengths of acids, possible method of

measuring 62

Summary —

And general discussion of results. . 87
Of neodymium spectra 40

Sulphuric acid solution —

Of uranous sulphate 85

Of uranyl sulphate 86

Temperature, effect of —

On absorption spectra 73, 98

On aggregates 83

On relative intensity of solvate
bands 74

Theory—

Of absorption spectra 89

Of dynamic isomerism 18

Unit of absorption S

Uranium —

Absorption spectrum of solutions of

certain salts of 43

Spectra, absorption centers of 45

Uranous —

Acetate, effect of the addition of

nitric acid 65

And uranyl bands 57. 94

Bromide, oxidation by hydrogen

peroxide 61

Chloride in acetone 82

Chloride in isobutyl alcohol 45

Chloride in methyl ester 45

Chloride in propyl alcohol 44

PAGE

Uranous chloride in water and methyl

alcohol 82

Chloride oxidation by hydrogen

peroxide 58

Chloride oxidized by hydrogen

peroxide 60

Oxidized to uranyl salts in solution 58
Salts, oxidation by nitric acid. ... 64

Sulphate in sulphuric acid 85

Sulphate, oxidation of 61

Uranyl —

Aggregates, selective reduction of. . 70

And uranous bands 57, 94

( 'hloride and nitrate in methyl ester 81

( hloride in acetone 81

Chloride in butyl alcohol 43

Chloride in ether 43

Chloride in ethyl ester 44

Chloride in formamide 44

Chloride in hydrochloric acid 86

Chloride in isobutyl alcohol 43, 81

( hloride in isopropyl alcohol 43

Chloride in methyl ester 44

Chloride in methyl ester, reduction of 69

Chloride in propyl alcohol 43

Nil rate in acetone 44

Nitrate in isobutyl alcohol 81

Nitrate in methyl ester 44, 81

Nitrate in nitric acid 86

Nitrate in propyl alcohol 44, 81

Salts in solution, reduction of 69

Salts, reduction in solution 69

Sulphate in sulphuric acid 86

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PLATE 28

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