STUDIES IN LUMINESCENCE

BY

EDWARD L. NICHOLS and ERNEST MERRITT

Professors of Physics in Cornell University

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

1912

CARNEGIE INSTITUTION OK WASHINGTON
Publication No. 15.2

■ *

PRESS OF GIBSON BROTHERS
WASHINGTON, D. C.

PREFACE.

The series of investigations described in this memoir was begun in 1903.

The authors believed that by the use of quantitative methods and par-
ticularly by the application of the spectrophotometer to the study of the
spectra of fluorescent and phosphorescent substances, something of definite
value might be added to the existing information concerning luminescence.

Spectroscopy, whether visual or photographic, is a method of high
precision where applied to line spectra, but in the case of the broad bands
of the spectra of fluorescent solids and liquids it affords but little informa-
tion beyond the approximate width and general location of the bands. The
spectrophotometer, on the other hand, enables the observer to determine
the distribution of intensities throughout the emission bands and the coeffi-
cients of absorption for the various wave-lengths of the corresponding
absorption bands. From the curves expressing the results of such measure-
ments, moreover, it is possible to locate with considerable accuracy the
crests of the bands. One may thus attain some detailed knowledge of the
laws of the radiation of luminescence, compare luminescence with the radi-
ation due to temperature, and obtain a basis for theoretical discussion.

A number of important portions of the work described in this volume have
been carried out at our suggestion by Doctors Frances G. Wick, C. A.
Pierce, Percy Hodge, and C. W. Waggoner, and by Messrs. H. E. Howe
and Carl Zeller. To these investigators and also to Prof. W. R. Orndorff,
who has repeatedly aided us by undertaking the preparation of fluorescent
compounds and by suggestions concerning the chemical aspects of the
problem, we desire to express our indebtedness.

The recent exhaustive, thorough, and discriminating review of the very
large literature relating to luminescence published by Professor Kayser
(H. Kayser, Handbuch der Spektroskopie, Bd. iv, Kap. v and vi) in his
Handbook of Spectroscopy, makes any extended bibliographic or historic
treatment here unnecessary and we have therefore given only such refer-
ences to previous researches as bear directly upon the subjects under
consideration.

The subject-matter contained in the several chapters appeared from
time to time, as each portion of the work reached completion, in a series
of papers in the Physical Review. In gathering this material together in a
single treatise we have recast and rearranged it, but have preserved the
original form of presentation in so far as it was found to be consistent with
our views after the completion of the work.

Grants which were received from the Carnegie Institution of Washington
in 1905, 1909, and 1 9 10 have greatly facilitated the prosecution of the experi-
ments described in this memoir and of others now in progress and have
furthered the preparation for investigations which it is proposed to under-
take in the near future.

Physics Laboratory of Cornell University,

May 23, iqii.

in

TABLE OF CONTENTS.

PAGE

Preface in

Chapter.

I. A Spectrophotometry Study of Fluorescence i

Rhodamin and resorcin-blau 12

Quinine sulphate 13

Chlorophyll 17

Canary glass 19

Fluorite 20

^Esculin 23

Summary 24

II. On the Absorbing Power and the Fluorescence of Resorufin. . . 25

Historical 26

Method of observation 27

Composition of resorufin 27

Absorption and thickness 28

Absorption and concentration 32

Fluorescence and concentration 33

Summary 38

III. The Luminescence of Sidot Blende 41

Luminescence excited by Roentgen rays 41

Photo-luminescence during excitation 42

Failure of Stokes's law 45

The phosphorescence spectrum during decay 45

IV. The Decay of Phosphorescence in Sidot Blende and certain other

SUBSTANCES 51

Karly stages in the decay of phosphorescence in Sidot blende 53

The study of phosphorescence in Sidot blende and certain other sub-
stances during the whole period of decay 58

Experimental method 58

Experiments with Sidot blende 59

Experiments with different phosphorescent substances 67

Summary 69

V. The Influence of the Red and Infra-Red Rays upon the Photo-
luminescence of Sidot Blende 71

Effect of the longer waves before excitation 71

Influence of the longer waves during excitation 73

Effect of the longer waves during decay 76

Variations of the effect with the length of the longer waves 83

VI. Variations in the Decay of Phosphorescence Produced by Heatino . . 85

Experiments with Sidot blende 85

Experiments with Balmain's paint 96

VII. Studies of Phosphorescence of Short Duration 109

Dr. Waggoner's studies in phosphorescence of short duration 109

Methods of measurement 109

Methods of preparing the phosphorescent compounds 110

Discussion of results 114

Effect of infra-red on the initial decay of Sidot blende 117

Decay curves for different wave-lengths 118

Spectrophotometry study of the cadmium-sodium compounds. ... 119

Summary 121

The experiments of Mr. Carl Zeller 121

The aniline dyes 122

The manganese-chloride group NaCl-MnCL 122

The cadmium group 124

Substances of slow decay 124

v

VI CONTENTS.

Chapter. page

VIII. Photographic Studies of Luminescence 125

The distribution of energy in the fluorescence spectrum and the

phosphorescence spectrum of Sidot blende 125

Phosphorescence at room temperature 128

Discussion of method 129

The phosphorescence spectrum during decay and the question of

two overlapping bands 131

Experimental 132

A photographic test of the effect of infra-red rays 134

The effect of temperature on the fluorescence spectrum 135

Discussion of results 136

IX. A Spectrophotometry Study of Certain Cases of Kathodo-Lumi-

nescence 137

Willemite 1 40

Sidot blende 141

Dependence of kathodo-luminescence upon current and discharge

potential 143

Conclusions 1 44

X. On the Electrical Properties of Fluorescent Solutions and Vapors 147

The influence of fluorescence upon electrical conductivity 147

Photo-active cells with fluorescent electrolytes (Dr. Hodge's experi-
ments) 152

Mr. Howe's experiments on fluorescent anthracene vapor 163

Apparatus 164

Observations and results 165

Conclusions 166

XI. On Fluorescence Absorption 167

XII. The Distribution of Energy in Fluorescence Spectra 175

Determination of the distribution of energy in the spectrum of the

comparison flame 175

Comparison of the fluorescence spectra with the spectrum of the

standard acetylene flame 179

The correction for slit width 182

The correction for absorption 1 85

The energy curves of fluorescence 186

XIII. The Specific Exciting Power of the Different Wave-lengths of

the Visible Spectrum in the Case of the Fluorescence of

eosin and resorufin 1 87

Coefficients of absorption 1 89

Computation of specific exciting power 191

XIV. The Theory of Wiedemann and Schmidt 195

XV. The Phenomena of Phosphorescence Considered from the Stand-
point of the Dissociation Theory 201

Summary of experimental laws 201

The decay of phosphorescence under simple conditions 202

Absorption effects 205

Influence of irregularities in distribution of the active material 208

Diffusion effects 211

Influence of ionic grouping 212

Hysteresis, temperature effects, etc., explained by ionic grouping. . 218

Summary 222

Index -?-'4

STUDIES IN LUMINESCENCE

BY

EDWARD L. NICHOLS and ERNEST MERR1TT

Professors of Physics in Cornell University

CHAPTER I.

A SPECTROPHOTOMETRY STUDY OF FLUORESCENCE.1

The law expressed by Stokes2 in his memoir entitled "The Change
of Refrangibility of Light," to the effect that in fluorescence the fluorescent
light is always of greater wave-length than the exciting light, has been called
in question by Lommel, who pointed out that for certain fluorescent bodies
there is an unmistakable overlapping of the regions in the spectrum occupied
by the exciting light and by the fluorescence which it produces. Lommel
made the further very important statement that for this class of substances
the character and composition of the fluorescence spectrum are independent
of the wave-length of the exciting light. Lommel's results, in so far as
they had to do with the non-validity of Stokes's law, were confirmed by
Hagenbach.3 A few years later Lubarsch4 published measurements in
confirmation of Stokes's law. A later paper by Lommel,5 in which he
described the fluorescence of the so-called chameleon colors, led Hagenbach
to new experiments, in the course of which he discovered what he believed
to be a source of error in his former measurements, and he reaffirmed the
law of Stokes for all such substances. In 1877, Brauner6 obtained results
in confirmation of Lommel's view. In 1879, Lubarsch7 published further
experiments on fluorescence, this time in favor of Lommel's results. Laman-
sky's in 1879 described measurements in confirmation of Stokes's law. In
a still later paper Hagenbach9 returned to the defense of Stokes's law as
against Lommel1" and Lubarsch,11 who in the meantime had published
further articles dealing with his objections and criticizing Lamansky's
method. Wesendonck1'2 in 1885 made observations with the sun's spectrum,
using two concave mirrors and a prism, in the course of which he obtained
conclusive evidence that the fluorescence of naphthalin-roth extended to
wave-lengths shorter than that of the exciting light. In 1886, Stenger13
took the question up at length. He found that whether he used Hagen-
bach's method of illuminating the free surface, Lommel's method of grazing
incidence through the side of a flask, or Lubarsch's fluorescent eye-piece,
his measurements confirmed Lommel as to the invalidity of Stokes's law
but not as to the independence of the fluorescent spectrum from the char-
acter of the exciting light. He also made experiments in collaboration with
Hagenbach, who was finally converted to the same view.

It is our purpose in this chapter to describe results obtained by the
application of the spectrophotometer to the measurement of the fluo-
rescence spectrum of those substances concerning the fluorescence of

'Physical Review, xvm, p. 403, and xix, p. 18.

:Stokes, Phil. Trans., p. 463, 1852.

3Hagenbaeh, Poggendorff's Ann., 146, pp. 65, 232, 373, 508.

4Lubarsch, Poggendorff's Ann., 153, p. 428, 1874.

'Lommel, Poggendorff's Ann., 159, p. 514, 1876.

6Brauner, Wiener Anzeiger, 19, p. 178, 1877.

'Lubarsch, Wied. Ann., 6, p. 248.

8Lamansky, Comptes Rendus, 88, p. 1192, 1879.

'Hagenbach, Wied. Ann., 18, p. 45.

10Lommel, Wied. Ann., 8, p. 244, 1879.

"Lubarsch, Wied. Ann., 9, p. 665, 1980; also Wied. Ann., 11, p. 68, 1880.

I2Wesendonck, Wied. Ann., 26, p. 521, 1885.

I3Stenger, Wied. Ann., 28, p. 201.

STUDIES IN LUMINESCENCE.

which, especially with reference to the validity of Stokes's law, the long-
continued discussion already described arose. But few attempts have
been made to apply the spectrophotometer to the study of fluorescence;
yet it is obviously possible to determine both the limits and the maximum
of a spectral region for which a curve of intensities can be plotted with
far greater accuracy than by the method hitherto pursued by all observers,
i. e., that of attempting to set the cross-hair in the eye-piece of a spectro-
scope in the region of greatest brightness, or at the point where the spectrum
ceases to be visible. Experimenters have perhaps been deterred from the
use of the spectrophotometer because of the faintnessof fluorescence spectra.
It is true that the fluorescent light from many substances is so weak as to
preclude all measurements of its spectrum; but it is also true, as we have
found in the course of the experiments to be described, that settings can
be made in cases where the brightness of the spectrum is far below that
necessary to arouse the sense of color and where the presence of light can
be detected only after prolonged shielding of the eye. The use of the

Fig. i.

cross-hair in such cases is out of the question, for the field is much too dim
to render it visible, while every attempt to illuminate it from the side would
flood the eye-piece with light sufficient to quench that under observation.

The instrument used in most of our observations was the spectropho-
tometer of Lummer and Brodhun. In many of our earlier measurements
the ocular lenses in the eye-piece were used, the eye being focused upon the
aperture in the eye-piece and not upon the face of the prism. By means of
metal screens attached to the collimator slits of the instrument the length of
slit was regulated so as to avoid overlapping of the spectral images and to
give two contiguous spectra in the field of view. One loses in this way the
advantage of the method of contrast, but the brightness of the field is
greatly increased. Later on the contrast field was employed because it was
found to be less fatiguing to the eye and on the whole more accurate. Since
in most cases it was desired to employ monochromatic light for the excita-
tion of fluorescence, the spectrophotometer was employed in connection
with a large spectrometer, as shown in Fig. i . The eye-piece and slit of the

A SPECTROPHOTOMETRY STUDY OF FLUORESCENCE.

EXCITING

LIGHT *-■

FLUORESCENT

SOLUTION

spectrometer were removed, and in place of the latter a Nernst filament (N)
was mounted vertically. This filament afforded a nearly linear source of
light, giving a sufficiently powerful continuous spectrum. The filament was
attached to the arm carrying the collimator tube so as to move with the
latter and to remain in the vertical plane passing through its axis. The
observing telescope was clamped, and different portions of the spectrum
were brought into the field as desired by movements of the collimator tube.
The liquid, the fluorescence of which was to be studied, was placed in a rec-
tangular cell (C), upon one face of which a real image of the spectrum was
focused. This face of the cell was provided with a metal screen having a
vertical opening i mm. wide, through which the light used for exciting fluo-
rescence could pass. This opening (see Fig. 2) was so placed that the exciting
light entering the cell would be parallel to the adjacent face of the cell and as
near to the same as practicable. In adjusting the metal screen one edge
of the slit or opening was made to exactly cover the glass forming this face
of the cell, as shown in the figure. The fluorescence spectrum was observed
from a direction at right angles
to the path of the exciting beam, £

and in order to bring the brightest
fluorescence regions of the liquid
into the field the vertical plane of
the collimator of the spectropho-
tometer was adjusted so as to bring
into the field of view the layer of
liquid lying next to the face of the
cell through which the exciting
light entered. By this arrange-
ment the width of the opening
through which the light entered
the cell and the width of the slit
of the spectrophotometer each
being 1 mm., the average depth
of liquid within which fluores-
cence was produced was approximately 0.55 mm. and the average distance
which the fluorescence light passed through the liquid before leaving the
cell was also 0.55 mm.

The source, A, of the comparison spectrum (Fig. 1) was an acetylene
flame, the light from which was reflected diffusely from the face of the
block of magnesium carbonate (M) mounted at an angle of 450 at the
end of the collimator slit. Measurements were made by varying the width
of the slit until the two regions of the spectrum under observation were
equally bright.

The substances specified by Iyommel as belonging to his first class, in
which it is possible to excite fluorescence by means of light of wave-length
longer than that of a portion of the fluorescence spectrum, and in which
the distribution of intensities in the fluorescence spectrum is independent
of the character of the exciting light, are naphthalin-roth, eosin, and
chlorophyll. To this list Stenger added the substance fluorescein. The last-
named substance, on account of its intense fluorescence and the location

2£

GLASS

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tu

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eo I

uj a
o-1

3

Fig. 2.

4 STUDIES IN LUMINESCENCE.

of the fluorescent band in the middle of the spectrum, in the regions of the
highest luminosity, was selected for detailed study for the purpose of test-
ing the conclusions reached by Lommel and the other investigators men-
tioned in the opening paragraphs of this chapter.

Ten cubic centimeters of alcohol were saturated with fluorescein at room
temperature and the solution was filtered. To 40 c. c. of distilled water
one drop of a normal solution of sodium carbonate (Na2C03) was added.
Two parts of the concentrated alcoholic solution were then mixed with 100
parts of the water thus rendered alkaline. The fluorescence spectrum of
this solution, excited by the undispersed rays of the acetylene flame, was
first measured, the distribution of intensities being compared, as in nearly
all our subsequent experiments, with the spectrum of the light diffusely
reflected from the surface of the block of magnesium carbonate shown in
Fig. 1. The absorption spectrum of the solution was then taken, the trans-
mission through the cell, which had a thickness of 1.1 cm., being meas-
ured by means of the spectrophotometer. The source of the transmitted
light was a second similar block of magnesium carbonate illuminated by
the same acetylene flame that served for the comparison spectrum.

Much stress having been laid by some of the previous observers upon
the influence of stray light, the following measurements were made. The
cell was filled with distilled water, set up precisely in the position in which
it had been placed in the study of the fluorescence spectrum, and similarly
illuminated by means of the acetylene flame. No measurable stray light
was found, but an exceedingly weak fluorescence spectrum due to the glass
walls of the cell was detected. Since the maximum of the fluorescence
spectrum of the glass was found to lie further to the violet than the fluores-
cence spectrum of the fluorescein, and since moreover it was of scarcely
measurable intensity, it was not deemed necessary to take further cog-
nizance of these sources of error.

To determine the fluorescence spectrum of the solution when excited
by monochromatic light, a mercury arc-light of the Lummer pattern was
used for excitation. It was found that the violet lines from the spectrum of
this arc produced fluorescence corresponding, as regards the position of the
maximum and the general form, with the curve previously obtained by
means of the light of the acetylene flame, but that the green line (X = 0.575)
excited no fluorescence. This latter result was to be expected, since this
line lies altogether outside of the absorption band of the solution in a region
for which almost complete transparency exists. The spectrum of the acety-
lene flame was subsequently tried as an exciting source, but it was too weak
to give easily measurable intensities of fluorescence. The slit of the spec-
trometer was then removed and a Nernst filament, .V, was mounted in the
axis of the collimator tube as shown in Fig. 1 . This filament was found to
be of abundant brilliancy. It was maintained at constant brightness by
means of a variable resistance, which was adjusted whenever the fluctua-
tions of an ammeter placed in the electric circuit indicated it to be necessary.

Measurements of the fluorescence spectrum of the solution were made
by means of the spectrum of the Nernst filament, using as exciting light
three nearly monochromatic regions of wave-length, X= 0.5 18 /1 to 0.536 n,
X= 0.487 ix to 0.502,11 and X= 0.460^ to 0.471 /i. The curves thus obtained

A SPECTROPHOTOMETRY STUDY OF FLUORESCENCE-

are plotted in Fig. 3, together with the curve of transmission for the solu-
tion. It will be seen from this figure that the maximum of intensity of the
fluorescence spectrum in these three cases lies in the same region, at 0.5 1 7 /x,
and that there is no evidence of any shifting of the fluorescence spectrum
with the wave-length of the exciting light. It is obvious, moreover, that
not only is it possible in the case of this solution to obtain fluorescence of
refrangibility greater than that of the exciting light, but that in the case
of the curve marked A the maximum of the fluorescence spectrum is of
shorter wave-length than the shortest wave-length used in excitation.
These curves likewise agree fully in character, and as regards the position
of their maximum, with that for the fluorescence spectrum of the same
solution when excited by the un-
dispersed light of the acetylene
flame. (Not shown in the figure.)
These curves for the fluorescence
spectrum do not correspond pre-
cisely with the typical curve, mean-
ing by that term the curve repre-
senting the distribution of intensi-
ties in the fluorescence spectrum
of the surface layer of the fluores-
cing liquid. It is possible, however,
in the case of a non-turbid medium,
to compute from the observed
curve the approximate form of the
typical curve. Fig. 4 shows graph-
ically the result of such a compu-
tation.1 Curve A gives the trans-
mission of a glass cell containing
a layer 1.1 cm. in thickness of the
fluorescein solution. The dotted
curve of similar form gives the
transmission corrected for losses
in the cell when filled with distilled
water. From this, by the well-
known law of variation of absorption

with the thickness, the curve D is found for a layer 0.055 cm- m thickness,
which is the estimated mean distance through the solution which the
fluorescent light passes before entering the slit of the spectrophotometer.
The curve B is the observed curve of the distribution of the intensities in
the fluorescence spectrum, and from this was computed the curve E, which
represents, as nearly as the accuracy of the data will allow, the typical curve
for this substance, corrected for absorption. It will be seen that in the
case of this solution, under the conditions of the measurements, the absorp-
tion of the fluorescent light by the solution produces only a slight shift of
the maximum toward the longer wave-lengths. When the fluorescent light
passes through a considerable layer of the solution the effect of absorption
is much more marked and there is a decided change of color. When a thin

'In this figure and in Figs. 5. 6, 7, and 8 the scale of wave lengths has been doubled.

00

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— D

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Tx \ A

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Fig. 3. — Fluorescein.

.^i

Fluorescence spectra obtained when the exciting light
lies in different regions of the spectrum. For
curve .1, the exciting light was confined to the
region marked .1 on the horizontal axis, etc.
Vertical scales arbitrary.

studies in luminescence;.

layer of the solution of fluorescein is viewed by reflected light its color is
green, whereas the fluorescence of a mass of the liquid appears decidedly
yellowish. This change is shown graphically in the curves of Fig. 5. In

: D

ir,n

/
/

/

1 ^

1 /
1 /

A

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60 80 .500/' 20 40 60

Fig. 4. — Fluorescein.

Typical fluorescence spectrum (EE).

this figure A is the transmission curve of the solution; B is the observed
curve of fluorescence when the slit through which the excited light enters the
cell is placed so that the fluorescent light passes through 0.055 cm- °f the

A

/\B

i

/

sC

/

b/

c/

*A

WAV F.

LENG1

"H

.4 50/^ 60 70

80 90 .500/' 10 20 30

Fig. 5. — Fluorescein.

-10 50 60 70

IvMect of absorption upon the fluorescence spectrum.

solution before exit ; and C is the fluorescence curve when the slit is shifted
to such a position that the fluorescent light passes through i cm. of the solu-

A SPECTROPHOTOMETRY .STUDY OF FLUORESCENCE.

tion. It will be seen that the maximum is shifted from 0.516 fx to 0.522 n,
and that while the two curves are nearly coincident on the side toward the
red the values on the other side of the curve fall off very rapidly as the
result of the increased absorption. The boundary of the fluorescence spec-
trum toward the violet in the one case would lie at about wave-length 0.505 11,
whereas in the thinner layer it would be visible to at least 0.490 y.. It is
obvious that the color of the fluorescence in the latter case will contain a
great excess of green.

The effect of diluting the fluorescent solutionis similar to that of diminish-
ing the distance through which the light passes. The results of observa-
tion upon a solution of fluorescein, diluted until the intensity of the fluores-
cence spectrum was diminished as far as would permit of satisfactory
readings, are shown in Fig. 6. The curve A A represents the transmission
of the cell filled with the dilute solution and BB is the distribution curve
of its fluorescence spectrum. The dotted lines show the corresponding

0

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—

1

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1
1
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1

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80

60

40

20

450." 60

70

80 90 .500" 10

20

30 40

50 60

70

Fig. 6. — Fluorescein.

Effect of dilution upon the fluorescence spectrum.

transmission curve and a portion of the fluorescence curve for the solution
before dilution. It will be noted that in this case, as in the case of the com-
parison of thick and thin layers of a given solution, the curves are coin-
cident toward the red, but that the dilute solution has its maximum shifted
toward the green, also that the ordinates on this side of the curve show
an increase indicative of the change of color, which, as is well known, is
always observed as the result of diluting the fluorescent solutions of this
substance.

Although, as has been shown in Figs. 4 and 5, the modifications pro-
duced by absorption in the curves of the fluorescence spectrum may be
very marked, the effect of absorption diminishes rapidly with dilution of
the solution. For example, if we apply the correction for absorption to
the curve for the dilute solution in Fig. 6 we find, as is indicated in Fig. 7,
that the change is insignificant. In this figure C is the observed curve for

8

STUDIES IN LUMINESCENCE.

the fluorescence of the dilute solution, A the transmission curve of a layer
i.i cm. in thickness, and B the computed curve for the transmission of the
mean layer of liquid through which the fluorescent light has to pass. The
correction for this absorption is indicated by means of the dotted line.

It having been established that the fluorescence of bodies of this class is
independent, as regards the distribution of intensities, of the wave-length
of the exciting source, it would be of interest to inquire whether the fluo-
rescent energy for a given wave-length of the fluorescence spectrum varies
with the wave-length of the exciting light when the energy of the latter
is constant, or whether it depends only upon the energy. The rigorous
determination of these relations involves a knowledge of the distribution of
energy in the spectrum of the exciting source, a difficult matter to determine
with accuracy for the shorter wave-lengths of the visible spectrum. The
curve which is shown in Fig. 8 may, however, be of some interest in this

i

B

\/^

A

\^_>/

«— ■"""

i '/
'/ /
1 /

'/ /
// /

i
i

ii

i

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i
i

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'/ I
i 1

i /

'/ /
'/ /
'/ /
'//

C

100

80

60

40

20

CO

80

40

.500/' 20

Fig. 7. — Fluorescein.

Typical fluorescence spectrum for dilute solution.

connection. It represents the intensity of fluorescence, taken at the maxi-
mum of the fluorescence spectrum of the solution of fluorescein, as a func-
tion of the wave-length of the exciting light. Curve A was taken with
the Nernst filament as a source, curve B with the acetylene flame. The
dotted line shows the absorption band for a layer of the solution 1.1 cm.
thick. It will be seen that fluorescence begins approximately at the wave-
length at which the solution begins to absorb, and that the maximum lies
well within the absorption band but is shifted to the red. The longer wave-
lengths within the band are more effective on account of their greater energy.
The difference in the form of the curves .1 and B is probably ascribable to
the different distributions of energy in the spectra of the sources of light
employed. The important questions of the actual distribution of energy
in fluorescence spectra and of the relative effectiveness of equal quantities
of energy when the wave-length is varied, in producing excitation, are con-
sidered at length in Chapters XII and XIII of this memoir.

A SPECTROPHOTOMETRY STUDY OF FLUORESCENCE.

In addition to the measurements on fluorescein the fluorescence spectra
of solutions of eosin and naphthalin-roth were also studied by the method
already described, and the transmission curves of the solutions were taken.

30

60

■to

20

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/ V

i

i
i

i
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\/

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/\ ^

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.430// 40 50 60 TO 80 90 .500," 10

Fig. 8. — Fluorescein.

20

30

40

Intensity of fluorescence as a function of wave-length of exciting light. Ordi-
nate's give the intensity of luminescence and abscissas the wave-length of the
exciting light. A refers to Nernst glower as source, B to acetylene flame.

Dilute solutions in alcohol were made, that of the naphthalin-roth being
about , rj0 saturated. The results obtained with these solutions, which are
shown in Figs. 9 and 10, afford additional corroboration of the statements

IUU

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80

1 \

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D

1 A

/ /V \

/ / T \

/ /A

60

/ /' \
/ / ' \
/ / " \

XI

/ / ' V v

' / ' \ \
/ ' \ \

40

j 1 l/\ \ \

Jl A r\ \ \

1

20

/
/
1

•B

C

/B

I

t

— r~

M--

|

1 f

III

1

.5/1

.6/4

Fig. 9. — Eosin.

Fluorescence spectra observed when the exciting light lies in different
regions of the spectrum. Curve .4 was obtained when the excit-
ing light was confined to the region marked .1 on the axis of wave-
lengths, etc. Vertical scales arbitrary.

IO

STUDIES IN LUMINESCENCE.

of Lommel. They are indeed in every respect analogous to those obtained
with fluorescein and lead to the same conclusions. Each solution was
excited by three distinct regions of the spectrum, one lying as far toward the
red as practicable, one toward the blue, and one at an intermediate wave-
length. Curves showing the distribution of intensities in the three spectra
thus produced are shown in the figures and it will be noted that, as in the
corresponding curves for fluorescein, the position of the maximum is entirely
independent of the wave-length of the exciting light and that the general
character of the curve remains unchanged. In the case of these two solu-
tions, as in that of fluorescein, it was possible to obtain a measurable amount
of fluorescence by the use of light of wave-length greater than that of the
maximum of the fluorescence spectrum. The form of fluorescence curve is
very similar for these three substances, but each has its own place in the

J

100

s'

D

80

/

/'

/

l\

/
/

/

60

\ V

\ /

f /

40

y
»\

20

_A

9 /

/ \

/

^C

'B

A

I

I

— -'

''\ R

1

/

5/'

Fig. io. — Naphthalin-roth.

Fluorescence spectra observed when the exciting light lies in different regions
of the spectrum. Curve A was obtained when the exciting light was
confined to the region marked A, etc. Vertical scales arbitrary.

spectrum. The maximum for fluorescein (Fig. 3) is at 0.5 1 7 fi, that for eosin
at 0.580JU, and that for naphthalin-roth at 0.594 /x. The position of the max-
imum of these three curves with reference to the absorption band appears
to vary with the different substances. The maximum for eosin coincides
approximately with the infra edge of the absorption band; that for fluo-
rescein lies slightly (about 0.05 /j.) toward the violet, while the maximum for
naphthalin-roth is much farther displaced toward the short wave-lengths.

Lommel's contention that it is possible to excite fluorescence in eosin
by means of the light of the sodium flame is fully confirmed by the data
plotted in Fig. 9, from which it will be seen that the exciting light by means
of which curve A was obtained had a mean wave-length almost precisely
equal to that of the sodium lines. Since it was found possible by means of
light, all of which was of greater wave-length than the maximum of the

A SPECTROPHOTOMETRIC STUDY OF FLUORESCENCE- II

fluorescence spectrum, to produce fluorescence of sufficient strength for
measurement with the spectrophotometer, it follows that observable fluo-
rescence can be produced by light of even greater wave-length than that
recorded in our diagram.

We deem the evidence already given in the foregoing paragraphs to be
conclusive, so far as these substances are concerned; but in view of the
differences of opinion among physicists as regards the validity of Stokes's
law we venture to add the following description of a determination of the
wave-length of the least refrangible monochromatic light which we found
capable of exciting fluorescence in the three solutions with which this
chapter deals.

To insure freedom from the existence of possible errors due to stray
light, two different methods were employed of avoiding it. In the first
the exciting light, before dispersion, was passed through a solution of the
substance to be examined, thus filtering out those rays particularly active
in producing fluorescence. The filtered light was then dispersed by means
of the large spectroscope already described, and a second solution was
subjected to an isolated, nearly monochromatic, region of the spectrum.
The wave-length of this region was increased until the last trace of fluo-
rescence, observed directly with the eye,

was about to disappear. To distinguish Table i.

between fluorescence and the presence
of light diffused from small particles,
the light was viewed at an angle of qo°
through a Nicol prism, by which means
diffuse light was completely excluded.
The limit of excitation thus determined
lay, as had been anticipated, farther to
the red than in the cases where a spectro-
photometrically measurable fluorescence

had been obtained, excepting in the case of eosin, where it was found to
coincide almost exactly with the ultra edge of the band used in exciting
the spectrum shown in curve A (Fig. 9).

The second method consisted in sending the isolated region of the
dispersed light to be used for excitation through a second spectroscope and
determining as before the limit of excitability. This method of removing
stray light has been extensively used and is well known to be effectual.
The source of light in both cases was an electric arc.

The results obtained by these two methods were identical and are shown
in Table 1.

These wave-lengths, all of which lie far to the red from the maximum
of the fluorescence spectrum, are indicated in Figs. 3, 8, and 9 by means
of the vertical lines marked /.

The results obtained with solutions of fluorescein, eosin, and naphthalin-
roth thus fully confirm the contention of Lommel1 that in the case of these
substances Stokes's law is not fulfilled. In the opinion of Lommel all other
fluorescent substances, with the probable exception of chlorophyll, conform
to Stokes's law, the shortest wave-length of the fluorescence spectrum being

Substance.

Maximum

wave-length

exciting

observable
fluorescence.

Fluorescein

Eosin

Naphthalin-roth

O.542
O.589
O.632

'Lommel, Poggendorff's Ann., 143, 159, and 160.

12

STUDIES IN LUMINESCENCE.

always of less refrangibility than the longest wave-length capable of exciting
fluorescence. We later extended our measurements to various other fluo-
rescent substances, determining in each case the location and character of
the absorption band with which fluorescence is associated and the form of
the curve of relative intensities in the fluorescence spectrum.

RHODAMIN AND RESORCIN-BEAU.

Of the substances examined rhodamin and resorcin-blau belong to the
same group as those previously examined, being organic dye-stuffs which
show fluorescence in solution. The fluorescence of these solutions was of
sufficient intensity to permit of the use of the method already described.
Two nearly monochromatic regions of the spectrum were selected in each
case, one lying well within the absorption band of the solution and the other
as far toward the red as it was possible to go without reducing fluorescence
to an extent which rendered measurements inexact. The fluorescence
spectrum was observed as before by means of a Lummer-Brodhun spectro-

100

80

60

40

20

,''-' T

A / i i I '

1 \ '
\ '

1/
7/

A

I
\

/ 1 \

1

\
1
1

\\

\

V
\

J j o\

t
\

\

\

[ ' ^^A '

\l

•

/

/
1

>B

n

/

/

B W

Fit]

.611

1 1.-

-Rhodamin.

Curve .1. Fluorescence spectrum when excited by green li«ht in region marked A
Curve B. Fluorescence spectrum when excited by blue light in region marked B.
Curve T. Transmission spectrum. Layer i.i cm. thick.

photometer from a direction at right angles to that of the exciting ray. The
solution was placed in a glass cell with plane faces, the exciting light being
introduced through a slit and passing through the layer of liquid lying next
to the wall of the cell through which the fluorescent light passed.

Figures 1 1 and 1 2 give in graphical form the results of these measure-
ments. The curves of the fluorescence spectra are of the same type as those
of fluorescein, eosin, and naphthalin-roth already cited. They rise from
within the region of the absorption band to a well-defined maximum, which
is located, generally speaking, near the infra edge of the absorption band;
beyond the maximum they fall away rapidly with increasing wave-length.
The comparison spectrum in these, as in the previous cases, was that ob-
tained from the light diffusely reflected by a block of magnesium carbonate
illuminated by an acetylene flame.

A SPECTROPHOTOMETRY STUDY OF FLUORIC SCENCE.

13

Neither of these solutions conforms even approximately with the law of
Stokes. Curve A of Fig. 12 (resorcin-blau) was obtained by means of light
none of which was of shorter wave-length than 0.642 fx, whereas the fluo-
rescence is of measurable intensity to 600 n. The rhodamin solution affords
a striking example of the non-validity of the law; for although the maximum
of the fluorescent spectrum lies at 0.554 /x, an observable fluorescence was
produced by means of very nearly monochromatic light, obtained by the
method of double dispersion, using two spectrometers in series, the shortest
wave-length of which was 0.602 yu. This limiting wave-length is indicated
by the line marked / in Fig. 11. This substance, like eosin, which has its
maximum of fluorescence at 0.580 /jl, will respond to the excitation of light
of the D lines of sodium or to even longer waves if of sufficient intensity.

The other substances to be considered in this chapter are more difficult
subjects for quantitative work because, being less strongly fluorescent, it

1

1

!

A

C

N

X

\
\

1 1 r\

' / /B\

\

\

I // ]

• 11 \ \
1 1 1 \
1 1 \

V

\
\

' 7 /

* i

/ r

•

1 \

\ !

*\ /

>.

ss\r

1

1

•-

~«.„J

— • — "* 1

1

100

PO

60

40

20

.6/i

.6/2 B A ,7 /A

Fig. 12. — Resorcin-blau in methyl alcohol.

Curve A. Fluorescence spectrum when excited by red light lying in region marked A.
Curve B. Fluorescence spectrum when excited by orange light in region marked B.
Curve C. Transmission spectrum of layer i.i cm. thick.

is necessary to have recourse to powerful illumination, and because in
several instances the fluorescence band lies near the limits of the visible
spectrum— either in the violet or in the extreme red.

QUININE SULPHATE.

The quinine sulphate subjected to measurement wyas prepared by making
a saturated solution in water containing a trace of sulphuric acid. The
solution was exposed to diffuse daylight in a cubical cell of glass, the faces
of which measured 8 cm., and the fluorescence was observed through a slit
in one of the faces at right angles to that exposed to the light. This slit was
placed as near the corner of the cell as possible, so as to bring the layer of
liquid lying next the glass on the exposed side into the field of view. The

14 STUDIES IN LUMINESCENCE.

reference spectrum, as usual, was obtained from the light diffusely reflected
from a block of magnesium carbonate, but in this case the surface of the
block was illuminated by daylight, so that the fluctuations of intensity
might affect both spectra equally and in the same sense.

The transmission of the solution was determined by placing a second
block of magnesium carbonate behind the cell and determining the intensity
of the light transmitted by the liquid in the various regions as compared
with that reflected from the first block. The instrument used in this and
in some of the subsequent measurements described in this chapter was a
spectroscope whose collimator was furnished with a double Vierordt slit.
Light for the comparison spectrum was introduced by means of a right-
angled prism and the adjustment of intensities was made in the usual
manner by means of micrometer screws.

A second set of measurements with this substance was made with the
Lummer-Brodhun spectrophotometer. A mercury-arc lamp was used in
place of the Nernst filament of our previous experiments and the ultra-
violet line 0.365 ju, isolated by dispersion with the large spectrometer, was
employed for excitation. The comparison spectrum in this case was the
magnesium block illuminated by means of the acetylene flame.

It is obvious that owing to the different distribution of intensities in the
spectra of daylight and of the acetylene flame these two sets of observations
would not be strictly comparable. In order, however, to make them at
least approximately so, the following correction was applied to the obser-
vations by daylight. From certain data obtained by Vogel1 and published
by him many years ago, a curve was plotted giving the distribution of
intensities in the spectrum of diffuse daylight with clouded sky, the spectrun
of a petroleum-gas flame being taken as the standard. Vogel 's measure-
ments agree as well as could be expected, considering the fluctuating char-
acter of daylight, with those subsequently made by W. H. Pickering2 and
by Nichols and Franklin.3 As the sky was overcast at the time of making
the observations on quinine sulphate just described, it was assumed that
the quality of daylight would on that occasion be represented with sufficient
accuracy by means of this curve. By means of a similar curve, giving the
distribution of intensities in the spectrum of the acetylene flame as com-
pared with the petroleum flame, it is possible to determine the relation
between daylight and the acetylene flame. Vogel's curve for daylight-
petroleum and the computed curve for daylight-acetylene are given4 in Fig. 13.
By means of the latter it is possible, from the observations upon quinine
sulphate, to compute a curve in which the light of the acetylene flame
reflected from magnesium carbonate is the standard. This curve (B, Fig.
14) corresponds precisely, as regards the location of its maximum, with
curve A in the same figure, which shows the results of the measurement of
the fluorescence of this substance when excited by means of the ultra-violet
of the mercury lamp. Since these two sets were made with different types
of spectrophotometer their agreement affords a desirable check upon the
performance of the two instruments.

■Vogel, Berliner Monatsberichte, p. 801, 1880.

2W. H. Pickering, Proc. Am. Acad, of Arts and Sciences, vol. 25, 1880.
3Nichols and Franklin, American Journal of Science, vol. 38, p. 100, 1889.

♦The two sources being so adjusted as to have the same intensity at the D line, the curve shows the ratio
of intensities for other wave-lengths.

A SPECTROPHOTOMETRY STUDY OF FLUORESCENCE.

15

*V-

20

■ ■'

'

Day Light
Petroleum

Voge

)

10

1 \

\ 1

9

1

Day Ligh-T
Acetylehe

\ fv

XI

.6//

.all

Fig. 13-
Curves for daylight-petroleum and daylight-acetylene

/\

/
/

/

r 1

\ \

1 i r

i !'

it \

\

\

1 1

1 1

\

4U

'

1 1

\l \b

\

/

h k

j

\y

V A

1

.4/1

.5/1

Fig. 14. — Quinine sulphate in water.

Curve .4 . Fluorescence spectrum when excited by Hg line, 0.3650 p.
Curve B. Fluorescence spectrum when excited by daylight.
Curve T. Transmission spectrum. Layer 8 cm. thick.

1 6 STUDIES IN LUMINESCENCE.

The curve T gives the transmission of the solution as measured by
the method already described. It was not found possible, owing to
the fact that this solution remains transparent almost to the limits of
the visible spectrum, to go very far into the absorption band; but the
observations suffice to locate the infra edge of the band very closely. To
gain further information concerning the nature of the absorption band a
series of photographs of the transmission spectrum were taken by means of
a Rowland grating with sunlight as the source of illumination. The infra
edge of the band, as shown by the disappearance of theFraunhofer lines, was
found to lie in the region indicated by the spectrometric measurements
shown in Fig. 14. Absorption became almost complete in the neighborhood
of the 77 lines and continued throughout the ultra-violet, at least up to the
point where glass becomes opaque.

In order to determine if possible the position of the ultra edge of the band,
the quinine solution was placed in a cell with quartz walls and photographs
were taken, using an arc light into which zinc had been introduced as a
source. Lines due to this metal were distinguishable in the comparison
spectrum as far as wave-length 0.2558 /x, to which point the opacity of the
solution of quinine sulphate was found to be complete.

It will be seen from the curves A and B (Fig. 14) that the fluorescence
spectrum of quinine sulphate is of the same type as that of the various
fluorescent dye-stuffs. It consists of a single band with a sharply defined
maximum at 0.437 fi. Since the curves obtained with daylight and with
monochromatic light from the mercury arc are identical as regards the
position of the maximum and in general form, and since they are of the
same type as the various other fluorescence spectra already described, it is
evident that quinine sulphate belongs to the same class as fluorescein, eosin,
etc. It is true that in the case of this substance it is possible to trace the
fluorescent light throughout nearly the entire spectrum, a point which is to
be considered further in a subsequent paragraph, but beyond 0.5/1 intensities
are very small.

Contrary to the view expressed by Lommel, the fluorescence of quinine
sulphate appears to be independent of the wave-length of the exciting
light. Since, however, this is one of the substances which has been cited
in support of Stokes's law, it was deemed important to determine as care-
fully as possible the longest wave-length of monochromatic light capable of
exciting fluorescence. This is a matter of considerable difficulty on account
of the weakness of the effect. By means of the method of double dispersion
already described, the electric arc being used as a source, monochromatic
light thoroughly free from all stray radiation was obtained. When this
was used for excitation the last trace of observable fluorescence was found
to disappear at wave-length 0.420 yu.

In no other case have we found this limiting wave-length to lie so close
to the ultra edge of the fluorescence band. It will be noted, however, that
in making measurements by daylight readings were obtained at wave-
length 0.41 n and the fluorescence under strong excitation is traceable still
further into the violet, certainly almost, if not quite, to 0.40 /z. While
quinine sulphate then approaches more nearly to conformity with Stokes's
law than the other substances that we have studied, the evidence is dis-

A SPECTROPHOTOMETRY STUDY OP FLUORESCENCE.

17

tinctly in favor of the view that it, like other fluorescent materials, is
capable of being excited by all wave-lengths of light lying in or on the
infra edge of its absorption band ; and that the longest wave-length capable
of producing an observable fluorescence is appreciably less refrangible than
the shorter wave-lengths of its fluorescence spectrum.

CHLOROPHYLL.

The fluorescence spectrum of chlorophyll was studied by Stokes,1 who
found, in addition to the usual red band, a fainter excitation in the green
of the spectrum, Hagenbach2 subsequently made an exhaustive study of
this substance. It was his opinion that it did not conform to the law of
Stokes. Lommel3 placed chlorophyll in his first class, to which belong all
substances whose fluorescence is independent of the wave-length of the
exciting light.

The solution of chlorophyll used in our measurements was made by
digesting green leaves in absolute alcohol and filtering. The transmission
curve (Fig. 15) shows four well-defined bands, of which the one to which

t
1

1

-— ^

*

1
1
1

1
1

1
1

1 1
1 1

.' \

/
1
1

1

I \

1

1
1
1

1

1 f

1
1
1
1

j

\a

1

1
1

\

\

\
\

1
1
1

1

\J

I
1
1
1

_ /*

n

1

/

1
/

\.

1

1
1

1
I

A

~ -'' \ fx"'

A(H£. Arc)

\
\

.„--

1

1

80

60

40

20

.5/X

Fig- 15-

.6/J .7/1

-Chlorophyll (fresh alcoholic solution from green leaves).

Curve A . Fluorescence spectrum when excited by Hg arc.
Curve T. Transmission spectrum of a layer i.i cm. thick.

fluorescence is due is more intense and broader than the others. Measure-
ments made in the extreme red appeared to indicate the presence of still
another region of diminished transparency which it was not possible to map
with the spectrophotometer. In the hope of determining more definitely
the character of this band and of ascertaining whether the series of striking
absorption bands in the visible spectrum extends into the infra-red, we
requested Mr. W. W. Coblentz, who was engaged in the study of the infra-
red by means of a radiometer and a mirror spectrometer with rock-salt

'Stokes, Philos. Trans., 1853.

2Hagenbach, Poggendorff's Ann., 141, p. 245.

3I,ommel, /. c.

18

STUDIES IN LUMINESCENCE.

prism, to explore the spectrum of this solution. The results of his measure-
ments are plotted in Fig. 16. His observations extend from 0.61 ^ to 1 .45 /i,
at which wave-length the alcohol in which the chlorophyll was dissolved
becomes so nearly opaque as to prevent further readings. The absorption
band at 0.675 M is clearly shown; also the band at the extreme edge of the
visible red 0.745 M- The latter, however, is of little intensity. In Mr.
Coblentz's measurements comparison was made between a cell filled with
the chlorophyll solution and the same cell when filled with alcohol in which
no chlorophyll had been dissolved. The curve therefore indicates the
effect of the chlorophyll and other dissolved matters upon the transparency
of the alcohol. It will be noted that between 0.8 /x and 0.9 /x the material
in solution has no absorbing power. At the latter wave-length absorption
again begins to show itself and increases steadily to 1.45 /x, where the trans-
mitting power of the solution is only 60 per cent as great as that of the
alcohol itself.

r^J

r

80

\

i\

60

! x

|

40

|

\

|

20

\

J

!

v

1

\J

Au

.811

1.2/(

1.0 /i

Fig. 16.

Transmission of chlorophyll in the infra-red. (Measurements by W. W. Coblentz.)

In determining the fluorescence curve A (Fig. 15), which was obtained
by using the green line of the mercury arc (0.546 /j.) for excitation, we were
able to trace the fluorescence to wave-length 0.624 M» which lies well beyond
the ultra side of the absorption band. This had not been found possible in
the case of the fluorescence spectra previously described, partly on account
of the greater width of the bands and partly for the following reason. The
fluorescence spectrum of chlorophyll is doubtless traceable to an unusual
distance toward the violet because the maximum lies in the extreme red,
in a region of very low luminosity. Fluorescence in this region, to be
appreciable, must be of great intensity, and on the side toward the violet
the rapidly falling intensity is largely counterbalanced by increase in
luminosity. The same is true of the infra side of the fluorescence band
of substances like quinine sulphate; so that, although the curves are of
the same type as those described above — fluorescein, etc. — the fluorescence

A SPECTROPHOTOMETRY STUDY OP FLUORESCENCE.

19

can be followed nearly through the spectrum. The curves of resorcin-blau,
fluorite, and sesculin afford other examples of this phenomenon. Where, on
the other hand, the maximum is in the middle of the spectrum, as in the case
of rhodamin, eosin, fluorescein, etc., the fluorescence soon becomes too small
for measurement on both sides of the maximum because of the diminution
of luminosity toward the ends of the visible spectrum.

The green fluorescence described by Stokes was faintly discernible under
strong illumination, but its intensity would not permit of measurements.

The question of the applicability of Stokes's law to this solution was
tested by observing with monochromatic light, by the method already de-
scribed, the longest wave-length which was capable of producing appreciable
fluorescence. The limiting wave-length (0.720 //), which gave faint but
unmistakable fluorescence (see line /, Fig. 15), lies on the infra side of the
maximum of the curve A.

100

80

60

40

20

T___

/\

1

\

i\

/•

//J

\a

V

/

1

^»

/

/

/
1

N^E

1

1

1

1

A

/

\

v .

/

I

B

Fig. 17.-

.5/i

-Canary glass.

.6/1

Curve .4. Fluorescence spectrum when excited by violet light, X =0.407 p.

Curve B. Fluorescence spectrum when the exciting light lay in the green, in the region marked D.

Curve T. Transmission spectrum for a layer 7 cm. thick.

CANARY GLASS.

In our experiments upon canary glass one corner of a rectangular slab
1 cm. in thickness and 7 cm. wide was used. This was mounted in front
of the slit of the Lummer-Brodhun spectrophotometer in place of and in a
position corresponding to the cell employed in the study of the fluorescent
solutions. Fluorescence was excited in the manner already described, by
means of light entering the glass at right angles to the axis of the collimator
tube. Measurements were made using the Hg line 0.407 n (see curve A,
Fig. 17) and green light from the spectrum of the Nernst filament (curve B).

22

V

A SPECTROPHOTOMBTRIC L'DY OF FLUORESCENCE.

23

sharp! the white J orite. The broadening of the band

upon this side may result in a sligh hifting of the fluorescence. It has
already been shown by £ 1 ». Knoblauch- that fluorescent

substances such I naphtha n-roth have the position of the fluo-

trum shifted by dis them in different liquids; and that

theshift 1 Is with that of th< (sorption band previously described

by Kundt.3

This sub ' typical of his second class,

the characterise which an cence spectrum independent of

the wave-length of th< 1 an absorption band beginning

.it th.it point in th< rum at wh first trace- of fluorescence shows

W

T

\

• ■

N.

fin hor se chestnut).

Curv. I

trn when li^ht.

1 111 thick.

itsib and extending ird the viole so that there is no overlapping of

the baud oi' the 1 xciting light with tin- uor< jcence spectrum.

Our measurements of the fluoi of aesculin were made upon a

solution consisting of water in whiebfreshly broken twigs of the horse-
chestnut tree had been immersed. W n freshly prepared this shows the
well-known fluorescence characterise ulin, but the solution loses

its activity upon standing. We w< ble to procure pure aesculin, but

the results. -.,» far .i- the form and a position of the fluorescence curve
are concerned, would probably la- the ime had measurements been made
upon the solution of the chemically pr>ared substance. The fluorescence
curve, Fig. 21, is of the usual form ad its position with reference to the

vr. Wic<l Ann . j8, p. 201.
oblaucb, Wied. Ann . s4. p. 193.

Kundt. Wied. Ann.. 4, p. 34, 1878.
I.ommel, PoggendorfT's Ann., 143, p. 38.

22

STUDIES IN LUMINESCENCE.

.4/1

.5U

Fig. 19. — White fluorite.

Curve A. Fluorescence spectrum when excited by the carbon arc.
Curve B. Fluorescence spectrum when excited by Hg arc.
Curve T. Transmission spectrum of a layer 0.8 cm. thick.

100

eo

to

40

?0

A

/ \
/ \

I

RO

J----

'"

p.n

\
\

s r
/ 1

t 1

A \

\

\

/

/

\ \

40

\\

\ \
\\

20

\\b

| J

\

J

/ ' /i

-n_ T

•

/' / \ I
jl 1 11

hi \\

k^^

N
N
V

s
\
\ ■

4 \\

' r I \ \
' 1 h 1 \

/ I 1 r \ \

- y

/''

■^

\y

'11 w

II \ \

1 /

N.

^J_A.

!

^~-*—_ ^B

.<IJ

.5 ft

Fig. 20. — Greeu fluorite.

Curve A. Fluorescence spectrum when excited by diffuse daylight.

Curve B. The same repeated on another day.

Curve T. Transmission spectrum of a layer 1.72 cm. thick.

A SPECTROPHOTOMETRY STUDY OF FEUORESCENCK.

^3

sharply as is the case of the white fiuorite. The broadening of the band
upon this side may result in a slight shifting of the fluorescence. It has
already been shown by Stenger1 and by O. Knoblauch2 that fluorescent
substances such as eosin and naphthalin-roth have the position of the fluo-
rescent spectrum shifted by dissolving them in different liquids; and that
the shift corresponds with that of the absorption band previously described
by Kundt.3

^SCULIN.

This substance was cited by L,ommel4 as typical of his second class,
the characteristics of which are a fluorescence spectrum independent of
the wave-length of the exciting source and an absorption band beginning
at that point in the spectrum at which the first trace of fluorescence shows

uu

r\ i j

T

80

\ /

-A/

A

* o

/ i

/ 4
/ \

i
u

I

\a

fi

fi

.5

M

Fig. 2i. — -■Esculin (from horse chestnut).

Curve A. Fluorescence spectrum when excited by arc light.
Curve T. Transmission spectrum of a layer 8 cm. thick.

itself and extending toward the violet, so that there is no overlapping of
the band of the exciting light with the fluorescence spectrum.

Our measurements of the fluorescence of sesculin were made upon a
solution consisting of water in which freshly broken twigs of the horse-
chestnut tree had been immersed. When freshly prepared this shows the
well-known fluorescence characteristic of aesculin, but the solution loses
its activity upon standing. We were unable to procure pure aesculin, but
the results, so far as the form and composition of the fluorescence curve
are concerned, would probably be the same had measurements been made
upon the solution of the chemically prepared substance. The fluorescence
curve, Fig. 21, is of the usual form and its position with reference to the

■Stenger, Wied. Ann., 28, p. 201.

jO. Knoblauch, Wied. Ann., 54, p. 193.

3Kundt, Wied. Ann., 4, p. 34, 1878.
*Lommel, Poggendorff's Ann., 143, p. 38.

24 STUDIES IN LUMINESCENCE.

transmission curve T is similar to that of the substances previously examined.
On the infra side, as has been pointed out by Lommel, it is possible to
trace the fluorescence throughout the entire spectrum ; but this, as in the
instances already considered, is due to the greater luminosity as we move
toward the middle of the spectrum rather than to any unusual maintenance
of the intensity of the fluorescence in that direction. The band falls off
quite sharply on that side, differing not at all in this respect from the bands
of other substances, such as quinine sulphate and fluorite. The measurement
of the transmission showed a very dilute solution, in which the maximum
of absorption is scarcely more marked than in the case of the bands of the
specimens of fluorite already described.

From the location of the absorption band with reference to the maximum
of fluorescence it would be safe to predict for this substance, on the basis
of previous experience, a wide divergence from the law of Stokes. Deter-
mination of the limiting value of the exciting light is somewhat difficult
to make. It was found, using the spectrum of the arc lamp as a source,
that the fluorescence due to light of wave-length 0.45 /x could still be
discerned.

It is not possible, with the spectrophotometer, to make a complete
exploration of the absorption band of aesculin, since the band extends into
the ultra-violet. Photographs of the absorption spectrum of this solution,
made by Miss Eleanor Burns at our suggestion, show this band to be com-
paratively narrow, its ultra edge lying at about 0.34 n, beyond which there
is no absorption capable of detection by photographic means.

There is nothing in these measurements which would lead one to place
sesculin in a class different from that of the foregoing substances. It is
distinguished, as regards the character of its fluorescence spectrum, from
quinine sulphate and fluorite, the other two substances on our list the fluo-
rescence of which lies in the blue, only in that the maximum lies at a some-
what greater wave-length. The curve A, Fig. 21, was obtained by the use
of the direct light of the arc, undispersed. Since the curve corresponded
in every form to the general type it did not seem necessary to repeat the
observations with exciting light of other composition.

SUMMARY.

From the measurements described in this chapter the following conclu-
sions may be drawn :

1. Eosin, naphthalin-roth, fluorescein, rhodamin, resorcin-blau, quinine
sulphate, chlorophyll, canary glass, green fluorspar, white fluorspar, and
sesculin all exhibit fluorescence of the same type.

2. The fluorescence spectrum in general consists of a single band situated
near the infra edge of the absorption band with which fluorescence is
associated.

3. The position of the maximum of the fluorescence band is in all cases
independent of the wave-length or composition of the exciting light.

4. The distribution of intensities in the fluorescence spectrum is inde-
pendent of the wave-length of the exciting light.

5. Stokes's law holds for none of the numerous fluorescent substances
thus far examined.

CHAPTER II.

ON THE ABSORBING POWER AND THE FLUORESCENCE OF

RESORUFIN.

The experiments described in this chapter were undertaken at the
suggestion of the authors by Miss Frances G. Wick. An account of them
was first given in the Physical Review.1 The chief object was to determine
whether the typical fluorescence spectrum changes with the concentration
of the solution or whether the shift of the band referred to in Chapter I is
caused by absorption only. The substance studied was resorufin, whose
strong fluorescence in the yellow-red is conveniently excited and readily
observed.

It has frequently been observed that the color of the light emitted by a
fluorescent solution is altered by a change in the concentration of the
solution. A dilute solution of fluorescein, as indicated in Chapter I, gives
a green fluorescence, while the light emitted by a concentrated solution of
the same substance shows a distinctly yellow tinge. In other words, an
increase in concentration causes the maximum of the fluorescence spectrum
to shift toward the longer wave-lengths.

Such a shift might result from a real change in the period of vibration of
the fluorescent molecules, i. e., the form of the typical fluorescence spectrum
might depend upon the concentration of the solution. But the observed
change in the fluorescence spectrum might equally well result from a change
in the absorbing power of the solution. The light emitted by portions of
the active substance in the interior of the solution must pass through the
solution before emerging, and is therefore weakened by absorption.
Since the maximum of the fluorescence spectrum does not occur at the
same wave-length as the maximum absorption, being always slightly dis-
placed in the direction of the longer waves, it is clear that different portions
of the fluorescence spectrum will be affected by absorption in varying
degree; the shorter waves will always be absorbed most strongly. The
maximum of the fluorescence spectrum is therefore always shifted toward
the red to some extent, and the increased absorbing power of concentrated
solutions causes the shift due to this cause to increase with the concentration.

The experimental work naturally falls under three heads, as follows:

i. The relation between absorption and thickness, to test whether or
not the absorption of a fluorescent solution obeys the exponential law of
optically perfect substances.

2. The relation between absorption and concentration, to test the
application of Beer's2 law, i. e., that an increase in the concentration of a
solution is equivalent to an increase in thickness.

3. The study of fluorescence spectra of solutions of resorufin of different
concentrations to determine whether or not there is any shift in the maxi-
mum of fluorescence as concentration changes.

'Frances G. Wick, Physical Review, xxiv, p. 356.
2Beer, PoggendorfF's Ann., 86, p. 78.

26 STUDIES IN LUMINESCENCE.

HISTORICAL.

In 1888 B.Walter,1 using the Vierordt spectrophotometer, found the ratio
of the fluorescent light emitted to total incident light absorbed, Fl/ A, to
increase with dilution. This law had been previously conjectured by
Lommel,2 whose mathematical theory Walter3 used in verifying his own
results. In connection with this work Walter tested Beer's law of absorp-
tion for fluorescein by four measurements. He found the law to hold true
for dilute but not for concentrated solutions.

Walter briefly states his conclusions as follows:

"Ability to excite fluorescence, Fl' A, in the most concentrated solutions,
is infinitely small or zero. After Fl/A obtains a measurable value it
increases in proportion to dilution to a certain dilution called the 'critical
point.' For greater dilutions Fl/A remains constant."

In explanation of the above, Walter4 advances a molecular group theory
which is of interest, preceding as it does the present theory of ionization
given by Buckingham.5 Walter summarizes his theory in the following
statements :

1. Every separate molecule of a given substance in solution absorbs,
so long as it remains in the separate condition, the same fraction of the light
falling upon it, no matter how great its distance from other molecules in the
solution may be.

2. Every separate molecule of a given fluorescent substance in solution,
so long as it is in the separate condition, changes the same fraction of
absorbed light into fluorescent light, no matter how great its distance from
neighboring molecules may be.

3. As soon as molecules begin to combine in groups the validity of these
statements ceases. Fluorescence entirely ceases in such a group and
absorption extends over wave-lengths which a separate molecule is not able
to absorb.

Walter's idea of the separate molecule seems to correspond closely with
what is now known as the ion.

In 1894 E. Buckingham0 performed a series of careful experiments to
discover whether or not fluorescent substances are in a state of ionization.
His investigations led to the conclusion that, at least in some cases, fluo-
rescence is due to ionization and produced only by that part of a solution
which is ionized. Certain ions, along with their other well-known optical
properties, possess this property of fluorescence.

The conclusions drawn from the spectrophotometric work described in
Chapter I are found to apply also to resorufm. For example:

1. The characteristic fluorescence band is situated near the edge of the
absorption band and is steeper on the side toward the violet.

2. As fluorescent light passes through greater thicknesses of liquid, the
position of the maximum in the fluorescence spectrum is shifted toward
the red.

>B. Walter. Wied. Ann., 34, 1888. 6E. Buckingham, Zeitschrift fiir physikaliscbe
-Lommel, Poggendorff's Ann., 160, p. 76, 1877. Chemie, 14, p. 129, 1894.

3B. Walter, Wied. Ann., 36, p. 502, 1889. 6E. Buckingham, /. c.
«B. Walter, Wied. Ann., 36, p. 518, 1889.

ABSORBING POWER AND FLUORESCENCE OF RESORUFIN.

27

R.C30rx>f(r».

3. The effect of diluting a fluorescent solution is similar to that of
diminishing the distance through which the fluorescent light travels.

METHOD OF OBSERVATION.

The instrument used for this work was the spectrophotometer of Lummer
and Brodhun.

The source of light was an acetylene flame 6" (Fig. 22) from which light
was diffusely reflected by a block of magnesium carbonate, n, mounted at an
angle at 45 ° with axis of the collimator A . For observations on transmission
a similar block of magnesium carbonate was placed in front of the slit D.

The work was performed in a room with black walls, the acetylene flame
being the only source of light. Screens were adjusted so that no stray light
could strike any part of the instrument.

The solution under consideration was placed in front of slit D, the width
of which was kept constant during a
single set of observations. The ocular
lenses of the telescope were removed,
the eye being focused upon the face
of the prism magnified by the objec-
tive of the telescope T.

In making observations the width
of slit C was adjusted by means of a
micrometer screw until the central
band of light reflected from KL was
brought into the best possible coinci- 1 — ytlT

dence with that transmitted from D
through the upper and lower parts of
the cube. The results given represent
an average of from 5 to 10 settings,
the zero point of the screw attached to
C being carefully determined and care
being taken to avoid errors due to lost
motion.

The hydrogen lines of a vacuum Fig

tube, together with a few of the Fraun-
hofer lines, were used for calibration.

22. — Apparatus arranged for
measuring fluorescence.

COMPOSITION OF RESORUFIN.

Weselski's diazo-resorufin,1 the fluorescent substance used in this investi-
gation,2 has the formula

I '

c o c

„ : 1 p .
I '_ I

■Berichte der deutschen chetnischen Gesellschaft, 22, 3036.

2 Acknowledgment should be made to Prof. W. R. Orndorff for his kindness in preparing the resorufin used
in this work.

o=c

(J— OH

28

STUDIES IN LUMINESCENCE.

Dissolved in absolute alcohol it is bright red in color, with a sharply defined
fluorescence band extending from 0.54 p. to the limits of the visible spectrum
in the red. The transmission spectrum given in Table 2 and Fig. 23 shows
the absorption band corresponding to this, and also a smaller absorption
band in the violet.

Different concentrations of resorufin were used. These are merely rela-
tive and are indicated as "concentration §, J, |, /,;, J.,, t.}A, and ^os," the
original solution being taken as ] . All curves corresponding to the above
concentrations are indicated by the letters A,B,C, D, E, F, and G respectively.

Table 2.

Transmission through layer of resorufin 1.075 cm. thick. Concentration |. X = Wave-
length. 70 = Intensity of light before transmission. 1= Intensity of light after
transmission.

I

/

X

h

/

— = Transmission.

X

h

I — = Transmission.

0.384M

h

/.

45

6

0. 1 1 1

0.48^

45 -72

32.05 0

70

39

46

12

.261

•492

45-72

28.87

63

394

46

20.25

.438

.508

44-7

24.4

538

407

46

16

•347

.524

44-7

'7-57

394

413

46

13.62

•297

•542

46.27

1 1

241

42

46

9-75

•213

.562

46.41

7-52

162

429

46

9.95

.217

.589

46 . 2 5 1 1 . 64

253

438

45 5

12. 1

.266

.614

46.55

32.27

706

448

45-5

14.15

•32

.646

46.62

45.62

981

457

4505

22.27

•497

.69

44- 52

44

99

.468

4505

25.97

•579

.72

41.25

4"

99

ABSORPTION AND THICKNESS.

The first problem under consideration was, as stated above, to find
whether a fluorescent solution acts like an optically perfect substance in
obeying Lambert's law of absorption, i. e., whether

where To and / are the intensities before and after transmission, and x is
the thickness of the absorbing layer.

■■

—

0

c

/

r 1

c

/

i
a

/

1

c
c

/

0

p

\1

1.

,/

\.<

0

-v

Wovo l_«r.gtK ^

Fig. 23.

Transmission spectrum of layer 1.075 cm. thick. Concentration i.

ABSORBING POWER AND FLUORESCENCE OF RESORUFIN.

29

To test this law the coefficient of absorption, /3, was calculated from
observations taken with cells of different inside thickness, the measurements
of these cells being made with great care.

The larger cell was measured with micrometer calipers. Thickness of
glass = a, fi (Fig. 24). Outside thickness of that part of the cell used for
transmission = C. Inside thickness = T.

T = C— (a+jS) = 2.o5 cm.

In the case of small cell (Fig. 25), calipers could not be used to measure
the thickness of glass. The cell was placed under a traveling microscope
to measure the average inside thickness of the top. The outside thickness
at the points indicated in the figure was measured with calipers.

T=T'-\-(n — m) = i.oj5 cm.

It was assumed that the thickness of glass from top to bottom of cell re-
mained constant, since broken pieces of the same glass varied not more than
0.01 mm.

To determine the transmission, the cell containing the resorufin was
placed in front of slit D (Fig. 22), care being taken that no direct light
from S should strike the cell, and light from the block of magnesium car-

a

A

t—

c

—i

>

fr—T— »

S-- -

- -m

«. — T >

i —

rv -

— »

Fig. 24. Fig. 25.

bonate, in front of slit D (not shown in the figure), was transmitted through
it. The reading of C was taken to get the relative intensity, /, of the light
transmitted for different wave-lengths. After each observation the cell
containing resorufin was removed and an identical one filled with absolute
alcohol put in its place. The reading of C in this case gave the intensity of
the incident light before absorption, I0. From these observed values of 7~o
and /, together with the thickness of the absorbing layer, the coefficient
of absorption was computed. If the loss by reflection may be regarded as
the same for the cell containing resorufin as for that filled with alcohol — and
the difference must be extremely small — it will be seen that the influence of
reflection is to introduce a factor, 1 — R, in the case of both settings, and that
this factor will disappear when the ratio is taken.

As a check upon these results a different method of observation was
used. Keeping D (Fig. 22) constant, the reading C\ was taken for the
light alone. A reading of Ci was then taken with alcohol in front of C and
resorufin in front of D.

Q

— = Incident light (70)

C2
D

= Transmitted light (/)

D

G

J

c2

c2

/„

D

3°

STUDIES IN LUMINESCENCE.

£-'

As a further check the positions of alcohol and resorufin were reversed, the
alcohol being placed in front of D and the resorufin in front of C. Then

D =J J_ Co

C*2 /o Cl

It will be noticed that in this method also the disturbances due to reflec-
tion from the surfaces of the cells are eliminated.

From these data the value of the coefficient of absorption for each wave-
length was found to be practically the same as that found b)r the first

0.4

0.4// 0.5// 0.6//

x= points for cell 1.075 cm. tnick
• = 2.05 "

0.7//

Fig. 26.

Coefficient of absorption as computed from cells of different thickness.

method and independent of the thickness of the cell. Absorption curves
plotted from average values obtained by both methods, using cells of two
thicknesses, are identical, as shown in Fig. 26. l The absorption coefficients
for different concentrations and also the data from which the computation
was made are given in Table 3.

Table 3.

Coefficient of absorption computed from cells of different thickness. Concentration
= 1/16. 70 = Intensity of light before transmission. /= Intensity of light after
transmission. /3 = Coefficient of absorption.

X

Thickness of cell =

1.075 cm.

Thickness of cell =

= 2.05 cm.

7o

I

(3

la

/

(3

0.429M

79-9

35.8

O.7476

8l.8

19.9

O.6884

.433

78.8

48

4

•4539

80.O

2?

9

•5495

•443

80.2

53

6

•3746

82.4

33

7

•4363

•457

79-5

62

5

.2223

80.6

48

3

•2495

.468

81.6

66

1

. 1805-

80.4

54

5

. 1702-

.48

78.9

70

0

"752

82.3

61

3

. 12672

•492

76.1

64

8

•15

80.5

57

0

.1678

.508

76.5

57

5

■2653

79-7

43

1

•2993

■524

76.8

45

8

.4808

78.9

29

5

.4786

• 542

75.6

34

5

•7283

78.5

•7

3

.7369

.562

75-9

25

4

1 .022

78.7

10

0

1 .03042

.589

75-8

30

9

0.8354

79-4

18

9

0.6979

.614

75-1

63

4

•'579

79.8

54

0

. 1899

.646

73-6

72

2

.0169

80.9

75

8

.0316

.69

64.9

63.8

.0148

7'-3

69

2

.0144

'Figures 26, 27, and 28 give only relative values of coefficients, since they are drawn from data computed
upon the basis of ordinary instead of natural logarithms. To get from the curves the absolute value of
the coefficients given in the tables multiply by 2.3.

2These values are average results from a number of observations.

ABSORBING POWER AND FLUORESCENCE OF RESORUFIN.

31

It appears, therefore, that the absorption of resorufin is in accordance
with Lambert's law. Deviations from this law, if present, are at least too
small to be detected by the experimental methods used.

Me

Vx

y& 3/\6

Concentration
Fig. 27.

Curves showing effect of change in concentration upon coefficient of absorption1 of

wave-lengths marked on curves.

1.0

c
0

Q.

(0

O

C .5
'0

0

0

a>
•00

(0

I_

>

<

Me

Ye,

Cor

icer

i
nrai

ion

Vz

Fig. 28.
Average coefficients of absorption1 for different concentrations taken from Table 4.

■See footnote on page 30.

32

STUDIES IN LUMINESCENCE.

ABSORPTION AND CONCENTRATION.

The second set of experiments was undertaken to study the relation
between absorption and concentration and thus test the validity of Beer's
law for a fluorescent substance, i.e., "increasing the concentration of a
solution is equivalent to a like increase in thickness."

The same method of observation was employed for this as for the pre-
ceding work, no changes in the apparatus being necessary. The coefficient
of absorption was found for different concentrations, the solution being
placed in a cell 2.05 cm. thick if dilute, and in cell 1.075 cm- thick if con-
centrated. This change of cell is permissible, since, according to the results
obtained above, Lambert's law of absorption applies to resorufin. The
absorption coefficient for different wave-lengths was thus obtained for
seven concentrations.

Table 4.
Coefficient of absorption for corresponding wave-lengths in different concentrations.

0.413M

\

1 1

4 8

1

1 if

1

3J

1

1

T2¥

2.254

I. 88l4 1. 2132 O

7125 0

3107 O

'444

O.O276

.42

2.806

2.2793 1.4237

8533

4360

1984

■077?

.448

2-53

I.5364 O.7935

3565

i860

0897

.0368

•48

1.336

O.6934 O.3864

1734 !

0749

0586

.0367

.524

3. 162

I.8584 I. 1247

5349

2783

1453

.0692

. 562

3-473

3.0797 2.0I7I

989

5441

3036

.1442

.614
Average

0.3864

0.3082 O.I55O

0995

O484

0524

.0265

2.2782

i . 6624 I . 0 1 62 + 0

5313 0

2683 O

1417

O.O597

Figs. 27 and 28 give the results of these observations. It will be noticed
from these curves that the coefficient of absorption, in the case of dilute
solutions, increases in direct proportion to concentration. For concentrated
solutions this proportion fails, the concentration increasing more rapidly
than the absorption. This is true for all the wave-lengths; in every case
the curve starts out as a straight line for dilute solutions and bends down-
ward as higher concentrations are reached.

Table 5.

Coefficients of absorption computed from average of careful observations for
concentration ^ and average absorption concentration curve. (Fig. 28).

1 1

1

1

1

I

1

2 4

8

1 e

3*

64

1 28

0.524M

2.1986 I. 6047I

O . 984 I

0. 51313 0

2 599

0.12995

O.O65I

.542

3.266 2.385

1-455

O.76245

38398

.19251

.O9637

. 562

4.3608 3.175

1.946

I .0177

5154

.2 5783

.1288

.589

2.7324 1.995

1 .219

O.63733

3227

. I6146

.0807

.614

0 49335 0.36087

0.22157

0. 1 15322

O5842

.0292

.OI46

.646

0.95588 0.0698

0.0428

0.0223

01 13

.00575

.00283

ABSORBING POWER AND FLUORESCENCE OF RESORUFIN. 33

Fig. 28 shows the average coefficient of absorption for the seven wave-
lengths as a function of the concentration. This curve is used later for
finding the coefficient of absorption for different dilutions, especially care-
ful measurements for the concentration ,V, being taken as a basis of cal-
culation. The values thus obtained, given in Table 5, were used in all
subsequent computations involving the coefficient of absorption.

The above results agree, in every respect, with those given by Walter.1
Beer's law is true for dilute solutions, but fails for greater concentrations,
as is indicated by the deviation of the curve from a straight line. The
straight form of the curve (Fig. 28) up to concentration | corresponds to
what Walter calls "complete solution," in which he says the molecules are
in a "separate state." Concentration § may be regarded as his "critical
dilution," where a change seems to take place in the condition of the fluo-
rescent substance. Solutions more concentrated than § correspond to those
called by Walter "incomplete," in which "molecular groups" exist.

Interpreted according to the ionization theory, the curve (Fig. 28)
indicates that in dilutions less than £ a state of complete or nearly com-
plete ionization has been reached. At this point a change takes place,
more resorufin being contained in the solution than is ionized. As the
concentration increases, more and more of the solution remains undis-
sociated. It appears that the undissociated resorufin is not only incapable
of fluorescence, but is also much less effective in causing absorption than
is the dissociated substance.

FLUORESCENCE AND CONCENTRATION.

For observation of the fluorescence spectrum the spectrophotometer
was adjusted as before. In front of slit D (Fig. 22) was placed a glass cell
5.4 cm. long, containing the resorufin solution. This cell was entirely
covered with black paper, except for a space about 1.5 cm. high across the
bottom of the face next to the exciting lights', and two narrow strips, x and y,
The opening x was to allow light to enter the collimator slit, while y,
directly opposite, was used only for adjusting. The source of illumination
used to produce fluorescence was a bank of four acetylene flames S'.
Between these and the fluorescent solution was placed a glass cell filled
with water to prevent the heating of the resorufin. The comparison source
was another acetylene burner S, light from which was reflected into the
slit C by a block of magnesium carbonate n. The cell containing the
resorufin was so placed that light from the whole layer of solution next the
inside surface of glass came through the slit D of the collimator.

It is clear that a portion of the fluorescent light is absorbed by the solu-
tion before reaching D, and attention has already been called to the fact
that this absorption will be different for different parts of the fluorescence
spectrum. If the fluorescence is measured in the manner indicated above
it is therefore necessary to apply a correction for absorption before the
typical fluorescence spectrum can be determined. Two methods of making
this correction have been used. In the first method the necessary cor-
rection was computed as follows :

IB. Walter, Wied. Ann., 36, pp. 502 and 518, 1S89.

34

STUDIES IN LUMINESCENCE.

Let P be any point in the slit (Fig. 29). The source of the light reaching
P is a cone in the fluorescent liquid converging toward P. The angle of
this cone is determined by the aperture of the object glass, EF, of the
collimator. Any part of this cone cut off by the glass walls of the cell or
by the upper surface of the solution is supplied by total reflection.

Consider a disk bounded by two circular sections of this cone at distances
x and x+dx respectively from the apex P (Fig. 30). The amount of
fluorescent light reaching P from such a disk as this will be independent
of x, provided no absorption takes place, for, while the area of the section
varies as x\ the intensity of the light reaching P from each small portion
of the disk varies as 1 .vL>. The fluorescent light that would reach P from
such a disk if there were no absorption is therefore kdx, where k is a func-
tion of the wave-length, X.

Fig. 29.

Fig. 30.

vSince the light emitted by each section of the cone is in part absorbed,
we have for the light reaching P from one of the disks

di^kdx.e-f*

and the total lisbt reaching: P from the whole cone is

= k
Jo

txdx

where a is the thickness of the fluorescent solution.

and k— -
0 1-

Hence

1

k.1-

,-fia

While the i in this expression represents the light reaching one infini-
tesimal portion of the slit, it is clear that the total light reaching the slit
is proportional to i. It is also evident that k is proportional to the total
amount of light, /, of the wave-length considered, that is emitted by the
fluorescent substance per unit volume. Hence

kpl

J

,-/3a

The curve showing the relation between /and X is the "typical fluorescence
spectrum."

To investigate the effect of concentration upon the position of the
fluorescence band observations were made in six concentrations. Table 6
gives values of observed fluorescence before any correction is made for
absorption. Table 8, graphically represented in FAig. 31, gives the same
sets of observations so reduced in scale as to be comparable in intensity
with each other. This reduction was made by multiplying the data for
each concentration by such a factor as to give wave-length 0.589 /j. the
same value as that of the observed fluorescence of the corresponding con-
centration in Table 9.

ABSORBING POWER AND FLUORESCENCE OF RESORUFIN.

J 3

Table 6.

Observed fluorescence intensity before correction is made for absorption. Meas-
urements made according to Method I; iluorescence excitation extending over
whole face of cell.

X

1

4

1

8

2

1 B
2. I

1

3 2

1
<; 1

1

t 2 8

0.524M

1.77

2

2.08

2.4

.542

i

86

2. 1

2.2

2

2.13

2.41

. 562

3

46

4.67

5.61

7.66

7-43

7.27

.589

30

3

46.4

56.6

34-9

23.2

16.2

.614

79

5

95-5

95 3

34.0

17.63

11. 6

.646

86

6

76.3

66.3

'7-7

8.9

6.88

.69

59

3

42.4

46.2

12.7

7.28

Table 7.

Observed fluorescence intensity reduced to such a scale as to make intensity of
wave-length o. 589 n the same as observed Iluorescence in Table 9.

X

1
4'

1

8

1
1 n

A

1

1 28

0

524/x

5.38

4.08

3.19

3 -32

3.58

3.12

542

5.65

4.28

3-34

3

32

3.66

323

562

10.5

9 53

8.53

12

7

12.8

9-73

589

92.2

94.6

86.1

57

9

40.4

21.8

614

241 .6

195

145

56

5

30.3

15.6

646

263

'55

101

29

4

■5-3

9.22

69

180

86.5

70.3

21

0

12.5

Corrected Iluorescence.

Table 8.

Value of f calculated from Table 6 according to
formula/ = 7/3/(1— e-0«).

Table 9.
Intensity of fluorescence of different concentrations at wave-length 0.589 ft.

1 1

2 4

1

8

1
1 <>

1
3 2

1

B4

t
1 2R

Observed fluorescence . 75.5 92 . 1 7

Corrected fluorescence,
i. e., value of k as ob-
tained from formula. 206 184

94.64
115

86.06
54.8

57-95
22.5

40.0I
I I .O

21.68
4.86

STUDIES IN LUMINESCENCE.

The curves in Fig. 31 show the observed values of / as a function of \
for six different concentrations of the solution. The shift in the maximum
caused by a change in concentration is here well marked.

Table 8 gives the result of correcting values in Table 6 for absorption
according to the above formula, the value of /, which is proportional to
actual fluorescence intensity (see page 34), being found in each case. These

C 15

10

o

-o

4>

O

bA

c

/ /D

vK

// /e
///ft

^

0.70JJ

Observed fluorescence intensity, showing shift of maxi-
mum, with change in concentration.

055>J
Fig- 3-

0.60^1

0.65>J

1'luorescence curves corrected for
absorption.

values are plotted in Fig. 32 according to an arbitrary vertical scale which
makes the height of each curve approximately proportional to intensity
of fluorescence for the corresponding concentration. It will be observed
that these curves are similar in form, the maximum wave-length being
approximately the same, about 0.595 fi.

For purposes of comparison a series of measurements was made to
determine the relative intensity of different concentrations at wave-length
0.589 n, keeping all conditions constant. Table 9 shows the results. In
Fig. t,^ the curve marked O gives the actual observed fluorescence. The

200 ""* c

•50

100

32 ie

Fig. 33. — Concentration and fluorescence.
Intensity of observed and corrected fluorescence for different concentrations at wave length 0.589 n.

ABSORBING POWER AND FLUORESCENCE OF RESORUFIN.

37

too

" corrected" curve, C, has for ordinates the value of / computed from the
formula (page 34) and shows the relative intensity of actual fluorescence in
different concentrations.

The following method of obtaining the corrected fluorescence curves
was used as a check upon the first, the final results being obtained by a
graphical rather than a mathematical procedure.

The apparatus was set up as before, except that the light exciting fluo-
rescence, instead of striking the whole face of the glass, was allowed to
strike only a vertical section 1 mm. in width. The side of the cell next to
the exciting light S' was covered with a black paper screen having a vertical
opening 1 mm. wide, through which light was admitted. This opening
was set at different distances from the end of the cell and the fluorescence
intensity was measured for three or four different positions. In order
to make possible accurate measurements of the distance from the edge
of the glass, a scale was etched
upon the face of the cell just above 150
the level of the collimator slit

The effect of absorption upon
the position of the maximum of
the fluorescence spectrum is well
shown in Fig. 34, in which the
three curves were taken with the
opening at different distances
from the face of the cell.

In order to correct for absorp-
tion, curves similar to those
shown in Figs. 35 and 36 were
plotted for each solution. These
curves show the variation in the
intensity of the fluorescent light
with the thickness of the liquid
through which the light passes.
From the intercept of one of these
curves upon the y axis the intensity
of fluorescence can be found when
the thickness becomes zero. In

such a case there is no absorption. The error of extrapolation was reduced
to a minimum by taking one point very close to the edge of the glass.

The corrected values of fluorescence intensity thus obtained were used
in plotting the accompanying fluorescence curve in Fig. 36. The same
procedure was followed in the case of each solution.

The observations and results obtained by this graphical method are
given in Table 10. The corrected fluorescence spectra obtained in this
way are plotted together in Fig. 37, the vertical scale being such that
values for wave-length 0.589 n are the same as the values for the corre-
sponding concentration in Fig. 32.

A comparison of the curves of Figs. 32 and 37 shows that the results
obtained by the two methods are the same. The typical fluorescence
curves are all similar in form and the position of maximum fluorescence

Fig- 34-

Observed fluorescence produced by light from i mm.
section oassing through different thicknesses of solu-
tion. Thickness of D\ = i mm.; of Di=i cm.; of

Us =2 cm.

STUDIES IN LUMINESCENCE.

(about 0.595 n) remains constant for all dilutions. The shift in the observed
fluoresence maximum is therefore due entirely to absorption.

This fixed position of maximum fluorescence seems to be consistent
with the theory of ionization. According to Buckingham, ions may have
some of the same optical properties as other substances and fluorescence

C

ISO

100

50

0)

>

A

^^x

\pr-r

Fig. 35-

0.55/X 0.60/1

Fig. 36.

0.6 Sp

Variation in intensity of observed fluorescence
from 1 mm. section transmitted through
different thicknesses of solution.

Fluorescence corrected for absorption by
graphical method. Concentration = {.

15

10

is one of these properties. If only the ions fluoresce the position of maxi-
mum fluorescence would evidently remain constant whether the material
of the solution were all ionized or not.

This fixed position of maximum fluorescence is also consistent with the
theory suggested in Chapter I,1 that fluo-
rescence is caused by an unusual kind of
dissociation similar to that produced in a gas
by the action of Roentgen rays. That part
of the solution which, during the process of
change, produces fluorescence does not give
evidence of any effect due to material not
dissociated beyond that of absorption.

SUMMARY.

The results of the above investigation upon
resorufin, as a typical fluorescent substance,
taken in connection with the work of B.
Walter and with the experiments described
in Chapter I, seem to establish the truth of
the following statements:

1. Fluorescent solutions are optically
perfect substances,/, c, they obey Lambert's
law.

2. Beer's law, i. c, that increase of concentration is equivalent to
increase in thickness, is true for dilute but not for concentrated solutions.

3. A change in the concentration of a fluorescent solution has no effect
upon the typical fluorescence spectrum.

'Nichols and Merritt, Physical Review, xix, No. 1, 1904; xxir, No. 5, 1906.

B

I'-

ll D

ly f

0.55,/U 0.60JJ

Fig. 37-

0.6 5ju

Fluorescence curves corrected for ab-
sorption by graphical method. Ver-
tical scale same as for Fig. 32.

ABSORBING POWER AND FLUORESCENCE OF RESORUFIN.

Table io.

Observed fluorescence of section i mm. wide from which light is transmitted through
different thickness of solution. The column marked o thickness gives fluorescence
intensity corrected for absorption by graphical method, i. e., by extrapolation
from curves similar to those in Fig. 35.

X

Concentration -§.

Concentration ,j .

! . 5 mm .

5 mm.

1 cm.

0

. . 5 mm .

5 mm. 1 cm.

0

0.524/x

'•75

...

1-5

• 542

2. 52

1.6

..2

2

2.83

2.1 1.3

3

. 562

■4-3

3

1.95

12.5

9.07

4.1 2.32

.3.8

.589

66.2

41-5

29.6

114

7'-4

45.9 21.4

103

.614

94 3

70.7

6. .2

103

88.1

73.7 64.6

95

.646

42.4

42

42.5

43

5'-4

48-7 j 54-3

52.3

\

Concentration |.

Concentration ,'„.

1.5 mm.

5 mm.

1 cm.

0

1 mm.

1 cm.

2 cm.

0

0.524M

2.65

2

1 73

2.5

2.83

3

3-5

542

3 9

2

2.66

4

5.92

2.97

2. S3

8

562

34 15

12. 1

7-52

50

34.8

8.62

5.56

42

589

96

69.7

49-3

109

132

73-9

55 4

144

6

102

795 63.3

117

614

79 -72

68.4 56.5

90

121

1 10

99-6

124

646

34-7

33.6 31

36

67.3

72.7

60.3

70

X

Concentration ,;', .

Concentration , !iK.

1.5 mm.

5 mm.

1 cm.
3

0

1 .5 mm.

5 mm.

1 cm.

0

0 . 508/u

3-47

381

4

.524

4.83

3-75

3 75

4

4'7

4-47

3 45

4-5

•542

6.16

4-9

423

7

6.52 6.1

5 33

6.8

. 562

62

38.5

25-5

76

25.2

23-7

'7

26

.589

146

138

125

150

49.2

49-5

47-4

49-5

.614

1 10

105

96. 1

113

39-7

37-7

38.3

39-5

.646

58.1

50.2

3'-7

60

27.8

2?

27-5

28

CHAPTER III.

THE LUMINESCENCE OF SIDOT BLENDE.1

Among the substances whose luminescence is sufficiently bright for
spectrophotometric study, Sidot blende or phosphorescent zinc sulphide
seems especially well suited to bring out the relationships that doubtless
exist between different types of luminescence ; for not only is this substance
excited to luminescence by all known exciting agents — light, Roentgen
rays, radium rays, cathode rays, etc. — but the stimulating effect of heat
and the property possessed by the red and infra-red rays of suppressing
phosphorescence are exhibited in unusual degree. It is for this reason that
we have chosen this substance for spectrophotometric study.

The material used in these experiments was in the form of a screen, of
the kind frequently used in exhibiting the properties of radium and for
numerous experiments in which it is desirable to have a fluorescent sub-
stance in convenient form. The experimental methods were similar to

Fig. 38.

The uppercurve shows the luminescence
spectrum of Sidot blende during ex-
citation by Roentgen rays. The lower
curve shows the phosphorescence 5
seconds after excitation.

4-0

0-77=1

20

0.46

0.50

0 54

0.58

0.62 JU

those described in Chapters I and II, a Lummer-Brodhun spectrophoto-
meter, with an acetylene flame as a comparison source, being used to measure
the intensity in different parts of the luminescence spectrum. In certain
portions of the work special devices were required which will be described
in their proper place.

LUMINESCENCE EXCITED BY ROENTGEN RAYS.

The screen was placed in front of the collimator slit of the spectrophoto-
meter at a distance of only a few centimeters, while the Queen self-regulating
tube used for excitation was about 20 cm. behind the screen. Roentgen
rays of moderate "hardness" were used. No systematic experiments to
determine the effect of varying hardness upon the form of the luminescence
spectrum have yet been made, but a few preliminary tests indicate that
the effect is not great. Corrections due to fluorescence excited in the
glass of the spectrophotometer, which we had at first thought would be
necessary, were found to be negligible.

The upper curve in Fig. 38 shows the luminescence spectrum observed
during excitation, while the lower curve shows the distribution of intensity

JAn account of the experiments described in this chapter was presented to the American Physical Society
at the Philadelphia meeting December 30, 1904. Physical Review, XXI, p. 247; XXII, p. 279; XXIII, p. 37.

41

42

STUDIES IN LUMINESCENCE.

in the phosphorescence spectrum, observed 5 seconds after excitation had
ceased. Owing to the weakness of the phosphorescence excited by Roentgen
rays, the latter curve was determined with some difficulty, the readings at
the red end of the spectrum being especially uncertain. The indications
of a second maximum in the phosphorescence spectrum at 0.62 n or beyond
are therefore not to be regarded as entirely trustworthy.

PHOTO-LUMINESCENCE DURING EXCITATION.

In the experiments upon photo-luminescence during excitation the carbon
arc was first used as an exciting source. The arrangement of apparatus
is shown in Fig. 39. The zinc sulphide screen was placed in a light-tight
box B, and the exciting light from an arc A , after dispersion by the prism P,
entered the box through an opening at the left. A second opening in B
allowed the fluorescence light to enter the slit of the Lummer-Brodhun
spectrophotometer Si. The comparison source was an acetylene flame F,

200

/

\

+'

/

/

*'

-4

\
\

120

eo

40

*1

V

/

\\

\ v

\

\
\

\ \
\ \
\ \

\ \

\
\

Y

\

\\

\ \

\
\

1,

Fig- 39-

.440 .480 .520/1- .560 .600

Fig. 40.

Luminescence spectrum of Sidot blende
during excitation by the violet bands
in the arc.

whose rays were reflected into the comparison slit by the block of magne-
sium carbonate, M. The second spectrophotometer S2 and the shutter C
were used in later experiments on phosphorescence, but not in the experi-
ments now considered.

Luminescence was excited only by the rays at the violet end of the arc
spectrum, and especially in the region of the violet bands. It is to be noted
that on account of the absorption of the glass prism and lenses the spectrum
extended only a short distance into the ultra-violet.

Observations taken to determine the luminescence spectrum when the
zinc sulphide screen was excited by the violet bands of the arc are plotted
in Fig. 40. Owing to the impurity of the exciting spectrum a certain
amount of blue and green light reached the screen even when the attempt
was made to excite by violet only. vSince this stray light was in part
reflected into the slit of the spectrophotometer with the luminescence light
of the same color, it was necessary to make a correction to remove the error
due to this cause. The correction was determined by replacing the zinc

THE LUMINESCENCE OF SIDOT BLENDE. 43

sulphide screen by a block of magnesium carbonate, whose surface was of
approximately the same color and roughness as that of the fluorescent
screen, and by measuring the light received from this block at different
points throughout the spectrum. The results of such a series of measure-
ments is shown in the lower broken line in the figure. It will be noticed
that the correction is inappreciable for the longer wave-lengths, but be-
comes important in the violet. The upper broken line in Fig. 40 shows
the intensity of the light reaching the spectrophotometer from the zinc
sulphide screen, being the combined effect of luminescence and reflected
light; and the heavy curve, obtained by subtracting the ordinates of the
lower curve from those of the upper, shows the corrected luminescence
spectrum during excitation.

The curves in Fig. 40, like those shown in the previous chapters, are
expressed in terms of the acetylene flame as a standard. In other words,
each ordinate represents the ratio of the intensity of the luminescence light,
of the particular wave-length considered, to the intensity of the light of the
same wave-length from the acetylene flame; the curves are not energy
curves.1

When the screen was excited by the arc as described above no trace
of the violet luminescence, which had been so prominent with Roentgen-
ray excitation, could be observed. It seemed not unlikely, however, that
the absence of the violet band was due to the fact that the ultra-violet rays
suitable for exciting it had been removed from the light from the arc by the
glass prism and lenses of the dispersing system. We therefore rearranged
the apparatus so as to use a quartz prism and quartz lenses, while a spark
between metal terminals was substituted for the arc. Since the excitation
in the region studied was solely by ultra-violet rays that were incapable
of passing through glass, it was a simple matter to test for errors due an
impure spectrum, or to any other source of stray light, by inserting a piece
of glass in the path of the exciting rays. This test showed that stray light
was in no case present in appreciable amount.

This ultra-violet excitation developed strong fluorescence in the extreme
violet, similar in color to that produced by Roentgen rays. But the green
band that had been excited by the visible rays of the arc was relatively
very weak. In spite of its faintness, however, the presence of the green
band could be readily detected, for while the violet luminescence died out
almost immediately when excitation ceased, the green persisted as a slowly
decaying phosphorescence observable for several minutes.

The similarity between the effects of ultra-violet light and those of
Roentgen rays as exciting sources is worthy of note. In each case the chief
luminescence is in the extreme violet, and is of short duration. But in
each case also this is accompanied by luminescence in the green, which is
relatively faint but of long duration. As is illustrated in numerous other
cases, the Roentgen rays, when comparable at all with rays of light, are
rather to be compared with ultra-violet light than with the rays of the
visible spectrum.

The agreement between the luminescence spectrum excited by Roentgen
rays and that observed in the case of ultra-violet excitation is not exact.

■For a description of a method of reducing such curves giving the distribution of energy in fluorescence
spectra see Nichols and Merritt, Physical Review, xxx, p. 328; also Chapter XII of this memoir.

44

STUDIES IN LUMINESCENCE.

We are of the opinion that the difference is due to the fact that the violet
luminescence does not consist of one band only, but of at least two, which
are excited by Roentgen rays and ultra-violet light in different relative
intensity. The matter will be made clear by reference to Fig. 41. Curve I
shows the luminescence spectrum when ultra-violet rays from the iron spark
were used for excitation. The rays used were those giving most intense
luminescence, and did not lie much beyond the edge of the visible spectrum.
The shape of this curve suggests that it is made up of two overlapping
bands, one having a maximum not far from 0.48 ;u, while the other has its
maximum near 0.42 ju. In curve II the exciting light was at the extreme
ultra end of the iron spectrum, and in curve 77/ the magnesium lines near
0.33 ix were used in excitation. In these curves also there is every indication
that the spectrum consists of at least two bands.

Unfortunately we have not been able to separate the bands more

completely. It is to be
remembered that the re-
solving power of a spec-
trophotometer is at the
best small, and that in
experiments of the kind
here described the small
intensity of the light
studied prohibits the use
of a narrow slit. Lumin-
escence bands may some-
times be separated by a
proper choice of the excit-
ing light. In the case
of Sidot blende the green
band is very readily ob-
tained alone by using the
carbon bands of the arc
for excitation. But the
two violet bands in the
luminescence of this sub-
stance are so close to-
gether that their regions of excitation appear to overlap throughout nearly
their whole extent. The most that we were able to do was to show that
ultra-violet rays of different wave-length produced different relative inten-
sities in the two bands, as illustrated by curves I and III of Fig 41.

The broken line, curve IV of Fig. 41, shows the luminescence spectrum
excited by Roentgen rays. This is the same curve that appears in Fig. 38,
and is introduced in Fig. 41 to facilitate the comparison of the effects of
the ultra-violet light and Roentgen rays. It will be observed that each of
the curves in Fig. 41 might well result from the superposition of three bands,
whose maxima lie approximately ato.42^, 0.48^ and 0.51 p.. In the Roent-
gen-ray luminescence the green band is relatively stronger than in the case
of excitation by ultra-violet light, while the band at 0.48 /x appears to be
entirely absent. In the luminescence produced by the magnesium spark

\

bO

x--x-\-

X

N

\

40

\ V

\ \

\ Q

20

K\

\ \

\>II
\>l

VV-«

~~*~~

*-■

N-iH

0.4-6

0.50

O.

54/i

Fig. 41.

Fluorescence spectrum of Sidot blende when excited by the ultra-
violet rays of the iron spark (curves / and //), by the ultra-
violet of the magnesium spark (curve ///), and by the Roentgen
rays (curve IV).

THE LUMINESCENCE OF SIDOT BLENDE. 45

(curve III) the green band, while still observable, is not so strong as in curve
IV, while the band at 0.48^ is readily detected. Curves I and 77 (iron
spark) show scarcely any trace of the green band, but the bands at 0.48^ and
at 0.42 ju are well marked.

FAILURE OF STOKES'S LAW.

As already stated, the green band is most brilliantly excited by the
violet, although rays from all parts of the ultra-violet spectrum are also
capable of producing a considerable effect. We thus see that there is the
same general relation between the position of the luminescence spectrum
and that of the exciting light as in the case of fluorescence. In all the cases
of fluorescence thus far studied we have found that the two spectral regions
overlap. It therefore seemed interesting to see whether the same is true in
the present case.

To settle this point it was necessary to use in excitation a far purer
spectrum than that employed in our other experiments with Sidot blende.
The rather crude dispersing system shown in Fig. 39 was therefore replaced
by a large spectrometer. A spectrum of the arc was thrown on the colli-
mator slit by the aid of a prism and single lens, and after a second dis-
persion in the spectrometer, light reached the phosphorescent screen as a
sharply focused band. Using a shutter described below so as to observe
the phosphorescence immediately after the exciting light was cut off, and
making observations with the unaided eye instead of with the spectrophoto-
meter, it was found that phosphorescence was unquestionably present for
exciting light lying between 0.470 \x and 0.497 M- Since the phosphores-
cence spectrum can readily be followed beyond 0.46 \x there appears to
be the same violation of Stokes's law in the phosphorescence of Sidot
blende that we have previously found in the case of fluorescence.

THE PHOSPHORESCENCE SPECTRUM DURING DECAY.

Although the phosphorescence of Sidot blende can be detected in a dark
room for several hours after excitation has ceased, it remains sufficiently
bright for spectrophotometric measurement only for a few seconds. In
order to determine the law of decay for different wave-lengths and especially
to determine the change, if any, in the phosphorescence spectrum during
decay, the following procedure was followed :

A shutter, shown by the broken line C in Fig. 39, and sliding upon vertical
guides, was attached to the box containing the phosphorescent screen.
When this shutter was raised it permitted the exciting light to enter the
box, as in the experiments just described, but at the same time closed the
opening in front of the collimator slit. When the shutter was dropped it
first cut off the exciting light and then, a moment later, opened the window
in front of the collimator. The observer was in this way protected from
the brilliant luminescence produced during excitation, but was enabled to
observe the phosphorescence immediately after excitation had ceased. The
comparison slit being set to some suitable reading, the observer recorded
upon a chronograph the instant when the two parts of the spectrophoto-
meter field appeared equally bright. By means of a suitable electric contact
on the shutter a record was also made at the time when the exciting light

46

STUDIES IN LUMINESCENCE.

was cut off. In this way the time required for the phosphorescent light
to fall from its initial value to any given final value could be conveniently
measured.

The determination of the instant when the rapidly changing phospho-
rescence became equal to the constant comparison light would at first
appear to be a difficult observation, and one not capable of great accuracy.
As a matter of fact these observations were fully as reliable as ordinary spec-
trophotometrie settings, and at these low intensities far less trying to the eye.

The most serious difficulty encountered in these measurements arose
from the unsteadiness of the exciting light. When constancy is essential
the arc, even under the best of conditions, leaves much to be desired.
After numerous unsuccessful attempts to obtain steady conditions we
abandoned all special efforts to keep the excitation constant, and arranged
to make observations only when the exciting light, either by adjustment
or by accidental fluctuation, had reached a certain definite intensity. In
order to accomplish this a second spectrophotometer (S2 in Fig. 39) was
used. Light from the luminescent screen was thrown into the collimator
by means of a mirror as shown in the figure. The comparison light came
from the acetylene flame F after reflection from a mirror and a block of
magnesium carbonate not shown in the figure. One observer at the eye-
piece of S2 waited until the intensity of fluorescence reached such a value as
to give equality of the two fields, and then, with suitable warning, dropped
the shutter C. The second observer at Si then made a chronograph record
as before described. Numerous determinations, often twenty or more, were
made in this way for each point of the curves described below.

Table i 1 .

The observed time, in seconds, during which the phosphorescence fell from

its initial intensity to the intensity at the top of the column.

Wave-length.

/ = ioo.

7=60.

7=30.

7 = 10.

7=5.

O aG\u

0.458

0.99

2.07

3«3

3-4"

2.91

7 1
10.4

472
483

497
5'2

528

535
547
553

568

572
592

O.OO

•49'
. 502
.290
.00

0.00
.456

•95
1 .00
0.682

0.745

1.63

1.80

1 49

0.223
0.00

0.838

2-55

7-5

0.302
0.00

1 .30

0.713

4-1

A preliminary set of measurements in which the duration of excitation
was varied from 3 seconds to 30 seconds showed no variation in the time
of decay. We conclude that the full effect of the exciting light is produced
in less than 3 seconds.1 In the subsequent experiments the duration of
excitation was never less than 5 seconds.

The results of one set of determinations made in this way are given in
Table 1 1 and are plotted in Fig. 42. In curve / the width of the comparison

'Later experiments (see Chapter IV) show that the long-time phosphorescence of Sidot blende is very
noticeably influenced by a change in the duration of excitation.

THE LUMINESCENCE OF SIDOT BI,ENDE.

47

slit was kept at ioo divisions of its micrometer screw, and the time required
for the phosphorescence of any given wave-length to fall from its initial
value to this intensity was determined as described above. The curve
is obtained by plotting wave-lengths as abscissas and times as ordinates.
In curve 77 the width of the comparison slit was 60 divisions ; in curve 777,
30 divisions; and in curve IV, 10 divisions. Observations made at 5 divi-
sions are not included on the plot. Points lying on the horizontal axis in
Fig. 42 were located by determining by trial the wave-length for which
there was a balance in the spectrophotometer the instant after dropping the
shutter. These points are probably somewhat less reliable than the others.
The significance of these results is better shown by plotting them in a
different way. In Fig. 43 wave-lengths are plotted as abscissas as before,
but intensities, instead of times, are plotted as ordinates. Each curve in
this figure shows the distribution of intensity in the phosphorescence spec-
trum at some definite time after the removal of the exciting light

.460 .480

.500 .520 .540

Fig. 42.

.560

.580 -600

Curves showing the time required for the phosphorescence to fall from
its initial intensity to a given final intensity. The final intensity is
kept constant throughout, each curve.

No curves have been plotted for intervals of more than 1.75 seconds
after excitation had ceased, since our data furnished so few points for the
later curves that their form would be largely a matter of conjecture.1 As
already stated, however, a curve of the type shown in Fig. 42 was deter-
mined for an intensity of 5 divisions. The maximum ordinate of this
curve was 10. 1 seconds and occurred at 0.506 ju; i. e., at the same wave-
length as in the case of the curves shown in 42. Comparison with Fig. 40
shows that the maximum of the luminescence spectrum during excitation
also occurs at this wave-length. In the case of the green band of Sidot
blende we conclude therefore that the maximum of the fluorescence spectrum
occurs at the same wave-length as that of the phosphorescence spectrum,

'In a series of observations of the kind recorded in Fig. 42 the number of points that could be located was
limited both by the difficulty of maintaining constant conditions during the three or four hours of observa-
tion, and by the endurance of the observer's eye.

48

STUDIES IN LUMINESCENCE.

100

and that no change that could be detected by our apparatus occurs in the
position of this maximum for 10 seconds after excitation has ceased.1

Fig. 43 also shows that if any change occurs in the form of the phospho-
rescence spectrum during decay, this change is extremely small. In other

words, the different wave-lengths
of the phosphorescence spectrum
decay at the same rate. Table 12
will showr to what extent this con-
clusion is justified by the data.
Each column of this table refers to
the wave-length stated at the top,
and in it are tabulated the intensi-
ties for this wave-length at different
intervals after the end of excitation,
these intensities being expressed in
terms of the intensity at the end of
1.75 seconds as unity. If the phos-
phorescence spectrum remains ab-
solutely unaltered during decay,
all the numbers in this table that
lie in a given horizontal line should
be equal.

The numbers in Table 12 corre-
sponding to 0.0 second are uncer-
tain ; first, because the observations
by which the initial phosphores-
cence was determined were them-
selves especially uncertain; and second, because the steepness of the curve
(Fig. 43) in the neighborhood of 0.48 /j. and 0.54 /x would cause a slight
change in the manner of plotting to produce a large change in the ordinates
at these wave-lengths. If the ratios tabulated for 0.0 second are left out
of consideration, the agreement between the remaining ratios, at 0.5 second

.460

.500 .540

Fig- 43-

.580//

Phosphorescence spectrum of Sidot blende at 'lif-
erent times after removal of the exciting light.

Table 12.

Intensity of phosphorescence at different intervals after excitation
ceased, expressed in terms of the intensity at the end of 1.75 sec.

had

0.48/x

0. 50^

0.52/x

0.34M

0.56^

0.0 sec.

7.8?

5-2

4.8

0.5 sec.

3-5

3-3

3 '

3.0

2.8

1.0 sec.

1.9

2.0

1.8

1.8

1.9

1 .75 sec.

1 .0

1 .0

1 .0

1 .0

1 .0

and 1 .0 second, is as close as could be expected. We conclude that, although
there is some indication of more rapid decay at the ultra edge of the phos-
phorescence spectrum, the probability is that all parts of the spectrum decay
at the same rate, and that the form of the phosphorescence spectrum
remains unchanged.

'Studies of the phosphorescence spectrum of Sidot blende by a photographic method, which made it
possible to determine the position of the maximum at various times up to 90 seconds after excitation had
ceased and which confirm this conclusion and extend it, will be described in Chapter VIII.

the luminescence; of sidot beende. 49

While the result obtained in this one case is not sufficient to establish a
general law, we are nevertheless of the opinion that the behavior of Sidot
blende is typical, and that in no case of phosphorescence is there any change
in the form of a band during decadence. In complex cases of phosphores-
cence we do not imply by this that the phosphorescence spectrum as a whole
remains unchanged in form, but rather that the distribution of intensity
in each band is unaltered. If the phosphorescence consists of several
bands, it is to be expected that the different bands will decay at different
rates. In fact Sidot blende itself furnishes an extreme illustration of this,
for the violet bands die out in one or two tenths of a second, while the green
band persists for hours.

Numerous cases in which the color of a phosphorescent substance seems
to change as the phosphorescence dies out at first appear to contradict
this view. We think, however, that all cases of this kind may be shown
to belong to one of the two following classes :

i . Cases of real color change ; for example, anisic acid at low tempera-
tures, where the phosphorescence changes from blue to greenish yellow.1
Such cases are probably due to the presence in the luminescence spectrum
of several phosphorescence bands, which decay at different rates. In the
case of anisic acid the results would be explained by the presence of a
brilliant but rapidly decaying band in the blue, and a persistent band of
smaller initial intensity in the yellow.

2. Cases where the apparent change in color is due to the fact that the
color sense in the eye is either weak or entirely absent at low intensities.
At very low intensities all colors appear to the eye as gray. However
brilliant may be the color of the phosphorescence light initially, it loses this
color and changes to a gray or faint white as it becomes fainter. But
such a change as this is in the retina, and does not indicate any change
in the phosphorescence spectrum.

It is interesting to note that attention was called to both of these causes
of change of color during decay by the elder Becquerel.2

The data of Table 1 1 might also be used to study the law of decay of
phosphorescence, i. e., the relation between intensity and time. More
reliable results are obtained, however, by studying one wave-length at a
time, since the data necessary for plotting a decay curve may in this case
be obtained more quickly and are, therefore, less liable to error due to
fluctuating conditions. Measurements of phosphorescence spectra during
decadence, in the case of other substances, are described in Chapters IV
and VII.

Nichols and Merritt, Physical Review, iS, p. 355, 1904.

-Ed. Becquerel, Comptes Rendus, 49, p. 27,1859; Ann.de Chimie et de Physique, series 4, 62, p. 20, 1 861.

CHAPTER IV,

THE DECAY OF PHOSPHORESCENCE IN SI DOT BLENDE AND
CERTAIN OTHER SUBSTANCES.'

The decay of phosphorescence was first studied by E. Becquerel.2 In
the case of short-duration phosphorescence the phosphoroscope was used
for this purpose and the intensity of phosphorescence was measured for
different speeds of the rotating disk. Becquerel regarded it as probable
that the law of decay was of the form

I = hc-ai (i)

and found that the observations were in fact fairly well represented by
an exponential expression. In discussing these observations, however,
Becquerel tacitly assumed that no appreciable time was required for the
exciting rays to produce their full effect. Later investigations have shown
that this assumption is not justified. Since a change in the speed of the
phosphoroscope altered not only the time that elapsed between excitation
and observation, but also the duration of exposure, it is probable, therefore,
that the initial excitation was less at high speeds. Attention was directed
to this point by E- Wiedemann and later by H. Becquerel, who states that
a recomputation of the data shows a less satisfactory agreement with the
exponential law than at first appeared.

For the long-time phosphorescence of the phosphorescent sulphides E.
Becquerel proposed an empirical expression of the form

rn(c-ht) = cr0m (2)

For each of the seven substances tested this expression was found to show
a fairly good agreement with the experimental results throughout a con-
siderable range. In one case, namely, that of a calcium sulphide prepara-
tion giving an orange-red phosphorescence, the expression represented
the observations with considerable accuracy throughout the whole range,
the value of ;;; being 0.5. But in most cases it was not possible to find
values of m and c which would make the formula fit the experimental data
for the whole time of decay. The values of m that suited the observations
best lay between 0.5, for the calcium sulphide just mentioned, and 0.806
for another calcium sulphide having an orange-yellow phosphorescence.

The same empirical formula has since been very generally used, among
others by Darwin3 in 1881, and Ch. Henry4 in 1892. The former worked
with Balmain's paint and found the best value for m to be 0.86. It can
not be said, however, that the experimental results were represented very
accurately by the formula. The substance used by Henry was Sidot blende,

'An account of the work described in this chapter was presented to the American Physical Society at the
meeting held on Oct. 28, 1905. Physical Review, xxn, p. 279.

2BecquereI, "La Lumiere." See also Comptes Rendus, 51, p. 921, i860, and Annales de Chimie et de
Physique, series 4, 62, p. 5.

••Philosophical Magazine, 11. p. 209, 1881.

4Comptes Rendus, 115, p. 505, 1892.

51

52 STUDIES IN LUMINESCENCE.

several different samples of which were tested. Henry states that in the
case of one specimen the results were represented by an exponential law
(eq. i) for 14 seconds, while other preparations obeyed the law

,-0.5936^ + 27-l8) = l6475 (3)

Henry appears to have been convinced of the correctness of the latter law
and proposed a new t)^pe of photometer which made use of the gradually
decaying phosphorescence of Sidot blende in the measurement of faint
sources of light.1 While the fact that the constant in eq. (3) is given to
five significant figures indicates an accuracy that is unusual in photometric
measurements, the lack of experimental data in the paper referred to
makes it difficult to form an independent opinion of the significance of the
conclusions.

The decay of phosphorescence was considered from the theoretical stand-
point by H. Becquerel2 in 1891. Upon the assumption that the light
emitted during phosphorescence was due to molecular vibrations set up
by the action of the exciting light and afterwards gradually dying out, it
was shown that the law of decay would be determined by the nature of the
damping forces. If the vibrations meet with an opposing force propor-
tioned to the speed it was shown that an exponential law of decay would
result ; while if the resistance is proportional to the square of the speed the
law of decay would take the form

7=(a-r-V (4)

It will be noticed that the empirical law proposed by E. Becquerel reduces
to (4) in case ^ = 0.5.

In the case of a substance whose phosphorescence spectrum contains
several bands Becquerel proposed the expression

in which there is one term in the summation for each band. Upon testing
this law with the data obtained by E. Becquerel for a calcium sulphide
giving blue phosphorescence it was found that the results could be expressed
by the use of two terms in the above series with great accuracy. The
existence in the spectrum of this substance of two bands possessing inde-
pendent properties could be demonstrated in various ways.

In the derivation of the law proposed by H. Becquerel the assumption
is that the vibrations set up by the action of the exciting light continue
during several minutes or even hours. This would imply either that the
vibrating atoms or molecules exist during this time without collisions with
other molecules, or else that such collisions are without effect upon the
vibrations. Neither of the suppositions seems to us tenable. But the
law nevertheless appears to be of very general application. We shall
show later that the same law may be derived from entirely different theo-
retical considerations. (See Chapter XV.)

"Comptes Rendus, 115, p. 602.
JComptes Rendus, 113, p. 618, 1891.

DECAY OF PHOSPHORESCENCE IN SIDOT BLENDE. 53

In all of the experiments upon the decay of phosphorescence with which
we are familiar it is the total light that has been measured; so far as we
are aware no attempt has been made to determine the law of decay for
different portions of the phosphorescence spectrum. This fact complicates
the problem greatly, for in most cases of phosphorescence the spectrum
consists of two or more bands, which, in general, decay at different rates.
It can hardly be expected, therefore, that measurements of the total light
will be found to obey a simple law. The difficulties resulting from the
complexity of the luminescence spectrum were recognized by E- Becquerel
and were several times mentioned in the course of his classic researches.
In discussing his experiments on decay he suggested an expression of the
form

I = Ae-at + Be-iii+ ... (6)

for the intensity of the total phosphorescence, with one term for each
band in the spectrum. But the difficulties of computation were such as to
lead him to abandon this expression and to employ instead the empirical
expression of eq. (2). Recognition of the fact that each band has its own
rate of decay is also implied in the law proposed by H. Becquerel (eq. 4).

While it is possible to test the correctness of an expression of the form of
(4) or (6) by comparison with measurements of total intensity, such a test is
difficult, and can not be altogether satisfactory; for with an expression
containing several terms the number of constants is so great that the law
may be made to fit almost any data. It is clear that a much more severe
test of any given law of decay may be obtained from experiments with a
substance having only one band in its phosphorescence spectrum. The
Sidot blende screen used by us in earlier experiments was found to have
three bands in its luminescence spectrum. But since the two violet bands
do not appreciably overlap the green band the spectrophotometer enables
the behavior of the latter band alone to be conveniently studied. The
matter is still further simplified by using the violet rays of the arc in excita-
tion, since these rays, while they produce a brilliant green luminescence, are
incapable of exciting either one of the violet bands.1 In view of the bril-
liancy of the green band, its long duration, and the ease with which it maybe
isolated, this band seems well adapted to the study of the decay of phos-
phorescence.

EARLY STAGES IN THE DECAY OF PHOSPHORESCENCE IN SIDOT BLENDE.

Our previous experiments on Sidot blende have shown that in the case
of the green band the phosphorescence spectrum shows no measurable
change in form during the first 10 seconds of decay. In other words, the
rate of decay is the same for all wave-lengths. To fix the behavior of the
whole band it would be sufficient, therefore, to determine the law for a
single wave-length. We have, however, made measurements at three
different wave-lengths, namely, at 0.483 ju, 0.512 n, and 0.547 M- The first
of these wave-lengths lies near the ultra edge of the band ; the second is not
far from the maximum; and the third is near the red edge.

'See Chapter III of this memoir.

54

studies in luminescence;.

The methods of measurement were the same as those described in Chap-
ter III. The experimental data obtained are contained in Tables 13, 14, 15.
In each case it is the average of from ten to twenty observations that
is recorded.

In the set of observations contained in Table 13 the curves for 0.547^,
0.483 ijl, and 0.512 fi were determined during the forenoon in the order just
stated. The data for 0.5 1 2 n with weaker excitation were observed during
the afternoon of the same day. The observations recorded in Table 14
were taken in a different manner, the measurements at a given intensity
being made for each of the three wave-lengths in succession. The obser-
vations of December 13 (Table 15) were made in a similar manner. The
first three curves of Table 13 are plotted in Fig. 44, while the second and
fourth curves of this table are shown in Fig. 45.

Table 13. [Nov. 25.]

Time in seconds required for the phosphorescence to fall from its initial intensity to
the intensity I. The computed values of / are derived by substitution in the
following equations:

For X = 0.483^1 i/|

X = o.5I2m i/I

X = o.547/x i/I

X = 0.512,1* (weaker excitation) i/|

7 =0. 102 + 0.059^
I =0.074 + 0.045/
I =0.096 + 0.055/
J =0.092 + 0.052/

/

X=o

483M

X=o

512m

X = o

547m

X = 0.5I2M

Weaker excitation.

/ sec.

/ sec.

/ sec.

; sec.

/ sec.

I sec.

1 sec.

t sec.

101

Obs.

Comp.

Obs.

Comp.

Obs.

Comp.

Obs.

Comp.

O.63
O.83
..38

O.58
O.85
I .20

81

61

O

41

0

44

O.56

O.58

O.72

O.69

41

0

99

O

92

I.84

..83

I . 10

I .09

I.24

I.23

21

I

88

1

98

3-25

3-23

2.07

2.23

2.42

2.45

I I

3

16

3

39

4-95

5.IO

3.87

3-74

4.03

4.04

6

6

55

5

23

7.42

7.5O

5.82

5-7'

6.22

6.92

Table 14. [Dec. 10.]

Time in seconds required for the phosphorescence to fall to the intensity I. The
computed values of / are derived by substitution in the following equations:

For X = o. 512/i 1/^/7=0.078+0.034/ X = o.547/j 1 /-y// =0. 104+0.042/

I

X = 0.483m X = 0.5I2m

X = 0.547/i

t sec.
Observed.

t sec.
Computed.

t sec. / sec.
Observed. Computed.

/ sec. / sec.
Observed. Computed.

Si. 4
61 .4

31-4
11.4

O.60
O.77
2.40
413

I.06 O.98 O.30 0.22
I.35 I . 49 O.55 O . 62
2 . 98 3 . 02 I . 81 1.8.1

6.47 6.46

4.62 4 .58

DECAY OF PHOSPHORESCENCE IN SIDOT BLENDE.

55

Table 15. [Dec. 13.]

Time in seconds required for the phosphorescence to fall to the intensity /. The
computed values of t are derived by substitution in the following equations:

For\ = o.483M 1 /V/ =0. 104+0.070/
X = 0.5 12m i Hi =0.094+0.046/
X = o.547m 1 /Vi" =0. 1 18+0.070/

I

\ = 0

483M

X = 0

512^

X = 0

547^

i sec.
Observed.

t sec.
Computed.

/ sec.
Observed.

/ sec.
Computed.

/ sec.
Observed.

t sec.
Computed.

81.4
61 .4

314

21 .4
II. 4

7-4

0.45
O.69

O.37
O.74

O.45
I .06
I .46
2.78
4.46

O.36
1.08
I .6l
2.76
3.80

O.25
O.88
I.30
2.6l
4.58

0. 14

O.87
I .40
2-55
3- 57

2.60
4.48
5-97

2.67

4-44
6.00

The data of the preceding tables have been used to test the applicability
of each of the several proposed laws of decay to the case of a single band.
It was found that the results can be closely represented by an expression
of the form given in eq. (4). To determine the constants a and b of this
equation a convenient graphical method was employed, in which the values
of // V7 were plotted as ordinates and times as abcissse. vSince eq. (4) may
be written in the form

the points located in this manner should lie on a straight line. A straight
line -having been drawn as nearly as possible through all the points, the
slant of this line and its y intercept at once gave the values of b and a respec-
tively. The values of a and /; determined in this way are given in each'of
the tables, and the values of / computed from the above equation are
tabulated for comparison with the times observed.

V

/

1

\

,.^

.?

<*

\

B

\+

B>^

A\\

<y^

40

Fig- 44-

Curves .4, B, and C show the decay in
the phosphorescence of Sidot blende
for the wave-length 0.483 /u, 0.512 p,
and 0.547 ix, respectively. The curves
A', B' , and C are obtained from A.
B. and C by plotting Z~J- instead of
the intensity of phosphorescence, /.

3 4

Seconds

In Figs. 44 and 45 the values of / * have been plotted as described
above. It will be noticed that in each case the points lie very nearly upon

56

STUDIES IN LUMINESCENCE.

a straight line, and that there is nothing to indicate any systematic vari-
ation from linearity. In the case of one curve, namely, that for 0.483 /z
in Table 14, the points located in this way did not lie even approximately
on a straight line. This curve, therefore, does not have the form corre-
sponding to eq. (4). But in the case of the other curves, including one
taken at this same wave-length (Table 13), the agreement between the
observed and computed values of / is satisfactory, the differences being
only such as might well result from accidental errors. It seems probable,
therefore, that the observations at 0.483 ju in Table 14 were subject to some
unusual source of error.

In Fig. 44 it will be noticed that one point of the line A ' (indicated by a
question mark) falls below the line, i. c, the observed time of decay was
considerably greater than that computed from an assumed linear relation
between 7_i and /. Discrepancies of the same kind will be found for the
smallest values of / in the first curve of Table 13 and for the first and last

•00

80

60

40

20

**

A

^

J/^

\B \

^S^

^*

^\^*

f=====

=3

Fig. 45-

Curves A and B show the decay of phos-
phorescence at 0.512 m for two dif-
ferent intensities of the exciting light.
In the curves A' and B' I '*- is plotted
instead of the intensity /.

3 4

Seconds

curves of Table 15. No such discrepancies are present in any of the four
curves taken at 0.512 /j. — i. e., near the maximum of the phosphorescence
spectrum. If these cases of disagreement between the observed and com-
puted values of t are not due to accidental errors, they show the beginning
of a systematic variation which will be dealt with at length in a later
paragraph.

The theory discussed in Chapter XV leads us to expect that under ideally
simple conditions the decay of phosphorescence should be in accordance
with the equation

where

—7= = a-\-bt
V/

0 =

no^lka

It is to be remembered that if / refers to the intensity at some particular
part of the phosphorescence spectrum, k must be regarded as a function
of the wave-length.

The fact that the experiments indicate a linear relation between the time
and /~- has already been pointed out, and reference to the tables on pages

DECAY OF PHOSPHORESCENCE IN SIDOT BLENDE- 57

54 and 55 shows that the differences between the computed and observed
values of /, which are usually small, are entirely unsystematic. It is
possible to test the law still further by considering the values of the con-
stants a and b. Except in the case of the last curve of Table 13, the data
refer to curves of decay for different wave-lengths, but with the same
excitation. a and «0 are therefore constant, while k depends upon the
wave-length. It will be noticed that

a 1 II 1

This quotient should therefore be a constant for all curves taken with the
same excitation and under similar experimental conditions. The values
of the ratio a/b for the three curves of Table 15 show considerable vari-
ation from equality, being 1.5, 2.0, and 1.7. The values of the ratio for the
first three curves of Table 13 are 1.73, 1.75, and 1.65. In view of the
difficulty of maintaining constant conditions in the experiments on phos-
phorescence the agreement between these three values is highly satisfactory.
As already stated, the observations at 0.483 ju in Table 14, probably on
account of some experimental error, can not be represented by the expres-
sion here considered, so that the value of a/b for this curve can not be deter-
mined. But for the curves taken at 0.512 n and 0.547 M (Table 14) the
quotient a/b has the values 2.32 and 2.43, respectively. Here, too, the
agreement is all that could be expected.1 Our results therefore afford
strong confirmation not only of the conclusion that all parts of the green
band decay at the same rate, but also of the general theory of phosphor-
escence that we have used in deriving the law of decay.

The data permit still another test of the general theory, the result of
which is less satisfactory. It will be remembered that in the case of the
fourth curve of Table 13 the exciting light was less intense than that used
with the other curve for 0.512 (jl. The observations for the fourth curve
were made on the same day as those for the second curve, and although the
former were made in the afternoon and the latter in the forenoon, every
attempt was made to keep the comparison source the same in the two cases
and to have all the other experimental conditions as nearly as possible
constant. If we compare the two curves for 0.5 1 2 y, in Table 13, we see
that a and k have the same value for both, while n0 is different. It would
seem reasonable, however, to regard the constant b as independent of 11 0.
This constant should therefore have the same value for both curves; in other
words, the straight lines obtained by plotting 7~- should be parallel. As a
matter of fact the values of b determined for the two curves are 0.045, for
the second curve of Table 13, and 0.052 for the fourth. The fact that the
two values are unequal is shown in Fig. 45 by the lack of parallelism of the
two lines A' and B'.

Both the theory of Becquerel and the simple theory first discussed in
Chapter XV fail to explain this result; which, however, is in accord with the
modified theory of Wiedemann and Schmidt discussed in the latter part of
Chapter XV.

'There is no reason why the value a/b for Table 13 should be the same as those for Table 14 or 15, since
the observations were made several weeks apart, and with no attempt to keep the intensity of the com-
parison source the same in the different cases.

58 STUDIES IN LUMINESCENCE.

THE STUDY OF PHOSPHORESCENCE IN SI DOT BLENDE AND CERTAIN OTHER
SUBSTANCES DURING THE WHOLE PERIOD OF DECAY.

In the experiments just described the phosphorescence of Sidot blende
was followed by the spectrophotometer for about 10 seconds after the
cessation of excitation. At the end of this period the intensity of phos-
phorescence had fallen to about 4 per cent of its original value. Although
it was possible to see the phosphorescent light in the spectrophotometer
for a much longer period than this, the extreme faintness of the field made
accurate measurements out of the question. In attempting to determine
the law of decay throughout a wider range we have therefore been com-
pelled to abandon the use of the spectrophotometer and to measure the
total light.

Measurements of the total intensity do not, in general, afford a satis-
factory means of determining the law of decay of phosphorescence, since, as
has already been pointed out, the phosphorescence spectrum usually con-
tains two or more bands, which decay at different rates. We have already
referred to the difficulties that are met with in interpreting the results of
such measurements. In the case of Sidot blende the conditions are, how-
ever, relatively simple, for the phosphorescence of the green band is so much

-R

i=

Fig. 46.

more prominent, both in duration and in intensity, than that of the violet
bands that the presence of these does not appreciably affect the total
intensity. Measurements of the total phosphorescence of Sidot blende are
therefore practically measurements of the intensity of the green band only.
If all parts of this band decay at the same rate, measurements of total in-
tensity will give the same results as measurements taken at some particular
portion of the band. Our earlier observations have shown that all parts of
the green band of Sidot blende do decay at the same rate for at least 10 seconds.
It seems not unlikely, therefore, that this equality of rates will be maintained.
Although the present section deals chiefly with the phosphorescence of
Sidot blende, the results obtained in the study of several other substances
are also included.

EXPERIMENTAL METHOD.

The apparatus used is shown in Fig. 46. The white screen was removed
from a Lummer-Brodhun photometer and the Sidot blende screen, s, was
put in its place, the active surface of the screen being toward the left. The
screen was usually excited by a Lummer mercury lamp, L, of the "end on"
form. In some instances the carbon arc, and in other cases the spectrum
of the carbon arc, was used for excitation. The exciting light could be cut
off by means of a shutter, S, which at the same time made a record on a

DECAY OP PHOSPHORESCENCE IN SIDOT BLENDE- 59

chronograph. When the mercury lamp was used in excitation a piece of
blue glass, B, was placed between the lamp and the screen, so that only the
violet lines in the spectrum were used.

The right-hand side of the screen was of white paper and could be
illuminated by means of the acetylene lamp A. Two pieces of green
glass, G, served to produce a sufficient color match. The acetylene flame
A was in a metal box, and the light used emerged from a small opening
immediately in front of the central part of the flame. Small changes in
pressure were therefore without effect upon the intensity of this comparison
source.

The procedure in taking observations was as follows:
One observer, with his eyes suitably protected from stray light, watched
the decay of the phosphorescence after the shutter had been closed, and
recorded by means of a key the instant at which the phosphorescent light
and the comparison field were equal. The second observer then shifted
the position of the comparison flame A to a slightly greater distance from
the photometer, and the time when equality was again reached was recorded
as before. In this way it was often possible to get as many as 15 points
on the curve of decay, although a smaller number than this was more
usual.

EXPERIMENTS WITH SIDOT BLENDE.

In preliminary experiments the light from the carbon arc was used in
excitation. It was thought that simpler conditions would be obtained
if exciting light having only a small range of wave-lengths was used, and a
spectrum of the arc was therefore formed, the violet end of which was used
for excitation. The gain in the intensity of phosphorescence which re-
sulted from cutting off the red and infra-red rays was very noticeable.

In our previous experiments with Sidot blende it was found that the
decay of phosphorescence during the first 7 seconds was in accordance
with the law

(a+btY

so that upon plotting the curves with T and I~- as coordinates a straight
line was obtained. Since the distance between the photometer screen
and the acetylene flame is proportional to I~-, it was convenient to plot
the results of the present experiments in the same way.

The general character of the decay curves when plotted in this manner
is exhibited by the curves of Fig. 47. The violet end of the carbon arc
spectrum was used in excitation, and the duration of excitation was varied
as indicated.

Inspection of Fig. 47 shows that while each curve starts as a line which
is nearly straight in the neighborhood of the origin, it soon begins to bend
over and ultimately changes into a second straight line having a different
slant. This behavior is especially noticeable in the case of long exposures.

If phosphorescence is due to the recombination of ions that have been
separated by the action of the exciting light, as assumed in the theory of
Wiedemann and Schmidt, it appears that the coefficient of recombination
does not remain constant during the whole period of decay. During the

6o

STUDIES IN LUMINESCENCE.

first 6 or 7 seconds this coefficient is practically constant, as was shown by
the results of our previous experiments. A change then begins which con-
tinues for about 20 or 30 seconds, after which the curves indicate a new
coefficient, smaller than before. This new coefficient, as is shown by the
constant slant of the curves of Fig. 47, remains constant during the rest of
the period of decay, or at least until the phosphorescence has become too
faint to measure. Whatever theoretical interpretation of the curves is

400

i

B

c

s

D

f

<f .

/

^^^

•&<y''

300

/

0*

/ /

j$

''^^^^

200

//

r ^s^

w

&r

100

w

Fig- 47-

Effect of duration of excitation.
Violet end of carbon-arc spec-
trum used in excitation. The
curves were taken in the
order indicated by the letters.
The times of excitation were
as follows: Curve A, 0.9.;
B, 2.4 sec; C, 4. 5 sec; D, 9.2
sec; E, 17.5 sec.

20

40

60 80

Seconds

100

120

adopted it is clear that the decay of phosphorescence involves two distinct
processes which merge into one another. In each of these the decay obeys
the simple law already referred to, but the rate at which the phosphorescence
dies out is greater in the first process than in the second.

It will be observed that the slant of the curve, for each of the two pro-
cesses, is a function of the duration of excitation.

400

/vr

300

200

100

D^

Zrf

^B

A^

Oj>

Fig. 48.

Effect of duration
of excitation.
Violet light of
mercury arc
used for exci-
tation. The
curves were
taken in the
order by the
letter The
times of excita-
t ion were as
follows: Curve
A, 27 sec; B,
10 sec; C, 3.1
sec; D, 1.2 sec.

20

40

60

80

100

120

140

160

Seconds

In Fig. 48 is shown a series of curves taken under the same conditions as
those of Fig. 47, except that the mercury arc was used in excitation instead
of the violet end of the ordinary arc spectrum. One is immediately struck
by the difference between the two sets of curves. In Fig. 48 there is a
strong tendency toward parallelism in the parts of curves corresponding
to the second process. In Fig. 47 no such tendency is observable.

DECAY OF PHOSPHORESCENCE IN SIDOT BEENDE.

6l

We were at first inclined to ascribe this difference to the fact that different
kinds of exciting rays had been used in the two cases; but the two sets of
curves differ in another respect, namely, in the order in which the curves
were taken. In the first case the curves of short excitation were taken
first, while in the latter case the curves corresponding to long excitations
were the first observed. It appeared possible that the difference in the
form of the curves was due to this difference in sequence rather than to the
difference in exciting light.

To test this matter the observations plotted in Fig. 49 were made. It
will be noticed that the dotted curves are similar to those in Fig. 48, while
the full curves are similar to those of Fig. 47. The mercury arc was used
in excitation in all cases. But curves taken after the screen had been sub-
jected to the long excitations corresponding to curves C and D differ widely
from the curves taken with approximately the same excitation previous
to C and D. A comparison of curves A and E illustrates this point, the
the duration of exposure being exactly the same in each of these two cases.

Fig- 49-

Effect of duration of
excitation. Mercury
arc. The curves were
taken in the order in-
dicated by the letters.
The times of excita-
tion were as follows:
Curve .4, 4.3 sec; B,
8.2 sec; C, 16.0 sec;
D, 56.0 sec; E, 4.3
sec; F, 1.4 sec; G,
1 .0 sec.

These results indicate that some change is produced in the phosphorescent
material by the action of the exciting light, and that this changed condition
persists for a considerable period after all visible phosphorescence has
ceased. In other words, the effect of a given excitation in producing phos-
phorescence depends upon the previous history of the phosphorescent
substance.

If the screen is allowed to rest in the dark for a number of hours this
semi-permanent effect of exposure in part dies out. But rest alone does
not restore the screen completely, even if continued for several days. The
effect of rest was also found to be somewhat uncertain, being much greater
on some occasions than on others.

Several methods of restoring the screen to a standard condition were
tried. Heating the screen to the temperature of boiling water for several
minutes and then cooling it again to the temperature of the room seemed
effective. But this method required considerable time and has not been
thoroughly tested. Cooling the screen to the temperature of liquid air and
afterwards warming it gradually to the original temperature seemed to be
without effect.

62

STUDIES IN LUMINESCENCE.

The well-known effect of the red and infra-red rays in suppressing the
long-time phosphorescence of various substances led us to think that these
rays might also prove effective as a means of restoring the screen to a
standard condition. This conjecture proved to be correct, and exposure
to the red and infra-red rays of a 50 c.p. lamp at a distance of about 20 cm.
was found to be both convenient and satisfactory. A piece of ruby glass
placed in front of the lamp served to remove the more refrangible rays.
An exposure of a few seconds to the rays that passed through the ruby glass
was sufficient to bring the screen into what seemed to be a definite standard
condition. A longer exposure was, however, ordinarily used.

The changes produced by excitation and the effect of the red and infra-
red rays are illustrated by the curves shown in Fig. 50. Curve A shows
the behavior of the screen when exposed for 10 seconds after resting in the
dark for 24 hours. Curve B is that corresponding to an exposure of 2
minutes; curve C was taken with an exposure of 10 seconds immediately

Fig. 50. — Illustrating
relative effect of rest
and exposure to infra
red.

A, 10 sec. excitation after
rest of 24 hours in the
dark. B, 2 min. excita-
tion.

C, 10 sec. excitation im-
mediately after B.

D, 10 sec. excitation after
exposure of 4 min. to infra-
red from 50 candle-power
lamp.

Curves A', B' , C, D' cor-
respond to .4, B, C, IK
except that / is plotted
in place of l~ Js .

after curve B; and curve D, also with an exposure of 10 seconds, was taken
after the screen had been exposed to the red and infra-red rays for 4tninutes.1
In some respects the behavior of the screen is analogous to the magnetic
behavior of iron. When iron is magnetized a certain residual magnetization
remains after the removal of the magnetizing force, and the effect produced
by a subsequent magnetizing force depends upon the magnetic history of
the specimen. Similarly some change is produced in Sidot blende by excita-
tion which does not immediately disappear upon the removal of the excit-
ing light, and which modifies the effect produced by subsequent excitation.
The analogy is rendered more striking if this property of Sidot blende is
exhibited in a different manner, as in Fig. 51. In the curve plotted in this
figure the abscissa of each point shows the duration of excitation, while
the ordinate gives the corresponding intensity of phosphorescence after 30
seconds decay. The resemblance of the curve to a hysteresis loop for iron
is striking.

SOC»

\1

Ay

C

B

^^

j4p/ \

iA\

V ^

B

/■;,

sC"

\P

A^

^d

BO

■ H«

MO Sec.

ao

SO

oo

■Experiments dealing with the effect of the infra-red rays during and after excitation will be found in
Chapter V of this memoir.

DECAY OF PHOSPHORESCENCE IN SIDOT BLENDE.

63

It seems possible that the action of these rays in destroying the residual
effect in the phosphorescent substance is similar to the effect of jarring or
tapping in destroying the residual magnetism of a bar of iron. Ignorance
of the existence of hysteresis would evidently lead to confusing results in
the case of either of these two classes of phenomena. Our delay in recog-
nizing" the effect of previous history has in fact made it necessary for us to
discard all of our earlier observations.

1

-j^^^

^^^j^5**

JS ,

^-J^

'^ — '

0/

/ r

/ 1

.

ill

6 /

bf

Fig- 5i.

Hysteresis loop. Ordinates
give the intensity of phos-
phorescence 30 seconds
after the end of excitation.
The curves from which these
points were determined
were observed in the order
indicated by the arrows.

20 40 60 80

Duration of excitation

100 sec

In our later work the screen was exposed to the red and infra-red rays
as described for one minute before each exposure. With this precaution
to avoid the effects of hysteresis the curves shown in Fig. 52 were taken
to determine the effect of varying times of exposure.

Effect of duration of excitation. Violet of mercury arc used for excitation. Screen exposed to
infra-red for i minute before each curve. The times of excitation were as follows:
Curve A, 1.2 sec; B, 5.4 sec; C, 12.0 sec; D, 37 sec; E, 60 sec; F, 15 min.

Since our previous experiments have shown that the curves are accurately
straight in the neighborhood of / = o, it is possible to determine the initial
intensity of phosphorescence by prolonging the curves in each case until

64

STUDIES IN LUMINESCENCE.

they strike the vertical axis. From the intercept thus determined, which
is equal to h~K the initial intensity can be computed. From the results
obtained in this way from the data shown in Fig. 52, curve A in Figs. 53
and 54 has been plotted. In these, two figures / has been plotted instead
of /-*.

It will be noticed that the intensity of phosphorescence at first increases
rapidly with increased duration of exposure, but that after an exposure
of 2 or 3 minutes is reached there is little further change. The phospho-

so

A,

—

I

cr

IE

C

b

01

Il2

e

1

I

N.

Tr^r

-

15 Mir

Fig- 53-

Effect of duration of excitation. Curve A shows initial intensity of phosphorescence (/) as a function
of the time of excitation. Curvesa.b, and c show the decay of phosphorescence after excitations of 15
minutes, 5 minutes, and 1 minute, respectively.

rescence may be said to be saturated so far as the effect of duration of
excitation is concerned. Not only is the initial intensity unaltered by
longer excitation, but the form of the decay curve also remains constant,
as is indicated by curves a and b in Fig. 53. (These curves correspond
to curves F and E of Fig. 51.)

We have also studied to some extent the influence of the intensity of
the exciting light upon the form of the decay curves. In order to vary

Fig- 54-
Effect of duration of excitation. A portion of the same curve shown in Fig. 54 plotted to a larger scale.

the intensity of the exciting light several metal stops were prepared, which
could be placed immediately in front of the mercury lamp. The apertures
of these varied from i mm. in diameter to the full size of the mercury-lamp
tube, namely, 15 mm. To determine the intensity of the exciting light
corresponding to each of these the following photometric method was
used: The phosphorescent screen was removed from the photometer and
a piece of white cardboard was put in its place. This being illuminated

decay of phosphorescence; in sidot blende.

65

by the violet light from the mercury arc passing through the stop whose
constant was to be determined, the intensity was measured by shifting
the position of the comparison flame on the opposite side of the photometer.
vSuitable glass screens were used to equalize the colors on the two sides.
To avoid errors resulting from the flickering of the mercury arc ten settings
were made for each determination.

400

'«

55-

Effect of varying the intensity of the
exciting light. Exposure in each case
20 seconds. The relative intensity of
the exciting light is marked on each
curve.

60 80

Seconds

In Fig. 55 decay curves are shown for different intensities of the exciting
light, the excitation in each case lasting for 20 seconds. In Figs. 56 and 57
similar sets of curves are shown for which the excitations were respectively
40 seconds and 2 minutes.

A study of these curves shows that there is some approach to intensity
saturation; in other words, the intensity of phosphorescence is nearly

400

300

200

100

Fig. 56.

Effect of varying the intensity
of the exciting light (as indi-
cated on each curve). Ex-
posure in each case 40 seconds.

20

40

60 80

Seconds

100

120

proportional to the intensity of the exciting light for small values of the
latter, but increases less rapidly than the excitation when the exciting
light is strong. This point is well brought out by curve A in Fig. 58, in
which the ordinates are proportional to the initial intensity of phosphores-
cence, 70. The values of 70 were determined from the data of Figs. 55, 56,
and 57 by extrapolation, upon the assumption that the relation between

66

STUDIES IN LUMINESCENCE.

/ and I~- is linear for small values of /. Since the early portion of each
decay curve is chiefly determined by the first two or three points, which
are the most difficult to observe, the values plotted for I0 are subject to
considerable error. Curve A is nevertheless reasonably smooth and
indicates nearly exact proportionality between intensity of excitation and

20 4-0 60 80 100

Seconds

Fig. 57-

Effect of varying the intensity of the exciting light (as indicated on each curve).
Exposure 2 minutes in each case.

initial phosphorescence. It is only with the most intense excitation used
that saturation begins.

It is to be observed that in most cases the values of I0 that are computed
from the data of Fig. 55 lie on the same curve as those obtained from
the data of Fig. 57. A well-marked difference is noticeable only in

300

200

100

^^ X

J3--"""

/ M

X'~''

X

Fig: 58.

Effect of the intensity of the excitation upon
the initial intensity of phosphorescence. The
points marked by dots are for an exposure of
2 minutes; those indicated by crosses are for
an exposure of 20 seconds. In curves B and
C the ordinates show the intensity of phos-
phorescence 1 minute after excitation hail
ceased.

10 20 30

Intensity of excitation

40

the case of the points corresponding to intense excitation. In other words,
for weak excitations the intensity of the initial phosphorescence is the same
after an exposure of 20 seconds as for 1 or 2 minutes. There at first appears
to be a contradiction here to the results shown in Figs. 53 and 54. But
while this may be due to the uncertainty in the values of I0, it is readily
explained upon the assumption that a weak excitation produces its full

DECAY OF PHOSPHORESCENCE IN SIDOT BLENDE- 67

effect more promptly. The form of the curves shown in Figs. 53 and 54
is probably largely dependent upon the intensity of the exciting light.

The ordinates of curves B and C (Fig. 58) show the intensity of phos-
phorescence 1 minute after the exciting light was cut off. For curve B the
exposure was 2 minutes, while for curve C the exposure was 20 seconds.
The effect of duration of exposure is here well marked.

In the case of exposures lasting for several seconds or more the phenomena
are manifestly complicated by the fact that the semi-permanent change,
to which we have already referred, is taking place in the active substance
during the time of excitation. It seemed probable that the relation
between intensity of excitation and intensity of phosphorescence might
prove simpler if the duration of excitation was reduced to a minimum. A
series of curves was therefore taken with a spark as exciting source. Pre-
liminary trials showed that the most intense excitation was furnished by
discharging eight large jars through a spark gap about 2 cm. long with one
cadmium terminal. The distance of the spark gap from the screen was
varied from about 10 cm. to 35 cm. A single spark at 10 cm. distance gave
an excitation approximately equivalent to 30 seconds exposure to the mer-
cury arc.

Experiments with the practically instantaneous excitation produced by
a single spark showed that the phosphorescence was proportional to the
intensity of excitation, not merely initially but throughout the whole
period of decay. In other words, it was possible to bring a decay curve
determined with the spark at a distance d2 into coincidence with the curve
corresponding to the distance d] by multiplying each ordinate 1 \ by the
ratio df/d]2.

EXPERIMENTS WITH DIFFERENT PHOSPHORESCENT SUBSTANCES.

It is natural to inquire whether the complex phenomena exhibited by
vSidot blende are peculiar to this particular material, or whether its behavior
is typical of a large class of phosphorescent substances. In order to test
this matter we have determined the decay curve under similar conditions
with three other substances, namely, " Emanations-pulver, M1 willemite,
and Balmain's paint. Characteristic curves for these three substances,
together with a representative curve for Sidot blende, are shown together
in Fig. 59. It will be noticed that the curves are all of the same type. In
each case the decay is at first rapid and apparently according to the same
law that was found to hold during the early stages of decay in the case of
Sidot blende. After 20 or 30 seconds the curves begin to bend, and finally
become straight lines whose slant is less than that of the earlier part of the
curve.

Upon plotting the data of E. Becquerel2 in the same manner we find that
in several cases the curves are of exactly the same type as those obtained
by us. Three such curves are shown in Fig. 60. In fact, all the data
recorded by Becquerel in his papers on this subject give curves which
show the same general characteristics, although in several instances the
curves are not so smooth as those shown in this figure.

'Obtained from Leppin and Masche, who do not state the composition of the powder. The chief constit-
uent is, however, zinc sulphide, so that the substance appears to be a modified form of Sidot blende. It is
said to be especially sensitive to the influence of the radio-active emanations.

*Becquerel, I.a Lumiere. Also Annales de Chimie et de Physique, Series 3, 62, 1861.

68

STUDIES IN LUMINESCENCE.

It is interesting to note also that the data of Darwin1 on Balmain's paint
give a curve similar to those obtained by us with this substance when
plotted in the same way- Two decay curves taken by E. Wiedemann2 with
Balmain's paint also show the same characteristics.

Cy

A

B^^

D

•

•

a£^~~~~

^*~~~"~5

^* (j^»

/vr

400
300
200
100

20 4-0 60 80 100 120 140 160 160

Seconds

Fig- 59-

Typical decay curves for various substances. Curve .4, Sidot blende; curve 1>, "Emanations-pulver;"

curve C, Willemite; curve D, Balmain's paint.

It appears therefore that the decay curve for Sidot blende is similar in
its main features to the curves obtained for a large number of other phos-
phorescent substances. In fact, we do not know of any case of long-time
phosphorescence in solids which shows a different type of curve.

300
200
100

cf

B,

?

o -,

A^

Fig. 60.

Decay curves plotted from the data of E.
Becquerel. (Ann. de Chimie et de
Physique, series 3, vol. 62, 1861.)

Curve A, Sulfure de calcium lumineux

vert (2, p. 69;.
Curve B, Sulfure de strontium lumineux

vert (p. 71).
Curve C, Sulfure jaune-orange vif (p

68).

200 400 600 800

Seconds

1000

The peculiar behavior of Sidot blende which we have compared with
magnetic hysteresis is also exhibited by willemite and Balmain's paint, as
illustrated in Figs. 61 and 62. In each of these cases it is evident that the
effect of a given excitation is dependent on the previous history of the sub-
stance. We have not yet had an opportunity to test this phenomenon in
other substances.

'Darwin, Philosophical Magazine, n, p. 209, 1881.

!E. Wiedemann, Zur Mechanik der Leuchtens, Wied. Ann., 37, p. 177, 1889.

DECAY OF PHOSPHORESCENCE IN SIDOT BLENDE.

69

SUMMARY.

The most important points brought out by the experiments here de-
scribed may be briefly stated as follows :

1. Form of Decay Curve. — The curve obtained by plotting the values of
7~* as ordinates and the corresponding values of / as abscissas is a straight
line for small values of / ; it changes to a curve concave toward the axis of t
as / increases; but for still larger values of / the relation between Z~* and

/vr

500

400

300

200

100

A/

/?>

B /

//

& E

v

cy

/>' *

ft '

/ r-4'

\ /'V

/ / * P

f/s

10 20 30 40

Seconds

50

60

100

200 300

Seconds

400

Fig. 61. — Willemite.

Excited by a spark between iron
terminals. The curves were taken
in the order indicated by the let-
ters and with the excitations
stated below:

Curve A, 0.7 sec; B, 3.4 sec; C,
9.0 sec; D, 60.0 sec; E, 3.9 sec:
F, 1.0 sec.

Fig. 62. — Balmain's paint.

The curves were taken in the order
indicated by the letters, and with
the following excitations:

Curve A, 5.9 sec; B, 12.0 sec; C,
26.0 sec. ; D, 42.0 sec; E, 6.4 sec

/ is again linear, and remains so until I becomes too small to measure. In
other words, the decay curve, when plotted in this way, consists of two
straight portions which gradually merge into one another.

2. Effect of Duration and Intensity of Excitation. — Not only the intensity
of phosphorescence, but also the form of the decay curve, is dependent on
the intensity and duration of excitation. The slant is altered in each of
the straight parts of the curve by changing either of these two factors in
the excitation.

70 STUDIES IN LUMINESCENCE.

3. Hysteresis. — The behavior of the phosphorescent substance with a
given excitation depends upon its previous history. Some semi-permanent
change is produced by excitation which persists for several hours, or even
for several days, after visible phosphorescence has ceased.

4. Effect of Red and Infra-red Rays. — In the case of Sidot blende the
semi-permanent condition produced by excitation may be destroyed and
the screen restored to a standard state by brief exposure to the red and
infra-red rays.

CHAPTER V.

THE INFLUENCE OF THE RED AND INFRA-RED RAYS UPON
THE PHOTO-LUMINESCENCE OF SIDOT BLENDE.i

The effect of the red and infra-red rays in suppressing the phosphorescence
of various substances has long been known,2 and has frequently been
utilized in the study of the infra-red spectrum. The effect is exhibited
by vSidot blende more strongly perhaps than by any of the other phos-
phorescent sulphides. In Chapter IV we have called attention to another
effect produced by the longer waves, namely, the restoration of a screen of
Sidot blende, after the excitation and complete decay of phosphorescence,
to a standard condition, so that the result of a subsequent excitation shall
be unaffected by the previous history of the substance. While this new
effect is doubtless connected in some way with that first mentioned, the
nature of the relationship between the two is by no means clear. For this
reason, and because of the bearing of the phenomena upon the general
theory of luminescence, we have investigated the influence of the longer
waves upon the luminescence of Sidot blende under a variety of different
conditions.

The work naturally falls under several heads, as follows:
i . The effect upon the luminescence of Sidot blende of exposure to the
longer waves before excitation.

2. The effect of the longer waves during excitation.

3. The effect of the longer waves after excitation, i. e., during the decay of
phosphorescence .

4. The influence upon the effect studied of the wave-length of the red
and infra-red rays used.

THE EFFECT OF THE LONGER WAVES BEFORE EXCITATION.

Experiments described in the preceding chapter indicate that when Sidot
blende is excited to luminescence " some change is produced in the material
by the action of the exciting light, and that this change persists for a con-
siderable period after a visible phosphorescence has ceased. In other words,
the effect of a given excitation in producing phosphorescence depends upon
the previous history of the phosphorescent substance. If the screen is
allowed to rest in the dark for a number of hours this semi-permanent effect
of exposure in part dies out. But rest alone does not restore the screen
completely even if continued for several days." An exposure of a few
seconds to the rays from a 50 c. p. lamp seen through ruby glass is, however,
sufficient to restore the screen to what seems to be a definite standard
condition.

'The substance of this chapter essentially as here given appeared in the Physical Review, xxv, p. 362.
-References to the literature of the subject are given by Dahms, Annalen der Physik, 13, p. 425.

71

72

STUDIES IX LUMINESCENCE.

The phenomenon in question is illustrated by the curves of Fig. 50, p. 62.
If we compare curves A, C, and D, all corresponding to the same excitation,
it is clear that exposure to the longer waves before excitation exerts a very
marked influence upon the rate at which the phosphorescence, excited after
this exposure, will decay. While the semi-permanent change produced
by excitation is partly lost as the result of prolonged rest in the dark, rest
alone is not a very satisfactory means of restoring the substance to a stand-
ard condition. Thus a rest of 24 hours brings about a change in the decay
curve following a 10-second excitation from C to A. Rest for several days
would shift the curve somewhat farther to the left. But even a rest of
several weeks failed to bring the decay curve as far to the left as curve D.

Curve D, Fig. 50, was taken after an exposure of 4 minutes to the longer
waves. A very much shorter exposure would have been nearly, if not quite,
as effective. This point is brought out by the curves of Fig. 63, which were
taken to determine the way in which the effect depended upon the duration
of exposure to the longer waves. In taking these curves the procedure was
as follows : In each case the screen was first exposed for 2 minutes to the

Fig. 63.

Effect on the decay curve of exposure to the infra-
red for different times.

Curve /, 10 seconds exposure after 48 hours rest in the
dark; II, 2 minutes exposure; ///, 10 seconds ex-
posure after 1 second infra-red; IV, to seconds
exposure after 3 seconds infra-red; V, 10 seconds
exposure after 60 seconds infra-red.

00 S rC

mercury arc ; the phosphorescence was then allowed to decay for 2 minutes,
at the end of which time, while still visible, it was too faint for measurement.
The screen was then exposed to the rays of a 50 c.p. lamp at a distance of
5 inches, a piece of ruby glass being interposed between the lamp and the
screen ; the duration of this exposure was 1 second for curve 7/7, 3 seconds
for curve 1 1 r, and 60 seconds for curve 1 '. After this exposure to the longer
waves the screen was excited by the mercury arc for 10 seconds, and the
decay curves shown in Fig. 63 were observed by the procedure described
on page 58.

It will be observed that even an exposure of only 1 second is more effec-
tive than 48 hours rest. It will be noticed also that 3 seconds exposure to
the longer waves is nearly as effective as an exposure of a minute.

With red and infra-red rays of less intensity a longer time is required
to destroy the effect of previous excitation. The curves of Fig. 64 were
taken with a procedure similar to that for Fig. 63, except that the distance
of the 50 c.p. lamp from the Sidot blende screen was 30 inches instead of 5
inches. Exposure for 60 seconds to these less intense rays produces as

influence; of red and infra-red rays upon sidot blende. 73

great an effect as a similar exposure to the stronger rays. But this is not
true for shorter exposures. While the ultimate effect of the weaker rays
is apparently the same, more time is required to produce the change when
the rays are of small intensity ; approximately, at least, the change produced
depends upon the product of intensity and duration of exposure.

The experimental data bearing upon this phase of the subject are so
meager as to permit of only the most general conclusions, and additional
experiments are much to be desired. So far as they go, however, the results
indicate that the condition in which the material is left after the excitation
and decay of phosphorescence is an unstable one, due perhaps to some new
grouping of the molecules of the phosphorescent material. During rest in
the dark accidental disturbances of various kinds may cause the substance
to return more or less completely to its normal condition. The effect of
rest is therefore uncertain, depending as it does upon the extent to which
various obscure and perhaps unrecognized agencies are active ; but certain
waves lying chiefly in the infra-red region of the spectrum have a definite
and positive effect in restoring the substance to its normal condition.

7

s

*

/

/

/"

J /'
1 7y

/

II-

/ ft

^^

i

/.

Fig. 64.

Effect on the decay curve of exposure to the infra-
red for different times. The intensity of the
infra-red rays was here only about j,, of the inten-
sity used for the curves of Fig. 63.

Curve/, 10 seconds exposure after 48 hours rest in
the dark; II, 2 minutes exposure; III, 10 seconds
exposure after 60 seconds infra-red; IV, loseconds
exposure after 15 seconds infra-red.

20

40

60

GO Sec. I0»

INFLUENCE OF THE LONGER WAVES DURING EXCITATION.

We have seen that a condition is developed in Sidot blende by excitation
which is favorable to the production of strong luminescence by a subsequent
excitation. A long excitation is therefore more effective than exposure
to equally intense exciting rays for a shorter period ; for the favorable con-
dition developed in the early stages of excitation makes the exciting rays
that act later more effective. The luminescence of such a substance during
excitation — i. e., the fluorescence — will be relatively weak when excitation
first begins and will increase in intensity as the exposure continues, reaching
a steady value only after a considerable time. In Sidot blende, with the
exciting light used in most of our experiments, 3 or 4 minutes were required
to reach a steady value.1

The steady condition finally reached is manifestly characterized by
equality in the rates of development and decay of the condition favorable
to luminescence to which we have just referred. If we think of this favor-

■See the preceding chapter of this memoir.

74 STUDIEvS IN LUMINESCENCE.

able condition as being due to some new grouping of the molecules, then
the condition of steady fluorescence is reached when these favorable groups
are being broken up, either spontaneously or through the action of some
outside agent, just as rapidly as they are formed by the action of the excit-
ing light.

It is clear that the intensity of steady fluorescence will be made less by
any agent which increases the rate at which the assumed favorable group-
ing is destroyed. Now it is precisely this effect that is exerted by the
red and infra-red rays; and we should therefore anticipate that the fluo-
rescence of vSidot blende would be diminished by the action of longer rays.

This effect of the longer waves, which does not appear to have attracted
much attention heretofore, may readily be made very marked indeed.
Thus the rays from a projecting lantern after passing through a sheet of
hard rubber 0.2 mm. thick are able to reduce the fluorescence of Sidot blende
so greatly as to leave the intensity only a few per cent of its normal value,
and this too with very intense excitation. If the ultra-violet rays of a
spark are used for excitation numerous lecture experiments may be devised
for demonstrating the existence of the invisible rays at the two ends of the
spectrum. As compared with the experiments first proposed by Dahms,1
in which the effect of the infra-red rays upon phosphorescence is utilized,
this procedure has the advantage of giving a persistent rather than a fleet-
ing effect.

In studying the influence of the longer rays upon fluorescence we have
directed our attention especially to the distribution of the effect through-
out the fluorescence spectrum. While the result of exposure to longer
waves is to diminish greatly the total brightness of the fluorescence light,
it might be that in certain restricted regions of the spectrum the intensity
would be increased rather than diminished, or at least that the effect of the
infra-red rays would vary greatly in magnitude in different parts of the
fluorescence spectrum.

For the study of this phase of the subject the Sidot blende screen was
mounted in front of a Lummer-Brodhun spectrophotometer, with an acety-
lene flame as a comparison source as in our previous work. The infra-
red rays from an arc fell upon the screen after passing through hard rubber.
The intensity of fluorescence was then measured in different parts of the
spectrum, first with and then without the action of the longer waves, the
excitation remaining constant. As exciting source a mercury-vapor lamp
was first used, the lamp being made of the so-called "Uviol" glass, which
possesses an unusual transparency to the ultra-violet rays.

In Fig. 65, curve /, shows the ordinary fluorescence spectrum of Sidot
blende produced by the mercury lamp, while curve 77 shows the spectrum
as modified by exposure to the infra-red. In the case of curves /' and 77' a
sheet of ordinary glass was interposed between the lamp and the screen, so
that the ultra-violet rays were in large part removed from the exciting light.
It is clear that the ultra-violet rays of the Uviol lamp introduce a band
at about 0.49 /x which overlaps and distorts the usual green band at 0.51 n.

A more annoying source of disturbance in these experiments, however,
was the light reflected from the screen, which was mixed with fluorescence

■Dahms, /. c.

INFLUENCE OF RED AND INFRA-RED RAYS UPON SIDOT BLENDE. 75

light and practically inseparable from it. By making observations only
at points lying between the bright lines of the mercury spectrum we had

Fig. 65.

Effect of infra-red on fluorescence. Sidot blende excited by
" Uviol " mercury -vapor lamp.

Curve /. Fluorescence spectrum.

Curve //. Fluorescence spectrum when screen is exposed to

infra-red during excitation.
Curve /'. Fluorescence spectrum with plate glass between screen

and mercury lamp.
Curve II'. Same as /', except that screen is also exposed to

infra-red.
R. Reflection of exciting light from white surface.

M M .48 .50 SZ _B4 .bfe/Z.

expected to be untroubled by reflected light. But owing either to optical
imperfections in the apparatus or to the existence of a faint continuous
spectrum in the light from the lamp, there was 18
always enough reflected light in the field of the
spectrophotometer to be an important and dis-
turbing factor. To get some idea of the inten-
sity and distribution of this reflected light we
made the observations plotted as curve R in Fig.
65 with a screen of MgO on cardboard instead
of the Sidot blende screen. To avoid confusion
this curve is displaced downward in the plot.
The intensity for points on curve R should be
read from the right-hand side of the figure.

The irregular distribution of the reflected
light and the great uncertainty in its measure-
ment make the experiments plotted in Fig. 65
of little quantitative value. Especially is this
true for the violet end of the spectrum, where
the reflected rays are of great intensity. We
could not even feel sure that the longer waves
produced any effect at all in this region.

With a different zinc sulphide screen, the
so-called "Emanations-pulver" referred to in
Chapter IV, the conditions were somewhat
more favorable. The curves in Fig. 66 show
the ordinary fluorescence spectrum (curve /) ;
the fluorescence spectrum with exposure to
weak infra-red rays (curve 77) ; the fluorescence
spectrum during exposure to strong infra-red
rays (curve 77/) ; and the reflected light deter-
mined as before. Different intensities of infra-red were obtained by using
in one case one piece of black rubber and in the other case two pieces
between the arc lamp and the screen. By this procedure it is possible to

Fig. 66.

Effect of infra-red on fluorescence
of "Emanations-pulver," ex-
cited by Uviol lamp.

76

STUDIES IN LUMINESCENCE.

determine the ratio of the effects produced by strong and weak infra-red,
in spite of the uncertainty in the value of the reflected light. Thus the
difference between the ordinate of curve I and the corresponding ordinate
of curve II is a measure of the effect produced by weak infra-red ; reflected
light, since it affects both measurements, is eliminated by taking their differ-
ence. Similarly the difference between the ordinates of curves / and III
measures the effect of the stronger infra-red. It is interesting to note that
the ratio of these effects is nearly constant throughout the green band.
Beginning at 0.562 /* and running toward the violet the ratio has the values :
1.32, 1.36, 1.3 1, 1.22, 1.94. Except for the last point, which is so near the
edge of the band that the intensity of fluorescence is small, the values are
constant to within observational errors. This fact adds another to the

many that have been observed in the course of our
work on luminescence to indicate that each band
in a luminescence spectrum behaves as a unit —
that whatever affects one part of the band affects
all other parts of the band in the same proportion.
A more satisfactory method of studying the
effect in question is to use only ultra-violet light
in excitation. All troubles due to reflected light
are in this case removed. The results of this pro-
cedure in the case of the original Sidot blende used
in our earlier experiments are shown in Fig. 67.
An iron spark was used as an exciting source, a
spectrum being formed by a quartz train and only
the ultra-violet rays used. The source of infra-
red was an arc lantern whose rays passed through
a sheet of hard rubber 0.2 mm. thick. The effect
of exposure to longer rays during excitation by the
ultra-violet rays of the iron spark was to change
the fluorescence from curve I to curve 77. The
diminution in intensity brought about by exposure
to the longer rays, expressed as a fraction of the
ordinary fluorescence at the same wave-length, is
given in curve III.
More extended experiments are required to determine whether the change
in the effect from 38 per cent at 0.546 fx to 20 per cent at 0.480 ju is real or
the result of errors. It does not seem likely, however, that experimental
errors alone can account for so great a change. In interpreting the results
we must bear in mind the fact that the spectrum shown in Fig. 67 obviously
consists of two overlapping bands, and that the infra-red effect may differ
for the two. It is highly probable also that still another band is present
at 0.49 n, as was found to be the case with ultra-violet excitation in the
experiments plotted in Fig. 65; and for this band the effect of the longer
waves may be different still. It is clear that further experiments on this
branch of the subject are needed.

EFFECT OF THE LONGER WAVES DURING DECAY.

In studying the effect of the infra-red rays upon the decay of phosphores-
cence two methods were used. In the first of these the intensity- of the

Effect of infra-red upon fluores-
cence of "Emanations-
pulver" excited by ultra-
violet rays of an iron spark.

INFLUENCE OF RED AND INFRA-RED RAYS UPON SIDOT BLENDE. 77

total phosphorescent light was measured by a photometer at different
times after excitation had ceased, as described in Chapter IV. The violet
end of the carbon arc spectrum was used for excitation, and a 50 c.p.
incandescent lamp as a source of infra-red rays. In these experiments a
cell containing a solution of iodine in carbon disulphide was used instead
of hard rubber to remove the visible rays. The distance of the lamp
from the Sidot blende screen was about 60 cm.

The curves of Figs. 68 and 69 show some of the results obtained by this
procedure. In Fig. 68, curve I is the ordinary decay curve without expo-
sure to infra-red. In the case of curve 777 the infra-red rays were allowed
to fall on the screen when the decay had proceeded for about 32 seconds.
In curve II the infra-red rays were turned on about 4 seconds after the

JL

■

V

u^°

K

1

(

/
/

/

S j

^

1

1
t S"/

Y.

Fig. 68.

I nfluence of exposure to infra-red
during decay upon the form of
the decay curve.

Curve/. Ordinary decay curve.

Curve //. Screen exposed to
infra-red after decay had pro-
ceeded for 4 seconds. Infra-
red cut off at / = 19 seconds.

Curve ///. Screen exposed to
infra-red after decay had pro-
ceeded for 32 seconds.

?o

40

60

60

100

ISO Sec. 140

end of excitation and were cut off again at the end of about 19 seconds.
The great increase in the rapidity of decay brought about by infra-red rays,
even when of such small intensity as those used in these experiments, is
clearly shown.

There was no indication in these experiments of any temporary increase
in the brightness of phosphorescence when the rays were first turned on,
as has been noted by many observers in the case of Balmain's paint and
other phosphorescent sulphides. Dahms has already called attention to
this peculiarity of Sidot blende. The effect of the longer waves in suppress-
ing phosphorescence has sometimes been explained by assuming that these
rays act in the same way as does a rise of temperature; i. c, that they
accelerate the process which causes phosphorescence, so as to produce a
brief flash, due to the sudden liberation of the energy stored during excita-
tion, followed by a complete loss of luminescence when the stored energy
has been used up. While this explanation of the phenomenon may be
correct for the other phosphorescent substances it can not be applied with-
out essential modification to the case of Sidot blende. It has recently been
shown, however, by Ives and Luckiesh1 that under some circumstances a

Physical Review, xxxir, p. 240, 1911. The specimen of Sidot blende that was tested by Ives and
Luckiesh differed from that used by us and other observers in the fact that the law of decay after one minute
was I~~0,*7 = a+bt instead of / — '* = a-\-bt. It thus forms the only exception known to us to the law of de-
cay discussed in Chapter XV.

78

STUDIES IN LUMINESCENCE.

flash may be observed immediately after exposure to infra-red rays even
in the case of Sidot blende. These observers find that "there are two stages
in the decay of phosphorescence, merging gradually one into the other. In
the first stage the effect of long waves is to cause a sudden drop in intensity ;
in the second stage the long waves cause a temporary increase of brightness
before decay. In the intermediate stage the accelerated decay appears to
the unaided eye to be delayed in starting." With intense infra-red rays
the preliminary flash could be made to occur as early as 30 seconds after
the end of excitation.

Perhaps the most striking feature of the curves in Fig. 68 is the nearly
exact parallelism that exists between the later part of curve 7/7 (after the
infra-red rays were cut off) and the straight part of the ordinary decay
curve. The action of the longer waves appears to be to bring the material
quickly into the same condition as regards ability to emit light that it
would have acquired at the end of a much longer period of ordinary
decav. The results of Ives and Luckiesh are not in accord with this view.

11

/

>

/if

If

y^

[/

1 /

1

*

;

u

Fig. 69.

Influence of exposure to infra-red during decay
upon the form of the decay curve.

Curve /. Ordinary decay curve.

Curve //. Screen exposed to infra-red after the
decay had proceeded for 18 seconds.

Curve ///. Screen exposed to infra-red during exci-
tation and for first 16 seconds of decay.

Curve IV. Screen exposed to infra-red during exci-
tation and for first 9 seconds of decay.

ZQ

40

CO

60 C«c. 100

In the brief abstract of their work referred to on page 77 they state that
"the decay curve after brief action of red or infra-red does not correspond
with the original curve with origin shifted."

In the case of curves III and I V of Fig. 69 the screen was exposed to
infra-red during excitation and during the early stages of decay. The
infra-red rays were cut off at the points indicated by the break in the curves.
It will be noticed that the latter portion of curve III is nearly parallel to the
straight part of the ordinary decay curve; but in curve IV, where the infra-
red was cut off earlier, the straight part of the curve is not even approxi-
mately parallel to curve I. The results shown in Fig. 69 are thus in agree-
ment with the statement of Ives and Luckiesh.

For the early stages of decay a number of curves were taken by means
of the spectrophotometer, the method being that described in Chapter III.
With this method it is impracticable to follow the decay for more than a
few seconds, since the illumination of the spectrophotometer field soon
becomes too faint for accurate measurements. The method possesses a
great advantage, however, in the fact that the effect of the long waves can
be determined for different parts of the phosphorescence spectrum.

INFLUENCE OF RED AND INFRA-RED RAYS UPON SIDOT BLENDE. 79

For the curve shown in Figs. 70 and 71 the exciting light was the violet
of the carbon arc spectrum. For the curves of Figs. 72 to 75 a spark between
cadmium terminals was used in excitation. The intensity of phospho-
rescence has been plotted in all of these curves instead of the reciprocal
square root, although in some cases the latter value has been plotted also.

100r
I

Fig. 70.

Influence of exposure to infra-
red on form of decay curve.

Curve /. Ordinary decay curve

for X =0.497 tx.
Curve /'. Screen exposed to

infra-red immediately after

end of excitation.

6 Sec?

100

I

\

f

/

tJU

\

&-'

40

\I

\l

to

Fig. 71.

Curve /. Ordinary decay curve for X =0.497 M-
Curve /'. Screen exposed to infra-red after decay
had proceeded to A.

O, Sec. S

Fig. 72.

Decay curves with and with-
out infra-red. X =0.5460.

8o

STUDIES IN LUMINESCENXE.

In all these figures the curve marked I was taken without infra-red1 and
that marked /' with infra-red. In general the exposure to infra-red began
at the instant the excitation ceased, the shutter being arranged so that the
same movement that cut off the exciting rays allowed the infra-red rays to
fall upon the screen. In the case of Fig. 71 exposure to the infra-red did
not begin until about 1.4 seconds after the end of excitation. Numerous
curves of this kind were taken in which the exposure to infra-red began at
different times after the beginning of decay. All of these curves show the
same sudden drop in intensity at the instant that the long waves begin to
act.

The curves of Figs. 70, 71, and 72, except for the method of plotting, are
quite similar to the curves for the total light obtained by the photometer
method first described. Apparently the action of the longer waves during
the first few seconds of decay is quite similar to its action later. It will

be observed that the curves of Figs.
70 to 72 refer to regions of the phos-
phorescence spectrum either near
the maximum or toward the red
edge of the band.

Figs. 73, 74, and 75 show the in-
fluence of the longer waves upon
those regions of the fluorescence
spectrum lying near and beyond the
violet edge of the green band. Here
the effect seems to be entirely dif-
ferent. At 0.445 M (Fig. 75) the
effect of exposure to infra-red is to
retard the decay of phosphorescence
instead of to accelerate it. In the
region lying between the green band
(0.51 /x) and the violet band (0.45 /j.)
the effect of the infra-red is at first
to retard the decay and later to ac-
celerate it (Figs. 73 and 74).

Owing to the faintness of the spec-
trophotometer field and the rapidity
with which the phosphorescence decays it is difficult to determine the form
of the curves in the early stages of decay with accuracy. Especially is this
true in the blue and violet, owing to the small luminosity of this region of
the spectrum. Each point plotted represents, however, the average of a
number of separate readings. For each pair of points the observations
for the time of decay with and without infra-red were taken alternately.
No special precautions were taken to keep the exciting source constant.
The slight initial curvature of the line I~- in Figs. 70 and 73, where the
exciting light was from the carbon arc, is perhaps to be explained as the
result of variations in this source. In the case of the other observations, in
which a spark was used in excitation, the points for I~* lie reasonably well

'The carbon arc was used as a source of infra-red, a piece of dense ruby glass serving as a filter. Red
light, and perhaps a little yellow light, was therefore present in addition to the infra-red. There has been
nothing in our experiments to indicate that the presence of these visible rays modifies the results in any way.

CO
60

\

A

\ \

\

\

1

so

oO

A

s\

S

^

SO

c

10

\

_l

u\

\^X

Fit

4- Sec. 5

73-

Decay curves with and without infra-red.
X =0.474 M-

influence; of red and infra-red rays upon sidot blende. 8 1

upon a straight line, and thus give a check upon the accuracy of the obser-
vations.

The remarkable reversal in the effect of infra-red in passing through the
luminescence spectrum received ample qualitative confirmation. With
the spectrophotometer set for some region in the blue or violet the bright-
ness of the field increased noticeably for a few seconds when the screen was
exposed to infra-red during decay, even when the exposure first began
several seconds after the end of excitation. The same effect was observed
in the case of " Kmanations-pulver " and Balmain's paint. In the case of
the latter substance the flash that accompanied exposure to the longer
waves developed more slowly and lasted longer than in the case of Sidot
blende.

Two interpretations of the results brought out in Figs. 70 to 75 suggest
themselves. Neither, however, is wholly satisfactory.

t '£ 3

Fig- 74-

Decay curves with and without infra-red.

•4 Sec. «5 « ^seey

Fig- 75-
X =0.464 m (Fig. 74) and X =0.445 m (Fig. 75).

In all of these experiments the luminescence spectrum consisted of two
bands, namely, the green band at 0.5 1 /x and the violet band at about 0.45 /x.
It is possible that the infra-red rays retard the decay of phosphorescence
in the case of the violet band and accelerate it in the case of the green band.
In the curves of Fig. 75 we are dealing with the violet band only; in Figs.
70, 71, 72, and 74 the light is almost entirely from the green band; but at
wave-lengths 0.474 M (Fig. 73) and 0.464 /x (Fig. 74) the light entering the
collimator slit comes partly from one of these bands and partly from the
other. The violet band apparently decays more rapidly than the green
band. (Compare the slant of the line for J~* in Fig. 75 and Fig. 72.)
If the violet band is initially the brighter of the two the retarding effect
of the infra-red upon the decay of this band will predominate in the early
stages of decay. Later, when the violet band has nearly died out and the
light is chiefly due to the green band, the opposite effect will predominate.
The two curves / and /' will therefore intersect, as shown in Fig. 72 and

Fig. 74-

Two objections may be urged to this explanation. If the light in the
case of Figs. 73 and 74 is from two bands that decay at different rates we
should hardly expect the relation between / and J~* to be as simple as the

82 STUDIES IN LUMINESCENCE.

linear relation that holds for the green band alone. Yet the deviation from
a linear relation in both these eases is well within the errors of observation.
Again, if the infra-red rays increase the brightness of the violet band after
excitation has ceased it would seem reasonable to expect a similar effect
during excitation. Yet the effect during excitation (Fig. 67) is nearly the
same for both bands.

We were first led to expect increased brightness in the violet during
exposure to the infra-red, and to undertake experiments in the hope of
detecting such an effect, as the result of an entirely different line of reason-
ing. Looking upon phosphorescence as due to the recombination of ions
dissociated by the action of the exciting light, we may explain the fact
that the phosphorescence light is of greater wave-length than the exciting
light (Stokes's law) briefly as follows : Dissociation results from the violent
resonant vibration of a neutral molecule of the active substance under the
influence of the exciting waves. The wave-length of maximum resonance
and therefore maximum excitation is determined by the natural period of
vibration of the active molecule, which is influenced to some extent, but
not greatly, by the surrounding solvent. The charged ions resulting from
excitation will, however, be attracted by the neutral molecules of the sol-
vent and will form the nuclei of heavy aggregations of molecules; and
recombinations of the ions will therefore occur under conditions which
make the resulting vibrations longer, on the whole, than the period of the
active molecules before dissociation; hence the well-known displacement
of the luminescence spectrum with reference to the absorption spectrum.

Now the effect of the infra-red rays may be to so shake up the molecules
of the solvent as to prevent the loading down of the ions by the attraction
of neutral molecules, or to destroy such heavy aggregations if already
formed. Under the influence of the infra-red, therefore, the light emitted
will be due largely to the vibrations that occur during the recombination
of unloaded ions and will be of the same wave-length as that which the
active substance absorbs. If the screen is exposed to infra-red rays after
excitation we should expect a decrease in the intensity of phosphorescence
throughout the phosphorescence band due to the breaking down of the
groups of molecules referred to above. But owing to the resulting increase
in the number of unloaded ions we should also expect the emission of light
whose wave-length is that of the resonant absorption band of the substance.
Now the absorption band always lies on the ultra side of the luminescence
band, and usually the two bands overlap. (That this is the case with Sidot
blende is shown by the fact that this is one of the substances for which
vStokes's law, in its strict form, is violated.) Exposure to infra-red should
therefore produce increased intensity near and beyond the violet edge of the
phosphorescence band; which is exactly what we have observed.

In the region where the absorption and emission spectra overlap, the
effect will be more complicated. While the light in this region due to the
ordinary luminescence band will diminish, there will be at the same time
a temporarily increased emission due to the recombination of the unloaded
ions that are shaken loose by the infra-red vibrations. A bright flash
immediately after the exposure to infra-red, followed by decay more rapid
than the normal, is therefore to be expected in the intermediate region
corresponding to Figs. 73 and 74.

INFLUENCE OF RED AND INFRA-RED RAYS UPON SIDOT BLENDE. 83

The effect of the longer waves during excitation is unfortunately as hard
to reconcile with this explanation of the phenomena as with that first
suggested.

VARIATIONS OF THE EFFECT WITH THE LENGTH OF THE LONGER WAVES.

In the case of several phosphorescent substances, including Sidot blende,
the effect of rays of different wave-length in suppressing phosphorescence
has been studied photographically by Dahms.1 Our own results, obtained
by an entirely different method, in general confirm his conclusions in a very
satisfactory way.

The arrangement of apparatus was similar to that used in our first experi-
ments on the decay of phosphorescence.2 A spark was used for excitation
and a shutter was so arranged that the screen was exposed to the infra-red
rays and the phosphorescent light allowed to fall on the slit of the spectro-
photometer at the same instant that the excitation was brought to a close
by short-circuiting the spark. A Nernst glower was used as a source of
infra-red rays. This was mounted in the place of the slit of a large mirror
spectrometer having a quartz prism.3 The Sidot-blende screen was covered
with black paper except for a narrow rectangular region having about the
same width as the Nernst glower. The adjustment of the spectrometer
having been determined by observations in the visible spectrum, wave-
lengths in the infra-red were computed from the angle of deviation. The
effect of different rays from the Nernst glower was measured by the differ-
ence between the times required for the phosphorescence to fall from its
initial intensity to a definite final intensity with and without exposure to
the rays to be tested. Each point of the curves shown in Fig. 76 is deter-
mined from the average of ten observations with infra-red and ten without,
the observations being made alternately. The difference between the two,
expressed as a fraction of the normal time of decay, has been plotted for
the different wave-lengths used, which ranged from 0.6 /x to 2.3 /x. The
observations refer to the region of maximum intensity in the phosphores-
cence spectrum (0.512 xx).

Referring to Fig. 76, it will be seen that the effect of the longer waves is
observable to some extent in the visible region. A maximum is reached at
about 0.9 /jl, followed by a minimum at about 1.0 /x and another maximum
at 1.3 xx. From 1.2 /x on the observations were repeated under slightly
different conditions the following day (see broken line). In this case the
chief maximum appears to lie at 1.37 /x. The results are probably in all
cases uncertain to the extent of 2 or 3 per cent, and errors are especially
likely to be serious in regions where the effect is small. For this reason we
can not feel certain of the third maximum at 2 . 1 8 xx, although the probability
is that it really exists. As far as any important effect is concerned, how-
ever, our results confirm the conclusion of Dahms that the action does not
extend beyond 1.5 xx.

It can scarcely be doubted that absorption of the active rays is necessary
before they can produce any effect upon phosphorescence. It seems

'Dahms; Annalen der Physik, 13, p. 425.
:See Chapter III of this memoir.

3Tests (by direct eye observation) with a rock-salt prism showed that no effect was observable for wave-
lengths longer than those transmitted by quartz.

84

STUDIES IN LUMINESCENCE.

probable, therefore, that Sidot blende possesses broad absorption bands
with maxima not far from 0.9 ju and 1.35 fx. Experiments with other phos-
phorescent substances having ZnS as their base will be necessary to deter-
mine whether the absorption that determines this effect on phosphorescence
is characteristic of the solvent (ZnS) or of the dissolved metal causing lumi-
nescence. It is interesting to note, however, that in the absorption

26
24
22

20

15

16

+-> 14
C
(D
U 12

£ '0

8

6

4

2

0

-2

Fig. 76.

Effect of infra-red rays of different wave-length. Ordinates represent the percentage diminution in the

time of decay under the influence of infra-red.

spectrum of sphalerite (ZnS) Coblentz has found evidence of bands
at about 0.9^1 and 1.4 ju. While Coblentz's work does not indicate great
absorption at these points, it must be remembered that they fall in the
most intense region of the spectrum of a Nernst glower, so that the total
amount of energy absorbed might be very considerable.

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CHAPTER VI.

VARIATIONS IN THE DECAY OF PHOSPHORESCENCE PRODUCED

BY HEATING.

In connection with the investigations described in Chapters IV and V
Dr. C. A. Pierce, during the years 1906-1908, made an extended study of the
phosphorescence of Sidot blende and of Balmain's paint at temperatures
ranging from room temperature to 3oo°C. The present chapter contains a
summary of his results.1

EXPERIMENTS WITH SIDOT BLENDE.

The apparatus used was similar in principle to that described in Chap-
ter IV, p. 58, but was modified to adapt it to the conditions of the experi-
ment. The substance studied was the phosphorescent zinc sulphide already
referred to as "Emanations-pulver." It was placed in a shallow dish and
heated by means of an electric furnace capable of being readily removed and
replaced in its position relative to the powder without disturbing the latter.

The temperature of the powder was measured by means of a thermo-
junction embedded in the mass. Phosphorescence was excited by a mer-
cury-arc lamp of the L,ummer type.

By means of properly placed mirrors the exciting light was reflected upon
the surface of the powder and the phosphorescent light was reflected
through a double window of mica and plate glass into the field of a Lummer-
Brodhun photometer. The adjustments were such that the conditions of
illumination and observation were the. same whether the powder was within
the furnace or outside. The comparison light was an acetylene flame with
diaphragm. A color match was obtained by the interposition of colored
glasses and the balance of illumination in the field of the photometer by
moving the comparison light.

The apparatus was arranged so that a single observer, working in a dark
room, could automatically record all observations such as the times of open-
ing and closing the shutter of the exciting lamp, the beginning and end of
heating, the times at which photometric observations were made, and the
position of the comparison light on the photometer bar. The observer
could likewise make all manipulations during a complete run without taking
his eye from the observing telescope. Before each set of observations
the powder was exposed to infra-red rays for 1 minute and allowed to re-
main in darkness for 5 minutes before excitation in order, as already de-
scribed in Chapter IV, to secure a standard condition. In the present case
the action of the infra-red was not sufficient to destroy all thermo-lumines-
cence when the powder was heated to 3500 C, but the effect was so reduced
that even after the strongest previous excitation the thermo-luminescence
could not be measured with the photometer. The infra-red rays, which

>C. A. Pierce. Physical Review, xxvi, pp. 312 and 454.

86

STUDIES IN LUMINESCENCE.

were from a 16 c.p. incandescent lamp with a screen of thin vulcanite,
heated the powder slightly, but the wait of 5 minutes allowed it to cool
to approximately room temperature.

The effect of temperature upon the decay curve, when the Sidot blende
is excited and allowed to decay at the same temperature, is shown in Fig. 77.

Fig. 77-

Effect of temperature during excitation and decay. Excited 40 seconds. Curve A, temperature 210 C;
curve B, temperature 370 C; curve C, temperature 6o° C; curve D, temperature 850 C

The curves are plotted with distances of the standard lamp from the photo-
meter screen as ordinates and time reckoned from the end of excitation as
abscissas. The ordinates are therefore inversely proportional to the square
roots of the intensities. Curves ^1 and B are concave downward through-
out, while C and D are approximately
straight lines. In fact, the points on
curve D indicate an upward bending.
The initial intensity evidently increases
and the decay becomes more rapid as
the temperature is raised.

To further test the form of the curve
of decay at high temperatures, a series
of runs was made with constant length
of excitation and constant temperature.
Four runs of the series are shown in
Fig. 78. If the curves had been plotted
with the actual times as abscissas, they
would coincide. In order that the eye
may be able to distinguish readily the
points on each curve, the four curves
are separated by plotting curve D with
the abscissas marked on the figure and
displacing the remaining curves each
5 seconds farther to the right. It is
evident that the curves are straight lines throughout the time of observa-
tion, but it is probable that they would show a downward bending nearer
the origin. Studies of curves of very rapid decay, to be described in
Chapter VII, greatly strengthen this view. The conclusion that at room

1

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Fig. 78.

Effect of temperature during excitation and
decay. Excited 400 seconds. Tempera-
ture 930 C

DECAY OF PHOSPHORESCENCE PRODUCED BY HEATING.

87

temperature the observed part of the decay curve is concave downward
and becomes less concave as the temperature is raised is substantiated by
several series of curves which are not reproduced.

A possible explanation of the changes in the decay curve that occur
when Sidot blende is excited at different temperatures may be deduced as
follows : Let it be assumed that the law of decay for a single band in the
spectrum is1

I=(a + bt¥

Let it be assumed that the phosphorescent spectrum of Sidot blende con-
sists of two bands, and let curves AB and CD, Fig. 79, represents the decay
of these bands. The decay of the total intensity can be computed by the
equation2

L^ia+bty]

D

and is represented in

Fig.

OP. This curve is concave

79 by the curve
downward throughout but approaches a straight line with increase in time.
From Figs. 77 and 78 it can be seen that the effect of a higher temperature
is to hurry the decay. Hence the slope of the lines representing the decay

D

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Fig. 79.

AB, decay of one band at a certain temperature;
CD, decay of the other band at the same tem-
perature; OP, decay of the total intensity due to
the two bands; EF, decay of the first band at a
higher temperature; GH, decay of the other band
at the higher temperature; MN, decay of the
total intensity at the higher temperature.

of the separate bands3 will be greater at a higher temperature. MN, Fig.
79, represents the decay of the total intensity at the higher temperature.
M N is concave downward throughout, but if the portion occurring within
the first few seconds were not drawn, the remainder of the curve would

■The law proposed by H. Becquerel. See Chapter XV.

2The law proposed by H. Becquerel for the decay of the total intensity when the phosphorescent spectrum
consists of more than one band.

aThe existence of two bands in the fluorescence spectrum of Sidot blende has been demonstrated in
Chapter IV. The presence of twosuchbandsinthe case of the powder studied by Dr. Pierce was established
by him by means of spectrophotometric measurements. Their maxima were found to be approximately
at 0.53 j» and 0.41 m- The same bands might therefore be expected in the phosphorescence spectrum.

88

STUDIES IN LUMINESCENCE.

appear to be a straight line, while under the same conditions the curve OP
would still be seen to be concave downward.

The curves in Fig. 79 represent closely the facts deduced from Figs. 77
and 78. They also agree with the observations recorded in Chapter IV,
which show that the decay curve consists of two straight lines gradually
merging into one another. By changing the slopes and intercepts of the
lines in Fig. 79 representing the decay of the single bands, a decay curve of
the total intensity can be derived which will exactly lit the deductions
mentioned above. Dr. Pierce's later experiments on the decay in Sidot
blende (Chapter VIII) nevertheless make it seem very doubtful whether
this explanation of the form of the decay curve, and of its dependence upon
temperature, can be made to accord with all the experimental facts.

The effect of heating the Sidot blende after it has been excited at room
temperature and allowed to decay at room temperature to a small fraction
of its initial intensity of phosphorescence is illustrated in Figs. 80, 81 , and 82.
The result of this process is the well-known phenomenon of thermo-lumines-
cence. The curves are plotted with intensities as ordinates and times, meas-
ured from the moment of heating, as abscissas. Higher temperatures increase
the intensity of the outburst of light, and cause the maximum to occur
sooner after the beginning of heating and to die away more rapidly.

In the case of Fig. 80 the initial intensity immediately after excitation
was somewhat greater than I =12. The intensity was allowed to decay to
/ = 0.8, when heating was begun. In the case of Fig. 82, the initial intensity
was somewhat greater than 1 = 37.

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Fig. 80.

Effect of heating when the phosphorescence has decayed to an intensity equal to 0.8. Excited 10
seconds at room temperature. Curve A, heated to 3070 C; curve B. heated to 2700 C;
curve C, heated to 208° C; curve I), heated to 1550 C; curve E, heated to i)<)° C.

The effect of heating after excitation may be considered in one of two
ways. Either it suddenly releases the energy represented by the phos-
phorescence, or else it sets up some new reactions in the powder. Though
the decay had reached a low intensity before heating was begun in the runs
shown in Figs. 80, 81, and 82, yet at this low intensity the decay was slow;
hence there may have been considerable energy still left, which heating

D£CAY OF PHOSPHORESCENCE PRODUCED BY HEATING.

89

released to give the flash peculiar to thermo-lumineseence. If this is the
case, the areas between the curves in either Fig. 80, 81, or 82 and the coor-
dinate axes should be equal to each other. It is impossible to get experi-
mental data with which to draw the curves to the axis, so the author pro-
jected the curves tentatively. This is not an entirely rash thing to do,

Fig. 81.

Curves similar to those in Fig. 81. Excited 80 seconds at room temperature. Curve A, heated to
308° C; curve B, heated to 2670 C; curve C, heated to 206° C; curve D, heated to 155.50 C;
curve E. heated to 99° C.

because a slight variation in the prolongations will have effect; furthermore,
while the low intensities were not measurable, one could nevertheless get

Fig. 82.

Curves similar to those in Figs. 80 and 81. Excited 320 seconds at room temperature. Curve A,
heated to 309° C; curve B, heated to 2660 C; curve C, heated to 207° C; curve D, heated to
153° C; curve F, heated to 980 C

some idea of the rapidity of decay by noticing how rapidly the photometer
screen became dark.

Fig. 83 shows the areas plotted with temperatures as abscissas.

If the areas had been equal to each other for a given excitation, each of
the curves in Fig. 83 would have been a straight line parallel to the tempera-
ture axis. Aside from the low values at 3000, which are probably due to

90

STUDIES IN LUMINESCENCE.

the approach to temperatures at which the substance loses its power to
phosphoresce, the curves suggest by their approximation to horizontal
lines that the function of heating is merely to release the stored energy
more suddenly.

If this explanation of the effect of heating is correct, then it would be
reasonable to expect that in the case of the curves of Fig. 77 the decay would
be more rapid the higher the temperature, which effect has already been
pointed out.

Fig. 84 shows the change in the maximum ordinate of the curves of thermo-
luminescence as the temperature is raised. No weight is given to the exact
shape of the curves, the points being connected merely to aid the eye in
distinguishing them. The curves are in accordance with what would be
expected if the function of the temperature is to liberate suddenly the
phosphorescent energy. If this is the case, the greater the temperature
the quicker the energy will be liberated and the brighter the flash. The
relation of maximum intensity to temperature is shown more clearly in
Fig. 85.

To ascertain the rapidity with which the powder within the furnace was
heated, measurements were made for each of the temperatures at which

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Fig. 84

Fig. 85.

Fig. 83. — Areas between the thermo-phosphorescence curves and the coordinate axes. Curve .4, excited
320 seconds (from Fig. &2) ; curve B, excited 160 seconds; curve C, excited 80 seconds (from Fig. 81) ;
curve D, excited 40 seconds; curve E, excited 20 seconds; curve F, excited 10 seconds (from Fig. 80).

Fig. 84. — Change in the maximum intensity as the temperature is raised. Maximum intensity vs. time of
maximum intensity measured from the beginning of heating. Curve A, excited 320 seconds (from
Fig. 82); curve B, excited 160 seconds; curve C, excited 80 seconds (from Fig. 81); curve D, excited
40 seconds; curve E, excited 20 seconds; curve F, excited 10 seconds (from Fig. 80). Each point on
a curve is for a given temperature. On each curve the lowest point is for 90° C. and for the other points
in consecutive order 155°, 207°, 2670, and 3080 C, respectively.

Fig. 85. — Increase of maximum intensity of thermo-Iuminescence with increase of temperature. Curve A,
excited 320 seconds (from Fig. 82); curve B. excited 160 seconds; curve C, excited So seconds (from
Fig. 81); curve D, excited 40 seconds; curve E, excited 20 seconds; curve F, excited 10 seconds
(from Fig. 80).

curves of thermo-Iuminescence had been obtained, and from these curves
were plotted showing the rise of temperature of powder and the variation
in the temperature of the furnace. Fig. 86 gives the curves for two of these
temperatures. The general conclusion from these measurements was that
the powder reached a constant temperature in a constant time independent
of the temperature. The periods of the two galvanometers used in the
temperature measurements were too short to affect the shape of the curves
appreciably.

The method of heating employed possesses several advantages. The
temperatures are known and can be accurately reproduced as many times as
desirable. Further, the gradual heating allows the flash to be followed in

DECAY OF PHOSPHORESCENCE PRODUCED BY HEATING.

91

the photometer. For some other work a strip of platinum which supported
a thin layer of the powder was heated by passing an electric current through
the strip. The flash in this case occurred too rapidly to be followed, the
maximum intensity being the only measurement possible.

Fig. 86.

Curves .4 and C, variation in temperature of furnace when
the powder is put in. Curves B and D, increase in tem-
perature of the powder. Curves .4 and B do not become
identical because they were not taken simultaneously and
the temperature changed slightly between the two runs.

It is not easy to estimate the effect of gradual heating as compared with
more nearly instantaneous heating. The outside layer of the powder is
subjected to the temperature of the furnace, which does not vary widely.
It is only the inside layers which are heated as slowly, as the curves in Fig.
86 indicate. It is not believed that any material change is introduced in

Fig. 87.

Effect of varying the length of excitation. Ordinates show the intensities of luminescence, and abscissas
the times after heating began. The length of excitation and the temperature of the furnace are as
follows:

Curve 6, 320 sec, 153. o° C; curve 5, 160.0 sec, 155.5° C.; curve 4, 80.1 sec, 155. 50 C; curve 3,
40.0 sec, 155. 50 C; curve 2, 19.9 sec, 155. 50 C; curve 1, 10.0 sec, 155. o° C.

the relation of the various curves, though the actual form of the curves
may be changed more or less.

The effect of varying the length of excitation is brought out in Figs. 87
and 88. As in previous curves, the phosphorescence was excited at room

92

STUDIES IN LUMINESCENCE.

temperature and allowed to decay to 7 = o.8 before heating. The points
are joined by straight lines to aid the eye in following the individual curves.
Two effects are noticeable at a glance. The maximum intensity increases
with excitation and is shifted to the right, i. e., comes at a later time. The
effect of saturation is shown. This is brought out more clearly in Fig. 89.

Fig. 88.— Curves similar to Fig. 87. Effect of varying the
length of excitation. The length of excitation and the
temperature of the furnace are as follows: Run 6, 320.1
sec, 309° C; run 5, 159.9 sec, 3080 C; run 4, 80.2 sec.
3080 C; run 3, 39.9 sec, 3080 C; run 2, 20.0 sec, 309"
C; run i, 10.0 sec, 307° C.

Fig. 89. — Saturation effect. 7M vs. length of excitation.
Curve A , temperature 3080 C : curve B, temperature 2670
C; curve C, temperature 207° C.; curve D, temperature
155 5 C; curve E, temperature 980 C

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Fig 88. Fig. 89.

To get the maximum point in each run, smooth curves, not shown in
the figures, were drawn through the different points. These curves show
that increasing the excitation beyond a certain length does not increase
the energy manifested as thermo-luminescence. That saturation takes
place is further shown by the time that luminescence lasts. Figs. 87 and 88
show that the duration of luminescence has about reached a maximum.
The areas included between the curves and the coordinate axes also show

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Fig. 90.

Effect of delay in heating. Time measured from the end of excitation. Excited 320 seconds at room
temperature. Temperature of furnace 3030 C Curve A, decay at room temperature. The time
between the end of excitation and the beginning of heating is as follows:

Curve B, 2.5 seconds; C, 21.4 seconds; D, 41.3 seconds; E. 81. 1 seconds; F. 162.0 seconds; G, 321.6
seconds; //, 631.8 seconds; /, 1280.0 seconds.

that further increase with increased excitation is limited. The shifting
of maximum intensity resembles an inertia effect; the longer the excitation
the longer it takes the temperature to produce the maximum thermo-effect.

DECAY OF PHOSPHORESCENCE PRODUCED BY HEATING.

93

The effect of delay between the end of excitation and the beginning of
heating was investigated at some length. The general character of the
results obtained is given in Fig. 90 and the subsequent diagrams.

Fig. 90 shows the way in which the outburst of thermo-luminescence
diminishes in intensity, in the case of a substance excited for a given time
and subsequently heated in a furnace of given temperature, as the interval
of time before the beginning of heating is increased.1 Figs. 92 and 93. in
which time is measured from the beginning of heating, indicate clearly
a shift in the time of reaching the maximum of intensity of thermo-
luminescence in the same direction as that already noted in the case
of increased length of excitation. The energy of phosphorescence does
not manifest itself so rapidly after
long delay in heating or long excita-
tion as after short delay or short ex-
citation. For any given excitation
and temperature the curves tend to
coincide after a given time, which
may be taken to indicate that the
interval of time occupied by an out- \
burst of thermo-luminescence is in-
dependent of the time which has
elapsed since excitation.

Table 16 gives the approximate
duration of the flash of thermo-
luminescence from nine sets of
observations under varying condi-
tions of excitation and heating. Fig. 91.1

While the times stated are neces-
sarily somewhat inaccurate, they indicate that brief outbursts follow
short exposures and high temperatures of the furnace and vice versa.

■X

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f

400

800

1200 Sec.

Fig. 92.

Effect of delay in heating. Time
measured from beginning of
heating. Excited 320 seconds
at room temperature. Tem-
perature of furnace 03° C
The time between the end of
excitation and the beginning
of heating is as follows:

Curve 1, 2.0 sec; curve 2, 21.9
sec; curve 3, 41.5 sec; curve
4,8i.5sec; curves, i62.osec;
curve 6, 332.0 sec.

JThe location of the crests C, D, E, F, G, H. I (Fig. 90) is along a curve which suggests in its form the
ordinary curve of phosphorescence, and when we apply the usual criterion, i.e., plotting I~ -^ and times, we
get the curve shown in Fig. 9 1 , which has the significant form already discussed in Chapter IV of this treatise .

94

STUDIES IN LUMINESCENCE.

Fig. 94 gives in graphical form a summary of the results of nine sets of
observations upon the decrease in maximum intensity of thermo-lumines-
cence with delay in heating. The curve G is from data plotted in Fig. 90.
In Fig. 95, which is from the same set of observations, the time of maximum
intensity is measured from the beginning of heating in order to exhibit
more clearly the shift of the maximum as the result of delay in beginning
heating.

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Table 16.

Fig- 93-

Time of

Temp, of

Time required for in-

excitation.

furnace.

tensity to reach I.

sec.

°c.

sec.

IO

94

1=1.25 60

10

200

45

10

302

32

40

92

7 = 2.5 ' °°

40

200

80

40

300

37

320

93

120

320

202

75

320

303

40

Curves similar to Fig. a.-?. Kxcited 40 seconds at
room temperature. Temperature of furnace 94°
C. The time between the end of excitation and
the beginning of heating is as follows:

Curve A, 1.9 sec:; curve B, 22.1 sec. curve C, 42.1
sec; curve D, 82.6 sec; curve E. 162. 1 sec.

In Fig. 96 is shown the maximum intensity plotted against length of
excitation. These curves are similar to those shown in Fig. 89, but cover
a larger range of conditions. They indicate the same tendency to satu-

Fig- 94-

Decrease in maximum intensity
with delay in heating. The co-
ordinates are I* and time of I»
measured from the end of excita-
tion. The length of excitation
and the temperature of the fur-
nace are as follows:

Curve A, 320 sec, 930 C

B, 40 sec, 920 C.

C, 10 sec, 940 C.

D, 320 sec, 2020 C.

E, 40 sec, 2000 C.

F, 10 sec, 2000 C.

G, 320 sec, 3030 C.
//, 40 sec, 3000 C.
/, 10 sec, 3020 C.

ration with increasing excitation whatever be the temperature and the
delay of heating; a point which would undoubtedly have been better
shown had a greater number of lengths of excitation been investigated.

Fig. 97 shows the relation between maximum intensity and temperature
for the nine sets of runs. These curves correspond to those shown in Fig.
84. In some of the cases but one point can be given, the other runs show-

DECAY OF PHOSPHORESCENCE PRODUCED BY HEATING.

95

ing no measurable intensity or else no maximum. The relation between
the maximum intensity and the temperature, beyond the fact that the
intensity increases with the temperature, is not clear.

40 Sec

Fig. 95-
Maximum intensity vs. time of maximum intensity measured from the beginning of heating.

For description of curves see Fig. 94.

Fig. 96.

Maximum intensity vs. length of excitation. The temperature of the furnace and the time between the
end of excitation and the beginning of heating are as follows:
21.6 sec. Curve //, 2000 O, 21.9 sec.

41-7 1. " 41-5

81.4 J, " 81.6

161. 6 K, " 161. S

321.0 L, " 3217

636.0
II55-0

c. C

jrve

iV7.93"C, 21

S

N, " 41

.8

0, " 82

.0

P, '• 162

.0

0. 322

.0

Curve A, 320 sec

21.7 sec.

B,

41.3

c.

8t.3

D,

162 .0

E,

321.8

F,

6318

G,

1280.0

ff.40

sec.

, 22.1 sec.

I,

41.8

./.

81.6

K,

161 .9

L,

3219

M.

642 .0

N,

1230.0

0, 10

sec.

21.8 sec.

P.

41-7

Q.

81.7

R,

161 .4

s.

3'9-5

Fig. 97-

Maximum intensity vs. temperature of furnace. The length of excitation and time between end of

excitation and beginning of heating are as shown herewith.

In an earlier paragraph mention was made of the existence of two bands in
the phosphorescence spectrum of the sample of Sidot blende used in this study .

96 STUDIES IN LUMINESCENCE.

If there are two bands, it is natural to expect some indication of their exist-
ence in the curves of thermo-luminescence. Referring to Fig. 92, there will
be found evidence of two bands. After the intensity has reached a maximum,
it falls off rapidly at first, then there is an indication of a slowing up in the
decay followed by relatively rapid decay. Whether there is a complete
bending back in any of the curves can not be stated, because the points are
too far apart. Whether a complete bending back is possible depends on the
relative intensities of luminescence of the two bands and the relative rapidity
of decay. It has not seemed advisable to search for more certain indications
of two bands in the thermo-phosphorescence because of the difficulty of
increasing the number of observations taken in a given time. In part II
of this chapter, curves will be shown in the case of Balmain's paint, which
decays slowly enough to be more completely studied. This substance has
two bands in its phosphorescence spectrum, and the curves of thermo-
luminescence show two maxima under certain conditions of excitation,
temperature, and delay in heating.

EXPERIMENTS WITH BALMAIN'S PAINT.

In the experiments with Balmain's paint the apparatus used was the same
as that used in the study of Sidot blende. A mercury lamp was used to
excite the calcium sulphide, which was in powder form. Since the lamp was
made of ordinary glass and the light from it was reflected at a mirror before
reaching the powder, the excitation was chiefly due to the visible spectrum
in the mercury arc. Before each excitation the powder was exposed to
infra-red rays in an attempt to bring it to a standard condition.

The action of infra-red rays upon the phosphorescence of calcium sulphide
is not so strong as upon Sidot blende. In the latter case, as has already been
shown in Chapter V, phosphorescence due to a very long excitation can be
destroyed almost immediately by infra-red rays, and only the highest allow-
able temperature, just under dull red heat, is able to produce thermo-
luminescence without renewed excitation. In the case of calcium sulphide
a very long exposure to infra-red rays was necessary to destroy the phos-
phorescence, and no exposure was found to be long enough to suppress the
thermo-luminescence completely. At the beginning of the experiments
on calcium sulphide this fact was not recognized. If it had been, the
powder could have been brought to an approximately standard condition
by heating to a temperature a little higher than the highest temperature
at which thermo-luminescence was to be studied. Fortunately, in every
case the powder was exposed for one minute to infra-red rays of constant
strength, hence it was always brought to a semi-standard condition. No
extended attempt was made to compare the two methods, since the blue
phosphorescence of calcium sulphide is a difficult color to measure in the
photometer.

The fluorescence spectrum of this sample of calcium sulphide is shown
in Fig. 98. This curve was obtained by comparison with the light reflected
from the surface of a block of magnesium carbonate illuminated by an
acetylene flame. Two bands are indicated by the curve, one with a maxi-
mum at about 0.41 /* and the other with a maximum at about 0.54^.

DECAY OF PHOSPHORESCENCE PRODUCED BY HEATING.

97

The dotted vertical lines show the wave-lengths of the lines of the mercury
arc. The figure indicates the presence of stray light in the speetro-photom-
eter, which lowers the zero line with respect to the curve and unduly
exaggerates the ratio of the maximum of the band at 0.41 /x to the maximum
of the band at 0.54^. The
band in the blue is, how-
ever, evidently much more
intense than the other.
This fact was shown by
the color of the initial phos-
phorescence, which was
blue.

The effect upon the de-
cay curve of varying the
length of excitation is
shown in Fig. 99. These
curves are plotted with
distances of the standard
light from the photometer
as ordinates and time
measured from the end

of excitation as abscissas. For short excitations the curves are concave
downward throughout, but for longer excitations the bending is concen-
trated in the first part of the curves. These curves are similar to those
obtained with Sidot blende, except that the phosphorescence lasts much
longer. Hence it is possible to get points relatively nearer the origin. This

12

J

T"

\

1

1

8

I

1

l
1

\

\

1
1

\

1
1

\

p

0

l

\

f-

30^°-

8

a;

>

AS

.50

SA AJ

Fig. 98.

Fluorescence spectrum of calcium sulphide excited by the visible
spectrum of a mercury arc.

/60. ZOO
Fig. 99.

362 Sec.

Effect of varying the length of excitation. Excitation and decay at room temperature.
Curved, excited 14.4 sec. Curve E, excited 59.9 sec.

B, " 30.1 F, 150.3

C, " 30.1 G, " 300.0

D, " 60.2 H, " 600.0

Curves .4, B, D, and F were taken on the second day and the remainder on the first day of the run.

fact allows one to make a tentative deduction regarding initial intensity
and length of excitation at room temperature. A study of Fig. 99 indicates
strongly that the initial intensity is greater the longer the excitation, which
is the impression obtained when getting the curves. Though it is impossible,

98

STUDIES IN LUMINESCENCE.

with this apparatus, to get a measurement much nearer the origin than
0.8 of a second, yet the eye is sensitive to part of the change of intensity
before this, giving one a means of estimating roughly the amount of change
before the lirst measured point. The saturation effect is prominent, as
was the case with Sidot blende. Saturation is shown both by the change
of initial intensity and by the change in slope of the curves as the excitation
is increased.

In Fig. 99 some of the curves were obtained one day and the remainder
on another day. One would expect curves B and C to coincide. The
difference between them is probably due to the fact that the infra-red
exposure does not reduce the powder to a standard condition. Curves D
and E agree more closely than B and C. A curve, not shown in the figure,
excited for 300 seconds, coincides with G. This coincidence may have

D

p3

rft

^"5

/ 4

^>o'

D2

5

5

f/Z

•^5

4r/j|

T r/

s^i

3

tv

QAJ-!

2

4M

P5

^3

100

20 40 60 80

Seconds

Fig. 100.

Decay curves at different temperatures. Excited 2 minutes.

120

Curve 1, temp, o

2,

3.

4.

5.

excitation and decay, room temp.
66° C.
102°

145°

180°

been due to chance, but it is what would be expected, because the effect
of previous history becomes of less importance as the length of excitation
is increased.

That the lack of agreement of curves B and C is due to the previous
history of the powder is substantiated by the fact that curve A was observed
immediately before curve B, and curve G immediately before curve C. No
deductions can be made for curves D and E, because the history of the
powder previous to the excitation for curve E is not known. Curve D
followed B immediately, but some preliminary work without infra-red was
done before the infra-red treatment preceding curve E.

The changes produced in the decay curve by varying the temperature at
which excitation and decay take place are shown in Fig. ioo. The part
that previous history plays in these curves can not be estimated accurately,

DECAY OP PHOSPHORESCENCE PRODUCED BY HEATING.

99

but its effect is small, because the excitation, which is relati\rely a long one,
was not changed during a set of curves. The curves were taken in the
order that they are numbered. Curve I, taken at room temperature, is
similar to the typical decay curves shown in Fig. 99. As the temperature
is raised the curves pass through a series of changes. Curve 2 begins con-
cave upward, but changes during the decay to concave downward. Curve
3 is practically a straight line throughout. As the temperature is raised
still farther, the curves again become concave downward throughout,
differing not widely from the typical decay curves at room temperature.
Another set of curves similar to those in Fig. 100 is shown in Fig. 10 1,
where the length of excitation is 10 minutes. The double bending1 is again
exhibited in curve 2.

40

80

200

?40

120 160

Seconds
Fig. 101.

Decay curves at different temperatures. Excited 10 minutes.
Curve 1, temp, of excitation and decay, room temp.

2. " " ' 420 C.

3. " " " " " 74°

4. " " " " " IOI°

5. " " '* " " 124°

The effect of previous history is shown in Figs. 102 and 103. These
curves might readily be mistaken for curves showing the effect of varying
the length of excitation. Fig. 103 also shows the effect of previous history,
modified, however, with the effect of temperature.

The decay of phosphorescence in the case of Balmain's paint was studied
some years ago by F. J. Micheli,2 who has published decay curves of several
phosphorescent substances, among them Balmain's paint. For the sake of
comparison, several of these curves have been replotted in Figs. 105 and 106
with i/V-f and time as coordinates. Fig. 105 shows the effect of varying the
length of excitation.

Fig. 106 shows the effect of varying the tempera-

■When taking the runs on which Fig. 99 is based, no importance was given to the single curve which shows
double bending. Later, the runs on which Fig. 100 is based were made and the apparatus was rearranged
for study of another substance before the significance of the double bending was recognized. On looking
over all the curves taken on Balmain's paint, other indications of double bending were found, but none of
them so pronounced as those shown above.

-Arch, des Sci. Phys. et Nat., 12, p. 5, 1901.

IOO

STUDIES IN LUMINESCENCE.

ture at which excitation and decay take place. These curves and the
curves given earlier in this article agree generally. Fig. 106 shows the
effect of excitation and decay at a temperature lower than any used in this
article.

Effect of history previous to excitation. Excitation and decay at room temperature. Excited 30 seconds.
Curve .1, excited after action of infra-red; curve B, excited immediately after curve A; curve (',
excited immediately after curve B; curve D, before exciting for curve D, the powder was excited for 10
minutes and allowed to decay to an intensity corresponding to D =9 in the figure, at which time.the
excitation of 30 seconds was begun.

In the case of calcium sulphide, it is not necessary to use a spectrophoto-
meter to prove that there is more than one band included in the phenom-
enon of phosphorescence, for under the influence of heat the color of the
powder can be seen to change from blue to green.

Fig. 103.

Curves of Fig. 102 repeated at a temperature of 1330 C

It has already been shown that the form of the phosphorescence decay
curve may be closely approximated on the basis of two bands, each follow-
ing the law

T

/ =

(a+bt?

This explanation was carried to some length in the earlier part of this
chapter. But there seems to be no way of combining two decays, each of
which follows this law so as to produce an upward bending. If it be sup-
posed that one band increases in intensity as the other decreases, then an

DECAY OF PHOSPHORESCENCE PRODUCED BY HEATING.

IOI

upward bending is possible; but a downward bending is impossible unless
both the bands subsequently decay together. This suggests that one of
the bands may be due to some secondary effect, instead of being produced

t)

D

±o

(>

6

A

B

c

<T

*^r}

4

'D

Z

0 20 40 60 80 100 120 140

Seconds
Fig. 104.

Effect of history previous to excitation. Excited 30 seconds. Before each excitation the powder wa3
excited for 10 minutes and allowed to decay to an intensity corresponding to IJ =9. at which time the
excitation of 30 seconds was begun.

Curve C. excitation and decay at room temp.

B, " 88° C.

A. " 133°

8 10

Minutes

Fig. 105.

Effect of varying the length of excitation. Excitation and decay at a temperature of 1S0 C. (replotted from

data of Micheli).
Curve A, excited 5 seconds; B, 20 seconds; C, 300 seconds. Another curve excited for 60 seconds coin-
cided almost exactly with curve C.

%1

■

4-

A1

,s

r

J

2 <

J

k*

^

r* '

<

?

r

0

2

4

a

?,

1

0

1

I

\

f

/

6

I

d

Urn

Fig. 106.

Effect of varying the temperature at which excitation and decay take place. Excited 300 seconds

(replotted from data of Micheli).
Curve A, temp, of excitation and decay, — 21° C.

B, " " ioo°

C, • " " 18°

by the exciting light. Such a band would increase in intensity a certain
length of time, then decrease in intensity. If we suppose that the first
band follows the law suggested, and that the second band follows the same

102 STUDY IN LUMINESCENCE.

law after it has reached its maximum intensity, then the discussion pre-
viously given still holds good, the supposition being that the second band
is completely formed before the first point on a curve can be measured. If,
however, conditions can be changed so that a point can be observed before
the second band is completely formed, then the curve will show initially
an upward bending, as in curve 2, Figs. 99 and 100. If a band is formed
by some secondary effect it will usually be present in the fluorescence
spectrum because the excitation is continued long enough for the band to
be formed.

The effect of previous history can be explained without adding any com-
plications to the ideas already set forth. In the case of Sidot blende, it
was undoubtedly the band of longer wave-length that decayed more slowly.
The same is true, probably, for calcium sulphide, for the color changes
from blue to green when the powder is heated. In the case of the decay
curve at room temperature, the intensity is too small for the change in
color to be recognized by the eye. Assuming that the band of longer
wave-lengths decays more slowly, and that this band is due to some
secondary effect, then the action of repeated excitations without inter-
vening treatment with infra-red might be to increase the intensity and to
decrease the rapidity of decay of the second band without introducing any
change in the rapidity of decay of the first band. Some indications of
these effects can be seen in Figs. 10 1 and 102.

It is unfortunate that the curves in Figs. 101 and 102 were not taken
in the reverse order, i. c, excitation of long duration followed by shorter
excitations. Such curves, according to the ideas just set forth, should
show parallelism after the decay had proceeded some time. Several of the
sets of curves described in Chapter IV were taken in this order and they
exhibit parallelism after decaying a short time. To explain completely
the curves in Figs. 99 and 100 on the basis of a second band due to secondary
causes is impracticable, because the problem is complicated by the addition
of the effect of temperature. One assumption will necessarily have to be
added, i. c, that there are certain temperatures at which either one or the
other of the bands is most readily formed. This is a possible explanation
of the fact that the decay curve becomes a straight line at one temperature.

A number of sets of observations were made to show the effect of heating
after decay has begun. In these experiments the powder was exposed to
infra-red rays, then excited and allowed to decay at room temperature, heat-
ing being begun at a definite point on the decay curve. Figs. 107 and 108,
in which the coordinates are intensity and time, show the effect of varying
the length of excitation. The double flash mentioned in the case of Sidot
blende is plainly evident in the curves, and in the experiment it was evident
to the unaided eye. The color also varied, being blue for the first flash and
a yellowish green for the second. At the longest excitation the second
flash is barely visible in the photometer, the only evidence of its existence
being a slowing up in the rate of decay. As the length of excitation is
decreased, the second flash becomes, relative to the first, greater and greater.

At lower temperatures the second flash becomes less evident, and this
peculiarity of the curve for 600 seconds excitation, as shown in Figs. 107
and 108, no longer exists. For the lower temperatures the second flash for

DECAY OF PHOSPHORESCENCE PRODUCED BY HEATING.

103

600 seconds excitation is greater than that for any shorter excitation at the
same temperature. At a temperature near ioo° C, the second flash is no
longer evident in the curves.

40

80

120

160 2.00 Z44

Fig. 107.

060 Sec.

Effect of varying the length of excitation. Excited and allowed to decay at room temperature to / =2.9,
when heating was begun. Temperature of furnace, 2770 C.
Curve A, excited 600.2 sec. Curve D, excited 60.7 sec.

B, " 300.0 E, " 30.1

C, " 150. 1 F, " 14.8

60

Fig. 108.

HO

Sec.

Curves similar to Fig. 107. Temperature of furnace, 3390 C
Curve 1, excited 600.0 sec. Curve 4, excited 59.8 sec.

2, " 300.0 5. " 29.8

3, " 150.1 6, " 14.2

io4

STUDIES IN LUMINESCENCE.

It is evident that two bands are represented in the preceding curves.
Any one of these curves is probably made up by the superposition of two
curves, each of these component curves representing the flash for one of the
bands. If one considers the first flash in Figs. 107 and 108, it will be seen
that the curves resemble closely corresponding curves obtained with Sidot
blende. The maximum intensity increases with length of excitation and
occurs later and later. The areas included between the curves and thecoor-

150 300

Seconds

Fig. 109.

Maximum intensity of the first flash plotted against length of excitation.
Curve i, temp, of furnace 3390 C (from Fig. 108).

2. " 2770 (from Fig. 107).

3. " " " 222°

4. 148°

dinate axes increase with the excitation. The effect of saturation is shown
both by the change in areas and by the change in the maximum intensities.
The observations have been plotted in Fig. 109 so as to show the relation
between maximum intensities and lengths of excitation, in which case if one
considers the first flash alone the curves are similar to those already given
for Sidot blende. The effect of changing the temperature of the furnace
is shown in Fig. no.

too

Fig. no.

aximum

intensity of the first flash plotted against
temperature of the furnace.

Curve A, exc
B,

c,

D.

E.
F.

ted 600 sec.

300

150

60

30

•5

The second flash does not always follow the laws of the first flash, as can
be seen in Fig. 108. As the length of excitation is increased, the maximum
intensity of the first flash occurs later and later, while the maximum of the
second flash occurs earlier and earlier. This effect is not well defined in
Fig. 107, due perhaps to the fact that the flash is of smaller intensity and
consequently more difficult to follow accurately. The effect of saturation

DECAY OF PHOSPHORESCENCE PRODUCED BY HEATING.

1 05

in the case of the second flash is shown in Fig. in. These curves indicate
that an increase in the length of excitation beyond a certain length does
not increase the maximum intensity, but decreases it.

150

300

Seconds

600

Fit

in.

Maximum intensity of the second flash plotted against length of excitation.
Curve A, temp, of furnace, 3390 C (from Fig. 108).

B, " " " 2770 (from Fig. 107).

C. " " " 222°

The relation between the maximum intensity of the second flash and
the temperature is shown in Fig. 112. This figure corresponds closely to
Fig. 1 10, which shows the corresponding relation for the first flash.

Fig. 113 shows the effect of delay in heating and Fig. 114 the same

30
20
10

I

1

1

B/3

'cy

d/

.

J*

!j^-°

Fig. 112.

Maximum intensity of the second flash plotted against
temperature of the furnace.
Curve B,

C.

o,

E,
F,

excited 300 sec.

150

60

30

15

100-

200v

300

f6

ID

ko

4i

c"

? V

I

D

11

l\

A

\
I \

T

\

\\

F

A

\

9

\

?

&

^

V

^

k

/

V

>— ' ■

-\

0

— f

w-

—6

Xi-

— &

I

(

Sec)

10

00

Fig. 113.

Effect of delay in heating. Time measured from end of excitation. Excited 60 seconds at room temperature.
Temperature of furnace, 2280 C. Curve .1, decay at room temperature. The time between the end
of excitation and the beginning of heating is as follows: Curve B, 2.1 seconds; C, 25.6; D, 101.7; E
203.2; F, 401.7; G, 801.7.

io6

STUDIES IN LUMINESCENCE.

curves plotted with time measured from the beginning of heating. Here
again, if one considers the first flash alone, the curves are similar to those
shown in the ease of Sidot blende. The longer the delay in heating, the
less intense the flash and the later the maximum intensity of each flash.
Furthermore, the time of decay at one temperature and constant length
of excitation is constant. It is difficult to make any deduction from the
points representing the second flash.

Another set of curves similar to those shown in Fig. 1 13 is shown in Fig.
115. These curves show that the second flash becomes relatively larger
with respect to the first as the excitation is shortened, and under suitable
conditions, as in curve C, may become considerably larger than the first.

80

120 /eo

Fig. 114.

Effect of delay in heating. Time measured from the beginning of heating

ZOO

2.40

Same curves shown in Fig. 113.

It is difficult to say how much difference exists between the behavior of
Sidot blende and Balmain's paint. The decay curves at room temperature
are very much alike. As the temperature is raised both decay curves
become straight lines, but Balmain's paint shows a transition through a
double curvature decay before reaching the straight line decay, while Sidot
blende does not exhibit this phenomenon. Above the temperature at
which the decay curve becomes straight, Balmain's paint shows a decay
approximating the decay at room temperature, while in the case of Sidot
blende the decay is too rapid to be followed with the available apparatus.
The decay of Sidot blende is so rapid that one can not get enough points

DECAY OF PHOSPHORESCENCE PRODUCED BY HEATING.

107

on a curve to say positively that it does not exhibit double curvature at
high temperatures. In the cases where the powder is heated after excita-
tion, Balmain's paint shows a double flash under some conditions, and one
flash under other conditions, while Sidot blende shows one flash under all
conditions, unless it be admitted that there are indications of a double flash
in some of the Sidot-blende curves, in which case the two substances show
substantially the same curves. Considering the first flash only, there is

12

8

B

100

200 300

Fig. 115.

I

40O Sec.

Effect of delay in heating. Time measured from the end of excitation. Excited
30 seconds at room temperature. Temperature of furnace, 3380 C.
Curve .4, decay at room temp.

B, waited 2 sec. after excitation before heating.

C, waited 400 sec. after excitation before heating.

perfect agreement between the curves of Sidot blende and Balmain's paint.
The action of infra-red is more marked in the case of Sidot blende, but
otherwise there is no marked difference. The fluorescence spectrum shows
two bands in the case of each substance, but there is one point of difference,
in that the band of shorter wave-length is more prominent in the case of
Balmain's paint and less prominent in the case of Sidot blende than the
band of longer wave-length.

CHAPTER VII.
STUDIES OF PHOSPHORESCENCE OF SHORT DURATION.

By the methods described in Chapters IV and VI of this memoir it is
difficult to make observations of phosphorescence within less than 0.4
second of the close of excitation. This suffices for the determination of
the form of the curve of decay with the exception of the region very near
the origin. There are, however, many substances exhibiting brilliant initial
phosphorescence where the effect fades to unmeasurably small intensity
within a few hundredths of a second.

It is of considerable interest and importance not only to determine the
curves of decay of such substances, but also, since Lenard1 in a recent paper
has questioned the linear relation between I~* and time during the so-called
first process of decay, to study the earliest portions of the first process in
the case of phosphorescence of slow decay.

B

Fig. 116.

Messrs. Waggoner and Zeller at our suggestion have investigated these
questions at some length, using a new type of phosphoroscope of especial
design and working independently of each other. The present chapter
contains a summary of their researches.

DR. WAGGONER'S STUDIES IN PHOSPHORESCENCE OF SHORT DURATION.2

METHODS OF MEASUREMENT.

The phosphoroscope used in these measurements is shown in Fig. 116.
It consists of a disk D, fastened to a cylinder L, rotating about a horizontal
axis. On the inside of L is a shaft K which rotates at the same speed as
the cylinder. On the end of K is a plane mirror, placed 450 to the axis of
the shaft, and by use of the mechanism at C the position of the mirror,
relative to any point on the disk D, may be shifted while the disk is rotating.
The disk has an opening at 0, through which the light from the spark E
may pass at each successive revolution and excite the specimen placed at F.
If the mirror is in the position shown it will reflect into the slit of a spec-

]Lenard, Annalen der Physik, xxi, p. 641, 1910.

= C. W. Waggoner, Physical Review, xxvu, p. 209.

109

IIO STUDIES IN LUMINESCENCE.

trophotometer the light which comes from the phosphorescent screen F
while it is being excited by the spark. By moving the rod R the mirror
is turned so as to reflect the light from the screen F into the spectrophoto-
meter some time after excitation, and in this way the intensity of the phos-
phorescence after successive excitations may be measured by the spectro-
photometer AB. The cylinder L is driven by a motor belted to it over the
pulley P. S is a worm driving a cog wheel, which serves as a device for
recording the speed by making an electric connection with a chronograph
once in every hundred revolutions of the disk. Current for the motor
was taken from a direct-current source having a special device for main-
taining constant voltage, and the speed of the motor was found to remain
almost constant. The disk D has a second half-disk fastened to the first
(not shown in the figure) by which it is possible to change the size of the
opening at O. The essential advantage of this phosphoroscope over others
is that the decay of the phosphorescent light may be studied without chang-
ing the time of excitation. Another feature of this method is that settings
on the spectrophotometer for any curve may be repeated as often as desired
and the time taken for the determination of each setting may be as long
as desired. The results shown in the curves that follow were determined
from two different settings of the spectrophotometer for each point, and
the plotted point represents the average.

The source of excitation was the spark between iron terminals. To
produce this spark an induction coil was connected to a sourceof alternating
current of 60 cycles frequency, a small condenser being connected in mul-
tiple with the spark gap. In order to study the effect of decreased time of
excitation, it was found necessary to increase the frequency of the spark,
this being accomplished by connecting it to a source of alternating current
giving 140 cycles. With this higher frequency the chances of the spark
exciting the screen at each turn of the disk is greater ; the higher frequency
spark was also used as a check on the curves taken at the lower frequency.

The light from an incandescent lamp connected to a constant potential
source was used for comparison in the spectrophotometric measurements.

METHODS OF PREPARING THE PHOSPHORESCENT COMPOUNDS.

The phosphoroscope just described, although it has a considerable range
as to speed, is especially adapted to compounds whose phosphorescence
decays rapidly. A number of such compounds were prepared and were
studied along with several specimens of willemite whose decay was of suit-
able rapidity.

In preparing the compounds the methods given by Wiedemann and
Schmidt1 and Andrews2 were followed. The ease with which these com-
pounds may be prepared and the intensity of the phosphorescent light
given off when they are excited by the spark seem to warrant a rather
detailed account of their preparation.

ZnCl<i.-\- M nSOi.* — Some zinc chloride prepared from metallic zinc was
dissolved in a small quantity of distilled water. A small trace (usually less

'Wied. Ann., 54, p. 604, 1895. 2Science, xix, March 1904.

'MnSOt was selected as an active salt in a number of cases, since it has been found by Lenard and Klatt
(Annalen der Physik, 15, p. 243, 1904) that with this salt the intensity of phosphorescence is less affected
by varying the concentration than with any other active agent.

studies of phosphorescence; of SHORT DURATION. I I I

than i per cent) of MnS04, dissolved in water, was added to the ZnCl2
solution and the whole brought to the boiling point. An equal volume of
sodium silicate, as the flux, was then added and the whole evaporated to
dryness. On being exposed to the spark, faint green phosphorescence
could be seen. It was then placed in a porcelain crucible and heated to a
bright red for 2 hours. When cool it showed pale green fluorescence when
exposed to the spark, and when the exciting source was cut off it was found
to have considerable bright green phosphorescence. A sample of the
powder at this point was kept and marked ZnCl2 No. 1. The remainder
of the powder was heated to a bright red for 3 hours and when cool showed
both fluorescence and phosphorescence of greater intensity than did ZnCl2
No. 1. A sample of this was saved and marked No. 2. The remaining
powder was heated 2 hours at a bright red; on examination when cool it
was found to have lost some of its original phosphorescent intensity. This
was marked ZnCl2 No. 3.

CdCk+xMnSOi— Substituting CdCl2 for ZnCl2, but giving it exactly
the same treatment, it was found that it was necessary to heat the compound
at a bright red for 3 hours before the phosphorescence was very intense, and
that subsequent heating had no marked effect on its phosphorescent in-
tensity when exposed to the spark. The fluorescence was pink and the
phosphorescence dark red.

CdSOi + xMnSOi. — Some CdS04 was dissolved in water with a small
trace of MnS04 and the whole heated to dryness without adding the flux.
The resulting white powder showed pale yellow fluorescence, and an orange
yellow phosphorescence of much longer duration than the zinc compounds.

ZnSOi + xMnSOi. — ZnS04 substituted for CdS04 and treated in the
same way resulted in a white powder which showed pink fluorescence and
bright red phosphorescence, but the intensity was too small to be measured
by the method used.

Some "chemically pure" calcium sulphide was purchased with the view
of trying to prepare compounds of CaS and MnS04, but it was found to be
already an active phosphorescent compound having a brownish-red color.

Two large pieces of willemite, showing brilliant green phosphorescence
with the iron spark, were studied. It was found that their rates of decay
were not the same. The sample having the slowest decay was marked
willemite No. 5. The sample having the very rapid decay was crushed
into a line powder, a sample made into a screen and marked willemite No. 1 .
Part of the powder was heated to a bright red heat in a porcelain crucible for
45 minutes. This was marked willemite No. 2. The remainder of the
powder was heated for 2 hours at a bright red, and marked willemite No. 3.

One ZnCl2-f-.vMnS04 sample made by W. S. Andrews, Schenectady,
N. Y., was also studied.

For purposes of study, screens were made by placing the phosphorescent
powder on small squares of heavy dark brown cardboard, the cards having
been covered first with white "zapon" varnish.

Among the preparations just described was one of cadmium sulphate
and manganese sulphate made by simply evaporating to dryness a water
solution of the two salts. After studying this compound it was put aside
for several months and when taken up later for further work it had lost

112 STUDIES IN LUMINESCENCE-

its power of phosphorescence under excitation by the iron spark, but on
being heated it regained its former brilliancy. This led to the suspicion that
these simple salts are photo-luminescent only when in the anhydrous condi-
tion, and subsequent tests showed this to be true of all the compounds studied.

The fact that moisture affects the photo-luminescence of cadmium salts
may be shown by exciting the anhydrous phosphorescent compound by an
ultra-violet source and allowing the light to decay for several seconds.
Now if a drop of water be placed on the decaying compound the residual
light will flash out brilliantly for an instant and then disappear very rapidly.
It was first thought that this phenomenon was due to the changes which
take place within the salt as it takes up water of crystallization. The
thermo-phosphorescence of the cadmium compounds, however, is quite
marked, and Dr. Kortright, of the Department of Chemistry, West Vir-
ginia University, suggested that the effect might be partly due to thermo-
phosphorescence brought about by the heating which accompanies the
process of hydration. To obtain some idea of the rise in temperature,
several grams of anhydrous salt wrere placed in a test-tube with a ther-
mometer, and water was added. The rise of temperature was found to be as
much as 50 C, which would account in a large measure for the phenomenon
purely on the basis of thermo-phosphorescence. Alcohol, which is not a
solvent for cadmium sulphate, was tried, but gave no effect.

The fact that these cadmium compounds do not show photo-luminescence
when in the crystalline state led the writer to try some of the original cad-
mium sulphate from which the compounds with manganese were made ; and
when the water of crystallization was driven off, the salt was found to be
highly phosphorescent. The color of the phosphorescence was similar to
that when a manganese salt had been added, but was less intense. When
subjected to kathode rays the sulphate showed more brilliant fluorescence
but less phosphorescence than when excited by the spark.

An attempt was first made to purify the original cadmium sulphate by
forcing it out of water solution with sulphuric acid, but this treatment
increased rather than decreased the intensity of phosphorescence.

Another method was to precipitate the cadmium with hydrogen sulphide
as cadmium sulphide, wash it with water till free from sulphates, then re-
dissolve the sulphide in sulphuric acid, crystallizing the salt formed. This
sulphate still gave a brilliant yellow phosphorescence under both the iron
spark and the kathode rays.

The next method was to crystallize the salts in fractions from water
which was slightly acidified with sulphuric acid to prevent hydrolysis.
The mother liquor was saved in this case and the crystals redissolved and
crystallized out. It was found that the phosphorescence of the successive
crystals, as they separated out from the retained mother liquor, decreased rap-
idly in intensity and at the end of the seventh fractionalization was almost
nil, thus indicating the presence of an impurity less soluble than the cadmium
sulphate. This view was supported by a micro-chemical test made by Dr.
Wilkinson1 showing the presence of sodium and a slight trace of zinc.

In casting about for a cadmium sulphate free from phosphorescence
numerous samples were tried and it was found that the "tested purity"
cadmium sulphate made by Eimer and Amend was free to a remarkable

■Journal Physical Chern., xin, p. 719, 1909.

STUDIES OF PHOSPHORESCENCE OF SHORT DURATION. 113

degree. The phosphorescence, under the iron spark, of the anhydrous
powder was so small that it was necessary to stay in the dark room a long
time before the eye became sensitive enough to detect the phosphorescence
at all ; and even then one could not be sure of the color. The analysis of this
material given on the bottle was as follows: " Cd, 52.84 per cent; S03, 47.15
per cent; As, none; Zn, none." This material was tested by the kathode
rays and it was found to show neither fluorescence nor phosphorescence.

That a zinc salt, when added to a cadmium salt, or vice versa, would
produce a phosphorescent compound has been pointed out by Wiedemann
and Schmidt.1 This fact led to the suggestion that the presence of small
traces of zinc in cadmium salt, or vice versa, might be detected by the
photo-phosphorescence which these compounds show, and some rough
tests were made showing photo-phosphorescence with less than one part
of zinc sulphate in 10,000 parts of the phosphorescent-free cadmium sul-
phate. The phosphorescent color was green, but the intensity was not
so great as when a sodium salt is added to a cadmium salt.

The pure cadmium sulphate is very susceptible to a small impurity;
how susceptible will appear from the following experiment: Some water,
which was twice distilled in an all-glass still, was placed in a bottle and
allowed to stand over night. The following day this water was used to
dissolve some of the pure cadmium sulphate and was then driven off.
When the cadmium sulphate was tried under the ultra-violet source it was
found to be highly phosphorescent. 200 c.c. of this same water when
evaporated down in a platinum dish left a residue so small that it could
not be seen except when the platinum dish was at red heat.

Cadmium Sulphate-Sodium Compounds.— Starting with the phosphor-
escence-free cadmium sulphate, a series of preparations was made by the
addition of various salts of sodium. The method used was that already
described. It was to dissolve the crystals of sodium salt and cadmium
sulphate in water; slowly evaporate the solution to dryness, and test the
compound for phosphorescence, when it reached the temperature of the
room, by exciting by the iron spark. The compound, from the original
crystals to the final test, was kept in a large platinum dish and every pre-
caution was taken to prevent impurities from entering the solution.

The water was twice distilled in an all-glass still and used immediately
in order to prevent it from taking up sodium from the glass container.
To produce an even and easily regulated temperature a small electric furnace
was made by wrapping "nichrome" ribbon around a porous battery cup;
this proved to be a very satisfactory arrangement, for the platinum vessel
could be placed inside of this furnace and covered over lightly to prevent
impurities entering from the air while evaporation was taking place.

Only readily soluble salts of sodium were used ; and each one was tested
in the anhydrous condition for the presence of phosphorescence. In one
or two cases it was necessary to recrystallize the salt to free it from the
impurity causing the phosphorescence. Several different percentages of
cadmium and sodium were tried and it was found that the intensity of phos-
phorescence depended somewhat upon the percentage of sodium present;
but so far as the tests were carried no change was produced in the color

>Wied. Ann., vol. 56, 1895.

ii4

STUDIES IN LUMINESCENCE.

of phosphorescence by rather wide variations in the percentages of the added
sodium. In most cases of the compounds recorded in Table 1 7 the amount
of sodium salt present was not far from 1 per cent.

An inspection of Table 17 shows that the color of the phosphorescence
of cadmium salts with sodium ranges from blue to yellow. In all but two
cases the intensity was very small — so small as to make it impossible to
obtain from these compounds reliable curves of decay with a spectro-
photometer. Doubtless all of these could be increased in intensity by find-
ing the suitable proportion of sodium.

Table 17.

Sodium salt added.

Phosphorescent color.

Spectrum.

Na2S04

Yellow.

Na2S2C),

No phosphorescence.

Na.SiO,

Blue.

0.486^, max. . . 5 IO.U,

0.604^

NaHP04

Faint green.

NaN03

Faint yellow.

Na2MiiO_,

Greenish yellow.

. 522,u, max. . . 566/u,

.616/j

NaFl

Greenish yellow.

474m, max. . .574^,

.6l2M

Na.Cr04

No phosphorescence.

NaOH

No phosphorescence.

NaCl

Green.

r1 1 f\ , 1 *■%"! /"1 v

■ 580M

. 5 1 <-ty/ , I n d X . . . ,

NaCK)3

Faint yellow.

Na,CO,

No phosphorescence.

NaBr

Blue.

.414M) max. . .48'oju,

. Goopi

Na,B407

Pale green.

Na2Cr07

No phosphorescence.

Na,S04Al,(S04)?

Faint green.

DISCUSSION OF RESULTS.

The results of the study of the substances just described are indicated
by means of decay curves plotted, like many of those which appear in
Chapters IV and VI, with I~- as ordinates, and times, counted from the
close of excitation, as abscissas. It will be seen that the curves have the
same general characteristics as those obtained by the measurement of
long-time phosphorescence. They may be regarded as consisting of two
straight lines merging into one another.

The curves in Fig. 1 1 7 are decay curves typical of three of the substances
studied. In the case of curve A (willemite No. 5) the decay was so rapid
that it was difficult on account of the small intensity of the light to follow
the "second process" very far.

In each curve one point has been plotted to the left of the zero on the
ordinates. This observation is made on the fluorescent light, i. e., it is the
intensity of the light coming from the screen when the mirror is in position
to reflect the light which comes from the screen during excitation. The
zero point, or the point where the exciting light fails to be reflected into
the spectrophotometer, was determined by placing a piece of white paper
in place of the screen.

Since the induction coil furnished at most 120 discharges per second it
will be seen that, for the time of excitation employed in the measurements
illustrated in Fig. 1 17, three sparks per revolution of the disk is the maximum

STUDIES OF PHOSPHORESCENCE OF SHORT DURATION.

"5

number that could pass while the screen is exposed. It would seem then that
the excitation might be uncertain and irregular. These excitations, how-
ever, follow each other very rapidly; for example, in curve B, Fig. 117, the
rotating disk makes a complete revolution in 0.108 second, and since 10 to
20 seconds were required to make a setting of the spectrophotometer, what-
ever changes take place in the spark during successive excitation, the reading
is an average value of the intensity at that point. In order to make sure
of this, readings were taken starting with the lowest intensity and ending
with the highest, then starting with the highest and following the decay to
the lowest value of intensity, the points plotted being an average of the two
settings for each point on the curve.

If the substances here studied exhibit the same hysteresis phenomena
that have been found in Sidot blende and several other substances,1 the
decay curves obtained by this
method are probably not the same
as would be observed if it were
possible to restore the substance
to a standard condition between
successive periods of excitation,
for example by the action of infra-
red rays. In using these curves
to test any theory of the decay of
phosphorescence, this fact must
of course be taken into considera-
tion.

The curves in Fig. 1 1 8 indicate
a much slower decay than those
shown in Fig. 117. The initial
decay or first process, however,
is quite rapid. These curves were
more difficult to obtain, since the
maximum of intensity lay in the
yellow-red, while the substances
whose curves are given in Fig.
1 1 7 have a green phosphorescence.
On this account the points are less
accurately determined than in

Fig. 117. It is evident, nevertheless, that the curves have the same
general character and consist of two straight lines merging into each other.

All the substances mentioned in this chapter are excited to fluorescence
by kathode rays. The mixture containing CdS04 was especially brilliant
and exhibited kathodo-luminescence of an intense yellow color. They
were also excited to some extent by X-rays, the CdvS04 again showing the
greatest intensity. In all cases the phosphorescence excited by X-rays
was too small to be measured with the present apparatus.

The curves given in Fig. 119 show the change in the decay of willemite
after heating. Curve L shows the decay of the untreated willemite.
Curve M shows the decay after heating the willemite for 45 minutes at a

A

r,

y

c^

-—

3 (

*

— -

—

— -

B

4b

>

Q

rf

■s

—

40
1

O

/

Vf

/

J

/

35

1

i

/

I

d

.02

04

Seconds

06

Curve A,
second.

Curve B,
second.

Curve C,

Fig. 117 — Decay curves.

Willemite No. 5, time of excitation 0.026
Andrews ZnCk, time of excitation 0.027
ZnCh No. 3. time of excitation 0.26 second.

'See Chapters IV and V.

n6

STUDIES IN LUMINESCENCE.

bright red heat. It is clear from the curves that the heat treatment has
decreased the initial intensity and made the decay less rapid; both these
changes may be observed by the eye alone if the two screens are excited
in a dark room. Willemite No. 3 which had been heated 2 hours was found
to give a curve whose points fell so nearly on curve M that it was not plotted.
The effect of heat upon the decay in ZnCl2+.rMnS04 is similarly
shown in Fig. 120. Curve H represents the decay in a sample of the sub-
stance taken as a dry powder, which was prepared by heating the mixture
only 2 hours at a bright red. Curve K represents the decay of a sample
heated 5 hours, and curve G after 7 hours heating. The curves show that
the first heating was not sufficient to bring out the initial brightness and
longer decay of the substance, and that there is a time limit to the heating

D

4t

~°E

^-0^

F

■ab

jS\/o

VT

44

4c

0 02 .04 .06

Seconds
Fig. 118.

Showing the decay in different substances.

Curve D, CaS, time of excitation 0.033 sec.

E, CdSOi, " " *' 0.032

F, CdCh, 0.031

.08

46

42

40

.01 02 .03 .04

Seconds

Fig. 119-

Showing effect of the heat treatment.

Curve L, willemite No. 1, time of excita-
tion 0.022 sec.

Curve M, willemite No. 2, time of excita-
tion 0.022 sec.

necessary to produce maximum intensity; for on heating beyond this the
initial intensity becomes less. From the behavior of this artificial com-
pound it would seem that the natural willemite shown in Fig. 119 had
reached a maximum heat treatment already, for further heating decreased
its initial intensity. The slope of the curves in Fig. 120 indicate little change
in the rate of decay after the initial drop; unfortunately the intensity be-
comes so small that it was found impossible to carry the curves farther.
The readings in this region represent the average of a number of settings.

The curves given in Fig. 121 show the effect of changing the time of
excitation in ZnClo No. 3. This was accomplished by making the opening
of the disk in Fig. 1 16 smaller. It was found that when the 60-cycle current
was used for the spark the curves taken with decreased time of excitation

STUDIES OF PHOSPHORESCENCE OF SHORT DURATION".

117

were more or less irregular. This was no doubt due to the fact that with
the small opening of the disk the irregularities of the spark were more pro-
nounced. However, on the 140-eycle current the curves were quite regular
and could be duplicated very closely. The curves given in Fig. 1 2 1 were
taken with the spark operated from the 140-cycle current. It will be noted
that a decrease in the time of excitation brings about a more rapid decay.

The curves in Fig. 121 also serve to confirm the results obtained with the
60-cycle current; for it will be seen that they have the same general shape
as those given for ZnCl2 No. 3 in Fig. 117.

.02 .03 .04 .05

Seconds
Fix. 120.

01 02 .03 04

Seconds
Fig. 121.

Carves showing effect of heat treatment. Showing the effect of changing the time of excitation.

Curve //, ZnCte No. i, time of excitation 0.031 sec. Curve .4. ZnCh No. 3, time of excitation 0.013 sec.

K, " " 2, " " " 0031 sec. B, 0.024 sec-

G, " " 3, " " " 0.031 sec. C, " 0.05S sec.

EFFECT OF INFRA-RED ON THE INITIAL DECAY OF SIDOT BLENDE-

The effect of infra-red on long-time phosphorescence is well known,1 and
it was natural to expect that short-time phosphorescence would be affected
in somewhat the same way. The method of experimentation followed was
to allow the light from the arc to fall on the phosphorescent screen through
a piece of very dense ruby glass, and to compare the shape of the decay curves
taken with and without the infra-red. It was found, however, that none
of the short-time substances were affected in measurable amount. There
was a slight indication of some change in the shape of the later portion of
the decay curve, but the error in setting the spectrophotometer in this
region, where the light is so faint, would account for the observed change.

While the effect of infra-red on the decay of short-time phosphorescent
compounds is so small that it can not be readily measured, the effect of
infra-red on Sidot blende is well marked.

■See Chapter V of this treatise.

uS

STUDIES IN LUMINESCENCE.

o

^

B

0

•u-6

/^_

A

I

Vf

}

^

f

i

Ad

.01

.02 .03

.06

.07 .08 .09

Iii Fig. 122, curve B, showing the decay under the action of the infra-red,
is the average of a number of determinations and the shape of the curve is
fairly definite. The data for curve A are, however, less reliable. There is
some doubt also as to the shape of the curve during the first hundredth of

a second. It was neces-
sary to change the posi-
tion of the disk on the
shaft of the phosphoro-
scope in studying the
effect of infra-red and it
is possible that the zero
given may be incorrect,
i. c, the points on the
zero ordinate and per-
haps the one following
may be in the fluorescent
light, and therefore the
real phosphorescent de-
cay may have started at
some later time than
the zero given. This
possible error in the zero
only applies to the curves taken in studying the infra-red and in no way
affects the other curves.

There is no question as to the effect of infra-red on the initial decay of
vSidot blende, but owing to the very slow decay the shape of the curves
may be more or less in error.

DECAY CURVES FOR DIFFERENT WAVE-LENGTHS.

In this determination the curves of decay were taken for different wave-
lengths of the phosphorescence spectrum of several substances. So far
as could be seen with the spectroscope all these spectra, at room temperature,
consisted of a single band, the location and extent of which in each case is
indicated in Table 18.

.04 .05

Seconds

Fig. 122.

Curves showing effect of infra-red on initial decay of Sidot blende.
Curve A, without infra-red, time of excitation 0.04 second.
Curve B, with infra-red, time of excitation 0.04 second.

Table iS.

CaS

0

.53 to 0.64 ix with

max. at 0

575M

CdCl2

•545 " -65 M "

tt tt

59 M

CdS04

.514 ' .623M "

t< tt

565M

ZnCl2 No

. 1

■49 '* -585M "

n tt

52 fi

Andrews'

ZnCl,

•49 -545M

tt a

52 m

Willemite No. 1

.46 " 06 m "

a tt

52 M

To determine the rate of decay of different parts of the band for any sub-
stance, two methods were used. One wTas to take a number of decay curves
at different wave-lengths of the band, and to plot a curve showing the inten-
sity, at a fixed time after excitation, for the different wave-lengths. In the
second method the mirror of the phosphoroscope was fixed so as to reflect the
light from the screen a certain time after excitation; then, by shifting the
telescope of the spectrophotometer, the intensity of the phosphorescence was

STUDIES OF PHOSPHORESCENCE OF SHORT DURATION.

119

determined for the different wave-lengths. The phosphoroscope was then
adjusted to give the decay at a later period, and the intensity for different
wave-lengths was measured in the same way. Both of these methods gave
the same results, i.e., that all portions of the band seemed to decay at the
same rate. The curves shown in Fig. 123 are typical for all the substances
studied. In none of the substances was there an indication of a shift of the
maximum of the curve as the decay went on. If any change occurs during
the decay it is too small to be detected by the method used.

SPECTROPHOTOMETRY STUDY OF THE CADMIUM-SODIUM COMPOUNDS.

The time of decay of these substances is so much longer than that of
the others considered in this chapter that the phosphoroscope already
described was not well suited to their study. A new instrument was
therefore devised for the purpose. This machine is a modification of the
phosphoroscope described by Kester,1 but is so constructed that it is possible
to determine curves of decay
without changing the time
of excitation.

Fig. 124 shows the top
view of the apparatus for
making the decay curves.
W is an iron pulley 45 cm.
in diameter mounted on a
vertical shaft. The mass of
the pulley being consider-
able, it acted as a balance
wheel on the driving motor
to keep the speed constant.
The speed was automati-
cally recorded on a chrono-
graph. The compound to
be studied was sifted lightly
over a strip of paper covered
with "zapon" varnish; the strip of paper was then placed on the rim of
the pulley. The spark gap S was mounted in such a way that it could
be moved to different points on the periphery of the wheel without in the
least disturbing the speed of the wheel or the continuity of the spark. This
made it possible to maintain a constant excitation and still obtain the inten-
sity of the phosphorescence at various times after excitation. The phos-
phorescent light to be studied enters the Lummer-Brodhun spectrophoto-
meter at Co. The comparison source is shown at G, and the telescope at T.

Fig. 125 shows the phosphorescence spectrum of a cadmium-sodium-
manganate preparation taken with the apparatus just described and with
an incandescent lamp as the comparison source.

Fig. 126 shows the phosphorescence spectrum of a cadmium-manganese-
chloride compound. The distribution of intensities in these two cases is
very similar and the spectra correspond closely with that of cadmium-

.50

Fig. 123.

Curve showing the phosphorescence spectrum of ZnCl: No.
at different times after the exciting light was removed.

'Kester: Physical Review, xxil, p. 280.

120

STUDIES IN LUMINESCENCE.

manganese-sulphate preparation previously observed, which showed a
band extending from 0.514 to 0.623 n, with a maximum at 0.565 /j.. It
would seem from these results that the manganese salts produce, when
added to a cadmium salt, a phosphorescent compound whose spectrum
shows the same maximum, and is independent of the salt.

B

,\>H

Ct W

Fig. 124.
Showing top view of phosphoroscope.

94

1

1

\

90

/

\

/

$

85

in

r

Ob

01

-t-
c

/

82

/

■j

1

IB

74

1

Wave leng

h

/

50 5^ 58 62

Fig. 126.

Showing the phosphorescence spectrum of
a CdS04-MnCl-2 compound, taken 1.44
seconds after excitation.

Of the sodium salts this is not true, for the colors are widely different.
An attempt was made to measure the spectrum of all the sodium compounds,
but'in most cases the intensity was far too small to measure.

48.2

48.0

/

+-

\

"en

c

V

•4-7.8

+-

f

474
47.3

/

vVave Ien6

I 1 °

th

52

54

56

58

60

62

Fig. 125.

Showing the phosphorescence spectrum of a Cd-Na2-
Mn04 compound, taken 0.374 second after excitation.

55
1

4 5

35

25

0 12 3 4-

Seconds
Fig. 127.

Showing curve of decay of a CdSOj-NaBr
compound.

Decay curves were taken from all the compounds whose intensity was
sufficient to measure. All these curves show the same characteristics when
plotted with the reciprocal of the square root of the intensity as ordinates

STUDIES OF PHOSPHORESCENCE OF SHORT DURATION.

121

and time in seconds as abscissas. Fig. 127, which gives the decay curve
for cadmium sulphate with sodium bromide, and exhibits the two straight
lines merging into each other, is a typical example.

SUMMARY.

The most important points brought out by the experiments here de-
scribed may be briefly stated as follows :

1. The decay curve when plotted with the values of /"- as ordinates
and corresponding values of t as abscissas consists of two straight lines
gradually merging into each other. In this respect the short-time and
long-time phosphorescent compounds seem to be similar.

2. The transition from fluorescence to phosphorescence is gradual, i. e.,
the curve shows no sign of discontinuity.

3. The shape of the decay curve and the intensity depend upon the time
of excitation.

4. The effect of heat treatment is such as to change both the intensity
and the rate of decay of phosphorescence.

5. The effect of infra-red on short-time phosphorescence, if it exists at
all, is very slight; but its effect on the initial decay of Sidot blende is quite
marked.

6. The experiments indicate that at ordinary temperatures all portions
of the phosphorescence band decay at the same rate.

THE EXPERIMENTS OF MR. CARL ZELLER.

The experiments described below deal with the phosphorescence of three
groups of compounds, namely, (1) the aniline dyes in their solid form; (2) a
group of manganese compounds of known percentage concentration and a
group of cadmium compounds, both prepared by C. W. Waggoner; (3) a
group of four phosphorescent sul-
phides furnished by Leppin and Ql
Masche, and prepared by the method
of Lenard and Klatt. —

The phosphoroscope described on
page 109 was used in connection with
a special form of photometer, the
arrangement of which is indicated in
Fig. 128.

In the diagram D is the revolving

disk of the phosphoroscope, S the (

spark gap, C the phosphorescent sub- -| / n \ \

stance, M the revolving mirror. The

photometer was made of two brass

tubes Z and X set at right angles, Fig. 128.

with a mirror A mounted at 450 at

the point of intersection of their axes. The mirror was a piece of microscopic

slide glass cut in the form of an oval with a hole in the center and then

silvered. This, when viewed from E, gave a contrast field with a patch

of phosphorescent light in the middle surrounded by a disk of light

'Carl A. Zeller, Physical Review, xxxi, p. 367.

122

STUDIES IN LUMINESCENCE.

reflected from the comparison lamp L. The color match was obtained
by using screens consisting of films stained with various aniline dyes on
glass. The lamp L was mounted on a photometer bar and could be moved
through a considerable range of distances.

THE ANILINE DYES.

The aniline dyes were dissolved in either zapon, gelatin, water glass,
or collodion, and flowed on glass. The glass proved to be too phos-
phorescent for its use; even oxidized brass emitted some light. Finally
black broad-cloth was used as a background, and the samples were tested

in powdered form, the phospho-
roscope running at 2200 R.P.M.
From a lot of fifty samples ob-
tained from Heller and Merz,
the following show both fluores-
cence (F) and phosphorescence
(P) in slight degree : Naphthyl
carmine, white; F blue, P very
slight. Naphthyl sodium di-
sulphonate, gray; F blue, P
slight. Phthalic anhydride,
white; F light blue, P slight.
Naphthol sodium disulphonate,
gray; /''blue, P slight. Naphthol
sodium monsulphonate, gray;
F blue, P slight. Naphthol so-
dium sulphonate, gray; F blue,
P trace. Beta-naphthol, white;
F light blue, P trace. Tetra-
chlor phthalic anhydride,
white; F blue, P slight. Alpha
naphthylamine; F violet, P
slight. Nitro-naphthylamine,
light yellow; F green, P slight.
Sulphonate of soda, reddish; F blue, P slight. Several of these show
marked fluorescence when dissolved. Eosin, fluorescein, and rhodamin, in
powdered form, show no trace of phosphorescence.

THE MANGANESE CHLORIDE GROUP NaCl-MnCl2.

The substances had been prepared by Dr. Waggoner by mixing solutions
of MgClo and NaCl in known proportions and evaporating to dryness.
The percentage of MnCl^ varied in different cases from 0.0 1 per cent to
2.0 per cent.

The samples were placed on brown cardboard with zapon varnish. The
phosphoroscope ran at 600 revolutions per minute, or one revolution in
o. 1 second. Time of excitation 0.025 second. The zero point was deter-
mined by readings taken at the point where reflected white light just
disappeared. In making a reading the mirror was turned just far enough
to shut off the direct reflected light. The standard was then moved until

•03 .04

Seconds

Fit

129.

Manganese-chloride group, ranging from 0.01
percentage concentration.

to 20

STUDIES OK PHOSPHORESCENCE OF SHORT DURATION.

123

a definite match was obtained. The rest of the readings were made by
moving the standard lamp one division and turning the mirror away from
the zero point. The dimness of the phosphorescent light and the limit of
the machine allowed but seven or eight points at most. The data plotted
are the averages of three or four readings. The phosphorescence of this
group is a pinkish red. In Fig. 129 the curves are plotted with time of
decay as abscissas and i/y/I as ordinates, i/\/I being proportional to the
distance of the standard lamp from the photometer screen. The lower and
upper parts of the curves are straight lines, becoming concave downward
as they approach each other. Several of the curves happen to have points
taken at the bend and would indicate two straight lines meeting. All of
the curves plotted with /"* as abscissas show the upper and lower parts as
straight lines, gradually merging into one another.

The curves are in fact of exactly the same type as the decay curves
obtained with Sidot blende.1 A glance at the curves in Fig. 129 shows a
marked tendency toward parallelism in both parts. According to the
assumption of Wiedemann and Schmidt, that the light is emitted during the

I

12

\

-

10
6
6

1

1

2

r

\

>

— ■

— <

l-Si

A

^
^S^

B/

/

C,

li

V

I23A56789
Percentage of concentration

10

Fig. 130.

Showing relation between intensity and percentage concentration.

.02 .04 .06

Seconds
Fig. 131.

Decay curves for three cadmium
compounds. The color of phos-
phorescence is green in each case.
Time of excitation = 0.025 sec.

A , CdS04 + MnSOj - phosphores-
cence; B, CdSOi + MnCl; C,
CdSOi +MnCl —different percent-
age.

recombination of the products of the dissociation produced during excita-
tion, this would indicate the same coefficient of recombination of the ions for
the lower parts of these curves, and also the same, but slower rate in the up-
per parts. In this group the compounds that have the greatest initial inten-
sity have the longest period of decay, although there seems to be no relation
between the initial intensity and the time of decay. The cadmium com-
pounds (Fig. 131) show just the opposite relation. The different slant in
the two parts of the curves indicates a different coefficient of recombination
and a different rate of decay for the two parts. The coefficient and the de-
cay are smaller in the second part, showing a much slower process in the
giving back of the stored energy.

The greatest intensity was shown by the compound of 0.8 per cent con-
centration. The relation between percentage concentration and initial

'See Chapter V.

I24

STUDIES IN LUMINESCENCE.

intensity is shown in Fig. 130 by plotting percentage concentrations as
abscissas and initial intensities as ordinates. The initial intensities were
obtained by extending the straight line of the first part of the curves in
Fig. 129 to the y axis and reading the y intercept from the scale.

THE CADMIUM GROUP.

The phosphorescence of the cadmium compounds is yellow-green. Fig.
131 shows the decay curves for three different compounds. These are
characteristic decay curves, and follow the general form. The compounds
CdS04+NaC03, CdS04-f-NaC103, CdS04+NaCl, CdS04-f-NaN03, and
CdS04+NaBr were too dim to measure, but when excited may be seen
in a dark room for several minutes. In this group the specimens that have
the most intense phosphorescence have the most rapid decay.

SUBSTANCES OF SLOW DECAY.

A number of phosphorescent sulphides, made by the method of Lenard
and Klatt,1 were obtained from Leppin and Masche, Berlin. They consist

of a sulphide, an active metal, and a flux.
Practically all long-time and short-time
decay curves have shown two distinct
processes, merging into one. By this
method it was possible, using long-time
specimens, to study the first part of the
first process, for the very first interval of
decay. The specimens were excited for
0.008 second and the first readings made
0.0014 second after excitation. The
curves shown in Fig. 132, plotted with
time as abscissas and I~- as ordinates,
are in all cases straight throughout the
range studied. These results, which are
in agreement with the results of Wag-
goner for Sidot blende, make it appear
quite improbable that the behavior of
the phosphorescent sulphides during the
early stages of decay can be represented
by an exponential law, as has recently
been predicted by Lenard.2 It should
be pointed out, however, that the slant of
the straight lines in Fig. 132 is such as to indicate an initial rate of decay
far greater than that corresponding to what is usually called the "first
process," or " Momentan-prozess," in the decay of substances of this
class. It is possible that under the conditions of these experiments the
decay curves of the phosphorescent sulphides consist of three portions,
each of which is linear, or nearly so, when 7_i is plotted against time.

1^

A/

/

8

a/

Qy

0

D,

02 04- .06

Seconds
Fig. 132.

A, Sr-Bi-Na phosphorescence, green; /?, Sr-
Zn-F phosphorescence, green; C, Ca-Bi-Na
phosphorescence, violet; I), Ba-Cu-Li phos-
phorescence, orange-red.

lSee Kayser, Handbucn der Spectroscopic, vol. 4. p. 750. :Lenard Annalen der Physik, XXI, p. 64t, 1910.

CHAPTER VIII.

PHOTOGRAPHIC STUDIES OF LUMINESCENCE.

Dr. C. A. Pierce, whose investigations of thermo-luminescence have
been described in Chapter VI, has attacked by a photographic method
some of the problems which arise in the study of the relations of fluorescence
and phosphorescence and in the consideration of the form of the curve of
decay. An account of his method and a summary of his results, which
have an important bearing upon these questions, are given in the present
chapter.1

THE DISTRIBUTION OF ENERGY IN THE FLUORESCENCE SPECTRUM AND THE
PHOSPHORESCENCE SPECTRUM OF SI DOT BLENDE.

The method used in the study of the distribution of energy in fluorescence
spectra consisted of photographing on the same plate the spectrum of the
excited substance2 and four spectra of an acetylene flame. Different in-
tensities of the light from this flame were used for the four spectra, but the
times of exposure of all four and of the fluorescence spectrum were the same.
Knowing the distribution of energy in the acetylene flame, :! the distribution
in the fluorescence light was obtained by photometric comparisons of the
spectra on the plate.

During the set of experiments the fluorescent powder was contained in a
square dish made of platinum foil. The foil was connected to and sup-
ported by copper leads, so that the temperature of the powder could be raised
and controlled by passing electric current through the foil. The temper-
ature was measured, if different from room temperature, by means of a
copper-constantan thermo-couple placed in the midst of the powder. The
blue lines of a mercury-arc lamp were used to excite the powder. The
various intensities of acetylene light were obtained by screening off all but
the center of an acetylene flame and moving the flame with the screen to
different distances from the slab of magnesia which reflected the light into
the slit of the spectrum camera. The camera consisted of a direct- vision
spectroscope set so that the spectrum could be focused on a sensitive plate
held in a plate holder.

The plates were handled and developed in complete darkness. The
developer was freshly mixed from a stock solution and of a standard
strength, and the length of development was timed, being for most of the
plates 8 minutes. The temperature was brought to 200 at the beginning
of development and the plates were rocked mechanically during the de-
velopment. The plates were fixed, at first for 30 minutes, after washing
in three separate waters, and the temperature of the fixing bath was brought

]See C. A. Pierce. Physical Review, xxx, p. 663; xxxn, p. 115.

2The substance employed was the " Emanations-pulver" used in the experiments described in Chapter IV.
3On the distribution of energy in the visible spectrum, by E. L. Nichols; Physical Review, xxi, p. 147.
Sept. 1905.

125

126

STUDIES IN LUMINESCENCE.

to 20° at the beginning of the fixing. Later not so much care was used in
the fixing because it was found to be inadvisable to intercompare spectra
on different plates. After fixing, the plates were washed in running water
for about 30 minutes, then thoroughly rubbed by hand and dried. Each
plate was dried in such a position that the last portion to drain was a part
upon which no measurements were to be made.

The distribution of denseness in any spectrum on a plate was obtained
by pushing the plate past a brightly illuminated slit placed squarely across
the spectrum, and measuring the transmitted light by means of a Lummer-
Brodhun photometer. The source of light to illuminate the slit and the
standard light consisted of two carbon-filament electric lamps with frosted
bulbs. The light back of the plate was concentrated on the slit by a
reflector. The current supplied to the lamps was held practically constant.
It was found by actual test that a change of 5 volts was necessary to vary
the relative intensities of the lamps enough to affect the settings of the
photometer appreciably. The lamps were run at 6 per cent above normal
voltage to increase the candlepower and were lighted only long enough to
make the measurements.

70
60

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50

54 .58

Fig- 133-

.62

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The wave-length measurements were made by calibrating in wave-
lengths the screw which pushed the plate past the slit on the photometer
bar. A zero wave-length was obtained on the plate by photographing the
three blue lines of the mercury arc superimposed on the different spectra.
These lines were in an entirely different region from that occupied by the
spectrum of the fluorescence light. The calibration of the screw was made
by photographing the entire visible spectrum of the mercury arc with the
spectrum camera and setting one line after the other directly in front of the
slit on the photometer bar. This slit was always opened just as wide as the
width of the lines in the spectra and was placed parallel to these lines. The
standard lamp was screened off by means of a variable slit, so that the pho-
tometer could be used on whatever part of the bar was desired.

Fig- J33 shows the curves obtained by plotting the light transmitted
through the spectra on one of the plates which had been exposed as de-
scribed above. Each exposure was 60 minutes long and was made with the

PHOTOGRAPHIC STUDIES OF LUMINESCENCE.

127

I

50

40
30
20
10

slit opened 40 units. Curves 1,2,3, and 4 represent the spectra of acetylene
light at intensities 32, 16, 8, and 4 respectively. Curve 7 represents the
spectrum of the fluorescence light. Curve 6 represents the distribution
of energy in the acetylene flame and is assumed to correspond to curve 3.
Then the energy curve corresponding to curve 2 will have ordinates equal
to 16/8 times the corresponding ordinates of curve 6, etc. Wherever curve
7 crosses curve 3, the energy for that wave-length is represented by the
ordinate on curve 6 for that wave-length. Wherever curve 7 crosses curve
2, the energy for that wave-length is 16/8 times the ordinate of curve 6 for
that wave-length, etc.

Since the curves intersect at only a fewpoints.it was necessary to get other
points on the desired energy curve by interpolation. This was done sys-
tematically and as follows: Suppose wave-length m is under consideration.
A curve is plotted with the intensities of acetylene, 4, 8, 16, and 32, as
abscissas and the intersections of the vertical at /n with the curves 1, 2, 3,

i

L

l\

\\ \'\i\\

\\ *

!)

t

1

»2

3

1=

4 8

16
Fig- 134-

32

and 4 as ordinates. From this curve is picked off the intensity of acety-
lene that would have coincided with curve 7 at wave-length ;ui. Fig. 134
shows six curves drawn in connection with Fig. 133. Curves 1, 2, 3, 4, 5,
and 6 correspond to wave-lengths ,0 = 0.50, 0.51, 0.52, 0.53, 0.54, and 0.55
respectively. The circle on each curve shows the point picked off. On
curve 6, Fig. 134, 23.5 is the abscissa corresponding to 2.8, the ordinate on
curve 7, Fig. 133, at ^ = 0.55. Hence the ordinate at ju =0.55 on curve 5,
133, which shows the distribution of energy of the fluorescence light is

Fig.

equal to

23-5

times 21.3, the ordinate on curve 6, Fig. 133, at ^ = 0.55.

Curve 5, Fig. 133, shows the energy curve of fluorescence as computed
from the other curves in Fig. 133. The curve has a well-defined maximum
at ^ = 0.55 and a minimum at /i = o.6o. The shape of the curve for wave-
lengths greater than yu = o.6o is uncertain.

Fig. 135 shows another set of curves corresponding to those in Fig. 133.
The length of exposure was 30 minutes for each spectrum with a slit width

128

STUDIES IN LUMINESCENCE.

of 20 units. The intensities of the acetylene flame were 32, 20, 12, and 6
for curves 1, 2, 3, and 4 respectively. Curve 6 is assumed to represent the
energy distribution in curve 4. Curve 5, which shows the distribution of
energy in the fluorescence light, is similar to the corresponding curve 5 in
Fig. 133. This shows that the curve is not materially influenced by two
factors, length of exposure and width of slit in the spectroscope.

The Sidot blende was excited by three blue lines of the mercury-are
spectrum. While these lines are in a different region of the spectrum from
that occupied by the fluorescence band, yet there was considerable halation
shown on the plates about the three lines and it was thought that this might
extend far enough to change the shape of the curves in the region of smaller
wave-lengths.

To test this matter a slab of magnesium carbonate was substituted for the
Sidot blende and was illuminated in the same manner by the blue and
violet lines of the mercury arc.

A plate was exposed to the light reflected from this slab and also, as in
the previous experiments, to four different intensities of acetylene light.
Curves showing the amount of transmitted light are given in Fig. 136.

.66

AZ .4-6 .50 .54 .58 .62

Fig. 136.

In this as in the previous figures curves 1 , 2,3, and 4 are for the acetylene
light. Curve 5 indicates the transmission of that part of the plate exposed
to the light of the mercury arc reflected by the magnesium carbonate.
vSince there is no indication of any effect it appears that halation from the
mercury lines is inappreciable for wave-lengths longer than 0.46 \x and that
no error from this source occurs in the curves marked 5 in Figs. 133 and 135.

Experiments were also made with very intense negatives in an endeavor
to obtain the shape of the energy curves for wave-lengths greater than 0.60 /i,
but on account of an apparent tendency to reversal in such negatives it was
not found possible by this method to extend observations to these longer
waves.

PHOSPHORESCENCE AT ROOM TEMPERATURE.

In order to obtain the distribution of energy in the phosphorescence
light, it was necessary to add to the apparatus already described a shutter
which would close the spectrum camera and excite the powder; then shut
off the exciting light and open the camera. The shutter was operated by
a constant-speed motor. Levers, springs, and triggers were so arranged

PHOTOGRAPHIC STUDIES OK LUMINESCENCE.

129

that the shutter operated in a small part of a second. The length of
excitation and decay could be increased or decreased together by changing
the speed of the motor, while either the length of excitation or the length of
decay could be varied alone by rearrangement of parts of the controlling
devices. The total time of exposure of the photographic plate to the phos-
phorescence light was made equal to the length of exposure of the acetylene
spectra.

With the apparatus described above, the photographic negative of
phosphorescence was obtained by light which varied in intensity, for the
phosphorescence decayed after the exciting light was closed off in the
manner characteristic of phosphorescent powders. However, if an energy
curve is obtained similar to that for fluorescence, it seems reasonably certain
not only that the energy curve does not change with decay, but also that it
does not change when the exciting light is shut off.

I

5

asJoJl

4^

V

^f^

<S

3

"i^

\ \

;

Cr^r

£v

0/ '

c

s>^

44-

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.52 .56

Fig- 137.

.60

Fig. 137 shows the energy distribution in the phosphorescence light of
Sidot blende. The powder was excited each time for 8.75 seconds and
allowed to decay 10.33 seconds. The powder was not exposed to infra-red
before each excitation. The effective length of exposure of the photo-
graphic plate was 60 minutes. Curves 1, 2, 3, and 4 show the transmitted
light of the photographic spectra corresponding to intensities of acetylene
equal to 6, 4, 2, and 1, respectively. Curve 5' shows the transmitted light
of the phosphorescence spectrum. Curve 5 shows the energy curve for the
phosphorescence light.

It is seen that curve 5 is similar in form to the corresponding curves for
fluorescence. The phosphorescence spectrum, at least in the early stages
of decay, is thus the same as the fluorescence spectrum.

DISCUSSION OE METHOD.

The chemical change made in a photographic plate depends upon the
wave-length and intensity of the incident light and upon the length of

130 STUDIES IN LUMINESCENCE.

exposure. If several spectra are photographed upon the same plate and
the length of exposure is the same for all the spectra, then the denseness of
the different negatives at any wave-length will depend only upon the
intensity of the incident light at that wave-length, assuming a small opening
of the slit to the spectroscope, uniform development, non-halation, etc.
Since intensity is proportional to the rate at which energy is received from
the source, the denseness of the different negatives at any wave-length
depends upon the energy received from the source at that wave-length.
By making several negatives of one source of light of known energy dis-
tribution at different distances from the spectroscope, curves can be drawn
showing the relation between energy received at any wave-length and dense-
ness of negative. If a spectrum of another light of unknown energy dis-
tribution is photographed on the same plate, its energy distribution can be
obtained by comparison of the denseness of its negative with the calibrated
densities. The densities can be compared by means of the light transmitted
through the negatives at different wave-lengths.

If the slit opening in the spectroscope is not very small, the denseness of
the negative at any wave-length will not depend alone upon the energy
received at that wave-length, but will also depend upon the energy received
at wave-lengths differing but little from the wave-length in question.
Consequently a small dimple in a curve might be obliterated by a wide slit
opening. The effect of slit opening can be tested by using different openings,
finally using the one which experience shows to be best.

The effect of halation is more troublesome than slit opening, and was
present more or less in all of the negatives. The effect of halation is to
broaden the band of energy distribution, but not to change the position
of the maximum point of the band. It is believed that halation in the
present experiments was not of sufficient effect to cause any serious error in
the curves.

The plates were developed immediately after the exposures, which were
made one directly after the other, so that any error due to continued chemi-
cal action after exposure in the film was eliminated as far as possible.

The photometric measurements were difficult to make because of the
large differences in the amount of light transmitted in different parts of the
negatives. At the beginning of the experiments, four settings of the pho-
tometer were made for each point measured. Later two settings were found
to be sufficient, one approaching uniformity of illumination from each
direction. The average of the two values calculated from the two settings
was taken as the true ratio of standard and transmitted lights. Remeasure-
ment of a set of curves never proved any exceptional accuracy, but always
gave the same type of energy curve with the maximum at nearly the same
wave-length. Consequently the measurements can be said to be substan-
tially correct.

These experiments show that the energy curve of the fluorescence light of
Sidot blende consists of a band extending from about ^ = 0.46 to ;u = o.6o,
having a maximum at ju = 0.55. There may be another band situated in the
region of longer wave-lengths. Furthermore, the energy distribution in the
fluorescence light and the phosphorescence light immediately after excita-
tion is the same.

PHOTOGRAPHIC STUDIES OF LUMINESCENCE. 131

THE PHOSPHORESCENCE SPECTRUM DURING DECAY AND THE QUESTION OF

TWO OVERLAPPING BANDS.

In Chapters IV, V, VI, and VII of this memoir, the characteristic form
of the curve of decay of phosphorescence for various substances and under
a variety of conditions has been given and it has been shown to consist of
two straight lines of different slopes which gradually merge into one another.
In Chapter VI it was shown, moreover, that Becquerel's1 expression for
the decay could be made to fit the actual curves by assuming the existence
of two overlapping bands having very different rates of decay.2

In Chapter IV measurements on the phosphorescence of Sidot blende
were described and it was shown that in the case of Sidot blende the spec-
trum did not change its form during the first few seconds of decay.3 A
similar result was obtained by Waggoner4 (see Chapter VII) for several
other substances exhibiting phosphorescence of short duration.

While the presence of overlapping bands of differing duration would
necessarily cause a gradual change in the distribution of intensities in the
phosphorescence spectrum as decay progressed, these measurements were
not decisive, as against the hypothesis of the existence of such bands. It
was found in the case of the Sidot blende that the "first process" of decay
lasted about 10 seconds, whereas the spectrophotometric measurements
covered only 3 or 4 seconds. Dr. Waggoner's determinations likewise apply
chiefly to the earlier stages of decay.

The experiments now to be described were undertaken to provide further
data bearing upon the two-band theory of phosphorescence decay. The
two rectilinear parts of the typical decay curve differ widely in slope.
Hence, on the assumption of two bands, most of the light before the bend
in the curve is due to the band corresponding to the steeper straight line
(see Fig. 79, Chapter VI). At the bend the light is due more or less equally
to both bands; and after the bend the light is due mostly to the second
band. With a method available for studying the light distribution before
and after the bend, the two-band explanation would be proved or disproved,
depending on whether a change was or was not found in the distribution of
the light.

As was shown in the first part of this chapter, the fluorescence spectrum
of the substance used consisted of one prominent, symmetrical, smooth-
sided band with a maximum at about ^ = 0.55 (see Fig. 133). The band
extends, approximately, from /jl == 0.46 to ^ = 0.60. Furthermore, the energy
distribution immediately after excitation (Fig. 137) is the same as in the
fluorescence spectrum.

The present work is concerned with the light distribution before and
after the bend in the decay curve.

•H. Becquerel. Comptes Rendus, 113, p. 618, 1891.
2See Chapter VI; also C. A. Pierce, Physical Review, xxvi, p. 312.
3See also Nichols and Merritt, Physical Review, xxi, p. 247.
«C. W. Waggoner, Physical Review, xxvn, p. 220.

132

STUDIES IN LUMINESCENCE.

EXPERIMENTAL.

The method employed to photograph the decaying band of phospho-
rescence was based on the assumption that the conditions and phenomena
could be reproduced indefinitely, a fact already established for the substance
under investigation. The phosphorescent powder, except in the case of Fig.
140, which gives the results obtained with Balmain's paint, was zinc sul-
phide, i. c, the sample of " Emanations-pulver" used in the investigations
described in Chapter VI.

The apparatus was that described on pages 130 and 131, but electri-
cally operated shutters were used to excite the powder, to expose the plate,
and to kill off the remaining phosphorescence with infra radiations before
repeating the excitation. With this apparatus the decay curve could be
photographed between any two points, time and time again, until an im-
pression had been made on the photographic plate. Since, at best, the
necessary exposure was very long, varying from a few hours to a much
longer time, no attempt was made to deduce the energy distribution, but

the photographic spectra
were compared with each
other, the principal weight
being attached to the posi-
tion of the maximum of
the band. Care was taken
to obtain negatives of
about the same average
density, and of not too
great density, so that no
complications could result
from widely different or
complete chemical change
of the films at any wave-
length. With these precautions, negatives were obtained which showed
definite maxima, and the results could be repeated as many times as desired.
The distribution of denseness on the photographic film was measured,
at first, with a photometer, as in the previous experiments; but this method
was soon abandoned because of the eye strain induced by comparing very
faint fields of light. The apparatus which was substituted consisted of a
brightly illuminated slit placed before a very sensitive system of thermo-
couples. The photographic plate was pushed past the slit by means of a
screw calibrated in wave-lengths. Since the deflection of the galvanometer,
which measured the current from the thermo-couples, depended upon the
length of time that light was allowed to pass through the slit, a pendulum
was made to light and extinguish the electric lamp which illuminated the
slit, at predetermined intervals. With this apparatus results were obtained
which were consistent with those obtained with the photometer, and the
results could be repeated.

Fig. 138 shows the data obtained by measuring a film with the pho-
tometer (curve .4)andwith the thermo-couple (curve B). In thisand in all

0

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Fig. 138.

PHOTOGRAPHIC STUDIES OF LUMINESCENCE.

*33

subsequent figures ordinates represent intensities of transmitted light.
These two curves are consistent as regards general shape and the position
of the points of minimum transmission. Since intensities of transmitted
light are plotted, the minima of the curves correspond to the maxima of
intensities of the spectra. Inspection will show that curve B is not the
exact duplicate of curve . 1 ; the ratios of the ordinates at long wave-lengths
are not the same as at shorter wave-lengths. This lack of similarity did
not exist in many of the comparisons and was, quite likely, due either to
eye fatigue or else to the gradual warming up of the plate under successive
exposures to the light which illuminated the slit.

Fig. 139 shows three curves corresponding to the decay of phosphores-
cence. In curve A, the Sidot blende was excited for 9.75 seconds and the
plate was exposed for 8.25 seconds immediately after excitation. The
powder was exposed for about 1 minute to infra-red rays and the process
was repeated over and over until a sufficient exposure had been secured.

P

°v

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^

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b'

AS

SO

.54 .56

Fig. 139-

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Fig. 140.

• 50/<

In curve B the powder was excited for 21 seconds, allowed to decay for 14
seconds, then the plate was exposed for 15 seconds. In curve B the x-axis
is raised so that curve B does not intersect curve A. In curve C the exci-
tation was 5.5 minutes, the decay 1.5 minutes, and the plate was then
exposed for 1 minute. The total time of exposure to produce the negative
from which curve C was made was 72 hours. The negative was faint and
the film was somewhat fogged, but remeasurements on the film always
gave approximately the same curve. In fact, curve C is the average of sev-
eral remeasurements. The minima of these curves occur at wave-length
M = 0.555 ±0.003.

The bend in the curve of decay of the phosphorescence of Sidot blende
occurs between 10 and 20 seconds after the end of excitation and is more
pronounced the longer the excitation. Hence curve A in Fig. 139 is due
chiefly to the light corresponding to the decay before the bend, while curve
B corresponds to conditions near the bend, and curve C corresponds to
conditions far beyond the bend. Or, in terms of the two-band theory,
curve A corresponds mainly to band 1 ; curve B, to bands 1 and 2 ; and curve
C to band 2 almost entirely. This set of curves shows no change in the
maxima of the spectra, and indeed no set was obtained which showed any
appreciable change.

134

STUDIES IN LUMINESCENCE.

The powder from which Balmain's paint is made, as will be seen by refer-
ence to Chapter IV, shows a more pronounced bend in the decay curve than
is the case with Sidot blende. Two determinations were therefore made
using this substance and showing the character of the spectrum before and
after the bend. For each curve (see Fig. 140) the excitation was 5 minutes.
For curve A, the plate was exposed 20 seconds immediately after excitation.
For curve B, the phosphorescence was allowed to decay 1.5 minutes, then
the plate was exposed for 1 minute. No shift in the minima of the curves
can be detected, while a decided change in the shape of the curves would
be expected if the two-band theory is correct.

A PHOTOGRAPHIC TEST OF THE EFFECT OF INFRA-RED RAYS.

The effect of infra-red radiations in suppressing phosphorescence and
fluorescence has been considered at length in Chapter V, in which exper-
iments were described indicating that all parts of a fluorescence band are
suppressed in the same ratio. As a check on that work and because, on
the assumption of the existence of two bands, one band would in all prob-
ability be suppressed more than the other, several runs were made in which

A

B

/ c

.46 .50 5 A .58 62,/i.

Fig. 141.

the fluorescence spectrum of Sidot blende under the influence of infra-red
rays was photographed. These rays were obtained from a 16 c.p. lamp
held 5 cm. from the powder. The visible rays were screened off by means
of thin rubber.

In Fig. 141 curve A corresponds to the fluorescence spectrum without
infra-red excitation, and curve B with infra-red. The exposure of the plate
in the latter was twice as long as in the former case. It will be seen that the
minimum is not changed appreciably from 11 — 0.555 by the exposure to infra-
red and that the curves agree very well in form, except at short wave-lengths.

In Fig. 142, curve A corresponds to the fluorescence spectrum without
infra-red; curve B to the same with infra-red; and curve C corresponds
to the phosphorescence spectrum just after excitation and is added for ease
of comparison. In each case the minimum of the curve occurs at n = 0.555
approximately.

The effect of infra-red excitation on the fluorescence spectrum, according
to all of the curves obtained, is to decrease the intensity of the band ; but

PHOTOGRAPHIC STUDIES OF LUMINESCENCE.

K->5

there is no change in the distribution of intensities that can be detected
by this method and no measurable shift of the maximum. This result is
in accordance with those recorded in Chapter V and it does not confirm
the two-band theory of decay.

THE EFFECT OF TEMPERATURE ON THE FLUORESCENCE SPECTRUM.

It has recently been shown1 that remarkable changes in the apparent
complexity of fluorescence spectra are in some cases produced by changes
in the temperature of the substance.

To determine whether such changes
as could be detected by the present
method take place in the fluorescence
band of Sidot blende when that sub-
stance is excited at different temper-
atures a series of photographs were
taken, the results of which are shown
in Fig. 143.

This figure contains four curves
corresponding to the fluorescence of
vSidot blende at different temperatures.
In each case the length of excitation
was varied so as to give, approximately,
negatives of equal density, hence the
dimming of the band at higher tem-
peratures is not evident, though it
occurred. In fact, the powder almost
ceased to exhibit fluorescence when the
temperature was raised sufficiently.

When the temperature was lowered the powder would again show the
same spectrum as before, provided a certain critical temperature had not
been exceeded. The .v-axis is changed for each curve, so that the points
may be entirely distinct from one another. Curves A, B, C, and D were
obtained at temperatures 220, 670, 88°, and 1200 respectively. No shifting
of the minimum can be seen.

•

Dv

C

B

A

0

.50

.54

Fig- 143-

58

.62/i

:Xicho1s and Merritt; Physical Review, xxxn, p. 38.

136 STUDIES IN LUMINESCENCE.

DISCUSSION OF RESULTS.

The accuracy of individual measurements with a non-direct method, such
as the photographic method described in this chapter, is not great. Con-
clusions must be drawn from the indications of many experiments, rather
than from the too exact interpretation of a single determination. A general
survey of the curves obtained, of which only few examples have been given,
shows that the minima, corresponding to the maxima of the usual curves of
distribution previously employed, occur within the limits X = 0.555 ^±0.003.
These limits were seldom exceeded and the variations were not consistent
with each other. For some sets of curves the limits could be contracted to
±0.002 n or possibly to ±0.001^. Hence, if the maximum of the band
changed under any of the conditions, it must have been within the limit
=±= 0.003 M and probably within even smaller limits. From certain considera-
tions, such as the bend in the decay curve of phosphorescence, and the
smooth, symmetrical shape of the whole band, one would expect, if the two-
band theory is correct, a considerable change in the band under the condi-
tions studied. No changes were found that were not explained by the
limitations of the method.

Hence the following conclusions are drawn from the work above :

1 . The fluorescence and phosphorescence bands of Sidot blende coincide
with each other.

2. No change in the position of the band of phosphorescence occurs with
decay in the case of either Sidot-blende or Balmain's paint.

3. No change in the position of the band of fluorescence occurs under the
action of infra-red rays.

4. No change in the position of the band of fluorescence occurs with a
change, between +200 and +1200, in the temperature of the powder.

5. No change in the shape of the band, discernible by this method, was
found under anv of the above conditions.

CHAPTER IX.

A SPECTROPHOTOMETRIC STUDY OF CERTAIN CASES OF
KATHODO-LUMINESCENCE.*

The spectrophotometric study of luminescence has thus far been confined
almost entirely to cases of photo-luminescence. It is of interest to inquire
whether exciting agents other than light will give luminescence spectra of
the same type, and to what extent the laws that have been found to hold
in the case of photo-luminescence possess a more general application. The
experiments on kathodo-luminescence described in the present chapter are
of interest chiefly because of the bearing of the results upon these and
similar questions. During the course of the work preliminary data have
also been obtained with regard to the de-
pendence of kathodo-luminescence upon
discharge potential and current strength.

The form of the vacuum tube used is
shown in Fig. 144, the substance to be
tested being placed at S. In order to pre-
vent disturbance from the fluorescence of
the glass the specimen in some instances
was placed in a metal inclosure which
protected the glass from excitation by the
kathode rays. The kathode, K, was a flat - —
aluminum disk. The fluorescent light was I,'

reflected directl v into the slit of the Lummer-
Brodhun spectrophotometer by a small J

mirror at the side of the tube. An acetylene
flame was used as a comparison source.
Current was furnished by a motor-driven
Hoi tz machine capable of giving a current of
0.6 milliampere. A galvanometer placed Fig. 144.

in the circuit next to the grounded pole of

the machine served to indicate the constancy, or lack of constancy, of the
current through the tube. In most cases relative values only of the current
have been recorded. The potential difference between the terminals of the
tube was measured by a Kelvin electrostatic voltmeter and served as a
sensitive indicator of changes in the vacuum. "-

Considerable difficulty was encountered in keeping the discharge through
the tube steady. Sudden changes occurred in the intensity of the fluores-

■The results contained in this chapter were in part presented to the American Physical Society, at the
meeting held in Washington, April 24, 1908, and were published in the Physical Review, xxvm, p. 349.

2Since the absolute values of the potential used are of 110 great significance in the present work, we have
plotted voltmeter readings rather than the actual values of the potential in the curves contained in this
chapter. If the potential in volts is desired thi-; may be found bv multiplying the voltmeter deflection by
385.

137

13S STUDIES IN LUMINESCENCE.

cence, even without any corresponding change in current or voltage, due
apparently to some erratic cause producing either a deflection of the kathode
rays or a change in the intensity of these rays. The most serious of these
disturbances were traced to irregular leakage from the wires leading to the
Holtz machine. This trouble was made more difficult to control by un-
favorable atmospheric conditions, the work being carried on in the spring ;
but the disturbances due to leakage were largely removed by using large
rods in place of wires, and by avoiding sharp corners. It proved advan-
tageous, also, to run the Holtz machine at full speed and to cut down the
potential difference between the terminals of the tube by means of resistance
in series. The necessary resistance was furnished by a tube containing
absolute alcohol. Flickering persisted to some extent even with these
precautions, and it was frequently necessary to discard the results of several
hours work. The results given here were, however, taken under favorable
conditions.1

It did not prove practicable to maintain the pressure in the tube constant
during the time necessary for spectrophotometric measurements through-
out the spectrum. Owing to a small leak, or to the development of gas
by the discharge, the pressure gradually increased during the course of a
series of readings, so that at the end of about an hour it was necessary to
reexhaust. In order to determine the luminescence spectrum corresponding
to constant pressure conditions the following procedure was adopted:

While one observer recorded the readings of the voltmeter and galva-
nometer and, when necessary, operated the pump, the other made settings
of the spectrophotometer as rapidly as possible, running back and forth
through the spectrum until the whole region had been covered a number
of times. Curves were then drawn for each wave-length at which settings
had been made by plotting intensities, as measured by the spectrophoto-
meter, against voltmeter readings. A series of such curves is shown at the
left in Fig. 145, which refers to a cadmium sulphate preparation. These
curves all have the same general shape, and in most cases each curve
contains enough points to determine its form with considerable deiiniteness.
After the curves had been drawn as accurately as possible the intensity
corresponding to any particular voltmeter reading could be determined
by graphical interpolation. The points which form the two curves to the
right in Fig. 145 (marked I. V. M. 30 and II. V. M. 17.5) were deter-
mined in this way for the voltmeter readings 30 and 17.5 respectively.
For these curves the abscissas are wave-lengths expressed in fractions of //
as indicated.

The procedure in the case of our experiments with willemite (Fig. 146)
and Sidot blende (Fig. 147) was exactly the same as in the case of cadmium
sulphate. A comparison of these three figures shows, however, that while
the voltage intensity curves in Figs. 145 and 146 are similar, being in each
case concave toward the horizontal axis, the corresponding curves in Fig.
148 are almost exactly straight. The difference results from the fact that in
the experiments corresponding to Figs. 145 and 146 the current through the
tube diminished rapidly as the voltage increased, whereas in the experi-

>It is probable that the trouble was due to electrostatic charges on the walls of the tube. A larger tube
so constructed as to have the kathode at a considerable distance from the walls would probably have been
more satisfactory.

SPECTROPHOTOMETRY STUDY OF KATHODO-LUMINESCENCE. 139

ments on Sidot blende (Fig. 148) the conditions as regards leakage were
more favorable and the current remained nearly constant. During the
observations corresponding to Fig. 148 the deflection of the galvanometer
varied from 32 for the highest voltmeter reading (33.8) to 37 for the lowest
voltmeter reading (11.7). In the experiments with willemite (Fig. 146)
the galvanometer deflection ranged from 7 to 25; and in the experiments
with cadmium sulphate the range was from 1 5 to 3 1 .

The results obtained are plotted in Figs. 145 to 148 and are further
discussed below.

CdSOi-\-xM nSOi. — Tliis substance was prepared by C. W. Waggoner, who
has determined its decay curve when excited by the iron spark.1 Excited by
kathode rays it gave an intense yellow fluorescence, with scarcely observable
phosphorescence. Close inspection of the powder showed occasional grains

e* o

Fig- 145-

CdSO* +*MnS04. The curves to the left show the relation between discharge potential and intensity for
different wave-lengths. Curves / and // show the luminescence spectra for discharge potentials of
1 1,600 volts and 6,700 volts respectively. Curves /// and IV show, to a larger scale, the luminescence
spectra excited by ultra-violet light and Roentgen rays respectivelj'.

which glowed with an orange or red light. At the end of our experiments,
after the powder had been bombarded by the kathode rays for about 10
hours, the surface was found to have acquired a ruddy brown discoloration.
Upon standing for several months the powder scarcely responded at all to
excitation by the iron spark, but recovered its activity after heating.

Inspection of curves / and II, Fig. 145, shows that the form of the fluo-
rescence spectrum is independent of the discharge potential, and therefore
of the velocity of the kathode rays, since it is clear that one of these curves
might be obtained from the other merely by changing the scale.

1 Waggoner, Physical Review, xxvir, p. 209, 1908. See also Chapter VII of this memoir.

I4<> STUDIES IN LUMINESCENCE.

The fluorescence spectrum was also determined for excitation by the
ultra-violet rays in the spectrum of the iron spark, and for excitation by
Roentgen rays, the results being shown in curves 77/ and IV, respectively.
The luminescence excited in both of these rays was weak and the spectrum
could be explored only with great difficulty. Curves/// and IV are drawn
to a much larger scale than curves /and // in order that they may be shown
in the same figure. In reality the intensity at the maximum of curve /
is 200 times as great as the corresponding intensity for curve ///, and
500 times as great as the maximum for curve IV. It will be observed that
the maximum of curve IV occurs at the same wave-length as in the case
of kathodo-luminescenee. In the case of photo-luminescence, curve ///,
there is an apparent shift toward the shorter waves. We are of the opinion,
however, that this shift is not real, but due to errors resulting from the
extreme faintness of the spectrum.

The substance continued to glow for a long time when excited by ultra-
violet light, but after excitation by kathode rays the phosphorescence
died out with great rapidity.

WILLEMITE.

The specimen which was tested was a part of the same piece that had
previously been used in determining the decay of phosphorescence in this
substance.1 No change was observable in the specimen, even after prolonged
excitation.

While the kathodo-luminescenee of willemite was not much more intense
than that of CdS04, the photo-luminescence, excited as before by the
ultra-violet rays from an iron spark, was found to be much brighter than
that of CdS04. The luminescence excited by Roentgen rays was, however,
only slightly more intense. Curves IV and V, Fig. 146, which show the
spectral distribution for the photo-luminescence and the Roentgen lumi-
nescence respectively, are plotted to a larger scale than the curves for
kathodo-luminescenee on the same figure, as it would otherwise be impossi-
ble to see the details of these curves. The maximum intensity for curve /
was in reality 120 times as great as that for curve IV and 1,500 times as
great as for curve V.

Inspection of the curves of Fig. 146 seems to justify the conclusion that
the luminescence spectrum of willemite has the same form whether the
exciting agent is ultra-violet light, Roentgen rays, or kathode rays. The
form of the spectrum is also seen to be independent of the potential difference
between the terminals of the tube, and is therefore independent of the
velocity of the kathode rays.

When excited by ultra-violet light the willemite used in these experiments
showed bright phosphorescence, whose rate of decay was so slow as to be
comparable with that of Sidot blende. But when excited by kathode rays
the phosphorescence was faint and disappeared within a few seconds. The
intense brilliancy of the kathodo-luminescenee during excitation makes this
difference especially striking.

'Nichols and Merritt. Physical Review, XXIII, p. 37, 1906. See also Chapter IV of this memoir.

SPECTROPHOTOMETRIC STUDY OF KATHODOLUMINESCENCE.

I4I

SIDOT BLENDE.

The specimens tested were pieces of the same screen that had been used
in the work on Sidot blende described in Chapters IV and V.1

Our first measurements of the kathodo-luminescence of this substance
were made in connection with this earlier work, and although the conditions
were not so definite as in the case of the more recent investigation, the
result of one exploration of the spectrum, shown in Fig. 147, will be of
interest for purposes of comparison. Kathode rays were developed in
these experiments by connecting the tube used with the secondary of an

30 V M

50 52 54 56 58 60 62 fa

Fig. 146. — Willemite.

The curves to the left are voltage-intensity curves for different wave-lengths. Curves /, //, and /// show the
luminescence spectra for discharge potentials of 13.500 volts, 9,600 volts, and 6,700 volts respectively.
Curves IV and V show, to a large scale, the luminescence spectra excited by ultra-violet light and
Roentgen rays respectively.

induction coil, the primary of which was supplied with alternating current.
The discharge was made approximately unidirectional by placing a rectify-
ing tube in series. The current was controlled by an alcohol resistance.
The current used being smaller than that employed in our more recent
work, less difficulty was met with in maintaining constant pressure in the
tube. That the pressure was nearly constant during the measurements
corresponding to Fig. 147 is shown by the fact that it was possible to run

' Nichols and Merritt, Physical Review, xxi, p. 247; xxu, p. 279; xxiii, p. 37; xxv, p. 362.

142

STUDIES IN LUMINESCENCE.

through the spectrum a second time with practically identical results.
The points marked by circles in Fig. 147 were determined first, while
those marked with crosses were taken afterwards, with no change in the
conditions, as a check. It is clear that the spectrum contains two over-
lapping bands, and the estimated curves for the separate bands are indicated
by the dotted lines.

Our more recent measurements with Sidot blende were made by the
same method as that used with CdS04 and willemite. The results are
shown in Fig. 148.

When subjected to the relatively intense kathode rays used in these ex-
periments the Sidot blende was found to undergo a rapid change, which was
manifested both by a discoloration of the surface and by a diminution in the

intensity of the luminescence.
In Fig. 148, curve III corre-
sponds to the same voltmeter
reading as curve I, but was
taken after the substance had
been excited continuously for
2 or 3 hours. Not only is the
intensity much diminished by
prolonged excitation, but the
whole character of the lumines-
cence spectrum is altered.

The band at about 0.5 /j,
which appears in Fig. 147 and
in curves I and 77 of Fig. 148,
appears to be the same as that
excited by light and by Roent-
gen rays.1 The band at about
0.455 M> is also excited by
Roentgen rays.2 But this band
is either not present in the
spectrum excited by ultra-
violet light, or is masked by
the other bands present in
that spectrum. The lumines-
cence of Sidot blende, as we have previously pointed out, is extremely com-
plex, and the green band at about 0.51^ is the only one which can be
readily isolated.

When excited by kathode rays the phosphorescence of Sidot blende is
faint and of relatively short duration.

The infra-red rays obtained by interposing a thin sheet of hard rubber
in the path of the rays from an arc diminished the kathodo-luminescence
of Sidot blende only slightly. A long series of settings, alternately with
and without infra-red rays, was necessary to make certain that any effect
was produced at all. Infra-red rays of the same intensity would have cut
down^the fluorescence excited by light to one-half its normal intensity.3

'Nichols and Merritt, Physical Review, xxi, p. 247, 1905, Figs. 30, 31, and 35. See also Chapter III, Fig. 40.

:See Fig. 41, p. 44.

'Physical Review, XXV, p. 362, 1907. See also Chapter V of this memoir.

S9/t .60

Fig. 147. — Sidot blende.

SPECTROPHOTOMETRY STUDY OF KATHODO-LUMIXESCENCE.

H3

DEPENDENCE OF KATHODO-LUMINESCENCE UPON CURRENT AND
DISCHARGE POTENTIAL.

Since kathodo-luminescenee results from the bombardment of the lumi-
nescent substance by the kathode rays it is natural to expect a simple relation
between the intensity of luminescence and the velocity and number of the
kathode-ray particles. The experiments here described show that the form
of the luminescence spectrum is independent of both these factors. The
experiments show also that the intensity of luminescence is increased by
increasing either the velocity of the rays or the current in the tube. But
the conditions of the experiments were unfortunately not such as to permit
definite conclusions to be drawn regarding the quantitative relations.

140

120

15 20

25 30V-M.

Fig. 148. — Sidot blende.

.58//

The curves to the left are voltage-intensity curves for different wave-lengths. Curves I and II show the
luminescence spectra for discharge potentials of 11.600 volts and 7,700 volts respectively. Curve II I
shows the luminescence spectrum for 11,600 volts after the substance had been altered by the action of
the kathode rays.

Several attempts were made to determine the relation between intensity
and current at a constant discharge potential. The current could be varied
through a wide range either by changing the speed of the Holtz machine
or by altering a resistance in multiple with the tube. The results make it
seem probable that the intensity of luminescence is proportional to the
current; but erratic changes in the conditions in the tube, due probably to
static charges on the walls, made it impossible to get entirely consistent

144 STUDIES IN LUMINESCENCE.

results. For a time the conditions would apparently remain constant and
the intensity was found to vary in the same ratio as the current. But
some source of disturbance soon developed, so that the next observations
were discordant. Some change in the form of tube or the source of current
supply will be necessary before this question can be definitely settled.

Working with the relatively slow kathode rays developed by ultra-violet
light, Lenard has found the intensity of luminescence, /, to be related to the
discharge potential, V, by the equation

I = CQ(V-V0)

where Q is the density of the kathode stream, C a constant, and 1'0 a mini-
mum potential below which no luminescence is produced.1 The existence
of a sharply defined lower limit to the potential that is capable of producing
luminescence is called in question by Wehnelt,2 who used in his experiments
a much greater density of the kathode rays. On the other hand a definite
lower limit to the velocity has been found by Rutherford3 in the analogous
case of luminescence produced by the a rays of radium.

Our own experiments with Sidot blende appear at first glance to confirm
the conclusions of Lenard, for the voltage-intensity curves, Fig. 148, are
straight lines cutting the horizontal axis at a point corresponding to a volt-
meter reading of about 1 2 divisions. In the case of the experiments with
Sidot blende the current in the tube varied only slightly. We are not
justified in concluding, however, that the density of the kathode stream
was constant, for the fraction of the current that is carried by the kathode
rays is known to vary as the vacuum changes, being practically zero at
high pressures and nearly 100 per cent when the pressure is low. In our
experiments it was not practicable to determine the amount of this change.
It seems scarcely probable, however, that the change was unimportant.
If the curves of Fig. 148 could be corrected so as to refer to a constant
density of the kathode stream it appears probable that they would differ
greatly from the curves plotted.

CONCLUSIONS.

In so far as it is possible to generalize on the basis of the small number of
experiments here described, the following conclusions appear to be justified :

1 . In cases of kathodo-luminescence the distribution of intensity through-
out each band of the luminescence spectrum is independent of the discharge
potential, and therefore independent of the velocity of the kathode rays.

2. In cases where a band is capable of being excited by light and by
Roentgen rays as well as by kathode rays, the form of the band and the
position of its maximum are the same for all these modes of excitation.

We have shown in Chapters III and IV that the distribution of intensity
in any given band is independent of the wave-length of the exciting light.4
It appears probable, therefore, that the above conclusion may be stated
more broadly, and that the distribution of intensity in each band of a
luminescence spectrum is independent of the nature of the exciting agent
by which the luminescence is produced.

■Lenard, Annalen der Physik, 12, p. 449, 1903.

:Wehnelt, Berliner physikalische Gesellschaft, 5, p. 225, 1903.

3Rutherford, Radioactivity, second edition, p. 547.

'Nichols and Merritt, Physical Review, xviii, p. 403; xix, p. 18.

SPECTROPHOTOMETRY STUDY OF KATHODO-EUMINESCEXCE. 1 45

3. Phosphorescence following excitation by kathode rays is less intense
and more fleeting than the phosphorescence excited in the same substance
by light.

The explanation of this fact is probably to be found in the relatively
slight penetrating power of the kathode rays. The excitation is thus con-
lined to a thin layer at the surface of the active substance, while in the case
of photo-luminescence the excitation extends to a considerable depth. As
will be pointed out in Chapter XV of this memoir,1 the decay of phosphor-
escence would be more gradual in the latter case.

4. The effect of infra-red rays upon kathodo-luminescence of Sidot
blende during excitation is small compared to the same effect upon the
photo-luminescence of this substance and produces a barely observable
decrease in the intensity of luminescence.

■Nichols and Merritt. Physical Review, xxvii, p. 373, 1908.

CHAPTER X.

ON THE ELECTRICAL PROPERTIES OF FLUORESCENT SOLUTIONS

AND VAPORS.

THE INFLUENCE OF FLUORESCENCE UPON ELECTRICAL CONDUCTIVITY.

The effect of light and of Roentgen rays upon the conductivity of solutions
has been tested by Cunningham,1 in the case of five solutions, one of which,
namely, uranyl nitrate, was fluorescent. In the experiments with light an
arc containing iron was used as a source, and the solutions were contained
in a cell having a quartz window, the object being to obtain intense ultra-
violet rays. Although some indication of increased conductivity was ob-
served, the observations were rendered so uncertain by the heating effect
of the rays from the arc that the author did not regard the results as
reliable.

Experiments of a similar character were undertaken by Regner2 with
especial precautions against the disturbances due to the heating effect of
the source. No change in conductivity as great as o.i per cent could
be detected, although the substances tested included those for which
Cunningham had found indications of a positive result. Several fluorescent
substances were tested by Regner, also with negative results.

In a paper by Burke;i on the change in absorption due to fluorescence,
mention is made of preliminary experiments upon the conductivity of
fluorescent substances; but the author states that difficulties were met with
which led him to abandon these experiments and to take up the study of
absorbing power instead.

Our own experiments upon this subject were undertaken with the feeling
that the increase in conductivity due to fluorescence was probably very
small, comparable rather with the conducting power produced in gases by
the action of Roentgen rays than with ordinary electrolytic conductivity
in solutions. In attempting to detect so small an effect it seemed advisable
to work with solutions in which the normal conducting power was as small
as possible. In the case of all the substances thus far tested we have there-
fore used absolute alcohol as a solvent.4

The accurate measurement of the resistance of alcoholic solutions offers
considerable difficulty ; for while the conducting power is too great to permit
the employment of an electrometer method, such as would be used with
gases, it is too small to be determined with accuracy by the methods gen-
erally used with aqueous solutions. No benefit was to be gained in the
present experiments by increasing the cross-section of the liquid tested,
and so reducing its resistance; for if the thickness exceeded a rather small

•J. A. Cunningham, "An attempt to detect the ionization of solutions by the action of light and
Roentgen rays," Proc. Cambridge Philosophical Society, II, pp. 431-433. 1902.
-Physikalische Zeitschrift, 4, 862, 1903.
3Burke, Philos. Transactions, 191A, p. 87.
'In the experiments of Cunningham and Regner the solvent was water.

147

148

STUDIES IN LUMINESCENCE.

value the absorption of the liquid made it impossible to excite the whole
mass to fluorescence. After trying a number of different methods without
satisfactory results we abandoned the attempt to accurately measure the
total resistance, and directed our attention to the measurement of the
change in resistance due to illumination. The most satisfactory method
that we have found for this purpose is that of the ordinary Wheatstone
bridge, using a direct current and a galvanometer of high resistance.

The form of the cell used to contain fluorescent solutions is of consider-
able importance in its bearing upon the sensitiveness of the method. Among
the galvanometers available for use with the bridge, that which seemed
best suited for the work was one having a resistance of about 10,000 ohms.
It was therefore desirable to have the resistance to be tested as near to
this value as other conditions would permit. On the other hand, the layer
of liquid tested should be so thin that it can be excited to strong fluores-
cence throughout its thickness. Although we did not find it possible to

satisfy both of these conditions, they were approxi-
mately met by the form of cell shown in diagram
in Fig. 149.

The figure shows both a perspective view and a
horizontal section of the cell. A thick piece of plate-
glass, G, was cut of such a size as nearly to fill the cell
and was held in position, tightly pressed against the
two electrodes a, b (of platinum foil), by the corks C, C.
Although the drawing is in other respects approximately
to scale and nearly actual size, the thickness of the
electrodes a, b is greatly exaggerated. The thickness
of the layer of liquid between G and the walls of the
tube wras determined by the thickness of the electrodes
and was about o. 1 mm. In the section shown in Fig.
149 the portions occupied by the solution are left un-
shaded. The length of the electrodes was about 20
mm. and their distance apart 2 mm.

The fluorescent substances tested were eosin, fluor-
escein, rhodamin, napthalin roth, and cyanin. As
already stated, the solvent was in all cases absolute alcohol. The solutions
were made quite concentrated, so that fluorescence was confined to a thin
layer at the surface. The concentration was so adjusted as to make this
fluorescent layer approximately 0.1 mm. thick.

The cell containing the solution to be tested was made one arm (C) of a
bridge. The resistance of the arm .1/ in series with C was in all cases 9,000
ohms, while the resistance of the third arm, R, was 5,000 ohms. The
fourth arm, X, was varied until an approximate balance was obtained1
and the apparent resistance wTas computed by the formula C = MR/N.
Current was furnished by two gravity cells in series.

Polarization in the fluorescent solution made it impossible to obtain the
true resistance by this method, and the apparent resistance of the cell
(computed as if polarization were absent) is doubtless in error by 50 per

ZSp£Z=ZZZZZZ

<m ■'

•■777,:.;,,^L^!pz

6

a 5

SECTION

Fig. 149.

•These resistances were chosen not because they gave the highest attainable sensitiveness, but because
they were the most suitable ones that were available at the time.

ELECTRICAL PROPERTIES OF FLUORESCENT SOLUTIONS. 1 49

cent or more. But the sensitiveness of the arrangement to changes in the
resistance of the test cell was high. In order to avoid disturbances due
to variations in the polarization E.M.F. it was found necessary to keep both
the battery circuit and the galvanometer circuit closed. Even the small
change made in N during the adjustment of the balance produced a con-
siderable alteration in the polarization, so that the adjustment often had to
be continued for an hour or more before the needle of the galvanometer
was sufficiently steady for observations to be begun. During this time
the cell was protected from the action of light by an opaque screen. When
the conditions had become steady, or more frequently when the motion
of the galvanometer needle was reduced to a slow uniform drift, the screen
was removed and light from an arc was allowed to fall on the cell. After
the effect of illumination had been noted, and when the needle had again
settled down to a steady drift, the screen was replaced, and the throw of the
needle in the opposite direction was observed.

In the measurement of so small an effect as that here considered it is
clear that the heating effect of the rays from an arc is likely to produce
serious errors, for the change in conductivity due to rise in temperature is
in the same direction as the change that we are attempting to detect. For
this reason the use of the direct rays of the arc, even at the distance of a
meter, was entirely out of the question. Even when a water cell was inter-
posed in the path of the rays the needle was displaced through several
hundred divisions. That this movement of the needle was due to rise in
temperature, and not to the effect sought, was indicated by the fact that the
change persisted after the rays were cut off; it was necessary to wait at
least 15 minutes before the original balance was approximately restored.

In order to avoid the disturbances due to rise in temperature we dispersed
the rays from the arc by a prism and used only those portions of the spec-
trum that were most effective in producing fluorescence. With this arrange-
ment the effects observed were much smaller than before, but they were
entirely free from any indication of temperature changes. Upon removing
the screen, so as to illuminate that part of the fluorescent solution lying
between the electrodes, a throw of the galvanometer needle was observed,
and if originally free from drift the needle vibrated about a new position and
finally came to rest. Continued illumination of the solution produced no
increase in the deflection. Upon replacing the screen a throw in the oppo-
site direction occurred, and the needle finally returned to its original reading.
If observations were made while the needle was in motion, these throws
were simply superposed upon the steady drift. In this case the effect of
illumination was measured by the average of the two throws.

vSo far as we were able to judge, the effect produced by light reached its
full value at once and ceased as soon as the light was cut off. The dis-
turbance of the balance of the bridge was always such as to indicate an
increase in the conductivity of the fluorescent solution. Upon repeating
the experiments with rays from different portions of the spectrum it was
found that a change in conductivity was produced only by those rays which
were able to excite fluorescence in the solution tested. And so far as we
could estimate the intensity of fluorescence by the eye, the rays which gave
the most intense fluorescence also produced the greatest change in con-

i5o

STUDIES IN LUMINESCENCE.

ductivity. In the case of eosin we were able to follow both effects to the
extreme edge of the violet, while illumination by the red of the spectrum
produced no effect.

In order to form an estimate of the magnitude of the change produced
we observed in each case the throw caused by increasing or decreasing the
resistance A7 by one ohm. By comparing this with the throw due to
illumination, it was possible to express the observed change as a fraction
of the normal (apparent) resistance. The results are given in Table 19.
The light used to illuminate the solution, in the case of the measurements
included in the table, was that which produced the brightest fluorescence.

We have also tested in the same way one solution that was not fluorescent,
namely, an alcoholic solution of fuchsin. The result was entirely negative.
No change in resistance due to illumination could be found in any part of
the spectrum, although the sensitiveness of the bridge was such that a
change of 0.008 per cent could have been detected. The solution was an
old one and had probably been made with commercial alcohol, since the
resistance, 90,000 ohms, was much lower than that of the other solutions.
For this reason we are not inclined to look upon this single experiment with
a non-fluorescent solution as possessing much significance.

Table 19.
Increase in electrical conductivity due to fluorescence.

Substance.

N

Apparent
resistance
of solution.

Throw pro-
duced by
illumination.

Throw pro-
duced by

increasing N
by 1 ohm.

Increase in

apparent

conductivity

due to
illumination.

Kosin

'55
45
300
360
170

300,000
1 ,000,000
1 50,000
125,000
270,000

111)11.

57
1
10
20
40

nun.
33
39
30
40

45

per ct.
1 . 1
0.05
0. 1 1
0. 14
0. 52

Napthalinroth. . .

Fluorescein

Rhodamin

Cyanin

In the preceding description of the experiments upon fluorescent solu-
tions we have ascribed the observed movements of the galvanometer needle
to a change in the resistance of the solution. The galvanometer deflections
undoubtedly indicate a disturbance in the balance of the bridge; but this
disturbance might equally well result from a diminution in the E.M.F.
of polarization. Without special modifications the method does not permit
a separation of these two effects. In the case of eosin, however, special
experiments were made bearing upon this point.

The experiments referred to were of two kinds. In the first of these a bridge
method was employed as before. The solution was contained in a glass tube
originally about 5 mm. in diameter. A portion of this tube had been drawn
down to a diameter of about 1 mm. and the current passed through the
liquid contained in this contracted part. The platinum electrodes were
placed in the larger part of the tube at each end. We were thus able (1)
to illuminate the solution about either electrode while the remainder of the
tube remained dark; or (2) to illuminate the contracted part of the tube

ELECTRICAL PROPERTIES OF FLUORESCENT SOLUTIONS.

151

while screening the electrodes. In the latter case the effect was observed,
as in the experiments already described. In the former ease only a very
small change could be detected.

The other experiments with eosin that bear upon the question of polari-
zation were of an entirely different character, and represent one of our
many early attempts to devise a satisfactory method of attacking the
general problem. Fig. 150 shows both the form of tube used to contain
the solution and a diagram of the connections. The platinum-wire elec-
trodes a, b were connected to the terminals of the secondary of an induction
coil, /, and to the two pairs of quadrants of an electrometer, E. The elec-
trode C at the center of the tube was connected to the needle. The
primary of the induction coil was excited by an alternating current of 120
cycles. When the solution was not illuminated the electrometer needle
stood nearly at its zero. Under these circumstances illumination of the
upper half of the tube pro-
duced a deflection in one
direction, while the illumina-
tion of the lower half gave an
approximately equal deflec-
tion in the opposite direction.
The direction of the deflection
in each case was such as to
indicate a decrease of resist-
ance. The spectrum of the
arc was used in these experi-
ments as in those made by the
bridge method; and the evi-
dence that the effect was due
to fluorescence, and not to rise
of temperature, was as con-
clusive as in the experiments
previously described. The
method wras less sensitive,
however, and was therefore used only in the case of eosin.

Shortly after the announcement of the results just described,1 Camichel,2
in a paper in the Journal de Physique, reported a series of measurements
with entirely negative results, and suggested that our precautions against
heating were insufficient and that the apparent change of conductivity
might be thus explained. It seemed desirable, therefore, particularly as
our experiments had been merely preliminary and prospective, to investi-
gate further. Dr. Percy Hodge, at our suggestion, took the matter in hand
and it was found, as will appear from the summary of his work given in the
second part of this chapter and from a later paper by Goldman,3 that the
effects of light were much more complicated than had been suspected.

Fig. 150.

■Nichols and Merritt. Physical Review, xix, p. 415, 1904.
2Camichel, Journal de Physique, IV, 1905.
'Goldman, Annalen der Physik, vn, p. 322, 1908.

152

STUDIES IN LUMINESCENCE.

PHOTO-ACTIVE CELLS WITH FLUORESCENT ELECTROLYTES (DR.

EXPERIMENTS).1

HODGES

^

V

Before going on to any further systematic investigation, Dr. Hodge
attempted to repeat as nearly as possible some of the experiments de-
scribed in earlier portions of this chapter.

The substance first chosen for experiment was eosin, and nearly all the
results to be described were obtained with this substance. As a solvent
absolute alcohol was used, and in most cases the solution was saturated at
room temperature.

The essentials of the apparatus used in these preliminary tests are shown
in Fig. 151. The source of light A was a Schuckert arc lamp inclosed in
the light-tight cover which is used with the lamp for projection. After

passing through a pair of condensers the
beam of light illuminated the slit S, then
went to a water cell II', and thence to a lens
and crown-glass prism, finally forming a
spectrum on the front of the light-tight box
D, containing the cell C. D was provided
with a sliding front by which the light could
be admitted to the cell C.

<For measuring the change of resistance
© ^> an ordinary Wheatstone bridge was used, the

cell being placed in one arm. The source of
current was a battery of gravity
cells, the number of which was
varied, as will be seen later. A
very sensitive Sullivan d' Ar-
sonval galvanometer of 1 , 100 ohms
was used with the bridge. The
galvanometer was fitted with a
concave mirror of 4 feet focus,
and its constant with the scale
at that distance was 90X10""
ampere per millimeter deflection.
The cell used to contain the solution was a small rectangular glass cell
6 cm. deep and 1 cm. from front to back on the inside. In this were placed
side by side, and at a distance of approximately a millimeter apart, two strips
of platinum foil o. 1 mm. in thickness, 5 cm. long, and 3 mm. wide. Behind
these in the cell was placed a piece of plate glass, holding the electrodes
pressed against the front wall of the cell, and itself held in place by brass
springs between it and the rear wall of the cell. A section of the cell is
shown in Fig. 152, the thickness of the electrodes and other dimensions of
the cell being considerably exaggerated. The dispersion of the prism gave
a visible spectrum at the point where the cell was placed about 4 cm. wide.
Thus the portion of the spectrum illuminating the narrow space 1 mm. wide
between the electrodes was quite small, and it was easy to select the part
of the spectrum producing maximum fluorescence without including the

■This section is a summary of a paper by Dr. Percy Horlge; see Physical Review, xxvi. p. 540; and
x-xviii, p. 25.

~1

£

O «

Fig- 151-

Fig. 152.

ELECTRICAL PROPERTIES OF FLUORESCENT SOLUTIONS. 1 53

rays of longer wave-length which would be likely to produce heat disturb-
ances. In the case of the eosin cell the light used was at the infra edge of
the green.

When the cell was first set up great difficulty was experienced in obtaining
a balance, the drift toward higher values of the resistance being quite rapid
and continuing for several hours. This was at first thought to be due
entirely to polarization, but it was found that if the top of the cell was
covered so tightly that evaporation was prevented the drift lasted for only
a short time, after which a comparatively steady condition was reached,
and accurate measurements were possible. At no time was the cell abso-
lutely free from a very slow drift, always toward higher resistance, and
probably due partly to polarization and partly to a very gradual decom-
position of the electrolyte. Evidence of the latter was seen in a very narrow
strip of colorless liquid which was observed along the inner edge of the
anode after several hours of application of the current. On account of
this decomposition most of the tests were made with cells which had been
freshly set up, the electrodes being removed and carefully cleaned between
tests.

The method of the experiment was as follows. After the apparatus had
been set up and the cell placed in its dark box the slide was raised and the
cell so adjusted in the spectrum that the liquid between the electrodes
became brilliantly fluorescent. The slide was then dropped, shutting off
the light, and a balance was obtained with the bridge. The resistance
of R\ (see Fig. 151) was in nearly every case 10,000 ohms, that of Ri was
50,000 ohms, and Ri was adjusted to suit the resistance to be measured.
As it was impossible to set the electrodes at exactly the same distance from
each other when the cell was set up at various times the apparent resistances
differed from each other greatly during different sets of readings. Also the
polarization E.M.F. in the case of eosin was found to be over two volts.
Therefore when two gravity cells were used the apparent resistance was
much higher than when four were used.

After the cell had stood in darkness long enough to obtain a fairly steady
condition of the bridge, the slide was suddenly removed and the cell
illuminated.

With two volts as the applied E-M.F. the result was an immediate and
very large decrease of resistance, the amount of resistance change required
in the variable arm to restore a balance indicating as high as 10 or 15 per
cent increase of conductivity. When the light was shut off the cell returned
immediately to nearly the same resistance that it had before illumination.
The test was repeated many times and on different days, and always with
about the same results. As the effect was much larger than any that had
been anticipated it seemed advisable to see if it could in any way be due to
heating of the liquid. With this end in view the temperature coefficient
of the solution was determined in the following manner: A balance was
obtained at the temperature of the room. Then the cell was surrounded
with ice water and a balance again obtained. Lastly the ice water was
drawn off and the cell allowed to regain the temperature of the room. The
first and last values of the apparent resistance were in turn subtracted from
the second and then averaged. The result showed a temperature coefficient

154 STUDIES IN LUMINESCENCE.

between zero and 20 degrees of not more than 1 .5 per cent per degree. The
effect of the light was also tried while the cell was in ice water and found
to be apparently about as large as at room temperature. These tests
seemed to preclude the possibility of any considerable portion of the effect
being attributable to heat. An additional reason why the effect should
not be attributed to heat was mentioned on page 149 of this chapter, namely,
the fact that the change of conductivity took place within an exceedingly
short time after the light was thrown on the cell, and after the light was
shut off the cell immediately resumed its former state. This fact was con-
firmed in the course of these experiments, the time required for the change
to take place being apparently considerably less than a second, and the
decay of the effect taking about the same time.

A series of tests was then made with the same cell and solution, but using
four gravity cells in series as the source of E.M.F. A difference of effect
from that obtained with two cells was expected, as with two cells only a
very small current could possibly have passed through the cell, since the
applied E.M.F. was not equal to the E.M.F. of polarization, while with
four volts the current flowing was of considerable magnitude. The effect
anticipated was one similar to the first but of smaller magnitude. When
the light was thrown on the cell, however, the surprising result was obtained
of a large increase of resistance instead of a decrease as in the former experi-
ments. The effects were produced as promptly as those of the preceding
experiments and died away nearly as quickly.

Here was something quite unexpected. If the remarkable decrease of
resistance observed in the first experiments was due to electrons set free by
ionization of the solution accompanying the phenomenon of fluorescence
it seemed highly improbable that an increase of resistance would be pro-
duced by light under any conditions whatever. Further, both effects were
entirely too large to be satisfactorily accounted for by any theory anal-
ogous to that applied to the phenomena of ionization in gases ; phenomena
which it might be supposed would very probably accompany fluorescence.

Again, if the decrease of resistance in the first case were due to ionization
of the nature of that produced in gases, it should be greater in dilute solu-
tions than in concentrated, since, owing to the greater penetration of the
light into a dilute solution the number of molecules affected would be as
great as in a concentrated solution, while, owing to the greater distance
between the molecules, the mean free path of the ions would be greater and
therefore recombination less rapid. A number of experiments were made
by diluting the saturated solution with two, four, and six parts by volume of
alcohol, and the effects were found to be greatly diminished, so that even in
the solution with two parts of alcohol there was a change of not more than
1 per cent in resistance.

In order to vary the conditions it was thought worth while to try next
a cell in which, as in Regner's experiments, the liquid could be kept moving
past the electrodes while being illuminated. All possible heat effects
would be in this way eliminated, and some light might be thrown upon the
nature of the phenomena which would not be brought out when the liquid
was at rest. A cell was therefore prepared in the following manner, again
copy the designing of one described on page 150.

ELECTRICAL PROPERTIES OF FLUORESCENT SOLUTIONS. 1 55

A glass tube of 5 mm. internal diameter was drawn down at its middle
point to a capillary about a millimeter in outside diameter and 1.5 cm. in
length. Into the larger parts of the tube at each end were led platinum
wires in the form of spirals to serve as electrodes. The upper end of the
tube was attached to a tubulated bottle to serve as a reservoir for the liquid.
To the lower end was attached a second capillary tube to hold the liquid back
and keep the first capillary full. With this apparatus the liquid could be
illuminated while flowing through the capillary at any rate desired. Numer-
ous tests were made with the apparatus, and in all cases the results were
entirely negative. The form of the cell was such, however, that its resist-
ance was much larger than that of any of the cells previously tried, and
as the sensitiveness of the bridge was much less under these conditions, the
results were looked upon with some suspicion.

Another form of circulation cell was therefore devised which was free
from the above objection.

A piece of glass tubing was obtained whose internal cross-section was
almost that of a figure 8, except that where the two loops of the 8 would
cross one another there was an opening between them about 0.3 mm. in
width. In a short length of this tubing were placed two No. 16 platinum
wires, each having a length of 5 cm. The wires were parallel to one another
in the two loops of the 8, and from the end of each a small platinum wire
was led out through the walls of the tube. There was thus left (see Fig.

» ' 11

I— /TVTv.

3E

Fig. 153- Fig. 154. Fig. 155-

153, which shows an enlarged cross-section of the cell) a narrow space
between the electrodes through which the liquid could flow, the electrodes
themselves almost filling the loops of the 8. Below the electrodes the tube
was drawn down to a coarse capillary to make it possible to keep the space
between the electrodes filled with liquid. The upper end of the tube was
attached to a tubulated bottle as in the last experiment. By illuminating
the contracted part of the tube between the electrodes a moving strip of
liquid about 0.3 mm. wide and o. 1 mm. from front to back could be excited
to fluorescence, while the current flow was from side to side of the strip at
right angles to its length.

Upon connecting the cell to the bridge its resistance reached a steady
value almost immediately after the current was turned on, and retained
the same value as long as the motion of the liquid continued.

The movement of the liquid was quite rapid, the time required to empty
the 400 c.c. reservoir being about 10 minutes. As the whole space between
the electrodes held not more than o. 1 c.c. a complete change of liquid
between the electrodes could not have occupied more than a fifth of a
second.

Numerous tests were made with this apparatus, but in no case was there
any evidence of a change of conductivity while the liquid was in motion.
When, however, the lower end of the tube was stopped the effects reappeared,
though they were not as marked as in the original type of cell. The sen-

156 STUDIES IN LUMINESCENCE.

sibility of the bridge as used in these tests was at least 0.0 1 per cent. There-
fore it seems fair to conclude that there could not have been any appreciable
effect due to electrons set free at the instant fluorescence began in any part
of the liquid, unless this effect required a considerable time to make itself
known.

Attention was next given to the effects at the two electrodes separately
and to the liquid between them, to find out, if possible, just where the phe-
nomena took place which caused the changes in conductivity.

As nothing but negative results were to be obtained from a circulation
cell the original type of cell was again adopted, and in the front of the
box D of Fig. 151 was placed a screen with a vertical slit cut in it about 4
mm. wide. By placing the cell behind this screen in such a manner that
only one electrode could be seen from the front of the box through the slit,
it was possible to observe the effect of illuminating one electrode at a time,
the other being in darkness behind the screen. With this arrangement and
two gravity cells the effect was tried on anode and kathode in turn. When
the anode was exposed the effect was very small, while on the contrary
when the kathode was exposed the effect was found to be about as large as
when both were exposed. As some of the liquid between the electrodes
was illuminated in either case it was not certain whether the effect was pro-
duced only at the electrode or whether the liquid at a distance from the
electrode also played its part in it. Therefore the question as to what
would happen when the electrodes were both covered and the liquid between
them exposed had next to be settled.

To this end several different arrangements of the cell were tried. The
first tests were with the same electrodes which had been used in the earlier
type of cell, but in front of each of them was placed a thin strip of hard
rubber 0.15 mm. thick, projecting just beyond the platinum strip so as to
completely screen it from the light, but so as to leave about 80 per cent of
the liquid between the electrodes exposed to the light.

With this arrangement no effect could be obtained by illumination. How-
ever, it was felt that the test was not conclusive, for the total thickness of
the film of liquid acting as conductor between the electrodes was that of
both the electrode and the rubber strip, and since the absorption coefficient
of a saturated solution of eosin is very large it was not at all certain that
the whole thickness of the layer was illuminated.

To overcome this difficulty another type of cell was constructed after the
following manner:

A block of hard rubber was sawed of such shape and size as nearly to fit
the rectangular cell used in most of the experiments. The block was then
clamped to a block of ebonite and with a f -inch drill two holes were bored
between the blocks in such a manner as to leave in each block a pair of
grooves of semicircular cross-section, and separated from each other by a
narrow strip approximately 2 mm. wide. The block that had been sawed
to fit the cell was then placed in the cell and two L-shaped electrodes were
placed in the grooves in the positions shown in Fig. 155, which illustrates
a section of the cell with block and electrodes in place. It will be seen
from the figure that the electrodes hold the ebonite block back from the
front wall of the cell, so as to leave a film of liquid of the thickness of the

ELECTRICAL PROPERTIES OF FLUORESCENT SOLUTIONS. 1 57

platinum strips between the two grooves when the cell is filled with liquid.
The film of liquid between the grooves could be perfectly illuminated, was
isolated from the electrodes, and at the same time included a large part of
the total resistance.

Careful tests were made with this cell and the results were entirely nega-
tive, though the resistance of the cell was quite low compared with others
that had been tried, averaging about 40,000 ohms with saturated solution,
and the sensibility of the bridge was as high as 0.005 Per cent. The con-
ditions within a few minutes after it was set up were very steady indeed,
and a slight change of conductivity would have been readily detected.

In addition to the saturated solution a solution of five parts alcohol to
one of saturated eosin solution was tried with both 2 and 4 volts E. M. F.,
but likewise with entirely negative results. From the results of these last
experiments it was evident that a change of conductivity in the liquid itself,
except at or very near the electrodes, did not. take place when fluorescence
was produced.

It had frequently been observed that, however smooth the strips of
platinum were when they were placed in the cell, and however tightly they
were clamped against the front wall, when the solution was poured in a
thin film of liquid immediately crept up between the electrodes and the
front wall of the cell. There seemed, therefore, to be two regions in which
the effects might be produced, either at the inner edges of the electrodes or
in the region in front of the electrodes, or in both places at once.

From a variety of minor indications which had been noticed from time
to time it seemed probable that the main effect was to be looked for in
the thin film and not at the edges of the electrodes. In order to test the
matter the effect was first tried of placing thin strips of ebonite in front of
the electrodes as in one of the previous experiments, but instead of cover-
ing the electrodes the ebonite strips were only allowed to cover the extreme
outer edges of the platinum strips, thus leaving nearly the whole of the
electrodes exposed to the light, but behind a layer of liquid a number of
times as thick as the capillary film formed when the electrodes were against
the front wall of the cell. The change of conductivity, though still present,
was found to be very greatly diminished.

As mentioned in connection with a previous experiment, the reduction
of the effect might easily have been due to the absorption of the light by
the thick layer of liquid in front of the electrodes, so that fluorescence did
not occur at the surface of the platinum strips.

A second arrangement was then tried in which the platinum strips were
separated from the front wall of the cell by strips of lantern-slide cover-
glass a millimeter in thickness. The inner edge of each electrode (see Fig.
I55) was allowed to project slightly beyond the edge of the glass. Thus
the capillary film between the electrode and strip of cover-glass could be
illuminated while the inner edge of the electrode was completely screened
from the light by a thick layer of liquid. When the light was turned on
this cell the effects were found to be present in as great a degree as when
the whole of the electrode was illuminated.

Lastly, the platinum strips were again placed against the front wall of the
cell and the inner edges covered, while the rest of the electrodes was illumi-
nated. Again the effect was present, as was to be expected.

158 STUDIES IN LUMINESCENCE.

The conclusion arrived at from the various tests was that, in the first
place, no effect is produced by light upon the conductivity of a fluorescent
solution unless a region very near the electrodes is illuminated; and that
further, this effect at the electrodes is only produced in a very thin film of
the liquid immediately in contact with the surface of the plates.

It may be well to state that, while no reference has thus far been made
to a variation of the effects in different parts of the spectrum, frequent tests
were made in all of the experiments described, to see if the effects could be
produced by parts of the spectrum which did not produce fluorescence.
In all of these tests the effects obtained were either very feeble or entirely
lacking. Thus, in the red, where the energy of the arc is the greatest, but
where no fluorescence was to be detected, no effects were observed which
were as great as 0.2 percent either way. The frequent references to fluores-
cence in connection with the effects observed seem therefore to be entirelv
justified.

The next step undertaken in the investigation was a study of the vari-
ation of the effects at anode and kathode with varying potential difference
at the terminals of the cell.

For this purpose the original form of cell was adopted, with electrodes
0.15 mm. thick and 3 mm. wide, placed next to the front wall of the cell and
3 mm. apart. The greater distance between the electrodes was adopted,
so that while one of the plates was being illuminated no stray light could
reach the other electrode. The greater thickness of the electrodes which,
in view of the results of previous work could not materially affect the
sensibility of the cell, served to keep the resistance of the cell as low as
possible.

The screen in the front of the light-tight box D was so arranged that the
cell could be moved into a position to expose either electrode to the light
without having to raise the slide in the front of the box to make the adjust-
ment.

As a source of E. M. F. a set of four sal-ammoniac cells was first used, but
as these were found to drop slightly in voltage during a run, a pair of storage
cells was substituted for them. The cells were connected to the ends of a
rheocord, consisting of 10 meters of manganin wire, and sliding contacts
were arranged so that any desired potential difference could be obtained
at the bridge terminals, up to the limit of the cells.

To the bridge terminals were also attached leads from a large Weston
milli- voltmeter, a resistance being placed in series with the meter, and the
whole calibrated to a total range of 15 volts. With this arrangement it
was found easy to obtain the potential difference between the terminals of
the bridge with an accuracy of 0.0 1 of a volt, and any given value of the
potential difference could be maintained constant throughout the time
needed for the readings.

For greater simplicity in computation, and to make it possible to main-
tain the potential difference between the terminals of the cell at a constant
ratio to the voltmeter reading, the bridge arms were arranged as follows
(see Fig. 151): R3 62,500 ohms, R^ 10,000 ohms, and Ri a 100,000-ohm
box to be used as the variable resistance. Thus, unless in a case where the
resistance of the cell was so great that the 10,000-ohm box had to be varied,

ELECTRICAL PROPERTIES OF FLUORESCENT SOLUTIONS.

159

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Potential difference in voits at cell terminals

Fig. 156.

the fall of potential across the cell terminals was nearly 86 per cent of the
voltmeter reading.

The change of resistance due to the illumination was estimated in two
different ways, depending on the magnitude of the effect. If the galvanom-
eter reading remained on the scale when the cell was illuminated, a reading
of the deflection was taken. The illumination was then stopped and a
second reading was taken and the two were averaged. A plug was then
inserted in the io,ooo-ohm box, thechangeof galvanometer reading obtained,
then the plug was removed, a second reading taken, and the two averaged.
Assuming that the galvanometer deflection was proportional to the change
of resistance in the io,ooo-ohm box, the number of scale divisions correspond-
ing to a change of one ohm or
one part in ten thousand was
easily obtained. As increase or
decrease in the apparent resis-
tance of the cell would produce
the same change in the potential
difference at the galvanometer
terminals as the same percen-
tage decrease or increase of the
resistance in Riy it was easy
to get quite satisfactory esti-
mates of the change produced
by the illumination. If the
effect of the light was great
enough to throw the galvanom-
eter off the scale, a balance
was restored by changing the
resistance in Ri. The light was
then shut off and a second
balance was obtained, and
from the average of the two
changes in R\ the change of
resistance of the cell was cal-
culated.

Curves were plotted, using
potential differences at the cell
terminals as abscissas, and F'g- !57-

change of conductivity, i. e., reciprocals of resistance changes, as ordinates.
Eight of these curves are shown in the accompanying figures. Each value
of the conductivity change is the average of at least two readings, taken
at intervals of 5 minutes time. These curves are typical of a large number
that were obtained in a similar manner.

Owing to the variation in the intensity of the arc light used, and to the
difficulty of setting up the cells in exactly the same manner each time, the
magnitude of the effects obtained varies considerably in different curves.
Nevertheless the character of the effects obtained did not vary greatly.

In curves 1 and 2, 3 and 4 (Figs. 156 and 157) the kathode and anode were
illuminated alternately throughout the experiment. Each value of the

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Potential difference in volts at cell terminals.

160 STUDIES IN LUMINESCENCE.

change of conductivity is the average of at least two measurements, made
at intervals of 5 minutes from the end of one measurement to the beginning
of the next. It will be noticed that the kathode shows at low voltage a
large decrease of resistance due to the light, followed by a considerable
increase of resistance as the voltage rises. On the anode the effect at low
voltage is just the opposite of that on the kathode and changes sign as the
voltage rises as in the case of the kathode. It will be noticed also that the
change of sign of the effect occurs considerably earlier in the case of the
anode than in that of the kathode. The result is that for a considerable
range of voltage both electrodes show a decrease of resistance with the light.
As this region happens to include the voltage obtained when two gravity
cells were used as the source of current, it is not surprising that a large
decrease of resistance was observed in the first experiments, when both
electrodes were exposed to the light at once.

As the curves showed that the effects on the two electrodes were in gen-
eral of opposite sign, it was thought advisable to arrange a cell so that one
electrode could be studied at considerable length, and with a certainty that
no complications would be introduced by stray light reaching the other
electrode.

In order to have the electrodes as far apart as possible, and still to keep
the resistance of the cell within reasonable limits, the electrode to be exposed
was alone fixed in the front of the cell, clamped to the front wall in front
of and very near the edge of a narrow strip of glass. The other electrode
was made L-shaped, nearly a centimeter being the length of each leg of the
L cross-section. This electrode was placed in the main part of the cell with
the base of the L toward the side of the cell away from the light source.
This arrangement insured a very large electrode surface and also an average
cross-section of the liquid between the electrodes sufficient to make up for
the greater distance between them. It also served to concentrate the
greater part of the resistance at the electrode to be illuminated.

As the kathode effect seemed the most promising for further study the
current was sent through the cell from the large electrode to the one to be
exposed.

A number of runs were made with this cell and from the results curves
were plotted, of which 5, 6, 7, and 8 are typical specimens (Figs. 158, 159,
and 160). It will be seen that the effects are considerably greater than
those obtained in the former experiments, and also that the form of curve
obtained is in general more regular than the others. The peculiar rise of
the effect after the large decrease shown in the first part of the curve does
not appear to be accidental, but is common to almost all the curves plotted
for the kathode effect.

Curves 7 and 8 were made to ascertain the effect which the previous
history of the cell might have on the action of the light. Curve 7 was ob-
tained in the usual way, and a run was made immediately after without
changing the cell, but beginning with high voltage and reducing the voltage
by the same steps that had been used in going up.

It will be noticed that the curves are very much alike, the one taken on
decreasing voltage showing a slight shift toward the right. For the last two
points on the return curve it was found impossible to obtain a balance of the

ELECTRICAL PROPERTIES OF FLUORESCENT SOLUTIONS.

161

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Potential difference in volts at cell terminals.
Fig. 158.

bridge, the condition of the cell appearing very unstable. At the three
voltages 1.03, 1.2 1, and 1.38 of the first of these two curves a phenomenon
was observed which occurred frequently throughout the experiments, at the
point where the effect reversed. When the cell was first exposed the gal-
vanometer showed an imme-
diate decrease of resistance,
followed in a few seconds, with
the light still on, by a large
increase. When the light was
shut off the cell showed the
two changes in the reverse
order.

This would seem to indicate,
at least in this region, a com-
bination of two effects, one
growing to a maximum more
rapidly than the other, and
dying away more slowly.

A few experiments were made
with fluorescein in absolute
alcohol, a trace of caustic soda
being added to produce fluo-
rescence. The effects obtained
were very similar to those
with the eosin, but not nearly
as marked.

Rhodamin was also tried,
with somewhat doubtful re-
sults, except that the effects
with very low voltages were
fully equal to those with eosin.

vSince there seemed to be no
doubt that the seat of the effect
was the thin film of liquid in
front of the electrode, and since
the current could not be sup-
posed to flow only to and from
this film without entering the
edge of the electrode, it did not
seem possible to account for
the changes in resistance ob-
served, except by the actual
creation of an electromotive
force at the surface of the elec-

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Potential difference in volts at cell terminals.
Fig- 159-

8 1.2 l.tfo' 2.0 2.4 2B 32 3.6

Po+ential difference in vol+s a+cell terminals

Fig. 160.

trode or near it, by the action of the light. So long as low voltages were
used, such an effect if it did exist would account for the change of resistance
when either anode or kathode was illuminated. For suppose the E.M.F.
produced to be such as to tend to make the exposed plate positive to the
unexposed one. Then if the exposed plate were made the kathode with

1 62 STUDIES IN LUMINESCENCE.

regard to the outside E.M.F., the E.M.F. due to illumination would act
with the original E.M.F., producing a greater current through the cell
and making its apparent resistance, as measured on the bridge, less.

If, on the contrary, the direction of the impressed E.M.F. were reversed,
the photo-E.M.F. would oppose the passage of the current and the apparent
resistance of the cell be increased.

The reversal of the effects when the voltage was raised is not easy to
account for. From a number of tests of the polarization E.M.F. of eosin
cells, made by comparing the E.M.F. with that of a standard cell by means
of a condenser, the effect of maximum polarization appears to be a back
E-M.F. of over 2 volts. This value determines the point where any con-
siderable current begins to traverse the cell, and hence it would be most
natural to expect that just here would occur any marked change in the
phenomena, such as the reversal of the illumination effect. This, however,
does not seem to be the case, for it will be noticed from the curves that the
reversal in the case of the kathode occurs at very nearly 1.7 volts, while at
the anode the voltage is much lower. That the effects at high voltages are
connected in some way with the polarization of the cell seems, nevertheless,
most probable, though in what manner has not as yet been determined.

Assuming that the effect at low voltage is due, as has been suggested, to
a photo-E.M.F. similar to those which have frequently been observed in
cells containing silver salts and many other electrolytes, attention was next
turned to the effect of light on the cell when no external E.M.F. was applied.

The same type of cell was used as had been employed in all of the experi-
ments on the kathode alone. The terminals of the cell were connected
directly to the Sullivan galvanometer which had been used with the bridge
in all the previous experiments.

Very considerable effects were produced by the action of the light, ample
in magnitude to account for the phenomena observed with the bridge at
low voltages.

A series of experiments was then undertaken to find if in any way this
photo-electric effect was intimately connected with fluorescence.

In the first place tests were made throughout the visible spectrum, and
curves plotted showing the variation of the current produced in the gal-
vanometer as a function of the wave-length of the exciting light. The effect
was found to increase regularly from the edge of the visible spectrum in the
violet to a pronounced maximum just before the absorption band was left
in going toward the red, namely, just at the infra edge of the green. The
decrease of effect from this point on toward the red was very sudden indeed,
and the effect was entirely lost before the edge of the visible spectrum was
reached. Three other substances, namely, rhodamin, fluorescein, and
naphthalin-roth were found to give similar effects, and in each one the
maximum results were obtained at the infra edge of the absorption band.
In rhodamin the effects were found to be enormously greater than with the
eosin, but with the other two substances the results were less satisfactory.

With rhodamin tests were made by a condenser method to test the mag-
nitude of the E.M.F. produced, and it was found to be as high as 0.2 of a
volt. In eosin it is probable that the maximum was not over half that
amount.

ELECTRICAL PROPERTIES OF FLUORESCENT SOLUTIONS. 1 63

Dilute solutions of rhodamin were also tried with very good effects. The
maximum dilution thus far tested is that of one part saturated solution to
ten of alcohol.

With dilute solutions of eosin, however, the effects were greatly reduced.
The extremely thin capillary film was found to be unnecessary, either in
eosin or rhodamin, though the effects produced when the electrodes were
more than 0.02 mm. back from the front of the cell were very small.

Tests were made of the absorbing power of both of the substances in
saturated solution, and it was found that practically no light penetrated
to a depth of 0.02 mm. in the region of the absorption band. Thus, if the
effect is produced at the surface of the platinum plate itself, a film of this
thickness or greater would completely screen the electrode from the action
of the light.

The results obtained by Goldman, whose paper is referred to on page 151,
agree in general with those of Dr. Hodge.

MR. HOWE'S1 EXPERIMENTS ON FLUORESCENT ANTHRACENE VAPOR.

The following is an account of an attempt to determine whether fluores-
cence has any effect on the electrical conductivity of anthracene vapor.
The expectation of a change in conductivity during fluorescence is based
on the theory that fluorescence is a dissociation phenomenon. Although
the results of the experiment were negative, it being impossible to detect
any conductivity of the vapor either unilluminated or fluorescent, the
accuracy of the work was such as to set a rather definite upper limit for any
effect that may be present.

Little work has been done on the conductivity of vapors as affected by
light. Henry2 could find no conduction of iodine vapor due to light. On
the other hand, J. J. Thomson3 states, without giving reference, that light
increases the conductivity of sodium vapor. Wiedemann and Schmidt,4 in
their paper in which they propose the dissociation theory, say that this does
not call for any ionization in the case of fluorescent vapors.

The desirability of anthracene vapor for this experiment is seen from the
following considerations :

1. The vapor is strongly fluorescent.

2. It has banded absorption and fluorescence spectra5 lying in the region
320-400 nn, light of these wave-lengths being transmitted by the glass used
in this experiment. Stark,6 in his work on band spectra, has concluded that
fluorescence and photo-electric properties are intimately associated with
absorption in bands shaded toward the red, and that the "carriers" of the
band spectra are the negative electrons, which are pulled out of their normal
position by the incident light, and upon their return to the position of
equilibrium liberate the energy of the fluorescent light.

3. If the separation of the electron is complete, photo-electric properties
would be exhibited. It is thus that Stark and Steubing explain the fluores-

'See H. E. Howe. Physical Review, xxx, p. 453. 'Wiedemann and Schmidt, Ann. der Phys., 56,

:Henry, Proc. Camb. Soc, 9, p. 319, 1897. p. 201. 1895.

3Tbornson, Cond. through Gases, p. 213. 5Elston, Astr. Jour., 25, p. 155, 1907.

'Stark, Phys. Zeit., 8, p. 81, 1907.

164

STUDIES IN LUMINESCENCE.

cence of solid anthracene.1 After investigating a number of substances,
they say: "It is highly probable that for organic substances, at least, the
photo-electric effect and fluorescence are intimately connected with each
other." Hence it would seem that the vapor of anthracene would be as
likely as any vapor to possess photo-electric properties.

APPARATUS.

The glass tube used to contain the anthracene is shown in Fig. 161. The
larger part of the tube was 15 cm. long and 2.5 cm. in diameter. The elec-
trodes between which the current passes consisted of an outer aluminum

Fig. 161.

cylinder, to which connection was made by sealing a platinum wire through
the glass at M , and an inner brass wire supported by a quartz tube Q and
visible through the slit in the outer electrode.

The quartz tube served as insulation for the brass wire electrode. Further
protection was afforded by a deposit of silver on the inside of the glass tube
around the quartz, this silver layer extending around to the outside of the
open end, where it was wrapped with tinfoil and grounded to form a guard-

Fig. 162.

ring. Thus no charge could leak over the glass surface from the outer
electrode.

The openings at the outer end of the quartz tube were sealed with de
Khotinsky cement. This would not stand heating, and the furnace was
arranged to inclose only the large part of the tube (see dotted outline in
Fig. 162). It was necessary to have the quartz tube fit snugly in place and
to provide a tight- fitting cap C (Fig. 161) at the inner end in order to pre-
vent the sublimation of the anthracene to the colder parts of the tube.

'Stark and Rteubing, Phys. Zeit., 9, p. 481, 1908. Also, Pochettino, Nuovo Cim., 15, p. 171, 1906.

ELECTRICAL PROPERTIES OP FLUORESCENT SOLUTIONS. 1 65

The tube, after being charged with anthracene, was exhausted to a
pressure of a few tenths of a millimeter, sealed off, and heated electrically.
Light from a carbon arc A (Fig. 162) was converged by the lens L, and could
be passed into the furnace through a mica window or could be cut off by the
screen 5. When the light was on, a bright cone of violet fluorescence could
be seen in the center of the tube.

The outer electrode was charged from a storage battery B, and the rate
at which the inner electrode acquired a charge was measured by means of a
sensitive Dolezalek electrometer, which was inclosed in a wire cage to pro-
tect it from the electrostatic disturbances. The end of the anthracene tube
projected through a hole in this cage, making the protection of the inner
electrode practically perfect.

OBSERVATIONS AND RESULTS.

The electrometer needle was charged to a constant potential (in most of
the work this was 60 volts), one pair of quadrants was grounded, and the
other pair connected to the wire electrode. This pair could be grounded
or insulated at will by means of the key K . When K was raised the motion
of the needle was noted by observing on a ground-glass scale the image of a
lamp filament formed by the small concave electrometer mirror.

If Vg be the potential of the charging quadrants ; K the deflection when
Vg is 1 volt; C the capacity of the charging system; Q the charge; D the
deflection; and /, the current; then

D = kXVg and Q = CVq
I = dQ/dt = C X dV/dt = k X C X dD/dt

on the assumptions that k1 is the same for the moving needle as for the
needle at rest and that C does not change with the position of the needle.
Such assumptions are allowable for slow motion and small deflections.

C was determined by dividing the charge with a cylindrical condenser of
30 cm. capacity, and was found to be 45 cm. or 5X 10-5 microfarads.

A charging rate of 1 division per second could easily be observed if
definite. This would mean a current 1.3 Xio~13 amperes. The insulation
was such that the rate of leak when charged to 50 divisions deflection was
about 10 divisions per minute, or one-sixth the rate mentioned as easily
measurable for charging.

At the low pressure used, anthracene vaporizes sufficiently to show fluores-
cence between 2000 and 2500 C. "90 per cent sublimed" anthracene was
used without attempt at purification, as it was easily obtainable and the
presence of impurities in the material does not seem to affect the fluores-
cence of the vapor. No attempt was made to shield the electrodes from
the light scattered by the end of the tube, nor to use monochromatic light.

In liquids there is a photo-electric effect, as has already been shown in
this chapter, dependent upon which of the electrodes is illuminated. The
possibility of the existence of such an effect in the vapor was not here
considered.

In most of the work the outer electrode was charged to 120 volts from
an ordinary storage battery. K was raised and the rate of motion noted
for the unilluminated tube. This was repeated with the light on.

'When the potential of the needle was 60 volts the value of k was found to be 375 mm. on a scale
one meter distant.

1 66 STUDIES IN LUMINESCENCE.

When the tube was newly made up the electrometer remained quiet under
all conditions, except for an occasional drift of not more than i division in 5
seconds. In the earlier work, after a few heatings, the electrometer began
showing a tendency to rise rather quickly to a more or less definite deflection,
regardless of conditions of potential or illumination. This peculiar effect
varied from run to run and became very great in a case or two when the
cement with which the tube was sealed cracked and admitted air. A brown
deposit was then left in the tube, apparently the result of a combination of
the oxygen of the air with the anthracene.

A possible explanation of this attainment of a steady deflection even
when the outer electrode was earthed is that the product of the decom-
position of the anthracene is deposited on the quartz tube separating the
silver guard-ring from the inner electrode and forms with these two metals
a sort of voltaic cell. This "cell" had a very high temperature coefficient
and in some cases the E.M.F. was so great as to throw the spot of light off
the scale.

After several trials it was found possible, by taking great care to prevent
the entrance of air, to get almost entirely rid of this effect. The elec-
trometer then behaved normally.

No ordinary conduction was found. If the light had any effect it was
too small to be detected. Thinking that the vapor under the action of the
light might be almost ionized, an attempt was made to help out the ioniza-
tion by exposing the vapor to the action of a small sample of radium
bromide. The ionizing rays from this substance did not pass through the
glass sufficiently to be of any use.

Higher potential was obtained by making up a set of small lead sul-
phuric acid storage cells in test-tubes. This battery, when put in series
with the one already at hand, gave from 360 to 540 volts, depending upon
the condition of the small cells. The sensibility was increased by charging
the electrometer needle to 120 volts. With the greater sensitiveness and
the unsteady potential furnished by the cells, the electrometer wandered
more or less in the neighborhood of the zero, but still did not indicate any
steady rate of charging such as would have been evident had there been
appreciable conductivity of the vapor.

In the freshly exhausted tube the pressure was so low that the application
of 400 volts caused a continuous luminous discharge through the tube, but
this soon disappeared as the pressure rose due to the vaporization of the
anthracene.

The number of trials was great enough to leave no doubt as to the be-
havior of the vapor under the various conditions mentioned.

CONCLUSIONS.

1. The conductivity of anthracene vapor at the temperature and density
used in this experiment is too small to measure in the manner described.

2. The effect of fluorescence, if any exists, is too small to be detected
by the very delicate method employed.

CHAPTER XI.
ON FLUORESCENCE ABSORPTION.1

In a paper published in 19042 we used the term fluorescence absorption
in referring to the increase of the absorbing power of a fluorescent substance
which results from fluorescence. Such an effect was first observed by Burke,3
who found a considerable increase in the absorption of uranium glass when
the glass was excited to fluorescence. The present writers observed the
same effect in solutions of fluorescein and eosin. The increase in the absorp-
tion appeared to be greater for those wave-lengths which corresponded
to the brightest regions of the fluorescence spectrum. Camichel,4 upon
repeating these experiments, was unable to detect any change of absorbing
power during fluorescence either in the uranium glass used by Burke or in
the fluorescent solutions tested by us. The question was again attacked in
1907 by Miss Wick,5 who made a detailed study of the phenomenon in the
case of an alcoholic solution of resorufin. Her results consistently showed an
increase in the absorbing power of the solution during fluorescence, and
were in complete agreement with the results obtained by us with fluores-
cein and eosin. More recently a method of detecting the effect, if it exist,
has been suggested by Wood,6 and a few trials of the method by him led to
negative results. The most recent experimenter in this field is Houstoun,7
whose very careful experiments also fail to give any indication of a change
in absorption due to fluorescence.8

The results obtained by ourselves, and especially those obtained by Miss
Wick, were so definite and positive that until recently we have been of the
opinion that the failure of others to observe the effect was due to the fact
that they had not chosen suitable conditions for the experiment. We were
led to suspect the existence of some systematic error, however, by the
results of a careful study of the collimator slit of the spectrophotometer
used in our experiments upon the distribution of energy in fluorescence
spectra.9 It was found that the screw was a very accurate one and that the
opening of the slit was very closely proportional to the reading of the micro-
meter screw. To test this point the slit was mounted in a lantern and the
enlarged image was measured for a large number of different settings. The
results are shown in Fig. 163.

We then tested the amount of light passing through the slit at different
widths by balancing two acetylene flames against each other, the adjustment
being made by varying the slit width in one case and by varying the dis-

1 Nichols and Merritt, Physical Review, xxxi, p. 500, 1910.
2Nichols and Merritt, Physical Review, xix, p. 397, 1904.
8Burke, Philosophical Transactions, 191a, p. 87, 1898.
^Camichel, Comptes Rendus, vol. 140, p. 139.
'Frances G. Wick, Physical Review, xxiv, p. 407, 1907.
•R. W. Wood, Phil. Mag., 16, p. 940, 1908.

'R. A. Houstoun, Proc. Royal Society of Edinburgh, 29, p. 401, 1909.

8Since the work described in this chapter was first published, still another article on the subject has
appeared, in which the results were also negative.
•See Chapter XII.

167

1 68

STUDIES IN LUMINESCENCE.

100

90

80

70
? 60

tance of the flame in the other. The experiments were performed in a dark
room with an elaborate system of screens to prevent reflections, and
several independent tests convinced us that the inverse-square law of dis-
tances was very exactly satisfied. If the intensity is computed by the law

of inverse squares, and if a
curve is plotted showing the
relation between intensity
and slit width, the results
obtained are of the type illus-
trated in Fig. 164.

It will be seen that for nar-
row slits the intensity of the
transmitted light is not pro-
portional to the slit width.
When the width exceeds a
few hundredths of a milli-
meter the line becomes
straight, so that equal incre-
ments in slit width corre-
spond to equal increments in
intensity. The conditions,
whatever they are, which
lead to the curve in the
neighborhood of the zero of Fig. 164 are equivalent in their effect to a shift
in the zero point of the screw by about 2 divisions. The intensity trans-
mitted by a slit 50 divisions wide is not twice as great as that transmitted
by one 25 divisions wide, but the ratio is in reality 48 to 23.

o

s:
—>

5

50

40

30

20 -

10

10

20 30 40 50 60

Drum reading
Fig. 163.

70

80 90 100

4S

s'

S

2

0 is

V
k 10

\

*M 5

*>*

j/s

»

2 1 6 O /O /Z /■-} 1/3 /& ZO 22
SLIT OP£ WAV OS IN HUNDREDTHS OP A MILLIf1£T£R

i*

Fig. 164.

These experiments were not made with the same instrument that had
been used by ourselves and later by Miss Wick, but refer to an exactly
similar Lummer-Brodhun spectrophotometer. It seems probable that
this effect, due possibly to diffraction or to reflection from the jaws of the

on fluorescence: absorption.

169

slit, is common to all instruments of this type. It is clear that if the effect
is disregarded, indications of fluorescence absorption may be obtained even
if no such effect exists. The method used by Miss Wick and by ourselves
involved three readings, namely, the intensity of the fluorescent light alone,
the intensity of the light transmitted by the solution when not excited,
and the combined intensity when light was transmitted through the solu-
tion at the same time that the latter was excited to fluorescence. The sum
of the first two readings being found greater than the third, it was assumed
that the transmission in the latter case was less than in the first. If, how-
ever, each of these readings of intensity had been too great by 2 divisions,
as indicated by Fig. 164, the result of such procedure would be to give an
apparent fluorescence absorption measured by 2 divisions.

While it is difficult to see how this source of error alone could account for
results of the character obtained by Miss Wick and ourselves, the detection
of one source of systematic error made it appear possible that other similar
errors might be present, and led us to take up the study of this question
anew. The result of the numerous experiments which will be briefly
described in this chapter has been to convince us that the phenomenon of

5

F,
) Exciting source

1 F2 I ^-Plane glass

""Ground glass

r-

1

6 C

Fi

d

F.

0

A

!ICE3 o

^Ground glass

Fig. 165.

Fig. 166.

fluorescence absorption either does not exist, or that the effect is so small
that the methods thus far used for its detection are inadequate.

In the first method used an attempt was made to obtain a photographic
record of the effect. The arrangement is shown in diagram in Fig. 165.

The large square cell Fh containing a solution of fluorescein, was excited
from above by a narrow beam of light, so that a central layer ab was excited
while the rest of the solution was not. Directly back of this was another
cell F2, also illuminated from above., so that the narrow vertical strip cd was
excited. The photographic plate P, suitably screened from all sources of
light except the fluorescence in Fi and F2, would be fogged nearly uniformly
over its surface by the light from ab if this alone was excited, and the fog-
ging would also be practically uniform if cd was excited. If, however, that
part of the liquid in Fi which is excited to fluorescence acquires the power
of absorbing the fluorescent light more strongly than before, then we should
expect ab to cast a shadow upon the photographic plate. While this
shadow could not be expected to be very sharp or very dense, the results
obtained by ourselves and Miss Wick indicated that under suitable con-
ditions it ought to be clearly visible in the negative.

170 STUDIES IN LUMINESCENCE.

Extended efforts were made to obtain such conditions of concentration,
thickness of layer, intensity of excitation, etc., as would bring out the
expected shadow on the plate P. Over 100 negatives were made and many
of these were under conditions which appeared to us to correspond to those
under which the spectrophotometer had indicated a large fluorescence
absorption; but in no case was a definite shadow observable.

The next method of testing the matter is shown in diagram in Fig. 166.
Three cells containing a solution of fluorescein, or in some cases resorufm,
were used as shown in Fig. 166. i*\ and F3 were excited by the same source
so as to eliminate errors due to variations in excitation; the source used
was sometimes a quartz mercury lamp and in other cases the tungsten lamp.
Since the solution was exactly the same in all three cells, and since Fi and
F3 were at nearly the same distance from the exciting source, the two colli-
mator slits were illuminated with almost equal brightness. Any slight
inequality was balanced by opening or closing one of the slits. This adjust-
ment being made, the exciting source was extinguished and light was sent
through the cell F3 from a small tungsten lamp T2. To balance this illumi-
nation, light from another tungsten lamp, after passing through the cell F->,
was reflected by a piece of plane glass into the second collimator slit. The
balance was obtained by adjusting the distance of T2, which slid upon a
graduated photometer track. When this balance was obtained the exciting
source was again started and the field of the spectrophotometer observed.
If the effect of fluorescence is to increase the absorbing power of F3 we
should expect that while fluorescence alone and transmission alone give a
perfect balance, there would be a lack of balance when excitation and trans-
mission occur simultaneously. When satisfactory conditions of steadiness
were obtained no such disturbance of balance could be observed. A sample
set of readings is given in Table 20. The small positive result obtained in
this case is smaller than the errors of observation. In other cases the results
indicated a small decrease in absorbing power during fluorescence. The
experiment was tried with solutions of different concentration and different
intensities of excitation. But when satisfactory conditions as regards stead-
iness were obtained no disturbance of balance could be detected which was
greater than the errors of observation.

Table 20.

Resorufin. Excited by Mercury Arc.

Slit in front of F3 set for equality of fluorescence alone. Readings 51.6, 49.7, 50.7,

50.7. Slit set at the average 50.7.
Lamp T2 set for F+T, i. c, fluorescence and transmission together. Distances

= 233,245. Average 239.
Lamp T2 set for transmission alone (7") 233, 233. Average 233.

F-\-T, 226, 228. Average 227.
Slit set again for equality of fluorescence alone. Readings 50.7, 50.4, 50.3, 50.7.
Slit set at the average 50.3.

F+T: 235, 239. Average 237.

T: 243, 245. Average 244.
F-\-T: 226, 237. Average 232.
Slit set for F alone: 50.8, 50.8, 50.7, 50.6. Average 50.8.

F+T: 238, 243. Average 240.5.

T: 240, 236. Average 238.
F+T: 243, 237. Average 240.
Average of all: Distance of lamp for T alone =238.3; for T+F = 2$6.o.
If this difference is real it indicates that the absorption of the solution during fluo-
rescence exceeds its absorption when unexcited by 1.7 per cent.

ON FLUORESCENCE ABSORPTION.

171

It was suspected that the phenomenon of fluorescence absorption appa-
rently demonstrated by Miss Wiek and ourselves might indicate not an
increase in the absorbing power of the solution but rather a decrease in its
power of fluorescing. The results obtained by us might be interpreted
equally well in either of these two ways. The fact that fluorescence absorp-
tion seemed to be in proportion to intensity of fluorescence in different parts
of the spectrum lent strength to this view. If the effect is a diminution of
fluorescence and not an increase in absorption the failure of our photo-
graphic tests could also be explained in an obvious manner.

If the fluorescence is diminished we should expect the diminution to be
observable not only along the line of the transmitted light but in other
directions. To test this matter we set up two large fluorescent cells F\
and F2 (Fig. 167) covered with black paper except on the sides toward the
exciting mercury arc, and having two openings Ox and 02 in the bottom
through which the light of the tungsten lamps 7\ and To, after reflection
from mirrors, might pass up into the cell. The balance having been ob-
tained with fluorescence alone, the lamp T\ was then turned on and we

T,
O

-0,

n

Mercury arc

01

f2

Td

-Green glass

Mercury arc
Blue glass

-O;

0

A

Plane glaso

<lTT]

M£C03

O

Fig. 167. Fig. 168.

tried to determine whether there was any change in the balance resulting
from extinguishing 7\ and at the same time lighting T2. The only effect
was a slight one, opposite in sign to that which would be indicated in fluores-
cence absorption, and due undoubtedly to a small amount of stray light
entering the slit after reflection from the walls of the cell.

It seemed to us possible that the effect might be analogous to the effect
of infra-red rays in suppressing fluorescence, and that possibly it might be
produced not by the visible rays which caused this annoyance through
stray reflection, but by the red or infra-red rays. We therefore interposed
ruby glass in the path of 7\ and T2, thus eliminating disturbances due to
stray light. Under these circumstances no disturbance of balance resulted
from extinguishing one lamp and lighting the other.

It seemed possible that while fluorescence absorption might really be the
result of a diminution in the intensity of fluorescence this decrease might
not be the same in all directions, but might be greatest in the direction in
which transmitted light proceeded. The arrangement of apparatus shown
in Fig. 168 was intended to test for the effect in directions nearly but not
quite the same as the direction of transmitted light. The light from the
tungsten lamp Ti passes through the cell Fi so as not to fall in the slit of the

172

STUDIES IN LUMINESCENCE.

instrument, but being nearly parallel to the axis of the collimator. The
small amount of stray light from 7\ which unavoidably did reach the slit
was balanced by light from the second tungsten lamp T2 as shown in the
figure. The results were entirely negative. Thinking that the intense
green line of the mercury arc might produce so much effect in weakening
the fluorescence in Fi that additional light from Tx would produce only a
slight effect, we interposed in some cases blue glass as shown, and in
order to diminish the excitation produced by the light from T\ we some-
times used green glass as indicated. The results, however, were in all
cases negative.

In Fig. 169 is shown an arrangement for testing the effect of transmitted
light upon fluorescence in case the direction of transmission is opposite to the
direction in which fluorescence is observed. No effect could be detected.

In Fig. 170 are shown the essential parts of an apparatus used in applying
the method suggested by Wood.1 A disk, shown in detail at the right of the
figure, was arranged so that the four sectors might either occupy the
position shown in the upper diagram, or might be shifted with reference to
each other so that the inner openings and the outer openings would be

T,
O-

Mercury arc
/2— Plane glass

x

Source

=&=-

Plane glass

'CD t2
- o

cMgC03

Screens

Collimator

</*-MgC03
'Ground glass

Fig. 169. Fig. 170.

alternate. This disk was mounted between the exciting source and the
fluorescent cell F, as shown. Light passed through the outer sectors to
excite fluorescence, and light passing through the inner sector, after reflec-
tion from a block of magnesium carbonate, passed through the cell to the
collimator slit. Having balanced the illumination against a standard
placed before the second collimator for the case where the inner and outer
sectors were open alternately, the disk was then changed so as to make the
excitation and transmission simultaneous. The results of several sets of
observations with this apparatus are given in Table 21. The observa-
tions indicate a slight negative effect, that is, the transmission of the solu-
tion appears to be greater during fluorescence. But the effect indicated is
so small that we are inclined to look upon it as resulting from accidental
errors.

A few measurements were made by a method essentially the same as that
used by us in 1904, except that the source of the light transmitted was the
fluorescence of a portion of the solution. A dilute solution of fluorescence
was contained in a cell 20 cm. long, so arranged that either the half nearest

■R. W. Wood, Philosophical Magazine, p. 940, 1908.

ON FLUORESCENCE ABSORPTION.

173

the slit or that at a distance could be screened from the exciting light, which
in this case was the mercury arc. The spectrophotometer was set for a
wave-length near the crest of the fluorescence spectrum and not overlapping
either of the mercury lines, so that errors due to stray light were excluded.
Upon screening that part of the cell lying nearest the slit the field was
illuminated by the fluorescence light of the distant part of the cell after
transmission through the rest of the cell T. Upon screening the distant
part of the cell the light received in the instrument was due to the fluo-

Table 21.

The numbers in the table give the width of the slit near F (Fig. 170) when set
to a balance with the standard in front of the other slit.

Fluorescein.

Moderate concentration. Wool
X =0.523/*

's method.

Resorufin. Dilute. Wood's method. T=i.02F.

X = OS99A«

Sectors open
alternately.

Sectors open
at same time.

Sectors open Sectors open
alternately. at same time.

IOO.9
97-7

104.2 Av.
103.5

IOl .58

101 .2
I00.2
IOO. 5

101 .2 Av.

IOO.78

64 . 6 66 . 2

65.2 64.4

67.3 65.4

66.8 Av. 65.98 65.7 Av.

65.42

100. 5
101 .7
100.0
100.4 Av.
Final av. =

IOO.65
101.11

102.0

98.8

IOO.7

98.7 Av.
Final av.=

IOO.05
= 1 00 . 4 I

66.6 66.8

66.4 65.9

66 . 5 66 . 2

66.9 Av. 66.68 66.3 Av. 66.30
Final av. = 66.33 Final av.=65.86
Fluorescence absorption = —0.014?".

Resorufin. Dilute

Wood's method. r = o.i8 F.

X=o.599M

Resorufin. Dilute. Wood's method. T = 7.2"j F.

X = 0. 599*1

113. 4
112.2
114. 7

Av.

113. 4

1 1 1 .2
1 12.4

1 12.3

1 1 1 .9 Av.

III. 95

20.6 19.0

20.1 I9.4

20.6 19.8
20.1 Av. 20.35 20.0 Av.

.9.78

112. 4 112.3

113. 8 113. 7
113. 3 113. 4
1 14.7 Av. 113.7 1 12.8 A v.
Final av. = 113.55 Final av.
Fluorescence absorption = —0.033

113.05
= 1 13 .0
T.

20.3 20.0

19.9 20.3

20.4 20.5
20.4 Av. 20.25 20.7 Av.
Final av. = 20.30 Final av.
Fluorescence absorption= —0.012

20.38
= 20.08

T.

rescence of the nearer part F. Upon removing both screens fluorescence
and transmission occurred at the same time C. If absorption is increased
by fluorescence we should have

F+T>C

The readings in one case are given in Table 22.

Table 22.

F
23.7
23-7
22.9

23-4

-C =

Average

T =
= 41.9-38

T C
23.2

23.4

23.6 41.2

22.9 41.2

Average 23.42
Zero 2.4

23.28 Average 41.2
2.4 2-4

F = 2l .02

F+T-

•20.88 38.8
.8=3.1=0.157".

174 STUDIES IN LUMINESCENCE.

The observations contained in Table 22 indicate a positive fluorescence
absorption of 15 per cent. But if we apply the additional zero correction
called for by the calibration curve of Fig. 164 the difference between F-\-T
and C is reduced to 0.9 division or 4.3 per cent. In other cases an equally
large negative result was obtained.

The results of all of these experiments, which have been repeated many
times, and performed with more precautions to avoid false results than can
be indicated in this brief account, has been to convince us that the previous
results of both ourselves and of Miss Wick are due to some systematic
error, and that the supposed increase in absorption due to fluorescence
either does not in reality exist or is too small to be detected by these methods.
We have not been able to determine the exact nature of the error which
led to our preceding results. The peculiar relation between slit opening
and intensity brought out in Fig. 164 will explain some of the results but
not all. Another source of error which might have been an important one
is that resulting from the neglect of the slit- width correction, to the impor-
tance of which attention is directed in Chapter XII of this memoir.

CHAPTER XII.

THE DISTRIBUTION OF ENERGY IN FLUORESCENCE SPECTRA.*

The energy of most continuous spectra is too feeble to permit of accurate
measurements excepting in the infra-red and the longer wave-lengths of
the visible spectrum, although we have a few determinations of the energy
of the visible spectrum of the acetylene flame by G.W. Stewart2 and by Cob-
lentz3 that extend beyond the green. The direct measurement of the energy
of even the brightest of fluorescence spectra, which are very small in intensity
as compared with those of our ordinary artificial light sources, is therefore
impracticable. We have shown, however, in previous chapters of this
memoir that it is possible to make quantitative spectrophotometric com-
parisons between fluorescence spectra and the spectrum of a standard such
as the acetylene flame. If the distribution of energy of the source used for
comparison be known, it is therefore easy to compute that of the fluorescence
spectrum. We have adopted this method in determining the energy distri-
bution in the fluorescence spectra of fluorescein, eosin, and resorufin, with
the results recorded below. The experimental work naturally falls under
three heads : ( i ) The determination of the energy distribution in the stand-
ard source; (2) the spectrophotometric comparison of the fluorescence
spectrum with the standard; (3) the measurement of the absorption of the
fluorescent liquid, in order that the observed curve of fluorescence may
be used to compute the typical curve.

DETERMINATION OF THE DISTRIBUTION OF ENERGY IN THE SPECTRUM OF

THE COMPARISON FEAME.

The comparison source used in the experiments to be described in this
paper was an ordinary flat flame from an acetylene burner, in front of which,
at a distance of 1.4 cm., was mounted a metal screen having a circular hole
0.6 cm. in diameter, so as to cut off the light from all but the brighter
central portions of the flame. This flame with its diaphragm was mounted
in a metal box having a circular window opposite the diaphragm. The
box was fastened to a base fitted to slide along a straight metal track. This
track was mounted horizontally in the same vertical plane as the axis of one
of the collimators of a IyUmmer-Brodhun spectrophotometer, and at such a
height that the axis of the collimator extended would pass through the win-
dow in the box and through the center of the diaphragm to the flame itself.

In front of the slit of the collimator was mounted a sheet of clear white
glass, the surface of which had been sufficiently roughened by grinding
with powered carborundum, so that at whatever distance the flame might
be placed the contrast field of the spectrophotometer would be of uniform
brightness throughout. The loss of light by the interposition of the ground

'The contents of this chapter first appeared in the Physical Review, xxx, p. 328.

JG. W. Stewart, Physical Review, xvi, p. 123.

aW. W. Coblentz, Bulletin of the Bureau of Standards, vn, p. 243.

175

1 76 STUDIES IN luminescence;.

glass was found to be about 40 per cent. Its transmission throughout the
range of wave-lengths used in our measurements was not measurably
selective.

To determine the distribution of energy in the spectrum of the light
received from the comparison flame after passing through the ground glass
and the optical parts of the spectrophotometer, this spectrum was carefully
compared wave-length by wave-length with the light received through the
other collimator of the instrument from a black body of known temperature.

The black body, Fig. 171, consisted of a tube of Acheson graphite about
50 cm. long, of 1.7 cm. bore and 4.0 cm. external diameter. In the middle
this tube was turned down for about 20 cm. until the thickness of the walls
was reduced to about 0.4 cm. and the thin-walled cylindrical chamber thus
formed was heated by means of an alternating electric current furnished by
a step-down transformer, of whose secondary circuit it formed the principal
part.

The ends of the cylindrical body were graphite plugs, each with an axial
hole 1 cm. in diameter. Through one of these passed a tube of fused quartz
containing a platinurn-rhodium-platinum thermo-junction of wires which
had been calibrated at the Bureau of Standards. This junction received
radiation from the surrounding walls and could be pushed in and out at
will so as to ascertain the range of temperatures within the black body.
Through the opening in the
other plug and through cor-
responding openings in dia-
phragms located nearer the pjg I7I
ends of the graphite tube,

light from the incandescent tip of the quartz tube reached the spectro-
photometer.

To reduce heat losses and prevent the too rapid oxidation of the graphite,
the tube was embedded to a depth of about 8 cm. in a mass of powered
magnesite, which was contained in a hollow cylinder of magnesium oxide
and asbestos such as is used for the packing of large steam pipes.

When the primary circuit was supplied with 80 amperes at no volts the
temperature of this improvised furnace, as indicated by the thermo-junction,
rose slowly to nearly 15000 C, at which temperature it remained with little
change for a considerable time. Temperatures were determined in the
usual way by means of a potentiometer and cadmium cell.

The arrangement of the apparatus is shown in Fig. 172, in which R is an
adjustable resistance, A is an a. c. ammeter, P is the primary coil of trans-
former, S is the secondary coil, D is a Dewar flask with the cold junction in
ice, a and b are the collimator slits of the spectrophotometer, L is the
Lummer-Brodhun prism of the spectrophotometer, 0 is the observing
telescope of the spectrophotometer, g is the ground glass in front of slit b.

The slits a and b were set once for all to convenient widths, b being 30
divisions =0.6 mm. in width, and a 0.06 mm. The adjustable diaphragm
in 0 was of the same width as slit b.

In this determination one observer made settings for wave-length and
watched the contrast fields of the spectrophotometer, while another recorded
the positions of the comparison flame when, for each region of the spectrum,

DISTRIBUTION OF ENERGY IN FLUORESCENCE SPECTRA.

177

equality had been reached. In the meantime a third observer followed the
changes of temperature with the potentiometer and recorded the E.M.F. of
the thermo-junction for each setting of the spectrophotometer. Readings
were begun when a temperature of 14100 (absolute) was reached. Sub-
sequently the current was slightly reduced and further sets of readings were
made throughout the spectrum.

t>

HE

E

^

FUme

Fig. 172.

From these data the distribution of energy in the spectrum of the
comparison flame was computed.
Wien's equation

I\=C\l 5e~

AT

Table 23.

was taken as giving the energy in the region of the spectrum at which
measurements were made. The accepted value, for an ideal black body,
of the constant C2 (i. e.,Ci = 14,500) was assumed to be applicable to the
present case, and the quantity C%/\T was calculated from the readings of
wave-length and temperature.
Since relative values only were
desired, the constant Ci was given
a value convenient for purposes
of computation.

The two slits of the spectro-
photometer were maintained at a
constant width throughout, and
the distance (d) of the comparison
flame was varied until the inten-
sities of the spectra were equal.

The energy of any given region of wave-length X was therefore propor-
tional to the ratio h/(i/d2) where h was the energy of the corresponding
region of spectrum of the light from the black body, computed as above.

Observations were made for twelve regions lying between 0.477 n and
0.656 m, and in the course of the determination the spectrum was traversed

Wave-length.

/A/(i/J!)

Wave-length.

/A/(i/J2)

O.656

248.0

0.534

58.70

.628

189.0

.52 1

48.50

.604

149.2

.508

42.50

•583

1 12.9

• 497

37-73

.565

87.0

.486

29-33

.548

69.2

• 477

21.25

178

STUDIES IN LUMINESCENCE.

four times. Good agreement existed between the various readings in each
region with the exception of the observations immediately following changes
made in the resistance of the primary circuit. These were rejected and all
other readings were used in the computations and averaged for each wave-
length separately. The results are given in Table 23 and are shown graphi-
cally in Fig. 173.

12

r,

h

(s

10

A

6

S

6

A

s

A

2

S^

r -Jy

7^

*£J^

A

^ -*

**■

.48 .50 .52 .54 .56 .58 60 .be -64 66^

Fig. 173-
Distribution of energy in the spectrum of the acetylene flame. The points marked S correspond to Stewart's
direct measurements with the radiometer. The points marked .4 are derived from Angstrom's measure-
ments of the Hefner lamp. The dotted line is computed from Wien's equation.

Table 24.

Spectrophotometric comparison of the acetylene comparison flame, viewed through
the circular diaphragm and ground glass, with flame of a Hefner standard lamp.

X

C2H2/Hefner.

C2H,/Hefner.

0.656M

O.916

o.534M

I.32

.628

.950

.521

1 43

.604

.990

.508

1 53

.590

I .00

•495

1 .69

.583

I .04

•487

..83

■ 565

I . II, I . 15

•477

1 .90

.548

1.25

Wien's equation has been employed, although in quite a different way,
by Knut Angstrom1 for the determination of the distribution of energy in
the spectrum of the flame of the Hefner lamp, and it is of considerable
interest to compare his determination, which involved neither the direct
measurement of temperatures nor observations upon a black body, with
the results of the experiment just described.

o

Angstrom, Nova Acta Upsaliensis, in, vol. xxn, 1904.

DISTRIBUTION OF ENERGY IN FLUORESCENCE SPECTRA.

179

20

For this purpose we made a careful spectrophotometric comparison between
our acetylene standard and a Hefner flame (see Table 24 and Fig. 174). From
Angstrom's curve of the distribution of energy in the spectrum of the Hefner
standard we then computed the relative intensities of our comparison flame
for several regions lying between 0.656 /1 and 0.483 ju. The points so deter-
mined are shown in Fig. 173 by crosses marked A.

Several points determined by Stewart by direct measurement with the
radiometer are also shown in Fig. 173, being marked S.

It is a matter of some interest to determine to what extent the visible
radiation from the acetylene flame corresponds to the visible radiation
from a black body. We find that the curve computed from Wien's equa-
tion is practically identical with our experimental curve from 0.56 n to
0.65 i*. But for wave-lengths less than 0.56 n the curve based upon Wien's
equation (shown by the broken line in Fig. 173) deviates considerably from
that determined by experiment. The acetylene flame appears to possess a
band of abnormally high radiating power in the region lying between 0.55 n
and the violet end of the
spectrum.

Since the first account
of our own work appeared
Coblentz1 has published
the results of direct
measurements of the
spectrum of the acetylene
flame with the vacuum
bolometer. While Cob-
lentz's results are in fair
agreement with ours in
the region from 0.48 /j. to
0.56 jit,they differ very appreciably for the longer waves, lying between our
own results and those of Angstrom.

COMPARISON OF THE FLUORESCENCE SPECTRA WITH THE SPECTRUM OF THE

STANDARD ACETYLENE FLAME.

Before making the final spectrophotometric determinations from which
the distribution of energy in the spectrum of various fluorescence spectra
were to be computed, a careful study was made of the effect of slit-width
upon the form of the observed curves.

In the ordinary use of the Lummer-Brodhun spectrophotometer slit a
(Fig. 172) would be of constant width and slit b would be varied. The
accuracy of the screw of the latter was therefore tested, as already described
at length in the opening paragraphs of Chapter XI, by mounting the
slit, which had been removed from the instrument, in the field of a pro-
jecting lantern and measuring the width of the slit-image, focused upon a
horizontal millimeter scale at a distance of about 8 meters. Variations
from constancy of the ratio of widths to micrometer readings were found
negligible for a range of two turns of the screw. Studies of the brightness
of the spectrum obtained when this slit was used showed, however, marked

10

p

0.50

0.54 0.58

Fig. 174-

0.62

0.66//,

'Bulletin of the Bureau of Standards, vol. 7, p. 243, 191 1.

i8o

STUDIES IN LUMINESCENCE.

deviations from the expected proportionality between the slit-width and
intensity, especially for widths of less than o.oi cm. (see Figs. 163 and 164
in Chapter XI).

It was therefore decided to avoid changes in slit width by using the
method of comparison used in determining the distribution of energy in the
spectrum of the comparison flame. In this method the two slits of the spec-
trophotometer remain unchanged in width, and equality of brightness is
obtained for each region of the spectrum by moving the comparison flame
along a bar or track parallel to the axis of the collimator. With the system
of screens which we employed to exclude stray light, the law of inverse

squares was found to hold
for the entire range of dis-
tances used in our experi-
ments.

The three substances
selected for measurements
were fluorescein in aque-
ous solution slightly alka-
line, eosin in alcohol, and
resorufin in alcohol.

The solutions in each case

£>Hl

* *»

I

6H-

Fig. 175.

were as dilute as was found practicable, so as to reduce the correction for
absorption to a minimum.

In the determination of the fluorescence curves the solution was placed
in a rectangular cell of white glass (/, Fig. 175) and was excited by the light
from a Cooper-Hewitt mercury lamp, C. II. The tube of this lamp was
vertical and mounted at a distance of about 30 cm. from the wall of the cell.
Only those portions of the tube were used which were nearly in the same
horizontal plane as the cell. The beam of exciting light entered the cell/,
Fig. 175, in the direction of the arrow at right angles to the axis of the
collimator.

The cell was inclosed within a metal box with black, matte, oxidized
surfaces, and having only the broad rectangular opening de for the admission
of the exciting light and a narrow, vertical, slit-like aperture opposite the
slit b through which the fluorescence was viewed.

The use of the mercury arc, with its almost complete absence of light in
the region occupied by the fluorescence bands to be measured, afforded
further protection against stray light. In the study of the fluorescein solu-
tion the additional precaution was taken of inserting a cell of ammonio-
sulphate of copper in water between the lamp and the fluorescent liquid,
thus cutting off the yellow and green lines of the arc almost completely.
The mercury lamp was fed from a storage battery of 120 volts with suitable
resistance in series, and under these conditions it furnished an exciting light
of unexpected constancy, surpassing in this respect any other source of suit-
able character and sufficient intensity with which we have had experience.

The arrangement of the apparatus for determining the fluorescence spectra
is shown in Fig. 175. The plan is similar to that used in comparing the
acetylene flame with the black body; but the photometer track carrying
the flame was mounted in line with collimator a, while the fluorescence cell

DISTRIBUTION OF ENERGY IN FLUORESCENCE SPECTRA.

181

was placed in front of slit b. In the figure, A is the comparison flame; si
and s2 are screens to prevent stray light from entering slit a; f is the cell of
fluorescent liquid, and C. II. is the Cooper-Hewitt mercury arc lamp.

The procedure was as follows:

vSlits a and b were set at equal widths of 50 divisions =0.05 cm. The
comparison flame was then moved up to a point on the track just in front
of the aperture in screen s2- The observing telescope was set for that
region of the spectrum corresponding with the maximum of the fluorescence
band to be measured. The fluorescent solution in cell / was diluted until
its spectrum for that region was slightly stronger than the corresponding
region in the spectrum of the comparison flame. By slightly shifting the

63y&

Fig. 176. — Eosin (to left) and resorufin.
The observed points are marked by circles.

.S4 S6A

Fig. 177. — Fluorescein.

The observed points are marked
by circles.

observing telescope in either direction two places could now be found, lying
a short distance from the crest of the fluorescence band, at which the two
spectra were of equal brightness. The circle readings of these positions
were noted, and the position of the comparison flame was read upon the
scale of the photometer track. The flame was then moved to a slightly
greater distance from slit a, and two new positions were found for the observ-
ing telescope, corresponding to points of equal brightness of the band
farther from the crest. In this way the entire band was explored several
times, and from these sets of readings the intensity of the band at various
wave-lengths was computed in terms of the corresponding intensities of the
spectrum of the acetylene flame.

1 82 STUDIES IN LUMINESCENCE.

From these values the observed curves in Figs. 176 and 177 were plotted.
The observed points are indicated by circles. The crosses and dots in this
figure refer to corrected values referred to in a subsequent paragraph. In
the case of eosin the wave-length of the crest is nearly the same as that of
the green mercury line. Measurements in the region of the crest were there-
fore rendered uncertain by the presence of stray light from the green line.
For this reason all the measurements in this region have been discarded.
Observations near the crest of the fluorescein curve were also somewhat
discordant, so that we do not regard this part of the curve as determined
with much accuracy.

To obtain from the observed curves the distribution of energy in the
fluorescence spectra it was necessary to make corrections for slit-width and
absorption, and to multiply the ordinates of each corrected curve by the
ordinates of the same wave-length in the curve giving the distribution of
energy in the spectrum of the acetylene flame.

THE CORRECTION FOR SEIT WIDTH.

In the spectrophotometric comparison of sources of light having con-
tinuous spectra and nearly the same luminosity curves the correction for
slit width disappears ; but in the case of spectra consisting of narrow bands
this is far from being the case. The slit-width correction used in the deter-
mination of the energy curves of incandescent solids does not apply, partly
because it is the luminosity of the rays rather than their energy that is
important, and partly because the distribution of luminosity in two sources
has to be considered. The slit correction applicable to the Lummer-
Brodhun spectrophotometer may be derived as follows : Let the luminosity
curve of the source Si, in front of the slit A, have the equation

when the distance of the source is such as to give the standard intensity,
which we shall call unity. If the distance is varied so that the intensity
becomes i, then (X being the wave-length)

The luminosity of the source S2 in front of the slit B will then be given
by the equation

L2=rf(\)

where r is the ratio of the energy of S2 at the wave-length X to the energy
of Si at the same wave-length, r is itself a function of X unless the two
sources are identical in quality.

Images of the slit A are formed in the focal plane of the telescope for each
wave-length of the spectrum of Si. If the spectrum is continuous, we
therefore have a series of overlapping images, forming a spectrum of greater
or less impurity according to the width of the slit A .

The light reaching the eye from the slit A will depend upon the width
of A, being proportional to this width if other conditions remain constant
But it will also depend upon the width of the aperture C at the principal
focus of the telescope, through which aperture the light used in making the
setting must pass. If the spectrophotometer is used without an eye-piece,

M 0 P RN Q

i I •' I

t

! I :

,.-v

DISTRIBUTION OE ENERGY IN FLUORESCENCE SPECTRA. 1 83

so as to obtain the benefit of the contrast field formed by the Lummer-
Brodhun cube, all the light passing through C is used in illuminating the
field, and the color is that resulting from mixing all the wave-lengths
present. When the instrument is set to a match we therefore have equality
between the total luminosity of the rays passing through C from A , and the
total luminosity of the rays passing through C from B.

An expression for the total luminosity of the rays from A may be found
as follows:

Let MN (Fig. 1 78) be the aperture at the focus of the telescope. It will be
convenient to express the width 2c of this aperture in terms of wave-length.
2c is therefore not a constant, even if the actual width
of the aperture is invariable, but depends upon the
dispersion in the region of the spectrum where the
observations are made. The widths of the slits A and
B, denoted by 2a and 2b, respectively, will also be
expressed in terms of wave-length. We shall consider
first the case where a<c. M' d' p" /?/v'Q'

Let the center of C (00' in the diagram) correspond piff 8

to the wave-length X. That image of the slit A which
is formed by light of wave-length X will have its center coincident with
the center of the aperture. The image formed by light of wave-length
\-\-x will be displaced by the distance x, so that its central line falls at
RR'. For values of x lying between + (c — a) and — (c — a) all of the light
forming the image of a will pass through the aperture. The total lumi-
nosity reaching the eye from such images will therefore be

£i=-7TJ f(\+x)dx=.-- {f(\ + x)+f(\-x)}dx

where d\ is the distance of the source Si from the slit and m is a factor, de-
pending upon the dispersion of the prism, such that ma is proportional to the
width of the slit in scale divisions. It is clear that 2am/ d2 measures the
intensity of the light entering the slit, the intensity being unity when the
source is at unit distance and the slit width one division.

For values of x lying between c — a and c-\-a only a part of the image v/ill
be transmitted, the transmitted fraction being

x-\-a — c

1

2a

The total transmitted luminosity from these images that are partially

transmitted will therefore be

20m fc+a ( '_ x-\-a— cy

( 'Yl-*±^)j/(X+:v)+/(X-*)jrf*
J c-a \ 2a /

2am rc+a a-\-c x

f l^-- \ \f(\+x)+f(\-x)\dx

J c-a I 2tf 2a I

d<2,
Upon expanding/ (X+.t) and/ (X— x) we have

^(X4-.v)+/(X-A-)=/(X)+.v/'(X) + :^:2•//, (X) + . • .+/(X)-.r/'(X) + ^r(X)- . . ,

2 2

= 2/(X)-f-*2/',(A)+...

For any ordinary case the terms in higher powers of x may be neglected

1 84 STUDIES IN LUMINESCENCE.

The two integrals therefore become

CC'a\f(\+x)+f(\-x)\dx= ( a[2f(\) + x'f"(\)}dx
Jo «~;o

x ,, (c-a)3 ,,

= 2(c-fl)/+- -./

and

Jc-a L 2 a 2d J 2 a L 3 J

_-r{(c+a)3_(c-a)I}/+(^>'-^>>]

2a\_ 4 -1

where /and/" are written for /(A) and /"(X) respectively.

Upon adding the two integrals and introducing the factor am/di2
we have

c(a2+c2)

"* di2 L - ' 3 J J diz ' ' "'
where

3
The form of the expression may be shown to be the same when a<c.
Similarly for that portion of the field which is illuminated by 52

*-?[«*+*&) VI

When the two fields are set to equality we have therefore

2ma r / . t ///i 2mb r /-la c2(y/)i

— |_2c/4-^J = ^[2cr/+^-^J

Expanding <r20/)/cX2 and remembering that the second term in the bracket
is in each case small we have:

di2 b\_ 2c/J L 2crj J

dfbL 2cf erf 2cr J

An approximate value of r, usually a close approximation, is obtained by
neglecting all terms after the first. This approximate value having been
plotted as a function of X, r' and r" may be determined; and since/' and/"
may be obtained from the luminosity curve of the source A , the correction
terms in the above expression can readily be computed.

In the experiments described in this paper the conditions were so chosen
as to make a = b = c. In this case the expression for r becomes

_aVr _a2 r" 2aVfl
~<VL 3 ' r" zrf A

If Ar is the increase in r which results from changing the wave-length
from X to X+2a, and if A/ is the corresponding increase in/, we have approxi-
mately :

r' = Ar/2a, f' = Af/2a

DISTRIBUTION OF ENERGY IN FLUORESCENCE; SPECTRA.

185

Fig. 179.

For purposes of graphical computation we may conveniently express the
approximate value of r" in terms of the distance DC, Fig. 179. If this dis-
tance be called br it may be readily shown that
r" = 28/a\

_di2 T _25r_iArA/~|

d*? L 3 r 6 r f J

In computing these correction terms the lumi-
nosity curve given by Tufts was used.1 The
values of Ar and br were taken from the curve
showing the relation between X and the approxi-
mate value of r.

The correction depending upon br is important
only when the curvature of the r curve is con-
siderable. In the present instance this correction
is large for regions near the crest of the curve,
but insignificant elsewhere. The importance of the correction is well shown
in the curves for resorufin (Fig. 176), where the points marked with crosses
show the curve after the slit correction has been applied. In the case of
eosin and fluorescein the region near the crest was uncertain for the reasons
already mentioned (p), so that the slit correction has not been applied in
this region.

The second correction term, depending upon the product ArAf, is negli-
gible near the crest of the luminosity curve of the source, where A/ is small.
The term is thus of no significance in the case of eosin and fluorescein.
Even with resorufin, where the whole luminescence spectrum lies well t? ths
infra side of the crest of the luminosity curve, this correction is important
only on the steep side of the curve. The result of applying the correction
is shown by the points in Figs. 176 and 177 that are marked with crosses.

THE CORRECTION FOR ABSORPTION.2

The fluorescent light which enters the slit of the spectrophotometer in the
foregoing experiments comes from a layer of liquid in the fluorescence cell
having a thickness determined by the opening de, Fig. 175, through which
the exciting light enters the cell. This layer, which is to be considered as
uniformly fluorescent, extends from x = X\ to x = X2, where x is the distance
from any point from which fluorescent light emanates to the wall of the
cell in the direction toward the slit of the spectrophotometer.

Let the intensity of light, of any wave-length X, sent toward the slit
from the unit layer be i, and from a layer of thickness dx, be idx. If the
coefficient of absorption of the liquid be a the light reaching the slit from
the layer dx is idxe~ax and the total intensity of light reaching the slit from
the whole of the excited layer is

<c

/=*v

Kdx = -[e-axl-e-ax*]
a

■Tufts's curve (see Physical Review, xxiv, p. 453) was obtained for an incandescent light. But the dif-
ference between the color of the acetylene flame and that of the glow lamp in the region considered is so small
that the error introduced is quite inappreciable.

-For a full description of the methods employed in determining the absorption of such solutions see
Chapter XIII.

1 86

STUDIES IN LUMINESCENCE.

In the cell used in these determinations .v2 was 36.5 mm. and .\\ was
2.5 mm.

The coefficient a was determined by measuring the transmission of a cell
containing a sufficient thickness of the fluorescent liquid to reduce the
intensity of light of the wave-length of the middle of the absorption band
to about 40 per cent. The correction for the losses of light in the glass and
the solvent was determined by measuring transmission through the same
cell when filled with absolute alcohol (or in case of fluorescein with water) .

The correction for absorption, like that for slit-width, is of importance
only in certain regions. The form of the fluorescence curves after the
application of both corrections is shown in Figs. 176 and 177.

—

A

Eosi

n

f\

F

luo

~esc

ein

1

1

1

\

esor

uf it

)

i

I

/

IU

1

Q C

)

)

<

> \

1

1

\

i

s

)

6

.50

.55

Fig. 180.

.60 M

THE ENERGY CURVES OF FLUORESCENCE.

From the data corrected in the manner indicated in the two preceding
sections curves for the distribution of energy in the fluorescence spectrum of
the three substances under investigation were derived by multiplying each
ordinate of the corrected curves of Figs. 1 76 and 1 77 by the ordinate of same
wave-length in the energy curve of the acetylene standard (Fig. 173).

The resulting curves are given in Fig. 180. They suggest by their form
a close resemblance between black-body radiation and fluorescence, except-
ing that the range of wave-lengths in the latter case is much smaller.
Attempts to find some simply modified form of Wien's equation which will
represent the results have thus far been unsuccessful.

CHAPTER XIII.

THE SPECIFIC EXCITING POWER OF THE DIFFERENT WAVE-
LENGTHS OF THE VISIBLE SPECTRUM IN THE CASE OF THE
FLUORESCENCE OF EOSIN AND RESORUFIN.

In the case of either a solid, like anthracene, or of a liquid such as one
of the fluorescent dyes, fluorescence may usually be excited by light of a
great variety of different wave-lengths. In fluorescent solutions there is
commonly a well-marked absorption band lying close to the fluorescence
band on the side toward the violet, and light of any wave-length within the
limits of this absorption band will excite fluorescence. In fact fluorescence
may usually be excited by light of much shorter wave-length, so that the
solution will be lighted up when exposed to an ultra-violet spectrum. Simple
inspection, however, is sufficient to show that the intensity of the fluores-
cence excited by various portions of a given exciting spectrum is widely
different. It is not clear whether this variation is due to the fact that
certain wave-lengths are particularly effective in exciting fluorescence,
or whether it results merely from the fact that the absorbing power of the
material varies for different wave-lengths. It is clear that light can not
produce excitation unless it is absorbed by the fluorescent solution. Dif-
ferences in absorbing power might therefore produce wide variations in the
apparent effectiveness of different spectral regions in producing fluorescence,
even if the specific exciting power, i. e., the fluorescence excited per unit of
absorbed energy, were in reality constant for all wave-lengths.

The determination of the relation between the specific exciting power and
the wave-length of the exciting light is a problem of some interest, whose
results possess also considerable significance on account of their bearing
upon the theory of fluorescence. The present chapter deals with the deter-
mination of this relation for eosin and resorufin.1

The exciting light was furnished by a Nernst glower which took the place
of the slit of a large spectrometer. A narrow region in the spectrum thus
formed was used in exciting the solution studied, and the intensity of
fluorescence produced was measured by a spectrophotometer as the wave-
length of the exciting band was varied.

The arrangement for exciting and observing fluorescence will be made
more clear by inspection of Fig. 181. The light of the Nernst glower,
after passing through the large spectrometer before mentioned, was reflected
directly upward by a total reflecting prism as shown in the figure, and passed
through a horizontal slit S into the cubical glass vessel which contained
the solution to be studied. The spectrometer was adjusted so as to bring
the spectrum in focus at the slit S. One vertical face of the cubical cell
was covered with a sheet of metal containing a slit S' as shown in the figure.
The exciting light passing through the slit S excited a narrow vertical

■An account of the experiments described in this chapter was given in the Physical Review, xxxi, p. 376,
and p. 381, 1910.

187

i88

STUDIES IN LUMINESCENCE.

strip in the liquid to fluorescence, and this was observed through the slit S',
in front of which the collimator slit of the spectrophotometer was placed.
The comparison source for the spectrophotometer, as in various experi-
ments described in previous chapters, was an acetylene flame sliding upon
a photometer track so that it could be set at different distances from the
ground glass in front of the slit. This procedure makes it possible to keep
the slit width constant and therefore eliminates the errors which might be
introduced in measurements where the variation of intensity is great.
The collimator slit of the spectrophotometer was broad and the instrument
was set so as to measure the central part of the fluorescence band.

The Nernst glower which furnished the exciting light was attached
permanently to the collimator tube of the large spectrometer. The wave-
length of the light used for excitation was varied by swinging the arm
carrying the collimator and glower, the other portions of the spectrometer
remaining fixed in position. At frequent intervals, corresponding to wave-
lengths of the exciting light which differed only slightly from one another,

To collimator

■>

of spectrophotometer

Exciting - >
light 5 ->

Fig. 181.

the intensity of the fluorescence excited was measured by the spectropho-
tometer. The cell containing the fluorescent solution was then emptied
and carefully cleaned and a piece of magnesium carbonate was mounted
obliquely in the cell so as to reflect light from the slit S into the spectro-
photometer. The intensity and range of wave-lengths of the light thus
reflected were measured for each of the spectrometer settings previously
used. Finally the absorption of a layer of the solution of known thickness
was measured for some definite wave-length in the spectrum. By a separate
series of measurements the relative reflecting power of the magnesium
carbonate was determined throughout the region used.

The procedure in computing results was as follows: To correct for the
unequal reflecting power of magnesium carbonate at different wave-lengths
the observed intensity, I0, of the exciting light was divided by the reflecting
power of magnesium carbonate for that particular wave-length. This
correction reached approximately 10 per cent as its highest value. Using
the values contained in Chapter XII for the distribution of energy in the
spectrum of the acetylene flame, we next determined the relative energy
of the light used for excitation as a function of the wave-length. It will
be noticed that this procedure not only takes account of the difference

SPECIFIC EXCITING POWER OP DIFFERENT WAVE-LENGTHS. 1 89

in quality and energy distribution between the Nernst glower and the
acetylene flame, but also eliminates any errors which might arise on account
of selective transmission or other causes anywhere in the apparatus.

The center of the slit S' through which the fluorescence was absorbed
was 8.2 mm. above the bottom of the cell containing the solution. On
the average, therefore, the exciting light had passed through 8.2 mm. of
solution before reaching the region whose fluorescence was measured by
the spectrophotometer. The light available at this point to produce
excitation was therefore less than that falling upon the face of the cell in
the ratio of one to e~°-82a. It was therefore necessary to compute this
factor e~° 'S2a for each wave-length and for this purpose we determined with
considerable accuracy the coefficient of absorption of both eosin and reso-
rufin for different wave-lengths.

COEFFICIENTS OF ABSORPTION.

The measurements, which were extended throughout the absorption
band for both dilute and relatively concentrated solutions of both sub-
stances, were made by comparing the intensity of the light transmitted by
a cell containing pure alcohol with the transmission, for the same wave-
length, when the cell was filled with the solution in question. For the con-
centrated solution two cells were used, the thickness of the absorbing layer
being 1 cm. in one case and 3 cm. in the other. The dilute solutions were
contained in a cell which gave an absorbing layer 29.5 cm. thick. The
source of light for transmission was an acetylene flame. The comparison
standard was also an acetylene flame, which was mounted so as to slide upon
a track as previously described.

The ratio of the intensity of the light transmitted by the solution to the
intensity of the light transmitted by the alcohol gave the percentage trans-
mission, from which, with the thickness of the cell, the coefficient of absorp-
tion, a, could be computed, a being defined by the expression

I = he~ax

where x is the thickness.

The results for eosin are shown in Fig. 182 and for resorufin in Fig. 183.
Curve I in each case gives the coefficient of absorption for the dilute solution
as a function of the wave-length, while curve II gives the coefficient of
absorption for the concentrated solution. In Fig. 182 the scale is fifty
times greater for curve I than for curve 77, and in Fig. 1 83 the scale for
curve I is ten times as large as that for curve II.

It will be seen that the absorption curve has very nearly the same form
for the dilute and concentrated solutions. This is especially true in the
case of resorufin, where each little ripple on the curve for the dilute solution
is reproduced on the curve for the concentrated solution. In Table 25
will be found the values of a for the two solutions, together with the ratio
of the two values. It will be seen that the ratio remains nearly constant
throughout the whole spectrum. The variation from constancy is most
marked at the two ends of the spectrum where the liability to experimental
error is greatest. The results in the case of resorufin appear to indicate
that the form of the absorption curve is not altered by concentration

190

STUDIES IN LUMINESCENCE.

throughout the range studied and are thus in agreement with the results
of Miss Wick.1

The appearance of the curve for resorufin suggests that the band is

.50 .52 .54 .56
Fig. 182. — Eosin.

.58A

^

—

0.8

7 i 1

G.
0.6

lf\\\ \

r \\\ \

/ U 1

iF

0.4

f~ |l~

\
\
\

\

11

\

\

w

1

1

1

\

\

O.cl

1
1

\

1

Id

.50

Curve / shows the coefficient of absorption as a func-
tion of the wave-length for a dilute solution.
Curve // shows the same thing in the case of a
concentrated solution. (The vertical scale in
curve / is fifty times greater than that of curve
//.) Curve F shows the distribution of energy
in the fluorescence spectrum of fluorescein.

.54-

Fig. 183.

.58
-Resorufin.

.62 A

Curve /, coefficient of absorption for dilute solu-
tion; curve //, coefficient of absorption for
concentrated solution. (The scale of curve /
is ten times greater than that of curve //.)
Curve // shows the energy distribution in the
fluorescence spectrum.

complex and that in addition to the principal maximum at 0.577 /z there
are secondary maxima of absorption at about 0.558 ix and 0.536 /x. If the
absorption of resorufin really consists of three overlapping bands, as seems
probable, our results indicate that all three of these bands are produced by
the ions.

Table 25.
Coefficients of absorption for resorufin.

X

a,

a2

Ratio.

Dilute.

Concentrated

O; : O!

0 Acrtu

0 . 0078

503

O. I 12

508

.0107

.125

n. 7

514

•'75

521

.0180

.232

12.9

527

.299

534

.0307

.385

12.6

54'

.0349

.414

11. 8

548

.0372

.448

12.0

557

.0467

.563

12.0

5<>5

.0522

.637

12.2

574

.0674

.807

12.0

583

.0448

.576

12.8

585

•557

593

.OI26

. 162

12.9

599

.084

604
605

. OO 1 7 .

.032

Frances G. Wick, Physical Review, xxiv, p. 356; see also Chapter II.

SPECIFIC EXCITING POWER OF DIFFERENT WAVE-EENGTHS.

191

In the case of eosin there is some indication of a secondary maximum in
the neighborhood of 0.49 n- The values of a for the two solutions and the
ratios for these two values are given in Table 26. It will be noticed that
the ratio is not nearly so constant as in the case of resorufin. There is
perhaps some slight indication that the secondary band at 0.49 /u changes
with the concentration at a different rate from the principal band, but it
is doubtful whether the results are sufficiently accurate to give any certainty
to such a conclusion. The small values of the ratio on the red side of the
band imply a slight shift toward the red in the case of the dilute solution,
but the region in which these small values in the ratio occur is the region
where the results are most liable to error.

In each case we have shown by the curve F the distribution of energy
in the fluorescence spectrum of the substance in question. The resemblance
is noticeable between this curve of energy distribution and the absorption
curve. Roughly speaking the one curve is the image of the other. In the

Table 26.
Coefficients of absorption of eosin.

X

a.

a2

Ratio,

Dilute.

Concentrated.

a2 : ai

0 . 48OJU

O.OI 16

O.356

30.7

491

.0153

.540

35

3

497

.0166

•597

35

9

503

.0192

.736

38

3

508

.0233

.870

37

2

514

.0328

I.274

38

7

521

•0473

I.765

37

3

527

•0533

I .892

35

6

535

.0444

I .422

32

0

54i

.0235

O.715

30

3

548

.0083

.250

30

1

absorption curve the side toward the red is steep, while the absorption dies
away gradually toward the violet. In the fluorescence curve the intensity
dies away gradually toward the red and stops abruptly on the violet side.
When the mechanism of fluorescence is more fully understood the reason
for this peculiar relationship between the two curves, which seems to be
a characteristic of the fluorescent substances of this class, will doubtless
be clear.

COMPUTATION OF SPECIFIC EXCITING POWER.

In the determination of the specific exciting power for eosin the solution
used was the same as that employed in determining the absorption curve,
for the more concentrated of the two solutions, shown in Fig. 182. The
value of a for each wave-length could therefore be read off at once from
that curve.

In the case of resorufin a different solution was used. Since, however,
the form of the absorption curve is the same for all concentrations, it was
sufficient to determine the absorption for one particular wave-length and
then to reduce the values read from the curve for that substance (Fig. 183)
in a constant ratio.

192

STUDIES IN LUMINESCENCE.

Having computed in this way the intensity of the rays which actually
reach the region in the solution under observation, and which are therefore
available for producing excitation, it was only necessary to find the amount
of energy absorbed in each case by multiplying I0e~° ,82a by a. Finally
the absorbed fluorescence, divided by the absorbed energy which produced
it, gave the quantity sought, that is, the intensity of fluorescence excited by
unit quantity of absorbed energy of the particular wave-length in question.

In the case of eosin the observed data, as well as the derived quantities

corresponding to several

steps in the computation,
are given in Table 27. In
the case of resorufin the
same thing is shown graphi-
cally (Fig. 184). The ob-
served points in each case
are connected by full lines,
while in the case of com-
puted values the points are
connected by broken lines.
In curve / we have the in-
tensity of the exciting light
after reflection from mag-
nesium carbonate. Curve
77 gives the intensity of the
light actually reaching that
flie ordinates of curve 77/ are

Curve III

62//

Fig. 184.

Resorufin.

part of the solution opposite the slit S' .

obtained by multiplying each of the ordinates of curve 77 by a.
therefore gives the energy actually observed from the exciting light for each
wave-length. When the ordinates of curve IV, which shows the observed
fluorescence, are divided by the ordinates of curve 77/ we have the quan-
tity desired, namely, the fluorescence excited per unit of observed energy
(curve V).

Table 27. — Eosin.

I

2

3

4

5

6

7

8

9

10

Intensity

Intensity

Reflecting

h corrected

Rnergv in

h
(Energy.)

Specific

X

of fluores-

of exciting

power of

for

C2H2

he-"*

afoe—"x

exciting

cence.

light.

MgCOs

reflection.

spectrum.

power.

0 . 479/*

2 .2

8.25

I02

8.08

I. 15

9 3

7.0

2.45

0.9

485

2.9

8.4

103

8.15

I.38

11 .2

7.67

3

53

0.82

49'

4 1

8-3

104

7.98

1.59

12.7

8.4

4

24

O.97

497

5-3

8.4

105

8.00

1 77

14.2

8.65

5

20

1 02

501

6.3

10. 1

10S

037

1 .90

17.8

10. I

(j

98

.90

509

8.6

8.9

1 1 1

8.00

2.13

17. 1

8.12

7

37

1.17

5M

10.4

11.2

1 1 1

10. I

2 .26

22.8

8..3

10

3

I .OI

521

1 1 .0

1 1 .0

1 1 1

9.9

2.40

23.8

5 .60

9

9

III

526

13.0

12.2

in

1 1 .0

2.58

28.4

5.98

1 1

4

I. 14

534

17.0

12.0

1 10

10.8

2.90

3' 3

9.76

13

8

1.23 I

540

18.3

12.5

109

11. 5

314

36.2

19 (i

14

7

I .25 1

547

1 1 .0

12.2

108

11. 3

3-4

38.4

31.0

8

07

1-37

5 56

4.8

12.7

107

11.9

3.84

84.5

42.7

3

«5

1.25

SPECIFIC EXCITING POWER OF DIFFERENT WAVE-LENGTHS.

193

The results for eosin are shown in Fig. 185 and for resorufin in Fig. 186.
In the case of the latter figure the values corresponding to those shown in
Fig. 184 are marked by circles. The crosses show the values obtained by
another set of observations which are not here recorded in detail. The
individual points on these curves show considerable erratic variation. In

£

0

0

0

0

0
4 '

0

c>^-"

D

Aj

\r

2

.4

8

J

0

J

z

M N^

Fig. 185. — Eosin.

Intensity of fluorescence excited per unit of absorbed energy of different wave-lengths. Curve A gives the
coefficient of absorption and curve F the energy distributed in the fluorescence spectrum.

many cases these can be traced back to irregularities in the original data,
doubtless due to some experimental errors. In making computations we
have made no effort to eliminate these errors by smoothing the curves, but
have taken each observation as it stood and carried through the work to

8

K j

6

x 5

0

4

"**o

0

0

« j

*"o

^^

y

0
2

.S

4

.J"

6

' .5

8

.6

&_

Fig. 186. — Resorufin.

Intensity of fluorescence excited by different wave-lengths per unit of absorbed energy. The circles and
crosses give the results of two independent sets of observations. Curve A gives the coefficient of
absorption and curve F the energy distribution in the fluorescence spectrum.

the final result. Smooth curves have, however, been drawn among the
computed points. The data and results recorded here represent the best of
a number of series of observations. While those not recorded gave more
irregular results than here shown they all agreed in indicating the same
general trend in the final curve.

194 STUDIES IN LUMINESCENCE.

In each of these two figures also we have plotted the carves for the coeffi-
cient of absorption (.1) and for the energy distribution in the fluorescence
spectrum (F). It would seem reasonable to expect that the curve of specific
exciting power might show some peculiarities either in the region of maxi-
mum absorption or in that of most intense fluorescence. It appears, how-
ever, that nothing of this kind occurs.

Before beginning these experiments we were of the opinion that either
the specific exciting power would prove to be constant, so that the same
quantity of absorbed energy would produce the same fluorescence regardless
of its wave-length, or else that the effectiveness of the exciting light would
prove to be greater for the shorter wave-lengths. The readiness with which
fluorescence is excited in the ultra-violet spectrum made the latter view
seem plausible. As it turns out, neither of these views is in accord with the
facts. We were unfortunately unable to extend the observations to the
ultra-violet spectrum. It seems clear, however, that if we confine our
attention to wave-lengths falling within the range of one absorption band
the light lying near the red side of the band is more effective in producing
fluorescence than that lying on the violet side, and the change in the specific
exciting power as we pass through the band is steady, without any indica-
tion of anything selective in the neighborhood of the region of maximum
absorption.

CHAPTER XIV.

THE THEORY OF WIEDEMANN AND SCHMIDT.

The most comprehensive theory of luminescence thus far proposed is
undoubtedly that first suggested by E. Wiedemann1 in 1889, and later
modified and extended by Wiedemann and Schmidt.2 According to this
theory some chemical or physical change is produced in a luminescent
substance during excitation, and the emission of light is an accompaniment
of the more or less gradual restoration of the modified substance to its
original condition. Thus, in the case of phosphorescence, it was suggested
by Wiedemann that a portion of the active material was changed by the
exciting light from the stable condition A into the unstable state B. Phos-
phorescence would then result from a gradual breaking down of the unstable
compound.3 Fluorescence maybe due either to the vibrations setup during
the change from A to 5, or to the fact that the reaction B ~~A proceeds, with
emission of light, during excitation as well as during decay. Thermo-
luminescence is to be explained as the result of some chemical change pro-
duced by heat, during the progress of which the molecules are thrown into
such violent vibrations as to bring about the emission of light. Lumines-
cence ceases in such cases when the change has been completed ; and some
outside stimulus is required, such as that furnished by kathode rays, to
restore the substance to the sensitive state. Not only is this explanation
of thermo-luminescence a plausible one, but in several instances evidence
has been found that the assumed change really occurs. In the paper
already referred to, Wiedemann and Schmidt have discussed all of the
various types of luminescence in considerable detail and have shown that
the theory proposed by them will account for the phenomena, at least
qualitatively, in a very satisfactory way.

It has generally been assumed that the change accompanying lumines-
cence is of a chemical nature. Various suggestions have been made in
regard to the character of the reactions produced by the exciting light (or
other cause), and attempts are not lacking to trace a connection between
luminescence and chemical constitution. It was pointed out, however,
by Wiedemann and Schmidt that in many cases there were reasons for
looking upon electrolytic dissociation as the more probable cause of lumines-
cence. In fact, since the electromagnetic disturbance that constitutes
light can get a hold on the molecules of the active material only by exerting
forces upon the electrical charges in the molecule, and will always tend to
separate the positive and negative parts, it appears probable that the first
effect of the exciting light is always to produce some type of electrolytic
dissociation, and that any chemical changes which may be exhibited are

IE. Wiedemann, " Zur Mechanik des Leuchtens," Wied. Ann., 37, p. 177, 1889.
=E. Wiedemann, and C. C. Schmidt, Wied. Ann., 56, p. 177. 1895.
3E. Wiedemann, /. c, pp. 224-225.

195

196 STUDIES IN LUMINESCENCE.

secondary effects.1 But although the exciting light causes a separation of
the active molecule into positive and negative parts, it appears unlikely that
these parts are the ions of ordinary electrolysis. There are many sub-
stances like fluorescein and eosin which fluoresce only when dissociated. In
such cases fluorescence can not be due to the recombination of ions; for
dissociation and recombination are taking place in an electrolytic solution
all the time, and if this were the cause of luminescence we should expect
the solution to glow continuously without the action of any exciting light.
Fluorescence in such cases must be due to some action upon the ions them-
selves. Now it can scarcely be doubted that the absorption of the exciting
light is the result of resonance on the part of the molecules or atoms of the
active substance; and although the vibrational energy thus imparted to
the molecules is rapidly transformed by collisions into translational energy
(heat), yet under favorable conditions the molecules might be thrown into
such violent oscillation as to be torn apart. It seems, therefore, that the
most plausible assumption to make is that the first effect of the exciting
light is to produce such violent vibrations as to liberate one or more electrons
from the molecule; in other words, to bring about dissociation similar to
that produced by Roentgen rays. For the sake of deiiniteness we shall
adopt this view in the discussion that follows.

Among the facts of luminescence that are satisfactorily accounted for by
the theory of Wiedemann and Schmidt is the law that the distribution of
intensitv and the wave-length of maximum intensity for each band in a
luminescence spectrum are independent of the mode of excitation.'2 Light
corresponding to any part of the absorption band may, if sufficiently intense,
produce dissociation ; and dissociation may also be brought about by vari-
ous other agencies, such as the action of Roentgen rays and kathode rays.
But the manner in which recombination occurs, and therefore the form of
the luminescence spectrum, will not depend in any way upon the manner in
which the dissociation was produced.

The theory also leads directly to the conclusion that the light emitted
during the luminescence of an isotropic substance is unpolarized, whatever
may be the condition of polarization of the exciting light, and this con-
clusion is in agreement with the experimental results in all cases where
polarization tests have been applied.

The law of vStokes that the light emitted during photo-luminescence is of
greater wave-length than the exciting light has always proved a stumbling-
block in the development of theories of luminescence, and at first glance
the difficulty appears to be as great in the case of the Wiedemann and
Schmidt theory as in any of the other theories proposed. Why should the
vibrations that occur on recombination differ in period from those origin-
ally set up by the exciting light? It is indeed possible that the latter are
forced vibrations, whose period bears no simple relation to the natural
period of the molecule ; but it is more reasonable to expect that vibrations
which bring about actual disintegration are produced by resonance. If

'It is to be observed that this reasoning does not apply to the case of kathodo-lumiiiescence, since there
is no reason why the kathode-ray bombardment should not directly cause chemical changes. The relatively
great chemical activity of kathode-ray excitation as compared with excitation by light isprobably connected
with this essential difference in the mechanism of excitation in the two cases.

2This law, first proposed by Lommel in the case of photo-luminescence, has been tested by the writers
for numerous cases of fluorescence excited by light of different wave-length (see Chapter I) and for several
cases of excitation by kathode rays and Roentgen rays (see Chapter IX).

THE THEORY OF WIEDEMANN AND SCHMIDT. 1 97

this is true we have to do with the natural period of the molecule in both
cases, and it would seem that the light emitted during luminescence should
have the same wave-length as the exciting light. In the discussion that
follows we venture to make a suggestion which offers an explanation of this
difficulty.

In the case of photo-luminescence in solids and liquids the active mole-
cule is always closely surrounded by other molecules. In general these
surrounding molecules belong to the solid or liquid solvent in which the
active material is dissolved. If we have to deal with the luminescence of a
pure substance (if such cases occur), the surrounding molecules are of the
same kind as those which participate in the luminescence phenomena.
But in either case the period of vibration will be different from what it
would be if the vibrating molecules were isolated. Since the change in
period will be relatively great for those molecules that are close to their
neighbors and smaller for those that are farther away, it is seen that the
natural period will vary through a considerable range as the active mole-
cules move about, and at each instant there will exist in the substance
molecules having all periods lying between certain rather wide limits. The
absorption spectrum of the substance therefore consists of bands rather
than of lines.1

When a molecule is dissociated by the action of the exciting light, the
two parts, being electrically charged, will be more strongly attracted by
the molecules of the solvent than was the original neutral molecule. Recom-
bination of the separated ions is therefore more likely to occur when the
latter are in the immediate neighborhood of the molecules of the solvent,
i. e., under conditions which make the period of the resulting vibrations
longer, on the whole, than the period natural to the active molecules before
dissociation. Since recombination can occur under a variety of conditions
a wide range of wave-lengths will be represented in the luminescence
spectrum ; the latter also will consist of bands rather than of lines. But
on the whole the wave-length of the light emitted during luminescence will
be longer than that of the exciting light. The same sort of thing will occur,
although in less marked degree, even when the active molecule is itself
electrically charged, as in the case of fluorescein. In this case, as well as
in that first considered, it may even be that each of the two parts into
which the active molecule is dissociated becomes a nucleus to which neutral
molecules are attracted, so that the ions become heavy aggregations of
molecules. This is the assumption usually made regarding the production
of ions in gases by the action of Roentgen rays, kathode rays, etc. If this is
the real condition of affairs, the reasons for increased wave-length in the
light emitted are still more evident.

It will be seen that this way of looking at the phenomena of photo-lumines-
cence gives what might be called a mechanical explanation of Stokes's law.
It does not lead us to expect, however, that Stokes's law will always be
exactly followed. The luminescence spectrum and the absorption spec-
trum may overlap ; in fact, it is to be anticipated that they will do so in the
majority of cases. This is in agreement with the more recent experiments
on this subject.

■Essentially this explanation of the broadening of spectral lines has been discussed in some detail by
Galitzin, Wied. Ann., 56, p. 78. 1895.

I98 STUDIES IN LUMINESCENCE.

Upon the basis of the dissociation theory of luminescence it is clear also
that we should expect no change in the form of the phosphorescence
spectrum during decay.1 The intensity of phosphorescence will depend
at each instant upon the number of recombinations that occur per second,
and will therefore diminish as the number of free ions becomes less. But
it seems probable that the number of recombinations that occur under
such conditions as to give light of a certain wave-length will still remain
the same fraction of the whole number. The case has some resemblance
to that of the distribution of velocities among the molecules of a gas ; if the
total number of molecules in a given volume is diminished, the number
having a given velocity will also diminish ; but this number will still be the
same fraction of the whole.

Probably the most serious objection to the form of the theory of
Wiedemann and Schmidt that is here considered is the absence of any
direct evidence of electrolytic dissociation during the excitation of lumines-
cence. The experiments of Howe,2 although carried out with apparatus of
high sensibility, fail to show any ionization in anthracene vapor when
excited to fluorescence. Ionization of gases by the direct action of ultra-
violet light has indeed been observed in numerous instances, but not under
conditions which indicate any connection with luminescence. No change
in electrical conductivity during excitation, such as might be expected to
result from electrolytic dissociation, could be detected in fluorescent liquids
by Cunningham, Regner, or Camichel. Our own experiments with
alcoholic solutions of the fluorescent dyes seemed at first to indicate an
increase in conductivity during fluorescence in some cases as great as 1 per
cent. But the work of Hodge and of Goldman has shown that these results
were in reality due to a change in polarization or to the establishment of a
photo-electric E.M.F.2

On the other hand, Lenard has found indirect evidence of an increase in
the conducting power of certain phosphorescent sulphides during lumines-
cence. The fact that luminescent substances usually exhibit strong photo-
electric activity, as was first pointed out by Elster and Geitel, also points
to a close connection between ionization and luminescence. Even the
absence of increased conductivity in fluorescent liquids is not so important
as would at first appear, for the solutions tested were those in which the
property of fluorescence resides in the ion. If a negative ion is dissociated
by the separation of an electron no change in the number of ions will result,
and the only change that could be expected in the conductivity is that
which might come from a change in the mobility of the ions. Whether the
mobility would be increased or decreased we have no means of telling; but
that the change would not be great seems reasonably certain.

While the failure of experiments to detect increased conductivity in
solutions thus appears to be without great significance, the fact that cells
containing fluorescent liquids give an E.M.F. , when one electrode is illumi-
nated, which is far greater than that of other photo-active cells, gives another
instance of a close connection between fluorescence and electrical effects.

!The hypothesis of molecular groups developed in Chapter XV in discussing the form of the decay curve
calls for a slight change in the form of the spectrum. But it seems probable that this change is so small
;js to be difficult of detection.

sSee Chapter X.

THE THEORY OF WIEDEMANN AND SCHMIDT. 1 99

It is to be remembered that the rays that produce photo-activity in such
cases are the same that cause fluorescence. Goldman has further shown
that the behavior of photo-active cells with fluorescent liquids is such as to
indicate the liberation of negative electricity at the illuminated electrode
at a constant rate depending upon the intensity of illumination. Goldman's
results are thus in complete accord with the view that fluorescence is
accompanied by ionization.

It appears, therefore, that while there is no direct evidence of electrolytic
dissociation in fluorescent solids and liquids, many phenomena have been
observed which make it seem probable that such dissociation exists. A
detailed study of this very interesting class of phenomena is much to be
desired, and will doubtless suggest some means by which a very fundamental
question in the theory of luminescence may be definitely settled.

In the case of gases the experimental results are less favorable for the
dissociation theory of luminescence, and in the case of fluorescence seem to
directly contradict it. Before accepting these results, however, as con-
clusive evidence of the falsity of the theory, it is desirable to consider the
nature of the differences which the theory would lead us to expect between
the behavior of gases and liquids.

It can scarcely be doubted that in the case of liquids or solid solutions
the presence of the solvent is favorable to the ionization of the solute. The
solvent acts as a catalytic agent, both for ordinary dissociation and for that
which we assume to be produced by light. To form a picture of the process
by which this catalytic action is effected, let us consider a molecule of the
solute (active substance in the case of luminescent substances) which is
subjected to some influence tending to separate it into positive and negative
parts. Such a tendency might result from a violent collision or from strong
resonant vibrations set up by light. As soon as the separation begins it
will be aided by the attraction of the neighboring neutral molecules of the
solvent, and when dissociation has actually occurred the ions are rendered
more sluggish in their movement, and are to some extent prevented from
recombining, by becoming attached to the solvent molecules. If we
imagine the solvent removed, so that we have to deal with a gas instead of
a solution, it is readily seen that the conditions are less favorable to ioniza-
tion for two reasons:

(1) The forces to be overcome in effecting dissociation are far greater,
since there are no solvent molecules to partially counteract the mutual
attraction of the ions ; and for the same reason the ions must be driven much
further apart in order that the separation may be complete;

(2) The tendency toward recombination is greatly increased, both
because of the greater effective attractive forces between the ions and
because of their more rapid motion.

It is not surprising, therefore, that spontaneous electrolytic dissociation
is very rarely observed in gases. HC1 in dilute solution is a good con-
ductor. But the same amount of HC1 in the form of a gas, occupying the
same volume as before, shows no conducting power except such as requires
the most refined apparatus for its detection. Does it not seem reasonable
therefore that the ionization produced in a gas by light should also be very
minute, so that we may hope to detect it only under especially favorable

200 STUDIES IN LUMINESCENCE.

conditions, and by means of apparatus even more sensitive than that thus
far used?

It may be also that some type of dissociation occurs in gases that is inca-
pable of producing a change in electrical conductivity, but nevertheless as
effective as complete dissociation in producing fluorescence. For the sake
of definiteness let is assume that fluorescence is due to the vibrations of one
ring of the Saturnian atom proposed by Sir J. J. Thomson. We can imagine
this ring set into such violent vibration by light of suitable period that an
electron is broken loose. This electron will later return and, by setting
up vibrations when it reenters the ring formation, will cause fluorescence.
But the separation of an electron from such a ring does not necessarily
mean dissociation in the electrolytic sense. In order that complete dis-
sociation should occur the electron must be driven away to such a dis-
tance as to be practically beyond the influence of the remainder of the
atom. The energy required to accomplish this is probably far greater
than that required to break down a ring; and complete dissociation, which
alone can manifest itself by an increase in electrical conductivity, may be
of relatively rare occurrence, even in cases of brilliant fluorescence. It
may be pointed out in passing that the partial polarization observed by
Wood in the light emitted by fluorescent sodium vapor is to be expected
if the view outlined above regarding the nature of fluorescence in gases is
accepted.

In the case of phosphorescent gases electrolytic dissociation is readily
detected so long as phosphorescence lasts. In such cases the dissociation
appears to be complete. The long duration of the after-glow in gases sug-
gests that the ions attach themselves to neutral molecules and in conse-
quence move slowly ; and the fact that the spectrum often consists of bands
rather than lines is in accord with this view. The decay of phosphores-
cence in gases follows the law that is predicted by the dissociation theory
in its simplest form,1 and seems to be almost entirely free from the dis-
turbing influences present in the case of solids.

While we are by no means justified in looking upon the theory of
Wiedemann and Schmidt as finally established, it appears to us, in the light
of the preceding discussion, that it is by far the most promising of the
theories thus far suggested, and that it affords a most useful guide in the
experimental study of luminescence.

■C. C. Trowbridge, Physical Review, vol. 26. p. 515, 1908; vol. 32. p. 129. 191 1.

CHAPTER XV.

THE PHENOMENA OF PHOSPHORESCENCE CONSIDERED FROM
THE STANDPOINT OF THE DISSOCIATION THEORY.

It is our purpose in this chapter (i) to derive the law of decay of phospho-
rescence which the dissociation theory would lead us to expect under such
ideally simple conditions as seem to exist in the case of gases; and (2) to
consider in turn the various modifying factors which may have an influence
on the phenomena of phosphorescence in solids, and to determine so far
as possible the nature of this influence. Comparison with experimental
results will then make it possible to form an opinion of the relative impor-
tance of the various factors considered.

It will facilitate the discussion if we consider first the requirements which
a satisfactory theory must meet. The most important experimental results
in the case of photo-luminescence are briefly mentioned below :

SUMMARY OF EXPERIMENTAL LAWS.

1. Stokes's Law.

2. If we isolate a single band of the luminescence spectrum it is found

that the distribution of intensity throughout the band is inde-
pendent of the intensity and wave-length of the exciting light.

3. The light emitted by an isotropic substance during luminescence is

unpolarized, no matter what may be the condition, as regards
polarization, of the exciting light.

4. During the decay of phosphorescence each band of the luminescence

spectrum behaves as a unit; i. c, the wave-length of maximum
intensity and the relative distribution of intensity throughout
the band remain unchanged.

These general laws were discussed in the preceding chapter and it was
pointed out that they are directly deducible from the dissociation theory.

To these four general laws must be added the following experimental
facts connected with the decay of phosphorescence :

5. Form of Decay Curve. The curve obtained by plotting the values

of I~- as ordinates and the corresponding values of /as abscissas
is a straight line for small values of t; it changes to a curve con-
cave toward the axis of / as / increases; but for still larger values
of / the relation between I~- and / is again linear, and remains
so until / becomes too small to measure. The form of the decay
curve is dependent on the intensity and duration of excitation,
the slant being altered in each of the straight parts by a change
in either of these two factors in the excitation.

6. Hysteresis. The behavior of a phosphorescent substance with a

given excitation depends upon its previous history. Some semi-
permanent change is produced by excitation which persists for
several hours, or even for several days, after visible phosphores-
cence has ceased.

201

202 STUDIES IN LUMINESCENCE.

7. Effect of Red and Infra-red Rays. In the case of certain substances
the semi-permanent condition produced by excitation may be
destroyed and the material restored to a standard state by a brief
exposure to the red and infra-red rays. The effect of the longer
waves during phosphorescence is to accelerate the decay. In
some cases the first effect is to increase the brightness of phos-
phorescence, this temporary effect being followed by decay more
rapid than the normal.

THE DECAY OF PHOSPHORESCENCE UNDER SIMPLE CONDITIONS.

According to the form of the Wiedemann and Schmidt theory that is
here adopted, the effect of the exciting light is to produce electric dis-
sociation of the active substance and the resulting negative and positive
nuclei exist for a time uninfluenced by their mutual attraction. The
vibrations that occur upon the recombination of the ions give rise to phos-
phorescence.

The simplest hypothesis regarding the law of recombination of the ions
in a luminescent substance is that which has been applied to the case of
ionization in gases.1 Let the number of positive ions present per cubic
centimeter at any time t be n. The number of collisions between a positive
and a negative ion will be proportional both to the number of positive ions
and to the number of negative ions; and a certain fraction of these col-
lisions will result in recombination. Since positive and negative ions are
present in equal numbers we have

dn/dt = — a n- 1 /n = c-\-at where c = 1 /n0

vSince the intensity of phosphorescence is proportional to the number of
recombinations per second

/ = kan- = ka/(c-\-at)2

This is one form of the empirical expression originally proposed by
E. Becquerel to express the decay of long-time phosphorescence, and is
the same law that was derived on the basis of entirely different theoretical
assumptions by H. Becquerel.2

In comparing our experimental results with the law just derived, it is
convenient to write the above expression in a different form, namely,

where

V/

a= U b = . '«

n<Ska ^

k

The decay of phosphorescence in gases appears to be strictly in accord-
ance with this law.3 Under certain special conditions as regards temper-
ature, etc., the law is very closely obeyed by zinc sulphide and even by
Balmain's paint.4 But in the great majority of instances the phosphores-

^utherford. Philosophical Magazine, vol. 44, p. 422, 1897.

2H. Becquerel, Comptes Rendus, vol. 113, p. 618, 1891.

3C C. Trowbridge. Physical Review, vol. 26, p 515; vol. 32, p. 129.

4C A. Pierce, Physical Review, vol. 26, p. 312 and p. 454, 1908. See also Chapter VI.

PHENOMENA OK PHOSPHORESCENCE-

203

cence of solids decays in accordance with a more complicated law. As
stated above, the curve obtained by plotting values of I~- as ordinates
and the corresponding values of / as abscissas is not linear throughout, as
the simple theory would lead us to expect. The curve is straight, both
for small values of / and for large values of /, but shows a sharp curvature
for intermediate values.

It has been suggested by H. Becquerel1 that the form of curve observed
in the case of solids is to be accounted for by assuming that the phosphores-
cence spectrum consists of two bands, each of which obeys the simple law,
but having different rates of decay. If a decay curve is plotted for each
of the two assumed bands the equations of the two curves will be

U h = ax+bJ

h 2 = a2-\-b2t

Fig. 187.

Illustrating Becquerel's explanation of the
form of the decay curve in solids.

and each of the two curves will be a straight line. But if we plot /
against /, where / is the observed total inten-
sity (7 = /i+Jr2) the resulting curve will not
be straight but will take the form shown by
curve C, Fig. 187. In this figure A and B are
the decay curves for the two bands taken
separately. In the case represented in the
figure the two bands are assumed to decay at
widely different rates, and the band which
decays more rapidly (.4) has initially the
greater intensity. It will be seen that under
these circumstances the decay curve for total
intensity has all the characteristics of the
curves determined by experiment. Becquerel
cites one substance for which the observed
intensity can be represented by the expression

/=i/(a1+M2+i/(a2+M)2

with remarkable accuracy. In the case of

this particular substance Becquerel also found independent evidence of the

existence of two bands in the phosphorescence spectrum.

This way of accounting for the form of the decay curve was also pro-
posed by Pierce in discussing the decay of phosphorescence in Emanations-
pulver, and has been shown by him to give a very satisfactory explanation
of the change in the form of the decay curve resulting from changes in
temperature.

If Becquerel's explanation of the form of the decay curve is correct we
should expect to find a change in the phosphorescence spectrum during
decay. For, referring to the figure, the light given out during the first
few seconds is chiefly due to band A, while that emitted during the later
stages of decay is almost entirely due to band B. Unless these two bands
are coincident some shift in the wave-length of maximum intensity is to
be expected. Our own experiments with Sidot blende show that no meas-
urable change occurs in the form of the spectrum, or the position of its
maximum, during the first 10 seconds, and the later work of Pierce shows
that the wave-length of maximum intensity is unchanged for at least 1

'Becquerel, /. c.

204 STUDIES IN LUMINESCENCE.

minute.1 The results of Waggoner and Zeller with short-time phosphores-
cence, although less accurate, point to the same conclusion.

In the case of our own observations on Sidot blende we have expressed
the opinion that the curves obtained are to be regarded as showing the
decay of the green band alone; for although Sidot blende also possesses
bands in the blue and violet, these decay so quickly and are of such small
luminosity that they can scarcely affect the curves to any appreciable
extent. In the later work of Werner2 special precautions were taken in
the choice of a substance and in the use of color screens to make certain
that one band only was studied ; yet the curves obtained are of exactly the
same type as those found by other observers.

The work of Werner, even more strongly perhaps than the work of
Pierce and our own work with Sidot blende, thus appears to discredit any
explanation of the form of the decay curve that is based upon the assump-
tion of two bands in the phosphorescence spectrum. It can not be denied,
however, that two bands may well be present, even when special precautions
of this kind are taken, and even when the spectrophotometer shows no
indication of a double band. The bands of a phosphorescence spectrum
are ordinarily so broad that two bands lying close together might readily
appear as one. If the case is one where the bands actually overlap it
would not be possible to make observations on one alone without working
at the extreme edge of the double band, where the intensity wTould prob-
ably be too small to permit of accurate observations. An extremely small
shift in the maximum might also fail of detection even by the photographic
method of Pierce. It does not at present appear possible to devise a method
of obtaining absolutely conclusive proof that Becquerel's explanation is not
correct. Since the assumption of two nearly coincident bands offers so direct
an explanation of the form of the decay curve, and since it seems certain
that curves may be plotted on the basis of this assumption which deviate
from the observed curves by less than the experimental errors, it must be
admitted that the hypothesis has much in its favor.

There are several reasons, however, for looking upon this explanation
of the form of the curve with suspicion. In the first place, it does not
account either for the remarkable changes produced in the decay curves
by varying the intensity and duration of excitation, or for the phenomena
of hysteresis that have been observed in almost all cases of long-time phos-
phorescence. It also leaves untouched the question of the effects pro-
duced by exposure to the longer waves. An even more serious objection
is the fact that it is necessary to assume the existence of two nearly coin-
cident bands in all cases of long-time phosphorescence ; for upon replotting
the decay curves obtained by different observers with /_i and / as co-
ordinates it is found that the curves are of the same type for all substances
thus far tested. It is hardly credible that this is accidental. Only two
explanations appear to be possible : either the existence of two nearly coin-
cident bands is an essential characteristic of substances showing long-time
phosphorescence, or else the peculiarities exhibited in the form of the decay

'This result has recently been confirmed by H. E. Ives and M. Euckiesh (Astrophysical Journal, vol
xxxiv. p. 17.}, iqii). who find that the phosphorescence spectrum remains the same for at least fifteen
minutes after the beginning of decay.

-A. Werner, Ann der Phys., 24, p. 164. 1907.

PHENOMENA OF PHOSPHORESCENCE. 205

curve are to be explained in some manner which does not involve the
assumption of two bands at all. While it may prove of interest at some
later time to develop the theory along the lines suggested by the first of
these alternatives, the present discussion will be based upon the acceptance
of the second, and we shall consider in what ways the form of the decay
curve, as well as certain other peculiarities of phosphorescence, may be
explained in substances possessing only one band in the phosphorescence
spectrum.

ABSORPTION EFFECTS.

If a homogeneous substance possessing only one band in its phosphores-
cence spectrum is uniformly excited throughout, and if the light emitted
by the interior portions suffers no diminution by absorption before reaching
the surface, then according to the dissociation theory here considered the
decay of phosphorescence will be in accordance with the law

V/

These conditions, however, can never be exactly attained. The exciting
light must be absorbed to some extent, for otherwise no energy would be
available to produce phosphorescence. The excitation will therefore be
greatest at the surface, and will diminish, at a rate determined by the
absorbing power of the material, as we pass to points within. Since the
ions are in consequence more numerous in the surface layers, and since
the number of recombinations per second, which determines the intensity
of phosphorescence, is proportional to the square of the number of ions,
it is clear that the light emitted during the early stages of decay will come
chiefly from the surface. As decay proceeds, however, the relatively high
rate of recombination at points where n is large will cause a rapid approach
to uniformity of ionic concentration, and the part contributed to the total
light by the interior of the mass will become increasingly important. At
first the intensity of phosphorescence is approximately proportional to
Vian'2, where V\ is the volume of the surface layer that is chiefly effective.
Later, when the light from the interior becomes comparable in intensity
with that from the surface, the phosphorescence will be approximately
proportional to Voari1, where Vo">Vi. In terms of the total number of
ions, N, the two intensities will thus be approximately proportional to
a/ V\ • N2 and a/ F2 • A2. So far as the slant of the decay curve is concerned
this is equivalent to a decrease in the coefficient of recombination and will
result in making the curve concave toward the axis of /. Absorption of
the emitted light will complicate the phenomena but will not modify the
general result. The effect of absorption is therefore to produce a change
in the form of the decay curve which is at least in the right direction to
account for the observed deviation from linearity.

In discussing the effect of absorption in detail we shall assume that both
the exciting light and the emitted light suffer absorption, the two coefficients
of absorption being /3 and ",' respectively. At any depth x below the sur-
face the intensity of the exciting light will be

E=E0e-fix

206 STUDIES IN LUMINESCENCE.

Assuming that ions are produced at a rate proportional to E and that
excitation has proceeded for a sufficient time to produce a steady condi-
tion, the number of ions per cubic centimeter at any depth x will be deter-
mined by the equation

^ =o = //£-a«o2 = //£o^v-a«03 «o= J~ e"w*

rfJ \ a

If w is the number of ions per cubic centimeter at the time / we have

i

n = — —

i/no+at

and the light emitted per unit volume, denoted by i, is

. . 2 pa-

i = paw = - — — -

(i;>io+at)-

p being the light emitted as the result of one recombination.

Since the emitted light suffers absorption, the amount contributed to the
total observed intensity by a layer of thickness dx will be

idxe yx = -r~. — ■ — r-
(i/no+aty

and the total intensity is

•» e~yxdx C* e~yxe~ffxdx

.co e~y*dx /'" e r*e "*dx

where

Putting
so that

a

a = \ -—

a/e~^/2 = 3

apt -^ /«\*

d z = e 2 dx and e yx

•(£)*

_ 2pa f° zm+ldz
~ p(at)m+2t'at {a+zY

where w = 2/7/3.

Successive partial integration gives

2 Sm+:?

2pa f i zm+i
I==~ p(at)m+2L^+^ (a+z)2 + w + 2-m+3 (a+z)3

W+2-W+3- w+4(a+s)4 "j,,

rm-M -i0

Upon putting in the limits this becomes
2 pa

1 =

{m+2

)/3'(a+a/)2LI^w+3'a + af w+3- m+4 \a+at) J

PHENOMENA OF PHOSPHORESCENCE.

20"

The series in the brackets has the value 1 for / = o and as t becomes large
approaches the series

2-3

1 +

+

+

= S

w-+3 W+3.W+4

If the decay curve is plotted in the usual way with 7_i and / as coor-
dinates it is clear that the change in slant in passing from small values of /
to large values will depend upon S and will be greatest when S is greatest.
The maximum deviation from linearity in the form of the decay curve
which can result from absorption will therefore be produced when m = o,
i. e., when T = o. To determine whether absorption is sufficient to account
for the observed type of curve we shall therefore consider this special case
for which the effect is greatest.

For m = o we have

2 pa /'° zdz 2 pa

(at)- J* "(a-J-^2

/ =

/3(a/)2*'«< (a + 2)2 j8(a/)

Putting in the limits this becomes

1 r z

)2l~ a + z

+ log (a+z)

]

al

1 =

ipa
0(a/)s

log

(.+^-

a J

at

a

.+*

a

Writing ft for at/a

1 =

2pa

i[log(l+*)__L]

This

The curve for 0 and /~*, computed in accordance with this equation, is
plotted in Fig. 188, the quantity 2/>a/a2/3 being put equal to unity,
curve may be looked upon as show-
ing the relation between /-* and /
for a/ a = 1 . The curve for any dif-
ferent value of a/a may then be
found by changing the vertical and
horizontal scales to correspond to
the change in a/a.

The curve of Fig. 188 is similar
in form to the decay curves observed
in the case of short excitation.
Pierce1 has also observed curves of
nearly this form with zinc sulphide.
It is possible, therefore, that the
relatively slight deviation from lin-
earity in such cases is to be ascribed
to absorption. But in the case of the majority of decay curves it is clear
that absorption is not to be regarded as an important factor; for no
change of scale can bring about any close resemblance between the curve
of Fig. 188 and the decay curves usually observed for long and moderately
long excitation.

8
I

vr

a e

Fig. 188.

»C. A. Pierce, Physical Review, xxvi, p. 314 (see Chapter VI, Fig. 77). In our own work with this same
substance, " Emanations-pulver," we found a decay curve of the usual type, i. e., with a well-marked "shoul-
der" at (=20 sec. (Physical Review, xxvin, p. 50, or Chapter IV, Fig. 59). It is interesting to inquire
whether the difference is due to the repeated heating and cooling to which Pierce's material was subjected.

2o8 STUDIES IN LUMINESCENCE.

INFLUENCE OF IRREGULARITIES IN DISTRIBUTION OF THE ACTIVE MATERIAL.

Luminescent substances are in most cases solid solutions. In fact it is
doubtful whether luminescence can occur in an absolutely pure substance.
Pure calcium sulphide for example is not phosphorescent; but the addition
of a small amount of some other metal, such as manganese or copper, gives
it the power to phosphoresce brilliantly. The method of preparing the
phosphorescent sulphides is such as to bring about a very intimate mixture
of the constituents, and it is natural to think of the manganese, copper, or
other active material as being dissolved in the sulphide.

So little is known regarding the nature of solid solutions that we can not
say with certainty whether such substances are to be regarded as strictly
homogeneous or not. Especially in the case of crystals it seems probable
that the molecules of the solute may not be uniformly distributed through-
out the mass of the solvent, but may to a greater or less extent collect in
groups or minute crystals. Any lack of uniformity in the distribution of
the active substance will cause a corresponding variation in the concentra-
tion of the ions produced by the exciting light; and if the nature of the
solvent is such as to permit of diffusion the phenomena will be complicated
by the fact that a redistribution of the ions will occur during excitation and
decay. Even without diffusion, however, the form of the decav curve will
be modified. For, since the rate of recombination is proportional to the
square of the ionic concentration the intensity of phosphorescence will
decay at different rates in different parts of the mass. The effect on the
decay curve will be similar to that produced by absorption; in fact, the decay
curve is modified by absorption of the exciting light only because of the
resulting lack of uniformity in the ionic concentration at different depths
below the surface.

Our complete ignorance of the distribution of the active material makes
it almost useless to attempt any exact treatment of the problem. It is
possible, however, without questionable assumptions or great analytical
complexity, to predict the general character of the effect to be expected.

Let n be the number of ions per unit mass at any point ; /; is then some
unknown function of the position of the point in question and of the time
that has elapsed since the decay began. The number of recombinations
per second will be an- per unit volume, and the number of recombinations
for the whole mass will hejairdr, where dr is an element of volume. Dis-
regarding the absorption of the emitted light we have

= ka ( ir<

Since ions are destroyed only by recombination

dN r 2,

= — a I ii-dr
dt J

where N is the total number of ions.

If n is the volume average of ;/, and n- the volume average of n2, so that

A — i /•. , , dN

n

= - I ndr=- , ii-= - I trdr, — = —am-

t *' t t J dt

PHENOMENA OF PHOSPHORESCENCE. 20Q

If we put

p=~ri, m2 = p(«)2 = p— and — - = -aTp— = A2

(«)- T" a/ T-1 T

A7 A^o t «-ro
Writing pi for the average value of p between o and /

i _ ,dN ka Jt '

N= — I=-k — = -•- —

i api a/ / /.— \ -s

A^o r

\N0 T )

i cx &

Pl= - I pj/, -J (pit) =p

t »7o a/

Although we are unable to determine p and pi as functions of / it is clear
that both will decrease as t increases. It is clear also that the change will
become less rapid as the decay proceeds, and that both p and pi will sooner
or latter become nearly constant. When /~* is plotted against / we shall
therefore obtain a curve which is concave downward and which finally
becomes a straight line. In other words, the decay curve will agree in
form with the curves determined by experiment.

The effect upon the decay curve of an irregular distribution of the active
substance may be illustrated by the following simple case : Let the active
material be uniformly distributed, except that small regions occasionally
occur where the concentration is abnormally large. The whole volume
may thus be divided into two parts, V\ and v2. The distribution of the
active material is uniform throughout each part, but the concentration in
vi is different from that in z>2. When the substance is excited to phosphores-
cence the ionic concentration will also differ in the two regions. Let the
number of ions per cubic centimeter at the end of excitation be ii\ in the
volume v\ and «2 in the volume i>2. Throughout the region vx the intensity
of the light emitted per cubic centimeter will be

ak
t =

(i/m+aty

and the total intensity at any instant due to this part of the whole mass
will be

1_(i/»i+a/)~2
The light emitted by the volume v2 is given by the similar expression

akvi
(i/w2+a/)2

2IO STUDIES IN LUMINESCENCE.

and the total intensity is

where

I = I +j0 = aj, ( l __| I \

i i . a , a

fli= , "2= -7= 61= , 02 = —;

It thus appears that the form of the decay curve is the same as though
the substance possessed two bands, coincident as regards wave-length, but
differing in rate of decay.

It is not at all unlikely that a condition approaching that assumed in
this illustrative case actually exists in most phosphorescent substances.
Upon considering the method used in preparing the phosphorescent sul-
phides, for example, it seems probable that the distribution of the active
substance will be far from uniform. When the mixture is first prepared,
and before calcination, the active material is unquestionably in the form
of small discrete masses distributed irregularly through the mixture. Upon
heating to redness diffusion will occur to a greater or less extent, depending
upon the temperature and the duration of heating. But even at high temper-
atures this will be a slow process, and considerable variations in concen-
tration are likely to remain even after prolonged heating. It is to be
expected, therefore, that the phosphorescent sulphides will contain numer-
ous nuclei of high concentration surrounded in each case by a region where
the concentration is relatively small.1

After calcination it is often noticed also that the phosphorescence is very
far from being uniform throughout the mass. Owing probably to accidental
differences in concentration, or to differences in the heat treatment, the
phosphorescence often differs greatly in intensity and even in color in
different parts of the same mass. A phosphorescent powder made from
such a mass, while presenting the appearance of homogeneity to the un-
aided eye, would differ greatly from point to point in the concentration of
the active material.2 Even if the mixture were so perfect that no irregu-
larities could be detected with the microscope, a wide deviation from line-
arity in the decay curve is to be expected. It is a significant fact that
with one exception all decay curves thus far recorded have been determined
with powders prepared in practically the same way that the phosphorescent
sulphides are prepared. In fact, most of the substances tested were sul-
phides. The exception noted above was natural willemite,3 in which case
the variation in brightness over the surface tested was plainly visible.

'Since a more complete diffusion of the active material will result from prolonged heating it is to be
expected, other conditions being the same, that the duration of phosphorescence will be prolonged by in-
creasing the time of heating. This agrees with the facts observed in the preparation of the phosphorescent
sulphides.

2 While we are chiefly concerned at present with the influence of lack of homogeneity upon the decay curve,
it can scarcely be doubted that the effect upon the phosphorescence spectrum is fully as important. For
some reason different parts of the mass are differently affected by the process of calcination. This may be
due to differences in concentration throughout the mass, or to the fact that the effect of the surrounding
gas varies from point to point. When such a mass is powdered and mixed the effect is the same as though we
were to make an intimate mixture of several entirely different phosphorescent substances. Each constituent
has its own band or group of bands, characterized by definite wave-lengths and periods of decay, which
differ according to the conditions of preparation. It is not surprising that the phosphorescence spectrum
of such a material is complex; and we can scarcely expect simple laws to apply to any of the phenomena
exhibited by such a mixture.

The decay of phosphorescence in a specimen of willemite possessing a long-time phosphorescence has
been studied by Nichols and Merritt, Physical Review, XXIII, p. 52, Fig. 53. Willemite whose phosphor-
escence dies out with great rapidity has been studied by Waggoner, Physical Review, XXVH, p. 209. See
also Chapters, IV and VII.

PHENOMENA OF PHOSPHORESCENCE. 211

It thus appears that irregularities in the distribution of the active sub-
stance are sufficient to explain the deviation from linearity in all the decay-
curves thus far observed. Such irregularities of distribution are not merely
probable, but in many cases are perfectly obvious. But while the dis-
tribution of the active material is probably in all cases an important factor,
it can not be the only factor of importance. With Balmain's paint, Pierce
has found that the decay curve, which possesses the usual shoulder at
ordinary temperatures, becomes almost exactly linear at a temperature of
74°, ' while for higher temperatures it again shows a curvature of the usual
kind. It appears highly improbable that such changes are brought about
by temporary changes in the distribution of the active substance. Non-
uniformity in the distribution of the active material also offers no explana-
tion of the phenomena of hysteresis or of the effect of the infra-red rays.

DIFFUSION EFFECTS.

Whenever irregularities exist in the distribution of the active substance
there will be a tendency for diffusion to occur. Under ordinary conditions
this tendency is probably neutralized by forces which act to keep the dis-
tribution unaltered, and so long as the substance remains in the molecular
form the condition is to be regarded as a stable one for the temperature and
pressure at which the phosphorescent substance normally exists.

But when the material is excited to luminescence a part of the active
substance will be dissociated, and since the resulting ions will possess a
different mobility and will be acted upon by different forces from those
that determine the behavior of the original neutral molecules, the condition
of equilibrium will be destroyed, and some change in the distribution of
the active substance is to be expected.

An exact discussion of the effects of diffusion would present great diffi-
culties; for the fact that diffusion and recombination occur at the same
time greatly complicates the analytical treatment. It is probable also
that in crystals the diffusion constant will be different for different direc-
tions. It is clear, however, that the influence of diffusion upon the form
of the decay curve must be similar to that produced by an irregular dis-
tribution of ions without diffusion. In fact, the discussion of the preceding
section applies without modification to the case of substances in which
diffusion of the ions may occur. Diffusion, however, will increase the
rapidity with which uniformity of ionic concentration is approached dur-
ing decay ; and since diffusion will be most active when large concentration
gradients exist, the effects of diffusion will be greatest in the early stages
of decay. In cases where diffusion is an important factor we should there-
fore expect a sharper curvature in the early part of the curve, and a more
rapid approach to linearity, than in cases where diffusion is absent.

It is not impossible that diffusion is sometimes important even when
there is nearly complete uniformity in the distribution of the active sub-
stance. Owing to the absorption of the exciting light the ionization pro-
duced in the surface layer will be more intense than that produced at points
beneath the surface. If the absorption is large a large gradient may thus

■Physical Review, xxvi, p. 458. See also Chapter VI.

212 STUDIES IN LUMINESCENCE.

be produced in the ionic concentration, and diffusion from the surface
layers inward will bring about a change in the decay curve similar in char-
acter to that caused by irregularities in the distribution of the active sub-
stance. Diffusion produced in this way would ultimately result in an appre-
ciable diminution in the surface concentration of the active material, and
we should therefore expect that the intensity of luminescence would be
diminished in such a substance by prolonged excitation. No effect of this
kind has been observed by us in the case of Sidot blende, but the obser-
vations of Werner1 with a SrZn compound show evidence of fatigue resulting
from prolonged excitation.

The change in the form of the decay curve due to changes in the duration
of excitation may be explained, at least in a general way, as a result of
diffusion. Diffusion of the ions will occur during excitation as well as
during decay. After prolonged excitation, therefore, the volume occupied
by ions will be greater than after short excitation, and the rate of decay — in
other words the slant of the decay curve — will be correspondingly reduced.2

Diffusion also offers an explanation of the phenomena of hysteresis in
phosphorescent substances. After prolonged excitation and subsequent
decay the neutral molecules that result from recombination will be dis-
tributed through a larger volume than before. Since the original distribution
of the active material was a stable one there will be a gradual return to the
normal distribution. But this will be a slow process and may well require
several days for its completion. In the mean time the material is in such
a condition that the decay following renewed excitation will be more
gradual than the normal, even for a short excitation. The spreading out
of the active substance by diffusion, which would normally require a long
excitation, has already been accomplished by the preceding excitation,
whose effects have not yet disappeared. Hysteresis effects such as those
discussed in Chapter IV are therefore to be expected.

If this explanation of hysteresis is correct the effect of the infra-red rays
must be to facilitate the return of the substance to its normal condition;
in other words, to increase the rapidity of the diffusion by which the original
distribution of the active material is restored. It is natural to expect such
an effect in substances which are able to absorb the infra-red rays. But if
the restoration is accomplished by the diffusion of neutral ions the rapidity
of the action is surprising, for with strong infra-red rays we have found that
an exposure of only a few seconds is sufficient to restore Sidot blende to
its standard condition.3

INFLUENCE OF IONIC GROUPING.

In discussing thermo-luminescence and the effect of infra-red rays upon
phosphorescence Wiedemann and Schmidt4 have suggested that some of
the ions produced during excitation form semi-stable combinations or

'A. Werner, Ann. cler Phys., 24, p. 164, 1907.

!If the volume occupied by ions is vi for short excitation and »j for long excitation, the ionic concentra-
tions will be n/vi and njvi respectively, and the two intensities of phosphorescence will be proportional, to
an2/vi and an/2/vi. Prolonged excitation therefore produces an effect which is equivalent to a diminution
in the coefficient of recombination.

aSee Chapter V.

"Ann. der Phys., 56, p. 247, 1895.

PHENOMENA OF PHOSPHORESCENCE. 213

groups with the neutral molecules of the solvent, and that these groups may
afterwards be broken down, and the ions liberated, by rise in temperature
or by the absorption of infra-red rays. The formation of such groups as
the result of ionization seems extremely probable, especially in the case of
solids, and can scarcely fail to be of importance in any satisfactory theory
of phosphorescence. Introducing such additional hypotheses as are neces-
sary to give definiteness to the suggestion of Wiedemann and Schmidt, let
us consider what the influence of such ionic groups will probably be.

We shall assume that the first effect of the exciting light is to produce
dissociation in a part of the active material. The dissociation assumed
may be either chemical or electrolytic1 and if of the latter type it may either
be similar to the dissociation of ordinary electrolysis, or may consist of the
expulsion from the molecule of one or more electrons, and thus resemble
more closely the ionization of a gas by X-rays. For the sake of definiteness
we shall assume that the effect of the exciting light is to produce such violent
vibrations as to liberate a single electron from the molecule.

The two ions produced in this type of dissociation will differ greatly in
mobility. The negative ion, owing to its small mass, will possess a velocity
hundreds of times greater than that of the heavy positive ion, and in con-
sequence will move about in the substance with considerable freedom.
While the electrons will at times attach themselves to the molecules of the
solid solvent this condition will usually be only temporary. We may
assume in general that a constant fraction of the whole number of negative
ions consists of electrons that are moving freely. The positive ions on the
other hand will possess only a small mobility. While some of these ions
will remain free, it is to be expected that many will attach themselves to
molecules of the solvent or to undissociated molecules of the active sub-
stance. It is to be noted that the small velocity of the positive ions makes
it probable that the groups formed by the union of a positive ion with a
neutral molecule will be more permanent than similar groups formed by the
negative ion'

The collisions between positive and negative ions, which lead to the more
or less gradual decay of the ionized condition after the exciting light has
ceased to act, will be of three different kinds: (1) collisions between a nega-
tive ion and a free positive ion; (2) collisions between a negative ion and a
positive ion that has attached itself to a neutral molecule of the solvent, and
(3) collisions between a negative ion and a positive ion that is attached to a
neutral molecule of the active substance. The number of modes of recom-
bination may in fact be greater than three, since the positive ion may become
the nucleus of more complicated molecular groups. But we shall restrict
the discussion to cases in which there are three modes of recombination.

When recombination occurs it is to be expected that vibrations will be
set up in the resulting neutral molecule, and these vibrations, in the theory
here considered, are assumed to be the source of the light emitted during
the phosphorescence. But the vibrations corresponding to the different

'Since the electro-magnetic disturbance that constitutes light can get a hold on the molecules of the active
material only by exerting forces upon the electrical charges in the molecule, and will always tend to separate
the positive and negative parts, it appears probable that the first effect of the exciting light is always to
produce some type of electrolytic dissociation, and that any chemical changes which may be exhibited are
secondary effects.

214 STUDIES IN LUMINESCENCE.

modes of recombination will probably differ in violence, in frequency, and
in radiating power.

Of the total number ;/ of positive ions at any time / let <pn be free, and
\pn attached to neutral molecules of the active substance ; then (i — <p — \p) n
will be attached to molecules of the solvent. <p and \p are proper fractions
whose sum is less than unity, and which, in general, are functions of I;
for the distribution of the positive ions will be subject to alteration during
the decay of phosphorescence as well as during excitation. The number
of recombinations between a free positive ion and an electron in unit time
will be proportional to the number of free positive ions <pn, and to the
number of free negative ions //, and may therefore be put equal to anpu2,
where ai is the coefficient of recombination. If the energy radiated as
light due to one recombination is pi the intensity of the phosphorescence
due to this mode of recombination will be

1 1 — piOLnpll2

Similarly

1 2 = p-id-2^ n- I?, = p3a3 {\ — <p — ip) n?

for the other two types of recombination.

The total phosphorescent light will thus consist of three parts. If the
different modes of recombination give rise to vibrations of different fre-
quency the phosphorescence spectrum will consist of three bands which
decay at different rates, and which may differ widely in intensity.

The determination of the law of decay of phosphorescence upon the basis
of the theory just outlined is thus seen to involve the determination of
n, ip, and \p as functions of /. In the general case the solution presents
difficulties that are wellnigh insurmountable. The problem may be sim-
plified, however, by an assumption which, while doubtless not exactly
true, probably gives in the majority of cases a close approximation to the
actual conditions. It is assumed, namely, that ai = a2 = a.-!; in other words,
the probability that a collision will result in recombination is the same for
the three types of collision. In this case

{i) — = — a<pu2 — a^ir — ad— ^ — \^)n2 = —an2

at

(2) n

i/wo+a/

where Vo is the number of positive (or negative) ions when excitation ceases.
To determine <p and \p as functions of / it is necessary to make some
assumption regarding the rate at which the free positive ions attach them-
selves to neutral molecules. Since the number of neutral molecules is
presumably large as compared with the number of ions, it is reasonable
to assume that the rate at which new groups are formed is proportional
to the number of ions that are still free to enter into such combination.
In other words, we may assume knpn and k<npn as the rates of formation of
new groups of the two possible types. Since the positive ions, whether
free or attached, are also recombining with the negative ions we have

(3) - <>") = -a-ipn2- (ki-\-k2)<pn

at

PHENOMENA OF PHOSPHORESCENCE. 215

While positive ions will occasionally break loose from the groups to
which they have attached themselves and return to the "free" condition,
this will occur only rarely on account of the sluggishness with which these
ions move. The small positive term due to this cause which should appear
in the right hand member of equation (3) is therefore negligible and may be
omitted. At high temperatures, however, and perhaps under other special
conditions, the omission of this term will no longer be permissible. If the
molecular movements are sufficiently violent, due to high temperature or
other causes, the formation of groups may even be prevented altogether.
Such cases will be considered later.

Equation (3) may be written

d dn d<p . ,.

-j\ipn) = ip jT+n— = —a<pir-{ki+k2)<pn
at dt at

Remembering that dn/dt = —an'1 this becomes

— =-(*, + £,),£ <p = <pQe-mt

where m = k\-\-k% and (po is the value of (p for / = o.

In the case of the positive ions that are attached to neutral molecules of
the active substance recombinations are occurring with negative ions at
the rate a^;r, while new groups are being formed by the attachment of
free positive ions at the rate ki<pn

.'. ■■-(4>n)=—ail/n2+ki(pn
at

or

dn d\j/
(5) 4 (U +« — =-aif,n-+ki<pn

Since

dn

, = — an1
dt

we have

d± =k^ = k^e-' /.*=- *«V-+ const.

Putting \po for the value of \p when / = o

(6) * = *o+^(i-e-w0

m

For the intensities of the three constituents of the phosphorescent light
we have therefore

.—mi

Il = pxaipn1 =

pia<poe~
(1 ;/,,+a/)2

M*«+JiW»(i-«""0]

h = p3a(i—tp—ip)nz =

(i/no+at)2
pza\i — ^0— knpa/m — kjipghn ■ c~mt)}

(j/tto+at)

2

2l6 STUDIES IN LUMINESCENCE.

Since we have usually plotted the reciprocal square root of the intensity
of phosphorescence, it will be convenient to use the same procedure here.

i/Vj> -t=L= eim'(i/Wo+a/)
, /— i/wo+a/

i/V/2=

i/VS-

V[^T^?oM(l-«"W,)j

i/wo+a<

v pa[i — 4*0— knpo/m — k<?o; m • e~mt\

Replacing certain groups of constant terms by single letters for brevity,
these expressions may be written

i/y/T^A^'ii/no+at)

i/np+at
^Ao-Boe-™1

I/V7

2:

i/V/3 =

i/np+at
^Az-Bze-

If a curve is plotted for i/V 7i in accordance with the above equation it
Is found to be of the type shown in Fig. 190 for 7 = 0. The curve for 1/ V fo
and i/V Ii is concave upward during the early stages of decay, becoming
practically straight when / is so large as to make e~mt inappreciable.

No decay curves corresponding to either of these types has been observed.
It must be remembered, however, that the effects of absorption, diffusion,
and irregular distribution have been neglected in the present discussion.
It is probable that one or more of these sources of disturbance are present,
in which case the theoretical decay curves would be modified in such a way
as to make them correspond more nearly with the curves actually observed.
If m is large, for example, the curve for 72-' will be straight except in the
immediate neighborhood of t = o. Irregularities in the distribution of the
active substance would then produce a deviation from linearity, as previ-
ously discussed. It appears probable that the observed facts, in the cases
of phosphorescence thus far studied, are best explained in this way. It is
interesting to note, however, that curves may be obtained as the result of
ionic grouping which agree very closely with the experimental curves, even
on the assumption of a uniform distribution of the active material. If, for
example, 7\ and 72 both contribute to the phosphorescence, while 1 3 is zero
or negligible

(7) I/V7= i/no+a/

<A+Be~mt

where

\po~\- — J B = pia<pu — p-2.a

PHENOMENA OF PHOSPHORESCENCE.

217

In case all three modes of recombination contribute to the phosphores-
cence we obtain a similar expression for Z-*, except that A and B now have
the values

A-p*(*>+ ^°) + ^1a(i-,0- kl<P°)

D . &1<£>0 k2(fO

3 = PiCUfo — ViO. Pz&~

m m

1

vr

/b

/c

D/

/ ?

O"

E

c

*uu

300

200

/

A

7

<

t — -f-

IUU

20

4.0

60

80

Seconds
Fig. 189.

100

120

14-0

1 60

Observed decay curves for different durations of excitation compared with curves computed

from the equation

i i/tii+at-

V/ " V B(y+e->»t)

The times of excitation and the relative numerical values of the constants used in computation are as follows:

Curve.
E
P
C
B

Excitation.
60 sec.
37
1 1 .6

54

y
0.36
0.4

O. I

o. r

1 /No

72
167
336
650

2.18I a =4.8
4 \m =0. 16

4-35J

In Fig. 189 several curves have been plotted for I~'J by means of this
equation. Values of the constants have been determined by trial so as
to make these curves correspond as nearly as possible with the series of
experimental curves obtained with Sidot blende by varying the time of
excitation.1 Observational points are indicated by circles. It will be
seen that, except in the case of one point on curve D, the agreement is highly
satisfactory.

To give an idea of the character of the decay curves which might result
from the presence of grouped ions (disturbances due to diffusion, absorp-
tion, etc., being neglected) we may write equation (7) in the form

1

V7

i/tio-\-at

(i/wo+a/).

V B Vt +

-mt

V5

'Nichols and Merritt, Physical Review, xxni. p. 45. See also Chapter IV.

218

STUDIES IN LUMINESCENCE.

where

T =

Vt +

-mt

and 7 = A 'B

The first factor alone plots as a straight line. The deviation from
linearity in the curve for I~^ is therefore determined by the second factor
T. Putting m = i we have computed T as
a function of / for several values of 7, the
results being plotted in Fig. 190. For
values of 7 ranging from o. 1 to 0.5 the
curves are of such a character as to cor-
respond with the experimental curves for
/~*. But for smaller values of 7 a double
curvature is shown which is not found in
any of the curves that we have deter-
mined. There is, however, an indication
of such a double curvature in some of the
results of Pierce. It is clear that disturb-
ances, due, for example, to irregularities
in the distribution of the active material,
might so alter the early part of the curve
as to eliminate any peculiarities of this
nature. In fact, there is at present so
much uncertainty regarding the relative
importance of the different factors that
influence the form of the curve for small
values of / that it is difficult to reach any
definite conclusions concerning this part

of the curve. Experiments bearing upon My + e-

the distribution of the active material, the rate of diffusion of the ions, and
related matters are greatly needed.

T

4

/

<

.fr

i

/

i

o.i

_

1/

/\

r =

0.2

z

///y^\

Jy y

r=l.

ao

60

4-0

Seconds
Fig. 190.

Curves showing relation between T and I for
different values of 7 where

I

T=

J-

HYSTERESIS, TEMPERATURE EFFECTS, ETC., EXPLAINED BY IONIC GROUPING.

To account for the hysteresis exhibited by phosphorescent substances,
in other words, the effect of a previous exposure upon the phosphorescence
produced by a given excitation, it is necessary to consider the essential
difference that probably exists between the groups formed by the union of
positive ions with neutral molecules of the active substance and those
formed by the attachment of positive ions with molecules of the solvent.
For the sake of brevity, as well as for the reasons that will appear shortly,
we shall refer to the former as favorable groups and to the latter as unfavor-
able groups.

We have already introduced the assumption that light is produced by
the recombination of a negative ion with a favorable group, while the re-
combination of the unfavorable groups, at least in some cases, gives out no
light. A difference is to be expected also in the behavior of the neutral
molecules that result from recombination in the two cases. A favorable
group consists in a positive ion attached to at least one neutral molecule
of the same sort. It seems natural to expect that forces similar to those
that hold together the molecules of a crystal may cause this grouping to

PHENOMENA OF PHOSPHORESCENCE. 2IQ.

persist even after recombination has occurred. In the case of the unfavor-
able groups this tendency to persist can scarcely be present in the same
degree if at all.

The condition of the phosphorescent substance is thus different after
phosphorescence has ceased from what it was before excitation. The
difference consists in the presence in the mass of a larger number of grouped
molecules of the active substance, which are so intimately connected that
when one member of the group is dissociated during subsequent excitation
its positive ion is in a position to form immediately one of the groups
favorable to phosphorescence. After the substance has been excited it is
therefore in a condition which enables a subsequent excitation to produce
a larger proportion of favorable groups than would be produced by a simi-
lar excitation of the fresh substance. In other words, the \J/0 of equation
(7) is increased.

During excitation we have dissociation and recombination taking place
at the same time; and the recombinations that occur during excitation
will bring about the same change in the condition of the substance that we
have assumed during decay. The value of \pn will therefore increase with
the duration of exposure to the exciting rays. Prolonged excitation, up
to the point where saturation is reached, also increases the number of ions,
i. c, the value of «0- A series of decay curves for different times of exposure
should therefore resemble the curves of Fig. 189, which are computed from
equation (7) by giving progressively increasing values to the constants
\po and n0.

The new condition in which a substance is left after phosphorescence
can scarcely be one of complete stability. The natural and stable arrange-
ment of the molecules is that of the substance before it has been disturbed
by the action of light. It is to be expected, therefore, that there will be a
more or less gradual return to the normal state after the light has ceased
to act. The recombination of the ions produced by excitation, with the
accompanying phosphorescence, forms only one stage in the complete
return to the normal state, and is followed by a more gradual breaking
down of the molecular groups resulting from recombination. We thus
have an explanation of the effect of rest. The effect of exposure to infra-
red rays and of elevation of temperature is to hasten the return to the
normal state by increasing the rate at which these groups disintegrate.

To account for the action of the infra-red rays during excitation and
decay it is only necessary to assume that these rays also have the power
of breaking down the "favorable groups." In the case of Sidot blende
the effect on the unfavorable groups appears to be inappreciable. Upon
exposure to infra-red during decay the first result is to diminish the number
of favorable groups, and to correspondingly increase the number of free
positive ions. This, by itself, will not greatly alter the intensity of phos-
phorescence; for in the phosphorescence of Sidot blende the recombina-
tion of a free positive ion appears to be nearly or quite as effective as the
recombination of a favorable group. But an increase in the number of
free ions causes an increase in the rate at which unfavorable groups are
formed. The positive ions that are shaken loose from the favorable groups
therefore pass quickly into the inactive condition, and a rapid diminution

220 STUDIES IN LUMINESCENCE.

in the intensity of phosphorescence results. The rate of decrease of
course depends upon the intensity of the active rays.

If the infra-red rays are allowed to act for a short time and are then
cut off, the condition of the phosphorescent substance will differ in two
respects from that which it would have reached during ordinary decay:
(i) the number of favorable groups is less than it would have been with-
out the action of the longer waves; (2) the number of free ions is, at least
to some extent, in excess of the normal. After the infra-red rays have
ceased to act, however, the free ions will soon form groups again, either
favorable or unfavorable, and the decay curve will quickly return to the
standard form. But the number of favorable groups will be less than if
the infra-red rays had not acted. The effect of exposure to the longer
waves is simply to bring the substance quickly into the same condition that
it would ordinarily acquire only after a much longer period of decay. The
theory here discussed is thus seen to be in agreement with the experimental
results recorded in Chapter V.

It seems probable that in some substances certain rays, presumably
in the infra-red, may have the effect of breaking down the unfavorable
as well as the favorable groups. In such cases exposure to these rays
would probably bring about an increase in the brilliancy of phosphores-
cence instead of a decrease. This is the case with certain of the phospho-
rescent sulphides. The same effect would be produced in substances where
the recombination of a free positive ion gives out more light than that of an
attached ion, i. e., where pi> Pz.

In cases like that of Sidot blende the form of the decay curve as modified
by exposure to infra-red rays may be determined as follows :

We shall assume that the rate at which favorable groups are broken
down is proportional to the intensity of the active rays Ir and the number
of favorable groups present (\pn). We therefore have

(8) - (\[>n) = — a\f/n- — Ripn — ki<pn

at

where R is proportional to Ir.

The destructive effect of the infra-red is so great, even for rays of small
intensity, while k\ is so small, that the last term knpn may in general be
neglected. It is only in the case of exposure to very weak rays, or with
substances which show the infra-red effect in small intensity, that the
omission of this term will lead to appreciable errors. Equation (8) there-
fore becomes

,dn dip , dip . „ , d\p „ ,

Y , -r« , = —ayri- + n , = — a\pn- — Rwn , =—R4>

dt dt dt dn

(9) ^ = ^i«

-Rt

where / is reckoned from the time when exposure to the infra-red rays begins.
If we take / as the time that has elapsed since the end of excitation we have

4/ = 4/ie-R{l-h)

PHENOMENA OF PHOSPHORESCENCE- 221

where /i is the time at which the longer waves begin to act and i/'i is the
value of \p at this time.

To determine the number of free positive ions pit after the infra-red rays
begin to act we have the equation

— (<pn) = —a.<pn~-\-R\pn — k«<pn
at

dn dip dip . .

^t:+» y. =—<pawt+n- = — a<pn- + R^n — k2<pn
at at dt

ft+k2<p = R + = R + 1e-R^
do) <p=<p1e-k«-»+1^-e-R«-»

K2 — K

An interesting special case, corresponding to the experimental condi-
tions in much of our work with Sidot blende, is that in which the exposure
to infra-red rays begins at once when excitation ceases, i. c, ti = o. In this
case

<Px = <Po ^1 = ^0 <p=<Poe~kit+^-(e-m-e-ktt) ^ = ^,e~Rl

K2 — R

I = p1a<pn-+p2a^n- = (A1e-Rl+Blc-k2t)n-
where

£2 p / Rfn \

Ai = p2(i2\(/o- hi = pia1[<po—- -J

K2 — R \ k2— RJ

For the special case where p\(x.\ = P2&2,

i i/m-\-at

7) =

V/ I ^o k

Vk2-R V k2-RJ

C2~ J^ \ K2

If the infra-red rays are intense so that R/kz is large
i i/no + at

Vl ^hfo/R • e~Rt + (^! + to)e-kil
and in the extreme case where R may be treated as infinite

i i/no+at

This gives a curve having the same form as the observational curve shown
in Fig. 72. The data of Fig. 70 will be found to give a curve of the same
form for i/V /', but the curve has been plotted only in the case of Fig. 72.

At high temperatures the increased activity of the molecular movements
will tend to prevent the formation of groups, or rather to destroy the
groups almost as quickly as they are formed. All those phenomena that
depend upon the existence of groups will therefore become less prominent

222 STUDIES IN LUMINESCENCE.

as the temperature rises. Sooner or later a temperature will be reached
where groups can no longer exist. At temperatures above this point we
should expect no effect from exposure to the infra-red, and all hysteresis
effects should disappear. The experiments of Pierce show that hysteresis
still persists in Balmain's paint at 1330 C. No other data bearing upon
this point are available.

It would be a matter of no great difficulty to develop the theory of ionic
groups in much greater detail than has here been attempted, for when
uncomplicated by the effects of irregular distribution and diffusion the
theory lends itself readily to analytical treatment. But in view of the
small amount of experimental data that are available for testing the con-
clusions it seems inadvisable to carry the development further at present.

SUMMARY.

In the case of a homogeneous substance having only one band in its
phosphorescence spectrum and uniformly excited throughout, the dis-
sociation theory first suggested by Wiedemann and Schmidt leads to a
linear relation between / and I~ j during the decay of phosphorescence.
The observed decay curve, however — i. c, the curve obtained when Z_J is
plotted as a function of / — usually shows a more or less sharp curvature in
the early stages of decay, and only later becomes straight. Using the dis-
sociation theory as a basis, we have discussed first the influence upon the
decay curve of various disturbing factors, such as absorption of the exciting
and emitted light, diffusion, etc. We have then considered the effect upon
the various phenomena of phosphorescence of the formation of complex ions,
which are assumed to be produced by the attachment of simple ions to
neutral molecules; and after the introduction of certain hypotheses regard-
ing the behavior of such ionic groups, an analytical theory is developed.

Briefly the results of the discussion are as follows:

While the observations thus far made on the decay of phosphorescence
are perhaps not sufficient to definitely disprove Becquerel's explanation of
the form of the decay curve, viz, that the curvature is due to the existence of
two bands in the phosphorescence spectrum, numerous objections to this
explanation are discussed. For example, the fact that the decay curve has
the usual form even when precautions are taken to study only a single band;
that the phosphorescence spectrum remains unaltered during decay; the
fact that the decay curve is of the same type for all substances thus far
studied; and the fact that Becquerel's explanation takes no account of
hysteresis phenomena, the effect of infra-red rays, etc.

2. It is shown that lack of transparency in the phosphorescent sub-
stance, resulting in the absorption of either the exciting light or the emitted
light or both, will cause a slight curvature in the decay curve. But the
effect can under no circumstances be sufficient to account for the observed
deviation from linearity.

3. Lack of uniformity in the distribution of the active material through-
out the mass of the phosphorescent substance will cause a deviation from
linearity corresponding to that actually observed. Under certain con-

PHENOMENA OF PHOSPHORESCENCE. 223

ditions the effect is the same as though the substance possessed two or
more bands, coincident as regards wave-length, but differing in rate of
decay. It is pointed out that such a lack of homogeneity is to be expected
from the method of preparation of phosphorescent substances, and has a
great deal to do with the complexity of the phenomena.

4. Diffusion of the ions will produce effects similar to those caused by
irregularities in the distribution of the active material without diffusion,
but the effects will be more marked. An explanation of hysteresis effects
may also be based upon the presence of diffusion.

5. The assumption of complex or grouped ions is shown to lead to a law
of decay which agrees closely with experiment. Ionic grouping also
accounts satisfactorily for the dependence of the decay curve upon the
duration of excitation and the previous history of the substance, and for
the effect of exposure to the infra-red.

The preceding discussion appears to justify the conclusion that the dis-
sociation theory proposed by Wiedemann and Schmidt accounts satisfac-
torily for all the phenomena of phosphorescence thus far studied. Addi-
tional quantitative data are greatly needed, however, in order to make pos-
sible the further development of the theory. The present difficulty is not
so much in accounting for the observed facts as in discriminating between
different hypotheses that are at present equally plausible, and in deciding to
what extent various recognized sources of disturbance are of importance.
The form of the decay curve, for example, may be accounted for equally well
by the assumption of two bands, by an irregular distribution of the active
substance, by diffusion, or by ionic grouping, and is unquestionably modi-
fied by absorption. No quantitative test of either explanation is possible
until the relative importance of the other factors is known. It may be, for
instance, that diffusion does not occur at all; and it is possible, but not
probable, that the disturbances due to lack of homogeneity are of no signifi-
cance. Experiments that will furnish a definite answer to questions of this
sort are of the greatest importance in the development of this or any other
theory of phosphorescence.

INDEX,

Absorption:

Beer's Law of, 25, 33, 39.
Coefficient of, 30-33.
Eosin, 189.
Resorufin, 189.
Fluorescence, 167-174.
-Esculin, fluorescence and transmission spectra of, 23.
Andrews, preparation of phosphorescent compounds,

110.
Angstrom, distribution of energy in flame of Hefner

lamp, 178.
Anthracene vapor:

Fluorescence of, 163.

Effect of fluorescence on conductivity of, 163, 106.

Balmain's paint:

Fluorescence spectrum of, 97.
Phosphorescence decay of, 67, 68, 211.
Effect of—

Delay in heating, 105, 106.
Infra-red rays, 81.
Previous history, 100.
Temperature, 97-101.
Time of excitation, 97, 101, 102, 103, 133,
134.
Hysteresis effect in, 63.
Law of, 202, 203.

Thermo-luminescence of, 102-107.
Becquerel, Ed.:

Cause of change in phosphorescence color, 49.
Phosphorescence decay, 51, 52, 53, 67, 202.
Becquerel, H., phosphorescence decay, 51, 52, 53, 131,

202, 203.
Beer's law, 25, 33, 39.
Buckingham, ionic theory of fluorescence, 26, 39.

Burke:

Change in absorption due to fluorescence, 147.
Fluorescence absorption, 167.

Cadmium salts:

Fluorescence of, 112, 113, 114.
Cadmium-sodium compounds —
Decay curve of, 121, 124.
Phosphorescence spectra of, 120.
CdSOi+xMnS'Ch:

Kathodo-luminescence of, 139.
Luminescence of, by ultra-violet light and by
Roentgen rays, 139.

Camichel:

Conductivity of fluorescent liquids, lol, 198.
Fluorescence absorption, 167.
Canary glass, fluorescence and transmission spectra of,

19.
Chlorophyll, fluorescence and transmission spectra of,

17, 18.
Coblentz, W. W.:

Absorption bands in spectrum of ZnS, 84.
Energy of acetylene spectrum, 175, 179.
Infra-red spectrum of chlorophyll, 17, 18.
Conductivity of solutions, effect of light on, 147, 150,

151, 198.
Cunningham, J. A., effect of light on the conductivity
of solutions, 147, 198.

Dahms, effect of infra-red rays on phosphorescence,

71, 77, 83.
Darwin, phosphorescence decay, 51, 68.
Decay of phosphorescence (see Phosphorescence).
Distribution of energy:

In acetylene flame, 125, 178, 179.
In Hefner lamp flame, 178.
In spectra of —
Eosin, 186.
Fluorescein, 186.
Resorufin, 186.
Sidot blende, 125-129.
Distribution of intensities in spectra of—
Daylight, 14, 15.
Eosin, 182. -
Fluorescein, 182.
Resorufin, 182.

Elster and Geitel, photo-electric activity of lumines-
cent substances, 198
Elston, spectra of anthracene vapor, 163.
"Emanations-pulver":

Decay of phosphorescence, 67.

Effect of infra-red rays, 75, 76, 77, 81.
Energy, distribution of (see Distribution).
Eosin:

Coefficient of absorption, 190.

Distribution of energy in spectrum, 186.

Distribution of intensities in spectrum, 182.

Fluorescence spectra, 9.

Fluorescence vs. exciting power of various wave-
lengths, 192, 193.

Photo-electric action, 159, 161.

Fluorescein:

Distribution of energy in spectrum, 186.
Distribution of intensities in spectrum, 182.
Effect—

Of absorption, 6.
Of dilution, 7, 8.
Spectra, 5, 6, 9.
Photo-electric action, 161.
Fluorescence, 25.

As a function of wave-length, 9.
Absorption, 167-174.
And absorption of —

Uranium glass, 19, 20.
Chlorophyll, 17.
Quinine sulphate, 16.
Effect of dilution on, 26, 35, 36.
Specific exciting power for various wave-lengths,

192-194.
Spectrum, 5, 6, 9, 10, 12, 13, 15, 17, 19, 22,[23, 87.
Effect of—

Solvent, 23.
Temperature, 135.
Distribution of energy in, 125, 126, 127.
Fluorite, transmission and fluorescence spectra, 21, 22.

Gibbs, R. C, fluorescence and absorption'of uranium

glass, 20.
Goldman:

Effect of light on ionization of liquids, 151.

Photo-electric effect, 163, 198.

Hagenbach:

Stokes's law, 1.

Fluorescence of chlorophyll, 17.
Henry, Ch.:

Conduction of iodine vapor due to light, 163.

Law of phosphorescence decay, 51, 52.
Hodge, P., photo-active cells with fluorescent electro-
lytes, 152, 162, 198.
Houstoun, R. A., fluorescence absorption, 167.
Howe, H. E., relation of fluorescence to conductivity,
163, 198,

Infra-red rays:

Effect in suppressing phosphorescence, 134, 135.
Effect on —

Decay of phosphorescence, 72, 73, 77-82, 84.
Phosphorescence when applied during excita-
tion, 75.
Intensity, distribution of (see Distribution).
Ionization:

Of liquids by light waves. 150, 151.
Theory of luminescence, 26, 39, 121.
Ives and Luekiesh:

Effect of infra-red rays on decay of phosphores-
cence, 77, 78.
Form of phosphorescence spectrum, 204.

Kathodo-luminescence:

Dependence of, upon current and potential differ-
ence, 143, 144.
Equation for intensity of, 144.
Of various substances, 139, 144.
Kester, phosphoroscope, 118.

Knoblauch, O., effect of solvent o« fluorescent spec-
trum, 23.
Kundt, effect of solvent on absorption bands, 23.

224

INDEX.

225

Lamanaky, Stokes's law, 1.

Lambert's law, applicable to fluorescent solutions, 31,

39.
Lenard:

Conducting power of phosphorescent sulphides, 198 .

Decay of phosphorescent sulphides, 124.

Equation for intensity of kathodo-luminescenee,
144.

Relation between I ■-' and time, 109.
Lenard and Klatt, long-time decay substances, 124.
Lommel:

Classification of fluorescent substances, 3, 23.

Effect of dilution on fluorescence, 26.

Fluorescence of quinine sulphate, 16.

Law of luminescence, 196.
Lubarsch, Stokes's law, 1.
Luminescence:

Kathodo, 137-144.

Thermo, 87-95.

Theory of, 195-200.

Micheli, F. J., decay of phosphorescence in Balmain's
paint, 99.

Naphthalin-roth:

Fluorescence of, 1.
Fluorescence spectra, 10.

Phosphorescence:

Apparatus for study of rapid decay, 109, 120.

Change of color during decay, 49.

Decay curves, 115, 116, 117.

Distribution of energy in phosphorescence spectra,

125, 127.
Effect of—

Heating, 86, 98, 99, 101, 102, 105, 106, 116, 117.
Infra-red rays, 62, 72-S4, 118, 133, 134, 135.
Intensity of excitation, 65-67.
Previous history, 100.
Time of excitation, 60-64, 97, 101-105, 117.
Hysteresis effect, 61, 63, 70.
Hysteresis vs. temperature, 222.
Laws of, 51-53, 202-204.
Of aniline dyes, 122.
Of cadmium compounds, 124.
Of manganese compounds, 122, 123.
Suppression of, 71, 77, 134, 135.
Theories of, 57, 59, 202-218.
Phosphorescent compounds, 110, 111.
Photo-electric effect:

Relation to fluorescence, 164.
With fluorescent electrolytes, 152-162.
Pierce, C. A.:

Becquerel's equation of decay of phosphorescence,

131.
Distribution of energy in spectrum of Sidot blende,

125-127.
Form of phosphorescence spectrum, 203.
Phosphorescence decay, 85-87, 202, 207, 211.

Quinine sulphate, fluorescence and transmission spectra
of, 15.

Regner, effect of light on the conductivity of solutions,

147, 198.
Resorcin-blau, fluorescence and transmission spectra

of, 13.
Resorufin:

Absorption of, 167-174.

Coefficient of absorption of, 30, 31, 190.

Effect of thickness on the fluorescence of, 37, 38.

Distribution of energy in fluorescence spectrum of,

186.
Intensity of fluorescence at various concentrations,

35,36.
Specific exciting power of various wave-lengths on,

193.
Transmission spectrum of, 28.
Rhodamin:

Photo-electric action of, 161.
Fluorescence spectrum of, 12.
Rutherford:

Kathodo-luminescenee, 144.

Law of recombination of ions, 202.

Sidot blende:

Distribution of energy in spectra, 125-127.
Luminescence —

Due to Roentgen rays, 41, 43, 143.
Due to ultra-violet light, 42, 44.
Kathodo, 143-147.
Intensity of vs. voltage, 143.
Thermo, 87-96.
Phosphorescence —

Decay of, 47, 48, 53-57.
Effect of—

Infra-red rays, 74, 75, 78-81, 118.
Various temperatures, 86, 128, 129.
Time of excitation, 135.
Spectrum —

Change of color, 49.
Effect of temperature on, 135.
Intensity of color, 45, 48.
Sphalerite, absorption bands of, 84.
Stark, ionization theory of fluorescence, 163.
Stark and Steubing, ionization theory of fluorescenco,

164.
Stenger:

Effect of solvent on fluorescence spectrum, 23.
Stokes's law, 1.
Stewart, G. W., energy of spectrum of acetylene, 175.
Stokes, fluorescence spectrum of chlorophyll, 17.
Stokes's law, 1, 11, 13, 16, 20, 24, 45, 196, 197, 201.

Thermo-lu minescence :

Effect of delav in heating, 93, 94.

In Sidot blende, 85-96.

Saturation effect, 92.
Thompson, J. J., influence of light on conductivity of

Na vapor, 165.
Trowbridge, C. C:

Decay of phosphorescence in gases, 202.

Ionic theory of phosphorescence, 200.
Tufts, luminosity curve, 185.

Uranium glass, fluorescence absorption of, 167.

Vogel, distribution of intensities in spectrum of day-
light, 14.

Waggoner, Dr. C. W.:

Decay of phosphorescence of Sidot blende, 136.

Of willemite, 211.

Form of phosphorescence spectrum, 204.

Phosphorescence of short duration, 109.
Walter, B.:

Coefficient of absorption, 33.

Effect of dilution on fluorescence, 26.
Wehnelt, kathodo-luminescenee, 144.
Werner, A.:

Form of phosphorescence spectrum, 204.

Luminescence fatigue, 212.
Wesendonck, Stokes's law, 1.
Wick, MissF. G.:

Absorption of resorufin, 25, 167, 190.

Fluorescence of resorufin, 25.
Wiedemann:

Phosphorescence decay of Balmain's paint, 68.

Law of phosphorescence decay, 51.

Theory of luminescence, 195.
Wiedemann and Schmidt:

Effect of infra-red rays on phosphorescence, 213.

Ionic theory of luminescence, 123, 163, 195.

Phosphorescent effect of zinc salts as impurities,
113.

Theory of decay of phosphorescence, 57, 59, 202.
Wilkinson, analysis of Cd compounds used in experi-
ments, 112.
Willemite:

Decay of phosphorescence, 67, 211.

Luminescence of, 140, 141.
Wood, R. W., fluorescence absorption, 167, 172.

Zeller, C:

Form of phosphorescence spectrum, 204.
Phosphorescence of aniline dyes, manganese com-
pounds, and cadmium compounds, 122-
124.
Zinc sulphide:

Law of decay of phosphorescence of, 202, 203.
{See also "Emanations-pulver" and "Sidot
blende.")

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