SUPPLEMENTARY INVESTIGATIONS

OF

INFRA-RED SPECTRA

Part V — Infra-Red Reflection Spectra
Part VI — Infra-Red Transmission Spectra
Part VII— Infra-Red Emission Spectra

BY

WILLIAM W. COBLENTZ

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

1908

CARNEGIE INSTITUTION OF WASHINGTON

Publication No. 07

The Plimpton Press Norwood Mtiss. U S.A

CONTENTS.

Part V. — Infra-red Reflection Spectra.

Page

Chapter I 9

Introduction 9

Chapter II. — Infra-red reflection spectra of various substances 10-20

Carbonates 10-13

Sulphides 13-15

Oxides 15-17

Aluminum silicates 18-20

Chapter III. — Minute examination of the reflection bands of quartz and of the

carbonates 21-24

Chapter IV. — Reflection spectra in the extreme infra-red 25-34

Apparatus and methods 26-28

Residual rays from mica 29

Residual rays from carbonates 29-31

Residual rays from cryolite 3Z~32

Residual rays from other minerals 32-34

Chapter V. — On regular and diffuse reflection 35~38

Part VI. — Infra-red Transmission Spectra.

Chapter I 41-58

Introduction 41

Group I: Transmission spectra of various solids 4i~47

Group II: Transmission spectra of various solutions 47-51

Group III: Transmission spectra of colloidal metals 51—53

Group IV: Transmission spectra of colored glasses 54~58

Chapter II. — Effect of special groups of atoms on radiant energy 59~68

Comparison of ultra-violet and infra-red 59

The hydroxyl group 59~6o

The effect of molecular weight of the maxima 60-64

Characteristic bands of quartz and of silicates 65-68

Part VII. — Infra-red Emission Spectra.

Chapter I 71-92

Emission spectra of metals in hydrogen 71-72

(a) Nickel arc in hydrogen 71

(b) Spark spectra of metals in hydrogen 72

Emission spectrum of the carbon arc 73_78

The investigations of Moll 76-78

Radiation at room temperature 78-81

Selective radiation from the Nernst glower 81-87

Radiation from metal filaments , , , , , 88—92

3

4 CONTENTS.

Part VII. — Infra-red Emission Spectra. — Continued. r,

Page

Chapter II. — Selective radiation from various solids 93-132

Introduction 93-°6

Radiation from electrically heated solids 96-1 1 1

Emission spectra of solids on Nernst "heater tube" 111-124

Relation between emissivity and energy consumption 1 24-1 29

Summary 129-132

Chapter III. — Radiation from selectively reflecting bodies, with special reference to

the moon 133-146

The effective temperature of the sun 133—135

The limiting temperature of the surface of the moon 135

Fall of temperature of the moon during eclipse 136-137

Reflection and radiation from the moon 138-139

Emission from a partial radiator 140-146

Appendix I: The effect of the surrounding medium upon the emissivity of a sub-
stance i47-i 5 1

Appendix II: Instruments and methods used in radiometry 152-176

I. The microradiometer 153

II. The radiomicrometer 153—155

III. The thermopile 155-158

Comparison of old and new form of thermopile 156-157

The Peltier effect 157-158

IV. The radiometer 158-165

Comparison of sensitiveness and area of vane 159-160

Comparison of sensitiveness and diameter of fiber suspension 160-162

Sensitiveness compared with wave-length of exciting source 162-163

The radiometer compared with the bolometer 163-165

V. The bolometer with its auxiliary galvanometer 165-175

Historical 165-169

Comparison of sensitiveness of various bolometer-galvanometer combi-
nations, 170

Comparison of a bolometer with a thermopile 170-172

Experiment with a sectored disk 172-175

VI. Summary 175-176

Appendix III: Additional data on selective reflection as a function of the atomic

weight of the base 177-180

Addendum 181-182

Index of substances 183

PREFATORY NOTE.

The present volume contains supplementary data on doubtful points,
which arose in connection with the preceding work on infra-red spectra.
The various phases treated, except Part VII, Chapter I, (4) and (5), and
Chapter II, were ready for publication during the summer of 1907, when
through the generosity of the Carnegie Institution of Washington it was
made possible to combine all the material into a new volume, which was
then delayed for the additional data on emission spectra of metal filaments
and insulators, thus rounding up the subject as completely as is possible
at this time. This completes, for the present, the "program of investiga-
tion." The subject, however, is not exhausted — not even thoroughly
initiated — for we know but little more of the cause of absorption and
of emission lines and bands, other than those due to several well-known

o

groups of atoms, than we did twenty years ago, when Julius and Angstrom,
independently, examined infra-red spectra. Each renewed effort is a step
in advance, as the present data on the effect of molecular weight illustrate.
The main problem is to obtain suitable material, which seems to be partly
a matter of chance. For example, one of the first substances ever
examined for selective reflection was quartz. It is the easiest obtainable,
and it illustrates the question of selective reflection better than any other
substance yet found, save carborundum, which was one of the latest min-
erals to be examined. On the other hand, after examining over 300 dif-
ferent substances, it has remained until the very last to find a series that
so well illustrates the effect of molecular weight as the data on the car-
bonates herewith presented. The reflection spectra of the carbonates and
nitrates in solution deserve further study. They are easily obtainable, and
their reflection bands, which are strong, lie in the region of the spectrum
where the radiation is quite intense, so that no serious difficulty need be
anticipated. Colloidal metals also deserve further attention. Artificial
substances, except carborundum, have never been examined, and it is
intended to make a study of the silicides, provided they can be melted
into homogeneous masses.

In Part III (Carnegie Publication No. 65) the discussion of the accu-
racy attainable refers to the region up to about 12 /*. Beyond this point
the absorption of the rock-salt prism increases very rapidly, and it was
not possible to attain great accuracy. In fact, the writer has given but
little weight to his isolated observations lying beyond 14 n, although others
have been able to verify them, a notable example being calcite (Carnegie

5

6 PREFATORY NOTE.

Publication No. 65, p. 70), where the probability of a band at 14 (i is
based upon two spectrometer settings and only three observations.

In Carnegie Publication No. 65, Appendix V, "Note on Blowing
Quartz Fibers," the originator of the method was then unknown to the
writer. Since then it has been found that in addition to shooting quartz
fibers the method of blowing them is fully described by Boys in the London
Electrician, p. 220, Dec. 11, 1896, "Blowing and Shooting Quartz Fibers."

Prof. E. F. Nichols has written me that he found the method in 1891,
and exhibited it at the jubilee celebration of the Physikalische Gesellschaft
in Berlin in 1896.

In Carnegie Publication No. 35, p. 51, line 10 from the bottom should
read: "Carbon dioxide is the only gas studied which has no strong absorp-
tion bands except at 4.5 \i and 14 /*."

W. W. COBLENTZ.
Washington, D. C, May, 1908.

PART V.

INFRA-RED REFLECTION SPECTRA.

CHAPTER I.

INTRODUCTION.

In the preceding volume, Part IV, the reflection spectra of several
groups of chemically related minerals were examined, including sulphates,
sulphides, and an extensive group of silicates. This examination did not
include the oxides and carbonates which were not obtainable at that time.
To examine the substances in chemically related groups seemed to be the
only logical way to gain insight into the mechanism of selective absorption
and of selective reflection. The main difficulty lay in obtaining material
of sufficient size to produce a suitable reflecting surface. A list of minerals,
which occur in sufficient size and homogeneity, was sent out to various
mineral dealers who very kindly submitted several large boxes of specimens
for selection. Even after this first elimination it was possible to obtain
only about 10 per cent of the number of minerals desired. In addition to
these minerals a considerable number of specimens were selected from the
collection in the U. S. National Museum.

The apparatus and methods employed were essentially the same as in
the preceding investigation. The spectrum was produced by means of a
rock-salt prism, and mirror spectrometer previously described. The re-
flecting power of the mineral was compared with that of a silver mirror.
The surfaces were ground plane, but were not always of the highest polish.
Since primarily we are only concerned with the accurate location of bands
of selective reflection, the question of polish is of secondary importance.

The spectrometer slits were 0.3 mm., or about 2' of arc, as in the
preceding work. The radiation from a Nernst "heater" was projected
upon the reflecting surfaces by means of a mirror having a focal length of
15 cm. and an aperture of 12 cm.

The reflection spectrum was explored by means of a Rubens thermo-
pile. Although its sensitiveness was greater than the radiometer used
previously, the unsteadiness at times, due to magnetic disturbances, neces-
sitated repeating the observations at each spectrometer setting. When
using the radiometer, it was rarely necessary to repeat the observations in
exploring the spectrum up to 9 or 10 fi, while in the present work, using
a thermopile, the region beyond 9 p. was examined under the greatest
difficulties, and in the case of the carbonates no attempt was made to
locate the band at 11.4^. When the reflection faces were less than the
standard size, 2 by 3 cm., which was frequently the case, the silver com-
parison mirror and the specimen were covered with diaphragms having
equal openings, so that equal areas of the two surfaces were exposed.

9

4
Fig. i

5 6 7 8

Smithsonite (a); Cerussite.

10/t

CHAPTER II.

INFRA-RED REFLECTION SPECTRA OF VARIOUS SUBSTANCES.

CARBONATES.

Smithsonite (ZnC03).
(Stalactitic crystalline mass. From New Jersey. Curve a, fig. i.)

The specimen examined was highly polished. The reflection curve is

strong, and is a complex of two
bands, as will be noticed in
nearly all the carbonates exam-
ined. The maxima occur at
6.65 and 7.05 //.

There seems to be a silicate
calamine (H20-2ZnOSi02) hav-
ing the same name. In fact
the sample was purchased as a
silicate. The present examina-
tion shows that the specimen ex-
amined is the carbonate known
by that name. In other words, this is an independent method of analyzing

such a mineral.

Cerussite (PbC03).
(From New South Wales. Curve b, figs. 1 and 2.)

This is a rare mineral and the present specimen was a fragment having
a surface about 1.5 by 2 cm.
The surface itself was cor-
rugated, with several plain
highly polished plates about
2 by 15 mm. It was the
only specimen obtainable
and no risk was taken in
attempting to grind it. The
reflection curve b, fig. 1,
therefore does not indicate
the true reflecting power.
The individual bands, how-
ever, are well resolved, the
maxima being at 6.9 and
7.25 p.. In fig. 2, curve c, the reflection curve is drawn on a larger scale,
which emphasizes these maxima.
10

15%

8 9 10/M

Fig. a. — Strontianite (a); Witherite (b)\ Cerussite.

CARBONATES.

II

Strontianite (SrC03).
(Massive specimen. From Hamm, Westphalia, Germany. Curve a, fig. 2.)

The specimen examined had a large, well-polished reflecting surface.
As in all the carbonates studied the height of the maximum reflection is
only about 30 per cent. This is the only carbonate examined which has
but one reflection maximum, which is at 6.74//. It will be noticed pres-
ently that these maxima shift toward the long wave-lengths with increase
in molecular weight. Since the intensity of the two bands is unequal, it is
possible that this inequality, combined with the shift of the maxima, is the
cause of the lack of resolution.

Witherite (BaC03).
(Massive, crystalline. Curve b, fig. 2.)

The reflection decreases normally from 4 to 6/( followed by strong
complex reflection band, with maxima at 6.78 and 6.98 fi. The latter band
is the more intense, which is just the opposite of the carbonates, having a
metal of less molecular weight. The specimen had an unusually high
polish, which accounts, in part, for the high reflection maximum.

Malachite (CuOCOvH20).
(Concretionary specimen. From Burra Burra, South Australia. Curve a, fig. 3.)

The specimen of malachite was highly polished. The reflection bands
are well resolved, while the second maximum is of the usual intensity for
carbonates. Beyond io/t there appears to be another band, but the un-

20%

4 5 6 7 8 9

Fig. 3. — Malachite (a); Azurite.

steadiness of the galvanometer prevented an accurate determination of
this question. The maxima of malachite occur at 6.66 and 7.3 ,u, with a
possible third band at 7.8 \i.

Azurite (3CUO2CO2H2O): Chrysocolla (CuSi03+H20).
(From Lyon County, Nevada. Curve b, fig. 3.)

The specimen examined was a mixture of azurite and chrysocolla, the
first mineral being present in only a small amount. No silicate bands at

12

INFRA-RED REFLECTION SPECTRA.

8.5 and 9 fi could be detected. The reflecting power is low, which is no
doubt due to the presence of the silicate. The maxima at 6.65, 7.3, and
7.8 y. are in common with those of malachite. The transmission curve of
azurite is given in the preceding volume. It shows the OH band at 3 \i
and the carbonate bands at 3.5 and 4 \i.

Dolomite [CaMg(C03)2].
(A plane cleavage piece. From Traversella, Italy. Curve a, fig. 4.)

Dolomite is of interest because it is a double carbonate of Mg and Ca.
The reflecting power is high. The first band is in common with that of

50%

40

C
.2 30

'w20
cc

10

°A

\

•

h

\

i

."7 "-**-

1

■

— -

> 5 6 1 8

Fig. 4. — Dolomite (a); Siderite.

IOJU

CaC03, while the second band is to be found in MgCOo. The maxima
occur at 6.58 and 6.95 fi.

Siderite (FeC03).
(Cleavage piece. From Allevard, France. Curve b, fig. 4.)

The reflection curve of siderite is composed of a complex maxima
similar to that of dolomite. The maxima occur at 6.6 and 7.1 /«.

Fig. 5. — Calcite (a); Magnesite.

Calcite (CaCOa).
(Curve a, fig. 5.)

The reflection curve is obtained from a natural cleavage face of Iceland
spar. The reflection band is complex with maxima at 6.6 and 6.85 fi.
The reflection bands in the deep infra-red will be noticed presently. It is

SULPHIDES. 13

desirable to examine aragonite, CaC03, which differs from calcite in its
crystalline form, to determine the effect of structure; this is given in
Chapter III. It was shown in Carnegie Publication No. 35 that isomeric
compounds have different transmission spectra; and one would expect also
the reflection spectra of isomers to be different. The unsteadiness of the
galvanometer prevented the location of the band, found by Aschkinass,
at 11.4/1. In this region the radiometer would have been more satisfac-
tory.

Magnesite (MgC03).
(Massive. Curve b, fig. 5.)

The sample examined was an opaque white mass which took a high
polish. The band of selective reflection is very similar to that of calcite
and consists of two maxima at 6.5 and 6.8 ;<, respectively.

As a whole the carbonates are conspicuous for a double band of metallic
reflection at 6.5 to 7 /i which previously had not been resolved in calcite.
The shift of these bands with increase in molecular weight of the metallic
ion will be discussed on a later page. That the shift is not due to a change
in the adjustment of the apparatus was verified at the completion of the
observations by examining several of the specimens in succession, when it
was found that the maxima coincided with the values first observed.

SULPHIDES.

The reflection spectra of sulphur and of the sulphides of Zn, Sb, Fe,
and Pb were described in the preceding volume. Sulphur and sphalerite
(ZnS) have a low reflecting power of only about 8 per cent throughout the
spectrum to 1 5 ft. The remaining minerals have a high reflecting power
of 32 to 36 per cent throughout the spectrum to 15 /.«, where the reflection
of stibnite, Sb2S3, seemed to decrease. The present examination includes
four new sulphides.

Molybdenite (MoS).
(Massive foliated. From South Australia. Curve a, fig. 6.)

This mineral is very soft, like graphite. The specimens were folded
and distorted so that it was not possible to grind a surface parallel to a
cleavage plane. The surface took a high polish, but no luster as is found
in the cleavage lamina. The reflection curve is fairly uniform beyond 4 ft,
which would indicate that the lack of polish had no serious effect beyond
this point. The reflecting power is low and uniform (18 to 20 per cent,
as compared with stibnite, 35 per cent) throughout the spectrum to 14 /*,
which is to be expected from its electrical conductivity. (For further
references to electrical conductivity, etc., see Carnegie Publication No. 65,
p. 93; also Konigsberger, Jahrb. Radioaktivitat & Elektronik, vol. 4,
p. 161, 1907.)

14

INFRA-RED REFLECTION SPECTRA.

Pyrrhotite [(Fe, Ni)S].
(Massive. From Sudbury, Ontario. Curve b, fig. 6.)

The specimen examined did not appear so highly polished as molyb-
denite. It is composed of a bright metal and a darker background. The
reflecting power is higher than that of MoS, and increases rapidly with
wave-length through the spectrum of 14 /i.

Chalcocite (Cu2S).
(Curve c, fig. 6.)

The reflecting power rises rapidly from 15 per cent at 1 /z to 45 per
cent at 4 /*, while beyond 7 /j. the reflecting power has a fairly constant

2 3 4 5 6 7 a 9 10 11 12 13

Fig. 6. — Molybdenite (a); Pyrrhotite (ft); Chalcocite (c); CoveUite.

/4/Z

value of 55 per cent. This substance is black, not unlike magnetite and
the Siberian graphite previously described. The polish was higher than
in pyrrhotite.

CJ COVELLITE (CuS).

(From Anaconda Mine, Butte, Montana. Curve d, fig. 6.)

This mineral is of a steel-blue color, and appears as compact as steel.
The reflecting power rises rapidly at 1 and 2 // and then more slowly to
14 p., where it amounts to 75 per cent.

As a whole, the examination of the sulphides shows that the sulphur
atom has merely reduced the reflecting power of the metal, but has not
brought about any bands of selective reflection in the region examined.
In the case of sphalerite (ZnS) the sulphur atom has introduced into the
metal a property which is to be found only in non-metals, viz, low re-
flecting power and selective absorption in the infra-red. Of the elements
which are on the border line between metals and non-metals, selenium
behaves like a metal in its high reflecting power (see fig. 7) and absence
of infra-red absorption bands, while iodine and sulphur (non-metals) have
absorption bands in this region.

OXIDES.

*5

Selenium (Se).
(Curve d, fig. 7.)

Selenium has practically the same reflecting power (18 to 20 per cent)
as molybdenite. Pfund (Johns Hopkins Univ. Circular No. 4, 1907)
has made a thorough investigation of the infra-red polarization of this
substance.

OXIDES.

In the previous examination only quartz (Si02) represented this im-
portant group of minerals.

Magnetite (Fe304).
(From Port Henry, Essex County New York. Curve a, fig. 7.)

The surface examined was a triangular crystal, face about 3 cm. on
an edge. The specimen was very homogeneous, but did not take a high
polish, sufficient to observe the reflected image of an object at a small
angle of incidence. The reflecting power rises uniformly throughout the
spectrum, showing that the observations are affected by the lack of polish.

Hematite (Fe203).
(From Cumberland, England. Curve b, fig. 7.)

The specimen examined was a very dense homogeneous concretion,
polished parallel to the radius of growth. The surface had a high polish
(better than that of magnetite), reflecting a strong image of a source at a

40%

:30

u20
0)

10

<L— —

^-* — — d— ~ — — :

. •>■ ■ • — . •

234-56 789/0//

Fig. 7. — Magnetite (a); Hematite (6); Chromite (c); Selenium.

/2

I3JU

small angle of incidence. The reflecting power, in the infra-red, is far
below that of magnetite, although it is higher in the visible. The reflecting
power is uniformly 12 per cent, from which it would appear that this is
the normal value for hematite.

Chromite (FeO, Cr203).
(From Lancaster County, Pennsylvania. Curve c, fig. 7.)

The surface of this dark specimen was mottled, parts of which took a
high polish. The reflecting power is uniformly 4 per cent throughout the
spectrum.

It is of interest to note that the iron oxides show no bands of selective
reflection in the region of the spectrum up to 15 fi; zincite, ZnO, fig. 9, is
another example.

i6

INFRA-RED REFLECTION SPECTRA.

SCHEELITE (CaWO-j).

(Massive. From Armidale, New South Wales. Curve a, fig. 8.)

The reflection power of this mineral behaves in the usual manner,
being low in the region of the spectrum up to n //, followed by a strong
complex band of selective reflection, the maximum of which extends from
ii. 3 to 12.5 n, the possible single maxima being at 11. 3, 11. 8, and 12.4 /<.
The reflecting power is unusually low on the long wave-length side of the
reflection band.

Zircon (ZrSiO*).

(Near Eganville, Renfrew County, Ontario. Curve b, fig. 8.)

The specimen examined was a large rectangular cleavage piece, about
3 by 3 cm., of brownish color, semitransparent in thin sections.

Although this is a double oxide of zircon and silicon, the reflection curve
is entirely different from the silicates previously studied. The strong band
of selective reflection occurs farther toward the long wave-lengths, the
single maxima (not well resolved) being at 10.1, 10.6, and 11 with a pos-

60%

50

40

c
o

"" 30

20

10

V*^

a

bj

\

a

\
6 \

I

N

p

\

J

1
\

-0

■ ■ 0—

—0—0-

6 7 8 9 '0

Fig. 8. — Scheelite (a); Zircon.

IZ

13

I4-/U

sible band at 11.7 /<. As a whole the reflection curve is exactly the same
as that of willemite, Zn2Si04, previously studied. In the latter, the bands
are well resolved and occur at 10.1, 10.6, 11. o, and n.6/<. From this it
would appear that in these two minerals the effect of Si02 is different
from that found in quartz and in the minerals commonly known as sili-
cates, viz, silicates of Ca, Mg, Fe, etc. Some of the latter, however, have
the last band lying close to the first band in the present silicates.

WULFENITE (PbMo04).

(Red Cloud Mine, Yuma County, Arizona. Curve a, fig. 9.)

This is a chrome-red crystal. A natural crystal face, 1.5 by 2 cm.,
having a high polish, but not perfectly plane, was examined. In spite of
this the reflection curve is one of the most remarkable yet found. The
band of metallic reflection is almost as strong as quartz. There are two

OXIDES.

!7

maxima, at 11.75 an^ I3-1 ,u> respectively. The reflecting power decreases
uniformly from 12 per cent at 4 ta to 10 per cent at 8 p, passes through a
minimum of 4 per cent at 10.8 /<, then suddenly rises to 78 per cent at
11.75//.

Rutile (TiO,).
(Graves Mountain, Lincoln County, Georgia. Curve b, fig. 9.)

The surface of the specimen examined was a natural crystal face, area
about 1.4 by 1.8 cm., having a high polish, but uneven and containing cracks.

The reflection curve is the most unusual of all examined in that the
band of selective reflection occurs almost at the working limit of a rock-
salt prism. The maximum is broad and is fairly well defined at 13.6 ft.

80%

3 4 5 6 7 8 9 10 II 12 13

Fig. 9. — Wulfenite (a); Rutile (6); Zincite.

14 ISfl

Zincite (ZnO).
(From Franklin, New Jersey. Curve c, fig. 9.)

The specimen examined was massive, having a red color and a dull
polish. The surface was full of cracks, which would materially reduce
the reflecting power. The reflecting power is low and uniform throughout
the region of the spectrum examined. No bands of selective reflection
were found. In this respect zincite is similar to iron oxide, but, on account
of its low reflecting power, it is to be classed with the " insulators," and
hence one would expect to find bands of selective reflection in the infra-red.

Corundum (AI2O3).
(Craigmont, Renfrew County, Ontario. Curve b, fig. 10.)

This specimen was an opaque crystal, of which a cleavage surface,
about 2 by 3 cm. in area, was examined. The surface contained striae,
but otherwise had a high polish. The reflecting power is unusually low,
except in the region of selective reflection, when it is quite high. The
region of selective reflection is wide with maxima at n.o, n.8, and 13.5 fi.

i8

INFRA-RED REFLECTION SPECTRA.

ALUMINUM SILICATES.
There is no special reason for thus classifying the following minerals,
except that most of them are quite unlike the commoner silicates previously

examined.

Cyanite (Al203Si02).

(Yancy County, North Carolina. Curve a, fig. 10.)

The color of this flat crystal was light green. The flat crystal face,
2-5 by 3.5 cm., was ground but did not have a high polish. The band of
selective reflection has three sharp maxima, at 9.3, 9.78, and 10.28^,

to%

50

c 40
O

0

® 30
u~

cc

20

10

A

0

'

\b

/

.. 1

>

6 7 8 9 10 II

Fig. 10. — Cyanite (a); Corundum.

12

13

14

15/1

respectively, and is not unlike that of willemite (Zn2Si04), except that in
the latter the whole band is shifted to longer wave-lengths with the maxima
at 10. 1, 10.6, and 11 ;t.

Beryl [BejAhKSiOa)*].
(From North Carolina. Fig. n.)

The surface examined was ground on a crystal face. The specimen
was opaque and green in color. The reflection curve follows the usual
course with a band of selective reflection extending from 8 to 10.5 [x. The

50%

40

c
.2 30

<» 20

10

ifc-s

10

'2

I3JJ.

Fig. 11. — Beryl.

maxima are not so high as usual with silicates, but they are sharp and
occur at 8.15, 9.2, 9.9, and 10.4/*.

SILICATES.

19

Topaz [(Al, F)2Si04].
(Villa Rica Mines, Geraes, Brazil. Curves a and b, fig. 12.)

The specimen from Brazil was a yellow rectangular prism with faces
1 by 2.5 cm. The natural face, which had a high polish, was examined.
The reflection curve a, fig. n, is low throughout the spectrum, except in

■80%

6

70

60

50

C
O

/ a
f

v_\

--i 40
O

I

V

(£30

1

20

\

10

11

V

3 '

t

5

i

7

Fig. 12

1

.-Toi

3 '

>az.

0

/

/2 /3 «

the region of selective reflection, which extends from 10 to 11.5 // with
unresolved maxima at 10.05, 10.6, 10.9, and 11.3 /*.

The white topaz examined, curve b, fig. n (in its highest part), is an
almost exact reproduction of the quartz curve, with the exception that the
latter is shifted to the longer wave-lengths. The white topaz, which was
ground and had a fairly high polish, has sharp maxima at 9.9, 10.45, an^
11 p. with a possible band at 12 \i. The band at 11 [i is in common with
that of several other silicates.

Sodium Silicate (Na2Si03).
(Curve b, fig. 13.)

The curve of liquid glass is similar to that of the glasses previously

examined. This is one of the simplest obtainable chemical compounds of

the silicates, and is further evidence that the silicon oxide radical is not so

constant in its behavior toward heat-waves as was found in the C02 radical

of the carbonates.

Spodumene [LiA1(Si03)2].

(From Pennington County, South Dakota. Curve a, fig. 13.)

This mineral is of the same composition as kuntzite, the transmission
curve of which is given on a later page. The specimen examined was a
polished cleavage piece from a large gray crystal. The maxima of the
selective reflection bands occur at 9.18, 9.7, and 10.4 pt, respectively.

20

INFRA-RED REFLECTION SPECTRA.

In conclusion, it may be said that, from all of the silicates examined,
there is no regularity in the position of the maxima such as obtains in the
sulphates and the carbonates. It is true that in a few cases the maxima
are in common, but, on the whole, the evidence indicates that there is no
uniform group of atoms acting in common as is found in the sulphates
and carbonates. In other words, the silicon radical is different in the

so;

40

c
o

01

QJ

.30

CC20

10

• /

A/

u

i

\

i \
I \

K

—

o

-o ^

a

r>^6

1 /

5 6 7 8 9 10 II

Fig. 13. — Spodumene (a); Sodium silicate.

IZfl

different minerals. This seems to be the necessary interpretation to be
given, for it has been impossible to work out a relation with the molecular
weight of the molecule, such as exists in the carbonates and sulphates, to
be discussed on a later page.

CHAPTER III.

MINUTE EXAMINATION OF THE REFLECTION BANDS OF QUARTZ

AND OF THE CARBONATES.

Having assembled a fluorite prism and bolometer for radiation work,
it seemed worth while to spend some time on the examination of the reflec-
tion bands at 6 to 7 p. in the carbonates, and at 8.5 to 9 /< in quartz. The
fluorite prism had a circular aperture of 3.3 cm., angle 6o°, and was per-
fectly clear. It was mounted on the spectrometer used in the previous
work and, for the regions of the spectrum examined, the dispersion was
from 4 to 6 times that of rock salt. On account of the large dispersion
and the small prism face, the deflections were only about one-tenth that
previously used. The glower of the Nernst lamp was used in order to
obtain a sufficiently strong source of radiation. The main objection in
using a glower is its narrowness, which requires greater care in maintaining
a constant adjustment. The bolometer strip was about 0.5 mm. or 4' of
arc, while the temperature sensibility was 1 mm. = 5° X io~5C. A radi-
ometer would have been more satisfactory, but the bolometer was con-
veniently at hand. With this greater dispersion it was found that the
reflection bands of some of the carbonates are quite complex, while in
others there is but one band.

Quartz.1

(Crystal cut perpendicular to optic axis. Curve b, perfectly clear specimen of

quartz glass, fig. 14.)

It has repeatedly been noticed that the various silicates have quite
different reflection spectra, while in the carbonates and in the sulphates
the spectra show great similarity. It was therefore assumed that the
silicon oxide radical is differently united in the different silicates. All the
evidence obtainable, without exception, of substances in the solid or liquid
(crystalline or amorphous) condition, shows that crystallographic form
does not explain these anomalies; neither will the slight impurities present
in many of the silicates explain them.

Pfund 2 found identical reflection bands of sodium-potassium tartrate
(C8H4K Na06 + 4H20) in the form of a polished crystal, and also in the
molten condition. His reflection maxima of molten nitroso-dimethylani-

1 The writer is indebted to Dr. Day and Mr. Sheppard, of the Geophysical Laboratory of
the Carnegie Institution of Washington, for the sample of quartz glass.

2 Pfund, Astrophys. Jour., 24, p. 31, 1906.

21

22

INFRA-RED REFLECTION SPECTRA.

80%

3
Fig. 14

3 9

■ Quartz: (a) Crystalline; (b) Amorphous.

iojul

line [(CH3)2NC6H4NO] coincide with the absorption maxima found by
the writer for a solid film of this compound. Furthermore, the writer
found that reflection spectra of solids in solution may or may not be iden-
tical with that of the solid, note-
worthy examples being the
sulphates of copper and of so-
dium. The cause of the reflec-
tion bands is therefore to be
sought within the molecule.

From the fact that the phys-
ical and chemical properties of
quartz-glass are different from
the crystal, one would infer
that there is a difference in the
molecular structure. This is
well illustrated in fig. 14. The
reflection spectrum of the amor-
phous material is entirely differ-
ent from that of the crystal. It
seems to have no connection with
any of the silicates examined. The maxima of the reflection spectrum of
crystalline quartz occur at 8.4 and 9.02 p., while those of the amorphous
quartz are found at 7.8, 8.4, and 8.8 n, respectively. The intensity of the
maxima of bands of the amorphous quartz is not so great, neither does the
reflecting power, in the region of the spectrum just preceding a reflection
band, fall to so low a value as in crystalline quartz.

In this connection it may be noticed that the various kinds of glass
(sodium silicates) previously examined have similar reflection spectra.

Calcite and Aragonite (CaC03).
(Fig. 15. Curve a, calcite; curve b, aragonite.)

From the fact that, although the carbonates examined belong to only
two crystal systems,1 the reflection spectra are different, one would infer
that the cause is not to be attributed to crystalline form. Previous exami-
nations have shown that isomeric substances have different spectra. One
would therefore expect to find the spectra of calcite and aragonite to be
different, as was previously found for orthoclase and microclene (KAlSi3Os)
at 8 to 10 /j.. The fact that the substances are inorganic, and that the re-
flection bands are far in the infra-red, is of interest but non-essential.

In fig. 15, curve a gives the reflection from the plane, highly polished

1 To the rhombohedral system belong CaC03 (calcite), MgC03, FeCC>3, and ZnC03; to
the orthorhombic system belong CaC03 (aragonite), BaC03, SrCOj, and PbC03. In the
former group the band toward the shorter wave-length appears to be the most intense, while
in the latter group the bands seem sharper and of more uniform intensity.

CARBONATES.

23

80%

70

60

50

<J

_0j40

cleavage face of Iceland spar. With the larger dispersion the band pre-
viously found at 6.6 jj. is now resolved into two bands with maxima at
6.5 and 6.6 /«, respectively. The
region at 7 /« is evidently still
more complex, but not resolved
even with this greater dispersion.
In curve b is given the reflection
spectrum of a clear crystal of
aragonite. The crystal face was
small, only about 6 by 15 mm.,
and since the area of the com-
parison mirror was not reduced
in like proportion, the reflecting
power of the maxima is really
higher than here given. In fact,
for a well-polished crystal the
reflecting power is no doubt as
high as for calcite. The impor-
tant point of interest in the pres-
ent examination is that aragonite has only two reflection maxima, of about
equal intensity, and located at 6.53 and 6.75 /«, respectively.

Magnesite (MgC03).
(Curve a, fig. 16.)

This sample was previously examined (fig. 5). In the present curve
there are three bands with maxima at 6.42, 6.65, and 6.9 p..

60%r

30

20

10

JN

k

1

\

\

j

1
t

v..

^

A

/

y\

b

\

/

f

1
\

\

\

/

V.

/•/

tfSy^

6.0

.2

.3

1.0

A .6

Fig. 15. — Calcite (a); Aragonite.

.2

7.4//

50

40

c
o

13 30

ce

20

10

A

r

1

\6

L

'N

\

I

*\j

V

\

\

\

J

V

\

1/

0.2 .4 .0 .a 7 .2

Fig. 16. — Magnesite (a); Smithsonite.

7.4/i

Smithsonite (ZnC03).
(Curve b, fig. 16.)

This sample was previously examined (fig. 1), when two maxima were
found. In the present examination the bands are more resolved, but the
maxima occur at 6.65 and 7.05^, respectively, as in the previous investigation.

24

INFRA-RED REFLECTION SPECTRA.

Strontianite (SrC03).
(Curve a, fig. 17.)

The band at 6.68 /* is not resolved even with the large dispersion of
fluorite. The band is symmetrical, as previously found in fig. 2.

Witherite (BaC03).
(Curve b, fig. 17.)

The pair of bands is somewhat better resolved, with maxima at 6.7
and 6.97 //, respectively, the same as was found in the previous examination
of this same specimen (see fig. 2). The reflecting power is slightly lower

4-d

30

(J

0)20

CC

10

*~\

1

\

6

//

\

\

/

V

2 .4 -6 .8 7 .2

Fig. 17. — Strontianite (u); Witherite.

7.4/U

but that is to be attributed to the difference in the adjustment, i.e., the faces
of the comparison mirror and of the substance may not have been in the
same plane. As a result they would not reflect the same part of the image
of the glower upon the spectrometer slit. This is of minor importance
since we are not concerned with the absolute reflecting power.

On a subsequent page it is shown that an emission band of Adularia
occurring at 2.9 p. is shifted to 3.2 fi in the absorption spectrum of the crys-
talline material. Using polarized energy, the reflection bands of the car-
bonates depend upon the direction of vibration of the incident energy.
The author therefore hopes to investigate water solutions to learn the be-
havior of these reflection bands, using polarized radiation.

CHAPTER IV.

REFLECTION SPECTRA IN THE EXTREME INFRA-RED.

From the foregoing work on selective reflection in which a single re-
flecting surface was used, it is apparent that by successive reflection of
heat-waves from several surfaces there will remain only the residual rays
lying in the region of selective reflection. It was noticed that the reflecting
power in the region of 4 to 8 p. is only about -^ that of the band of selec-
tive reflection. Hence, after reflecting from three surfaces the intensity

would be only -j^oo' an<^ a^ter nve reflections only xooVoo'

Rubens and Nichols x were the first to apply this method in locating
the maxima of the residual rays of a series of substances including quartz,
mica, fluorite, rock-salt, sylvite, crown and flint glass, sulphur, alum, and
calcite. By using a grating of fine wire they were able to extend their
observations to 61 //, the longest heat-waves yet identified. Of the above-
mentioned substances, only the first four were found to have bands of
residual rays in the extreme infra-red.

Aschkinass 2 did some further work on this subject, examining marble,
calcite, selenite, alum, sodium bromide, and potassium bromide. He
found a band of residual rays at 29.4 jj. in marble, and showed that similar
bands exist in the bromides, the maxima of which lie beyond 60 jj..

From the results obtained with the silicates (see Carnegie Publication
No. 65, pp. 80 to 90), especially glass, it becomes apparent that one can
hardly expect to locate bands of residual rays in the extreme infra-red, even
at 18 to 20 /<, unless they are much more intense than those found in the
region of 8 to 10^. For it is necessary to have several reflecting surfaces
to eliminate the large amount of energy of short wave-lengths as compared
with the small amount to be measured, of the long wave-lengths. As is
well known now, in the case of quartz, rock-salt, and sylvite, this is an
easy matter on account of the high reflecting power of these bands. For
example, for the band at 61 n, which was examined after reflection from
five surfaces of sylvite, it was found that the reflecting power of sylvite is
80 per cent. The galvanometer deflections were only about 5 mm. in
the maximum of the band. If the reflecting power were only one-half
this amount (cf. the silicates), the galvanometer deflection after five
reflections would be one thirty-second of 5 mm., which could not be

1 Rubens and Nichols, Ann. der Phys. (3), 60, p. 418, 1897.

2 Aschkinass, Ann. der Phys. (4), I, p. 42, 1900.

25

26 INFRA-RED REFLECTION SPECTRA.

observed with accuracy. In the case of the silicates (as compared with
quartz), where the height of the reflection band at 8 to 10 p is always
considerably less than 50 per cent, after three reflections the galvanometer
deflections would be only 1 or 2 mm. at 18 to 20//. It does not follow,
therefore, because no residual rays of a substance are to be found in the
region of 18 to 20 p, as was the case in the present examination, that no
bands exist, but that they are too weak to be measured. From the simi-
larity of the spectra, throughout the infra-red, of great groups of chemically
related compounds, and from the fact that quartz and mica have bands of
residual rays in the region of 18 to 20 p, it is to be assumed that the sili-
cates, in general, are selectively reflecting in this region. The fact that
no bands were found is to be attributed to the weakness of the reflection
bands.

As a whole, however, the reflecting power of some of these bands is
very high. One has really no conception of the state of affairs until he
examines a substance like quartz. The phenomenon is so easily observed
with quartz that it might well serve as a general laboratory experiment.
In regard to the ease of observing in the infra-red, the writer's experience
has extended from the optical region into the most remote infra-red, and
it may be said that more difficulty was experienced in the region of 12 to
15 p, on account of the absorption of the rock-salt prism, than in the
region of greater wave-lengths. The actual intensity in the emission
spectrum differs greatly throughout the spectrum. For example, Rubens
and Aschkinass (loc. cit.) compute that for a temperature of 20000 the
maximum emission at 1.5 p is 800,000 times as great as at 60 p.

APPARATUS AND METHODS.

In the present examination the usual methods of procedure were
employed. The spectrometer was the one used in previous work. The
spectrum was produced by means of a wire grating G, fig. 18. The grating
was made by the well-known method of winding two copper wires of the
same diameter on a brass frame. One wire was then unwound and the
remaining one was fastened to the frame by means of an electrolytic deposit
of copper. The strands were then cut from one side of the frame. The
wire was not of uniform thickness, so that the grating was far from perfect.
Such a grating has the well-known property of producing only the odd
order of spectra. On account of its imperfections it was not possible to
make accurate measurements on orders higher than the seventh, using a
Bunsen sodium flame. Two gratings were employed, the one (No. 2)
having a constant of .£=0.2120 mm., for the other £ = 0.3279 mm.,
determined with the sodium flame. The coarser grating was the more
uniform in its individual windings, but in one region the wires were more
closely but very regularly wound. The magnesite band at 29.4 p was
apparently double, at least very wide, just as though this grating spectrum

RESIDUAL RAYS.

27

contained "ghosts." For the main part of the investigation grating No. 2
was used, which on account of the finer wires was not so regular in its
winding. Using three reflections from quartz it was possible to observe
the third order maximum of the band of wave-length, 9.05 ll; in all other
cases, the third order spectrum was too weak for observation. A grating
having wires of more uniform diameter and more uniformly wound, e.g.,
wound on a screw thread, would no doubt have been more efficient in
producing a pure spectrum.

] ^2

N

Fig. 18.

The spectrometer arm carrying the Nernst "heater," the slit St and
the mirror, mu was movable. The plane of the grating was made normal
to the beam of light by placing a mirror on the wires and revolving the
grating until an image of the slit, S1} was projected back upon the slit.
It was not convenient to have the collimating mirror and the grating revolve
about the spectrometer axis, which is necessary to maintain the same grat-
ing constant. Accordingly the grating was kept in a fixed position and
the spectrometer arm, carrying the mirror and source, was revolved
about it.

The apparatus was calibrated by locating the maxima of the selective
reflection bands of substances previously examined by Rubens and Nichols
and by Aschkinass, viz, quartz at 8.7 and 20.75 <"> mica at J8-4 and 21.25 /*>
fluorite at 24.4 /x, and calcite at 29.4 \x. The calibration curves are shown
in fig. 19, where curve a is for the grating having a constant -KT= 0.3279 mm.
(for Na), and curve b is for the grating (No. 2) having a constant 2T=o.2i20
mm. (for Na). In this curve the abscissae are the maxima of reflection
bands and the ordinates are the rotations of the spectrometer arm from
the zero position. Blocks of wood with vertical metal plates, having
openings 2 by 3 to 4 by 5 cm. were mounted securely upon a board, as
shown at rv r2, rS) r4. The minerals were secured to the back of the metal

28

INFRA-RED REFLECTION SPECTRA.

plates by means of soft wax, the plane faces being, of course, placed over
the openings in the plates. In case less than four reflecting surfaces were
available, aluminum mirrors were placed in the remaining holders. A
short focus mirror projected an image of the slit, S2, upon an improved
iron-constantan thermopile of 20 junctions. The wire used in the ther-
mopile was only 0.075 mm- *n diameter. This eliminated heat conduction
and there was no drift of the zero. The galvanometer suspension weighed
about 10 mg., which eliminated the effect of earth tremors. This, how-
ever, necessitated lengthening the period to gain great sensitiveness. The

km. 19. — Wire-grating calibration.

Z8

3ZM

thermopile was in a metal case, wrapped in felt, and the complete outfit
was perfectly steady. It was, therefore, possible to increase the period of
the galvanometer to 20 to 30 seconds (single swing) when its sensitiveness
was f=5Xio~n ampere on a scale at 1 m. The resistance of the ther-
mopile was 8.9 ohms. The galvanometer resistance was 5 ohms, from
which it was computed that a deflection of 1 mm. = 7X io~7 °C. for a scale
at 1 m.

From this it will be seen that the temperature sensitiveness of the
instrument was as great as, and in some instances greater than, in previous
investigations. Hence, it appears that in numerous cases the absence of
reflection bands is to be attributed to some property in the material rather
than to a fault in the instruments.

The whole apparatus was, of course, thoroughly protected from stray
light by providing numerous black pasteboard screens.

RESIDUAL RAYS.

29

RESIDUAL RAYS1 FROM MICA.
Muscovite Mica [H2KAl(SiO\i):i].

In fig. 20 arc plotted the spectrometer arm rotations (abscissae) and the
galvanometer deflections in the grating spectrum of mica. The sharp
maximum at A is the central image, while Bx is the selective reflection band
(first-order spectrum) in the region of 9 /«. For this part of the curve
three reflection surfaces were used, while to obtain the part Cu C2 four
reflecting surfaces were employed. The latter bands are plotted to a
larger scale in C\ and C'v The maxima in the spectrum to the left (not

J

50

B,

mm

P

to

6 C'(C

1
2

n30

777777 jb.

O

0>

.f

\

0

1

J

V

*

tU

Q

Cz

0 1 8 3 4 5 6

Fig. 20. — Muscovite mica.

plotted) were identical with the above. The wave-lengths of the maxima
at C\, C2 are 18.4 and 21.25 t1 (Rubens and Nichols) and are used in the
calibration curve of grating No. 2, curve b, fig. 19.

RESIDUAL RAYS FROM CARBONATES.

Calcite (CaCOs).

(Grating No. 2. K = 0.2120 mm.

Fig. 21.)

The plane cleavage faces of 3 large crystals were used in this exami-
nation. The spectrometer slits were 4 mm. wide. In fig. 21 the central

1 It is a pleasure to note the general adoption of the expression " residual rays " for
the meaningless " reststrahlen," formerly used. The English vocabulary seems sufficiently
complete to enable writers to find equivalents to " etalon," " entladungsstrahlen," etc,

3°

INFRA-RED REFLECTION SPECTRA.

image A is plotted to one-fifth the scale of Bx and B2, while the scale of
C\, C'2 and D\, D'2 is five times that of Bu B2. The sharp maxima at
B1 B2 are due to the complex reflection band at 6.7 /x, previously studied.
In the present case the maximum occurs at i°4o/, which corresponds to
6.78//. The wave-length of the maximum at 50 45', Du D2 is 29.4 ix as
determined by Aschkinass (loc. ciL). He located the maximum at C1} C2
at 1 1.4 /1 in the rock-salt spectrum, but did not find it in the grating spec-
trum. From his observations he predicted a weak band in the region of
15 to 20//. The asymmetrical part of the reflection curve at +40 indi-
cates a possible band at 14 to 16 jx, beginning also in the transmission curve

2 10/

Fig. 21. — Calcite.

of calcite, previously studied (see Carnegie Publication No. 65, p. 70), where
the substance becomes entirely opaque at 14 [x.

The band at 11.4 ;x deserves further study because of the possibility of
its being dependent upon the direction of polarization of the incident
energy.1 In the transmission curve of calcite previously studied (see Car-
negie Publication No. 65, p. 70) it was found that the region of 9 to 14 it
is quite transparent. A small absorption band was found at 11.3 jx, which
is so inconspicuous that it would appear impossible for it to give rise to
selective reflection. Of all the substances examined this is the first example
(see, however, nitrosodimethyl aniline) where a small absorption band
apparently coincides with, or gives rise to, a reflecting band. In all other

1 While this paper is in press, Nyswander (Phys. Rev., 26, p. 539, 1908), using polarized
light, found that the band at 11. 3 n is due to the complete absorption (reflection) of the
extraordinary ray, the ordinary ray showing no trace of an absorption band in this region.
At 14. 1 M is a band due almost entirely to the complete absorption of the ordinary ray.

RESIDUAL RAYS; CRYOLITE.

31

cases the transmission curves show complete opacity in the region of a
selective reflection band even for the thinnest film yet examined, viz, glass
(see Carnegie Publication No. 65, p. 65). That the band at 29.4 jj. is not
influenced by the structure of the crystal is proven by the fact that Asch-
kinass found this band in white marble.

Magnesite (MgC03).
(Grating No. 2, fig. 22.)

In this examination three reflecting surfaces of the massive material
were used, one of which did not have a high polish. The maxima at
B1} Cv Z>! correspond to wave-lengths 6.7, 11.3, and 30.7/* (50 52'),
respectively. In fig. 22 the maxima C\, C'2 (=11.3/1) and D\, D'2
(=30.7 jj) are drawn to ten times the scale of Bu B2. The band at Dx is

90

777777

80

70

60

SO

w

c
o

o

0)

£40

30

20

10

A\

27C777

i

,

J

D'g

2

mm

//

^

/

t

\ j

B2

>.

A

&

A

\!

\

7

t

\

/ j

\

\

>

Du

\

:

7" 65432/ 0 1 2 3 4 5 6 7 8"

Fig. 22. — Magnesite.

plotted to the same scale as Bt but only two reflecting surfaces were used.
The magnesite maxima are identical with those of calcite (except at a
possible greater wave-length, at 30 fx) . This would seem to indicate that
coincidence of the reflection spectra of chemically related groups of sub-
stances is a property which obtains throughout the spectrum. This is to
be expected, but it seemed of interest to add further experimental evidence
to support this view, by extending the observations into the remote infra-
red. The bands at 29.4 y. are too weak to illustrate the effect of molec-
ular weight, noticed at 6.7 to 7.2 \i in the carbonates.

RESIDUAL RAYS FROM CRYOLITE (3NaF-AlF3).
(Gratings No. 1; slits 4 mm.; fig. 23.)

Reasoning from the fact that cryolite is a fluoride of Na and Al, and
from the behavior of fluorite (CaF2), it was hoped to find this mineral to

l^V

32

INFRA-RED REFLECTION SPECTRA.

have properties similar to fluorite. That the expectations were not ful-
filled is perhaps to be attributed to the fact that the specimen was an
hydrated alteration product, as will be noticed from its transmission
spectrum (fig. 26) , which shows that the mineral contained water.

In fig. 23 is given the reflection curve of cryolite, four large, well-polished
surfaces being used. The central image was quite strong. The first-order

-2 -/ 0 ♦/

Fig. 23. — Cryolite.

diffraction band to the right and to the left is quite strong, being measured
in centimeters instead of millimeters. This, of course, is partly due to the
coarser grating. The mean angular position of the maximum is 20 43'.
From the calibration curve the wave-length of this maximum is 15.1 /'.,
which is just half the value of the most intense band of fluorite.

RESIDUAL RAYS FROM OTHER MINERALS.

Diaspore [AIO(OH)].

In examining the following minerals grating No. 1 was used in order
to obtain an intense spectrum. The slits were 4 mm., which is a little
smaller than those used by others. In Carnegie Publication No. 65 the
reflection spectrum of an intense double band was found at 14 to 15 ft.
From its general trend it was hoped to find the reflection spectrum to con-
tain further bands. Only two reflecting surfaces were available. With
these, the maxima at 15 ;>. in the rock-salt spectrum were verified, but no
bands were detected beyond this point.

RESIDUAL RAYS; SILICATES. $$

Stibnite (Sb2S3).

Stibnite was previously found to have a high reflecting power up to
15 /«, where it seemed to decrease. In the present examination, using
three reflecting surfaces, this high reflecting power was found to continue
throughout the infra-red. A small maximum was found at 2 /1, which no
doubt belonged to the third-order diffraction band. When using alumi-
num mirrors a similar band was found at this point.

Spodumene [LiAl(Si03)2].

Three large reflecting surfaces were used. The galvanometer was
perfectly steady, so that 0.2 mm. deflections could have been read, but no
radiation was detected beyond that due to the reflection bands at 9 to 10 /«
previously examined with the rock-salt prism.

Celestite (SrSCX,). Selenite (CaS04+2H20).

Three large cleavage specimens of each of these were examined, using
slits 4 mm. wide. In the case of celestite the central image gave a deflec-
tion of 50 + cm., but nowhere could radiation be detected except in the
region of 9 to 10 /« previously studied. Aschkinass (Joe. cit.) has predicted
a band in the region of 50 jx for the sulphates.

Apatite [Ca6F(P04)3].

Three large ground and polished surfaces were used in this examina-
tion. No reflection bands were observed except at 9 ft, where an exami-
nation with a rock-salt prism showed a weak (20 per cent) double band.
Apparently the phosphates have no strong reflection bands throughout the
infra-red spectrum. This is just the opposite of what has been described
in Chapter II on the oxides, where very intense bands were frequently
observed.

CYANINE (C29H35N2I).

(Grating No. 2; slits 4 mm.)

This substance was melted between glass plates, which were then split
apart, thus leaving plane smooth surfaces. Three such films on glass were
examined. It was previously found (see Carnegie Publication No. 35,
p. 82) that cyanine is very opaque at 6 to 8 //. The central image gave
a deflection of 50 + cm., and there was still a little reflected energy to be
detected at 25 (i, indicating a high reflecting power, but no reflection bands
could be detected throughout the spectrum.

SILICATES.

The reflection curve of mica has already been described in connection
with the calibration of the instrument. This examination included also
serpentine, albite, and microcline. The grating spectrum showed the
reflection bands at 9 ji, but beyond this no appreciable radiation could be
detected. This, of course, was to be expected for serpentine, for which

34

INFRA-RED REFLECTION SPECTRA.

the rock-salt prism showed only weak maxima at 8 to 10 p.. In a previous
study of quartz with a rock-salt prism a narrow, quite intense band was
located at 12.5 /*, which had not been recorded by Rubens and Nichols.
In the present examination, using grating No. 1, the maximum was found
at 20 21' (=12.8//), while with grating No. 2 the maximum was located
at 30 20' (=12.5^). In table I are given the maxima of the reflection
bands of the minerals studied.

Table I. — Maxima of Reflection Bands.

Smithsonite, ZnCC>3

Cerussite, PbC03

Strontianite, SrCOs

Witherite, BaCOs

Malachite, CuOC02-H20
Azurite, 3 CuO-2 C02-H20 . . .

Dolomite, CaMg (C03)2

Siderite, FeC03

Calcite, CaC03

Aragonite, CaC03

Magnesite, MgC03

Scheelite, CaW04

Zircon, ZrSi04

Wulfenite, PbMo04

Rutile, Ti02

Cyanite, Al203-Si02

Corundum, A1203

Topaz (Al-F)2Si04:

Yellow

White

Spodumene, LiAl(Si03)2

Muscovite Mica, H2KAl3(Si04)3

Cryolite, 3 NaF- A1F3

Beryl, Be3Al2(Si03)6

Quartz:

Glass

Crystalline

6.65

6.9

6.68

6.70

6.66

6.65

6.58

6.6

7-05
7-25

6.98

7-3
7-32
6-95
7-i

6.4?: 6.5

7.8?
7.8

6.62

7.0?
6.9

11. o
"•75

10.28
i3-5

10.9
11. o
10.4
10.2
6.0
9.9

8.8

11. 4

11.6
i3-i

"•3

12.?

18.4

i5-i

10.4

11. 4

30.2

21.25

16.?

M

29.4

CHAPTER V.

ON REGULAR AND DIFFUSE REFLECTION.

From his previous work on the reflecting power of silicates the writer
concluded that a surface like the earth or the moon, which is composed of
silicates, must be selectively reflecting. From an examination of some of
the subsequent discussions of the reflecting power of the moon, it would
appear that a matte surface can not be selectively reflecting. In fact, the
whole misunderstanding seems to hinge on this point. Since the surfaces
of the minerals examined by the writer were, in nearly all cases, plane and
highly polished, there was no "diffuse reflection," which is an entirely
different question from the one of low, "practically zero," reflection, which
the writer found to be a common property of certain minerals, for certain
regions of the infra-red spectrum.

When energy is reflected from a plane smooth surface, it is commonly
called "regular" (or, less accurately, "specular") reflection. On the other
hand, energy reflected from a rough surface suffers "diffuse" reflection.
The rough surface is equivalent to numerous small, plane, reflecting sur-
faces, the planes of which lie in all directions. In "diffuse reflection" for
each infinitesimal surface, the ordinary laws of reflection are obeyed in
full, unless the linear dimensions of the reflecting surface or of the ragosi- -
ties or inequalities on it are small compared with the wave-length. How-
ever, the unpolished surface as a whole destroys all phase relation between
the particles in the reflected wave-front, which is no longer plane, but
irregular. (See Wood's Optics, p. 36.) This irregularity decreases as the
angle of incidence increases, so that for a given roughness we get regular
reflection. The long waves will be reflected first, then the shorter ones.
"Smoked glass, which at perpendicular incidence will show no image of
a lamp at all, will at nearly grazing incidence give an image of surprising
distinctness, which is at first reddish, becoming white as the angle in-
creases." (Wood's Optics, p. 37.)

The amount of energy reflected "regularly" from a plane surface will
depend upon the reflecting power of the substance. Now, the reflecting
power R of any substance is related to its index of refraction n and its
absorption coefficient k by the equation

For "transparent media," i.e., "electrical non-conductors" (see

35

36

INFRA-RED REFLECTION SPECTRA.

Drude's, also Schuster's Optics), the absorption coefficient is so low that
it is negligible, and the reflecting power is a function of only the refractive
index. Here the reflecting power is low, only 4 to 6 per cent, and de-
creases with increase in wave-length. All transparent media thus far
examined (except silver chloride) have bands of selective reflection. In
these bands the absorption coefficient k attains high values.

If k becomes sufficiently large (see Schuster's Optics, Pockel's Crys-
tallographie, 1906), of the order unity, the absorption affects the reflecting
power, and the heat and light waves no longer enter the substance, but are
almost totally reflected, as in metals, whence the name, "bands of metallic
reflection." For metals, "electrical conductors," the absorption coefficient

2 3 4 5 6 7 8 9 10 II 12
Fig. 24. — Reflecting power; gold, silver, quartz, carborundum.

I4-/1

is so large that nearly all the energy for all (gold and silver are exceptions)
wave-lengths is reflected. In other words, the reflecting power of plane
surfaces ("regular reflection") of "transparent media" (electrical non-
conductors) will be low in all regions of the spectrum, except where there
are bands of "metallic reflection." It is evident that the diffuse reflection
from rough surfaces of transparent media must also be selectively reflecting.
For electrical conductors the reflecting power is high throughout the infra-
red spectrum. The great dissimilarity in the reflecting power of these two
classes of substances is well illustrated in fig. 24, which contains the graphs
of the reflecting power of gold, silver, quartz, and carborundum.

Apropos of the illustration just quoted, of the reflection from smoked
glass, Very1 "records that the percentage of reflected rays, as measured by

1 Very: Astrophys. Jour., 8, p. 278, 1898.

DIFFUSE REFLECTION. 37

the bolometer, is especially large (perhaps two or three times the usual
proportion) in the narrow crescent moon where the rays suffer a grazing
reflection at a large angle of incidence, the emission under the correspond-
ing angle of emission being small." This is particularly noticeable in the
early forenoon. Very's1 recent discussion gives the impression that, in the
moon, "specular reflection" is something to be sought for at the angle of
reflection with the sun, at which the image of the sun in rays of 8.5 to 10 /<
may be isolated by using a screen with a pinhole aperture. Here "spec-
ular reflection" appears to be used in the sense of "regular reflection."
The writer is not discussing this limiting case, but it appears self-evident
that a matte surface like a plane surface can be selectively reflecting, and
hence that a surface of quartz, for example, which, if smooth, reflects like
a metal for wave-lengths 8.5 and 9.03 p. and like a transparent medium
for all other wave-lengths, must still reflect selectively when it is rough.2
Hence, in the stream of energy reflected in any direction the density will
be greatest for wave-lengths 8.5 to 9.03 p.. Of course these bands of
"metallic reflection" will now be less intense. Since the energy density
in every direction must be greatest for wave-lengths 8.5 to 10 p, one would
expect to detect this difference in any direction, and not simply at the
angle of regular reflection.

If, then, the eye were sensitive to the infra-red quartz would have a
"surface color" corresponding to wave-lengths 8.5 and 9.03 p. In speak-
ing of surface color, however, a sharp distinction must be made, for the
reflecting power of these bands is as great as that of metals, although the
substance is a non-conductor; whence one would expect the reflecting
power to be low. In the case of transparent non-conductors the surface
color is due less to reflection than to absorption, for it is due to absorption
that the reflected light is deprived of some of its constituents and becomes
colored. However, on the long wave-length side of the absorption band
the reflecting power is high, which contributes to the color. For example,
the aniline dyes, such as fuchsine, have a low reflecting power (as compared
with metals) and yet they possess surface color. Pigments belong to the
class of substances having "body color." On the other hand, metals,
such as gold and copper, also have a surface color, due to selective absorp-
tion. They are electrical conductors, however, and theoretically would
totally reflect all radiations for all wave-lengths. This has been established

1 Very: Astrophys. Jour., 24, p. 351, 1906. Here the writer is quoted as having found
" that common minerals reflecting diffusively" from 4 to 8 /j. have bands of metallic reflection
from 8 to 10 M-

This quotation is erroneous. The writer found that the reflection was "regular" for all
wave-lengths, but that from 4 to 8 /". the reflecting power is low, as in cases of transparent
media having low refractive indices, while from 8 to 10 /x the reflecting power is high, like
metals, hence called "metallic reflection."

2 See footnote, p. 146, for experimental evidence supporting these statements.

38 INFRA-RED REFLECTION SPECTRA.

for long wave-lengths.1 The reflection curves of such metals as gold and
silver, however, are entirely different from those of the electrical non-
conductors (see fig. 24). In the former the extinction coefficient is always
high, while in the latter the extinction coefficient fluctuates through a great
range. In quartz the ions have a proper period of undamped electrical
vibration which almost totally reflects the wave trains in the region of
8.5 and 9.03 n, while for the shorter wave-lengths the vibration periods
are more damped, the reflection is enormously decreased, and more of the
energy is absorbed. In the case of a reflecting surface composed of elec-
trically non-conducting material, of the total energy falling on the surface,
the reflected energy in and near the visible spectrum will consist of that
reflected from the surface and that part which enters the particles and is
returned, due to internal reflection. In the region of 8 to 10 \i (for sili-
cates) almost all the observed energy will be reflected from the surface,
and hence none will be reemitted due to internal reflection. As a whole,
then, what the writer found is that the silicates reflect like "transparent
media" for all wave-lengths up to 8// and like metals from 8.5 to 10 fi.
In other words, it may be said that in quartz (Si02) the silicon ions retain
the proper period of undamped electrical vibration which they would have
in the metal, silicon, for wave-lengths 8.5 and 9.03 p., while, for all other
wave-lengths, the vibration periods are more damped and the reflecting
power is decreased.

1 Hagen and Rubens: Ann. der Physik, 8, p. 1, 1902.

PART VI.

INFRA-RED TRANSMISSION SPECTRA.

CHAPTER I.

INTRODUCTION.

With the accumulation of data the evidence becomes more and more
convincing that there are no strong bands of selective absorption (except
in the colored glasses given on a later page) in the region of short wave-
lengths, near the visible spectrum, such as are to be found beyond 6 /*.
Material suitable for investigation is obtained with difficulty. The present
contributions to the subject of transmission spectra is rather meager in
volume, but several of the substances show peculiarities worth recording.

The apparatus and methods of examining the following material, con-
sisting of a rock-salt prism and mirror spectrometer, were described in
Carnegie Publication No. 65. Instead of the radiometer, an improved
Rubens 1 thermopile was used to measure the energy in the transmission
spectra.

GROUP I: TRANSMISSION SPECTRA OF VARIOUS SOLIDS.

Molybdenite (MoS).

(From Yorkes Peninsula, South Australia. Foliated, distorted; thickness, J =0.05 and 0.31

mm.; fig. 25.)

In the preceding work it was noticed that the sulphides are, in general,
quite transparent. Sphalerite (ZnS) was found to have wide absorption

JU

* a

a .

• — 1 —

25

§20

<n
to

E /5
£,0

b

5

5 6 7 8 9

Fig. 25. — Molybdenite.

10

12

13

I4JU-

bands at 3 and 15 fx. On the other hand, stibnite (Sb2S3) is opaque to the
visible and has a second absorption band beyond 1 5 <x. In the interven-

1 See Appendix II.

4i

42

INFRA-RED TRANSMISSION SPECTRA.

ing part of the spectrum the absorption coefficient is very small, and the loss
of energy is due to the high reflecting power, which is about 40 per cent.
Molybdenite differs from stibnite in that it is very opaque throughout
the spectrum, examined to 15 p.. The two specimens examined were thin
folia, only 0.05 and 0.31 mm. in thickness (curves a and b, fig. 25). From
the greatly decreased transmission of the thicker piece, it will be noticed
that the loss of energy is due to absorption rather than reflection. The
absorption coefficient has not been computed, but an inspection of the
curves shows that it must be large. By increasing the thickness six times
the transmission is decreased 18 per cent, while in stibnite, by increasing
the thickness five times, the transmission is reduced only about 5 per cent,
in the region of general absorption.

90%

0/2345675
Fig. 26. — Kuntzite (a); Cryolite (6); Carborundum (c); Nitrocellulose.

(From Pala, California.

Kuntzite [LiAI(Si03)2].

Cleavage piece, parallel to m; transparent; ^ = 1.05 mm.
Curve a, fig. 26.)

This mineral, which is violet spodumene, is obtainable in large speci-
mens, hence it was of interest to determine its transparency to infra-red
radiation. It is lacking in the small band of Si02 at 2.95 /* and is more
opaque than quartz in the region of 1 //, so that it would not be more
serviceable than the latter in optical work.

VARIOUS SOLIDS. 43

Carborundum (SiC).
(Artificial product; t = i.2"j mm. Curve c, fig. 26.)

The specimen examined was a flat crystal which transmitted blue light,
and reflected yellowish-green light. It has remarkable optical properties,1
especially a high value for the refractive index. It is opaque in the ultra-
violet (Jewell) and beyond 2 p. in the infra-red. In this respect the trans-
mission curve c, fig. 26, is very remarkable as compared with all the other
substances examined, of which only one, viz, mellite, is as opaque to infra-
red rays. The reflection curve was examined previously; it also shows
remarkable properties.

Cryolite (3NaF • A1F3).
(From Ivigtut, Greenland. Massive, semitranslucent; £ = 2.3 mm. Curve b, fig. 26.)

From the composition of the material it was hoped to find this to be
more transparent to heat rays than usually has been the case with minerals.
Possibly a pure crystal of cryolite would be different. From the absorp-
tion bands (curve b, fig. 26) it will be noticed that this specimen contained
water of crystallization.

Thomsenolite (NaLiAlF6 + H,0), previously examined, is an alteration
product of cryolite, and from the similarity of the transmission curves it
would appear that this sample of cryolite had already undergone some
alteration into the hydrated mineral.

Nitrocellulose (Transparent "Celluloid").
(Curve d, fig. 26; £=0.138 mm.)

Transparent celluloid is a mixture of nitrocellulose [C12H16(NO2)4O10]
and camphor (C10H16O). The transmission curve which was obtained
with a fluorite prism and bolometer is conspicuous for two regions of
great absorption, with maxima at 3 //, 3.43 /*, and 6 //.

Camphor is a very complex carbohydrate. Its alcoholic properties,
OH groups, should cause an absorption band at 3 p. (2.95 /* in Carnegie
Publication No. 35, p. 58, for rock-salt prism). Its ketone properties,
CO groups (perhaps aldehyde properties, CHO groups), should cause a
strong band at 6 p.. Excellent examples of OH groups are (see Carnegie
Publication No. 35, p. 108) carvacrol and thymol, of CHO groups are
benzaldehyde, cuminol, eucalyptol, colophonium, and Venice turpentine,
and of both these groups is terpineol. The band at 3.43 y. is characteristic
of the CH3-groups.

Excepting in intensity, the transmission curve is similar to that of col-
lodium (Carnegie Publication No. 65, p. 60), which is also a complex
compound of cellulose. It seems rather remarkable that the characteris-
tic vibration period of these groups of atoms is not affected by the com-
plexity of the compound.

1 Jewell: Physical Review, 24, p. 239, 1907.

44

INFRA-RED TRANSMISSION SPECTRA,

Siderite (FeCOs).
(From Allevardj France. Cleavage piece; < = i.2 mm.; grayish color; translucent

to transparent. Fig. 27.)

This specimen was examined in connection with the carbonates pre-
viously studied. The complex band, with maxima at 3.4, 3.9, and 4.6 /*,
is in common with the other carbonates. In a very thin section of calcite

25%

20

c
o

"</>

.52 IS
£

(0

C
£/0

I-

/

^

\

1

I -

\

/

\

/

\

Vs.

/

\

■

\

2 3 4-

Fig. 27. — Siderite.

6/i

(CaC03) this region was resolved into sharp bands, having maxima at
3.44, 3.93, and 4.6 ix. This sample furnishes additional proof that the
large groups of chemically homologous compounds have similar absorption
spectra.

WULFENITE (PbMo04).

(From Red Cloud Mine, Yuma County, Arizona. Deep-yellow color; t = 2.i mm.

Curve a, fig. 28.)

This mineral is of interest in connection with a study of the oxides,
e. g., Ti02, Si02, etc. The transmission curve a, fig. 24, shows wide

30

25

C20
,0

V)

'E 15
w

c
<d

2 3 4-567

Fig. 28. — Wulfenite (a); Phosphorus.

IOJLL

absorption bands at 3.15 and 6.5^, and complete opacity beyond 9 /*,
where there is a band of metallic reflection.

VARIOUS SOLIDS.

45

Phosphorus (P).
(Solid film melted between plates of fluorite. / = o.8 mm. (?). Curve b, fig. 28.)

Phosphorus was examined in order to answer an inquiry whether it
could be used for prisms contained in vessels with quartz or fluorite
windows. The yellow stick phosphorus was used. It was melted under
water (at 440 C), poured on a glass plate, and pressed into a thin sheet
by means of a second plate of glass. This thin sheet was then placed
between two plates of fluorite and the edges covered with soft wax. As
will be noticed in curve b, fig. 28, the transmission curve shows bands due
to water which adhered to the phosphorus. The latter melted during the
latter part of the examination. The transmission curve includes the
fluorite plates, which were not clear, and hence increase the observed
absorption. QuARTZ (Si02)>

(i=3 mm. Curve b, fig. 29.)

The sample examined was used as a window on the spark-tube in the

emission spectra of metals in hydrogen, Part VII. The curve shows that

emission lines, if of appreciable intensity, would be transmitted out to 4 ;«.

100%

2 3

Fig. 20

4-5678

— Glass (a); Quartz (b); Fluorite.

2/1

Glass.
(Thickness = 0.7 5 mm. Curve a, fig. 29.)

This sample was examined in connection with prospective work on
radiation from incandescent lamps. The transmission is uniform (80 per
cent) out to 2 to 2.4 p.. From this it will be seen that the maximum of
the energy spectrum of the filament will not be affected by the glass bulb.

46 INFRA-RED TRANSMISSION SPECTRA.

Fluokite (CaF).

(Curve c, / = 2.s8 mm.; curve d, t = io mm.; curve e, / = i.84 mm.; curve/, /=3-85 mm.

light-green color; fig. 29.)

On account of the increased scarcity of this material, it is of interest
to determine the effect of inclusions upon the transmission. In fig. 29,
curve c was a perfectly clear specimen; while curve e contained numerous
small inclusions, or, perhaps more exactly, small cleavage planes. The
latter when held at a distance of 30 to 50 cm. from the eye appeared quite
blurred. Nevertheless, throughout the infra-red the loss of energy is only
about 2 per cent greater than that of clear fluorite. This is to be expected;
for each cleavage plane reflects some of the light, and the magnitude of
this reflection, since it depends upon the refractive index, decreases with
increase in wave-length. Curve / shows the transmission of a specimen
of light yellowish-green fluorite (see Carnegie Publication, No. 65, p. 69,
for another example) which has an absorption band at 1.4 /<, hence not
suitable for a prism. This specimen was free from inclusions.

Carbon (C).
(Curves a, b, c, lampblack; curve d, diamond; fig. 30.)

The commonest and most conspicuous example of the effect of structure
upon absorption is to be found in carbon in the form of lampblack and

100%

t=. 009mm

3 4 5 6 7 0

Fig. 30. — Carbon (a), (6), (c); Diamond.

of diamond. Since the observations on these two substances are published
in separate and rather isolated places they are incorporated here for com-
pleteness of illustration of the effect of structure. The transmission curves

VARIOUS SOLUTIONS. 47

a, b, c, thickness 0.038, 0.023, and 0.009 mm., respectively, of lampblack
are due to Angstrom.1 The transmission curve of diamond is due to
Julius.2 The lampblack is opaque to the visible and shows an increase
in transmission with wave-length. The diamond crystal is transparent to
the visible and has well-defined absorption bands at 3, 4, and 5 \i and
complete opacity beyond 8 /*. A band of metallic reflection is predicted
at 12 n, which happens to coincide with the band of carborundum.

It seems rather remarkable that diamond should have one of its prin-
cipal absorption bands in the region where carbohydrates have a wide
band of great transparency.

GROUP II: TRANSMISSION SPECTRA OF VARIOUS SOLUTIONS.

The data presented here were obtained several years ago (but not pub-
lished) in connection with an investigation of methods of measuring radi-
ant efficiencies.3 One of the methods of measuring the efficiency of an
illuminant, i. e., the ratio of the light emitted to the total radiation, is to
absorb the infra-red by means of a water cell. The idea persists even to
this day that a water solution of potassium alum absorbs more heat than
does clear water, although Donath and others demonstrated, long ago, that
this is not the case; the data herewith presented is further proof of this
fallacy. During the present year (Phys. Zeitschrift, 1907) measurements
of radiant efficiencies have been made using a water solution of ammo-
nium-iron alum, and it was claimed that the solution was more opaque
than pure water. A transmission curve of this substance will be shown
presently which indicates a greater opacity than water. The iron alum
oxidizes readily and it is difficult to keep the solution clear.

The problem was recently presented to the writer by the Astrophysical
Observatory to suggest a substance (see asphaltum) that absorbs all the visi-
ble or all the infra-red energy, the dividing line being 0.76 /*. Such a sub-
stance would of course be an ideal energy filter; but no such substance is
known. Indeed, from all the substances examined, it appears that none
(at least none of the common ones) have large absorption bands near the
visible. Beryl 4 is recorded as having a large band at 0.89 //, when ex-
amined in polarized light. Cyanine would be a fairly good material for
absorbing the visible, but it is very opaque in the region of 6 to 8 ft. Io-
dine would be much better material for absorbing the visible, and trans-
mitting the infra-red. Its absorption band, at about 7.3 ft, is very weak.
The absorption band of iodine in the visible spectrum corresponds closely
with the sensibility curve of the eye. Since we are concerned only with the
light that affects the eye, the fairest rating of efficiencies would be to com-

o

1 Angstrom: Wied. Annalen, 36, p. 717, 1889.

2 Julius: Konigkl. Akad. Wiss. Amsterdam, Deel I, No. i, 1892.
'Nichols & Coblentz: Phys. Rev., 17, p. 267, 1903.

4 Konigsberger: Ann. der Phys. (3), 61, p. 687, 1897.

48

INFRA-RED TRANSMISSION SPECTRA.

pare the energy that affects the normal eye to the total energy. This would
lower the present efficiencies of sources that are rich in the red, which
color affects the eye but little.

The use of iodine in solution is prohibited by the selective absorption
of heat rays by all known solvents. On the other hand, solid iodine
evaporates readily, so that it would be impossible to keep a solid film of
uniform thickness for any great length of time.

ASPHALTUM.

(Curve c, f = o.i mm ; 0 = 0.0005 mm.; 6 = 0.03 mm.; d = 0.005 mm.; fig. 31. For the complete
curve, to 14 /x, see Carnegie Publication No. 35, p. 75, and fig. 44, p. 198.)

The next best substance to iodine for absorbing the visible and trans-
mitting the infra-red is asphaltum, which is a solid hydrocarbon, previously
examined. This substance can be formed into films of any desired thick-
ness. In fig. 31 curves a and b are due to Nichols.1 From the curves it

■100%

Z 3JU. 4

Fig. 31. — Asphaltum.

7U

will be seen that a thickness can be selected which fulfills the conditions
of complete transparency beyond 0.7 jx better than any other common
substance. Even the thick film (curve c, / = o.i mm.), which transmitted
a trace of red, shows great transparency, after passing beyond the effect
of the absorption band in the visible. A film absorbing up to 0.65 or 0.7 ;i
would be practically transparent in the infra-red (see Carnegie Publica-
tion No. 35, p. 17), where, for a very thin film of water, the small absorption
band at 4.75 /1, similar in intensity to those of asphaltum, has entirely
disappeared. By using a clear rock-salt plate, covered on both sides to
prevent the action of moisture, a fair standard could be produced. The

1 E. L. Nichols: Phys. Rev., 14, 204, 1902.

VARIOUS SOLUTIONS. 49

asphaltum may be applied to the rock-salt as a thin varnish, the volatile
parts of which evaporate in a short time.

Asphaltum varnish, applied in an extremely thin coating, forms an excel-
lent protecting surface for radiometer or radiomicrometer windows made
of rock-salt. It might also be used advantageously as a covering for
rock-salt prisms to protect them from moisture. The pair of films of
asphaltum to be used for such purposes can be made extremely thin, and
are almost transparent, as shown in fig. 31, curve d, which was of a light
amber color showing but little absorption in the visible spectrum. The
film was prepared from commercial asphaltum varnish, which is a very
thick solution, by diluting it with gasoline. This precipitates the very
insoluble constituents, which apparently are simply held in suspension,
and by experimenting on the amount of gasoline to be added, a film or
pair of films of any desired thickness may be produced by a single dipping
of the plate or prism. Of course, a thicker film may be formed by using
a thicker solution, or by applying several coats of the thin solution. By
filtering the dilute solution through cotton wool or through filter paper
the black insoluble asphaltum particles are removed. The dried film is
then homogeneous and perfectly free from even traces of black particles.
The gasoline must be quite free from moisture, otherwise there is likely to
be a slight action on the rock-salt surfaces, and the dried film will not be
so homogeneous as that obtainable on a glass plate. It may be possible
to apply a homogeneous film of paraffin or of ozokerite, which is white,
and which, on account of its low refractive index, will reflect less than a
rock-salt surface; but neither of these substances are as satisfactory as
asphaltum purified, as indicated above. Such surface coverings for hydro-
scopic substances are, of course, useful only where the radiometer and
prism are used in making relative comparisons, i. e., it is not adapted to
making spectral energy measurements of sources of radiation, where the
absorption of the apparatus and the atmosphere must be eliminated.1
Curve d is for two films on a plate of glass, 0.175 mm- thick. The values
are obtained by dividing the observed transmission, when the glass was
coated with asphaltum, by the observed transmission through the clear
glass. The fact that the transmission is greater than 100 per cent is due
to the higher reflecting power of glass, for which no correction was made.

1 Since writing this, Dr. A. Trowbridge, at the Washington Meeting of the American
Physical Society, April, 1908. described the transmission of extremely thin films of collodium
(Carnegie Publication No. 65, p. 60, fig. 46) and their application to rock-salt as a protecting
surface. For the region of the spectrum up to 6 m there are no prominent absorption bands,
and the small ones at 3.3 p. have almost entirely disappeared in the thinnest films, which are
of the order of a light-wave in thickness. Collodium is said to be slightly hydroscopic, but
whether it is sufficiently so to become separated from the rock-salt is unknown. At this
meeting it was learned that at the Astrophysical Observatory it is proposed to use asphaltum
varnish to protect the large rock-salt prism which is about 18 cm. high.

5°

INFRA-RED TRANSMISSION SPECTRA.

Alum Solution.
(Saturated solutions at o° C. ; cell i cm. thick; fig. 32.)

In fig. 32 are given the transmission spectra of water (0-0-0-0) and
of solutions of the alums of potassium (x-x-x), ammonium (•_._•_•), and
of ammonium-iron. The curves are due to E. F. Nichols1 and are in-
cluded here because of their bearing upon the present data. The trans-
mission curves of the solutions of potassium and of ammonium alum are
identical with that of water. The ammonium-iron alum [(NH4)2Fe2(S04)7]

80

5 .7 9 /. / 1.4-jU

Fig. 32. — Water 0-0-0; Potassium alum
x-x-x; ammonium alum •-•-.-.; ammo-
nium iron alum (lowest curve).

shows greater opacity, which from the trend of the curve appears to extend
into the visible spectrum, due perhaps to a trace of iron oxide which is
avoided with difficulty. The transmission curve of a clear plate of alum
4 mm. in thickness is shown in curve a, fig. 33.

Borax and Potassium Permanganate.
(Cell 1 cm. thick; glass walls; fig. 33.)

In fig. 33 are given the transmission curves b of borax (Na2Br407) and
curve d of potassium permanganate (KMnOJ. The latter solution was
not saturated. The results show that there is no increased absorption of
the solution over that of pure water, except in permanganate, which, of
course, is almost opaque to the visible.

Lanthanum Nitrate (LaNOs); Didymium Nitrate [Pr,Nd(NC>3);i];

Sulphuric Acid (H2S04); Liquid Glass (NaiSiOs).

(Cell 1 cm. Concentration of nitrates unknown. Fig. 34.)

In fig. 34 are given the transmission curves, a of sulphuric acid, b of
lanthanum nitrate (solution), c of didymium nitrate (solution), and d of

1 E. F. Nichols: Phys. Rev., 1, p. 1, 1896.

COLLOIDAL METALS.

51

liquid glass. The sulphuric acid is more transparent than water, while
the didymium nitrate has a sharp absorption band at 0.76 ft. The latter

90%

1.7, U

Fig. 33. — Potassium alum (a); Borax solution (b);
Water (c); Potassium permanganate solution.

Fig. 34. — Sulphuric acid (a); Lanthanum nitrate (6);
Didymium nitrate (c); Liquid glass.

have absorption bands in the visible, so that they would not be useful for
efficiency work. The same is true of liquid glass.

Chlorides of the Yttrium Group (Sc.Yt, La,Yb); Neodymium Nitrate [Nd(N03)3].
(Cell i cm. Concentration unknown. Fig. 35.)

These solutions, like the preceding ones, were obtained from the Chem-
ical Laboratory of Cornell University, and the concentration was unknown.
They were examined to learn whether the sharp absorption bands in the
visible are also to be found in the infra-red. It will be noticed that the
didymium nitrate (curve b, fig. 35) has two bands at 0.78 and 0.98 p,
while the yttrium group of chlorides have a band in common with didym-
ium at 0.98 pt.

GROUP III: TRANSMISSION SPECTRA OF COLLOIDAL METALS.

The metals and non-metals present two distinct types of absorption, of
reflection and of emission spectra. The metals (electrical conductors), even
in very thin films, are extremely opaque to all radiations throughout the
spectrum, except gold, silver, and copper, which have narrow transparent
bands at 0.32, 0.5, and 0.6 p, respectively.1 The opacity is really due to

1 Hagen & Rubens: Ann. der Phys., 8, p. 432, 1902; Javal: Ann. de Chim. et Phys. (8),
4, P- 137, 1895.

52

INFRA-RED TRANSMISSION SPECTRA.

80%

70

60

50

C

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their high reflecting power, which is uniform throughout the infra-red.
Their emission (arc and spark) spectra consist of numerous fine lines,
which occur throughout the visible and ultra-violet part of the spectrum.

No strong lines have yet been found in
the infra-red, except in the alkali metals.
On the other hand, the non-metals (insu-
lators) have absorption bands throughout
the spectrum. Their reflecting power is
low, and in some compounds is highly
selective in the infra-red. They have
emission lines which extend far into the
infra-red. The elements on the border
line between metals and non-metals are
of peculiar interest, and the reflecting
power of their compounds deserves further
study. A notable example is silicon,
which from the known properties of non-
metals would be expected to have a uni-
5A formly low reflecting power throughout the
fig. 35- — chlorides of Yttrium Group (a); infra-red; in the form of a compound,

Neodymium nitrate. o-^> ,i n . o i

SiG2, the reflecting power at 8.5 and 9 jj.
attains a value as high as that of metals. The elements iodine, selenium,
and sulphur have a uniformly low reflecting power. It would be inter-
esting to learn whether only the compounds of non-metals have bands
of selective reflection in the infra-red; all of our present data indicate
that only in a compound (cf. Si02) do the ions of non-metals attain a
freedom such as obtains in metals.

Silver.
(Colloidal film, on glass. Curve b, fig. 36.)

The film examined was made1 by Prof. R. W. Wood, and was of a
deep ruby-red color. In fig. 36, curve b, is given the transmission curve
of a thin uniform film of colloidal silver, deposited on glass. The trans-
mission begins in the red, and increases uniformly throughout the infra-red
to 4 fi, beyond which it was not possible to extend the observations on
account of the opacity of the glass. This is exactly the reverse effect
observed with a metallic film (see fig. 24), where the transparency lies in
the violet and the opacity extends throughout the infra-red. The film of
colloidal silver behaves like a turbid medium, which has the property of
increasing in transparency with wave-length. This is in agreement with
Garnett,2 who concludes from the optical properties of Cary Lea's so-called
solutions of allotropic silver that they consist of small spheres of silver, i.e.,

1 Cary Lea's Allotropic Silver, Amer. Jour. Sci., vol. 37, p. 746, 1889.

2 Garnett: Proc. Roy. Soc. (A), 76, p. 370, 1905-06; Phil. Trans., A, pp. 385-420, 1904.

COLLOIDAL METALS.

53

normal silver in a finely divided state, little if any being in true solution
(molecularly subdivided). He further concludes that the color of ruby
gold glass is due primarily to the presence of small spheres of gold; the
irregular blue and purple colors, sometimes exhibited by gold glass, are
then explained by the presence of crystallites (caused by the coagulation
of gold spheres) which reflect but do not transmit red light.

100%

90

80

70

60

C

o

if) 50

'e

It)

30

20

10

d^ {Au (Colloidal)

0 .5 / 2 3

Fig. 36. — Gold; Silver; Nickel.

In view of the fact that colloidal metals appear to behave exactly the

reverse of films of a metal in the normal state, the subject deserves further

consideration.

Gold and Nickel.

In fig. 36, curve a gives the transmission (Hagen and Rubens, loc. cit.)
of a metallic film of gold, which has the well-known transmission band in
the yellowish-green part of the spectrum. Curves c and d give the trans-
mission of suspensions, in water, of colloidal gold and nickel, respectively,
studied photometrically by Ehrenhaft.1 The gold transmitted red, while
nickel (also Pt and Co) have a brown color by transmitted light. As in
the present work, the behavior of the colloidal material is exactly the
reverse of the normal metal film, thus placing it under the class known as
non-metals or " insulators."

1 Ehrenhaft: Ann. der Phys., II, p. 489, 1903.

54

INFRA-RED TRANSMISSION SPECTRA.

GROUP IV: TRANSMISSION SPECTRA OF COLORED GLASSES.

Cobalt Glass.
(Fig. 37. Thickness, 2.43 mm.)

Colored glasses have been but little studied. Nichols1 examined cobalt
glass, and found an absorption band at 1.4 ;i. This is one of the few
substances having a prominent band near the visible spectrum. Garnett
(loc. cit.) concludes that the deep blue color of cobalt glass can not be
due to small diffused spheres of metallic cobalt, which would give a reddish
color to transmitted light, but that the metal is in the form of discrete
molecules (amorphous).

For the present work a fluorite prism and bolometer were used (see
Appendix II).

2 3

Fig. 37. — Cobalt blue glass.

In fig. 37 is given the transmission curve of a cobalt blue glass which
showed three weak absorption bands in the visible spectrum. In the
infra-red there is a larger absorption band at 1.5 p., a smaller band at
2.1 fi, and second large band at 3.5 p. The small band at 3 /i is due to
the glass itself, and is also found in quartz. Beyond 4 p. the opacity is
due to the glass, as found in clear specimens.

The behavior of this glass is entirely different from the red glasses to
be noticed on a following page. It is not like an optically turbid medium,
unless we consider it to have more than one region of selective absorption,
which is in line with the conclusion arrived at by Garnett {loc. cit.).

"Nichols, E. F.: Phys. Rev., 1, p. 1, 1893.

COLORED GLASSES.

55

Blue-violet Glass.
(Schott's1 No. F 3086; ^ = 2.58 mm.)

In fig. 38 is given the transmission of a plate of blue-violet glass. There
is a large absorption band at 0.5 to 2 //, followed by smaller bands at 2.1
and 3.5 /i. In the deep infra-red such a band would cause selective re-
flection. In the present case the absorption band is due to the metal
behaving somewhat like an optically turbid medium. Using a pocket
spectroscope close to a Nernst glower, it was found that a trace of yellow

Fig. 38. — Blue-violet glass.

at 0.56 n and of red at 0.707 /x was transmitted, the intensity of the maxima
being of the order of 0.01 per cent. In the violet the transmission was
30 per cent. This region was studied with a spectrophotometer. Ang-
strom has very ingeniously applied this glass in measuring the violet radia-
tion from the sun; using an absorption cell of water, 1 cm. thick, in addition
to this glass plate, all the radiation beyond 0.5 is absorbed.

Green Glass.
(Schott's copper oxide, No. 431 III. Fig. 39. Thickness ^ = 3.43 mm-)

The specimen examined showed an absorption band in the violet and
a second one in the red. There appears to be a small band at 2 /*, beyond
which point there is the usual absorption of uncolored glass. The band
at 2.95 p, found in silicates, e.g., mica, seems to be intensified by the pres-
ence of coloring substance, while the transmission seems to be terminated
at a shorter wave-length (also due to the coloring substance) than would
be found in a clear plate of glass of the same thickness.

Ruby Glass.

(Fig. 40. Curve a, Schott's monochromatic red, No. 2745; ^=3.18 mm.

Curve b, impure red; / = 2.48 mm.)

The monochromatic red glass examined is used in the Holborn and
Kurlbaum pyrometer. The second sample, curve b, transmitted impure

1 See Zsigmondy, Zs. fur Instrk., 21, p. 97, 1901.

56

INFRA-RED TRANSMISSION SPECTRA.

red; it was taken from a Le Chatelier pyrometer and is described as a
copper oxide glass. Although thinner, the second sample is the more
opaque in the region of i to 2.5 ,u. The transmissivity of the monochro-
matic red glass is very unusual. In the region of 1 to 2.5 p. the metal
which is used to color the glass renders it more permeable to heat waves
than is ordinary, visually transparent glass. The band at 2.9 p appears
to be intensified by the presence of the metal, which behaves like the

100%

90%

80

70

60

c
0

' <f>
(0 50

'I

(0

1-
t-

30

20

10

/'I

/

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1

1

I

l\

1

j

1

l\

1

1

J

*M

Fig. 39. — Green glass.

12 3 4-

Fig. 40. — Ruby glass (a) and (6); Sphalerite.

colloidal suspensions just mentioned. In fact, it is a pertinent question
whether the red color in glasses is due to the presence of metals, such as
copper and gold, in the colloidal condition. The monochromatic red
glass was found to have a uniform reflecting power (about 4 to 5 per cent)
throughout the spectrum to 8 [x. This seems to show that the transparent
region at 1 to 2 ti is not due to the same cause that produces an abnormal
transparency on the short wave-length side of a region of anomalous dis-
persion.

Black Glass.

(Fig. 41. Curve a, £ = 2.48 mm.; curve b, Schott's Rauchglass No. 444, III, t—s-6 mm.)

Curve a gives the transmission of a dark "neutral" glass, colored with
the oxides of cobalt and nickel. This glass has a uniform absorption
throughout the visible spectrum. There are small absorption bands at
2 and 3.5 p., respectively. Curve b gives the transmission of a very dark
glass, which shows an absorption band in the region of 1.5 ,«.

Considered as a whole, these glasses can be divided into two groups,

COLORED GLASSES.

57

viz, (i) glasses having absorption bands in the visible and in the infra-red

spectrum, and (2) glasses having a wide absorption band in the optical

region, followed by an increase in transparency with wave-length. The

red glasses belong to the latter group, and

their behavior is more nearly like that of

optically turbid media, the color being due

to the presence, in the glass, of small

spheres of metal (see Garnett, loc. cit.).

To the first group belong the cobalt blue

and Schott's blue-violet glass. If the color

in some glasses is due to the metal, one

would expect great opacity in the infra-red,

as is known for a thin film of the metal. If

the metal is present in a colloidal form, the

present examination of colloidal silver

would indicate great transparency, with the

possibility of small absorption bands in the

infra-red. The whole question is in an

unsettled state, and an investigation of the

transmission of various glasses, blown into fig. 4i- — Black glass.

thin films, as described in Carnegie Publication No. 65, page 65, seems

highly desirable.

In leaving this subject it is of interest to note that a specimen of garnet
(Carnegie Publication No. 65, p. 59) was found to have a wide absorption
band, with complete opacity extending from 1.2 to 2.6 p. This is one of
the few substances having a large absorption band near the visible spectrum.

Sphalerite (ZnS).
(Cleavage piece; ^=1.53 mm.; transparent; slight yellowish tinge; curve c, fig. 40.)

This substance was previously examined (Carnegie Publication No. 65,
p. 63), using for the purpose a rock-salt prism and a radiometer. In the
present examination, the same mirror spectrometer, but a fluorite prism
and a bolometer were used. On account of the small dispersion at 1 \x
the rock-salt prism is not well adapted for work in this region. The
transmission curve of sphalerite appears to have a band at 1.8 /<. On
examination of the original data and curves, it appears that in redrawing
the curve for publication the draftsman placed the point at 1.3 (i a little
too high (should read 59 per cent), which in the reduced illustration makes
a depression at 1.8 p..

The curve obtained with the fluorite prism is given in fig. 40, and
represents conditions more accurately. A Nernst glower was used as a
source, which gave large deflections in this region of the spectrum.

The transmission curve increases uniformly from 47 per cent at 0.6 fi
to 56 per cent at 1 p., then decreases uniformly to 54 per cent at

58 INFRA-RED TRANSMISSION SPECTRA.

1.8 n, beyond which the transmission drops rapidly to a minimum at 2.9 [i,
previously found. Using this larger dispersion, there appear to be no
absorption bands up to 1.9 /*. The study of zinc sulphide is of interest in
connection with luminescence of one form of this substance known as
Sidot blende. In their study of the rapidity of decay of phosphorescence
in Sidot blende, when subjected to infra-red rays, Nichols and Merritt1
found that the rate of decay was the most rapid when exposed to infra-red
rays of wave-length 0.9 n and of wave-length 1.37 [i. From this it would
seem that the screen of Sidot blende possessed broad absorption bands
with maxima in the region of 0.9 p. and 1.37 ll. The luminescence of
Sidot blende appears to be due to some metal dissolved in it. From the
foregoing experiment on sphalerite (ZnS) it appears that these absorption
bands are not due to the ZnS, but rather are to be sought for in the dis-
solved metal (see cobalt glass) , and in the cement, in case a glue is used to
secure the powder in the form of a " screen." The present study of colored
glasses gives us some idea of what to expect in solutions of metals (oxides
of metals?) in glasses; while the phenomenon maybe further complicated
by the molecular structure of ZnS, which in the form of Sidot blende
may have absorption bands not found in sphalerite.

1 E. L. Nichols and E. Merritt: Phys. Rev., 25, p. 362, 1907.

CHAPTER II.

EFFECT OF SPECIAL GROUPS OF ATOMS OF RADIANT ENERGY.

In the present chapter it is purposed to discuss some of the relations
found among the infra-red spectra of the various substances examined;
and incidentally to explain some apparently inconsistent statements made
in Part I on the effect of certain particular groups of atoms in producing
characteristic absorption bands.

COMPARISON OF ULTRA-VIOLET AND INFRA-RED.

Hartley1 found that an open chain of CH-groups, as well as a union of
C and N atoms, produced no absorption bands in the ultra-violet. Neither
could he identify any absorption band, of any of these substances, with the
carbon atom in the benzene ring; the molecules behaved as though there
were no special atoms present. This is just the opposite of what was found
in the infra-red. But the length of the infra-red spectrum examined is 60
times as long as the ultra-violet. It is, therefore, possible to find relations
in the infra-red which, on account of the narrowness of the spectrum, may
not be found in the ultra-violet. In the infra-red there are transparent
regions between characteristic absorption bands, which are often several
times the width of the entire ultra-violet spectrum. The petroleum distil-
lates (chain compounds) have a wide transparent region between 4 and 5 [i}
just as Hartley found in the ultra-violet. In general, if we were to examine
a narrow region of the spectrum, e.g., the ultra-violet, or at 4 /1 in the infra-
red, it appears to be a matter of chance whether or not absorption bands
will be found there. In the visible spectrum many substances have ab-
sorption bands, i.e., are colored. If the eye were sensitive to rays of
wave-lengths 3 to 3.5//, all the carbohydrates would appear "colored,"
and the position of the maximum of the absorption band would be just
as irregular as those found in the visible and in the ultra-violet spectrum.

THE HYDROXYL GROUP.

During the preparation of the third volume of Kayser's "Handbuch
der Spectroscopic" only the writer's "Preliminary Report on Infra-Red
Absorption Spectra"2 was available to the compilers. They call attention
to the fact that "the writer found an effect due to special groups of atoms,
particularly OH and CH3. Although many substances, which contain the

1 Kayser: Spectroscopy, 3, p. 169 and p. 170.

2 Coblentz: Astrophysical Journal, 20, 207, 1904.

59

60 INFRA-RED TRANSMISSION SPECTRA.

OH-group, have an absorption band between 2.9 and 3 p, Coblentz ques-
tions whether they are due to the OH-group. On the other hand, he
thinks the CH3-group causes the band at 3.43 p."

This inconsistency is easily explained. One need but examine the
third volume of Kayser's Spectroscopic, pages 81, 83, 84, 88, 283, etc.,
and see the numerous exceptions to "Kundt's Law," to be convinced that
the announcement of that "Law" was premature. Knowing how subse-
quent investigators were misled by this announcement, it seemed to the
writer that, so long as there were any serious exceptions, the effect of
special groups of atoms on heat-waves should be doubted until the evidence
was much greater than obtained at that time. The mineral brucite
[Mg(OH2)] caused this doubt; for if there is an effect due to the OH group,
one would expect to find it in such a simple compound as this one. The
first examination of brucite showed no band at 3 p, but a wide band with
a maximum at 2.6 p. Since then this mineral has been re-examined, using
a larger dispersion, and the large band was resolved into its components,
with maxima at 2.5, 2.7, and 3 p. In other words, this mineral has the
characteristic band of the hydroxyl group. In the meantime, numerous
other substances (see Carnegie Publication No. 65) containing OH-groups
and water of crystallization have been examined. In Carnegie Publication
No. 65, the exceptions to the rule that the OH-group has a characteristic
absorption band at 3 p were found among minerals of which the chemical
constitution is in doubt. The absorption band at 3 p, found in substances
containing hydroxyl groups, is, therefore, to be ascribed to that group of
atoms. The band at 3 p found in the alcohols belongs, therefore, to the
OH-group, and one ought not expect (as the writer stated in Carnegie
Publication No. 35, p. 108) to find a second band at 6 p (which is found
in water), if the 3 p band is due to the OH-group. The OH group
apparently does not produce a harmonic series of bands such as are to
be found in compounds containing CH2 or CH3-groups. Water also has
a harmonic series of bands, but it has not yet been shown to be due to
the OH-group.

THE EFFECT OF MOLECULAR WEIGHT OF THE MAXIMA.

In Part I a search was made for a shifting of the maximum of an ab-
sorption band with an increase in the number of atoms, or especially of
groups of atoms, in the molecule. The evidence was very contradictory.
In the case of the methyl derivatives of benzene, the maximum of the
band was found at 3.25 p in benzene, at 3.3 p in toluene (C6H5CH3), at
3.38 fx in the xylenes [C6H4(CH3)2], and at 3.4 p in mesitylene [C6H3(CH3)3].
In other words, by substituting three CH3-groups for an H atom, we have
shifted the maximum from 3.25 to 3.4/'. Several gases showed a shift of
the band, in the region of 3.2 to 3.5 p, toward the long wave-lengths with
an increase in the number of H atoms. On the other hand, several nitrogen

EFFECT OF MOLECULAR WEIGHT.

6l

50%

derivatives of benzene showed a shift of the maximum toward the short
wave-lengths. An examination of two petroleum distillates, C8H18 and
C24H50, showed no shift of the maxima, and on the whole the evidence of
a shift, due to a change in molecular weight, was inconclusive.

In Carnegie Publication No. 65, p. 56, the sulphates showed absorption
and reflection bands which seemed to shift toward the long wave-lengths
with increase in molecular weight. This data will be discussed in the
present paper.

Ever since the announcement of "Kundt's Law" various observers
have tried to find relations among the absorption bands, viz, such that
with an increase in molecular weight the band shifts toward the long
wave-lengths. The results have just been discussed in the first part of
this chapter. There is a physical basis for expecting a shift of the band
with change in molecular weight. For example, the sulphate of the metals
K, Rb, and Cs have been compared by Tutton,1 who has shown that both
as regards crystalline form, specific gravity, thermal expansion, and corre-
sponding refractive indices the Rb salt lies between the K and Cs salts.

In Carnegie Publication No. 65, the sulphates of Mg, Ca, Sr, Cd, and
Ba were found to have the maximum of
their absorption at 4.5, 4.55, 4.6, 4.6, and
4.63 «, respectively. The same shifting of
the maximum was observed in the reflec-
tion bands of SrS04 and BaS04; in the
former the maxima are at 8.2, 8.75, and
9.05 fi} while in the latter the maxima are
shifted to 8.35, 8.9, and 9.1 //.

In the present paper all the available
data have been compiled, to show this shift.
In fig. 42 is given a composite of the re-
flection curves of the carbonates [(Mg, Ca,
Zn, Ba, Pb) C03] described on a previous
page. The maxima are evidently double,
although it is not very apparent in some
curves. As a whole, however, the location
of the maxima of PbC03 is so much farther
in the infra-red than in MgC03 that there
can be no doubt that the shift is a real one.

In fig. 43 are given the graphs of the
maxima of the reflection band plotted
against atomic weight of the basic atom in the molecule. The carbon-
ates of Mg, Ca, Zn, Sr, Ba, Fe, Cu, Pb are included. In this figure the
two maxima of the complex band at 6.4 to 7.2 [x are plotted. When the

Fig. 42. — Reflection curves of carbonates.

See Miers's Mineralogy.

62

INFRA-RED REFLECTION SPECTRA.

maximum is not sharp and well denned, several readings were made and
plotted in the figures. This method of reading a flat band establishes a
limit within which the maximum may lie. The two graphs are approxi-
mately parallel, and show a uniform shift of the maxima of Mg, at 6.55
and 6.86 ft to 6.94 \i and 7.18 fi, respectively, in Pb. The carbonates of Fe

6.4 6.6 6.8 7.0

11.4 ll.b 11.8/1- 14.0 14.2 14.4 14.6 14.8 l5jU

Fig. 43. — Maxima of reflection bands of carbonates.

and Cu lie farthest from the curves; but they belong to a different group
in Mendelejeff's table. The graphs at n /< and 14 p. are due to Morse,
see Appendix III.

In fig. 44 are plotted the maxima of the reflection bands of the sulphates
[(H2, Mg, Ca, Sr, Ba)S04], previously examined (Part IV). Here there are

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— Maxima of reflection bands of sulphates.

EFFECT OF MOLECULAR WEIGHT.

63

four bands. As in the preceding, several readings of each ill-defined
band are plotted in order to get the limits between which it extends. It is
of interest to note that one of the H2S04 bands, at 8.62 /1, falls on the graph
of the second maximum.

In fig. 45, the maxima of the absorption bands of the sulphates [(Mg,
Ca, Sr, Cd, Ba) S04] are plotted (see Part III, Carnegie Publication No.
65, for data). It will be noticed that the graph of the band at 4.5 fi is
much steeper (the horizontal scale is different for these bands) than are
the ones farther toward the infra-red. The maxima of the absorption
bands lie close to the graph drawn through them.

Fb .,.Pb

riMmc in Max

45 46 4-.7/J. 45 4-.9 5.0 6.0 6.2 6.4 6.6 6.8/Z

Fig. 45. — Maxima of absorption bands of sulphates.

The second and third maxima of the absorption bands at 6.2 and 6.5 jj.
are quite close to the graph, except SrS04, which has but one maximum
lying midway between the two graphs. From this it would appear that
the SrS04 band may be the mean of two unresolved bands (see, however,
Part V, Chapter III). The slant of these graphs is less than in the pre-
ceding one.

For a discussion of the graph at 45 jj. see Appendix III.

Considered as a whole, the data presented demonstrate very conclu-
sively the influence of the molecular weight of the basic atom or "ion"
upon the position of the maximum. The results explain why the increase
in the number of the groups of atoms (see carbohydrates) had no effect
upon the position of the maximum. This is to be expected if the cause
of the band is due to the group of atoms; but the position of the band

64

INFRA-RED REFLECTION SPECTRA.

depends upon the metallic (basic) atom to which the group of atoms is
united to form a compound.

It is a remarkable fact that nearly all the substances examined, espe-
cially the oxides, have a region of strong selective reflection from 8 to 1 2 ;i.
Drude1 has shown that the optical properties {e.g., dispersion) lead to the
conclusion that each atom is a union of many independently vibrating
elementary masses or ions. In the ultra-violet the absorption band is due
to the sympathetic vibration of particles which have a charge and a mass
identical with the negative ion or "electron," while in the infra-red the
absorption and reflection bands are due to positive ions ("ponderable
atoms") which have a mass of the order of magnitude of the atom. From
this standpoint, one would expect to find a shift of the maximum toward
the long wave-lengths, as we increase the atomic weight of the element
which is attached to the radical.

The latest theoretical discussion of this subject is due to Einstein,2 who
shows how the theory of radiation, especially Planck's, leads to a modi-
fication of the molecular-kinetic theory of heat, and clears up points here-
tofore difficult. He also shows relations between the thermal and optical

properties of solid bodies.

Table II.

Element.

Atomic heat =

atomic weight X

specific heat.

X calculated.

Substance.

X observed.

X calculated.

S and P

Fl

O

5-4

5

4

3-8

2.7

2-3

1.8

P-

42

33
21
20

i5
13
11

CaFl

NaCl

KCl

CaCG3
Si02

24; 31.6

51.2

61.2

6.7; 11.4; 29.4

8.5; 9.0; 20.7

33; >48

>48

>48

12; 21; > 48

20; 21

Si

B

H

C

From the value of the atomic heat of a substance, he computes the
maxima of the bands of infra-red selective reflection. The observed
reflection bands and the same as calculated by Einstein are given in table II.
From this table it will be seen that carbon (diamond) would have a large
reflection band at 11 //, which is possible, as will be noticed in the trans-
mission curve, fig. 30.

This table shows that certain calcite maxima are due independently to
C at 1 1.4 [x and to O at 21 /t. In quartz the band at 20 /1 is due to Si at
20 n and O at 21 p.. From this line of reasoning, it would appear that the
large reflection band of carborundum at 12.5 [i is due to C, and that there
is another band at 20 \i due to Si. A similar computation for selenium
and iodine indicates a reflection band at about 42 /«.

1 Drude: Ann. der Phys. (4), 14, 677, 1904.

2 Einstein: Ann. der Phys. (4), 22, p. 181, 1907.

CHARACTERISTICS OF SILICATES. 65

CHARACTERISTIC BANDS OF QUARTZ AND OF SILICATES.

About half the minerals known are silicates. Their percentage com-
position has been ascertained by quantitative analysis, but little has been
possible, as yet, in establishing their constitutional formula?. In organic
chemistry the constitution of compounds has been determined by vapor
density determinations, by the preparation of series of derivatives, by the
replacement of certain constituents by organic radicals, or by studying
their physical properties in solution. In mineralogy this has, as yet, not
been possible, and the constitution of many of the minerals has been
derived mainly from analogies with other substances which are better
understood. The silicates are sometimes grouped as follows: disilicates
(RSi205) are salts of disilicic acid (H2Si205), and have an oxygen ratio of
silicon to bases of 4: 1; polysilicates (R2Si308) are salts of polysilicic acid
(H4Si308), with oxygen ratio of 3:1; metasilicates (RSi03) are salts of
metasilicic acid (H2Si05), oxygen ratio of 2: 1; orthosilicates (R2Si04), salts
of orthosilicic acid (H4Si04), oxygen ratio of 1:1.

Among the complex silicates, and often among the simple ones, not
only is the actual molecular structure in most cases doubtful, but even the
simple empirical composition of many species is still unsettled. A single
species may vary greatly in composition. This has been explained by
regarding the different forms as derivatives of a normal salt in which
various atoms or molecular groups may enter. It is not surprising then,
in the present limited study of their reflection spectra, to find no very
important relations among the reflection bands of silicates.

From previous work it is difficult to decide what one ought to expect
in the question of the coincidence of bands in SiO, and in silicates. In
the case of the carbonates there are no bands (except a small depression
at 4.6 /j. in common with CO) coincident with those of C02; and we might
expect a similar condition to obtain in the silicates and Si02. On the
other hand, in the carbohydrates having a structure indicating CH2 and
CH3 groups, there is a coincidence of certain characteristic bands with
those of ethylene (C2H4) and ethane (C2H6). The introduction of oxygen
atoms, however, modifies the spectrum, and, as a rule, there is no longer
a coincidence with the former bands. Furthermore, in S02 the maximum
at 8.7 11 is in coincidence with a similar band found in many sulphates,
while the band at 10.4 /« is in common with a similar one in fuming sul-
phuric acid and is probably due to S04. From this it is difficult to predict
what one ought to expect in the case of Si02 and of the silicates. From
previous observations that groups of chemically-related substances have
similar absorption spectra, one would expect to find a similar condition to
obtain in the various groups of silicates. The present investigation is not
extensive enough to draw general conclusions. Moreover, the constitution
of the minerals is in many cases in doubt, so that the lack of similarity in

O

66

Rn-Silicates l

Willemite Zn2Si04

Spodumene LiAl(Si03)2
(Sodium Silicate Na2Si03)
Enstatite MgSi03
Pectolite HNaCa2(Si03)2

Amphibole Ca(Mg3)(Si03)4

Talc H2Mg3(Si03)4

Serpentine HjMgsSi^Og

Deweylite H4Mg2Ni2(Si04)3-
4H20

Apophyllite (H,K)Ca(Si03), •
H20

R" '-Silicates

Topaz (Al,F)2Si04

Cyanite (A10)2Si03

' Orthoclase KAlSigOg

Microcline KAlSisOg
Albite NaAlSi3Og

INFRA-RED REFLECTION SPECTRA.

7JU 8/U 9yU IOjU

a

[ Muscovite H2KAl3(Si04)3
S !

§ Biotite (H1K)2(Mg,Fe)2(Al,Fe)2
I (Si04)3

Tourmaline (Na,Mg,Al,Fe.)
(BOH)(Si04)8

Natrolite Na2Al2Si3Oio+HtO
Datolite CaB2(Si04)2

Beryl Be3Al (Si03)6

R^-Silicates.

Zircon ZrSi04

g ["Quartz (crystal) SiQ2

-a

■a
o

^ Quartz (glass)

IIJU

I2JU

1

1

1 1

*

-

1 1

1

1

'

-

—

#■

n

1 1
1

, 1

1

—

—

1
, 1

r

1

1 ,

1

—

—

1 r \

1 1

1

—

_ 1

1

1 1

1 f
1

1

1 ,

-

i i

1

_

i i i

I 1

1

1

f 1

—

1

f f

*

-

1 ,

1

I

1,

i

7fl 8^ 9 ft

Fig. 46. — Line spectra of silicates.

IOJU.

IIJJ

I2,U

1 R", Valence of metal is indicated by the exponent.

CHARACTERISTIC BANDS.

67

spectra of supposedly related silicates is not due to a fault in the criterion
for judging such relations. The reflection bands of the various silicates
examined are represented by lines in fig. 46. The lines broken at the top
indicate that the reflection bands are not well resolved. The silicates of
the divalent metals, R "-silicates, are metasilicates, while the Rm-silicates
are orthosilicates, as noticed on a previous page. Since there is a band (at
10 //) in common with both groups we are at liberty to assume that the
division into ortho- and meta-silicates is not a characteristic in the spectra.
Again, the groups of chemically related minerals may or may not have
similar spectra. Thus, the pyroxenes and micas have dissimilar spectra.
Serpentine and deweylite (and the feldspars) have similar spectra. Ortho-
clase and microcline, which are isomeric, show their structure by their
dissimilar spectra. The amphibole seems out of place. Spodumene, talc,
serpentine, deweylite, apophyllite, microcline, albite, and muscovite have
a group of atoms in common, which causes a band at about 9.7 p.. Am-
phibole, topaz, microcline, albite, tourmaline, natrolite, datolite, and beryl
have a group of atoms causing a band at 10 y..

Serpentine, deweylite, topaz, and albite have a common radical causing
a band at 10.5 y.. Quartz glass, datolite, orthoclase, and possibly amphi-
bole, have a band in common at 8.8 \i.

The group of triplets in willemite, topaz, and zircon is unusual, the
maxima being, on an average, at 10. 1, 10.6, and 11 //, respectively. In
willemite (R11) the bands are sharp and well defined, in topaz (R111) the
bands are not so well resolved, while in zircon (R™) the bands are almost
indistinguishable. Whether this change in sharpness of bands is due to
the variation in valence is a pertinent question. Previous work indicates
that the variation in the sharpness of the bands is due to the amount of
oxygen present — oxygen sharpens the bands. In crystalline quartz and

Table III.

Compounds having the
following groups.

CH2 order CH3

NH2

CeHe

N02

OH

NCS

R-SO41

R-SiOx

R-CO31

Show characteristic absorption and reflection bands at:

3-43

2.96

3-25

7-47

2-95

4-78

4-55

3

6-5

8.68

6-5
8.8

11. 4

14

9.8

8.2

9-7
14

11.8

8.7
10
29.4

M

12.95

9.0
10.5

9.2
11

1 The position of the maxima in these compounds depends upon the atomic weight of the basic element, R,
with which the group or acid radical is combined.

68 INFRA-RED REFLECTION SPECTRA.

quartz glass there is a band in common at 8.4 fi, showing a common group
of atoms. This band, with the five groups in common with the silicates,
makes a total of six groups of bands, the radicals which cause them being,
as yet, undetermined. These bands are at 8.4, 8.8, 9.7, to, 10.5, and n /t.
This seems to substantiate the view expressed in Carnegie Publication
No. 65, p. 95, that in the silicates there appear to be several silicon-oxide
radicals, one or more of which are present in each mineral, or even in the
different specimens of the same mineral.

In table III are given the characteristic maxima of the reflection and
absorption bands of radicals studied in this and in previous work.

PART VII.

INFRA-RED EMISSION SPECTRA.

CHAPTER I.

EMISSION SPECTRA OF METALS IN HYDROGEN.

In Carnegie Publication No. 35 the writer presented the results of an
examination of the arc spectra of various metals. It was there shown that
only the alkali metals have strong emission lines in the infra-red. In the
case of metals like copper, iron, and zinc there was a weak continuous
spectrum in the region of 2 to 3 fi, which appeared to be due to the hot
oxides formed in the arc. If this be true, then one would expect to elimi-
nate this radiation by producing the arc in an atmosphere of hydrogen.
During the past year (1907) this work has been repeated, using an arc
inclosed in a metal case which was filled with hydrogen. A Rubens
thermopile was used to measure the radiation. The galvanometer had a
full period of 16 seconds; its sensitiveness was i=i.7Xio-10 for a scale at

1 m. When used with the thermopile the temperature sensitiveness was
such that a deflection of 1 mm. = iXio~5 ° C. for a scale at 1 m.

The spectrum was produced by means of the rock-salt prism and mir-
ror spectrometer (slits 0.5 mm.) which were described in Carnegie Publica-
tion No. 65.

(a) NICKEL ARC IN HYDROGEN.

With this outfit the emission spectrum of nickel in hydrogen was exam-
ined. The window of the metal case inclosing
the arc was of thin glass which absorbed all the
radiation beyond 3.5 ll. The metal electrodes
were about 1 cm. diameter, with the ends cut
wedge-shaped. This produced a long narrow
arc, an end-on image of which was projected
upon the spectrometer slit by means of a 12 cm.
focus, concave mirror.

The spectrum appeared continuous in the
red, where the maximum radiation was observed.
The results are shown in fig. 47, where a strong
emission is to be observed at 0.75 y.. Beyond

2 ll the radiation from the hot electrodes is indicated by the rapid rise in
the radiation curve.

The results show but little radiation in the region of 2 li, where pre-
viously a continuous spectrum was observed. It is also to be observed
that no strong emission lines occur at 1 p., where the alkali metals have

intense lines.

71

0

cm

to

1

0 3
0

1

u- 2
O

1

1

A

J

Fig. 47- ■

Nickel arc in hydrogen.

72

INFRA-RED EMISSION SPECTRA.

(b) SPARK SPECTRA OF METALS LN HYDROGEN.

For producing a high potential spark (arc) a 10,000- volt transformer,

with 100 volts, 1 to 9 amperes in the primary,
was used. From one to three glass condenser
plates, each having a capacity of 0.0028 M.F.,
was placed in parallel with the electrodes. This
produced a very intense arc. An image of the
electrodes was projected upon the spectrometer
slit by means of a short focus, concave mirror.
The electrodes were about 3 mm. diameter, filed
wedge-shaped. They were mounted in glass
holders which were fitted into a glass vessel of
about 200 c.c. capacity by means of ground
joints (designed by Dr. Nutting), which per-
mitted adjustment of the electrodes as shown in
fig. 48. The radiation passed through a quartz
window about 3 mm. thick, hence opaque be-
yond 4 ft. The hydrogen was prepared electro-
lytically in an automatic
glass generator shown in
fig. 49.

The spark (arc) spectra
of the following metals
were examined : alumi-
num, copper, and calcium. For electrodes of the
latter, thin pieces were sawed from a large bar of
the metal, which were then hammered into wires
about 2 to 3 mm. in diameter. These were kept
under oil to prevent oxidation. The result of the
examination was rather disappointing, for no ap-
preciable radiation could be detected up to 3.5 to
4 fi, where the electrodes gave from 3 to 10 cm.
deflections. This latter was eliminated by covering
the spectrometer slit over the whole length, except fjo. 49.— Automatic ciectro-

1 ,., v.i 1,1 t- lytic hydrogen generator.

about 2 mm., which permitted only the radiation

from the high potential arc (spark) to enter the instrument. The fact
that no deflections greater than 1 mm. were observed is not due to a lack
of sensitiveness of the instrument, and it may be concluded that no emis-
sion lines were produced which had an appreciable intensity. This is
rather surprising, for calcium is close to the group of elements having
emission lines in the infra-red.

Fig. 48. — Hydrogen spark tube.

THE CARBON ARC. 73

EMISSON SPECTRUM OF THE CARBON ARC.

The emission spectrum of carbon was previously examined by the
writer in connection with the emission spectra of metals and salts of metals
in the carbon arc. It was found that the violet part of the carbon arc had
several emission lines near 1 fx, followed by a weak continuous spectrum
with a maximum at about 2.5 it. There seemed to be a slight emission
band at 4.52 ti, which on subsequent examination could not be reproduced.
The writer, therefore, concluded that in the combustion of the carbon
electrodes no C02 or CO is formed, but that the carbon passes off as a
vapor.

Furthermore, it was found that when salts of the alkali metals (Na,
K, Li) were present, even in small quantities, there was a strong emission
line at 4.52 fi.

The radiometer used was not very sensitive, while the conclusion that
the carbon arc was mainly vapor of carbon did not seem satisfactory,
hence the work was repeated, using electrodes of ordinary gas carbon and
also of pure Acheson graphite. The latter is difficult to burn even when
the length of the arc is only 2 mm. long.

In the present examination a Rubens thermopile was used. The
galvanometer period was 10 seconds, and its sensitiveness was about
2.5 Xio~10 amperes. The spectrometer slit was 0.1 mm. wide and 2 mm.
long, and an image of the blue vapors of the arc was projected upon it by
means of a short focus mirror. The carbon electrodes were of the usual
diameter, 12 mm., while two sizes (5 and 10 mm. diameter) of graphite
electrodes were used. An examination was also made using one carbon
and one graphite electrode.

In fig. 50 are given the emission curves of the carbon arc, a and b for
a solid electrode with soft core, using 15 amperes (D. C), while curve c is.
for hollow electrodes (2 mm. holes). The latter was examined to learn
whether the air, drawn in through the hole, had any effect — which appears
negative.

In curves d and e are given the emission curves observed at various
times, under various conditions, when the hollow carbons contained the
salts of the metals NaCl and KC1. The length of the arc was from 5 to
7 mm. and in some instances was as much as 12 mm. On the whole,
whatever the current — from 5 to 15 amperes — there is always an emis-
sion band at 4.5 pt when salts of the alkali metals are in the arc, while the
pure carbon (on 15 amperes) had little if any selective emission in this
region.

An examination was then made of the emission of pure graphite (Ache-
son's). The direct-current arc was maintained with difficulty and could
not be made longer than 4 to 5 mm.

In fig. 51 are given the observed results for the emission of pure graphite

74

INFRA-RED EMISSION SPECTRA.

in the region of 4.5 \x. The purity of this material may be judged from
the entire absence of the sodium lines; only the violet carbon bands were

4

Cm

u

c

._ 2

o

0 /

<D
Q

ATi

2 3 4

Fig. 50. — Emission of carbon arc.

CM

visible. The lower curves are for a current of 7 amperes (120- volt cir-
cuit), while the two upper curves are for a current of 8 amperes, all of

6mm

z4

E 2
.5 3

C 2
o

•.£ /

<J

£2

Q /

- « 1 * « ■—*

. •

. ■ » • ' — ■

2mm

1
2mm

I

O

I

3 4 5 6JU

Fig. 51. — Emission of graphite arc (8 amperes).

which were obtained under various conditions. On account of the diffi-
culty in maintaining a steady arc, no attempt was made to map a strong
emission band extending from 1.2 to 1.7 fx and a second band extending

GRAPHITE.

75

from 2 to 3 jb. The latter is the one always observed and generally
attributed to oxides, in the case of a metallic arc.

It is to be observed that the deflections are only a few millimeters as
compared with the large deflections when salts of metals are present.

An examination was then made of the graphite arc, using a current of
4 amperes. The length of the image of the arc projected upon the spec-
trometer slit (which was also the length of the arc itself) was only from
2 to 3 mm. The arc was very unsteady and difficult to keep burning.
The results are given in fig. 52, curves a (4 amperes) and d. Curve a is
for an arc-length of 2 mm., while d is for a length of about 5 mm. For
the shorter arc the maximum is shifted toward the long wave-lengths.

In this same figure, curve b gives the emission when the cold (negative)
electrode was a gas carbon
(with soft core) and the posi-
tive electrode was pure graph-
ite. The current was varied
from 5 to 6 amperes, and the
length of the arc was from 4
to 5 mm. There is an emis-
sion maximum at 4.6 /i, as
was found for the two graph-
ite electrodes. For compari-
son of position and intensity,
the emission band of carbon
dioxide in a Bunsen burner
is given in curve c, fig. 52.
The results with the pure
graphite show that there is a sharp emission band at 4.65 /* (4.55 fi for a
longer arc) , where previously, for a larger current, there was none. Since
all the observations extend over the same range of length of arc, it appears
that the occurrence of the emission band depends upon the current and that
it disappears for a large current. This is certainly extraordinary. Since
the observations on graphite were made in quick succession on the same
day, as given here, and since no changes were made in the apparatus, the
lack of an emission band for a current of 7 amperes is not to be attributed
to a fault in the adjustments. To attribute the occurrence of emission
bands, when salts of metals are present, to some "catalytic" action caused
by the presence of these metals does not elucidate matters very much.

Paschen was the first to observe that the emission band of carbon
dioxide at 4.3 jx shifts to the long wave-lengths with rise in temperature.
At that time the dissociation of carbon dioxide at high temperatures was
but little investigated. Since the absorption band of carbon dioxide is at
4.3 n and that of carbon monoxide is at 4.6 /*, if the emission band is due
to a pure thermal effect, then one would expect to find that, with rise in

SJU 4 SJU

Fig. 52. — Emission of graphite arc (4 to 6 amperes).

76 INFRA-RED EMISSION SPECTRA.

temperature and the consequent dissociation of C02 into CO, the maxi-
mum of the emission band of C02 will shift toward that of CO. The writer
has observed this emission band of C02 under various conditions of tem-
perature. In the emission spectrum of the Hefner lamp it was found at
4.36 fi. In the arc spectra of the salts of the metals previously studied it
was found at 4.52 ;x. In the vacuum-tube radiation of CO and CO, the
maximum was found at 4.75 /«. In the acetylene flame it was found at
4.4 ft, while in the present examination it occurs at 4.55 to 4.65 ;i. It has
always appeared to the writer that this shift of the emission maximum
with rise in temperature is due to dissociation of C02 into CO, and no
satisfactory data has yet been found to refute this assumption. The
vacuum-tube radiation of CO and C02, where the maximum emission
occurs at the same place for both gases, seemed to prove the point. The
present data furnish further evidence that the cause is due to dissociation.
For, using a graphite arc on 4 amperes, when the arc is short, and the
hottest, the maximum occurs at 4.65 fi. When the salts of the metals are
introduced into the arc they are ionized and the metals carry the greater
part of the current. The resistance is reduced, and hence the tempera-
ture (and dissociation of C02) is reduced. Here the maximum is at a
shorter wave-length corresponding to less dissociation of C02. While
this explains certain points it fails to account for the lack of an emission
band when the current is large, 8 to 15 amperes. It is hardly permissible
to assume that neither CO or C02 is formed at this high temperature and
that the electrode disappears in the form of vapor, though the writer
made this assumption at the conclusion of his previous examination of
the carbon arc.

The change in density of the gases in the two cases will not account
for the observations, for even the smallest amount of CO and C02 is capable
of emitting perceptible radiation, as was found in the experiments with the
vacuum tube. As a whole, the mechanism producing this radiation is not
understood and further experiments will be needed to gain insight into
this subject.

THE INVESTIGATIONS OF MOLL.

While the above experiments on the carbon arc were being concluded
the investigations of Moll1 on infra-red metallic spectra appeared in print.
He examined the emission spectra of the salts of the metals of Na, K, Rb,
and Cs in the carbon arc, using for the purpose a rock-salt prism, a ther-
mopile, and an automatic device for recording the galvanometer deflections.
The apparatus was a little more sensitive than the radiometer previously
used by the writer, while his dispersion was almost twice as great, and
with the automatic device he was able to explore the region beyond 2 /i
more thoroughly than was possible by making personal observations. In

1 Moll: Onderzoek van Ultra-Roode Spectra, by W. J. H. Moll. Dissertation, Utrecht,
1907; Proc. Amsterdam Acad., Feb. 21, 1907.

INVESTIGATIONS OF MOLL.

77

the region from 2 to 4 /i he succeeded in resolving the emission into sepa-
rate bands, which the writer, from the smallness of the deflections and the
limited number of observations, recorded as a continuous spectrum. He
found the C02 band at 4.44 n when he used the salts in the arc, and also
for various kinds of carbon electrodes. His energy curves for sodium and
for potassium are given in figs. 53 and 54, respectively, in which the reso-
lution of the energy in the region of 1.5 to 4 y. into separate bands is well
illustrated. The automatic apparatus succeeded in doing what physical

180

n

240

n

100

90

80

70

60

3">

'to
C50
a)

40

30

20

10

1 80

n

25

4 5

C02

I I I I 1 I I'l t I 'I I I'l ]' II I I I I'! ' J

I.54Q 1.535 1-530 I.5ZS 1. 520

A

T

— i — r
1.5 2

0.75 / 1.5 2 3

Fig. 53. — Emission of sodium (Moll).

-I —
4/1

endurance would hardly permit, in mapping this region. No curves are
published for the carbon electrodes, but considerable comment is made on
the fact that in every case he observed an emission band of C02, while
the present writer did not find the band, except when using salts in the
arc. From the aforesaid observations on the radiation from graphite, it is
evident that the conditions were different in the two cases. The writer
had observed the same phenomena as did Moll, who, in his comments,
missed the point emphasized by me, viz, that whether or not the spec-
trum at 2 to 4 ;« is a complex of small emission bands, the emission bands
of the alkali metals at 0.76 to 1// are the most intense in the whole spec-

78

INFRA-RED EMISSION SPECTRA.

trum, indicating a much higher temperature (if the radiation is purely
thermal) than would be the case if the emission bands at 2 to 4 /z were
the more intense.

100

90

80

(0

60

m

c

.2

4-0

30

zo

10

A

n

620

rx3zo

^J

rn — 1 — r-| — l 11 — 1 11 — 1 11 — — rn — r i | v \ 'i i

1.540 1.535 /.530 1.525 1.520

95

\)

\A

co2

~r

~r

-I \ 1 1 —

1.5 Z 3 4JU

/I 0.75

Fig. S4- — Emission of potassium (Moll)

RADIATION AT ROOM TEMPERATURE.

The distribution of energy in the spectrum of a complete radiator has
been determined for temperatures varying from 3730 to 18000 abs., and
the constant in the "displacement law" -^max T=A has been found to be
2930 (2940, Lummer and Pringsheim; 2920, Paschen). For bright plati-
num the value of this constant is about 2630. The measurements of
Lummer and Kurlbaum1 show that at low temperatures (4920 abs.) the
emissivity of iron oxide is 0.3, while bright platinum is only about 0.04
that of a complete radiator at the same temperature. The spectral distri-
bution of radiation at low temperatures has been given but little attention.
This is due in part to the impracticability of reducing the temperature of
the radiation meter below that of the room. The problem is therefore
reversed, in that the radiation meter (bolometer, radiometer, or thermopile)

1 Lummer & Kurlbaum: Verb. Phys. Ges. Berlin, 17, p. no, 189S.

RADIATION FROM RUBENS THERMOPILE. 79

is allowed to radiate to what is generally the source, which is at a much
lower temperature. The first examination was made by Langley:1

He located the maximum galvanometer deflection in the region of 8 //,
but adds that the position of " the maximum depends upon a single obser-
vation of some delicacy, which is liable to subsequent correction." He
further adds that for this temperature difference of — 180 the maximum
ought to be at 10 to 11 a. This would indicate that there was some con-
fusion of ideas as to what was really taking place. The energy emitted
by the vessel of ice and salt is very small in comparison to the amount it
absorbed from the bolometer. Hence the galvanometer deflections are a
measure of the radiation emitted by the bolometer to the vessel of snow
and ice, and the observed maximum is for a temperature of — 20 instead
of —1 8° C. However, since a bolometer strip is always at a higher tem-
perature than the surrounding atmosphere, it is quite probable that,
although the observations were made during zero weather, the maximum
observed is really due to a higher temperature of the source (bolometer
strip) than — 20 C. In fact, from the "displacement law," which was
then unknown, and from our present knowledge of radiation of different
surfaces, it is quite possible that the temperature of the bolometer was as
high as 20 to 300 C.

Mendenhall and Saunders 2 state that they found the energy curve of a
complete radiator at — 900 C. From their discussion of the energy curve
of a body at the temperature of boiling liquid air where the emission maxi-
mum should lie at 30 /1 (which, of course, is possible if one could reduce
the temperature of the radiometer below this point in order to make
measurements), it would appear that they, too, had not considered their
radiation curve to be due to the bolometer strip. Moreover, the use of a
bolometer to deduce a distribution of energy curve at low temperatures,
where the bolometer itself becomes the source, is not very satisfactory, due to
the fact that the temperature of the radiating strip can not be determined.

Lummer and Pringsheim3 obtained a small portion of the spectrum
energy curve of a screen at room temperature, which they used during
their radiation experiments. The first really lucid discussion of the sub-
ject is due to Stewart,4 who found the spectrum energy curve of a Nichols
radiometer radiating to a receiver at liquid-air temperature. The maxi-
mum deflections observed were 4 mm. The temperature of the room
(and radiometer vane) was 240 C. or 2970 abs. The observed maximum

1 Langley: Amer. Jour. Sci., 31, p. 1, 1886. "The radiator was the bolometer itself at a
temperature of — 2° C, and the source radiated to was a vessel of snow and salt at — 200 C,
thus determining the distribution of energy in the spectrum of a surface below the freezing-
point of water."

2 Mendenhall & Saunders: Astrophys. Jour., 13, p. 25, 1901.

3 Lummer & Pringsheim: Verb. Phys. Ges., 2, p. 163, 1900.

4 Stewart: Phys. Rev., 17, p. 476, 1903. (In this paper figs. 4 and 6 are interchanged.)

8o

INFRA-RED EMISSION SPECTRA.

to

c

of his energy curve, corrected for slit-width, occurs at 9.2 ,u, while the
value computed from the "displacement law," using the temperature
2970 abs. and A = 2920, would be found at 9.8 ;x. If we compute the
value of this constant from the observed values of >}max and T, then
^ = 2722, which is about 6 per cent lower than for a complete radiator.
From experiments on the radiation from plane surfaces, such as a radi-
ometer vane, this departure from the radiation of a full radiator is to be
expected. Then, too, this form of radiation is complicated by the window
before the radiometer vane. In general, the intervening material between
the source and the receiver would have no effect. For very delicate radi-
ometers, however, the writer found (Carnegie Publication No. 35) that the
inequality in the radiation from the radiometer window had a great effect
upon the position of the vanes. Just how much this would affect the
vane in the present case is unknown. In the present investigation a Ru-
bens thermopile (20 junctions of iron-constantan) was allowed to radiate
to a copper vessel at liquid-air temperature. A mirror spectrometer and
rock-salt prism, described in Carnegie Publication No. 65, were employed
to produce the energy spectrum. The only changes introduced consisted

in replacing the Nernst heater
by a short focus mirror, which
projected an image of the bot-
tom of the vessel at liquid-
air temperature, upon the
spectrometer slit. The gal-
" vanometer used with the ther-
mopile had a sensitiveness of
i=2Xio-10 amperes and a
full period of about 12 sec-
onds. The deflections were
very unsteady, due to air cur-
rents caused by the evapo-
ration of liquid air. The
thermopile was placed in the
position formerly occupied by
the radiometer and was cov-
ered by the inner metal shield,
with a slit 1 by 15 mm. area.
The whole was inclosed in a
metal tube covered with felt.
This formed a more perfect black body than is possible with a radiometer.
The receiver to which the thermopile radiated consisted of a thin cylindri-
cal copper vessel about 4 cm. diameter and 12 cm. long (covered on the
inside with copper oxide and lampblack) , which was suspended in a vessel
of liquid air, as shown in fig. 55. The receiver was kept stationary and

CD

i.

CD
O
CD

Fig. 55-

THE NERNST GLOWER.

8l

the height of the vessel of liquid air was regulated. The temperature
changed so rapidly that it was necessary to begin taking readings with the
liquid air just touching the bottom of the vessel, when fairly consistent
results were obtained. The observations are plotted in fig. 56. Curve a
is plotted through the mean values of the observations, while curve b is
drawn through the highest observed values. The two curves are fairly
consistent, considering the difficulties in making the observations. In the
region of 6 // atmospheric absorption reduced the deflections.

3

y

„ ,

6'

2

/ y

' ~dr'

s.

-4

/

/

^5

* v

mm

w *
4o

O

«*—

Q>
O

-/

/

/

>^r*

©\;

S 1

4

^

^fct

___.

y/

•

1 1

,

3456789/0//
Fig. 56. — Emission spectrum of Ruliens thermopile.

12

13,

Y

Curves a' and b' are the two lower curves after correcting for variation
in slit-width, i.e., reducing them to a normal spectrum. The maximum
of these radiation curves lies at about 9.6 /«. The temperature of the
thermopile was 210 C, or 2940 abs. Substituting these values in the
"displacement" formula, the constant is .4 = 2822, which is about 3 per
cent smaller than that of a complete radiator. For the latter, the maxi-
mum would occur at about 9.95 jx. It appears that the thermopile ap-
proaches very closely to that of a Kirchhoff radiator.

SELECTIVE RADIATION FROM THE NERNST GLOWER.'

The study of the radiation from metal filaments of incandescent lamps
is of interest in connection with the speculations as to whether the great
light emissivity (high luminous efficiency) is due to an abnormal emission
in the visible spectrum, with a corresponding suppression of the radiation
in the infra-red, or whether the effect is due to the high temperature at
which the lamp is burning. In the latter case, the distribution of energy
in the spectrum may be uniform (no discontinuities), but a great deal of
it will lie in the visible spectrum. From a theoretical consideration2 of

1 A detailed description of this investigation is given in the Bulletin of the Bureau of
Standards, vol. 4, p. 533, 1908.

2Aschkinass, Ann. der Phys. (4), 17, p. 960, 1905; Einstein, Ann. der Phys. (4), 17, p-
132, 1905; 22, pp. 191, 569, 800, 1907.

82 INFRA-RED EMISSION SPECTRA.

the fact, that the filaments are metallic, electrical conductors with a high
reflecting power in the visible, and probably reflect uniformly high in the
infra-red, one would expect the distribution of energy in the spectrum to
follow a law similar to that of platinum, but with different constants.
The results thus far obtained from a study of such metals as tungsten
and osmium support this hypothesis. On the other hand, in the case of
oxides, which conduct electrolytically at high temperatures, there is no
data to form even a working hypothesis. All the oxides thus far examined
have no strong absorption bands near the visible spectrum; the only ex-
ception being the oxides of the rare earths, such as cerium, thorium,
lanthanum, didymium, erbium, etc., the compounds of which have strong,
sharply defined absorption bands in the visible, and at least some of these
have absorption bands in the infra-red. It seems to be a characteristic of
the oxides, that they have a low reflecting power (like transparent media,
electrical non-conductors) throughout the infra-red to about 8 n beyond
which point they have strong bands of metallic reflection. In this region
of metallic reflection, the emission will be suppressed l in proportion to the
reflecting power. In the rest of the spectrum, the emission will be pro-
portional to the absorbing power (general absorption), while at the point
where there is a band of selective absorption in the transmission spectrum,
there will be an emission band in the emission spectrum, provided the
radiation is a purely thermal one, following Kirchhoff's Law.

In the case of the Auer mantle the emission spectrum2 is a series of
emission bands at i to 2 /1, with practically no emission in the region from
4 to 7 n, while beyond 9 fi the spectrum is continuous, and is apparently
as intense as that of a complete radiator at the same temperature.

In the case of the Nernst glower, which is a combination of the oxides
of cerium, thorium, and zirconium, belonging to the "rare earths," the
compounds of which are noted for their strong absorption bands, one
would expect the emission spectrum to show strong, sharp emission bands;
at least one would hardly expect the emission to follow the same general
law of spectrum energy distribution as is known for metals. This, how-
ever, has been done in the past, notably by Lummer and Pringsheim,3
and by Mendenhall and Ingersoll.4

The first two investigators, from a rather cursory examination of the
Nernst filament, under normal power consumption, found a smooth con-
tinuous curve, with a maximum at wave-length ^max=i.2 /«. From this
and from the Wien displacement law, ^max T= const, (const. = 2940 for
"black body," = 2630 for platinum), assuming that the Nernst glower

1 Aschkinass, Verh. J. Phys. Ges., 17, p. 101, 1898. Rosenthal, Ann. der Phys. (3), 68,
p. 791, 1899.

2 Rubens: Phys. Zeit., 6, p. 790, 1905.

3 Lummer & Pringsheim: Verh. Deutsch. Phys. Gesell., 3, p. 36, 1901.

4 Mendenhall & Ingersoll: Phys. Rev., 24, p. 230, 1907; 25, p. i, 1907.

THE NERNST GLOWER. 83

belongs to the same class of radiators as platinum and a "black body,"
they computed 4^=2450° abs., and >}min=22oo° abs.

Their computed energy curve of a "black body" having its maximum
emission at 1.2 a departs considerably from the observed curve. Men-
denhall and Ingcrsoll compared the emission of the Nernst glower in terms
of a constant comparison lamp, at a certain wave-length in the visible
spectrum, at the melting points of gold and of platinum. From this they
extrapolated on a straight line assuming that Wien's equation holds for
the glower, and found the temperature at normal or any desired power
consumption. This leads to erroneous values (which are probably too
high, as will be shown presently), due to the fact that the spectrum is the
composite of numerous emission bands, which rapidly increase in intensity,
in the short wave-lengths, with rise in temperature. They found the
normal temperature to be 23000 abs., disagreeing with a recent determina-
tion by Hartman,1 who, by means of thermocouples of different thickness
placed against the glower, correcting for heat conduction by extrapolating
to a temperature corresponding to an infinitely thin couple, found the
temperature to be 1800 abs. Although this method had previously been
extensively used, with fair success, in measuring temperature of gas-
flames, it is not suited to the glower, in which there is no layer of hot gas
to even partially compensate for the heat lost by conduction.

That the Nernst glower emits selectively in the visible spectrum, has
been shown by Kurlbaum and Schulze,2 who found a minimum of emission
at 0.52 n, which became fainter with rise in temperature, and disappeared
entirely at high temperatures.

To this brief review of what has been done on the Nernst glower may
be added a paper by its inventor,3 who showed that the conductivity is
electrolytic, while Kaufmann4 showed that in spite of the entirely different
inner mechanism of conduction of a gas in a vacuum-tube, and in a Nernst
glower, the electrodynamic phenomena are nevertheless very similar.

The present investigation of the Nernst glower consists in mapping
the distribution of energy in the spectrum, varying the power consumption,
and hence the temperature.

The apparatus used in this work consisted of the mirror spectrometer,5
used in the preceding experiments, a perfectly clear fluorite prism, having
an angle of 6o° and circular faces ^ mm. in diameter, and a bolometer 8
with a hemispherical reflecting mirror. The bolometer strip and spectrom-
eter slit were 0.6 mm. wide, or about 4' of arc. The upper part of the

1 Hartman: Phys. Rev., 17, p. 65, 1903.

2 Kurlbaum & Schulze: Verh. Phys. Gesell., 5, p. 428, 1903.

3 Nernst: Zeit. fur Electrochemie. 6, p. 41, 1899.

4 Kaufmann: Ann. der Phys. (4), 2, p. 158, 1900; 5, p. 757, 1901.

5 For adjustments see "Investigations of Infra-red Spectra," Carnegie Institution of
Washington Publication No. 35, 1905.

6 Described in this volume, Appendix II.

84 INFRA-RED EMISSION SPECTRA.

spectrometer, containing the collimating mirrors, and the Wadsworth
mirror-prism outfit, were entirely inclosed by a thin sheet-metal box, lined
with black velvet. The spectrometer slit was covered with a clear plate
of fluorite. The openings in the top, for adjusting the mirrors, were closed
with soft wax, while the hole admitting the axis for rotating the mirror-
prism table was tightened with a nut and packing.

Within the box, and below the level of the mirrors, were placed vessels
containing phosphorous pentoxide and sticks of potassium hydroxide,
which entirely eliminated the absorption bands of C02, and water-vapor
from the emission curves. A water-cooled shutter was placed before the
spectrometer slit, and the Nernst glower, inclosed in an asbestos case to
prevent air-currents, was placed close to the shutter. Observations were
also made without inclosing the glower, to prove that the effects observed
are not due to stray radiation from the asbestos case. In spite of all that
has been written about bolometers, it may be added that there was no
"drift," unless a poor storage battery was accidentally used.

The auxiliary galvanometer of 5.3 ohms resistance, with a single swing
of 4 to 5 seconds, had a current sensibility of i=i.6 to 1.5 Xio~10 ampere.
A greater sensitiveness for the same period would have been possible, by
using a lighter suspension. The present suspension of ten needles was
just heavy enough so as not to be affected by tremors.

Using a battery current of 0.04 ampere the computed temperature
sensibility was 5°Xio~5 C, which was generally far in excess of that re-
quired. The deflections were reduced to 14 to 15 cm., by inserting resist-
ance in series with the galvanometer. The individual readings would
vary by 1 mm. or less than 1 per cent, which is as close as the nature of
the work required, since the actual deflections were as high as 2000 cm.
Furthermore, at high temperatures (especially when operated above nor-
mal power consumption) the filament would deteriorate in emission by
that amount during the series of observations.

The calibration curve of the fluorite prism was constructed from the
refractive indices, found by Paschen,1 which, after plotting all the obser-
vations made by different observers, seems to be as close to the most
probable values as observations will permit. Unfortunately the dispersion
curve passes through a double curvature at 1.5 //, just where the energy
spectra have their maximum. In the region of 1.5 ,«, the correction for
purity, so-called "slit-width" correction, is a maximum.

The wave-lengths in the calibration curve were plotted to the fourth
decimal place, so that there is a certainty of the values to at least the
second decimal place. This, however, is of less importance than the
value of the slit-width correction, which was made according to Paschen,2
the values being obtained from a curve plotted on a large scale to insure

1 Paschen: Ann. der Phys. (4), 4, p. 299, 1901.

2 Paschen: Ann. der Phys. (3), 60, p. 714, 1897.

THE NERNST GLOWER. 85

an accuracy greater than required in the work. In a few cases a correction
was made for the reflecting power of the silver mirrors, but it was found
negligible except in the visible spectrum.

With this apparatus, a series of energy curves was obtained, varying
the power-consumption from 16 watts (the lowest at which the glower
would conduct on no volts) to 123 watts, which is far above the normal.
The energy curves, which were continuous, underwent great variations
in appearance with rise in temperature. At 2.5 and 3.5 /i depressions
would generally appear in the curves, which could not be attributed to
experimental errors, and since previous work by others seemed to show
that the spectrum is continuous, an attempt was made to locate the disa-
greement in the apparatus (in the calibration, or in the slit-width correc-
tion curve), but nothing would account for it until the filament was run on
a 2000- volt transformer which permitted the heating of the glower at a very
low power-consumption. At the lowest temperature the glower was a
grayish red. The results are given in fig. 57 (no-volt A. C. glower No.
118), and are entirely different from anything hitherto recorded in the
emission of solids in the infra-red. At the lowest temperatures (8oo° to
9000 C.) the bands in the region of long wave-lengths are the most in-
tense. As the temperature increases, the bands in the region of 2.5 /x in-
crease very rapidly in intensity, so that by the time the temperature has
increased to 1000 to 11000 the intensity of the group of bands at 2 tt is
far in excess of those at 5.5 /x.

The depression at 3 to 3.5 ;x is to be noticed, for it persists even at
normal power-consumption. The region at 2.5 \x is also to be noticed, for
the curves at higher temperatures often show a slight depression, not attrib-
utable to experimental errors. As the temperature rises (curve e, fig. 57)
new emission bands appear, notably at 2.5 and 4 ;x. This shift of the maxi-
mum of intensity of the bands, with increase in temperature, is to be ex-
pected, if the emission is a purely thermal one, following Kirchhoff's law,
and is the most conspicuous illustration yet recorded.

In fig. 58 are given the emission curves for the glower (200 volts, serial
No. 118) at 19.6 and 102.5 watts, respectively. It is to be noticed that the
emission curve has already become smooth and continuous, with but two
maxima, at 1.4 and 5.5 /«, respectively. In this figure curve b is one-tenth
the scale of curve a.

On the whole, from whatever point of view we consider the data at hand,
it is evident that even after the emission spectrum has become apparently
continuous it does not follow so simple a law as has been established for
solids emitting continuous spectra. It is also evident that any estimation
of the temperature of the glower based on these laws will lead to erroneous
results. From a commercial point of view the efficiency of such a radiator,
in which the emissivity is abnormally high at 0.6 to 0.7 /x, while the maxi-
mum at 1.2 ti is abnormally low, must be much higher than that of some

86

INFRA-RED EMISSION SPECTRA.

2345676 SJU

Fig. 57. — Emission spectrum of Nernst glower, o = 2, b = 3, c = 4.2, d = 6.2,
e= 7.1, and/= 10.6 watts, respectively.

THE NERNST GLOWER.

87

substance having a radiation law similar to platinum, in which case, in
order to attain a similar intensity in the visible, the maximum at 1.2/*
rises to extremely high values.

The results obtained are for filaments made from different lots of
material, the 220-volt glower being at least 5 years old. The observa-
tions have been made at different dates, on different filaments, all in dupli-
cate, some quadruplicate, and there is reason for feeling that what has

cm. De.fl.

140

Fig. 58. — Emission spectrum of Nernst glower; (a)= 19.6 watts, (6)= 102.5
watts; Xmax = I-45M and 1.32 /J., respectively.

been observed is a reality. No doubt on examination of other makes of
filaments, there will be found a variation in the finer details of the emission
bands, at low temperatures, but the gross results can hardly be modified
without employing a much larger dispersion and a much narrower bolom-
eter. Some of these bands coincide with certain ones found in the emission
spectrum of the Nernst "heater," the results of which are given in Carnegie
Publication No. 35.

88 INFRA-RED EMISSION SPECTRA.

RADIATION FROM METAL FILAMENTS.

The measurement of very high temperatures is based upon an extra-
polation of the laws governing the energy emitted by a body with change
in temperature. Our knowledge of these laws is confined to the radiation
from platinum and from a uniformly heated cavity (so-called black body)
which is the nearest approach to a complete radiator. The remarkable
progress made in the development of processes requiring an accurate
knowledge of the temperatures involved makes it imperative to study the
laws of radiation of various substances with variation in temperature.
The object of this and of subsequent investigations is to gain an insight
into these laws. In order to determine these radiation laws it is generally
necessary to study the spectrum energy curves, using for the purpose a
prism that is transparent to heat rays, and some sort of very sensitive heat-
measuring device, such as, for example, a bolometer or a thermopile. It
is also possible to study the total radiation emitted. The chief difficulty
in studying these so-called radiation constants of substances lies in the
impossibility of determining the temperature of the radiating surface.

The curves showing the distribution of energy in the normal spectrum
of all solid bodies thus far studied are unsymmetrical with respect to the
maximum, having the appearance of the probability function, modified by
suitable constants. The solids heretofore studied, e.g., platinum, in which
it was possible to determine the approximate temperature, have spectrum
energy curves, which are represented fairly well by the function,

(i) £=ClA-ae-C2/;r

In the case of a complete radiator,1 or so-called "black body," the
exponent 0=5, while for platinum, a=6.

In order to determine the constants of the above equation from the
spectrum energy curves, it is necessary to know the temperature of the
radiator. Fortunately the index a may also be obtained from the spectrum
energy curve in which the temperature T is constant (E= galvanometer
deflections, X = wave-length, are variable), without knowing the actual
temperature, for it can be shown from equation (1) that the ratio of the
emissivities (the observed bolometer-galvanometer deflections) for any
two wave-lengths, X and ^max, is:

E \ A,™,, *

(2)

* ''max

E

max

max e

from which the value of a may be obtained. It was found by Paschen

1 Paschen: Ann. der Phys. (3), vol. 58, p. 455; vol. 60, p. 663, 1897; (4), vol. 4, p. 277,
1901. Lummer & Pringsheim: Verh. deutsche phys. Gesell., 1, p. 215, 1899.

METAL FILAMENTS.

89

that for carbon, platinum, etc., the value of a was in agreement with that
obtained from a knowledge of the temperature of the radiator.

With this equation it is possible to obtain some idea of the probable
total emissivity of a radiating body, as to whether it is proportional to the
4th power (a— 1 = 4 for a black body, a— 1 = 5 for platinum), or to some
higher power of the absolute temperature. Of course the assumption is
made that the emissivity function is similar to that of platinum and of a
black body. The appearance of the energy curves for various tempera-
tures will give some clue as to the admissibility of this assumption, which
is nothing more than has been made by previous observers. How far this
assumption falls short of the observed facts, may be seen in figs. 57 and 58
for the Nernst glower, which has a spectrum composed of numerous
emission bands. With substances whose energy spectra undergo no change
in contour with change in temperature, it does not seem unreasonable to
apply our knowledge gained from the behavior of platinum under similar
conditions, especially since the filaments are metals, electrical conductors,
which, theoretically,1 should have similar emissive properties. That the

20 40 60 80 100

Fig. 59. — Radiation constant (a) of Nernst glower.

120 Waits

method is open to criticism is admitted, but until a better one is suggested,
the present method is the only one available without a knowledge of the
temperature of the radiator. The apparatus used in this work consisted
of a mirror spectrometer, a fluorite prism, and a bolometer, mentioned in
the description of the results on the Nernst glower.

The variation of the radiation constant a for the Nernst glower with
rise in temperature is shown in fig. 59, from which it will be seen that it
decreases from a value of 7 at 18 watts to 5.3 at 80 to 120 watts. These
values are taken from the smooth energy curves, such as those shown in

• 1 Aschkinass: Ann. der Phys. (4), 17, p. 960, 1905. Einstein: Ann. der Phys. (4), I7> P.
132, 1905; 22, pp. 191, 569, 800, 1907.

9o

INFRA-RED EMISSION SPECTRA.

fig. 58. The curve for 102 watts was irregular in outline and the values

of a obtained at different wave-lengths
(indicated by crosses, circles, dots,
squares, etc.) undergo great variations.
On the whole, it is evident that it is
hardly permissible to obtain this con-
stant on the assumption that the ra-
diation law of the Nernst glower is
similar to that of platinum or of a
complete radiator. The energy curve
of a complete radiator having a maxi-
mum emission at 1.45 p. falls far be-
low the Nernst glower curve (a, fig.
58) at 5.5 n, which shows that the
emission at 1.45 /* is not as intense as
it should be if the emissivity were
similar to a complete radiator. For
the same reason, the spectral energy
curve of a complete radiator having
a maximum at 1.32 ,« lies far above
the glower curve (b, fig. 58) at 5.5 fx.
In fig. 60 is shown a series of
energy curves of an incandescent fila-
ment (3 cm. long) of osmium, when
on an energy consumption of curve
a=2.44 and curve 6=7.38 watts, re-
spectively. The corresponding tem-
peratures, observed with an optical pyrometer, using red absorption glass,
are 16070 and 20000 K.66AC., respectively, while the computed tempera-
tures, on the assumption that the radiation constants are the same as for
platinum (for X =1.35 and 1.2 /*), are 16700 C. and 19070 C, respectively.

Fig. 60. — Radiation from osmium.

c 1

-4-J

05

c
o
u

c
o

2 -5

-a

DC

£*j Tungsten

-2 0

S

•

•

?-

eX

§

X

ffl

0

12 3 4-56 7 8 Waits

Fig. 61. — Radiation constant (a) of osmium.

These filaments were in glass bulbs similar to that of an incandescent
lamp, and had short side tubes with fluorite windows. Fig. 61 shows the
value of a of osmium (of fig. 60) for variation in energy consumption.

METAL FILAMENTS.

91

Except for the lowest temperatures the value of a obtained by this method
is about 6.4. The surface became roughened, which lowers the value of
this "constant." The weakness of this method lies in the difficulty of ob-
taining the values of the emissivities, particularly the maximum of the
energy curve. Since the filaments are so narrow that the prism-face is
only partly covered, the height of the ordinates will depend upon the cone
of energy falling upon the prism face. For this reason the energy curves
of the different substances can not be compared. The value of a for a short
tungsten filament (see fig. 61) was found to be 7.3 for one energy curve.
Further experiments on another filament gave a value of a =6.88 for the
mean of 18 computations and 5 energy curves. The values of a for an
untreated carbon filament, for various values of energy consumption, are
plotted in fig. 62. As with osmium, the temperature was raised from a

5<*

TO
10

C

o
O

c
o

<?

^°

K

S9

ST--

~-— _°

~x~ _

X

•

-

0 —

— ox^

1234-56789 Watts
Fig. 62. — Radiation constant (a) of untreated carbon filament.

low red to the normal condition. Apparently the emissivity constant, a,
drops from 6.4 at about 9000 to 5.3 at 18000 to 20000 C. Paschen found
no such variation for the specimens of carbon examined. Neither did he
find a variation in a for the oxides of copper and of iron. On the other
hand, Lummer and Kurlbaum, by measuring the total radiation emitted,
found that with rise in temperature the emissivity of these oxides approached
that of a complete radiator. This difference may, of course, be possible,
just as in the present examination of the Nernst glower the value of which
was found to vary, while Mendenhall and Ingersoll,1 by using a total
radiation method of comparison with platinum, could not establish with
certainty any variation of a with temperature. An inspection of figs. 58
and 59 shows that this may be possible, especially in the latter where the
energy curve appears to have two maxima. For the "Helion" filament,
which is a carbon filament upon which is a deposit of silica, the constant
a=8.3 when new, and decreases to 6.3 after aging. For tantalum the
value of a is 6.5. From the energy curve of the acetylene flame published
by Stewart (Phys. Rev. 16, p. 123) the value of a is about n. A plati-
num strip, 50 X 1.5 X 0.02 mm. inclosed in a glass bulb with a fluorite
window, was also examined. Estimating temperatures from the position

1 Mendenhall & Ingersoll: Phys. Rev., 25, p. 1, 1907.

92 INFRA-RED EMISSION SPECTRA.

of the ^max (^max T= 2620) it was found that the " constant " a decreased
from a — 8.5 at 9000 C. to a = 6.3 at iioo0 C.

Using metal filament lamps (commercial no- volt) under "normal"
working conditions, for metallized carbon a — 6.1, for tantalum a = 6.3, for
tungsten a = 6.6, and for osmium a = 6.9. In all cases the value of a
was found to decrease with rise in temperature.

Theoretically, this variation in a must occur at some stage in the tem-
perature (either an abrupt decrease at some fixed temperature, or a uni-
form decrease throughout the range), otherwise a temperature would be
attainable at which the total radiation is greater than that of a complete
radiator at the same temperature.

In the " black body " the reflection is zero. The Nernst glower, as
well as the oxides in general, have a very low reflecting power; hence, if
the radiating layer is of sufficient thickness, the emissivity of the oxides
must be almost as great as that of a" black body," as has just been found
for the Nernst glower, at high temperatures. Some oxides, having absorp-
tion bands in the visible spectrum, e. g., Ce02, when in thin layers (Wels-
bach mantle) and at high temperatures will have a higher luminous effi-
ciency than the same material in thick rods, e.g., the Nernst glower.
In the next chapter it will be shown that the greater the electrical con-
ductivity the more continuous will be the emission spectrum of the oxides.
Furthermore, mixtures of oxides like mixtures of gases in a vacuum tube
have composite emission spectra. This indicates a possible method of
analysis of mineral solutions, and it is hoped to make a further examina-
tion into this question.

CHAPTER II.

SELECTIVE RADIATION FROM VARIOUS SOLIDS.

INTRODUCTION.

Our knowledge of the emission of radiant energy from various sub-
stances with change in temperature is extremely limited, being confined
to platinum in the case of metallic electrical conductors, to several gases
in vacuum tubes, to water-vapor and carbon dioxide in the Bunsen flame,
to carbon, to the oxides of copper and iron, and to the radiation from a
uniformly heated cavity or complete radiator. The emission spectra of the
Bunsen flame and of gases in vacuum tubes were found to be composed
of sharp emission bands superposed upon a weak continuous spectrum.
The solids were found to have smooth continuous emission spectra, and it
seems to be the general expectation to find (see Kayser's Spectroscopic,
vol. 2, pp. 135 and 284, also Rudorf, Jahrb. Radioakt. und Elektronik, 4,
385, 1908) that all solids emit continuous spectra.

To Paschen is due the credit for the first systematic study of the spectral
distribution of radiant energy from various solids, and from the Bunsen
flame. Subsequent work by others has been but little more than the
establishment of the so-called radiation constants to a greater number of
significant figures than was possible by Paschen, with the facilities at his
disposal. Great credit is due to Lummer and Pringsheim for establishing
the limits within which the radiation laws, notably Wien's law, hold. It
must be said, however, that Paschen had these limits partly established;
but he insisted that the discrepancies between theory and experiment were
due to errors of observation. In this brief summary it is not possible to
present the subject fully, but after working over the data, one can not help
feeling that it is extremely unfortunate that the results of these investiga-
tions are beclouded with controversies as to whom belonged the credit for
doing or suggesting this, that, or some other thing in connection with the
work.

The best proof of Kirchhoff's law of the proportionality of emission and
absorption is due to Paschen,1 who found that the intensity of the emission
of the C02 band at 4.4 /*, when using a column of gas 7 cm. long, was as
great as for a column of gas ^ cm. long. In other words, the intensity
of the radiation was as great as that of a complete radiator for the same

1 Paschen: Ann. der Phys. (3), 53i P- 26, 1894.

93

94 INFRA-RED EMISSION SPECTRA.

wave-length and at the same temperature. Unfortunately the number of
radiating substances of which it is possible to determine even an approxi-
mate temperature is extremely small. Hence, the work presented on the
following pages can not be more than a qualitative proof of Kirchhoff's
law of proportionality between emission and absorption. It has numerous
applications, however, particularly in studying substances having sharp
emission (hence sharp absorption) bands. Numerous substances are given
here, of which it has not been practicable to study the absorption spectra.
Their emission spectra give us some idea of the nature of their absorption,
the only difference being that the emission bands are more intense, due to
the greater thickness (and the higher temperature) of the substance exam-
ined. Another feature of the results obtained, which is new, is the extreme
sharpness of the emission bands. Moreover, the maxima of the emission
and absorption bands coincide, although the temperatures at which the
two sets of observations were made differ by 800 to 10000 C. The posi-
tions of the sharp, well-defined maxima are not affected by change in
temperature. This is in marked contrast with the results of Konigsberger1
for the limited region of the visible spectrum, in which he found that for
certain selectively absorbing substances the maximum of the absorption
band shifts toward the long wave-lengths with rise in temperature, and
with the results of Paschen on the emission bands of C02 and water-vapor,
which shifted with rise in temperature, some toward the long, others toward
the short wave-lengths. The data may also prove to be of use in deter-
mining whether pleochroism is an inter- or intra-molecular phenomenon.
For example, the absorption spectrum of adularia shows a band at 3.2 //,
which in the emission spectrum is shifted to 2.9 p.. The latter band is
characteristic of silicates, whether in the emission or absorption spectrum.
The present work may be considered an examination of the emission spectra
of electrical insulators, or transparent media. The only previous work
done on this subject is due to Rubens,2 who examined the radiation from
the Auer mantle, as well as mantles composed of the pure oxides of cerium
and of thorium. The difficulty experienced by him was the elimination of
the emission spectrum of the hot gases which was superposed upon that
of the oxides composing the mantle. Hence, if he had examined a mantle
of zirconium oxide he would not have been able to detect an emission
band which occurs at 4.3 /«.

The substances used in the present investigation were in the form of
solid rods made in an oxy-hydrogen flame or in the form of thick layers
of the substance placed upon a heater. The rods were heated by an electric
current from the secondary of a 2000-volt (300-watt) transformer. These
rods had, of course, to be heated initially with an alcohol or blast lamp
until they became conducting, just as is necessary with the Nernst glower.

1 Konigsberger: Ann. der Phys. (4), 4, p. 796, 1901.

2 Rubens: Phys. Zs., 6, p. 790, 1905.

VARIOUS SOLIDS. 95

The resistance placed in the primary circuit of the transformer to regulate
the current, and the low capacity of the transformer, acted as a "ballast"
to the radiating substances. The rods were provided with platinum ter-
minals, sealed in the ends, and were securely mounted in incandescent
lamp sockets. After heating them until they became conducting, they
were mounted securely before the spectrometer slit. The substances that
could not be melted and formed into rods were made into a paste and
spread upon the "heater tube" of a Nernst lamp. The "heater tube"
consisted of a hollow porcelain tube about 5 cm. long and 8 mm. diameter
covered with a coil of fine platinum wire, and was used in preference to a
platinum strip on account of its rigidity and ease in handling. It gave
the same results as the same material on a platinum strip. The spectrom-
eter, the fluorite prism, and the bolometer were previously used in ex-
amining the Nernst glower. It is important to notice, however, that by
inclosing the optical parts of the instrument it was possible to entirely
eliminate the atmospheric absorption bands, which had not been done
successfully by previous experimenters. The emission curves are, there-
fore, within experimental errors, an exact portrayal of the distribution of
the energy emitted in different parts of the spectrum. Unfortunately it is
not possible to determine the temperature of the radiating body, the thick-
ness of which would have to be specified, in many cases. A thermocouple
can not be applied on account of the loss of heat by conduction. It is, of
course, absurd to attempt to measure temperatures with an optical py-
rometer. For example, the rod of oligoclase was a perfectly transparent
glass, and emitted no light other than that due to sparking of the platinum
terminals, although at a temperature of 11 00 to 12000 C. Nevertheless, a
substance such as iron oxide would have emitted light when at the same
temperature, while both emit strongly in the infra-red. A further example
is the rod of topaz, which was an opaque white mass. On withdrawing it
from the oxy-hydrogen flame, it was accidentally stroked with the iron
forceps, when the parts so stroked emitted a dull red light, due to the
greater emissivity of the iron oxide, while the untouched parts retained
their usual white color.

Instead of temperatures, the energy consumption is given; also the
dimensions of the rods, the lengths being the distances between the platinum
terminals. The diameters are in some cases not very accurate, owing to
the irregularity of some of the filaments. The voltage was measured with
an electrostatic voltmeter, joined to the terminals of the rod. The current
was measured with a milliammeter, of a suitable range to insure accuracy.

It is manifestly impossible to have the smoothness of the surface and
the thickness of the radiating layer the same for all substances. The
distance (about 2 cm.) of the radiator from the slit and the bolometer
sensibility will also vary. No attempt has therefore been made to obtain
the emission curves of the various substances at the same temperature,

96

INFRA-RED EMISSION SPECTRA.

and to plot them to the same scale. The purpose of the present examina-
tion is to map the distribution of energy in the various spectra, hoping that
at some future time it may be possible to make a more extended study of
certain substances which show unusual emission spectra.

RADIATION FROM ELECTRICALLY HEATED SOLIDS.

Prominent among this group are a series of silicates, which have an
emission band in common at 2.88 p. (characteristic of Si02) which is as

/ 2 3 4 5 6

Fig. 63. — Emission spectrum of albite.

8JU

sharp as any yet found in gases. The absorption spectra of many of these
compounds are recorded in Part III of these investigations (Carnegie
Publication No. 65).

In order to give the reader some idea of the conditions under which

SILICATES.

97

the data was obtained, a rough estimate is made of the temperature at
which a complete radiator would emit light of a color similar to that given
out by the substance under investigation. The fact that soot from the gas
flame was burned off the rods not emitting light would indicate that the
temperature was above 6oo° C, and in all cases the estimated temperature
is greater than 8oo° C.

The length of the rods used depended upon the melting-point. For
example, the "soft glass" rod was very short on account of its softening
at the temperature necessary to keep it electrically conducting. The ends
of the rods were shielded from the spectrometer slit. All these curves are
reduced to the normal spectrum by dividing the observed galvanometer
deflections by the slit-width expressed in wave-lengths. In this work the
unsteadiness of the bolometer prevented an accurate mapping of small
emission bands occurring beyond 6 /*.

Albite (NaAlSiaOs).
(Rod 8 mm. long, tapering from i.8 to 2.8 mm. in diameter. Energy supplied, 7.1 and 8.2
watts. Curves a and b, fig. 63. Temperature 8oo°. Transmission given in Carnegie
Publication No. 65, p. 65.)

The rod was translucent and seemed to emit no light except that re-
flected from the platinum terminals. The emission band at 2.88 /* is

0 1 2 3 4 5 6 7

Fig. 64. — Emission spectrum of orthoclase (var. adularia).

more intense than the one found in orthoclase, just as was found in the
absorption spectrum. Two other bands are noticeable at 4.1 and 4.5 /*,
respectively.

98

INFRA-RED EMISSION SPECTRA.

Orthoclase (var. Adularia) [KAlSisOs].

(Rod 8 mm. long, 2 mm. diameter. Energy supplied, 6.2 and 8.9 watts. Curves a and b,
respectively, of fig. 64. Transmission, Carnegie Publication No. 65, p. 64.)

This substance emitted a little more light than albite, although it was
more transparent. The emission band at 2 j.i is prominent. The one at

0 / 2 3 4 5 6 7 8U

Fig. 65. — Orthoclase (a); alumina and silica (6); alumina and feldspar.

2.88 n is shifted from its position at 3.2^ in the absorption spectrum,
from which it would appear that the group of atoms causing the absorp-
tion is different in the two cases. An examination of the transmission
spectrum using polarized light will be necessary to determine the true

SILICATES.

99

position of the absorption band. The band at 4.1 and 4.5 /x are in common
with the other feldspars.

Orthoclase (Feldspar) [KAlSi308].

(Rod 7 by 2.2 mm. Energy, 7.5 watts. Curve a, fig. 65. Transmission, Carnegie Publica-
tion No. 65, p. 64.)

This sample was translucent, due probably to air-bubbles. Its color
on 7.5 watts was slightly red, due in part to reflected light from the elec-

3 4-56

Fig. 66. — Oligoclase.

trodes. The emission bands at 2, 2.88, 4.1, and 4.5 /* are in common with
the preceding, with the possibility of a small band at 3.1 p., to be noticed
in fig. 66.

100

INFRA-RED EMISSION SPECTRA.

Oligoclase

3 NaAlSia08 '
CaAkSiaOg

(Rod 9 by 2.8 mm. Energy supplied. Curve a = 8.6, 6 = 13.3, and £ = 29.4 watts.
Curves a and b, fig. 66. Transmission, Carnegie Publication No. 65, p. 64.)

' The original crystal, as well as the glass rod, were perfectly transparent.
The rod showed no color on suddenly throwing off the current. The
platinum terminals melted on 16 watts, but the rod showed no color. As
in the preceding feldspars there are bands at 2, 2.88, 3.1, 4.1, 4.5, and 7 ;x.
The absorption band at 4.8 /j. seems to be shifted to 4.5 /* in the emission
spectrum. Curve c shows the distribution of energy when the rod (another
sample, plotted to a different scale) was heated until viscous, see fig. 91.

cm
60

50

40

>

.52 30

20

10

V

1

1

\

\

\

A

T •

I

y

t

1 •

TO

•\

1

\

2 3

Fig. 67.

4 5

Amphibole.

8JU

Amphibole (Tremolite?) [CaMg3(Si03)4].

(Rod 6 by 1.5 mm. Energy supplied, 2 (?) and 4.8 watts. Curves a and b, fig. 67.

Transmission, Carnegie Publication No. 65, p. 64.)

At the highest energy consumption the color of the filament was a light
yellow, corresponding to a temperature of 1200 to 13000. The emission

SILICATES.

IOI

curve is quite different from the preceding, due in part to the higher tem-
perature. The absorption band at 6 ^ appears as a depression in the
emission spectrum.

WOLLASTONITE (CaSi03).

(Rod 10 mm. long, tapering from 3 to 4 mm. diameter. Energy supplied, 18 watts.

Curves a and b, fig. 68.)

This rod was made from a pure transparent glass, supplied by Dr.
Allen of the Geophysical Laboratory of the Carnegie Institution of Wash-
ington, which became a white crystalline mass on melting in the oxy-
hydrogen flame. It was rendered conducting with difficulty, and could
not be heated uniformly.

2 3 4 5

Fig. 68. — Wollastonite.

The silicate band at 2.9 p is prominent, while a new band appears at
3.7 p. Curve b gives the emission from the coolest side of the rod, while
curve a represents the emission from the hottest side. The latter is an
excellent illustration of the rapid increase in intensity with temperature,

102

INFRA-RED EMISSION SPECTRA.

of the emission bands on the side of the spectrum toward the short wave-
lengths. Further examples will be found in figs. 77 and 78. In this and
in the preceding substance the emission bands are not so sharp as usual,
which may be due to the greater molecular weight of the base.

Porcelain (Pyrometer Tubing).
(Hollow rod 15 by 2 mm. (hole 1 mm.). Energy supplied, 9.2 watts. Fig. 69.)

The sample of pyrometer tubing examined was heated to a light-red
color (14000) to keep it conducting. The energy spectrum is marked for

Fig. 69. — Porcelain.

its strong emission, with sharp maxima at 1.8, 2.1, 2.83, 3.7, 4.1, and
4.5 ft, and with indications of bands beyond 6 ft.
a hydrous aluminum silicate.

Porcelain is made from

Glass ("Soft Glass").

(Rod 3 by 2 mm., heated to dull red color. Curve a, fig. 70. Transmission,

Carnegie Publication No. 65, p. 65.)

The substance examined was a piece of ordinary "soft" white glass

tubing, drawn into a solid rod. There are emission bands at 2, 2.86, 3.6,

4.4, and 5.5 pt.

Magnesia (Pyrometer Tubing).

(Hollow rod, 12 by 1.5 mm. (hole 1 mm.). Energy supplied, 6.6 watts. Curve b, fig. 70.)

This material is used to insulate thermo-couples, and conducts only at
high temperatures. It is probably a mixture of magnesium oxide (see
fig. 80) and silica. The temperature was probably 12000 to 14000. The

COBALT GLASS.

103

emission spectrum is conspicuous for two regions of strong emissivity at
1.6 and 5 n, respectively, with a deep depression at 3.5 pt. The emission
bands at 1.6, 2.7, 5, 5.5, and 7.1 /1 are not well resolved, but their presence
can not be doubted.

2 3 4 5 6

Fig. 70. — Glass (a); Magnesia.

9/1

Glass (Cobalt Blue).
(Rod 12 by 2 mm. Energy supplied, 5.4 and 7.5 watts. Curves a and b, fig. 71.)

This rod was heated to a dull red. The emission spectrum is quite
different from that of soft glass, but there are not such prominent bands

>6

14

12

10

X

OJ o

*w

r\

a ,

I \

A

v

^>

A

\
\

\

V

s.

1'

1

r

\

1/

2 3 4-5

Fig. 71. — Cobalt glass.

&JM

104

INFRA-RED EMISSION SPECTRA.

as one might expect from a knowledge of the absorption bands at 1.6 and
2.2 fi to be noticed in fig. 37. The silicate band at 2.87 /< is prominent.
Other bands occur at 2, 3.6, 4.6, and 5.5 /<, respectively.

Spodumene [LiAl(Si03)2].

(From Pennington County, South Dakota. Irregular rod, 12 by 1.5 mm. Energy

supplied, 5.7 and 9.5 watts. Curves a and b, fig. 72.)

This mineral contains cerium, lanthanum, and other "rare earths" as
impurities. It can be melted in a blast flame. Considerable light was
emitted, due to the presence of impurities. The emission spectrum shows
the silicate band, sharply defined at 2.88 /«, with other bands at 2, 4.1, 4.5,
4.9, and 6.5 //.

4- 5

Spodumene.

Beryl [Be3Al2(Si03)fi].
(Rod 10 by 3 mm. Energy, 7 to 8 watts. Curve a, fig. 73.)

The rod was an opalescent milky glass, although the original crystal
was a transparent green. The temperature was probably 11000. The
emission spectrum is unusual in appearance, with a sharp maximum
superposed upon it at 2.8 fi. Other ill-defined maxima appear to be at
1.7, 2.4, 2.9, 3.6, 4.4, and 4.8 ;«.

TOPAZ.

!°5

Rutile (Ti02).

(Flat plate 8 mm. long, tapering from 1.5 to 1.8 mm. wide and 0.25 mm. thick. Energy

supplied, 6 watts. Curve b, fig. 73. Transmission, Carnegie Publication No. 65, p. 67.)

This mineral was heated to a bright red color corresponding to a
temperature of perhaps 10000. The emission spectrum shows maxima at
2.4, 3.2, 5,5, and 7.0//. The transmission spectrum is too low to show
these as absorption bands; but the band at 3.1 /« is visible in the trans-
mission spectrum of brookite (Ti02). This substance is a good conductor
of electricity at this temperature, but a very poor radiator of light rays.

3 4 5 6

Fig. 73. — Beryl (a); Rutile.

'M

Topaz [(AlF)2Si04].
(Rod 13 mm. long, 2 to 2.5 mm. diameter. Energy supplied, 0.05 and 0.07 ampere, —
probably 15 to 21 watts. Curves a and b, fig. 74.)

This rod was built up from the massive transparent mineral, and
rendered conducting by the vapor set free from the albite cement used to
secure the platinum terminals to the rod. The vapor left a narrow con-

io6

INFRA-RED EMISSION SPECTRA.

ducting streak along one side of the rod, which heated the topaz. The
curve is conspicuous for the sharpness of its emission bands, which occur
at 1.4, 2.85, 4.1, 4.5, and 7.5 //.

SILICATES.

IO7

Apatite [Ca5F(P04)3].

(Rod 12 by 1.5 mm. Energy supplied, 9.3 and 11.8 watts. Curves a and b, fig. 75.

Transmission, Carnegie Publication No. 65, p. 58.)

The emission spectrum shows emission bands at 2.9 and 4.5 fx. The
rod was made from a dark massive specimen which may have contained
silica, whence the band at 2.9 /j. in both the emission and absorption spec-

345

Fig. 75. — Apatite.

8JU.

trum. On the other hand, many oxides, known to be free from silica,
have a strong emission band in this region, from which it would appear
that this band is characteristic of the oxides.

Aluminum Oxide (AI2O3).
(Rod 14 mm. long, tapering from 2.5 to 3 mm. in diameter. Energy supplied, 0.025 and

0.115 ampere. Curves a and b, fig. 76.)

The rod was built up from the pure powder in an oxy-hydrogen flame.
The emission curve is conspicuous for the great variation in intensity of
its emission bands which occur at 1.4, 2, 3.1, 4.7, 5.2, and 7-5(?) /*• The
emission curve differs but little from the one of topaz except that the latter
has the silicate band at 2.85 11. In fig. 65, curve c shows the radiation
from a rod made from alumina and 1 per cent of feldspar, and curve b,
fig. 65, gives the radiation from a rod made of alumina and 1 per cent of
silica. In both cases the silica band at 2.87 fi is superposed upon the
spectrum of the aluminum oxide.

io8

INFRA-RED EMISSION SPECTRA.

Zirconium Oxide (Zr02).

(Rod 5 by 1.4 mm. Energy consumption, 3.5 (ooo°), 4.8, 5.8, 7.5 watts, curves

a, b, c, d, fig. 77; 9.6, 1 2.1, and 14.5 (14000) watts, curves a, b, c, fig. 78.)

The specimen examined was a fragment from a furnace wall. It
probably contained some "binder," although from the curve for the pure

material (fig. 79) the foreign substance contributed but little to the emission
bands. The fine platinum terminals were wound around the ends of the
rod which was about 10 mm. long (5 mm. between the terminals).

The curves are conspicuous for their sharp emission band at 4.3 fi,

ZIRCONIUM OXIDE.

109

which remains superposed upon the continuous background even at a
bright yellow heat, corresponding to a temperature of about 14000.

2 3 4 5 6

Fig. 77. — Zirconium oxide.

This series of energy curves is one of the best illustrations yet recorded
of the gradual shift of the maximum of intensity of emission toward the
short wave-lengths with rise in temperature. On an energy consumption

no

INFRA-RED EMISSION SPECTRA.

of 3.5 watts, the maximum of the envelope of these emission bands lies at
4 to 5 //, and shifts steadily to the short wave-lengths, being at about 1.8 fi
for an energy consumption of 14.5 watts. The pure oxide is not an efficient
radiator of white light, and only becomes so when a small amount of cerium
or thorium oxide is added, which combination is the Nernst glower already

2 3^-5

Fig. 78. — Zirconium oxide.

mentioned. Aside from the sharp emission lines at 2.8 and 4.35 fi there
are wide hazy bands at 2 and 2.4 /x (appears on 7.5 watts), while from
5 to 6 n there is a wide band which is evidently unresolved, the maximum
shifting toward the short wave-lengths with rise in temperature. The
extraordinary rapidity which characterizes the growth of the emissivity at

MAGNESIUM OXIDE. Ill

1.5 to 2 ll is worthy of notice. In one case where the ratio of emissivity
at 4.3 {x for a given increase in energy is 50, it is almost 200 at 2 /*. In
fig. 77 the curves are all for practically the same sensibility, 71, of the
instrument. In fig. 78 the sensibility of the instrument was 60. These
units are arbitrary, being the galvanometer deflections obtained by un-
balancing the bolometer by a constant amount. For this substance the
radiating rod was short and the voltage too low to be measured with the
electro-static voltmeter. The ordinary voltmeter used affected the tem-
perature of the radiator slightly, so that the energy consumption recorded
is not so accurate as in the other observations.

EMISSION SPECTRA OF SOLIDS ON NERNST "HEATER TUBE."

In examining the radiation from solids placed upon a heater it is pos-
sible to have some of the radiation from the latter transmitted through
the former. If the substance to be examined is in the form of a fine powder
and the layer is from 0.5 to 1.5 mm. thick there is but little chance for
the radiation to be transmitted from the heater. This fact enables one to
study the selective emission of solids which can not be formed into solid
rods, and is applied in studying the following list of substance.

Zirconium Oxide (Zr02).

(Layer of oxide 0.8 to 1.5 mm. thick, on Nernst heater tube from which the clay

covering had been removed. Fig. 79, curves c, b, and c.)

The zirconium oxide was heated to a dull red, which was somewhat

lower than for the rod heated electrically (curve a, fig. 77). Curves a and

c give the emission of different samples of the commercial product, the

electrical properties of the latter being very bad when used for the body

of a glower. Curve b shows the emission of a sample which has good

electrical properties. It contained less than 0.01 per cent of silica, and

was the purest obtainable sample. From this it is evident that the sharp

maxima are not due to silica. The emission spectra are very similar and

are conspicuous for two sharp emission bands, with maxima at 2.78/1

(curve a, maximum at 2.83 //) and at 4.3 ft, respectively. Smaller maxima

appear at 2, 2.4, 3.2, 4.7, and 5.4/*. From a comparison of these curves

with those given in figs. 77 and 78 it appears that in the latter sample the

"binder" used, probably yttria, has but little influence upon the sharp

emission bands.

Magnesium Oxide (MgO).

(Curve c, fig. 80.)

The magnesium oxide (curve c, fig. 80) spectrum shows two wide
bands at 3 and 5.3 n, respectively, with a smaller band at 2.1 it. Some-
what similar conditions were observed in fig. 70.

In fig. 80, curve a gives the emission spectrum of the material used in
a Nernst glower placed upon a heater-tube. The general outline, and

112

INFRA-RED EMISSION SPECTRA.

especially the emission bands, are to be observed in the radiation curves
of the glower, given in fig. 57. In curve b, fig. 80, is shown the emission
spectrum of a commercial "heater-tube." The covering is some refractory
substance, perhaps containing aluminum oxide and a silicate binder.
There is a sharp band at 2.85 p. and hazy bands at 4.7, 5.3, and 6.3 ;i.

2 3 4 s

Fig. 79. — Zirconium oxide.

7JU

Uranium Oxide (U203); Cerium Oxide (Ce02); Thorium Oxide (Th02).
(Curve a = U203; &=Th02; c = Ce02; fig. 81.)

The uranium oxide is a greenish-brown powder which gives a smooth
continuous spectrum with hazy maxima at 2.8 and 3.4 ft, respectively.

The thorium oxide, curve b, shows no marked emission bands, and the
whole spectrum is suppressed to 7 /*. Beyond this point the emissivity is

CERIUM AND THORIUM OXIDES.

IJ3

high, corresponding closely to that of a complete radiator, as was shown
by Rubens, using a gas-mantle of this material.

The cerium-oxide curve shows a strong emission at 2 to 3 /*, as was
found by Rubens, with possible bands at 4.4 and 7.5 p.. It was shown by
Rubens that beyond 7 // the emissivity approaches that of a complete
radiator.

0/234 5. 6 7 8 9/1

Fig. 80. — Nernst glower (a); Nernst "heater-tube" (6); Magnesium oxide.

In the Auer gas mantle (90 per cent Th02 + 1 per cent Ce02) the cerium
oxide acts as a sensitizer does on a photographic plate, by making an ab-
sorption band in one region of the spectrum without affecting the rest.

The energy curve of an electrically heated filament of the same com-
position as the Auer mantle is given in the Bulletin of the Bureau of
Standards, vol. 5, No. 2, 1908. The energy curve is entirely different

ii4

INFRA-RED EMISSION SPECTRA.

/ 234 567 8 9M

Fig. 8i. — Uranium oxide (a); Thorium oxide (6); Cerium oxide.

from that of the Auer mantle, being similar to that of the Nernst glower.
This is no doubt due to the greater thickness of the radiating layer, which
emits a more nearly saturated radiation in the region of i to 3 /<, as well
as in the remainder of the spectrum.

0/234567
Fig. 82. — Beryllium oxide (a); Silica (b), (c); Feldspar.

YTTRIUM OXIDES.

115

Beryllium Oxide (BeO); Silicon Oxide (Si02); Vanadium Oxide (V206).
(Curve a=BeO; b and c = Si02; fig. 82.)

The beryllium-oxide emission spectrum is smooth, with two wide
maxima at 3.5 and 5 /*, respectively. The temperature was such that a
faint red showed through the interstices of the layer of white oxide.

The silicon-dioxide (quartz, French flint) curve shows bands at 2.2,
2-&3> 3-7> 4-4> and 5.3 /<, which coincide with the absorption bands (see
Carnegie Publication No. 65, p. 21). For curve c the layer of silica was 1.4
mm. For curve b (thinner layer) the surface temperature was about 8oo°.

3 4 5 6

Fig. 83. — Yttrium oxide.

The vanadium oxide in the preliminary examination showed no emis-
sion bands. The substance is a black powder, which melted at a low red
heat, and no further examination was made into its emission spectrum,
which was similar to that of carbon. Curve d, fig. 82, shows the emission
band of pure feldspar placed on the heater.

n6

INFRA-RED EMISSION SPECTRA.

Yttrium Oxide (Y203).
(Curves a and b, fig. 83; curves a, b, c, fig. 84.)

The surface color in the two cases (fig. 83) was a deep and a bright
red, corresponding to a temperature of 900° to iooo0. The two curves
are similar in appearance, showing emission maxima at 2, 2.76, 3, 3.6, 4.6,
and 6.9 ft, respectively, the latter band being unusually sharp. It will be

70

4 5

Yttrium oxide.

shown presently, in fig. 92, that this sharp band may be due to a suppres-
sion of the radiation at 6 fi, caused by a band of metallic reflection at this
point. However, bands of metallic reflection are not common at these
short wave-lengths, so that the maximum at 6.9 // may be a true emission
band.

In fig. 84 are shown the emission curves of three additional samples of

ERBIUM OXIDE.

117

yttrium oxide, of unknown purity, all of which were a beautiful yellow
color, as compared with the sample given in fig. 83, which was a yellowish-
white. These samples were obtained by fractional precipitation from the
sulphate of yttrium by means of oxalic acid. The precipitation, however,
was not carried out to the extent of obtaining yttrium, erbium, and ytter-
bium oxides separately. In these curves the prominent bands of fig. 83,
with maxima at 2 and 2.75 fi, are suppressed, and the small bands of the
latter at 3 and 6.8 li are the most prominent. These three samples were
heated so that the surface appeared a dull red, the sample for curve b
being the hottest. The emission bands occur in two groups, at 3 and
6.8 li, respectively, just as was found in the oxides of zirconium and mag-
nesium. There are sharp maxima at 2.8, 3.15, and 6.75 ju, and smaller
bands at 2, 3.9, and 5.2 li, respectively.

3 4 5 6

Fig. 85. — Erbium oxide.

Erbium Oxide (Er203).
(Curves a and b, fig. 85.)

Curve a shows the emission of a layer of the oxide formed by decom-
posing a solution of the nitrate on a strip of platinum, heated electrically.
Curve b gives the distribution of energy of a layer of the oxide on a heater-

n8

INFRA-RED EMISSION SPECTRA.

tube. The two spectra are similar, with a sharp emission band at 2.85 [i.
Other maxima appear at 2, 3.2, 4.1, 5, and 7.5 li. The general outline of
the spectrum is similar to that of yttrium.

Neodymium Oxide (NdO); Manganous Oxide (MnO).
(Curve a, NdO; curve b, MnO; fig. 86.)

The neodymium oxide was deposited in a thick layer upon a strip of
platinum, by evaporation from a solution of the nitrate. The radiation
curve shows maxima at 3, 4.4, and 4.83 ll. Beyond 6 ll the emissivity is
strong and not unlike that of cerium and thorium. The scale of emissivity
is one-half of the curve for MnO.

/ 2 3 4 5 6

Fig. 86. — Neodymium oxide (a); Manganous oxide.

8/1

The manganous oxide was a grayish-brown color. The thickness of
the layer upon the "heater-tube" was about 1.2 mm. The radiating
surface was a dull red. The spectral radiation curve is uniformly smooth
throughout its whole length, with but a slight depression at 3.2 11 to be
noticed in numerous oxides. In these two curves the sensibility of the
instrument is different, as is frequently the case.

Zinc Oxide (ZnO); Lead Oxide (PbO).
(Curve a, ZnO; curve b and c, litharge (PbO), curves e and/, platinum; fig. 87.)

The zinc oxide became a yellowish-green on heating, resuming its
former white color on cooling. In spite of this selective emission in the
visible spectrum, the distribution of energy in the infra-red is uniform,
with the usual depression at 3.2 ti. Lead oxide ("litharge") melts at a low
temperature. On heating the color changes from orange to deep red.
Curve b, fig. 87, shows the distribution of energy for the oxide after it had
melted into a smooth mass and solidified. Curve c shows the emissivity

OXIDES.

II9

from an unmelted surface. The depression at 3.3 // is marked, and at
5.5 ;« there is a possible emission band. The thickness of the layer was at
least 1.2 mm.

Curves e and /show the emission of a strip of platinum. No depression
appears at ^ n, from which it would appear that the depression at 3.$ n
is a characteristic of the oxides and not due to absorption in the instrument.

/ 2 3 4 5 6 7 8JU

Fig. 87. — Zinc oxide (a); Lead oxide (6), (c); Platinum.

Iron Oxide (Fe203); Copper Oxide (CuO).
(Curves a and b, Fe203; curves c, d e, CuO; fig. 88.)

The iron oxide used was red hematite, or "rouge." The layer on the
heater-tube was 1.5 mm., heated to redness, 8oo°. The energy curve is
smooth, except a depression at 3.2 /<. The copper-oxide layer was 1.5 mm.,
heated to a deep red. The energy curve is smooth, except the slight
depression at 3.2 /*.

Cobalt Oxide (C02O3); Chromium Oxide (Cr203); Stannic Acid (Sn02).
(Curve a = Co203; curves b and c, Cr203; curve d, Sn02; fig. 89. Sensibility 80, 85,
and 73, respectively. Temperature 8oo° to 9000).

The cobalt-oxide curve is smooth throughout, except the depression at

Chromium oxide is green in color, and emits a fairly smooth spectrum
with a possible maximum at 5 [i. The depression at 3.2 fx is prominent.
Stannic acid is grayish-white in color, but, unlike many of the white

120

INFRA-RED EMISSION SPECTRA.

oxides, it emits a continuous spectrum. The depression at 3.2 p. is small.
The transmission bands in cassiterite, Sn02, were found to be small.

12 3 4 5 6 7

Fig. 88. — Iron oxide, (a) (6); Copper oxide.

8/1

0/234567 8/J.

Fig. 89. — Cobalt uxiue {a)\ Chromium oxide (b), (c); Stannic oxide.

Nickel Oxide (NiO); Calcium Sulphate (CaS04+2H20).
(Curves a, b, c = NiO; curve <f = CaS04; fig. 90. Temperature about 9000. Trans-
mission, Carnegie Publication No. 65, p. 18; curve b, fig. 2.)

The nickel-oxide surface was rougher than for the other oxides,
hence not so uniformly heated. At the highest temperature the de-

CALCIUM SULPHATE.

121

pression at 3.3 /j. is very marked, while at the lower temperatures it is
not so deep.

The calcium sulphate used was a thick, smooth layer of "plaster of
paris," which dehydrated (at least in part) at the red heat used. The
emission bands at 2, 3.2, 4.65, and 6.3 jx coincide in intensity and position
with the absorption bands found in previous work (see Carnegie Publica-
tion No. 65, fig. 4). The band at 4.65 /« is shifted from its position at

/ 2 3 4 5 6 7

Fig. 90. — Nickel oxide (a), (b), (c); Calcium sulphate.

8M

4.55 fi in anhydrite (CaSOJ, but coincides with the partially dehydrated
selenite (CaS04+2H20) given in Carnegie Publication No. 65, figs. 3 and 4.
The absence of the band usually found at 2.8 it may indicate that it is
not due to water. Paschen found an emission band of water at 2.83 // in
the Bunsen flame.

Calcium Oxide (CaO); Tricalcium Phosphate [Ca3(PO«)2].
(Curves a and 6 = CaO; curve c = Ca3(P04)2; fig. 91.)

The surfaces of these two substances were at a dull red heat. The
calcium-oxide layer was somewhat cracked, but not sufficient to interfere
with the radiation. The layer of oxide in each case was about 1.2 mm.

The calcium-oxide emission curve is conspicuous for its two sharp
maxima, at 2.8 /x and at 4.75 ;i, respectively, and a high emissivity at 8 ll,

122

INFRA-RED EMISSION SPECTRA.

which is not unlike that of cerium and thorium oxides, observed by Rubens.
Smaller bands appear at 2.4, 3.3, and 4 fx. The calcium oxide was heated
to a bright red before mounting it upon the heater-tube, and was apparently-
free from the carbonate (fig. 92). Since there are no emission bands
belonging to that substance, it appears that the water used in making the
CaO into a paste was entirely expelled.

0/234567
Fig. 91. — Calcium oxide (a); (6); Tricalcium phosphate.

The tricalcium phosphate has two marked emission bands, at 2.85 and
4.75 li, respectively, and smaller bands at 2 /x and 6.2 /«. The emission
spectrum is unusually similar to that of apatite (fig. 75).

A chemical analysis of calcium oxide, by Dr. H. C. P. Weber, showed
no weighable amount of silica. This shows that the band at 2.8 11 is not
due to silica.

Calcite (CaCOs).
(Curves a and b, fig. 92. Transmission, Carnegie Publication No. 65, p. 70.)

The sample examined was a layer of finely ground white marble. The
surface color in the two cases corresponded to about 9000 and 10000, re-
spectively. The emission curve is of interest on account of the two types
of emission it contains. In the region of 6.7 /« calcite has a band of strong
selective, "metallic," reflection. In this region of the spectrum the emis-
sivity is proportional to the reflecting power, thus placing in the class with

CALCIUM CARBONATE.

123

metals. Here the emission is actually suppressed, and we have an emission
minimum instead of a maximum, as will be described in the following
chapter. In the remaining part of the spectrum the emission is propor-

345

Fig. 02. — Calcite.

tional to the absorption. The emission bands at 2, 2.7, 3.5, 3.9, 4.7, and
5.5 /j. coincide with the absorption bands previously observed. The ap-
parent maximum at 7.3 // is due to the suppression of the radiation at
6.8 p. This fact can, of course, only be determined from a knowledge of

124

INFRA-RED EMISSION SPECTRA.

the reflecting power of the substance under examination. The maximum
reflection and the minimum emission do not coincide on account of the
high reflecting power on the side toward the long wave-lengths, which
suppresses the emission curve, thus shifting its minimum farther into the
infra-red. The only other example heretofore investigated is by Rosen-
thal,1 for quartz, which will be described in the next chapter. The band
at 4.7 // is of interest since it occurs in CO and C02 in the vacuum tube
radiation (see Carnegie Publication No. 35), and is in common with the
carbonates and the sulphates.

RELATION BETWEEN EMISSIVITY AND ENERGY CONSUMPTION.

The substances just described must have one or both of two kinds of
spectral energy distribution, due (1) to the general absorption which is
present to some extent, however small, and which gives rise to a continuous
spectrum, and (2) to bands of selective absorption which give rise to
emission bands.

The object in examining the isochromatic radiation curves of the
aforesaid solids is to determine whether or not the observed sharp emis-
sion bands behave like spectral lines, or like bands which include a con-
siderable portion of the spectrum. If the observed bands behave like
those of a gas, the emission must be proportional to the energy supplied.

If the radiation is similar to
the complex and highly damped
emission of a solid, e.g. plati-
num, the isochromatics can not be
straight lines, but must be similar
j^2.42j/i tQ ^Qgg 0f platinum.

A complete radiator emits
energy, however small the
amount, of all wave-lengths,
whatever its temperature above
the absolute zero. The isochro-
matic energy curves must, there-
fore, all begin at the origin of
the energy axis; and they may
have a double curvature. This
is well illustrated in fig. 93 for
the isochromatic radiation curve
of platinum at 2.752 p.. The
platinum strip was 50 by 1.5 by 0.02 mm., in an exhausted glass bulb, with
a fluorite window. In this figure the graphs of wave-lengths X= 1.804 P- and
^=1.968 11 intersect at 2.8 watts, showing that the maximum of the energy
curve lies between these two wave-lengths; temperature about 11000 C.

12 3 4 5 Waits

- Isochromatic radiation curves of platinum.

1 Rosenthal: Ann. der Phys. (3), 68, p. 791, 1899.

EMISSIVITY AND ENERGY CONSUMPTION.

125

It is interesting to note that the point of inflection in the curves oc-
curs when the isochromatic wave-length is identical with the wave-length
^max) (Emax). This, of course, is due to the well-known property of spec-
tral emission of a complete radiator, or a metal, in which the emissivity
in the short wave-lengths increases more rapidly than on the long wave-
length side of the maximum emission.

On the other hand, the graph showing the relation between the emis-
sivity of a spectrum line and the energy supplied should intersect the
energy axis at a distance from its origin, corresponding to the energy (a
finite amount) required for excitation, which is different for different
spectral lines. This is a well-known property of spectral lines, being inde-
pendent of the wave-length.

Furthermore, if we follow the common line of reasoning (see Kayser's
Spectroscopy, vol. II, pp. 59, 245, and 331) and consider the separate
lines as a part of an energy curve, obtained by drawing the envelope
through the highest points of the separate emission bands, then the maxi-
mum of the envelope must shift
toward the short wave-lengths
with increase in energy con-
sumption, and the slant of the
isochromatics must be similar
to those of platinum (fig. 93).
It is difficult to conceive how
this is possible with discrete
spectral lines which require the
application of a certain amount
of energy to excite them. The
change of the emission curves
of the Nernst glower from a
discontinuous into a continuous
one has already been noticed.
They illustrate this envelope
type of energy curve just men-
tioned. But it seems more
probable that this is due to the
rapid growth of the general
emission of the intervening fre-
quencies, which, with a doubtful broadening of the emission bands, oblit-
erates the selective emission at high temperatures. In fig. 94 are shown
the isochromatic energy curves of a Nernst glower, the values being
taken from fig. 57. In fig. 95 are given a series of isochromatics for a
no-volt glower 1.4 cm. long and 1.4 mm. diameter. The current was
supplied from a 2000- volt 600- watt transformer on a no- volt circuit.
The energy supply was regulated by means of resistances in the primary.

2 4 6 8/0 iZW&ttS

Fig. 94. — Isochromatic radiation curves of Nernst glower.

126

INFRA-RED EMISSION SPECTRA.

The voltage was obtained with a multiple-cell electrostatic voltmeter. The
curves for the intensest part of the spectrum pass through a double cur-
vature and have the general outline of that of platinum. The normal
burning is 80 watts and above that point the isochromatics appear to

cm
1500

1400

10 20 30 40 50 60 70 80 90 100 Watts

Fig. 95. — Isochromatic radiation curves of Nernst glower.

show a slight increase in curvature. The intersection of the graphs for
wave-lengths X= 1.206 [i and k = 1.633 P- at 73 watts 1 shows that the maxi-
mum of the spectral energy curve for this power consumption lies between

1 None of the isochromatics, given in this paper, have been corrected for slit-width,
which would change the observed energy consumption a few per cent.

NERNST GLOWER ISOCHROMATICS. 1 27

these two points, as previously observed. This method of locating the
maximum eliminates the correction for slit-width, and is an independent
proof of the previous observations that the maximum of the energy curve
for normal burning (80 watts) can not lie at such short wave-lengths as
was observed by previous investigators. This, of course, is on the assump-
tion that the glowers were of the same material, the base of which is zir-
conium oxide with a small per cent of thorium or yttrium oxide.

In Carnegie Publication No. 35, p. 318, it was shown that in the case
of vacuum-tube radiation the intensity of the emission lines is proportional
to the energy consumption. The graphs there obtained, showing the
relation between current and emissivity of a spectral emission line, are
curved, due to the fact that Ohm's law does not hold for the vacuum-tube
discharge. For the present examination, instruments were available to
measure the energy consumption, when the graph ought to be a straight
line, provided the partition of the energy emitted is in discrete lines. As
a typical, selectively radiating solid, oligoclase was chosen on account of
its sharp emission bands, and also on account of its homogeneity. A rod
2.5 cm. long and 2 mm. diameter was prepared in an oxyhydrogen flame.
The spectrometer slit was reduced to 5 mm. in length, which permitted
the entrance of radiation from only about 5 mm. of the central part of the
rod, which was a perfectly clear glass, free from air-bubbles. At the
highest temperatures this central part showed a peculiar faint white "lumi-
nescence" similar to the intense white noticeable in quartz when heated in
the oxyhydrogen flame. The rod was thickened at the ends which seemed
to prevent internal reflection of the radiation from the platinum terminals.

The distribution of energy from the central portion of this rod is shown
in curve c, fig. 66, on 29.4 watts. The ends of the 0.3 mm. platinum
terminals within the glass rod were red hot. The rod was viscous, indi-
cating a temperature of at least noo° to 12000; but no light was emitted
other than the hazy white glow already mentioned, which is in marked
contrast with the radiation from the platinum electrodes. A similar
example is given in Wood's Optics, page 457, where it is stated that sodium
sulphate, in a loop of platinum wire, heated in a blast lamp, emits but
little light, although the wire glows vividly.

In fig. 96 are given the isochromatic emission curves of oligoclase at
wave-lengths 2.048, 2.905, 4.445, and 6.082 //, respectively, for different
values of power consumption. The graphs are for the same rod, under
the same conditions of galvanometer sensibility and distance of the radiator
from the silt. Two additional series of observations on different days,
and using different adjustments, were made at wave-length 2.905 fx. Only
one of these graphs was parallel with the one given in fig. 96, showing that
there is a variation in the slant of the isochromatic curve, under different
conditions. It will be noticed that, throughout the range investigated, the
change in emissivity is proportional to the energy supplied, just as is true

128

INFRA-RED EMISSION SPECTRA.

of gases. It is possible that for some wave-lengths the thickness of the
radiator was not sufficient to emit a saturated radiation, and this may ex-
plain why the emissivity at 2.048 p. apparently does not follow the same
law as do the other emission bands. In order to have a displacement of
the maximum of emission, just as is known for solids emitting continuous
spectra, it is necessary that the intensity of the emission at the short wave-
lengths increase more rapidly than it does in the long wave-lengths. The
2.048 [i isochromatic slants only a little less from the normal than does

16

id

Fig. 96.

20 22 24 Watts

■ Isochromatic radiation curves of oligoclase.

^=6.082 /<. In this region of the spectrum there is a weak general ab-
sorption, while the other wave-lengths are the maxima of selective emission
(absorption) bands, and it is possible that what corresponds to the emis-
sivity constant a of a complete radiator is different for the two kinds of
radiation found in this substance. It is possible that the isochromatic at
2.048 n undergoes a sudden change, curving sharply upward, at a higher
temperature. The same is true of the platinum isochromatic at 1 ft. In
fact, it appears that the emissivity at 2.048 \x must suddenly change in
intensity, unless oligoclase is entirely different from the other substances
examined ; for, like the others, in the oxyhydrogen flame, it emits an intense

ISOCHROMATICS OF OLIGOCLASE. 1 29

white light, although, as shown in curve c, fig. 66, the emissivity at 2 /1
is still weak when the temperature is close to the melting-point.

In oligoclase it is not possible to locate so definitely, if at all, the position
of the maximum of the envelope by the intersection of the isochromatics;
for the lines show no tendency to curve, as in platinum, although at closely
the same temperature. By the extrapolation of the 2.9 p and the 4.4 p.
isochromatics, the intersection (obtained after correcting for slit-width) is
found to be at about 8.5 watts. The maximum emission would then lie
at about 3.5 /*, which is the position of the maximum of a complete radi-
ator at 8500 abs. (5800 C). On 29.4 watts, when the oligoclase was
already viscous, indicating a temperature of noo° to 12000 C. (melting-
point 13000), the maximum emission would have to lie at a much smaller
wave-length than 3.5 ;i, say at 2 //, which is inconsistent with the observa-
tions that the emission curve is but little different in outline from those
obtained at lower temperatures, — there being no indication of a shifting
of the energy distribution. It would therefore appear that it is not per-
missible to consider the envelope, drawn through the emission maxima, as
a criterion for judging the temperature of a substance like oligoclase. On
the whole, it appears that the general emission, as distinguished from the
bands of selective emission, is less intense in oligoclase than in most of the
other silicates studied. Such a substance, if it would withstand high tem-
peratures, would most nearly approach the ideal light producer. For, since
the absorption coefficient is small throughout the region, to 3 /*, where, in
a continuous spectrum, the greatest amount of energy is emitted, the emis-
sion spectrum would remain discontinuous, and only at the highest tem-
peratures would the general emission become of importance, when a large
amount of the energy radiated would be of wave-lengths affecting the eye.

SUMMARY.

In general, the results on the oxides furnish an excellent illustration
of the shifting of the maximum of intensity of emission toward the short
wave-lengths with rise in temperature, just as is known for solids emitting
continuous spectra. In this respect the various emission curves of zirco-
nium oxide are particularly conspicuous. In addition to what may be
termed " general emission," in which the maximum shifts with rise in
temperature, the curves of zirconium oxide are unique in having a sharp
band of " selective emission " which does not shift nor broaden with rise
in temperature. The results are not unlike those obtained by Anderson 1
for erbium oxide, in the visible spectrum. He found that the emission
spectrum was not continuous, but consisted of bright bands superposed
upon a continuous faint background. With rise in temperature the bands
became more hazy in outline, and at very high temperatures the spectrum
became continuous. If, according to Stark's theory, the continuous spec-

1 Anderson: Astrophys. Jour , 26, p. 73, 1907.

130 INFRA-RED EMISSION SPECTRA.

trum is due to the presence of a large number of free electrons, this is to
be expected. At low temperatures the electrical conductivity is small, and
the emissivity is confined to particular bands caused by certain groups of
electrons. With rise in temperature more electrons, extending over a wider
range of wave-lengths, are excited to activity and the separate emission
bands become merged into a continuous spectrum. This is not always
true, however, in the present work. For example, the sharp emission band
of zirconium oxide at 4.3 tx seems to retain the same intensity, superposed
upon a continuous background of increasing intensity, irrespective of the
temperature. In fact, many of the emission bands are as sharp as those
found in gases.

Many of these oxides have an emission band in common in the region
of 2.85 fi, from which it would appear that this band may be due to the
oxygen atom which is in common with all of them. Furthermore, the
emission spectra, when smooth, generally show a depression at 3.2 ll, for
which no satisfactory explanation has been found. There are no atmos-
pheric absorption bands in this region, and fluorite is not known (see
fig. 29) to have absorption bands at this point (Paschen's absorption curve
for a 4 mm. plate shows an increase in absorption of 2 per cent from 2.75
to 3.2 ll, maximum at about 3.1 ll). The emission spectrum of platinum,
carefully examined at the same time, showed no depression. The emission
curve of the bare "heater-tube," which consists of a porcelain tube wound
with fine platinum wire, likewise gave a smooth emission curve similar to
that of the platinum strip. On the whole, the present data indicate that
the depression is a characteristic of the oxides. In other words, the oxides
have a small (sometimes large) characteristic emission band at A=2.8 to
A=3 ll and a second group of bands at 4.5 to 5 ll. If this be true, then it
will be necessary to assign the cause of the selective emission to the element
common to all the oxides, viz, to oxygen. It is well known that groups of
atoms have characteristic bands; but, heretofore, no data has been at
hand which, in any way, indicated the possibility of the characteristic
bands being due to a particular atom in the group.

The tentative conclusion that these emission bands in the oxides are
due to oxygen atoms is perhaps to be expected, for the presence of traces
of the universal impurity, silica, will not explain matters. The oxides are
the most important and the most stable of all the important groups of
chemical compounds; and the spectra of substances belonging to a particu-
lar group are similar. But few substances are inert to the action of oxy-
gen. The spectrum of oxygen (see Carnegie Publication No. 35, p. 49)
shows absorption bands at 3.2 and 4.75 ll. The emission spectra of CO
and C02 show bands in the region of 2.7 and 4.75 ll (Carnegie Publication
No. 35, p. 313). Oxygen in a vacuum-tube showed a strong emission band
at 4.75 li, which, at the time the research was made, was ascribed to CO
or C02 supposed to be formed by the electrical discharge. In view of the

SUMMARY. 131

fact that, with rise in temperature, the C02 emission band shifts toward
that of CO, at 4.6 pt, and that eventually all three gases, C02, CO, and O,
when radiating by electrical excitation in a vacuum-tube, have their im-
portant emission band in common at 4.75 n, it does not seem unreasonable
to assume that the latter maximum is due to the oxygen atom set free
during the time intervening between dissociation and recombination,
brought about by the electrical discharge in the carbon oxides.

In a broad sense the intensity and sharpness of the emission bands
are a function of the electrical conductivity. The best insulators (strong
bases), e.g., the refractory silicates, the oxides of aluminium, zirconium,
erbium, etc., have the sharpest emission bands, while the best electrical
conductors, such as the oxides of cerium, iron, zinc, etc., have no sharp
emission bands. The molecular weight of the base seems to affect the
sharpness of the bands to as great an extent as does the electrical con-
ductivity, the sharpest bands occurring as a rule in oxides of low basic
molecular weight. These results, if true, are to be expected from our
knowledge of the emission, absorption, and reflection of electrical con-
ductors and insulators.1

Paschen, using the spectral energy curves of the oxides of iron and
copper, found no variation of the emissivity constant, a, with rise in tem-
perature. On the other hand, Lummer and Pringsheim, using the total
radiation from these substances, found that the emissivity increased with
rise in temperature. Such a change in emissivity is to be expected, for it
can be shown that at the highest temperatures the emissivity constant, a,
must decrease, otherwise a point would be reached where the emissivity
is greater than that of a complete radiator. In the present curves, where
sometimes at high temperatures the emission seems to be distributed into
apparently two bands, the "constant" a, derived from the spectral energy
curve of one band, may be different from that obtained from the total
radiation measurements. This is illustrated in the present study of the
Nernst glower, where Mendenhall and Ingersoll,2 using total radiation,
found no certain variation in a with rise in temperature.

From one line of theoretical consideration, one might expect to find,
with rise in temperature and the accompanying increase in the electrical
conductivity of the oxides, that the reflecting power increases. If this be
true, then the spectral emission ought to become more continuous, as is
found in the Nernst glower, and the emissivity constant, a, should retain
a high value similar to that of metallic electrical conductors. The value
of a from the spectral radiation curve was found to decrease with rise in
temperature, while Mendenhall and Ingersoll found no certain variation
in a, when measuring the total radiation. It is evident that experiments

1 This subject has been thoroughly treated by Aschkinass: Ann. der Phys. (4), 17, p.
960, 1905.

2 Mendenhall & Ingersoll: Phys. Rev., 24, p. 230, 1907; 25, p. i, 1907.

132 INFRA-RED EMISSION SPECTRA.

on the reflecting power of oxides at high temperatures will be necessary.
The emissivity of the sharp spectral lines in a magnetic field will also
require examination, although, on account of the wave-lengths involved,
the possibility of obtaining results is not promising.

It may be added that the well-defined maxima of emission are not
affected by change in temperature, which is in marked contrast with the
results of Konigsberger1 for the limited region of the visible spectrum.
Whether the emission maxima at 2.85 fi and 4.75 /x are due to the presence
of water, in the various minerals, remains to be determined. If they are
due to water, then one would expect to find them in calcium sulphate
(CaS04+ 2H20), as well as in calcium oxide (CaO; probably some CaOH).
But in the sulphate no band was found at 2.9 n, where the emission should
be the most intense if due to water. The band at 4.75 \i is characteristic
of the sulphates (see Carnegie Publication No. 65). On the whole, the
assumption that these bands are due to the presence of water is no more
satisfactory than to ascribe them to the common constituent, viz, oxygen.

The isochromatics of oligoclase show that the emissivity is proportional
to the energy in-put, thus differing from the other solids investigated ; but,
in view of the fact that in the oxyhydrogen flame the oligoclase emits
an intensely white light, it appears that the emissivity must suddenly
undergo a change in the visible spectrum, and perhaps form a more con-
tinuous spectrum in the infra-red.

In considering the emissivity of these oxides in connection with the
radiation from the sun which is a mixture of these substances in various
conditions of temperature and physical state, and remembering that the
tendency of the solids is to emit a continuous spectrum at high tempera-
tures, it forms an interesting field for speculation as to what one ought to
expect for the composite radiation from the solar surface. The available
data show that gases radiating in a vacuum-tube and the metallic vapors
in the arc have their strongest emission lines in the region of 1 \i. In the
spark discharge the metallic vapors appear to have their maximum energy
in the ultra-violet. With data provided by the Astrophysical Observatory
(see vol. 2 of the Annals), giving the distribution of energy in the normal
solar spectrum, it has not been possible for me to compute a consistent >^max
for different wave-lengths, nor a uniform value of the radiation constant a
(the value of a varied from 21.9 at 0.4 n, 15.6 at 0.5 /*, 11. o at 0.6 /*, 7.8 at
0.7// to 5.4 at 1.2 /i), by the methods given on a previous page, and it
seems evident that these laws are not applicable. If, as the data herewith
presented on the emissivity of the oxides seems to show (assuming the solar
surface to be solid, electrolytic conductor as compared with a pure metal
with high reflecting power) these radiation laws do not hold, how much
less must they be true in the case of the solar surface in its real condition.

1 Konigsberger: Ann. der Phys. (4), 4, p. 796, 1901.

CHAPTER III.

RADIATION FROM SELECTIVELY REFLECTING BODIES, WITH
SPECIAL REFERENCE TO THE MOON.

In Carnegie Publication No. 65, p. no, on infra-red reflection spectra,1
the writer showed that all the silicates examined have a region of strong
selective reflection in the region of 8 to 10 jj. and discussed the effect this
would have upon a reflecting surface like the earth, or the moon, which is
composed largely of silicates. Since this discussion has brought forth
comments favorable and unfavorable, in print and in private communica-
tions, the purpose of the present paper is to summarize, in a general way,
the present data bearing upon the subject, and to discuss certain points
not mentioned in the previous communication. The computed temper-
atures of the sun and of the moon derived in the present paper will be used
in the subsequent calculations, which will be found to be in slight disagree-
ment with the writer's previous results. As a whole, it will be shown how
easy it is to obtain estimated values which ultimately can have but little
meaning other than a guide in experimental work.

THE EFFECTIVE TEMPERATURE OF THE SUN.

It has been shown by Poynting2 that, when a surface is a complete

radiator and absorber, its temperature can be determined at once by the

fourth power law, if we know the rate at which it is radiating energy. If

it radiates what it receives from the sun, then a knowledge of the solar

constant enables us to find the temperature, which will be the highest the

surface can attain when it is receiving heat only from the sun. Knowing

the solar constant and the radiation constant (cr=5.32Xio~5 ergs Kurl-

baum)3 Poynting computes the effective temperature of the sun. If 5 is

the radius of the sun's surface, R is the radiation per square centimeter;

then the total rate of emission is 47:s2R. This must equal the radiation

passing through the sphere of radius r, at the distance of the earth, and

with a surface 4xr2 gives

47rs2R = 47cr2S

where S is the solar constant. Hence R = 46000 S. Using the solar

1 Investigations of Infra-red Spectra, Parts III and IV, published by the Carnegie Insti-
tution of Washington, D. C, December, 1906.

2 Phil. Trans. Roy. Soc. Lond., vol. 202 A, p. 525, 1903.

3 Kurlbaum: Wied. Ann., 65, p. 748, 1898.

*33

134 INFRA-RED EMISSION SPECTRA.

constant 5=2.5 gram calories per minute = 0.175 Xio7 ergs per square
centimeter per second, R = 0.805X10" ergs.

If we equate the sun's radiation to odi, where a is the radiation constant,
we get 6, the "effective temperature" of the sun, that is, the temperature
of a full radiator which is emitting energy at the same rate.

Thus, 5.35X10-5 6i = 0.805 Xio11; whence 6=6200° abs.

With each new determination, however, the solar constant is being
reduced from the former high value, so that a more probable value1 is
about 2.1 gram calories per square centimeter per minute.

Using this value of 5= 2.1 = 0.147 Xio7 ergs per square centimeter per

second,

7t = o.o676Xiou,

whence

0=5980° abs.

This is somewhat closer to Wilson's2 value (5773° abs.), which he
obtained by making a direct comparison of the radiation from the sun
with that from a "full radiator" 3 at a known temperature.

Warburg4 has also computed the probable temperature of the sun.
He assumes that the rate of radiation per degree is constant, and that the
Stefan-Boltzmann law is applicable at all temperatures. For 5=2.54
gram calories per second, the computed temperature is 6256° abs., and for
5=2.17 gram calories per second, a temperature of 6014° abs.

Fery and Millochau5 have measured the temperature of the solar surface
with a pyrometer. The mean value of the observed temperature, after
correcting for atmospheric absorption, is 5620° abs. For different parts
of the solar disk the temperatures vary from 5888° to 5963° abs.

Abbot (loc.cit.) gives a solar spectrum energy curve, deduced from
bolometric measurements, in which the maximum occurs at 0.49 ti, with
a possibility of the maximum8 lying at about ^ = 0.46/*. The equation
^max T= const. = 2920, using the value of ^=0.46//, gives a black-body

1 Abbot : Smithsonian Miscellaneous Collections, vol. 45, p. 81, 1903. See also his later
results, Annals Astrophys. Obs., vol. 2, p. 99.

2 Wilson: Proc. Roy. Soc, vol. 69, p. 312, 1901.

3 Poynting (loc. cit.) very aptly says: "A surface which absorbs and therefore emits every
kind of radiation, is usually described as 'black', a description which is obviously bad when
the surface is luminous. It is much better described as 'a full absorber' or 'a full radiator,'"
i.e., a complete radiator, as distinguished from a partial radiator or so-called "non-black
body." In justice to Kirchhoff who was the first to give a clear discussion of this subject, it
should be called the Kirchhoff radiator.

4 Warburg: Verb. Deutsch. Phys. Ges., I, p. 50, 1899.

6 Fery & Millochau: C. R., 143, pp. 505, 570, 731, 1906.

6 The curve as drawn by Abbot is very asymmetrical and depressed on the side of the short
wave-length. From the data given, it is possible to draw the radiation curve more symmetri-
cal, as one would expect it to be, which shifts the maximum to about 0.46 n. See also his later
results, Annals Astrophys. Obs., vol. 2, which have been published since writing this paper.

LUNAR TEMPERATURE. 135

temperature of about 63000 abs. One of the most refractory substances
farthest removed from a full radiator in its emissive power is bright plati-
num. For this substance the constant is 2630 instead of 2920. If the
radiation from the sun is purely thermal it must lie between the "black
body" and bright platinum in its radiating properties.
If the sun radiates like platinum, from the equation

^max T = 2630, using imax= 0.46 n T = 57000 abs.

The value of the sun's temperature, viz, 59000, used in the present compu-
tations, is about the mean of the " observed " and the computed tempera-
tures. From the results given in the preceding chapter, it is evident that
the true temperature of the sun can not be determined by these methods.

THE LIMITING TEMPERATURE OF THE SURFACE OF THE MOON.

Poynting (loc. cit.) further shows that when there is no conduction
inwards from the surface, the highest temperature of a full radiator is
attained when its radiation is equal to the energy received. Equating the
energy to the solar constant, using 5=0.175 Xio7 ergs

5.35 Xio-504 = 0.175 Xio7

whence a ,Q ,

o =426 abs.

which is the upper limit of the temperature of the moon, assuming that it
absorbs all the energy received from the sun. If part of the energy is
reflected and only a fraction x of that falling on it is absorbed, then the
effective lunar temperature is 426 -\/x abs. From Langley's1 estimate that
the moon absorbs x=| the energy it receives, Poynting {loc. cit.) computes
the upper limit of temperature of the surface exposed to a zenith sun to be

0=426X (|)i=4i2° abs.

But this upper limit to the temperature of the hottest part of an airless
planet is never attained because the moon turns the same face to the
earth instead of to the sun. He shows that, if N is the normal stream of
radiation from a unit of surface of the moon immediately under the sun,
the normal stream from the equivalent flat disk is Nd=%N.

" The effective temperature of the flat disk is therefore V§ tnat of the surface imme-
diately under the sun at the same distance from it. Then the effective average temperature
is 412 X -\/i = 3710 abs. The upper limit then, to the average effective temperature of the
moon's disk, is just below that of boiling water."

If we use 5 = 0.147 X io7 instead of 0.175 X IC)7> 0 = 4080 abs. and the
"effective average temperature" is 408 V§ • I = 350° abs. or 820 C. for
the full moon. This assumes no conduction inwards. Evidently there

1 Langley: Nat. Acad. Sci., vol. 4, part. 2, p. 197.

136 ESfFRA-RED EMISSION SPECTRA.

is some heat conducted inwards, for the results of Langley Qoc. cit.) show1
that the surface of the full moon is about 3000 abs.

FALL OF TEMPERATURE OF THE MOON DURING ECLIPSE.

As already mentioned in Carnegie Publication No. 65, page no,
Langley made observations on the eclipse of the sun on September 23,
1885, which indicate a sudden and very rapid fall of energy received from
the moon at the beginning of the eclipse, with some indications of a rise
nearly as rapid after its conclusion. In Carnegie Publication No. 65,
page 113, are plotted his observed galvanometer deflections during the
progress of the eclipse. The observations were interrupted by the forma-
tion of clouds just at the predicted time for the moon to leave the umbra.
The curve shows that in the short time of about 1.5 hours, in passing from
the penumbra to the umbra, the radiation from the west limb has fallen
from a maximum to a zero value. In other words, the fall of temperature
is practically coincident with the change in illumination, and at first ap-
peared to the writer2 to indicate that the greater part of the observed
energy is due to reflection. That the moon at mid-eclipse is still as warm
as the earth is shown by the fact that the galvanometer gave zero (or only
small positive) deflections. If the moon had been cooler than the earth,
then the deflections would have been negative. Subsequent search of the
literature on the subject shows that Very3 found appreciable radiation from
the moon during totality. The following computations show that this is
to be expected. After about n days of insolation the temperature of
the sun-lit surface of the moon will be fairly constant. From the surface
inwards there will be a layer which may be considered at a uniform tem-
perature for the period of 1.5 hours, as compared with n days. If,
then, the moon were suddenly eclipsed, the fall of temperature of the
surface with time (x=o} 0=f(t)\ assuming at time t = o, that #0 = 300° abs.)
is found from the equation

ax2 at

where k = conductivity, d = density, and 5 = specific heat. The first term
in the equation represents the energy lost by conduction (it is assumed

1 Very, Astrophys. Jour., 8, pp. 273 and 274, 1898, however, records "inferred effective
temperatures" as high as 4550 abs. for limited regions of the moon, and (loc. cit., p. 286) a
mean surface temperature of +970 C. The fact that the moon absorbs energy from the sun
(the maximum of which is of short wave-lengths) and emits energy of wave-lengths from 8 to
10 m, where, on account of the probable high reflecting power, the emissivity is very low, would
explain why Mr. Very has found a temperature which is much higher than that of a complete
radiator under similar conditions.

2Phys. Rev., 23, p. 247, 1906.

3 Very. Prize Essay on the Distribution of the Moon's Pleat, p. 40. Professor Very has
called my attention to an error in Carnegie Publication No. 65, p. 112, third line from the
top: viz, "Harrison (not Langley) reminds the reader ..."

LUNAR EMISSIVITY. 137

that there is conduction in only one direction, hence the differential terms
in y and z disappear). The second term represents the loss of heat by
radiation from the surface; while the last term represents the change in
temperature at the surface. The dependent variable enters here as a
fourth power, and makes the solution difficult. The computations are
made for instantaneous eclipse, while in the actual case the shadow moves
slowly across the surafce. Hence the error due to neglecting conductivity
is partly compensated by the slow eclipse.

By neglecting conductivity from the interior, the temperature of the
surface will fall more rapidly and we have a steeper temperature decadence
curve. The solution of the equation is very simple if we neglect conduc-
tion, and is

0«

6 =

0£°t

ds

for I in minutes, (7=76.6X10 12 gram calories per square centimeter per
minute, d=2, and 5 = 0.2 (approximate values for rock material). Hence

»-T 30O°

vi + 0.0155/

In fig. 97 are plotted the temperature decadence curves for the radiation
constant o, for 0.3 a (emissivity of iron oxide) and for 0.1 0. The curves
show that for a full radiator the temperature would fall from 3000 to 2800
(room temperature when the bolometer would be in temperature equilib-
rium with the moon) in 17 minutes, for 0.3 a in 55 minutes, and for 0.1 a
in 140 minutes. The solution for 0.1 a and 0.3 a is, of course, only ap-
proximate since the emissivity varies more nearly as the fifth power instead
of the fourth power, as here used; and the computed temperatures should
be somewhat higher. The computation is also made (fig. 97), assuming
the temperature to be 3500. Here, for a complete radiator the temperature
would fall from 3500 to 2800 in 38 minutes, and for 0.3 a in about 130
minutes. By including the conductivity term these periods would be con-
siderably prolonged. The present solution is close enough, however, to
show that the temperature (3000) decadence curve is not coincident with
the eclipse curve, unless the emissivity is of the order 0.3 a. Hence the
writer's1 previous surmise that at the beginning of totality radiation should
still be appreciable is substantiated, although the observations made by
Langley seemed to contradict it. Very's observations2 show that during
totality of the eclipse of January 17, 1889, the radiation from the umbra
of the eclipsed moon was about 1 per cent of the heat which was to be

1 Phys. Rev., 23, p. 247, 1906.

2 Very: Distribution of the Moon's Heat, p. 40.

138

INFRA-RED EMISSION SPECTRA.

expected from the full moon. Boeddicker1 also records that the minimum
of the heat-effect falls decidedly later than the minimum illumination.

350
Abs

20

300
Abs

280

REFLECTION AND RADIATION FROM THE MOON.

Using the data just computed, we are now in a position to make a
rough comparison of the relative amounts of energy reflected by and radi-
ated from the moon. In
the short wave-lengths the
reflecting power depends
upon the refractive index,
while at 8 to io n the re-
flecting power will depend
upon the refractive index
and the extinction coefficient
(see Drude's Optics, p. 336),
and at all times will have a
high value, except at 7 a.
The reflecting power of the
moon is variously recorded 2
from 0.09 to 0.23, which val-
ues are several times higher
than the refractive index of
ordinary rocks permits.
This seems to indicate in-
ternal reflection.

The question is therefore
reduced to finding the ratio
of the energy emitted by
the moon to the energy of
the sun, reflected from the
lunar surface. For this pur-
pose the temperatures of the
moon and of the sun (3500
abs. and 59000 abs., respect-
ively) given on the preceding
pages are employed. The
values are taken slightly lower, although it makes but slight difference in
the final computation. We are not so much concerned with the question
as to where the maximum emission of the moon occurs as we are in the
fact that, in the region of 8 to 10 [x, where Langley observed radiation from
the moon, there is also reflected energy from the sun. From the transmis-

260

240

220

200

k

l\

i\

\ \
\

\
\
-\

I

\
\
\

\

:

r

\
\
\
\

1
\

\
\

^^0.3

or

•

\

V

^0.3 0

—0.10

\

\
\
\
\
\
\
\

:

1

r
\

\

20 40 60 80 100 120 i40mcnu{es

Fig. s>7. — Lunar temperature decadence during eclipse.

1 Boeddicker: Trans. Roy. Dublin Soc, 3, p. 321, 1885.

2 Very: Astrophysical Jour., 8, p. 276, etc., 1898.

LUNAR EMISSIVITY. 139

sion curves of the water vapor (curve c, fig. 102) it will be seen that in the
region from 5 to 8 /j. nearly all the energy will be absorbed by the earth's
atmosphere. The reflecting power of the moon for visible rays, according
to Langley (loc. cit.), is only -g"oo1oo"ff ^u^ sunlight. Assuming that at 9 fi
the reflecting power of the silicates is, on an average, 10 times that at 0.5
to 4 ft, this value becomes 5^oo"o or 0.00002. (This seems a fair estimate
of ratio of the reflecting power of the silicates at 1 and 9 fi. In the
absence of better data this solution can be only a rough approximation.)
Using the above values for the temperature of the moon and of the
sun, from Planck's1 formula for the distribution of the energy in the spec-
trum of a complete radiator (which, of course, the moon is not)

Ek = clX-*(ec*nT— i)-1

we can obtain the ratio of the intensities for the two temperatures T=t)^o°
abs. and T2=$goo° abs. from the formula

Where c2= 14,500 and X=g p. The ratio is 0.00316. But the moon, not
being a perfect radiator, will have a smaller emissivity at 8 to 10 fi. If
its surface were iron oxide,2 its emissivity would be only 0.3 that of a full
radiator, and, for the region at 9 /<, judging from the drop in the emission
curve (see Carnegie Publication No. 65, p. in) and the high reflecting
power of the silicates, the emissive power may be less than this, say 0.1.
This ratio of the emissive power of the moon to that of the sun will then
be 0.000316, which is 16 times (0.000316 -=-0.00002) the reflected energy
of the sun from the moon. If we had taken 3000 as the temperature of
the moon, then this ratio would be (0.00014 ^0.00002) = 7 instead of 16.
Computations3 like these, which require all sorts of assumptions, can be of
little value ultimately. Any computation can not be more than a rough
approximation, for the reflecting powers, observed up to 4 fi, will be too
high, due to internal reflection. In the region of 8 to 10 p. (for silicates)
there can be but little if any internal reflection. Hence, the ratios just
obtained are too low, but how much so is difficult to estimate because
of the lack of data. We know that Langley observed also direct radia-
tion from the sun, in this region of the spectrum, and from existing data
of the radiation from the moon in this region we do not know how much of
it is selectively reflected energy from the sun. The amount reflected must
be small, but, since the total amount emitted is also small, it is important
to establish the fact that there is selective reflection in this region.

1 Planck: Verb. Deutsch. Phys. Ges., 2, p. 202, 1900.

2 Kayser: Spectroscopy, vol. 2, p. 80.

3 Very: Astrophys. Jour., 24, p. 353, 1906, computes a much greater difference between
the amount reflected and the amount emitted.

a

140

INFRA-RED EMISSION SPECTRA.

EMISSION FROM A PARTIAL RADIATOR.
We have now to consider a problem which has heretofore not been
discussed in connection with the radiation from the moon. Planck's
formula for the intensity of radiation at a given wave-length X in the spec-
trum of a complete radiator is

Ex^c1X-B(^XT—i)-1

For any radiating body the emission cf>x = AxEx, where A is the absorption
coefficient. But Ax=i—Rx (for "transparent media," i.e., non-metals),
where R is the reflecting power, and hence <£>X=EX (i— R^).

185

Defl in mm
150

6 7 8 9 10/1
Fig. 98. — Emission spectrum of copper oxide and of quartz, 3250 C (Rosenthal).

It follows therefore that a selectively reflecting body like quartz will
be a partial radiator1 and that if we compare its radiation curves with
that of a complete radiator, there will be minima of emission in the region

•Partial radiators may be divided into so-called "gray bodies" and bodies having a
highly selective emission. Quartz belongs to the latter class.

PARTIAL RADIATORS.

141

of 8.5 and 9.03 /jl. This was predicted by Aschkinass1 and experimentally
verified by Rosenthal,2 with quartz, mica, and glass (see fig. 98). The
latter found the observed emission curves of these substances to coincide
precisely with the computed curves, using the observed reflecting power
of these minerals and the observed emission curve of a full radiator at the
same temperature.

Since we have constantly before us examples of selective emission, such
as the Welsbach mantle, and high temperature radiation of such metals
as tungsten and tantalum, in the visible spectrum, it will be of interest to
consider the radiation of several substances having bands of selective
reflection in the infra-red. In fig. 99 is given an illustration of the marked
effect of selective reflection upon emission. The extraordinary reflection
of carborundum (SiC) has been found in only one other substance, viz,

50

E

40

'30

20

U

^>c~

N

/ "^

^ \

a/

"\ \

\c

1

NA

1

\ /
\ /

\

N?

\

\ /

\

a \

\ /

\ \

\ /

\/

A

/ \

/ \

/ \

/ \

/ \

C /

/ '

v

->

/

\

\
s

/

-x

/

^

_/

100%

90

80

ro

o

e-f,
O*

60 D
o
-*i

O

50 0>

-j
cr
o

40^

3
Q.

30 §
20

10

10

II

IZ

13

14

I5JJL

Fig. 99. — Reflection from carborundum (a); Radiation of complete radiator at 3000 abs. (6);
Radiation from carborundum at 3000 abs. (c).

quartz, and in the latter it is the result of two well-defined bands, at 8.5
and 9.03 ;x. The effect of such a band on the dispersion of this substance
must be very marked. In fact, the unusual dispersion in the visible spec-
trum found by others no doubt is greatly influenced by this band. In this
figure are given the computed energy curve for a perfect radiator (using
Planck's formula — any temperature might have been selected instead of
3000 abs.) and the computed energy curve of carborundum at the same

1 Aschkinass: Verb. d. Deutsch. Phys. Ges., 17, p. 101, 1898,
3 Rosenthal: Ann. der Phys. (3), 68, p. 791, 1899.

142

INFRA-RED EMISSION SPECTRA.

temperature, using the formula tf)x=Ex(i—R^), where R is the observed
reflecting power and E is the corresponding intensity of the full radiator.
It will be noticed that the radiation will be highly selective, and that at
10 p. it approaches closely to that of the complete radiator. If, then, one
were to observe the emission curve of carborundum, the maximum at 10 \i
would appear as an "emission band," but its explanation is different from
that of emission bands of hot vapors, e.g., C02 and H20 in the Bunsen
flame. In fig. ioo is given the observed reflection curve, a, of quartz, and
the computed emission curves of a complete radiator, b, and of quartz, c,
at a temperature of 4000 abs.

5

Fig. ioo.

8 9 10 11 12/JL

Reflecting power of quartz (a); Emission of complete radiator at 4000 abs. (6); Emission
curve (c) of quartz at 4000 abs.

In fig. 1 01 are given the computed emission curves, a and b, for a com-
plete radiator, for temperatures 3000 and 4000 abs., respectively, and the
corresponding emission curves, c and d, of quartz, at the same temperature
using the values of the reflecting power of quartz given by Rosenthal
(loc. cit.). These values are somewhat lower than found by the writer.
In general, the difference in the emissivity of the silicates and that of a
full radiator at 8 to 10 /j. would not be so marked as in quartz. For mica
(see Carnegie Publication No. 65) the emission minima would occur at
9.1 and 9.8 //, while for granite the emission minimum would extend from
8.5 to 10 (x. The combination would have an appreciable effect upon the
radiation curve of a surface like that of the moon. Even, if we exclude
atmospheric absorption, the emission curve (a, fig. 102) will not be smooth
and continuous, as some writers seem to think. If we superpose atmos-
pheric absorption, the emission curve of quartz (curve c, fig. 101) will be
somewhat as shown in curve b, fig. 102. Transmission curve c (fig. 102) is

PARTIAL RADIATORS.

J43

E

B

--^a

20

\
\
\
\

IS

\

f

^400*/46s

/

!

/

>

\>

i
/

\
\

/
/

/o

/

/
/
/
/

V-

300°/l 6s

9

^b^

^

N

V

/
/

s

/■ — '

5 6 7 8 9/0// /2/t

Fig. ioi. — Emissioa of complete radiator (a) and (6); Emission of quartz (computed).

for a column of air no meters in length, containing 1.2 mm. precipitable
water,1 and is due to Langley (loc. cit.). A thick layer would shift the
maximum at 8 ft toward 8.3 ft found in the moon's radiation curve. In
fig. 103, curves a, c, and d show some of Langley's observed lunar radia-

5 6 7 6 9 10 II I2jU

Fig. 102. — Emission of quartz (curves a and b); transmission of air (Langley).

tion curves, and as a whole there is a close parallelism between the theo-
retical curve b, from fig. 102, and the observed curves, at 10.7 ft, where we
have to consider only atmospheric absorption. From & to g ft, however,

1 Apparently water-vapor is more transparent than a film of the liquid, for a film of even
one-tenth this thickness is opaque to heat rays beyond 5 /*.

144

INFRA-RED EMISSION SPECTRA.

we have to consider the combined effect of atmospheric absorption, of
incomplete emission of the moon (emission minima from 8.5 to 9.8^),
and of selectively reflected energy from the sun. The latter will, no doubt,
vary the most in intensity. The computed emission curve is the most
intense at 10.2 /x, while the observed is the most intense at 8.3 \x (see fig. 103).
This is to be expected if the observed energy curve is the composite of the
selectively emitted energy of the moon, and the selectively reflected energy
of the sun, which is selectively transmitted by the earth's atmosphere.
The selectively reflected energy of the sun would to a certain extent fill up
the minima in the lunar emission curve, thus making it higher at 8.3 fx
than at 10.2 ft, and, as far as our present knowledge goes, would explain
the observed curves a, c, d, fig. 103, which lack a minimum at 8.5 fi. As

5 6 7 8 9 10 ii

Fig. 103. — Emission of quartz (6); Emission from moon (Langley).

a whole, from whatever standpoint we view this matter we come to the
same conclusion, viz, that in the region from 8 to 10 [x the energy emitted
from the moon consists of its own proper radiation and of reflected energy
from the sun.

To sum up, we know that Langley observed radiation from the moon
in the region of 8 to 10 fi. He observed also direct radiation from the sun
in this same region; but we do not know how much of this is superposed
upon the direct radiation from the moon due to the latter being selectively
reflecting in this region of the spectrum. As stated in Carnegie Publica-
tion No. 65, page 115, computations which require all sorts of assump-
tions will not settle the question. Bolometric comparison of the spectrum
energy curves of the sun and of the moon made at high altitudes will be
of greater service in clearing up the matter. Very's1 suggestion of search-
ing for a solar image in the lunar image, with a delicate heat-measuring
instrument covered by a screen with a pin-hole aperture, also deserves a
thorough trial. Exception has been taken to the writer's statement (loc.
cit.) that, of the radiation observed by Langley in the lunar spectrum at

1 Very: Astrophys. Jour., 24, p. 353, 1906.

LUNAR RADIATION. 145

8 to 10 n, we do not know how much is selectively reflected energy from
the sun. But the writer maintains that just as soon as we admit the
possibility of the moon being selectively reflecting in the region where it
has its own proper radiation, then the use of glass1 as an absorbing screen
for testing the quality of that radiation is inadmissible, although it does
serve as a rough test of the reflected energy at 0.5 to 3 y.. For this purpose
it has been serviceable, and when we consider the great difficulties under
which such work must be carried on, and the numerous corrections that
must be introduced (which at all times may be larger than the one for
selective reflection), the use of a glass screen is not objectionable.

Furthermore, we have noticed that the silicates reflect as a transparent
medium in all regions of the spectrum considered, except from 8 to 10 /*,
where it reflects like a metal. This means that the reflected energy in all
these regions except 8 to ioju will consist of two parts, viz, that reflected
from the outer surface and that due to internal reflection. From 8 to 10 \x
there will be but little, if any, energy due to internal reflection. Hence,
to use the reflected energy spectrum of the moon from 0.5 to 4 \i as a means
of estimating the amount of reflected energy at 8 to 10 p. is not a fair test,
and, as used (for want of better data) in the present paper, can only serve as
an approximation. As in the case of emission spectra, it ought to be possi-
ble to analyze a substance by means of its reflection bands. The constitu-
tion of an isolated body like the moon, shining by reflected light, might thus
be determined. This will probably never be possible since terrestrial atmos-
pheric absorption will interfere with the observations, while a far greater sen-
sitiveness in the radiometers will have to be attained than now is possible.

From whatever standpoint we view the matter, the conclusion is that
the lunar surface must be diffusively, selectively reflecting, however small.
Hence, in the stream of energy reflected in any direction from the lunar
surface the density will be greatest for the wave-lengths of the bands of
selective reflection. One would, therefore, expect to detect this difference
in all directions, and not simply at the angle of reflection as suggested by
Very (Joe. ciL). It is difficult to apply a thorough test which will deter-
mine whether, and how much of, the energy from the moon is due to emis-
sion, and how much is due to reflection of energy from the sun. A rigid
comparison of the energy curves of the sun and the moon in this region
would be of great value. The polarization of the radiation from the
moon might also be of use in deciding this point. Pfund2 has shown that

1 Various observers have compared the total radiation from the moon to that part which
is transmitted by glass. Glass being opaque beyond 4 jw. was assumed to absorb the proper
radiation from the moon, while the part transmitted by glass was considered to be reflected
energy from the sun. Evidently if there is selectively reflected energy of the sun beyond 4 m,
then it will be superposed upon the direct radiation of the moon, and the use of a glass screen
for testing the lunar radiation is inadmissible.

2 Pfund: Astrophys. Jour., 24, p. 19, 1906.

146 INFRA-RED EMISSION SPECTRA.

the bands of residual rays reflected from calcite are elliptically polarized.
Pfliiger1 studied the polarized radiation from tourmaline heated to high
temperature. Further than this, nothing is known concerning the polari-
zation of the radiation emitted by substances like the silicates; hence this
test might not be very decisive.

To subject the above conclusions to experiment in which the reflection
of a rough surface is to be measured will be accompanied with difficulties
because of the smallness of the surfaces that can be used.2 In the case
of the moon an image of the whole, or a greater part of the surface, may
be projected upon the spectrometer slit. This means concentrating radia-
tion which comes from a surface many miles in diameter, as compared
with a surface which, when produced in the laboratory, amounts to only
a few square centimeters.

1 Pfliiger: Ann. der Phys., 7, p. 800, 1902.

2 Since writing this, Dr. A. Trowbridge, at the Washington meeting of the American
Physical Society, April 24-25, 1908, described a series of experiments on the diffuse reflection
of infra-red energy, in which he showed that powdered quartz has minima of diffuse reflec-
tion in the regions where there are absorption bands (e. g., 2.95 /*), and reflection maxima
at 8 to 9 fi, just as obtains for plane surfaces. This is an excellent illustration of the dif-
ference of what corresponds to body color and surface color in the visible spectrum (see
Wood's Optics, p. 352). It also illustrates the question of diffuse selective reflection discussed
on a previous page.

APPENDIX I.

THE EFFECT OF THE SURROUNDING MEDIUM UPON THE
EMISSIVITY OF A SUBSTANCE.

In most of the problems in radiation from substances the surrounding
medium is air, and no account is taken of the refractive index, which is
taken as unity. In a remarkable research on "The Formation of River
Ice with Special Reference to Anchor Ice and Frazil," by H. C. Barnes,
it is shown that the surrounding medium plays an important part in the
rate of cooling of the earth's crust.

In Canada, as well as other localities having high latitudes, three kinds
of ice are observed, viz, sheet or surface ice, frazil ice, and anchor ice.

Surface ice is found only in still water, and is caused by the loss of
heat to the cooler atmosphere, by radiation and conduction from its sur-
face. Thickening of the ice-sheet takes place downwards by conduction
of heat through the ice to the air.

Frazil ice is the French-Canadian term for fine spicular ice, from the
French for forge-cinders, which it is supposed to resemble. It is found in
all rivers or streams flowing too swiftly for the formation of surface ice.
A dull, stormy day, with the wind blowing against the current, is produc-
tive of the greatest amount of frazil ice, which, like anchor ice, has a
tendency to sink upon the slightest provocation, and to follow submerged
channels until it reaches a quiet bay. Here it rises to the under side of
the surface ice, to which it freezes, forming a spongy growth, attaining
great thickness; in some cases the author observed a depth of 80 feet of
frazil ice.

Anchor ice, as the name implies, is found attached or anchored to the
bottom of a river or stream, and often attains a thickness of 5 to 6 feet.
It is also called ground-ice, bottom-ice, and ground-gru. In a shallow,
smooth-flowing river we are more likely to have anchor ice formed in
excess, whereas in a deep and turbulent stream we are likely to have more
frazil. In a river 30 to 40 feet deep anchor ice is almost unknown, al-
though large quantities of frazil are met with.

Barnes remarks that —

The various facts of common observation in connection with anchor ice points to radia-
tion as the primal cause. Thus it is found that a bridge or cover prevents the formation of
anchor ice underneath. Such a cover would act as a check to radiation, and reflect the heat-
waves back again to the bottom. Anchor ice rarely forms under a layer of surface ice covered
with snow. It forms on dark rocks more readily than on light ones, which is in accord with

H7

148 EMISSION SPECTRA.

what is known in regard to the more copious radiation of heat from dark surfaces. Anchor
ice never forms under a cloudy sky either by day or by night, no matter how severe the weather,
but it forms very rapidly under a clear sky at night. Anchor ice is readily melted off under a
bright sun. It seems highly probable, then, that radiation of heat supplies the necessary
cooling to the bottom of a river to form the first layers of ice, after which the growth or build-
ing up of the ice is aided by the entangling and freezing of frazil crystals, which are always
present in the water.

The author found that during rapid ice formation the water becomes
slightly undercooled to the order of a few thousandths of a degree, and
that the ice which is found is in a very adhesive state. On the cessation
of cold weather the temperature of the water rises slightly above the
freezing-point and the ice gradually melts. Anchor ice rises from the
bottom in mild weather and also in extreme cold weather under the in-
fluence of a bright sun, when it is dangerous to small boats. It is also
known to lift and transport large boulders. On the other hand, a bright
sun prevents the water from becoming undercooled and the formation of
frazil ice. The author's conclusion that anchor ice is formed by radiation
rather than by conduction is practically the same as that of Farquharson
in 1841.

At the request of the editor of the Monthly Weather Review, the writer1
has inquired into the aforesaid conclusions, which at first seemed unten-
able. After considering various experimental data, it appears to the pres-
ent writer that the explanation of Farquharson and of Barnes accounts for
the observed phenomena better than any of the other theories propounded.
Thus the loosening of the anchor ice under a bright sun is simple enough
from the fact that water is transparent to heat-waves up to 1 /*. The
thickness of the layer of ice that must be melted in order to overcome the
adhesion to the rock surface must be of molecular dimensions. In addi-
tion to this, there is the tension on the rock surface due to the buoyancy
of the ice, which also tends to melt the ice. The explanation of the for-
mation of anchor ice is more difficult, and the author's statement that
"it is not to be supposed, because a substance like water has been found
to be highly opaque to the radiation from hot bodies, that it will be the
same for cold body radiation" is a little startling, and not very clear, for
the transmissivity of any region of the spectrum is independent of the
temperature of the source. However, if total radiation is meant, then
such an interpretation is possible. In "Investigations of Infra-red Spec-
tra" (Carnegie Publication No. 35), several examples, e.g., methyl iodide,
are given, illustrating the latter case. There is no evidence, however, for
saying that "it is probable that water possesses an absorption band for
shorter heat-waves, but may become perfectly transparent for the longer
heat-waves." It is known that water is exceedingly opaque to heat rays
from 4 to 8 ft followed by a more transparent region from 8 to 20 /* (this

1 The Monthly Weather Review, p. 225, May, 1907.

EFFECT OF SURROUNDING MEDIUM.

149

was found by Rubens and Aschkinass1 for water-vapor, which behaves
like the liquid in its properties for absorbing heat-rays), beyond which
there is great opacity. In fig. 104 is given the transmission curve of a
column of 75 cm. of water-vapor (due to Rubens and Aschkinass, loc. cit.),
from which it will be noticed that there is a quite transparent region from
8 to 15 /*. They found that the heat waves at 51 \x were entirely absorbed,
while at 80 \x theory (see Drude, Optik, p. 359) predicts a band of metallic
reflection. Moreover, water differs from most other substances in that its
great opacity is due to numerous small absorption bands. Consequently
its absorption coefficient is smaller than that of a substance like quartz
which has bands of metallic reflection at 8.5, 9.02, and 20.75 /*• Hence,
there is no objection to saying that "the whole question of the formation

100%

10 II 12 13 14- 15 16

Fig. 104. — Transmission of water vapor.

of anchor ice depends upon admitting that the long heat-waves can pene-
trate freely through the water," for the maximum radiation of a body at
a temperature of o° C. lies in the region of the spectrum extending from
wave-lengths 8 to 20 p., and it is here that water has its greatest transparency
for long heat-waves.

It is difficult to conceive of a more complex form of radiation than the
one here involved. According to Provost's theory of exchanges, when two
bodies are at different temperatures the hotter receives energy from, and
imparts energy to, the colder by radiation, and vice versa. In the case of
the river, when the sky is clear, the water is radiating into space which is

1 Rubens & Aschkinass: Ann. der Phys. (3), 64, p. 584, 1898; also Ann. der Phys., 65,
p. 251, 1898.

i5o

EMISSION SPECTRA.

probably at absolute zero of temperature. The river-bed is radiating
energy into the water, and probably through it into space. Leaving out
of consideration the special nature of the emissivity of the two bodies
(water and river-bed), it has been established (see Drude's Optics, p. 462)
that the radiation from a "non-black body" is approximately proportional
to the square of the refractive index of the surrounding medium, which is
transparent, so that, from this standpoint, the emissivity of the river-bed
into the water would be greater than that of the water into the air. Of
course, if water were perfectly transparent, its emissivity would be nil,
and the problem would be less complex. Little is known concerning the
special nature of these two bodies, but from the fact that the anchor ice
separates so easily from the river-bed under a bright sun, it is evident that
the absorption coefficient of rock material is greater than that of water,
and hence, that its emissivity must also be greater. Hence, more energy
will be radiated from the river-bottom than from the water, into space,
the river-bottom will become the cooler, and finally a film of ice is formed
on it. During cloudy weather the temperature of the water-vapor in the
air is equal to or higher than that of the water and the river-bottom. There
is then an equality in the radiation, or an excess is being emitted from the
clouds to the earth. A certain amount will also be returned from the
clouds by reflection. Hence, the excess of radiation is toward the earth,
and since the temperature of the clouds is above the freezing-point no
anchor ice is formed.

To sum up, from this elaboration of the author's explanation, just
quoted, of the formation of anchor ice, it will be seen that it is not only
possible, but also highly probable that the cause is to be attributed to the
greater emissivity of the substances forming the bed of the river, and to
the greater transparency of water to heat-waves than is generally supposed
to obtain for that substance. It is difficult to conceive that such a condi-
tion can exist, but the magnitude of the heat transfer, required to bring
about this ice formation, must be exceedingly small, and the explanation
given accounts for all of the facts observed.

On a previous page the conclusion was reached that the energy re-
ceived from the moon will be the composite of the selectively reflected
energy of the sun and of the selectively emitted energy of the moon, which
is selectively transmitted by the earth's atmosphere. In the same manner
the earth would emit selectively in the region of 8 to 10 p and less than a
complete radiator at the same temperature. It is, therefore, hotter than a
complete radiator which emits the same amount of energy. All these ques-
tions are of extreme importance to the meteorologist who is concerned
with the cooling of the earth. It may be added, however, in conclusion
that the loss of radiation from the earth under the above conditions must
be exceedingly small. In the same manner the suppression and conser-
vation of energy in the region of 8 to 10 p. must be very small, so that it is a

EFFECT OF SURROUNDING MEDIUM. 151

matter of speculation whether the earth would be at a much lower tem-
perature if its surface were composed of material not having bands of
selective emission.

Rubens and Aschkinass (loc. cit.) found that after four reflections from
fluorite surfaces the deflection due to solar energy is reduced to zero,
showing that the opacity of the combined solar and terrestrial envelope is
practically total for wave-lengths 24 to 32 fi. In a recent communication
on the absence of very long waves from the sun's spectrum, Nichols1 found
that the atmospheric transmission for wave-lengths ^=51/1 can not be
greater than 3 per cent. This, however, does not indicate that the sun
is lacking in radiation of wave-lengths ^=51 /«, as the title of the paper
appears to imply, but that, whatever the intensity of the radiation emitted
at 51 n, it is almost entirely absorbed by the earth's atmosphere.

1 E. F. Nichols: Astrophys. Jour., 26, p. 46, 1907.

APPENDIX II.

INSTRUMENTS AND METHODS USED IN RADIOMETRY.

There are few fields of experimental investigation so beset with diffi-
culties as the quantitative measurement of radiant energy. This is due
to the fact that the radiation to be measured is generally from a surface
of which it is practically impossible to determine the temperature. The
measurement of radiant energy usually involves its transformation into
some other form, and the receiver used for this purpose is subject to losses
by heat conduction within, and by reflection, radiation, and convection
losses from its surface.

As a result of inquiry into the development of the various instruments
and methods used in measuring radiant energy, viz, the radiometer, the ther-
mopile, the radiomicrometer, and the bolometer with its auxiliary galvanom-
eter, the writer has accumulated extensive data, part of which is included
here, with the hope that it may be useful to others interested in the subject.

Various instruments have been devised, the sensibility, or the possible
chances for improvement, of which can be rated without further investi-
gation. That in many cases the sensitiveness has been overestimated, will
be noticed in the present paper. However, four instruments, viz, the
radiomicrometer, the thermopile, the bolometer, and the radiometer, have
been used extensively in radiation work, and in each case the inventor has
found qualities which seemed to him to render his instrument superior to
other types. But, to the writer's knowledge in no instance have all four
instruments been studied by any one person; and, perhaps, it should not
be expected. Each instrument requires a special mode of handling, and
has peculiarities which can be learned and controlled only after prolonged
use. This is particularly true of the radiometer and of the bolometer
with its auxiliary galvanometer.

Having already had considerable experience with radiometers, one of
which was the most sensitive yet constructed, the writer has, in this exami-
nation, devoted most of his attention to the bolometer. The investigation
originated for the most part from the question whether the radiometer
was selective in its action, in the region of the short wave-lengths. In
previous work it was found1 that the radiometer gave small deflections in
the violet spectrum of the arc where Snow,2 using a bolometer, found large
deflections.

1 See Investigations of Infra-red Spectra. Part II. Infra-red emission spectra. (Carnegie
Publication No. 35.)

2 Snow: Physical Review, 1, p. 32, 1893.

152

THE RADIOMICROMETER. 1 53

In the course of the discussion it will be noticed, as was previously
shown in a general way, that each instrument has some quality which
makes it useful for a certain kind of work. For measuring very narrow
emission lines and determining dispersion curves the bolometer is no
doubt the best adapted. For measurements requiring a larger receiving
surface, the linear thermopile is the more sensitive and the more precise.
It has the further advantage that there is no permanent current. On the
contrary, the bolometer has a current which heats the bolometer strip above
the temperature of the surrounding air. This causes air currents which
make the zero unstable. This is not true of the thermopile. Less is
known concerning the radiometer, which rivals the bolometer and the
thermopile in sensitiveness. Furthermore, the radiometer is not subject to
magnetic perturbations. Its window limits its usefulness to the region of
the spectrum up to 20 /<. The fact that it is not portable is a very minor
objection.

I. THE MICRORADIOMETER.

Since we are concerned with radiation meters of the greatest sensitive-
ness, the ingenious device of Weber,1 called the microradiometer, deserves
passing notice. The instrument is not unlike a combination of a differ-
ential air thermometer and Wheatstone bridge. Two arms of the bridge
consist of a thin glass tube containing a bit of mercury at the center with
a solution of zinc sulphate at the ends, into which dip platinum electrodes.
The ends of the bulbs are covered with rock-salt windows. If radiant
energy is allowed to enter one of these bulbs, the air expands and pushes
the liquids toward the opposite bulb. This will change the relative lengths
of the column of mercury and of the solution between the platinum termi-
nals, which means a change in resistance in the bridge arm, and a conse-
quent deflection of the galvanometer. The instrument was stated to be
sensitive to a temperature change of 0.000010, and while it is not adapted
to spectrum radiation measurements, it might be used in total radiation
work where an elaborate installation is not convenient. By making the
receiving bulb of opaque non-conductive material and by covering the
inside with lampblack, or platinum black, this would be as complete an
absorber ("black-body") as the thermopile or bolometer. Its efficiency
would, of course, depend upon the gas inclosed.

II. THE RADIOMICROMETER.

The radiomicrometer is essentially a moving coil galvanometer, of a
single loop of wire, with a thermo-junction at one end. This instrument
was invented independently by d'Arsonval2 and by Boys.3 The former

1 Weber: Archiv. Sci. Phys. et Mat. (3), 18, p. 347, 1887.

2 d'Arsonval: Soc. Franc, de Phys., pp. 30 and 77, 1886.

3 Boys: Proc. Roy. Soc, 42, p. 189, 1887.

154 RADIOMETRY.

used a loop, one part of which was silver and the other was of palladium;
the latter used a junction of bismuth and antimony, which was soldered
to a loop of copper wire.

The sensibility of the Boys instrument was given as To~ei>^~oTro to
9^~o'oV"^'0"o °f x degree. From subsequent work with other radiation meters
in which this high degree of sensitiveness has never been attained, it would
appear that the sensibility of the radiomicrometer was overestimated. It
certainly has never attained the sensibility of the radiometer, one example
of which, used by Nichols (loc. cit., Table V), was twelve times as sensitive
as the radiomicrometer of Boys. The latter gave a deflection of a little
less than i cm. per square millimeter of exposed vane for a candle and
scale, each at a distance of i meter. Paschen1 attempted to improve the
radiomicrometer, but out of about fifty junctions only three were useful,
and these were only three times as sensitive as that of Boys, while the
period was about 40 seconds. The long period is not always detrimental,
however, for the radiomicrometer is not subject to magnetic disturbances
and is a very useful instrument for work not requiring the highest attainable
sensitiveness.

The writer has indicated further improvements in the instrument (see
Carnegie Publication No. 65) and places it in a vacuum, which increases
the sensibility by at least 70 per cent. The instrument was about six
times as sensitive as that of Boys for a full period of 25 seconds. Para-
and dia-magnetism limited the sensitiveness to this value. The work
with this instrument brought out the fact that one is inclined to use too
strong field-magnets, and that further progress can be made by using
weak magnets, or narrow strong magnet, situated as far as possible above
the thermo-junction, so as to avoid the effects of para- or dia-magnetism.
The combination of the radiomicrometer and the radiometer is feasible,
although the writer found its usefulness as limited as that of the radio-
micrometer. When wires can be obtained more free from magnetic
material it will possible to construct a more sensitive instrument. It is
doubtful, however, whether it will ever surpass the bolometer used with a
galvanometer of the highest sensibility. With the radiomicrometer, Lewis2
was able to investigate infra-red emission spectra of the alkali metals,
which are weak in energy. Wilson3 and Julius4 have used the radio-
micrometer for total and spectral radiation work, and have found the
instrument highly satisfactory. The slow period and lack of portability,
mentioned by some writers, is certainly not to be weighed against its
indifference to magnetic perturbations and constancy of the zero reading.
Even a slow period is less objectionable than a quick period instrument

1 Paschen, Ann. der Phys. (3), 48, p. 272, 1893.

2 Lewis: Astrophys. J., 2, p. 1, 1895.

3 Wilson: Proc. Roy. Soc, 55, 1894; 58, 1895; 60, p. 337, 1896.

4 Julius: Handlingen 5, de Nederlandisch Natuur- en Geneeskundig Congres, 1895.

THERMOPILES.

155

with which just as much time is lost by repeating observations which may
be affected by the lack of constancy of the zero. The instrument is self-
contained, and where the greatest sensitiveness is not required, it deserves
a wider application.

Table IV. — Sensitiveness of Radiomicrometers and Rubens Thermopile.

Observer.

Full
period.

Area of
vane.

Deflections in cm /mm2
candle and scale alia.

Radiomicrometer:

Boys (Phil. Trans., 180 A, p. 159, 1 889)
Paschen (Wied. Ann., 48, p. 275, 1893)
Lewis (Astrophys. Jour., 2, p. 1, 1895)
Coblentz (Bulletin Bureau of Stand-
ards, 2. D. 470. 1006)

sec.
IO
40
20
40

25
14

mm2
4

cm.

0.9

3

i-3 (?)

3-6

6 (in vacuo)

16(250 cm. total deflection)
1 mm = 1.10 X io"6 C.

1.4

3
3

i6(?)

Thermopile:

Rubens (Wied. Ann., 45, p. 244, 1898)

In table IV are given the various radiomicrometers thus far described
and their sensitiveness, which is expressed in centimeter deflections per
millimeter of exposed vane, for a candle and scale each at a distance of
1 meter.

III. THE THERMOPILE.

From a historical point of view the thermopile has been in use from the
very beginning of radiant energy measurements, and in the hands of Tyndall
and other pioneers in this domain rendered excellent service in spite of its
great heat capacity. For spectro-radiometric work, however, only the linear
thermopile of Rubens1 is well adapted. This thermopile consists of 20 junc-
tions of iron and constantan wires about 0.1 mm. to 0.15 mm. thick (resist
3.5 ohms), and when used with a galvanometer, having a figure of merit of
i= 1.4X io-10 amp. (period = 14 sec), a deflection of one scale division indi-
cated a temperature change of i.i°X io-8. A candle at 5 m. gave a deflec-
tion of about 10 cm. or 250 cm. at 1 m. The area of the exposed face is
about 0.8X20 mm. The deflections were as rapid as a bolometer, and its
stationary temperature was reached in less time than the single swing of
the galvanometer needle. In other words, its heat capacity was so small
that it gave an accurate register of the energy falling upon it. In another
experiment, using a galvanometer sensitiveness of 2 = 5Xio~10 ampere, and
the scale at 1 m., 1 mm. deflection = 2. 2°Xio~6 C. The sensitiveness is
the same as that of the best bolometers yet constructed, while its simplicity
commends itself even in spectrum radiation work.

The general experience in this country, however, has been that the
commercial instrument does not fulfill all the excellent qualities claimed
for the one originally described. The wires are heavier than in the original
specifications, which makes the instrument sluggish.

1 Rubens: Zs. fur Instrumentenkunde, 18, p. 65; 1898.

156 RADIOMETRY.

The problem in thermopile construction is to have it of low resistance
(equal to that of the galvanometer), of low heat capacity and heat conduc-
tivity, and of high thermoelectric power. The latter requirement is fulfilled
by using iron and constantan. The heat capacity can be reduced by
using finer wire, say 0.06 to 0.1 mm. diameter, and by making the unex-
posed junctions smaller than the ones to be exposed. The latter are
soldered with quite large beads of silver, which are then flattened to present
a large surface. The unexposed junctions do not need this, and the small
bead formed by the fusion of the two wires (with a bit of silver solder if
necessary) can be hammered thin, in order to have it radiate rapidly. By
using finer wires the resistance will be increased if the dimensions of the
Rubens pile be retained. In the commercial instrument, at least one-
third of the wire is between the unexposed junctions and the binding-posts.
The greater part of this wire may be eliminated by making the supporting
frame narrower, while still retaining the original distance between the
exposed and the unexposed junctions. The elimination of this superfluous
wire will reduce the resistance by about one-third.

Comparison of Old and New Form of Thermopile.

In order to test these conclusions in regard to the use of finer wire, a
new iron-const ant an pile of 20 junctions, made of wire 0.08 mm. diameter,
was ordered from the makers of the original instrument. Although the
specifications were not completely fulfilled (the frame was nearly the same
size as the original, which increased the resistance to 9 ohms), the sensi-
tiveness was 1.4 times that of the old type which has resistance of 4.8 ohms
(wire about 0.15 mm.). By means of suitable switches the two thermo-
piles were connected to the same galvanometer, having a full period of
12 seconds (i=2Xio~10 amp.) and exposed to the radiation from a Nernst
heater. For all deflections, as large as 35 cm., the new thermopile showed
no drift greater than 2 mm., which may be attributed to the galvanometer.
On the other hand, the zero of the old thermopile would drift 0.5 cm. in
a 10 cm. deflection to 2.2 cm. in a 27 cm. deflection and would require
16 to 20 seconds for the deflection to return to its original zero.

The two instruments were then tested in a vacuum. The sensitiveness
of the old instrument was increased only 15 per cent, while no change in
sensitiveness could be detected in the new one, although two distinct tests
were made on different days, the pressure having been reduced to 0.01 mm.

The thermopiles are mounted on ivory frames and are covered with a
sheet of copper, one side having a slit, the other a funnel-shaped opening
(1 by 15 mm.) in it. The slit was covered and the radiation passed through
the funnel. The whole was suspended from a rubber cork in a wide-
mouthed bottle, which was exhausted with a mercury or a Geryk pump.
The source of energy was an incandescent lamp. With 200 ohms in
series with the galvanometer the deflections were about 10 cm. The fact

THERMOPILES. 1 57

that the sensitiveness of these thermopiles did not increase appreciably in
a vacuum is rather remarkable. Brandes (Phys. Zeit, 6, p. 503) found
that a single junction of 0.02 mm. wire became 18 times more sensitive in
a vacuum. Lebedew (Ann. der Phys., 9, p. 209) found that a 0.025 mm'
iron-constantan junction when black was 7 times, and when bright 25
times more sensitive at a pressure of 0.01 mm. than for atmospheric pres-
sure.

The Peltier Effect.

The result of the Peltier effect is to lower the temperature of the ex-
posed junction. Consequently, the thermopile does not give an accurate
record of the energy received. The actual error introduced has never
been determined. Since there is a possibility of using the thermopile for
quantitative work in place of the bolometer, it is desirable to learn the
degree of accuracy of this instrument.

The rate of generation of heat by the Peltier effect is proportional to
the current, while the generation of heat on account of resistance is pro-
portional to the square of the current. Jahn1 has shown that the heat
generated by the Peltier effect, determined experimentally, agrees, within
experimental error, with the value computed from the observed thermo-
electric power. The value for iron-constantan has never been determined
experimentally2 but from the work of Jahn it is permissible to compute
the heat generated in the thermopile by using the known thermoelectric
power which is about 50X10""8 volts.

Using a galvanometer of 5 ohms resistance and having a figure of
merit of 2 = 3 Xio-10 ampere per millimeter for a scale at 1 m., and an
iron-constantan thermopile of 20 junctions, wire 0.08 mm. and 5 ohms
resistance, 1 mm. = 2°Xio-8 C. The Peltier effect in calories is computed
from the formula

p=_TU dE
J ' dt

where 7^=274; 2=3 Xio-11 c.g.s.; /=5 sec; 7=4.2Xio-7 and dE/dt=
50X10-2 c.g.s. units;

P= ±5Xio~12 gr. cal.

The total weight of the junctions is about 0.01 gr. and the specific
heat is about 0.1 gram calorie. Hence the temperature change of the
exposed junctions is:

M= 5Xl° — =c°Xio-fl (for 1 mm. deflection)
0.1X0.01 3 v

1 Jahn: Wied. Ann., 34, p. 755, 1898.

2 Since writing this, it has been found that Lecher (Ber. Akad. Wiss. Wien, 115, p. 1505,
1906; Sci. Abstracts, 1083, 1907) has recently determined this constant to be 12.24 gr- ca^
per amp. hr., while the value previously computed was 10.5 gr. cal. per amp. hr.

158 RADIOMETRY.

and since the temperature of the unexposed junctions is changed an equal
amount in the opposite direction the total A*=i°Xio~8. But a deflec-
tion of 1 mm. = 2°X io~9 C, hence the error is 1 part in 200 under the best
theoretical conditions. In practice the temperature sensitiveness will not
be so great; it will be shown presently to be of the order 5°Xio"~6, whence
the Peltier effect would cause an error of 1 part in 500, or 1 mm. in 50 cm.,
which is as close as one can read such large deflections.

Since the Joule heat depends upon the square of the current it is negli-
gible. Further consideration of the thermopile as an instrument for
quantitative measurements will be found below in connection with the
bolometer.

It will be noticed presently that prior to his construction of the iron-
constantan thermopile, Rubens had used several very sensitive bolometers,
all of which were displaced by the thermopile. For exploring spectra
with very narrow lines, the linear bolometer is probably better adapted
than the pile which, however, may be covered with a diaphragm, having
a narrow slit. For extreme sensitiveness it equals the bolometer, and it.
is a noteworthy fact that all the investigations in the extreme infra-red
and ultra-violet parts of the spectrum, where the energy is weak, have
been accomplished by means of the thermopile. Unless one can build up
an elaborate permanent bolometric apparatus in a room not exposed to
direct sunlight, the thermopile will give the more reliable readings, as far
as the constancy of the zero is concerned. Whether or not the thermopile
will give a true reading of the energy falling upon it will depend upon
the manner in which it is employed. It requires no particular skill to
manipulate, and is easier to protect against temperature changes than
is a bolometer with its storage battery. The "drift" in bolometers to be
noticed presently was found in the old thermopile when used with a very
delicate galvanometer. This was not due to unequal increments of re-
sistance as in the bolometer, but to thermoelectric effects at the binding
screws, to the connecting wires moving in the earth's magnetic field, and
principally to the large heat capacity of the junctions. Most of these
disturbances, however, are small and easily avoided in the Rubens type of
thermopile. Since the bolometer strips and the balancing coils are of
dissimilar material, it is also as subject to thermoelectric disturbances.

IV. THE RADIOMETER.

The manner in which an interesting scientific toy can be made to
serve a useful purpose is well exemplified in the radiometer of Crookes,1
discovered about 1875. By fastening bits of pith (the one black, the
other white) at the ends of a long straw, which was suspended by means
of a silk fiber in a long glass tube, he was able to make measurements of

1 Crookes: Phil. Trans. (II), 166, p. 325 1876.

THE RADIOMETER. 1 59

radiant energy, even at that early date. Pringsheim1 simplified the instru-
ment somewhat, suspended the vanes bifilarly with silk thread, and used it
to investigate the infra-red spectrum of the sun to about 1.5 /*, produced by
means of a glass prism. From this, the first really useful radiometer was
developed by Nichols.2 The behavior of the radiometer has been worked
out theoretically by Maxwell3 in his paper on "Stresses in Rarefied Gases
arising from Inequalities of Temperature." Among other things he showed
that for two parallel disks very near each other the central points will pro-
duce but little effect, because between the disks the temperature varies uni-
formly, and only near the edges will there be any stress arising from an
inequality of temperature in the gas. It has been shown by others, especially
by Crookes and by Nichols, that the sensitiveness of the radiometer is a
function of the pressure of the residual gas, of the kind of gas surrounding
the vanes, and of the distance of the exposed vanes from the window. The
latter, on account of its absorption, of course limits the region of the spec-
trum which is to be investigated. If the vanes are not too close to the win-
dow, the deflections will be proportional to the energy falling upon one of

them.

Comparison of Sensitiveness and Area of Vane.

For veins of finite dimensions, such as must be used in practical work,
the writer has found that the deflections are proportional to the area of
the exposed surface of the vane. This is perhaps to be expected, although
there seemed to be some doubt. The curve of deflections and exposed
area of vanes (area 10.5 X 1.3 mm. for constant pressure of 0.02 mm.) does
not pass through the origin. One explanation may be that for infinitely
narrow vanes the graph is not a straight line, but curves as it approaches
the origin. Because of the impracticability of suspending vanes of different
widths successively from the same fiber suspension, at the same distance
from the window, using the same gas pressure for all vanes, it was necessary
to use one wide vane, with a slit before it, and vary the opening of the slit.
The source of energy (Nernst heater) was at a distance of 3 meters and,
hence, the width of the projection of the slit upon the vane was practically
the width of the slit, except for a very narrow slit when diffraction may
decrease the energy incident upon the vane. This, however, would dis-
place the graph still farther from the origin. The forces acting in a radi-
ometer are so complex and so little understood that no further examination
was made to ascertain the limits within which the above proportionality
holds. The test was made to reduce the deflections to unit area between
the above limits of exposed vane. It is of interest to note in this connec-
tion that in a bolometer the sensitiveness varies as the square root of the
area of the bolometer strip.

1 Pringsheim, Ann. der Phys. (3), 18, p. 32, 1883.

2 Nichols: Phys. Rev., 4, p. 297, 1897; Berl. Ber., p. 1183, 1896.

3 Maxwell: Collected Papers, 2, p. 681; Phil. Trans., Part I, 1879.

l6o RADIOMETRY.

In his earlier communications the writer held to the belief that the
weight of the vanes and their size were the most important factors in
determining the sensitiveness and period of a radiometer. However, so
many factors enter into the problem that it is difficult to decide this point,
and the following test may be of interest, showing that the sensitiveness is
determined principally by the diameter of the quartz fiber suspension.

Comparison of Sensitiveness and Diameter of Fiber Suspension.

Using the same vanes (0.5X9 mm. area) the sensitiveness and period
were found for a heavy and a light quartz fiber suspension. The main
difficulty was to insure that the vanes were at the same distance from the
window, and that the pressure was the same in the two cases. Hence these
qualities are only approximate. In table V, it will be noticed that in
changing from a heavy to a light fiber the sensitiveness is increased 4.5
times while the period was increased almost threefold. It further shows
that for the same (light) fiber the sensitiveness was doubled (36 to 71 cm.
per mm.) by changing the pressure and the distance from the window,
which was of fluorite, and hence opaque beyond 10 /i. The sensitiveness
of 71 cm. per square millimeter of exposed area is the highest on record.
This, however, was not the maximum sensitiveness since the pressure was
0.02 mm., while radiometers have their maximum sensitiveness at a pres-
sure of about 0.05 to 0.1 mm. At this pressure, however, heat conduction
would cause annoyance.

The vanes of this suspension were of platinum foil 0.01 mm. thick,
covered on one side electrolytically with platinum black, and then smoked
over a candle. (It is best to cool the gases from the flame by placing a
wire gauze or sheet of metal full of holes between the flame and the vanes
when smoking them.) These vanes were suspended by means of one of
the finest workable quartz fibers, and when within 3 mm. of the window
either one of the vanes would always approach and adhere to it, even at
atmospheric pressure. From tests with fluorite windows, which from
internal strains might be piezoelectric, and with rock-salt windows, when
bare, and also when covered with tinfoil, it was found that this effect is
not due to electrification. Starting with the vanes parallel, and at a dis-
tance of about 5 mm. from the window, it was found that as this distance
was decreased one of the vanes (generally the one to be exposed to radia-
tion) would approach the window, and for a distance of about 3 mm.
would turn until the plane of the vanes was at right angles to the window.
The observations extended over several months, and all evidence indicates
that this effect is due to gravitational attraction. As a result of this the
deflection of such a vane would not be proportional to the energy received.
The radiometer had no torsion-head to control the zero. However, for
general work with very sensitive radiometers a torsion-head would be nec-
essary, since the best pumps may leak, which will cause a slow "drift."

THE RADIOMETER.

161

It will be shown presently that this is about five times the sensitiveness of
Snow's bolometer, for which i mm. deflection (scale at 3 m.) indicated a
temperature difference of 7.5°Xio~6. In other words, this radiometer
would detect t 00q 000 degree rise in temperature. But the period of the
radiometer was 6 times that of the bolometer, which is its weakest point in
radiation work requiring a short period.

Table V. — Sensitiveness of Radiometers.

Pressure.

FuU
period.

Deflection per
square mm.

Remarks.

mm.

sees.

cm.

O.02

45

7-9

Vanes (area 0.5X 9 mm., weight 5 mg.) 3 mm.
from window.

.02

60

ii-5

Vanes closer to window.

SAME VANES (0.5X0 mm

.), FINER QUARTZ FIBER.

mm.

min.

cm.

0.02

2

36

Vanes 3 mm. from window candle at 3 m.

•°3

2-5

71

Vanes nearer window. This is the greatest
recorded sensitiveness. A deflection of 1
mm. on scale at 1 m. = 3.8°Xio-c.

.04

63-5

HEAVY VANES, AREA 1.3

Xio.s mm., WEIGHT 10 + mg.

mm.

sees.

cm.

O.02

40

5-5

Distance of vanes from window unknown.

.02

60 to 64

13-7

Vanes nearer window, hence longer period.

.026

36 to 40

8-5

Vanes farther from window than in preceding.

•035

25

3-3

Vanes still farther from window, which
shortens period and decreases sensitiveness.
For a pressure of about 0.05 mm. the
sensitiveness would be much greater.

A comparison can also be made (table V) between light vanes (0.5X9
mm.) and heavy ones (1.3 X 10.5 mm.) at the same pressure but having
different quartz-fiber suspensions. The results show that while the light
weight vanes are more sensitive than the heavy ones (see table V), there
seems to be no limit to the sensitiveness attainable in either case, pro-
vided one does not consider the period. The idea of not considering the
period of vibration with sensitiveness seems reasonable, for by sensitive-
ness is meant the minutest quantity of radiation one can detect, assuming
one is willing to wait for the deflection to reach a maximum. In table VI
are compiled the most notable radiometers used in radiation work. The
use of a candle as a standard of comparison is questionable, but since the
sensitiveness of the various instruments varies by a factor from 2 to 20,
it is sufficiently accurate for the present comparison. In this table it will
be noticed that for the same period the various radiometers vary in sensi-

162

RADIOMETRY.

tiveness by as much as 50 per cent. Porter's radiometer was the most

sensitive of the instruments having a period of 90 seconds. But he gained

little, on the whole, for the vanes were so light that he could work only

during the quiet hours at night. On the other hand, the writer,1 after

trying light vanes, adopted the heavy ones, and was thus able to work at

all hours, even with a large air-compressor in operation, in an adjoining

room.

Table VI. — Sensitiveness of Various Radiometers.

Observer.

E. F. Nichols:

Phys. Rev., 4, p. 297, 1897

Astrophys. Jour., 13, p. 101, 1901

Stewart:

Phys. Rev., 13., p. 257, 1901

Drew: (Phys. Rev., 17, p. 321, 1903)

Porter (Astrophys. Jour., 22, p. 229, 1905)

Coblentz

Abs. Spectra

Phys. Rev., 16, 20, and 22. (Vac. tube)..

Another vane

(Ibid.)

Full period.

mm. sec.
O 12

O

I

5

1
1
1

1
2

1

11

20

30
3°
30

5°
40

3°
10

Area of vanes.

sq. mm.
2X15

3-i

3°

7

3-6

15(10X1.5)
12
11 (nXi)

4-5

4-5

Deflections per
square mm . area
of exposed vane;
candle and scale
each at 1 m.'

5<?)

12.5
4.9

17
17.1

27-5
8 to 10
1 o to 12
52
7i
35

Sensitiveness Compared with Wave-Length of Exciting Source.

In his investigations of emission spectra of the alkali metals, using a
bolometer, and a prism and lenses of quartz, Snow2 found that the vapor
of the carbon arc had the larger portion of its energy concentrated in one
large band in the violet. The writer,3 using a radiometer, a rock-salt
prism, and a mirror spectrometer for investigating infra-red emission
spectra, found that the radiometer gave but small if any deflections in the
violet. The violet band is far enough from the reflection minimum of
silver not to be weakened by it, hence it appeared that the radiometer
might be selective in its behavior to radiant energy.

To test this point the following experiment was tried. The total
radiation from an aluminum spark (with glass plate condenser) on a
10,000 volt transformer was measured with a very sensitive radiometer
(period 65 to 70 seconds; sensitiveness 35 cm. per square millimeter) just
described, and with a bolometer, to be described subsequently. The win-
dow of the radiometer was of white fluorite 2 mm. thick, hence transparent
to the ultra-violet, but opaque beyond 10 ft. A large point of the energy
of the aluminum spark lies in the ultra-violet. The maximum energy of
the warm electrodes occurs at about 8 ft. The radiation from the spark

1 "Investigations of Infra-red Spectra," Carnegie Publication No. 35, 1905.

2 Snow: Phys. Rev., 1, pp. 28 and 221, 1893.

3 Carnegie Publication No. 35, Part II, 1905.

THE RADIOMETER. 1 63

passed through a quartz cell 8 mm. thick, containing distilled water which
absorbed all of the infra-red energy. The observations consisted in ob-
taining the ratio of energy transmitted by a glass plate 8 mm. thick (which
is opaque to rays shorter than 0.3 p) to the total energy of the spark.

Unfortunately, at the high sensitiveness required for ultra-violet radia-
tion the two instruments were not in perfect working order at the same
time. During the first test the bolometer had a short period, 10 seconds,
and caused trouble by the "drifting" of the zero with changes in the
spark, while in the second test the radiometer was leaking slightly, which
caused its zero to "drift." Then, too, the spark was by no means constant,
but, as will be seen presently, the ratio above referred to is about the same
for the radiometer and the bolometer, after correcting for the loss of 4
per cent by reflection at the nuorite window. The agreement is close
enough to show that the radiometer is not selective in its action and hence
is adapted to investigations in the ultra-violet.

In the first test the direct radiation from the aluminum spark (with
condenser) was compared with the part transmitted by glass. There was
some infra-red energy in this case which made the ratio lower than in the
second experiment. The direct deflections with the radiometer were
about 20 cm. The ratio of the deflection through a piece of plate glass
to the direct deflection varied from 17 to 19.5 per cent (mean about 18
per cent), while with the bolometer the same ratio varied from 16 to 20 per
cent, the mean being about 19 per cent. The bolometer followed the
flunctuations of the spark, hence the greater variations. The spark was
75 cm. from the radiometer and 25 cm. from the bolometer.

In the second test the infra-red radiation was absorbed by a cell of
water, with quartz windows. Plate glass was again used to absorb the
ultra-violet. In this test, the ratio of the radiation transmitted by the
glass to the total radiation was on an average about 65 per cent, while
the same ratio for the bolometer was 67 per cent. Correcting for the
loss by reflection at the window the ratio for the radiometer would be
about 69 to 70 per cent.

The results show that the radiometer is not selective, i. e., it is as efficient
in the ultra-violet as in the bolometer.

THE RADIOMETER COMPARED WITH THE BOLOMETER.

In discussing the merits of the radiometer, writers have generally
emphasized the fact that the radiometer is not adapted for quantitative
work since it cannot be calibrated. As a matter of fact, in reviewing the
work done in radiation it was found that even with the bolometer there
are only a few cases where the energy was obtained in absolute measure.
Even in the study of the laws of radiation from a hollow enclosure or
KirchhofT radiator (so-called "black-body"), the direct galvanometer
deflections were observed and reduced to a single standard of sensitive-

164 RADIOMETRY.

ness which was in arbitrary units. The same may be done with the radi-
ometer. Its sensitiveness is easier to control, since it can be made to
depend only upon the pressure of the residual gas — the constant of a
galvanometer varies continually. It is not affected by magnetic effects,
and a heavy vane is less affected by earth tremors than is a very light
galvanometer suspension. It is sensitive to temperature changes, but less
so than the bolometer, and it can be more easily shielded from temperature
changes than can a bolometer with its galvanometer, battery, etc. The
fact that it is not portable is not a serious drawback, since it is not usually
necessary to move the instrument. It has two disadvantages, viz, its
window, or preferably double window, is selective in its transmission, and
its period is somewhat longer than that of a bolometer and galvanometer
of equal sensitiveness. But the latter is nearly always drifting and to
repeat one's readings it takes as long for an observation as it does with a
radiometer.

Since the weight is of minor importance, tremors are avoided by having
the suspension weigh about 8 to 10 mgs. When used with a good mercury-
pump, it requires no attention after it is properly adjusted. A delicate
galvanometer requires frequent adjustment and in connection with a
bolometer the investigator's time is occupied principally with the care
of the instrument (at least that has been the writer's experience), which
should be a secondary matter. The two instruments are of the same
order of sensitiveness, with the possibility of the radiometer being the
more sensitive.

This is well illustrated in the test for their efficiency to ultra-violet
radiation, where both instruments were at about their maximum working
sensitiveness. The bolometer used was 0.22 by 10 mm. in area, resistance
2.8 ohms, and for a battery current of 0.04 amp. with a galvanometer
sensitiveness of i=i.5Xio~10 amp. (period 16 seconds) had a tempera-
ture sensitiveness of 9X10"" per millimeter deflection, on a scale at 1 m.
(see table VII). (A Nernst heater was also used in making the com-
parison.)

A candle at 1 m. gave a deflection of 45 cm. which, on the assump-
tion that the sensitiveness is proportional to the square root of the area
of bolometer strip, is 30 cm. per square millimeter. For the radiometer
having a vane 0.5 by 9 mm. a candle gave a deflection equivalent to 159 cm.
at 1 m. or, since the deflection is proportional to the area of the exposed
vane, 35 cm. per square millimeter (table VI). In other words, the radi-
ometer was 1.2 times as sensitive as the bolometer, or 1 mm. deflection
corresponded to 7.5°Xio~6C. (For a full period of 2.5 minutes its sensi-
tiveness was 3. 8° Xio~8 C.) Its period, however, was 4.5 times that of
the bolometer. This estimation of sensitiveness is based on the assump-
tion that the radiometer was as complete an absorber of energy as the
bolometer. Judging from its period, its efficiency is much lower than

THE BOLOMETER. 165

that of a bolometer, hence the radiometer must be sensitive to temperature
changes less than the value just given.

The sensitiveness of bolometers thus far attained is about 1 00q 000
degree per millimeter deflection. Paschen claims a sensitiveness of
10 0(H) (Too degree by reading to o.i mm. But the conditions are rare
when one can read to o.i mm., so that the estimate would seem too high.
As will be seen presently, the working sensitiveness is of the order of
x 00q 000 degree, or generally considerably less.

V. THE BOLOMETER WITH ITS AUXILIARY GALVANOMETER.

We have now to consider one of the most useful radiation meters yet
devised, viz, the bolometer which is simply a Wheatstone bridge, two
arms of which are made of very thin blackened metal strips of high elec-
trical resistance and high temperature coefficient, one or both of which
are exposed to radiation. When thus exposed, their temperature changes,
thus unbalancing the bridge, and the resulting deflection of the galvanom-
eter gives a measure of the energy absorbed. The maximum sensitive-
ness of the bolometer is limited by the size of the strip and is proportional
to the square root of the surface exposed to radiation. Any further gain
in sensitiveness must be attained by increasing the sensitiveness of the
galvanometer, which, for the moving magnet type, varies as the square of
its (undamped) period. The sensitiveness is also proportional to the
bolometer current, which is limited by the resistance of the bolometer
strips.

It will be noticed presently that the working sensitiveness of the vari-
ous galvanometers thus far used is of the order of 2X10-10 amp. per
millimeter deflection, while the temperature sensitiveness varies from
5°Xio-5 to 5°Xio~8 for 1 millimeter deflection for a scale at 1 meter.

Historical.

The various bolometer-galvanometer apparatus will first be noticed,
in so far as it relates to spectro-radiometric work.

The first great step in improving the moving magnet galvanometer is
due to Kelvin who decreased the weight of the moving parts to a few
milligrams, and introduced the astatic system of magnets. The main
problem in bolometer construction is to use strips of a metal having a high
resistance-temperature coefficient, a small specific heat, and low heat con-
ductivity. These metals are nickel, platinum, tin, and iron, but for vari-
ous reasons in mechanical construction, platinum is the most commonly
used.

The manner in which this instrument has been developed to its pres-
ent high sensitiveness is best illustrated by considering the various designs
of different investigators, given in table VII.

i66

RADIOMETRY.

Table VII. — Bolometer-Galvanometer Sensitiveness.

Observer.

Langley ( AnnalsAstrohpys
Obs.)

Abbot (Astrophys. J., 18,
p. i; 1903)

Angstrom:

Wied. Ann., 26, p.

253'. l885

Wied. Ann. 36, p.

715; i889

Wied. Ann., 48, p.

497; 1893

Julius (Licht und Warme-
strahlung, p. 31;
1890)

Helmholtz (Verh. Phys.
Gesellsch., Berlin,
7, p. 71; 1888)....

Lummer and Kurlbaum:

Zs. fur Instrumenten-

kunde, 12, p. 81;

1892

Wied. Ann., 46, p.
204; 1892

Rubens: Wied. Ann., 37, p.

255; l889

Wied. Ann., 45, p.
238; 1892

Galvanometer.

Resist-
ance.

ohms.

[.6

2.7

5
80

Full
period.

Current

sensitiveness

for 1 mm.

deflection.

Bolometer.

20

l6

4 to 5

1 to 5X10-10

5Xio-»

5.7X10-9
4Xio-°

8X10-9

1. 5X10-

3.2X10-10
3.2X10-1°

Resist-
ance.

ohms.
4

8.8

60

5-2
3

Area.

sq. mm.

0.05 to 0.02
Xl2

O.1X12

O.3X14

12X1X32

7X0.3X3S
3X0.2X10

Bolometer
current.

amp.
O.03

O.I33

0.006

0.2

BOLOMETERS.

167

Table VII. — Bolometer-Galvanometer Sensitiveness.

Temperature sensitiveness.

For 1 mm. de-
flection and
period given
in col. 3.

°C.
1X10-6

9X10-6

8X10-

3X10-

2X10-4
5X10-6

r Ibid.,
for a scale
at 1 m.

2.5X10-

Ibid., for

a scale at 1 m

and a full

period of

15 sees.

°C.

4X10-

Candle test.

a "2
2 ?

met.

u S

c o

m a

tn g

bo

met.

-a a
o> o

S J

o-S

800

60

. -A u

<V CZ2.

?£^

> c>2

IrS *.

cry 4)"

03

Remarks.

In practice for solar spectrum
work a full period of 3 sees,
and i=2.2Xio-9 ampere is
used. Balancing coils of plati-
noid. Bolometer is inclosed in
water jacket.

In practice drifting is minim-
ized by placing a short copper
wire in series with one of the
bolometer arms.

Surface bolometer; 23 strips of
tin foil; blackened with PtCl2
and soot. Balancing coils of
copper wire in separate box.
Sensitiveness 556X10-9 gr-
cals/cm2/sec.

Single Pt. strip 0.1X12 mm. in
spectrobolometer.

2 arms of tin foil in form of grat-
ing. Sensitiveness also given
as 127X10-9 gr- cals/cm2sec.

Bolometer strips of nickel 0.002
mm. thick. Balancing coils of
platinum wire in oil in sep-
arate box. Found deflection
proportional to current.

Surface bolometer, 4 similar
arms, so no compensating re-
sistances needed; diagonally
opposite arms exposed. Sen-
sitiveness also given as 533 X
io-9 gr-cals/cm2/sec.

Surface bolometer 4 similar
arms of platinum foil 0.001
mm. thick; 2 diagonally op-
posite arms exposed to radia-
tion surface consists of 1 2 strips
of platinum, 1 mm. wideX32
mm. long, separated by 1.5
mm., back of which openings
are 12 similar strips of the
diagonally opposite arm. Bo-
lometer currents as high as
0.04 ampere could be used.

Surface bolometer of tin foil 0.01
mm. thick; 7 strips in each arm.

2 arms of 0.04 mm. iron wire
hammered flat; 3 strips in
each arm.

i68

RAJDIOMETRY.

Table VII — Continued.

Observer.

Rubens: Wied. Ann., 45, p.
238; 1892

Rubens and Snow (Wied.
Ann., 46, p. 529;

1892)

Snow (Phys. Rev., 1, p. 31;

1893)

Aschkinass (Wied. Ann.,
55, p. 401; 1895)..

Donath (Wied. Ann., 58,
p. 609; 1896)

Paschen (Wied, Ann., 48,
p. 272; 1893)

Coblentz

Rubens Thermopile .
Coblentz Radiometer

Resist-
ance.

ohms.
80

I50
140

*6o

*6o

20

5-2

5-2

Galvanometer.

Full
period.

20
20

20

7

15

3°
34

10
16

14

70

150

Current

sensitiveness
for 1 mm.
deflection.

3-2XIO"

I.8X10-11
1. 5X10-H

6X10-H
6X10-12

Bolometer.

Resist
ance.

2.3X10-"
3-3X io-»
1.6X io-n
8.3X10-12

2.5X10-1°
1.5X10-1°

1.4X10'

-10

ohms.
80

80

75

13

5

12
8

2.8

3-5

Area.

sq. mm.
O.09X 12

O.09X 12
0.05 X 7

Bolometer
current.

amp.

O.O25

0.04

3X0.5X15

0.25 X7

0.22X 10

0.8X20

0.5X9

0.5X9

0.01 to 0.02

0.03

.02

0.02

♦Galvanometer; 4 coils 40 mm. external and 5 mm. internal diameter, 1,200 turns of graded wire in each coil;
moving system, 13 magnets in each group, 1 to 1.5 mm. long, on both sides of glass staff, and 0.3 mm. apart;
mirror 2 mm. diameter X 0.03 mm. thick. Total weight of system = 5 mg.

BOLOMETERS.

169

Table VII — Continued.

Temperature sensitiveness.

Candle test.

Remarks.

For 1 mm. de-
flection and
period given
in col. 3.

Ibid.,

for a scale
at 1 m.

Ibid., for

a scale at 1 m.

and a full

period of

15 sees.

0

&) V

a a
to 3

5

*-

a °

a 0
£ a
— S

3 -a

be

T3 fl
<u 0

Otj

Equivalent de-
flection per mmJ

area for candle
and scale at 1 m.

°C.

8X10-6

3XlO-B
8X10-6

3XIO-5

°C.
4X10-5

°C.

met.

I

met.

5

cm.

12

40
15

2.7
8-3

2 arms of 0.005 mni- platinum
wire hammered flat. Deflec-
tions of galvanometer pro-
portional to current through
bolometer and rise in temper-
ture of arms proportional to
incident energy.

For Hefner candle at 1 m.

2 bolometer arms of platinum
0.000936 mm. thick; balanc-
ing coils of German silver; 4-
coil galvanometer, total of
7200 turns, wound with two
sizes of wire. Magnet system
of 12 magnets 3 to 4 mm.
long; total weight of system
80 mg.

2 bolometer arms of 3 iron wires
hammered flat.

2 arms of platinum 0.17 mm.
wideX 0.0074 mm. thick; bal-
ancing coils (of platinum) of
elaborate design in separate
box. Drifting was serious.

Bolometer arms of platinum
0.001 to 0.0005 mm- thick.

Compensation resistances of man-
ganin wire, final adjustment
on platinum wire with mer-
cury contact.

Using an 8-magnet system and a
full period of 20 sees. i=8X
io-11 ampere, while a 6-mag-
net system had a sensibility of
i = 2.5Xio_n ampere for a
full period of 36 seconds.

2.4X10-5
1.2X IO-4

4X10-5

I

3
4

3
2.7

2-5

lXlO~6
84XIO-7

2.7 X io-6
2.1 X io~ 5

11 X io-6
8X10-5

9X10-"

9X10-°

I

5

2-3

3

1

(1?)
1

1

45

10
3°
35-5

3°

1.1X io-c

iXio-fi

IS
35
7i

t7-'5Xio-6

tfThese values are obtained by comparing the deflections per unit area with the bolometer.

1 70 RADIOMETRY.

Comparison of Sensitiveness of Various Bolometer-Galvanometer

Combinations.

The most important data on sensitive radiation meters, and particu-
larly that relating to bolometers, is given in table VII. It will be noticed
that the thermopile is as sensitive as the bolometer. The sensitiveness of
the radiometer is obtained by comparing it with the bolometer. Although
the radiometer is no doubt less efficient than the bolometer, it probably
absorbs as much of the incident energy. Since the radiometer deflections
were larger, per unit area, than the bolometer deflections, it is safe to
assume that the radiometer was just as sensitive as, if not more so than,
the bolometer. The long period is of course a serious objection in certain
classes of work. In table VII it will be noticed that the high temperature
sensitiveness of the various instruments has been attained by the use of
a highly sensitive, long period, galvanometer, by using a large bolometer
current and by placing the scale at a great distance from the galvanometer.
Assuming that the sensitiveness is proportional to the square of the period
(undamped) for a scale at 1 m. and a bolometer current of 0.04 ampere,
it will be noticed from column 10, table VII, that the temperature sensitive-
ness of the various instruments falls in two groups. To the first group
belong the earlier instruments of Rubens, of Snow, and of Paschen, with
a sensitiveness of about 5°X io-5 per millimeter deflection. To the second
group belongs a more sensitive combination of Paschen's, and the writer's
instrument, in which 1 mm. deflection corresponds to a rise in temperature
of ii°Xio~8 and o°Xio-6C., respectively. In other words, the instru-
ments of the latter group have the same sensitiveness, and any increase in
the same is to be attained by increasing the scale distance; the bolometer
current of 0.04 amp. is about the limit for accuracy. The sensitiveness
of the writer's instruments could have been further increased by length-
ening the scale distance to 2.5 m., when the temperature sensitiveness
would have been 3.6°Xio-6, and by doubling the galvanometer period,
when the sensitiveness would have been q°Xio-7 against Paschen's i°X
io-8. Such a computation is, of course, illusory on account of damping
in the galvanometer. On actual trial (but not for the magnet system
quoted) for a full period of 30 seconds the sensitiveness of the galvan-
ometer was i=7Xio-11 ampere.

Comparison of a Bolometer with a Thermopile.

The efficiency of the bolometer and the thermopile is reduced by losses
due to reflection and radiation from the receiving surface, and by heat
conduction to the unexposed parts. The loss of energy in the thermopile
due to the Peltier effect has been considered in discussing that instrument.
The loss of energy due to reflection is about 4 per cent (Kurlbaum, loc.
cit.). Assuming the bolometer to be made of platinum 0.5X0.002 mm.
cross-section, and the thermopile of 20 junctions of iron and constantan

THE BOLOMETER. 171

wire 0.06 mm. diameter, it can be readily shown that the cross-section of
the thermopile is about 56 times that of the bolometer, and from their
heat conductivities, for the same temperature gradient, that the loss of
heat by conduction in the thermopile is about 100 times that of the bo-
lometer. But the temperature gradient at the ends of a bolometer strip
carrying an electric current may be 500 to ioo° C, so that the heat lost by
conduction may be about the same for both instruments. Since the
temperature of the bolometer is from 500 to ioo° C. higher than the ther-
mopile, the loss of heat per second due to radiation in the former is from
2 to 3 times that of the latter. But the mass of the thermopile is 5 times,
while its specific heat is 3.3 times, that of the bolometer. Hence, to raise
the temperature of thermopile and the bolometer to the same extent, 16
(5X3.3) times as many heat units must be supplied to the former. Since
the loss by radiation is 3 times as great from the bolometer, it will require
about 5 times as long for the thermopile to reach a steady temperature.
In practice, however, on account of the blackening of the surface, the
bolometer is not so quick in its action as here computed.

From these considerations, as well as the mechanical difficulties in
constructing a thermopile of wire less than 0.05 mm. in diameter (and
keeping the resistance low) , it will be seen that the thermopile can not be
made so quick in its action as the bolometer and hence is not so well
adapted where a quick automatic registration of the galvanometer deflec-
tions is desired. But, as will be shown presently, since it is difficult to
read large deflections accurately in less than a 4 to 5 second single swing
of the galvanometer system, a thermopile of 0.06 to 0.08 mm. wire, which
attains a steady temperature in this interval of time, is not objectionable,
and, since it is less disturbed by air-currents (being at room temperature),
it may be the more reliable instrument (see table IV) . Neither instrument,
however, compares with the radiometer in steadiness. The amount of
work done on emission, absorption and reflection spectra, as well as the
accuracy attained, in the infra-red to 15 //, where the radiometer deflections
were, again and again, only a few tenths of a millimeter would not have
been possible with these instruments. In a recent examination of reflection
spectra of minerals, using a thermopile, the accuracy attainable without
repeating the readings several times was far from that of the radiometer,
although the actual deflections were larger.

The present experimental comparison of the thermopile, of 0.08 mm.
iron and constantan wire, and the platinum bolometer was undertaken in
order to determine the accuracy attainable in measuring a constant source
of radiant energy, and hence to learn the feasibility of substituting the
thermopile for the troublesome bolometer. Within experimental error it
has been established by Langley, by Rubens and by Julius that the bolom-
eter (galvanometer) deflections are proportional to the current flowing
through the bolometer, and also to the amount of energy falling upon the

172 RADIOMETRY.

bolometer strip. It remains, therefore, to determine whether the present
bolometer behaves likewise, and also whether the same accuracy is attain-
able with the thermopile.

To this end a bolometer was constructed with the greatest care. It
was annealed before adjusting the resistance of the strips, covered elec-
trolytically with platinum black (after the method of Kurlbaum) and then
smoked over wire gauze over a paraffin candle. The resistances of the
bolometer strips were 1.782 and 1.797 (A = 0.015) ohms, respectively.
After blackening them they were 1.766 and 1.818 (A = 0.052) ohms, re-
spectively. The width of the bright strips was 0.5 mm., which increased
to 0.56 mm. after blacking. The length was 11 mm. and thickness less
than 0.002 mm. The bolometer current was 0.04 ampere and throughout
the following experiments there was no difficulty due to air-currents, or
drift. Magnetic disturbances were at a minimum, and, as a whole, condi-
tions for accurate measurements were as perfect as one would expect.

The thermopile of 0.08 mm. wire (20 junctions covered with a slit
0.5 mm. wide) already described, showed a slight lag in registering the
energy received. Although this was not marked, there was a tendency
for the deflection to creep, instead of stopping abruptly as in the case of
the bolometer. This was most marked in large deflections, and necessi-
tated exposing the thermopile to radiation for a definite time (6 seconds)
and taking the zero at the expiration of an equal interval of time. The
results are given in table VIII. The last two values for the thermopile are
vitiated by radiation from the rotating disk, to be noticed presently. The
results show that there is no great difference in the two instruments. It
was necessary, however, to note the time of exposure of the thermopile,
which is not convenient for large deflections. The estimation of the
relative merits of the bolometer and the thermopile is, therefore, a personal
one, and from the experience gained it may be said that for measuring in-
tense sources the bolometer is the more accurate (when working to 0.5 per
cent) unless great precautions be taken in making the thermopile readings.
The theoretical temperature sensitiveness of the thermopile was con-
siderably greater than that of the bolometer, as was found on subsequent
computation. It may be added, therefore, that if the bolometer sensitive-
ness had been increased, by increasing the current through it, there would
have been greater unsteadiness in the galvanometer readings.

Experiment with a Sectored Disk.

In comparing the relative merits of the bolometer and the thermopile,
the simplest method appeared to be to reduce the intensity of the source
by a known amount, by using a sectored disk, the angular openings of
which are accurately known. It will be noticed that while the ratios of
energy transmitted by the sectored disk were in close agreement in any
series of measurements (see tables VIII and IX) the numerical values were

THE BOLOMETER.

173

in all cases higher than the true ones.

In other words, the disk transmitted
too much energy, or the apparent opening was larger than the true one.
It remained, therefore, to be shown whether this was due to diffraction (of
the very long wave-lengths) or to lack of proportionality in the registering
of the energy by the bolometer and the thermopile. The method of ob-
servation consisted in taking from 5 to 10 readings without the disk, then
a similar number with the rotating disk interposed, followed by a number
without the disk.

Table

VIII. — Comi

'AR1SON OI

Bolometer and Thermopile.

Direct deflection
(mean value).

Deflection with

disk, i8o°=4g.o

(mean value).

Ratio.

Direct deflection
(mean value).

Deflection with

disk, i8o° = 4o.o

(mean value).

Ratio.

Bolometer:

May 24, '07:
12.22
12.16

6.14
6.11

*5°-24
*5°-24

Thermopile:

May 24, '07:

I3-U
14.22
14.30

6.61
7.21
7.24

t5°-26

$5°-7
J50.6

* Nernst heater on 98 volts at 1 m. from bolometer. Galvanometer (full) period 8 seconds undamped; 50 ohms
in series. Total deflection about 80 cm. Bolometer is perfectly steady and comes to rest abruptly. Readings
vary from 0.1 to 0.7 per cent from mean of about 10 in each set. Temperature sensitiveness =6°X 10— 5.

t Conditions same as for bolometer. Thermopile deflection "creeps," and does not come to rest in same time
as galvanometer (on open circuit or with bolometer), due to its larger heat capacity. Galvanometer single swing
of 4 seconds increased to 6 seconds and is fully damped, due to lag of thermopile; 50 ohmsin series with galvanom-
eter. Temperature sensitiveness = 2°X io— 6.

J Source and disk nearer screen. Difficult to read deflection on account of "creeping," which amounts to 1 to
3 mm. Readings made at end of 6 seconds vary by 0.4 per cent from mean; 20 ohms in series with the galvanom-
eter. Temperature sensitiveness = 5°X io— 6.

The first test made was to determine whether the rotating disk (30 cm.
diameter, 1.3 m. from the bolometer) affected the instrument; and it was
found that the resulting deflections 1 to 2 mm. were no larger than those
due to stray radiation reflected from the stationary disk. A heavy black
cardboard shield was then placed between the bolometer and the disk
(0.5 m. from the disk) and similar screens were placed around the source,
which was 2 m. from the bolometer. No radiation was detected from the
stationary disk, whether the open or closed part of the disk faced the
bolometer; but unfortunately this test was not made for the moving disk.
The disk with the 2400 opening gave values 0.5 per cent too high (see
table IX). The results with the 1200 disk (6 openings of 200 each) were
in still greater error. The space between the bolometer and the shield
was then entirely inclosed, and with the disk close to the opening (7X10
cm.) in the shield the discrepancy became still greater. It was then found
that the increased transmission is due to the moving disk and depended
upon the distance of the disk from the screen.

It was further shown that the transmission was proportional to the
speed, so that the 2400 disk (true transmission 66.827 per cent1) gave

1 These disks and their constants were supplied by Dr. Hyde. Bureau of Standards,
Bulletin, 2, p. i, 1906.

174

RADIOMETRY.

values from 69.3 to 77.2 per cent. In fig. 105 are plotted the galva-
nometer deflections for different distances of the disk (abscissae) from the
screen. The latter was 80 cm. (source at 2 m.) from the bolometer, and
had an opening in it which was the size of the openings in the sectored disk.
No radiation was observed from the stationary disk, nor from the shields
back of it when the open sector was before the bolometer. The speed of
the disk was such as is used in photometry, and was kept constant for
each series of observations. In the lower curve for the 2400 disk the
speed was slow and there was a flicker on viewing it. The curves show
that the maximum radiation occurs when the disk is about 6 cm. from
the shield; and it disappears immediately on stopping the disk.

Table IX. — Reliability of Bolometer Measurements.

Direct deflection, a

Deflection with
disk, a

Ratio.

Direct deflection, a

Deflection with
disk. 0

Ratio.

Disk opening, 24o° = 66.827.

Disk opening, i2o° = 33.406.

9.42 cm.

9-35
11.82

9.11

8-94
I3-3I

6.33 cm.
6.28

7-95

6.12

6.04

8.99

b 67.23
b 67.25

667.25

667.2

C67.5

c 67.6

15.82
11.56
12.31
30.40

13-75
14-15
17.04

5-65
4.14

4.28

IO-53
4.70

4-75
5.68

635-7°
/35-75
£34-8
^34-7
*34-S
33-56
33-38

Disk opening, 1800 =50.125.

17.17 cm.
17.21

8.65
S.64

<*5°-37
d 50.20

a Mean of 6 to 10 readings.

6 Galvanometer period 8 seconds, vibration undamped; 20 ohms in series with galvanometer. Temperature
sensitiveness =6°X io— 6 C High values due to radiation from moving disk, which is 0.5 m. from screen.

c Galvanometer period 14 seconds and vibration just damped. Temperature sensitiveness =i°.2Xio- SC

d Nernst heater on 98 volts at 1 m. from bolometer. Galvanometer period 8 seconds. 30 ohms in series with
galvanometer. Total deflection about 80 cm. Individual deflections vary 0.2 to 0.5 per cent from mean.

e Galvanometer period 14 seconds.

/ Galvanometer period 8 seconds. Disk better shielded than in previous experiments and closer to screen —
6 cm. from it. Heater 150 cm. from bolometer; screen at 80 cm.

g 14 ohms in galvanometer circuit. Galvanometer period 14 seconds. Nernst heater on 74 volts.

h No resistance in galvanometer circuit; results show that high value is not due to lack of proportionality of
galvanometer deflections.

* Nernst heater on 95 volts. 20 ohms in galvanometer circuit. Total deflection about 45 cm.

The motor was run continuously for a complete series of measurements
and no deflections greater than i to 2 mm. were observed from it or the
disk, immediately after stopping the rotation. In these tests the motor
was shielded from the bolometer. On removing the shield and the disk
and on running the motor the deflections were from 1 to 3 mm. The
whole shows that the sectored disk is not as applicable as one would sup-
pose, unless one determines the corrections which in two series of experi-
ments were found to be in very close agreement.1 In the present curves

1 The sector was rotated at a higher speed than actually required with a bolometer. By
means of suitable pulleys the speed may be reduced to perhaps 35 that used in the present
test, which would decrease the errors.

SUMMARY.

175

the galvanometer was at its full sensitiveness — no resistance in series —
so that in the actual experiments (table IX) the error was less exaggerated.
For the 2400 disk where the error in the deflections was only 1 or 2 mm.
the correction reduces the observed values close to the true one.

From the consistency of the ratios in each series, which is of the order
of 0.2 to 0.3 per cent, it will be noticed that the bolometer is a very reliable
instrument in spite of its mechanical weakness.

6 8 10 IZ 14- IQ 18 .20

Fig. 105. — Radiation due to rotating sectored disk.

24 26C777

VI. SUMMARY.

The present paper deals with four instruments for measuring radiant
energy, viz, the radiomicrometer, the linear thermopile, the radiometer,
and the bolometer with its auxiliary galvanometer.

As a result of this historical inquiry and by experiment it was shown
that the radiomicrometer is capable of great improvement, by reducing
its weight, by lengthening its period, and by placing it in a vacuum. It
was further shown that on account of para- and dia-magnetism the sensi-
tiveness of the radiomicrometer is very limited, perhaps only a fifth of the
best bolometers described.

It was also shown that the Rubens thermopile is as sensitive as the
best bolometer, and that its heat capacity can be greatly reduced by using
thinner (0.06 to 0.08 mm. diameter) wires, which are made shorter, thus
keeping the resistance low. The computed errors due to the Peltier effect
are about 1 part in 300. The thermopile is not so well adapted as is the
bolometer for instantaneous registration of radiant energy and it does not
admit so great a range in variation of sensibility; but, on account of its
greater steadiness, it commends itself for measuring very weak sources of
radiation, e.g., the extreme ultra-violet and infra-red region of the spectrum.

By a direct comparison it was shown that the radiometer can be made
just as sensitive as the bolometer, but its period will be much longer, It

1 76 RADIOMETRY.

was found that the radiometer is not selective in its action, and, hence,
that it can be used for measuring ultra-violet radiation. The main objec-
tion to the use of a radiometer is its long period; but, since it is easily
shielded from temperature changes, and since it is not subject to magnetic
perturbations, this long period is of minor importance so long as we are
dealing with a constant source of radiation. In spectrum energy work its
usefulness is limited to the region in which the window is transparent —
to 20 // and from 40 to 60 [i by using quartz. The fact that the deflections
of the radiometer can not be obtained in absolute measure is a minor
objection, since in but few cases (thus far at least) has it been necessary
to thus obtain the deflections. The action of a radiometer is somewhat
analogous to a photographic plate, in that it will detect weak radiation,
provided one can wait for it, and, on account of its great steadiness, is,
of all the instruments considered, probably the best adapted in searching
for infra-red fluorescence.

A bolometer installation is so distributed that it is difficult to shield
from temperature changes. In spite of its small heat capacity the bo-
lometer has a "drift" due to a slow and unequal warming of the strips.
Air-currents which result from the hot bolometer strips also cause a varia-
tion in the deflections of the auxiliary galvanometer. Nevertheless, despite
these defects it is the quickest acting of the four instruments considered,
and is the best adapted for registering the energy radiated from a rapidly
changing source. For precision work it is necessary to keep the bolometer
balanced to less than 1 cm. deflection.

The auxiliary galvanometer is the main source of weakness in measur-
ing radiant energy, and in places subject to great magnetic perturbations
a period greater than 5 seconds, single swing is to be avoided. Hence,
although a greater sensitiveness is possible, the working sensibility of the
various galvanometers studied is of the order of i=2Xio-10 ampere per
millimeter deflection on a scale at 1 m. Under these conditions the various
bolometers used were (as a fair estimate of the recorded data) sensitive to
a temperature difference of 4°Xio-5 to 5°Xio~6 per mm. deflection, on a
scale of 1 m. The galvanometer sensibility was found to be closely pro-
portional to the period.

A direct comparison of the thermopile and the bolometer shows that
there is little preference, other than a personal one, in these two instru-
ments.

The use of a rotating sectored disk for reducing the intensity of the
source is liable to introduce errors, which must be taken into account.

APPENDIX III.

ADDITIONAL DATA ON SELECTIVE REFLECTION AS A FUNCTION
OF THE ATOMIC WEIGHT OF THE BASE.

As this work goes to press the experiments of Morse1 have been pub-
lished, and since it contains considerable new data for the region of the
spectrum from 10 to 15 fi, it is included here for the sake of completeness.

In that paper considerable comment is made upon the fact that the
writer (see Carnegie Publication No. 65) missed the reflection bands in
calcite and in magnesite, previously found by Aschkinass at n to 12//.
In reply it may be stated that these two substances were examined in the
preliminary work on reflection spectra, in order to get a check on the
calibration; and, on finding the reflected energy very weak, no attempt
was made to locate the bands known to be at 11 to 12 /<. In this work
a Rubens thermopile (heavy wires) was used, which was sluggish and was
disturbed by air-currents. Although the sensibility was higher than in
the radiometer previously used, the small deflections were not so reliable
and no attempt was made to locate weak reflection bands beyond n /«,
such as are found in the carbonates. This demonstrates the superiority
of the radiometer for measuring weak radiation.

The investigation of weak reflection spectra in the extreme infra-red is
accomplished under great difficulties, and Morse has done an excellent
service in obtaining data in this region of the spectrum. He used a 35 cm.
focal length mirror spectrometer as compared with the writer's 52 cm.
focal length mirrors. In the larger spectrometer the energy in the spec-
trum is much weaker, while the resolution is greater. The shorter focus
does not militate against the results, however, which show that the simple
atomic weight relation among the carbonates found by the writer at 6 to
8 p. holds for the long wave-lengths at n to 15 fi, where the dispersion is
considerably greater. The writer found the reflection band of the carbon-
ates at 6 fi very complex (see Chapter III) and it would be interesting
to learn whether the bands at 11.4 to 15 fi are likewise. In table X are
given the maxima of the reflection bands of the carbonates examined by
Morse. It contains one new substance, MnC03, not examined by the
writer. The values of the maxima of the first band are not always in
agreement, but this appears to be due to the difference in resolving power
of the two instruments. In fig. 43 are plotted the wave-lengths of the

1 L. B. Morse: Astrophys. Jour., 26, p. 225, 1907.

177

i78

SELECTIVE REFLECTION BANDS.

reflection maxima of the bands at 11.5 and 14.5 /x against the atomic

weight of the base. From this it will be seen that the rate of shift of the

band with increase in atomic weight of the base is greater for the bands at

1 1.5 and 14.5 pt than for those at 6.6 and 7.0^, just as was found for the

bands of the sulphates at 4.6 /ji, at 6.2 to 6.6 ft, and at 8.2 to 9.3 /« (see

figs. 44 and 45).

Table X.

Substance.

Magnesite . . .

Calcite

Aragonite. . . .
Rhodochrosite

Siderite

Smithsonite .
Strontianite .
Witherite . . .
Cerussite ....

Chemical
composition.

MgC03

CaCOs

CaCOs

MnCOs

FeCOs

ZnCOs

SrC03

BaC03

PbCOa

Atomic

weight of

base.

24.2
39-7
39-7
54-6

55-5

64.9

86.9

136.4

205.4

Reflection maxima.

Band i.

6.5 M

6.6

6.65

6.63

6.60

6.7

6.76

6.86

7.2

Band 2.

Band 3.

11. 2 M
II.31

"•55

11.47 +

11.47-

11.38

11.56

11.60

11.94

T3-9/*

14.2

14.2
14.0—

13-9-

13.6

14-37

14-5

14.8

By arbitrarily selecting KN03 and AgN03 from the nitrates, Morse
found the line drawn through the maxima of the reflection bands was
"approximately parallel" to those of the carbonates (see fig. 43). More-
over, the line drawn through the maxima of the sulphates, at 8.6 to 9 //,
is also closely parallel with those of the carbonates. From this he was
led to suspect that the shift of the band with increase in the atomic weight
of the base is of the same order of magnitude in carbonates, nitrates, and
sulphates. If the data had been plotted to a larger scale, as the accuracy
of the values of the maxima seem to permit, then the lines would not even
be approximately parallel, as will be noticed in figs. 43 and 44.

After examining the reflection and transmission spectra, one or both,
of over 300 substances, the observations lying in the region of the spectrum
from 0.5 to 30 + /*, I have found that it is an easy matter to work out all
sorts of fantastic relations, only to learn, after gathering more data, that
the whole thing was an illusion. For this reason it seems to me that the
linear relation between the weight of the element, combined with equal
amounts of oxygen in the acid radical found by Morse by arbitrarily
selecting maxima of reflection bands, is misleading. From the earliest
work of Abney and Festing to the latest (theoretical) work of Einstein, it
has been generally accepted that oxygen is the active element in causing
(at least in " sharpening ") certain bands, just as sulphur has been found
quite inactive. The great groups of chemically related compounds have
been found to have similar absorption and reflection spectra, but no sim-
ple relation could be established between the spectra of the groups of
compounds. The present simple relation results from selecting particular
reflection bands found in certain carbonates, nitrates, sulphates, and sili-

THE ATOMIC-WEIGHT LAW.

179

cates. Hence, the selection of KN03 (max. at 7.1 /«) from the nitrates by
Morse seems arbitrary, for Pfund (loc. cit., see Carnegie Publication No.
65) has given a large number of nitrates which have a band in common at
7.45 11. This seems to be a characteristic band of the nitrates, and AgNOs
might have been selected instead of KN03. A characteristic band of the
sulphates appears to be at 9.1 il (harmonic with the one at 4.55 li), while
Morse selected a less frequent one at 8.6 li. From the silicates, MgSi03,
with an insignificant band at 9.1 /<, was selected, while Na2Si03, with a
sharper band at 9.9 li, and Zn2Si04, with a group of still more intense
bands at 10.1, 10.6, and 11 li, respectively, were not considered.

In PbM04 the maxima lie in the region of 11.75 an<^ 13 /*> while in
CaW04 there is a large reflection band with maxima at about 11.4, 11.9,
and 12.5/*, respectively. These data including Morse's are plotted in
fig. 106 and tabulated in table XI, from which it will be observed that

Table XI.

Substance.

Chemical
formula.

Atomic weight
of base.

Weight with
48 gr. of O.

Position
of band.

Calcite

CaCOs
KNO3
CaS04
MgSi03

ZrSiCU

PbMoCu

CaWCu

39-7
38.9
39-7
24.2

90.6
206.9

39-7

1 2 gr. of C
14 gr. of N
24 gr. of S
28 gr. of Si

28 gr. of Si

72 gr. of M

138 gr. of W

6.6 /j-

7-iS (7-°5)

8.6

9.1

10, io.6, 11 m

"•75,13
11.4, 11.9, 12.5

Potassium Nitrate

Anhydrite

Enstatite

Zircon

Wulfenite

Scheelite

the graph is not a straight line, and that if any relation exists it is a very
complex curve, showing that for the region up to 10 /x the atomic weight
of the element in the acid radical has a great effect in shifting the maxi-
mum, while beyond this point the atomic weight of the element united
with oxygen is of minor importance. In fact, the base and the element
united with the oxygen in the acid radical seem to influence each other.
As an illustration of the arbitrariness in selecting bands to establish
relations like the aforesaid, calcite (CaC03) may be noticed, in which the
reflection band is complex with maxima at 6.4 (?), 6.5, 6.6, and 7 ll, while
in its isomer, aragonite, the maxima are at 6.4 (?), 6.52, 6.74, and 7 (?)/<.
In SrC03 the band with a maximum at 6.67 jx could not be resolved even
with a fluorite prism. Hence, one is at a loss to know which band (at
6 to 7 /i) to select from the carbonates to compare with the sulphates,
nitrates, and silicates. On the other hand, in the carbonates and in the
sulphates the maxima of the bands have been plotted in their order of
occurrence, which would seem to eliminate personal bias in the selection
of maxima. From this it would appear that the simple, linear relation
between the atomic weight of the base and the maxima of the reflection

i8o

SELECTIVE REFLECTION BANDS.

bands in the carbonates and in the sulphates is in agreement, at least as
a first approximation. Even with these data at hand, speculation in regard
to dynamical relations among atoms in the molecules had better be post-
poned until more data have been procured.

160

160

140

J 20

©

'§ 100

CD

C

£
o
<J

-t->

-C
CT)

'co

5

80

60

40

20

X X

*CaW04

CO

•ho

£

o5

o

X

PbMo 04

a:

"5

© c

3^*

V®" — "

"ST* •

xZrSi04

4 5 6 7 8 9/0//

Fig. io6. — Maxima of reflection bands.

/2

/3/-i

The data on the reflection bands of the carbonates at n to 15 p. is of
interest in connection with the question of the value of the extinction
coefficient necessary to give rise to selective reflection. In Carnegie Pub-
lication No. 65, page 70, is given the transmission band of a thin section
of calcite, from which it will be observed that the absorption band at 11.4 fi
coincides with the band found by reflection. It would seem desirable to
examine the transmission of thin sections of carbonates in this region of
the spectrum, using polarized energy, to compare with the intensity of
the bands found by reflection. The reflection bands at 14 /j. were fre-
quently found to be extremely weak, and in view of the importance of the
bearing of the results upon the whole subject, it seems highly desirable to
examine the transmission spectra of thin sections of these minerals (using
preferably polarized energy), to verify the aforesaid observations. (See
foot note on page 30.)

-0CA{

ADDENDUM.

Mr. Morse has recently presented "Additional observations on the
selective reflection of salts of oxygen acids" (Washington meeting, Amer-
ican Physical Society, April, 1908). He chose substances in which the
" weight of the acid-forming element is not greater than the weight of the
oxygen with which it is combined." These substances are: CaC03 with
a reflection maximum at 6.6 fi; KN03 at 7.1 fi; AgN03 at 8 fi; CaS04 at
8.6 fi; KC104 at 9 p; CaSi03 at 9.2 ft; KC103 at 9.9 fi; KMn04 at 10.9 fi;
PbCr04 at 11.5 ft; and CaTiOs at 14.2 ft. These maxima lie close to the
line drawn through them, when plotted against the weight of the oxygen.
He excluded PbM04, CaW04, etc., because the weight of the acid-form-
ing element is greater than that of the oxygen present. Of course, if we
admit the validity of such a procedure the rule is proven; but a rule loaded
down with exceptions can not prove satisfactory, and much as all spectro-
scopists wish to establish such a simple relation between spectra of different
groups of compounds, it should be along lines of less arbitrary elimination.

On the other hand, this simple atomic-weight relation seems to hold
for substances belonging to the same
group of compounds, even for re-
mote parts of the infra-red spec-
trum, as was shown by Nichols
and Day (Washington meeting,
American Physical Society, April,
1908). They found a band of
residual rays in SrC03 at 43.2 ft
and in BaC03 at 46.5 ft, for the
carbonates (see fig. 45).

If we take the ratio of the in-
crease in atomic weight of the base to the position of the maximum, as
read from the graphs in figs. 43 to 45, the value for the group of bands
at 6.5 ft is about 500 to 1, at 11.5 u it is 260 to 1, and at 45 ft it is 20 to 1.
The graph (fig. 107) of these values appears to be an hyperbola. If such
a relation really exists and if the carbonates have a band in the region of
30 ft, corresponding to the CaC03 and MgC03 bands, then the aforesaid
ratio of atomic weight to position of the maximum is about 90 to 1, and
one would expect to find a maximum for PbC03 at 31.5 ft and SrC03
at 30 p. However, from the variation in complexity of the bands at 6 to
7 fi and in intensity of the bands at 11 to 14 ft, it is evident that it is use-

181

-a

600

O 400

cc

200

\x

V

\

\

\v

X—

10 20 30 40 SOJU

Fig. 107. — Mean position of groups of residual rays.

1 82 ADDENDUM.

less to attempt to predict their behavior in the remote infra-red. On the
whole, the present data indicate that the greater the wave-length of the
maximum the greater the shift of that maximum with increase in atomic
weight of the base.

The latest conclusion (Morse, Phys. Rev. 26, p. 526, 1908) is that
" present data do not form any reasonable basis for assuming that all the
reflection bands in even the simpler oxygen acids are connected with each
other by such a simple relation as that found to hold roughly for the first
bands, and which may be found to hold also for the second bands in the
salts examined."

INDEX OF SUBSTANCES.

PAGE.

Adularia 98

Albite 97

Aluminum 72

Aluminum oxide 107

Ammonium alum 50

Ammonium-iron alum . 50

Amphibole 100

Apatite 33, 107

Aragonite 22, 34, 178

Asphaltum 48

Azurite 1 1 , 34

Beryl 18, 34, 104

Beryllium oxide 115

Borax 50

Calcite.12, 22,30, 34, 122, 178

Calcium 72

Calcium oxide 121

Calcium sulphate 1 20

Carbon 46, 73, 91

Carbonates 61, 62

Carborundum 36, 43

Celestite 33

Cerium oxide 112

Cerussite 10, 34, 178

Chalcocite 14

Chromite 15

Chromium oxide 119

Chrysocolla 11

Cobalt glass 54

Cobalt oxide 119

Copper 72

Copper oxide 119

Corundum 17, 34

Covellite 14

Cryolite 32, 34, 43

Cyanine 33

Cyanite 18, 34

Diamond 46

Diaspore 32

PAGE.

Didymium nitrate 50

Dolomite 1 2, 34

Erbium oxide 117

Feldspar 99

Fluorite 46

Glass 45, 102

Glass (colored) . .55, 56, 103

Gold 36, 53

Gold (colloidal) 53

Graphite (arc) 74

Hematite 15, 119

Iron oxide 15, 119

Kuntzite 42

Lanthanum nitrate .... 50

Lead oxide 118

Liquid glass 50

Magnesite 13, 23, 31

Magnesium oxide . .102, in

Magnetite 15

Malachite n, 34

Manganous oxide 118

Mica 29

Molybdenite 13, 41

Neodymium nitrate .... 51

Neodymium oxide 118

Nernst glower 81, 126

Nickel 53, 71

Nickel oxide 120

Nitrocellulose 43

Oligoclase 100, 129

Orthoclase 99

Osmium 90

Phosphorus 45

Platinum 119, 124

Porcelain 102

Potassium 77, 78

Potassium alum 50

Potassium permanga-
nate 50

PAGE.

Pyrrhotite 14

Quartz (crystalline) . 21, 34,

36, 45, 140

Quartz (amorphous) ... 21,

34, 115

Rhodochrosite 178

Rutile 17. 34, 105

Scheelite 16, 34

Selenium 15

Siderite 12, 34, 44, 178

Silicates 66

Silver 36, 52

Smithsonite . 10, 23, 34, 178

Sodium 74, 77

Sodium silicate 19

Sphalerite 57

Spodumene . . 19, 33, 34, 104

Stannic acid 119

Stibnite 33

Strontianite . n, 24, 34, 178

Sulphates 62, 63

Sulphuric acid 50

Tantalum 92

Thorium oxide 112

Topaz 19, 34, 105

Tricalcium phosphate . . 121

Tungsten 92

Uranium oxide 112

Vanadium oxide 115

Water 50

Water vapor 149

Witherite ... n, 24, 34, 178

Wollastonite 101

Wulfenite 16, 34, 44

Yttrium chloride 51

Yttrium oxide 116

Zincite 17, 118

Zircon 16, 34

Zirconium oxide. . 108, in

183

SUPPLEMENTARY INVESTIGATIONS

OF

INFRA-RED SPECTRA

Part V — Infra-Red Reflection Spectra
Part VI — Infra-Red Transmission Spectra
Part VII — Infra-Red Emission Spectra

BY

WILLIAM W. COBLENTZ

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

1908

MBL WHO! LIBRARY

lfilF 5