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RAYS OF POSITIVE ELECTRICITY

AND THEIR APPLICATION TO

CHEMICAL ANALYSES

MONOGRAPHS ON PHYSICS

EDITED BY

SIR J. J. THOMSON, O.M., F.R.S.

MASTER OF TRINITY COLLEGE, CAMBRIDGE
AND

FRANK HORTON, Sc.D.

PROFESSOR OF PHYSICS IN THE UNIVERSITY OF- LONDON

8vo.

RAYS OF POSITIVE ELECTRICITY AND THEIR
APPLICATION TO CHEMICAL ANALYSIS. By Sir
J. J. THOMSON, O.M., F.R.S., Master of Trinity College,
Cambridge. With Illustrations.

MODERN SEISMOLOGY. By G. W. WALKER, A.R.C.Sc.,
M.A., F.R.S., Deputy University Lecturer in Astrophysics
in the University of Cambridge. With Plates and Diagrams.

THE SPECTROSCOPY OF THE EXTREME ULTRA-
VIOLET. By THEODORE LYMAN, Ph.D., Assistant Professor
of Physics, Harvard University. With Diagrams.

THE EMISSION OF ELECTRICITY FROM HOT BODIES.
By O. W. RICHARDSON, F.R.S., Wheatstone Professor of
Physics, King's College, London. With Diagrams.

RELATIVITY, THE ELECTRON THEORY, AND GRAVI-
TATION. By E. CUNNINGHAM, M.A., Fellow and Lecturer
St. John's College, Cambridge. With Diagrams.

THE PHYSICAL PROPERTIES OF COLLOIDAL SOLU-
TIONS. By E. F. BURTON, B.A., Ph.D., Associate Professor
of Physics, University of Toronto. With Illustrations.

LONGMANS, GREEN AND CO.
39 PATERNOSTER ROW, LONDON, E.G. 4
NEW YORK, BOMBAY, CALCUTTA, & MADRAS

•RAYS OF
POSITIVE ELECTRICITY.

AND THEIR APPLICATION TO

CHEMICAL ANALYSES

BY >,-.•,;..,';

SIR J. J. THOMSON, O.M., F.R.S.

J J \\

MASTER OF TRINITY COLLEGE, CAMBRIDGE
PROFESSOR OF EXPERIMENTAL PHYSICS, CAMBRIDGE

WITH ILLUSTRATIONS

SECOND EDITION

LONGMANS, GREEN AND GO.

39 PATERNOSTER ROW, LONDON

FOURTH AVENUE & 30TH STREET, NEW YORK
BOMBAY, CALCUTTA, AND MADRAS

1921

QC7//
"T45

PREFACE TO SECOND EDITION

THIS edition contains a considerable amount of new
matter both in the text and in the plates. I have
paid special attention to those properties of the
Positive Rays which seem to throw light on the
problems of the structure of molecules and atoms and
the question of chemical combination. The hope
expressed in the first edition that the method of
Positive Rays would be of 'service in connection with
important chemical problems has been fulfilled to a
remarkable extent by the researches of Mr. Aston
and others on the determination of atomic weights
and the detection of isotopes. I am convinced that
as yet we are only at the beginning of a harvest of
results which will elucidate the process of chemical
combination, and thus bridge over the most serious
gap which at present exists between Physics and
Chemistry.

I regret the long delay in the issue of this edition ;
it has been due to the War and the pressure of many
duties. I have much pleasure in thanking Mr. W. H.
Hayles, of the Cavendish Laboratory, for his help in

the preparation of the plates.

J. J. THOMSON.

THE LODGE,

TRINITY COLLEGE, CAMBRIDGE.
Augusf, 1921.

4760 4 2

PREFACE TO FIRST EDITION

I HAVE endeavoured in this book to give some account
of the experiments on Positive Rays which have been
made at the Cavendish Laboratory during the last
seven years, and which have been the subject of papers
scattered through the Philosophical Magazine, the
Proceedings of the Royal Society, and the Proceedings
of the Cambridge Philosophical Society. I have, in
addition, included a short account of the researches of
Stark and others on the Doppler effect in Positive
Rays and of Gehrcke and Reichenheim's experiments
on Anode Rays, as these, those on the Doppler effect
especially, are very closely connected with the results
obtained by the very different methods described in
the earlier part of this book. I have described at some
length the application of Positive Rays to chemical
analysis ; one of the main reasons for writing this book
was the hope that it might induce others, and especially
chemists, to try this method of analysis. I feel sure
that there are many problems in Chemistry which could
be solved with far greater ease by this than by any
other method. The method is surprisingly sensitive —
more so even than that of Spectrum Analysis, requires

an infinitesimal amount of material, and does not require

vii

viii PREFACE

this to be specially purified : the technique is not diffi-
cult if appliances for producing high vacua are available.
I am glad to be able to take this opportunity of ex-
pressing my obligations to Mr. F. W. Aston, B.A.,
and Mr. E. Everett. My thanks also are due to
the President and Council of the Royal Society for
permission to use the blocks illustrating the Bakerian
Lecture.

J. J. THOMSON.

CAMBRIDGE,
4 October, 1913.

CONTENTS

PAGB

RAYS OF POSITIVE ELECTRICITY i

RECTILINEAR PROPAGATION OF THE POSITIVE RAYS . .__ . 5

DOUBLE AND HOLLOW CATHODES 5

; ON THE NATURE OF THE POSITIVE RAYS, THEIR DEFLECTION BY

ELECTRIC AND MAGNETIC FORCES 16

ELECTROSTATIC DEFLECTION OF THE PARTICLE . . . .19

WIEN'S PROOF OF THE MAGNETIC AND ELECTRIC DEFLECTION OF

THE RAYS .... 22

V EXPERIMENTS MADE BY THE AUTHOR ON POSITIVE RAYS . . 25

EFFECT AT VERY Low PRESSURES . 27

METHOD OF HOT CATHODES 35

ASTON'S Focus METHOD . . ... . . .36

DEMPSTER'S METHOD . . , ... J; .' ... . .., . . . 40

DISCUSSION OF THE PHOTOGRAPHS . . . . . 4!

Loss AND GAIN OF CHARGE BY PARTICLES * . . . 48

lONIZATION BY POSITIVE RAYS 54

^ SECONDARIES 60

NEGATIVELY CHARGED PARTICLES 70

MULTIPLY CHARGED PARTICLES 77

CONCENTRATION OF THE POSITIVE RAYS ROUND DEFINITE VELOCITIES 84

ORIGIN OF THE CHARGED ATOMS AND MOLECULES IN THE POSITIVE

RAYS 88

ELECTRIC FORCE IN THE DARK SPACE ...... 108

METHOD OF CONSECUTIVE SYSTEMS OF ELECTRIC AND MAGNETIC

FIELDS 117

METHODS FOR MEASURING THE NUMBER OF THE POSITIVELY

ELECTRIFIED PARTICLES 120

CHARGES CARRIED BY THE ATOMS FROM A MOLECULE OF A COM-
POUND GAS 128

x CONTENTS

PACK

RETROGRADE RAYS 134

ANODE RAYS . . . . . . . . . •. 142

DOPPLER EFFECT SHOWN BY POSITIVE RAYS , •-. . . . 148

POLARIZATION OF LIGHT FROM POSITIVE RAYS . . . .165

SPECTRA PRODUCED BY BOMBARDMENT WITH POSITIVE RAYS . 169
DISINTEGRATION OF METALS UNDER THE ACTION OF POSITIVE RAYS 171
ABSORPTION OF GASES IN THE DISCHARGE TUBE . . . - . 178
USE OF POSITIVE RAYS FOR CHEMICAL ANALYSIS . . ... . . 179

DISCUSSION OF PHOTOGRAPHS . * . ,.'*,•.. .188

EXAMINATION OF THE GASES GIVEN OUT WHEN SOLIDS ARE BOM-
BARDED BY CATHODE RAYS . * 190

NATURE OF X3, THE SUBSTANCE GIVING THE "3" LINE .. . 196

ORIGIN OF THE LINE #*/<>= 3-5 . . . , . . . 203

CONDENSATION OF GASES ON THE SURFACES OF SOLIDS . . 207

LINES DUE TO NEON . . . . . . . • . .212

DETERMINATION OF ATOMIC WEIGHTS BY POSITIVE RAYS . , . 216

STRUCTURE OF ATOMS AND MOLECULES . » ... . . . 222

INDEX 235

LIST OF PLATES

PLATE I (Fics. i, 2, 3, 4)
PLATE II (Fics. i, 2, 3, 4)
PLATE III (Fics. i, 2, 3, 4)
PLATE IV (Fics. i, 2, 3, 4)

PLATE V (Fics. i, 2)
PLATE VI (Fics. i, 2, 3, 4)
PLATE VII (Fics. i, 2, 3, 4)
PLATE VIII (Fics. i, 2, 3)
PLATE IX

/ At end of Volume

RAYS OF POSITIVE ELECTRICITY

THE positive rays were discovered by Goldstein in I886.1
His apparatus is represented in Fig. I ; the cathode K which
stretched right across the tube r was a metal plate through
which a number of holes were drilled, the diameter of the
holes being considerably less than the thickness of the plate ;
the axes of the holes were at right angles to the surface of the
plate ; the anode a was at the end of the lower part of the
tube. The pressure of the gas in the tube was so low that
when the electrodes K and a were connected with the ter-
minals of an induction coil and a discharge passed through
the tube, the dark space below the cathode was well developed.
Under these circumstances Goldstein found that slightly
diverging bundles of a luminous discharge streamed through
the holes in the cathode into the upper tube. The colour of
the light in these bundles depended on the kind of gas with
which the tube was filled : when it was air the light was
yellowish, when it was hydrogen, rose colour. These rays
can be shown very conveniently by the use of the tube
represented in Fig. 2 ; a form also used by Goldstein in his
earlier experiments. The cathode which fills the middle of
the tube is a flat disc with a hole in it ; a metal tube fitting into
the hole is soldered on to the cathode, the length of the tube

1 Ober eine noch nicht untersuchte Strahlungsform an der Kathode inducirter
Entladungen. « Berl. Ber.," XXXIX, p. 691, 1886; " Wied. Ann.," 64,
p. 38, 1898.
B

RAYS: OF POSITIVE ELECTRICITY

should be several times the diameter of the hole and its axis
perpendicular to the plane of the cathode ; the anode is a
wire fused into the upper part of the tube. When the pres-
sure of the gas is properly adjusted, the positive rays stream
through the tube into the lower part of the vessel while

\ /

FIG. 2.

the cathode rays shoot upwards. The contrast between the
colour of light due to the positive rays and that due to the
cathode rays is, when some gases are in the tube, exceedingly
striking. Of all the gases I have tried for this purpose neon
gives the most striking results, for with this gas the light due

COLOURS PRODUCED BY RAYS 3

to the positive rays is a most gorgeous red, while that due to
the cathode rays is pale blue ; with helium the positive rays
give a reddish light, while that due to the cathode rays is
green. The spectroscopic examination of the light due to the
positive and cathode rays reveals interesting differences which
we shall have to consider later ; we may anticipate, however,
so far as to say that the character of the light produced by
the positive rays is similar to that of the velvety glow which,
in an ordinary discharge tube with an unperforated cathode,
spreads over the surface of the cathode.

As in Goldstein's experiments these rays were observed
streaming through holes or channels in the cathode ; he
called them " Kanalstrahlen." Now that they have been
proved to be streams of particles, the majority of which are
positively electrified, it seems advisable to call them positive
rays, as indicating their nature ; the name Kanalstrahlen only
suggests the methods of demonstrating them.

Many important properties of the positive rays can be
easily demonstrated by the use of a tube like that shown in
Fig. 2. For example, when the rays strike against the glass
sides of the tube they make the glass phosphoresce. The
phosphorescence produced by the positive rays is of a different
colour from that produced by the cathode rays and is in
general not nearly so bright. With German glass the positive
and cathode rays both produce a greenish phosphorescence,
though the greens are of different shades. With some sub-
stances the contrast is much more striking : for example, with
fused lithium chloride the phosphorescence produced by the
positive rays is an intense red showing when examined by
the spectroscope the red lithium line; the phosphorescence
due to the cathode rays is a light blue giving a continuous
spectrum. The phosphorescence due to the positive rays is a
most valuable aid for studying the way the rays are deflected

4 RAYS OP POSITIVE ELECTRICITY

by electric and magnetic forces, and it is important to find
the substance which gives the brightest phosphorescence.
The substance which I have found most useful is willemite,
a natural silicate of zinc. The mineral should be ground into
as fine a powder as possible, the powder shaken up in alcohol
so as to form a suspension, which is allowed to deposit slowly
on a glass plate ; by this method the glass is covered with an
exceedingly even deposit of the willemite. After continued
exposure to the positive rays the brightness of the phosphor-
escence diminishes and ultimately disappears, so that for the
detection of these rays the willemite must be renewed from
time to time. Some substances deteriorate more rapidly than
others ; for example, zinc blende phosphoresces very brightly
under the positive rays, but, as far as my experience goes,
it deteriorates much more quickly than willemite, so that
when the observations have to last for any considerable time
the willemite is preferable. Since phosphorescence necessarily
involves the transformation of the material from one state to
another some decay is inevitable. A more sensitive, and for
many purposes more convenient, way of registering the de-
flection of the positive rays is to take advantage of the fact
that, when these rays strike against a photographic plate, they
affect the plate at the place of impact and thus a permanent
record of the position of the rays can be obtained. The action
of the rays on the plate differs from that of light, since they
do not use the whole thickness of the film but only a layer
close to the surface, so that it does not follow that the most
"rapid" photographic plates are the most sensitive to the
positive rays. The choice of the most suitable type of plate
is a matter of great importance in -many investigations. The
most sensitive plates for the detection of the positive rays
would be those having very thin films containing as much
silver as possible. I have tried the old Daguerreotype process

SANDWICH CATHODES 5

instead of the usual dry plate method, but without much
success. Schumann plates (Baly's " Spectroscopy," p. 359)
which are now in commerce are the most sensitive, but for
general use I have found Paget process plates the most useful,
they are sensitive and give well-contrasted photographs. The
plates known as " Imperial Sovereign " also give very good
results.

The positive rays gradually remove any thin deposit of
metal which may be on the parts of the tube against which
they strike. Such thin deposits can readily be produced by
running an electric discharge through the tube when it contains
gas at a low pressure, using for the cathode a piece of the
metal it is wished to deposit on the glass. The metal cathode
" splutters " and the metal is deposited as a thin layer on the
glass near the cathode.

RECTILINEAR PROPAGATION OF THE
POSITIVE RAYS

This can be shown by placing a solid obstacle in the path
of a pencil of positive rays : this casts a shadow on the part
of the tube which was phosphorescing under the impact of
these rays. Comparing the shape of the shadow with that of
the obstacle, it is found that the shadow is very approximately
the projection of the outside of the solid on the walls of the
tube by lines passing through the hole in the cathode through
which the pencil of positive rays emerges.

DOUBLE AND HOLLOW CATHODES

Goldstein l found that positive rays came freely from the
space between two parallel plates metallically connected to-
gether and used as a cathode for the discharge through gas
at a low pressure. The streams of positive rays are accom-

1 Goldstein, "Phil. Mag.," VI, p. 372, 1908.

6 RA YS OF POSITIVE ELECTRICITY

panied by cathode rays, and the discharge from a " sandwich "
cathode of this kind, through a gas where there is a marked
difference in colour between the luminosity produced by the
cathode and positive rays, presents some very interesting
features. Hydrogen, and to a still greater degree helium and
neon, are suitable gases for this purpose. When a cathode
formed of two parallel equilateral triangles connected together
by a wire is used for the discharge through helium at a low
pressure, the discharge near the cathode has the appearance
represented in Fig. 3. From the points of the triangle stream

FIG. 3.

pencils of luminosity showing the characteristic red colour of
the positive rays in helium, while the middle points of the
sides are the origin of streams of greenish luminosity, the
colour of the path of the cathode rays through helium. The
difference in the character of the rays is also made evident by
bringing a small magnet near the discharge tube ; the green
rays are visibly deflected by the magnet but no appreciable
effect is produced on the red rays. By using polygons instead

PHOSPHORESCENT FIGURES 7

of triangles, or scalene triangles instead of equilateral ones
very interesting distributions of the red and green pencils can
be obtained. Researches on these parallel cathodes have
been made by Kunz 1 and Orange,2 and they are often useful
for giving strong pencils of positive rays in definite directions.
Goldstein also found that positive rays come out freely
along the axes of hollow tubes when these are used as cathodes.
Thus if a hollow cylindrical tube of circular cross section is
used as a cathode the stream of rays when the planes of the
ends are at right angles to the axis is along the axis.
When the plane of one end is oblique to the axis there are
two streams at right angles respectively to the cross sections
as in Fig. 4. The directions of these streams do not depend
on the position of the anode.

•••:*.;

FIG. 4.

When the cross section of the tube is not circular but poly-
gonal very interesting phosphorescent figures are produced by
the rays coming from the tube. That represented in Fig. 5
was obtained by Kunz with a tube whose cross section was
an equilateral triangle.

1 Kunz, "Phil. Mag.," VI, xvi, p. 161, 1908.

1 Orange, " Proc. Camb. Phil. Soc.," XV, p. 217.

RA YS OF POSITIVE ELECTRICITY

Goldstein (" Phys. Zeitschrift," II, p. 873) has shown that
positive rays can be produced in a very simple way by using

FIG. 5.

two parallel wires as a cathode. The rays spread out from
the space between the wires in the manner illustrated in
Fig. 6. By using three or more such parallel wires for the
cathode very interesting patterns of positive rays can be
obtained.

FIG. 6.

Since perforated cathodes ot one form or another are used
in the great majority of experiments on positive rays, the
consideration of the action of these cathodes is a matter of
considerable interest and importance. The positive rays
passing through a hole in a plane cathode are not by any
means identical with those which would have struck the
site occupied by the hole had the cathode been continuous.

PERFORATED CATHODES g

The hole in the cathode produces a much greater effect
when an electric discharge is passing between the anode
and cathode than it does on the distribution of the lines of
electric force 'before the discharge begins to pass. There
are many points of interest in the behaviour of perforated
cathodes which are I think probably connected with the
question of the transmission of the electric charge from a
positively charged atom or molecule to a metallic electrode.

Thus, for example, Aston (" Proc. Roy. Soc.," 87, A., p. 437)
found that when a piece of perforated zinc was used for the
cathode the discharge passed more easily than with a con-
tinuous zinc cathode of the same area; and also that with
the perforated cathode the luminosity in the gas was greater
opposite the holes than opposite the zinc.

The path of the cathode rays has considerable influence
on the luminosity in the gas and on the ease with which the
discharge passes through the tube.
In an experiment made long ago by
Sir William Crookes with a tube like
that represented in Fig. 7, the dis-
charge went more easily along the
path (i) where the cathode rays do
not traverse the same path as the
positive column than along (2) where
the paths coincide. Again, when he
discharge is passing along a tube like
(2), if the cathode rays are deflected FlG 7

to one side by a magnet, the

luminosity of the positive column will come much closer up
to the cathode, the Faraday dark space is shortened, and when
the magnetic force at the cathode is strong the cathode fall
of potential is reduced. The Faraday dark space has its
origin in a slight ionization due to cathode rays which have

io RA YS OF POSITIVE ELECTRICITY

travelled through the negative glow, hence if the rays are
deflected the Faraday dark space will disappear.

The magnet will deflect the cathode rays when close to
the cathode and make them travel along curved paths instead
of straight lines ; thus in passing from the cathode to a point
in the gas they will travel along a longer path and therefore
produce more ions when the magnet is " on " than when it is
" off." The magnet produces a virtual increase in the ionizing
power of cathode rays close to the cathode ; such an increase
will be accompanied by a decrease in the cathode fall of
potential.

If the cathode particles started from the inside of a
Sandwich cathode their paths would not be straight lines, for
an electron shot from the inner surface of one plate would be
sent back by the other, and thus would pursue a zigzag path
before getting out from between the plates. This increase in
the length of path would tend to diminish the cathode fall of
potential.

When a positive atom gives up its charge to a metal, it
must, when close to the metal, regain an electron and become
neutral. If it comes close to a piece of metal at a place
where there is no electric force, then an electron in the neigh-
bourhood would run into the atom and might become attached
to it. If, however, there is an intense electric field close to
the metal the electron will acquire a high velocity and
instead of combining with the positive atom may shoot past
it. We see from this that the discharge of positive electricity
to the electrode may be hampered by a strong electric field,
such as might be produced by a double layer of electricity,
close to the electrode. Now, when the electrode is emitting
cathode rays there is at the seat of emission such a double
layer, which not only gives rise to an intense electric field
close to the cathode but also diminishes the electric field

PERFORATED CATHODES 11

in the gas beyond the double layer. The layer does two
things : (a) it makes it more difficult for the positive ions
to lose their charges : (b) it concentrates the electric field close
to the cathode. When this concentration is great the positive
ions will acquire by far the larger part of their energy close to
the surface of the cathode, and thus ions originating in different
parts of the dark space in front of the cathode would reach
the surface of the cathode with practically the same energy.
Again, the cathode rays which originated in the dark space
would only possess a very small fraction of the energy of
those which started from the cathode itself. The cathode
rays would thus consist of two groups — the energy in one
group being constant while that in the other would be
variable but small compared with the energy of those in the
first group. This is consistent with the behaviour of the
cathode rays coming from a continuous cathode. The
positive rays coming through a perforated cathode show a
wider variation in energy than is indicated by these
considerations.

Though a hollow or " sandwich " cathode may be sur-
rounded by the Crookes dark space there is in general
luminosity inside the hollow or between the plates, indicating
that in these regions there is, what there is not in the dark
spaces, recombination of ions or the neutralization of positive
particles by electrons. We might also expect that there
would be an accumulation of electrons between the plates,
for electrons shot out from the inner surface of one plate
would be stopped by the other plate. This accumulation of
electrons would tend to neutralize the drop of potential which
occurs at the surface of the plate and would make the
potential in the space between the plates approach that of
the metal part of the cathode. Thus we may regard the space
between the plates as a cathode without a sudden cathode

12 RAYS OF POSITIVE ELECTRICITY

drop of potential, or at any rate with a much smaller drop
than a metallic cathode. This cathode has also a plentiful
supply of electrons behind it to neutralize the positive
particles which come up to it. Since the potential drop is
much less abrupt, the electric field outside will be more
intense than that near a metallic cathode. As the space
between the plates is narrow, the gaseous cathode will be
small and the electric force will diminish rapidly as we recede
from it ; the region of intense electric force will extend to a
distance from the cathode comparable with the diameter of
the hole in the cathode. Ions produced at different places
along these lines of force would reach the cathode with
different amounts of kinetic energy so that there might be a
considerable variation in the velocity of the positive rays
coming through the channel ; this, as we shall see, is a
conspicuous feature in the behaviour of positive rays.

This variation in the velocity of the positive rays should be
accompanied by an associated variation in the velocity of
those cathode rays which are produced along the paths of
those lines of electric force which start from the channels, so
that we might expect these cathode rays to be much more
heterogeneous than those of the usual type. To test this
point I tried the following experiment —

The cathode was a perforated one of the kind used for the
production of positive rays. The cathode rays after passing
through a fine tube fell upon a screen covered with willemite.
At very low pressures the image on the screen was a bright
spot at the place where the axis of the tube struck the screen.
The spot was surrounded by a bright circle. If the cathode
rays varied considerably in velocity they would, if acted upon
by a magnet, be deflected by different amounts and the spot
would be drawn out into a line. It was found that under the
action of the magnet the luminosity had the following appear-
ance. There was a bright spot not markedly larger than the

ORIGIN OF CATHODE RAYS 13

undeflected one, and this was accompanied by a faint tail
where the deflection was greater ; this tail was due to cathode
rays which are slower than those producing the bright spot.
The tail was so much fainter than the head that it was
evident that by far the greater part of these rays possessed
the maximum velocity, and that though there were some
with smaller velocities these formed but a small fraction of
the whole group. We shall see that when the positive rays
are deflected by a magnet a spot of luminosity produced by
them is in general drawn out into a line of approximately
uniform luminosity, proving that the concentration of the
positive rays on any particular velocity is much less marked
than that of the cathode rays which give rise to phosphor-
escence on the screen.

We conclude from this experiment that the majority of the
fast cathode rays are produced quite close to the cathode and,
therefore, experience the full fall of potential. The cathode
rays starting from the metal itself need not be accompanied
by any equivalent of positively charged particles in the gas.
Those originating in the dark space would have a positive
particle corresponding to each cathode ray. lonization in the
negative glow would give rise to positive rays which would
have experienced the full fall of potential when they reached the
cathode, and since the electric field beyond the negative glow
is so weak the cathode rays due to this ionization would have
so little energy that they would probably escape observation.

Two causes for the emission of cathode rays from the
cathode itself suggest themselves. The first of those is the
impact of positively charged particles against the cathode.
We know by direct experiment (Fuchtbauer, " Ann. der Phys.,"
23, p. 301, 1907 ; Saxen, "Ann. der Phys.," 38, p. 319, 1912 ;
Baerwald, "Ann. der Phys.," 41, p. 643, 1913 ,-42, p. 1207, 1913)
that electrons are emitted by metals when these are bombarded
by positively charged particles. According to Baerwald, how-

14 RA YS OF POSITIVE ELECTRICITY

ever, positively charged hydrogen atoms must have an amount
of energy greater than that due to a fall through 900 volts
before the emission of electrons becomes appreciable.1 The
quantity of electrons emitted is much the same whatever the
metal may be against which the positive particles strike.
The energy possessed by the electrons when they are ejected
from the metal does not exceed that which would be acquired
through a fall of potential of about 20 volts. As the energy
possessed by the positively electrified particles in tubes of the
kind used to study positive rays is far greater than the
minimum of 900 volts required to develop electrons, part
at least of the cathode stream from the electrode must be due
to the impact of positively charged particles against the
cathode. As, however, we can get cathode rays with a
potential difference of less than 900 volts there must be other
agencies also at work : such, for example, as the ionization of
the molecules of the gas by the positive particles and the
incidence of radiation produced by the discharge, and having

the character of soft Rontgen radiation with a wave length
small compared with that of the type of ultra-violet light which
can get through quartz or even through fluorite. We know that
radiation of this type exists in the tube ; we know, too, that
radiation of this kind when it falls upon metals causes them
to emit a stream of electrons, so that part of the cathode
stream must be due to this cause. How much is due to this
and how much to the previous one has not yet been
determined. Wehnelt (" Ann. der Phys.," 41, p. 739, 1913) has
shown that any ultra-violet light which can pass through
fluorite does not produce an appreciable effect on the emission
of the cathode stream.

The places from which the positive rays originate can be

1 Horton and Davis (" Proc. Royal Soc.," 95, p. 333, have detected the
emission of electrons from a metal plate struck by positive helium atoms with
energy as low as 20 volts.

ORIGIN OF POSITIVE RAYS 15

traced in a very simple way by means of a screen covered
with a layer of fused lithium chloride. This substance when
struck by rapidly-moving positively- electrified particles
phosphoresces with a deep red light ; the red lithium line
being very prominent when the light is examined with the
spectroscope. When lithium chloride is struck by cathode
rays the phosphorescence is steely blue and the spectrum
is continuous. To explore the tube for positive rays a thin
rectangular strip of mica or metal covered with the fused
chloride is attached to a closed glass tube which contains
a piece of iron and can slide along the bottom of the discharge
tube. The strip can be moved to or from the cathode
by moving the piece of iron along the tube by a magnet. If
we start with the mica strip near to the cathode we find that
the anode side of the screen is a brilliant red, proving that
in this region there are plenty of positive rays moving up to
the cathode. When the strip is pulled further away from the
cathode the red light on the anode light persists and is quite
bright until the screen almost reaches the limit of the dark
space close to the negative glow, when it gets into the
negative glow the phosphorescence on the anode side
disappears. This shows that many of the positive rays start
from close to the junction of the dark space and the negative
glow. It is surprising to find how short is the distance which
the screen has to travel from the boundary of the negative
glow for the red phosphorescence to be quite marked. As
at this end of the dark space the electric force is very feeble,
the charged particles cannot have fallen through more than
a small fraction of the potential difference between the anode
and the cathode.

The negative glow is thus a most important place for the
manufacture of the positively charged particles which form
the positive rays ; the study of the positive rays enables us, as
we shall see, to determine the character of these particles.

16 RA YS OF POSITIVE ELECTRICITY

ON THE NATURE OF THE POSITIVE RAYS, THEIR
DEFLECTION BY ELECTRIC AND MAGNETIC FORCES

As cathode rays were proved to be negatively electrified
particles by the study of the deflections they experience when
acted on by magnetic and electric forces, and as these deflec-
tions gave the means of finding the mass and velocity of the
cathode particles, it was natural to attempt to apply the same
methods to the positive rays. It was not, however, until
twelve years had elapsed since the discovery of the rays that
any effect of a magnetic field on them was detected. A small
permanent magnet held near a bundle of cathode rays
produces a very appreciable effect ; it has, however, no ap-
parent action on the positive rays : as a matter of fact the
deflection of the positive rays due to a magnetic field is at
most about 2 per cent of the deflection of cathode rays in
the same tube. In 1898, however, Wien, by the use of very
powerful magnetic fields, proved that the positive rays were
deflected by magnetic forces.1

Before discussing Wien's experiments it will be convenient
to consider the theory of the deflection of a moving electrified
particle by a magnetic field. The force acting on the moving
particle is at right angles to the magnetic force, at right
angles also to the direction of motion of the particle, and is
equal to ellvsinfa where H is the magnetic force at the
particle, v the velocity of the particle, <£ the angle between H
and v, and e the charge on the particle. Since this force is
always at right angles to the direction of motion of the
particle it will not alter the speed of the particle but only the
direction in which it is moving. Suppose that the particle is
originally projected with a velocity v parallel to the axis of xy
and that it is moving in a magnetic field arranged so as to be

1 W. Wien, "Verb. d. phys. Gesell.," 17, 1898.

MAGNETIC DEFLECTION OF MOVING PARTICLES 17

very approximately in the direction of the axis of z, the
direction of the force along the particle will be parallel to the
axis of y> and this will be the direction in which it will be
deflected. If y is the deflection in this direction at the time /,
m the mass of the particle, H the magnetic force parallel to
the axis of Z, and e the charge carried by the particle, the
equation of motion of the particle is

Integrating this equation we get

w-f = *H^fc= ettdx . . . . (i)

dt 1 o dt Jo

if the origin of co-ordinates is taken at the point of projection ;
for since the particle was projected parallel to the axis of x,

-?- = o when x—o. Now if the deflection of the particle is small
dt

~ will, neglecting the squares of small quantities, be equal to

v, and -^to v -^-. On this assumption equation (i) may be
dt dx

written

mVdx~~Joe ' *'
hence if y is the deflection when x = I

mV = \'o{\*aeKdX}dX..

Integrating by parts we have

/i n

eUdx — I xeHdx
o Jo

or writing A for

e f

RA YS OF POSITIVE ELECTRICITY

A depends merely upon the strength of the magnetic field
and the distance from the point of projection at which the
deflection is measured ; it is quite independent of the charge,
mass, or velocity of the particle.

If the magnetic field is that between two poles of an
electromagnet placed close together and reaching up to the
point of projection of the particle, then if a is the breadth of
the pole pieces, H is approximately constant from x=o to
x—a and vanishes from x=a to x=l. Substituting this value
for H in the expression for A we find

when H is the magnetic force between the poles. When this
approximation is not sufficiently accurate and we have to
take into account the stray magnetic field beyond the poles
as well as the variation of the magnetic force between the
poles, A may be conveniently determined by the following
method.1 Wind a coil of triangular section DEF, the base
DF being equal to /, the angle EDF a right angle, and DE

FIG. 8.

small compared with the depth of the pole pieces of the
electromagnet. Place the coil so that DF is along the direc-
tion in which the particle is projected, D being at the
point of projection and F at the distance at which the
deflection is measured, connect up the coil with a ballistic

1 J. J. Thomson, " Phil. Mag.," VI, xviii, p. 844.

ELECTRIC DEFLECTION OF MOVING PARTICLES 19

galvanometer, or, what is more convenient, with a Grassot flux
meter, and determine the number of lines of magnetic force
which pass through this coil when the electromagnet is
made or broken ; from this number we can easily determine
the value of A. For if N is this number, then we see from
Fig. 8 that

N = f H X PN . dx

and from the figure

DE ~~ FD /
hence N = j* H . ^ (l-x)dx

Thus when N is known A can be determined at once.

ELECTROSTATIC DEFLECTION OF THE
PARTICLE

Let us suppose as before that the particle is projected
with a velocity v parallel to the axis of x: let the electric
force acting on the particle be parallel to the axis of z and
equal to Z, then the equation of motion of the particle under
the electric force is

When the deflection is small, ^ = vz -^ approximately,
and hence

«*%-*

or z = —

20 RA YS OF POSITIVE ELECTRICITY

where B =

Thus B is quite independent of the charge, mass, or velocity
of the particle, and depends merely on the distribution of the
electric field and the distance from the point of projection at
which the deflection is measured.

A very convenient method of producing the electric field
is to have two parallel plates perpendicular to the axis of z ;
in this case the electric field is approximately constant
between the plates and vanishes outside them. If b is the
length of the plates measured parallel to the axis of x, and
if one end of the plates just comes up to the point from which
the particle is projected, putting Z=Z from x=o to x=b, and

Z=<? from x=b to *=/, we find that B=Z£ ( I -
so that if z is the deflection when x—l

mv 2

The electric field is not absolutely constant between the
plates, it is greater close to the edges than in other parts of
the field, nor does it absolutely vanish at all places outside
the plates ; when great accuracy is required these points
have to be taken into account in the calculation of B. A
method by which this may be done was given by the author
in the "Phil. Mag.," VI, vol. xx, p. 752.

If the particle is acted on simultaneously by magnetic and
electric forces parallel to the axis of #, we may, if the deflec-
tions are small, superpose the effects due to the magnetic and
electric forces, so that the y, z deflections of the particle
parallel to the axis of y and z respectively are given by the
equations

DEFLECTIONS OF MOVING PARTICLE 21

= — A ......... (i)

Thus if a stream of charged particles of different kinds
(i. e. with different values of ejm) were projected from the
origin with different velocities parallel to the axis of xt in the
absence of electric and magnetic forces they would all strike
a screen at #=/at the same point. When, however, they are
submitted to the action of electric and magnetic forces they
get sorted out, and no two particles strike the same point on
the screen unless they are moving at the same speed and also
have the same value of ejm. If we know the deflected
position of the particle we can by equations (i) and (2)
calculate both the values of v and also the value of elm ; we
have from these equations

"*... .•--. ... (3.

Thus y\z will be constant for all particles moving with a
given speed whatever may be their charge or mass, hence all
such particles will strike the screen in a straight line passing
through the undeflected position of the particles.

Again, for the same kind of particle y*\z is constant
whatever may be the velocity of the particles, hence particles
of the same kind will all strike the screen in a parabola with
its vertex at the undeflected position of the particles, and
there will be as many of these parabolas as there are different
kinds of particles.

In the preceding investigation we have assumed that
the pressure of the gas was so low that we could neglect
the resistance the gas. offered to the motion of the positive

RA YS OF POSITIVE ELECTRICITY

particles through it. If this resistance is represented by a
retarding force equal to R times the velocity we can show,
if we neglect terms involving squares and higher powers of R,
that the term in A2/B, which is proportional to /, and which
in most cases is by far the most important, is not affected by
the resistance,

WIEN'S PROOF OF THE MAGNETIC AND ELECTRIC
DEFLECTION OF THE RAYS

W. Wien1 applied this method to demonstrate the
magnetic and electric deflections of the positive rays ; he
proved in this way that the positive rays contained electrified
particles, and the direction of the deflections showed that they
were positively charged. He calculated by the formulae we
have just given the values of e\m and v for these particles.

The method used by Wien is illustrated in Fig. 9.

FIG. 9.

The cathode K was an iron cylinder 3 cm. long with a
hole 2 mm. in diameter bored through it, the anode was at
the top of the tube. The lower end of the tube was made as
flat as possible so as to facilitate the observation of the spot
of luminosity produced by the impact of the positive rays on

1 W. Wien, " Wied. Ann.," 65, p. 440, 1898; "Ann. der Phys.," 8, p. 224,
1902.

DEFLECTION OF POSITIVE RA YS 23

the glass. The magnetic field was produced by an electro-
magnet whose poles were at N and S : it is necessary to shield
the part of the tube through which the discharge is passing
from the magnetic field ; if this were not done the discharge
would be so much affected by the magnet that trustworthy
observations would be impossible ; the tube was shielded by
surrounding it with thick sheets of soft iron. The electro-
static field was produced between two parallel metal plates
which were connected with the terminals of a voltaic battery.
When the magnetic and electric fields were acting, the round
spot of phosphorescence due to the positive rays coming
through the hole in the cathode was drawn out into a straight
band. Since the band was straight the velocities of the
different particles producing it would all be the same ; the
values of e\m for these particles would, however, all be
different. When the tube was filled with hydrogen, Wien
found that the value of ejm for the most deflected portion was
7545 ; the value of ejm for a charged atom of hydrogen in the
electrolysis of water is 10,000. In his first set of experiments
Wien found that on filling the tube with oxygen the value of
ejm for the most deflectible rays was 9800 in one experiment ;
in later experiments, after very pure oxygen had flowed
through the tube for a long time, he found on first passing the
discharge through the tube very much smaller values of ejm
than for hydrogen, but the higher values reappeared after the
discharge had passed for a short time.

The deflections of these rays by the electric and magnetic
fields show that they are positively charged particles, the
values of e\m obtained for these particles show also that they
are very much more massive than the particles in the cathode
rays for which e[m = 17 x io7. The displaced particles in
this experiment were spread out into a continuous straight
band, indicating, according to the theory of the effect of

24 RA YS OF POSITIVE ELECTRICITY

electric and magnetic fields on charged particles, that in the
positive rays there are particles giving all values of efm from
zero up to about 10,000. This would imply, assuming that
the charge on each particle is the same, that the masses of the
particles vary continuously from a certain value comparable
with the mass of an atom of hydrogen up to a value which is
very large in comparison with this mass. This continuous
variation in the value of ejm is contrary to what might be ex-
pected, for, from the molecular theory of gases, the masses
available in the gas would not vary continuously but would
increase by finite steps, the smallest step being the mass of the
atom of hydrogen : again the results of many different lines
of investigation lead to the conclusion that e like m does not
vary continuously, but that all electrical charges are multiples
of a unit charge whose value in electrostatic measure is
4*8 X io-10. Again it would appear from the uniformity of
the luminosity produced by the displaced positive rays that
there is no special kind of atom which is predominant among
these rays. For if there had been a great excess of particles
of one kind, these would have produced a very bright spot on
the glass if they had all been moving with the same velocity,
or a bright arc of a parabola if they had been moving with
varying velocities. The experiments now to be described,
which I made in 1906, show that the discrepancies between
the theory and the experiments are due to the pressure of the
gas in the discharge tube in Wien's experiments having been
so high that the particles forming the positive rays collided
with the molecules of the gas whilst they were passing through
the electric and magnetic fields. The effect of these collisions
is to ionize the gas so that the gas through which the positive
rays have to pass is full of charged particles, some charged
with positive, others with negative electricity. The result of
the presence of this electrification is that some of the positive

DEFLECTION OF POSITIVE RA YS 25

ray particles which were charged before they entered the
electric and magnetic fields have their charges neutralized
before they pass through them, and thus do not experience
the full deflection. On the other hand others which had got
neutralized before they entered the field strike against an
electron or atom and, losing an electron, get ionized by
the collision. In this way they acquire a positive charge in
the field and are deflected by an amount which depends
upon the stage in their journey at which they picked up the
charge. Thus the quantities we denoted by A and B (see
p. 21) vary from particle to particle, and the values of
elm cannot be obtained from equation of the type (3) and (4)
where A and B are calculated on the supposition that the
particles are charged for the whole of the time they are
between the poles of the magnet and the plates of the
condenser.

In my first experiments l on this subject the arrangement
was as follows: The cathode K (Fig. 10) had a hole bored

t
FIG. 10.

through it and in this hole a tube F with a very fine bore was
firmly fixed ; it is essential to the success of the experiment
that the bore of the tube should be exceedingly fine so as to

» J. J. Thomson, " Phil. Mag.," VI, xiii, p. 561. | 1 O 1

26 RA YS OF POSITIVE ELECTRICITY

get a small, well-defined patch when the positive rays strike
the screen, S. This was a flat glass plate uniformly covered
with powdered willemite which phosphoresces much more
brightly than glass when struck by the rays. M and N are
the poles of the electromagnet, and Px P2 the parallel metal
plates used to produce the magnetic and electric fields
respectively ; t,t, W,W are sheets of soft iron to screen the
discharge in the tube from the magnetic field due to the
electromagnet.

The effect observed on the screen depended to a very great
extent upon the pressure of the gas in the tube ; when this
was not exceedingly low, the phosphorescence under the
action of the magnetic and electric fields was drawn out into
two continuous straight bands as in Fig. 1 1. The value of ejm

FIG. ii. FIG. 12.

for the most deflected portion of the band a was io4, for that
of band b> 5 X io3. These correspond to the values of ejin for
the atom and molecule of hydrogen respectively, suggesting
that the one band is due to hydrogen atoms, the other
to hydrogen molecules, \yhen the tube contains helium

SEPARATION OF POSITIVE RAYS 27

there are three bands to be seen as in Fig. 12. The values of
e\m at the tips of these bands are respectively io4, 5 x io3,
2*5 X io3, indicating that we have here again bands due to the
atom and molecule of hydrogen, and in addition a new one
due to atoms of helium, for (as the atomic weight of helium
is 4) e\m for the helium atom is one quarter of that for
the hydrogen atom. It is remarkable that the slope of these
bands, and therefore, by page 21, the velocity of the particles
varies little if at all with the potential difference between the
anode and cathode of the discharge tube. This potential
difference may be increased three or four times without
producing any appreciable effect upon the slope of the bands
of phosphorescence. When air is in the tube, the appearances
of the bands is much the same as when the tube contains
hydrogen, though the phosphorescence is not so bright. The
most conspicious things on the screen in this case are the two
bands corresponding to the atom and molecule of hydrogen
respectively.

In addition to the two bands deflected in the direction in-
dicating a positive charge on the particles, there is another
fainter band deflected in the opposite direction which must
therefore be due to particles with a negative charge. The
value of elm for the tip of this band is io4, thus these negative
particles are not cathode rays for which ejm is 17 X io7, but
have a mass equal to that of an atom of hydrogen. The
existence of particles deflected in the opposite direction
to that of the majority of the particles had also been observed
by Wien.

EFFECT AT VERY LOW PRESSURES
When the pressure is reduced to as low a value as is
possible the appearance of the luminosity on the screen
entirely changes. At these low pressures it is exceedingly

28 RA YS OF POSITIVE ELECTRICITY

difficult to get the discharge to pass through tubes of
moderate size when the cathodes are made of aluminium or
any of the metals ordinarily used for this purpose, and there
is great danger of sparks passing through the glass and
breaking the tube. This can be avoided to a great extent by
facing the cathode with a thin layer of calcium, or smearing
the face of the cathode with the liquid alloy of sodium and
potassium. This reduces considerably the difficulty of
getting the discharge to pass and diminishes the risk of
perforating the tube. The appearance at these low pressures
when hydrogen or air is in the tube is shown in Fig. 13. It
will be noticed that the straight bands of phos-
phorescence have almost disappeared and that most
of the phosphorescent light is concentrated into
two parabolic curves which are connected with the
undeflected spot by straight faintly luminous lines.
The value of e\m for one parabola is io4, that for
* the other 5 X io3 so that they are due to the atom

and molecule of hydrogen respectively. At these
low pressures the luminosity in the negative direction dis-
appears. But both at the low and higher pressure there is,
even when the magnetic and electric fields are in action, an
appreciable amount of luminosity at the position occupied by
the undeflected spot.

When, as in these early experiments, the pressure is the
same in all parts of the tube, there is considerable advantage
in using very large glass vessels for the discharge tubes when
studying positive rays ; with large vessels the pressure can
be made very small before the tube offers great resistance to
the passage of the discharge through it. The increase in the
difficulty of getting the discharge to pass comes in at the
pressure when the dark space round the cathode reaches
the walls of the tube. When the tube is big the walls are far

POSITIVE RAY TUBES 29

away from the cathode and the pressure has to be exceedingly
low before the dark space reaches the sides of the tube. We
can work with much lower pressures with these large tubes
and therefore reduce the obstruction which the positive rays
meet with in their passage from the cathode to the screen.
Using vessels of about 2 litres capacity I have observed l on a
willemite screen the parabolas corresponding to carbon,
oxygen, neon, and mercury vapour as well as those corre-
sponding to the atom and molecule of hydrogen and the atom
of helium. The photographic plate is, however, for most
purposes a much more convenient detector than a willemite
screen. It is more sensitive, it gives a permanent record, and
measurements can be made with much greater accuracy
on the plate than they can on the screen. Before entering
into the discussion of the theory of the positive rays it
is desirable to describe the results obtained with the photo-
graphic method, as well as the experimental details by which
these results have been procured.

FIG. 14.

The apparatus now in use at the Cavendish Laboratory is
represented in Fig. 14. The discharge takes place in a large
glass flask A : a volume of from one to two litres is a

1 J. J. Thomson, " Phil. Mag.," VI, xx, p. 752, 1910.

30 RA YS OF POSITIVE ELECTRICITY

convenient size for this purpose. The cathode C is placed in
the neck of the flask. The position of the front of the
cathode has a very considerable influence on the brightness of
the positive rays and ought to be carefully attended to. The
best position seems to be when the front of the cathode
is flush with the prolongation of the wider portion of the
flask. The shape of the cathode is represented in section in
Fig. 15 : the face of the cathode is made of aluminium,
the other portion is soft iron. A hole is bored right through
the cathode to admit the fine tube through which the positive
rays are to pass. Care should be taken to bore this hole so
that its axis is the axis of symmetry of the cathode. The
tube through which the positive rays pass is fastened into the
cathode in the way shown in Fig. 15.

FIG. 15.

The bore of this tube will vary with the object of the
experiment. If very accurate measurements are required,
the diameter of the tube must be reduced to • i mm. or less.
With these very fine tubes, however, very long exposures
(ij to 2 hours) are necessary. The length of the tube is
about 7 cm. The tubes are prepared by drawing out very
fine bore copper tubing until the bore is reduced to the
desired size. The tube is straightened by rolling it between
two plane surfaces, and great care must be taken to get the
tube accurately straight, as the most frequent cause of
dimness in the positive rays is the crookedness of the tube.
After long use the end of the tube nearest the discharge tube
gets pulverized by the impact of the positive rays, and the

POSITIVE RAY TUBES 31

metallic dust sometimes silts up the tube and prevents the
rays getting through. The cathode is fastened in the glass
vessel by a little sealing-wax, and a similar joint unites it to
the ebonite box, UV. To keep the joints cool and prevent
any vapour coming from the wax, the joints are surrounded
by a water jacket J through which a stream of cold water
circulates.

The electric field is produced between the faces of L and
M which are pieces of soft iron with plane faces. These are
fitted into the ebonite box UV so that their faces are
parallel : the distance between the faces should be small
compared with their lengths. In many of the experiments
described subsequently the length of the faces was 3 cm. and
their distance apart 1-5 mm. Their faces are connected with
the terminals of a battery of small storage cells : in this way
any required difference of potential can be maintained
between them.

These pieces of soft iron practically form the poles of an
electromagnet, for the poles of the electromagnet P and
Q are made of soft iron of the same cross section as L,M ;
they fit into indentations in the outside of the ebonite box
and are only separated from the pieces L,M, by the thin
flat pieces of ebonite which form the walls of the box. This
arrangement makes the magnetic field as nearly coterminous
as possible with the electric, which is desirable in several
of the experiments. To screen off the magnetic field due to
the electromagnet, thick iron plates V,W, Fig. 14, are placed
round the neck of the tube.1

A conical glass vessel F 40 cm. long is fastened by wax

1 Though an increase in the distance of the photographic plate from
the cathode increases the deflection of the parabolas for the same
electric and magnetic fields, the definition is not so good. It is
advisable when sharp definition is very important to use strong fields
and place the photographic plate as near the cathode as is convenient.

RA YS OF POSITIVE ELECTRICITY

to the ebonite box while the other end is fixed to the

apparatus which con-
tains the photographic
plate. One form of this,
designed by Mr. Aston,
is represented in Fig. 16.
The photographic plate
is suspended by a silk
thread wound round a
tap T which fits into a
ground glass joint ; by
turning the tap the thread
can be rolled or unrolled
and the plate lifted up
nxXy) or let down. The plate

__ — ^^j GA slides in a vertical box B

made of thin metal ; this

" -- "Tl"""^
) \

FIG. 16.

the openings A which
are placed so that the
positive rays can pass
through them. The
openings are on both
sides of the box and
about 5 cm. in diameter.
When the silk thread is
wound up the strip
DEFG of photographic
plate in the box is above
the opening A, so that
there is a free way for

the rays to pass through A and fall on a willemite screen
behind it. This screen is not used for purposes of

POSITIVE RAY APPARATUS x 33

measurement, but only to see before taking the photograph
that the tube is giving an adequate supply of positive rays.
The box is sufficiently large to hold a film long enough for
two or more photographs ; if it is wished to take two photo-
graphs, the plate is lowered until the bottom half comes
opposite to the opening A, a photograph is taken in this
position, the plate is then let down still further until the top
half of the plate comes opposite to the opening, then a
second photograph is taken. This plan is convenient because
the deflections of the different kinds of positive rays differ
so much that it is difficult to measure them accurately when
they are all on one plate. For example, the magnetic deflec-
tion of the hydrogen atoms is about fourteen times that
of the mercury one, thus if the deflection of the hydrogen atom
is within the limits of the plate, that of the mercury atom
would be too small to measure accurately. When we can
take two photographs, however, without opening the tube,
we may take one with a small magnetic field to get the
deflection of the hydrogen atom, and the second with a much
larger one to get the deflection of the mercury one.

Two tubes containing coco-nut charcoal are fused to this
part of the apparatus ; by immersing these in liquid air the
pressure can be made exceedingly small. As the only
communication between this part of the apparatus and that
through which the discharge passes is through the long and
very narrow tube in the cathode, it is possible to have the
pressure on the camera side of the apparatus very much less
than the pressure on the side through which the discharge
is passing.

A Gaede pump worked by a motor is connected with
the discharge tube, and keeps the pressure in this part of
the apparatus at a suitable value.

When the rays in some particular gas are under examination
D

RA YS OF POSITIVE ELECTRICITY

a constant stream of this gas is kept flowing through the
discharge tube. The gas is stored in the vessel A, Fig. 17,

camera

FIG. 17.

over a column of mercury : this vessel is connected with
the discharge tube by the system shown in Fig. 17, where
BC is a long and fine capillary tube. When the tap T
is turned the gas has to pass through this capillary : it does
so exceedingly slowly. The rate can be adjusted by raising
or lowering a mercury reservoir connected with A ; this is
held in such a position that when the Gaede pump is in
action the pressure in the discharge tube is such as to give
well-developed positive rays.

The curves on the photographic plates made by the
positive particles are measured by the apparatus represented
in Fig. 1 8. The photographic plate is clamped in a holder
A, and the position of any point on it is determined by
moving the carrier C until the tip of the needle NN comes
just over the point in question. The carrier C has two

METHOD OF HOT CATHODES

movements, one parallel to the base BB, and the other, by
means of the screw S, at right angles to this direction ; the

position of the point is read off on the two verniers. The
plate is placed in the holder so that the direction of the
magnetic deflection is parallel, and that of the electrostatic
deflection at right angles, to BB.

THE METHOD OF HOT CATHODES

Another method by which positive rays with a great range
of velocities may be produced is to use for the cathode
a Wehnelt cathode — i. e. a spot of lime round a hole in a strip
of platinum, or a spiral of tungsten wire raised to incon-
descence by an electric current. The hot cathode emits
electrons, and these, when there is an adequate potential
difference between the anode and cathode, ionize the gas in
the discharge tube ; the positive ions produced by this
ionization move up to the cathode and pass through the hole
in the strip or the spaces between the wires. A fine tube
placed just behind the cathode isolates a thin pencil of
rays which pass through electric and magnetic fields as in

36 RA YS OF POSITIVE ELECTRICITY

the previous method. When the voltage between the cathode
and anode is less than a few hundred volts the positive ions
have not sufficient energy to affect the photographic plate ;
they may, however, after passing through the fine tube be
accelerated by inserting two parallel plates of fine wire gauze
between the end of this tube and the beginning of the electric
field. These plates are connected with some source of
constant difference of potential. In this way positive ions
produced with small difference of potential in the discharge
tube can be studied conveniently. The accelerating field can
be dispensed with if, instead of registering the rays by their
photographic action, we use the electrical method described
on page 124. The method of the hot cathode was first
employed in the Cavendish Laboratory by Professor Knipp1
who accelerated the rays and detected them photographic-
ally. It has also been employed by Dempster who used the
electrical method of' detection. The hot cathode method
has the merit of permitting the use of a much wider range
of pressures and voltages without changing the discharge tube
tfian the other method, and thus can be employed for special
investigations beyond the power of the first method. The
discharge tube, too, may be of much smaller dimensions,
a matter of importance in experiments when it is necessary
to keep it at a high temperature. The photographs which
have hitherto been obtained by this method are not, however,
comparable in clearness with those taken by the first
method.

A 'method has been devised lately by Mr. Aston ("Phil.
Mag./' Dec. 1919) which has the advantage of bringing
particles with the same value for ejm but with different
velocities together on the photographic plate and so avoid -

1 Knipp, "Phil. Mag.," VI, xxii, p. 926, 1911. Dempster, "Phil. Mag.,"
VI, xxxi, p. 438, 1916.,

ASTON3 S FOCUS METHOD 37

ing the weakening in intensity due to the spreading of these
particles over a considerable length of arc.

FIG. 19.

,
The elementary theory of the method is as follows :

Suppose that AB is a stream of positive particles, let one
of these particles be deflected through an angle 6 by an
electric field at O, when it gets to O' let it be deflected in the
opposite direction through an angle 99 by a magnetic field, its
path after leaving this field being along O'L. Let a particle
with a slightly different velocity be deflected by the electric
field along OO" and by the magnetic field along O" M, theryf.
O'L and O"M intersect at P, P will be a focus at which the 4*
rays with different velocities will overlap.

To calculate the position of this point we notice that "/
by equations <i) and (2) on page 21 if v is the velocity of the
particles

.^§,-1 tit,

If
and therefore

9 y^-l///J

(p2 = C 0
where C, C', C" are independent of v, hence

5^£=^. (i)

/)***** \ /

(p u
But remembering that <p and 6 are small angles we see from

the figure that

00' X 66 = 0'P% - 6)

or from (i)

P^=^_ (2)

OO' o> - 26

38 RA YS OF POSITIVE ELECTRICITY

When 99 = 26, O'P is infinite, i.e. the rays with different
velocities come out parallel, when 99=46 O'P=OO'.

When the particles are chiefly those which form the heads
of the parabolas, the particles with different values of ejm
will have approximately the same kinetic energy, and so will
be equally deflected by the electrostatic field, hence 6 will be
much the same for all these particles, so that by equation
(2) the foci will all be on the curve whose equation is
O'P(26— <p)=constant.

As 6 and 99 are small this will be approximately a straight
line in the direction given by 99=26, and passing through
the point given by 97 = 46 ; O'P=OO'.

The apparatus by which this method is carried out is
represented in Fig. 20, taken from a paper by Mr. Aston}
" Phil. Mag.," May 1920.

FIG. 20.

The discharge tube B is an ordinary X-ray bulb about
20 cm. in diameter. The anode A is an aluminium wire
surrounded by an insulated aluminium tube to protect the
glass walls. The aluminium cathode C, about 2*5 cm. wide,
is concave and placed just at the neck of the bulb. To
protect the opposite end of the bulb from being melted by
the concentrated beams of cathode rays, a silica bulb D
about 1*2 cm. in diameter is mounted as shown in the figure.

The arrangement of slits S1; S2, to produce the fine pencil

ASTOATS FOCUS METHOD

of positive rays is shown in Fig. 21. The slits, which are
•05 cm. wide and 2 mm. long, are about 10 cm. apart and can
be adjusted to be accurately parallel by means of their

FIG. 21.

diffraction patterns. The pencil of rays is split up into an
electric spectrum by passing between the plates Jj and J2, 5 cm.
long and 2'8 mm. apart, which can be maintained at any
required difference of potential. Kx and K2 are the diaphragms,
Kj is fixed and K2 mounted on the bore of a carefully ground
stop-cock. After leaving the diaphragm, the rays pass
between the pole pieces M of a large Du Bois magnet, these
are soldered into a brass tube O which forms part of the

FIG. 22.

camera N, which is made of stout brass tube 6*4 cm.
diameter. The arrangement for holding the photographic
plate W is shown in Fig. 22.

40 RA YS OF POSITIVE ELECTRICITY

The rays, after being magnetically deflected, pass between
two vertical brass plates about 3 mm. apart and reach the
photographic plate through a narrow slot 2 mm. wide and
11*8 cm. long cut in the horizontal metal plate XX. The
photographic plate, which is a 2 cm. strip cut lengthwise from
a 5x4 plate, is supported at its ends on two narrow
transverse rails which raise it just clear of the plate XX.
The plate is moved parallel to itself over the slot by
mechanism which is set in action by the torque rod V
working through a ground-glass joint. Y is a willemite
screen, and P a cap with a plate-glass back.

The adjustment of the plate-holder to make the rays
come to a focus on the plate was made by taking a series of
exposures of the hydrogen lines with different magnetic
fields on a large plate placed nearly vertically in the camera.
By developing this the actual paths of the rays could be
determined and the foci calculated. The final adjustment
was made by trial and error, and was exceedingly tedious, as
air had to be admitted and a new plate inserted after each
tentative small alteration of the levelling screws.

The plates were measured against a standard Zeiss scale
by a comparator. To measure faint lines it is necessary that
the magnifying power of the eye-piece of this instrument
should be very small, otherwise the edges of the lines are too
indistinct to be measurable.

Dempster (" Phys. Review," XI. p. 316) has employed a
method which had previously been used to determine ejm for
cathode rays. It consists in finding the strength of a uniform
magnetic field which will bend the rays into a circle of radius
a. If H is the strength of the magnetic field, v the velocity
of the particle

m H<7

e v

POSITIVE RA Y PHOTOGRAPHS 41

This method is applicable only when all the particles of
the same kind are moving with the same velocity ; if this
velocity is due to a fall through a potential difference V

1 ,«z,2 = v*,

and the preceding equation becomes

2V '

DISCUSSION OF THE PHOTOGRAPHS .

With the arrangement shown in Fig. 10 the appearance
of a typical photograph produced by the impact of the
positive rays on the plate when the pressure on the
camera side of the apparatus is reduced to about 'OOI mm.
of mercury is shown in Fig. i, Plate I. In this and the
following figures the deflection due to the magnetic field
is vertical, while that due to the electrostatic field is horizontal.
It will be seen that the curves on the plate are of two
different types.

i. A series of separate parabolic arcs, often of considerable
length. From the theory given on page 21 it will be seen
that each of these parabolas arises from particles having
the same value of ejm, and that these particles have retained
their charges throughout the whole of the journey through
the electric and magnetic fields. As the velocity of a particle
is by equation (3), p. 21, proportional to the tangent of the
angle which the line joining the origin to the point where
the particle hits the screen makes with the horizontal, it
follows there is a considerable range of velocities among the
particles having the same value of ejm. In many cases we
have velocities among the same kind of particles differing
so much that the velocity of the slowest ones is less than

42 RA YS OF POSITIVE ELECTRICITY

one-fifth that of the fastest. In some cases the parabolas
are of fairly uniform intensity along the whole of their length.
In others, as in that shown in Fig. 2, Plate I., the head of
the parabola (the part least deflected) is considerably brighter
than the rest of the curve, while sometimes, as in the case
represented in Fig. 3, Plate I., there are several spots of
maximum luminosity dotted along the parabolic arc.

With some exceptions (to be considered later) the heads
of all the parabolas are in the same vertical line, showing that
the minimum electrostatic deflection suffered by the particles
which produce these curves is the same for all the different
kinds of particles. By equation (2) page 21 the electrostatic
deflection is proportional to ejmv2. If the energy of the
particles is due to the fall of the charge through a potential
difference V

%mv2 = V . e

so that -^ = i /2V. Hence as the minimum electrostatic
mv^

deflection is the same for all the particles, we conclude that
the maximum potential through which they have fallen is
the same for particles of all kinds. It is natural to conclude
that this maximum potential is the difference of potential
between the anode and cathode of the discharge tube.
It is easy to verify that when the pressure is altered so as
to increase this difference of potential the deflection of the
heads of the parabolas diminishes.

2. Besides the parabolas there are on the plate a series of
straight lines connecting the parabolas with the origin.
These are due, I think, to particles which have been charged
during a part only of their passage through the electric and
magnetic fields. This might happen in two ways. A particle
which had got neutralized before reaching these fields might,
while passing through them, come into collision with an

POSITIVE RAY PHOTOGRAPHS 43

electron, get ionized, and acquire a positive charge, and
during the rest of its journey be deflected by the electric
and magnetic forces. Or again, a particle might be positively
charged when it entered the fields, attract an electron whilst
in them, get neutralized, and for the rest of its journey be
free from electric and magnetic deflections. This view of
the origin of these lines seems to me to be proved by the
following experiments.

As on this view these lines are due to particles which
get charged or discharged in the electric and magnetic fields,
their intensity, as compared with that of the parabolas, ought
to diminish if the length of these fields is reduced. To test
this I took a photograph when the lengths of the electric
and magnetic fields were reduced to i mm., the intensity
of the fields being increased in proportion so as to get
deflections comparable with those in the longer fields. With
this very short field the straight lines disappeared, and
nothing except the parabolas and the undeflected central
spot was to be seen on the photographic plate.

Another way of testing this view is to use magnetic and
electric fields, which are not coterminous. Let us suppose,
for example, that the magnetic field stretches beyond the
electric, on the camera side. There will be a part of the
field where the particles are exposed to magnetic but not to
electric forces. If a neutralized particle gets ionized in this
region, it will experience magnetic, i.e. vertical deflection
but no electrostatic or horizontal deflection. Thus with a
field of this kind we should expect the line due to particles
which acquired their charge whilst in the electric field to
have the shape shown in Fig. 23. The straight vertical stem
near the origin is due to the particles ionized beyond the
electric field, the piece running up to join the parabola, to
those ionized inside this field, the portion close to the parabola

44 RA YS OF POSITIVE ELECTRICITY

being due to particles which get ionized almost as soon as
they enter the fields. Photographs taken with 1;he magnetic

FIG. 23. FIG. 24.

field overlapping the electrostatic show this effect very
plainly; one of them is reproduced in Fig. 4, Plate I.,
another in Fig. I, Plate II.

Let us now consider the case of the charged particles
which get neutralized while passing through the field. The
part of the line near the origin will be due to particles which
get neutralized almost as soon as they enter the field. We
have supposed that the magnet was moved towards the
camera so that its field overlapped the electric on that side.
This will tend to make the electric field overlap the magnetic
on the other side, i. e. the side nearest the cathode, so that
when a particle first enters the field its deflection is mainly
due to the electrostatic force and is therefore horizontal ;
thus a particle which gets neutralized at the early stages of
its journey through the fields will have a horizontal displace-
ment abnormally large compared with the vertical ; while
those which get neutralized after leaving the electric field
will lose vertical but not horizontal deflection. The curves
produced on the photographic plate by the particles which
get neutralized will thus have a shape something like that
shown in Fig. 24. We see that with these overlapping

POSITIVE RA Y PHOTOGRAPHS 45

fields we can distinguish between the lines which are due to
particles which have gained a charge in their journey and
those which have lost one. The concavities of the two
curves are in opposite directions. These two sets of lines
are very prominent in photographs taken with apparatus in
which care has not been taken to make the fields coterminous ;
an example of this is shown in Fig. 4, Plate I. If the
fields are coterminous and uniform the two curves coincide
and are straight lines passing through the origin.

The rays when they travel through a gas keep passing
from a positively charged state into a neutral one and back
again to the positive charge. Sometimes instead of becoming
positively charged after being neutral they acquire a negative
charge, so that as the pencil of positive rays passes through
the gas it becomes a mixture of atoms and molecules, some
positively charged, others neutral, while some carry a negative
charge. This is very clearly shown by the following experi-
ment (J. J. Thomson, "Phil. Mag.," VI, xviii. p. 824, 1910).

The positive rays were produced in a tube made so as to
allow room for two electromagnets A and B, Fig. 25, to be

FIG. 25.

inserted between the cathode C and the willemite screen, S.
The magnets were placed so that the magnetic force due to

46 RA YS OF POSITIVE ELECTRICITY

the one nearer the cathode was horizontal and the deflection
due to it, therefore, vertical, while the force due to the
magnet next the screen was vertical and the deflection due
to it horizontal. The deflection due to the two magnets
could thus be separated and measured independently. The
effects observed when the magnets were applied separately
and then in succession are interesting. A typical case when
the pressure is such that the only spot visible is that due to
the hydrogen atom is represented in Figs. 26 and 27.

Fig. 26 gives the appearance of the screen when the

j !

a>
I

FIG. 26. FIG. 27.

magnet next the cathode is the only one in action, a is the
position of the undeflected spot, b that of the deflected, a and
b are connected together by a straight luminous band, the
luminous streak above a is due to negatively charged particles.
Fig. 27 gives the appearance when both magnets are on.
If there had been no loss or gain of charge the only effect of
the second magnet would have been to remove the spot b

LOSS OR GAIN OF CHARGE BY THE PARTICLES 47

horizontally to another place V ', and only two spots, a and b1 ',
would be visible. If, however, the pressure is not very low
there are, as a matter of fact, four spots, a, a', b, b{ ', on the part
of the screen corresponding to positive charges as well as
considerable luminosity over the rectangle with these points
as corners. Let us consider these points in succession ; b' has
experienced the full horizontal as well as the full vertical
deflection, it is therefore produced by particles which have
retained their charges while passing through both magnetic
fields. Let us now take b ; this spot has the maximum
vertical, but no horizontal, deflection. The particles producing
this spot must have been charged all the time they were in
the field of the magnet A, but have lost their charge before
reaching the field of the magnet B. This is an example of a
particle losing its charge on its way down the tube. Now
consider the spot a\; this has not been deflected vertically at
all, therefore it must be due to particles which were uncharged
when they were passing the first magnet A. On the other
hand it has experienced the full horizontal deflection, so that
the particle must have acquired a charge before reaching the
second magnet B : this is an example of a particle acquiring
a charge during its path. The appearance of the luminosity
due to the negatively charged particles shows that these too
gain and lose negative charges in their passage down the
tube.

When the pressure was lower than that in the case just
considered, though higher than that used in taking most of
the photographs reproduced in this book, the spots due to
oxygen, the molecule of hydrogen and the atom of hydrogen
could be distinguished easily, and it was found that each
one has its negative counterpart showing that all these
can receive a negative charge. We shall see later on that
the hydrogen molecule rarely receives a negative charge at

48 RA YS OF POSITIVE ELECTRICITY

the pressures at which most of the photographs are taken,
though at these pressures the negatively charged hydrogen
atom is represented on nearly every photograph. All the
spots showed the characteristics exhibited by the one spot,
due to the hydrogen atom, in the case previously considered.
In this case there are other transformations possible besides
the loss or gain of an electric charge. One of the particles
might, for example, begin its course as a molecule of hydrogen,
and in its path through the gas split up into atoms so that
the charged molecule would be represented by a charged
atom at the end of its path ; there is evidence of this on
some of the photographs which will be given later.

The preceding results were obtained when the pressure was
considerable ; when we reduce the pressure of the gas to the
lowest value we can reach by the use of charcoal and liquid
air, in the case first considered the luminosity is con-
fined to two spots, one corresponding to the undeflected
spot A and the other at b. All the luminosity inside the
rectangle has disappeared along with that arising from
particles carrying a negative charge. Investigations on the
loss and gain of charge by the positive rays have been made
by W. Wien ("Ann. der. Phys.," 39, p. 519, 1912) and by
Konigsberger and Kutschewski ("Ann. der. Phys.," 37, p. 161,
1912; Sitz. Heidelberg Akad. abh., I, 1912; Glimme and
Konisberger; Sitz. Heidelberg Akad. abh., 3, 1913).

It is natural to connect the loss of charge by the electrified
particles and the recharging of the neutral particles with
collisions between the particles and the molecules of the gas
through which they are moving, and to introduce quantities
analogous to the mean free path of a molecule of a gas to fix
the rate at which the particles pass from charged to the
uncharged state or vice versa. Thus we may introduce
the quantities ^ 1%, such that e~ x^ is the probability

LOSS OR GAIN OF CHARGE BY PARTICLES 49

that a charged particle will have retained its charge after
passing through a distance x} and that e~*l** is the proba-
bility that an uncharged particle will not have regained its
charge in the same distance. It is found too that the number
of particles, charged and uncharged, diminishes as the pencil
of positive rays passes through the gas ; we may, therefore, in-
troduce a quantity /such that if N0 is the number of particles
in the beams of positive rays when x = o, then N^"^ is
the number when the beam has passed through a distance x,
If every collision between a particle and a molecule of the gas
deprived the particle of its charge, if it were charged before
the collision, and charged it up if it were uncharged to begin
with, then if the collisions were analogous to those between

uncharged elastic spheres we should have ^ = A2 = — L__ where

IN JTo

N is the number of molecules per unit volume and S the sum
of the radii of the particle and a molecule of the gas through
which the particles are passing. It must be remembered,
however, that the particles in a pencil of positive rays are by
no means homogeneous ; some of them are atoms, others are
molecules, and in general the ., atoms and molecules of a
considerable number of different gases are present.

A pencil of positive rays becomes diffuse while passing
through a gas, showing that the direction of motion of the
particles is gradually altered by the collision ; the alteration is,
however, slight, even when the distance travelled is a consider-
able multiple of Ax and A2. The methods generally used to
detect positive rays only take into account the particles which
are moving in directions which make small angles with the
initial direction of the particles, so that if a particle were
deflected through a large angle by a collision it would escape
detection and would be counted as one of those absorbed by
the gas,

50 RAYS OF POSITIVE ELECTRICITY

An interesting feature of the transformations which the
particles undergo is that they are not accompanied by any
charge in the velocity large enough to be detected by the
methods hitherto employed ; right up to the place at which
they are absorbed the particles are moving with approxi-
mately their original velocity. This has been shown very
simply by Konigsberger and Kutschewski (" Ann. der Phys.,"
37, p. 161) by the following method : At two places, A and B,
in the path of a pencil of positive rays they deflected the
particles by magnetic forces and adjusted these forces so
that at a particular pressure the deflection at B just counter-
balanced that at A, thus the particles were not deflected after
passing through both the magnetic fields. They found that
if this adjustment were made for any particular pressure
of the gas through which the particles were passing it held for
all pressures at which the positive rays could be observed.
If the velocity of the particles were appreciably diminished
by a collision, then, since at the higher pressures the particles
would make more collisions in traversing the path from A to
B, the velocity at B would fall below that at A more at
high pressures than at low ones. But the deflections produced
by the magnetic fields both at A and B depend on the
velocity of the particles, and if a balance is obtained when
there is one proportion between the velocities it will be
disturbed when that proportion is altered. When we increase
the number of collisions the proportion must be altered if
there is any appreciable loss of velocity at a collision. The
fact that the balance is independent of the number of collisions
shows that the collisions are not accompanied by any
appreciable loss of velocity.

In the case of the a particles given out by radioactive
substances, which are also positively electrified particles
though their speed is much higher than that of positive

LOSS OR GAIN OF CHARGE BY PARTICLES 51

rays produced by electric discharges, there is a considerable
diminution in velocity before they cease to produce appre-
ciable effects. The difference can, I think, be explained
by taking into consideration the difference in the velocity
of the particles in the two cases. The absorption of an a
particle or a positive ray may be regarded either as the
result of an impact with a molecule, of such intensity that
the particle is deflected through a considerable angle, or as
a capture of the particle by a molecule ; in either case the
probability will diminish rapidly as the energy of the particle
increases. The charging and recharging of the positive rays
are the results of collisions of a much milder type, and it
is probable that the chance of such collisions is not diminished
by an increase in the kinetic energy of the particles to
anything like the same extent as it is for the more intense
collisions which result in absorption. The result would be
that the a particles would make far more of these minor
collisions before being absorbed at a major one than the
particles in the positive rays. We know, for example, from
the ionization produced by the a particles that these particles
make before they are "absorbed" as many as 100,000 or
more collisions. The measurements of Konigsberger and
Kutschewski (/. c.) show that the quantities we have called
Ax, A2 are of the same order as /; this means that the
particles in the positive rays only make a small number of
collisions before they are absorbed. Since the change from
the uncharged state to the charged one involves the ionization
of the gas through which the particles are passing, some
energy must be absorbed at these stages, though it need not
be more than that corresponding to the ionizing potential
of the gas, i. e. a quantity of the order of 10 volts ; hence, if
a particle were to make as many as ten of these collisions
before absorption, the loss of energy would only amount to

52 RA YS OF POSITIVE ELECTRICITY

some 100 volts, and as the original energy in these rays is
generally above 20,000 volts the diminution would have been
too small to be detected. The case, however, is very different
when we have 100,000 collisions as in the a rays ; here the
loss of energy is comparable with that possessed initially by
these rays.

Wien (/. £.) has determined the values of the quantities
we have denoted by Ax, A2 for hydrogen, oxygen, nitrogen,
both in the cases when the positive rays were made from
the gas through which they passed and also when they were
made from different gases. In the case of hydrogen rays
passing through hydrogen he finds that ^ (reduced to
atmospheric pressure on the supposition that it varies
inversely as the pressure) is

6 . 15 x io~5 cm.
and that ^2=34 • 8 X io~5 cm.

The beam of positive rays included both atoms and mole-
cules, of hydrogen, so that these values are intermediate
between the values of A for atoms and molecules.

The mean free path of a molecule of hydrogen through
hydrogen is according to the kinetic theory of gases io~5cm.,
and that of an atom of hydrogen through molecules of
hydrogen about 2X io~5cm. The values of A, though greater
than the ordinary free paths, are of the same order of magnitude,
so that a positive ray particle could not make many collisions
of the type of those contemplated in the kinetic theory ot
gases without altering its electrical state. An interesting
point brought out by Wien's experiments is that the values
of A do not seem to depend upon the electro-positive or
electro-negative character of the gas. He found that the
values of A when hydrogen positive rays passed through
oxygen, which is strongly electro-negative, were much the same
as the values when these rays passed through nitrogen, which

LOSS OR GAIN OF CHARGES 53

has much less strongly marked electrical properties. At first
sight it might have been thought probable that the chance
of a positive particle being able to take an electron from an
electro-negative gas like oxygen would be less than that of its
taking one from nitrogen and, therefore, that ^ for hydrogen
rays through oxygen would be much greater than its value
when the rays passed through nitrogen. Wien's experiments
show, however, that this is not the case, and, indeed, further
consideration would show that we should not expect it to be
so ; for the ionizing potential for oxygen, which is the measure
of the work required to take an electron from a molecule of
oxygen, is not greatly different from the ionizing potential
of nitrogen. The only effect produced by the electro-negative
or electro-positive property of a gas is that in the electro-
negative gases like oxygen, chlorine or iodine the negatively
electrified constituents in the positive rays are more pro-
nounced than in the other gases. These negatively electrified
rays are not by any means confined to the electro-negative
elements, for, as we shall see, hydrogen and carbon atoms
very often occur with a negative charge.

Let us now consider what occurs in the gas through
which the particles in the positive rays are passing when
these undergo the transformations we have just been
considering.

Let us take first the case when a positively charged particle
becomes neutralized. It does so by acquiring an electron
from the molecules of the gas through which it is passing.
This will result in a molecule of the gas having a positive
charge, or if the collision has dissociated the molecule one
atom of the gas will be positively electrified and another
atom neutral. Whichever view we take the loss of the positive
charge is accompanied by the formation of one positive ion
in the stationary gas.

54 RA YS OF POSITIVE ELECTRICITY

Again, when the uncharged particle acquires a positive
charge we can see that there must be one negative ion in the
stationary gas ; this may be either a negatively charged atom
or a negatively charged molecule according as the collision
which charges the moving particle does or does not produce
dissociation.

If the neutral particle acquired a negative charge, as it does
in some cases, one positive ion would be formed in the
stationary gas. Leaving out of consideration the negatively
electrified particles, we see that when a particle in the positive
rays has passed once into the uncharged state and back again
into the charged state it has produced two ions. Now
Seeliger (" Phys. Zeitschr.," 12, p. 839) found that when positive
hydrogen rays passed through hydrogen at the pressure of
T^j- of a millimetre of mercury each particle produced \ of an
ion per cm. of path. This number did not seem to vary
much with the speed of "the rays. The average distance
travelled by a particle between losing its charge and regaining
it is ^ + A2 (see p. 49) ; taking the values of Aj + A2 found
by Wien we find that at a pressure of yJ-F of a mm.

AJ + A2 = 30 cm. approximately,

so that owing to the transformations from the charged to the
uncharged state each particle would produce 2 X ^V == rs i°ns
per cm. or about J of the number found by Seeliger. We
must remember that, as we have seen, the rays by these
collisions lose only an exceedingly small fraction of their
energy, so that their energy is practically intact when they
are absorbed. If Seeliger's numbers are right little of this
energy can be spent in ionizing the gas ; it may perhaps
be spent in dissociating the molecules of the gas into
uncharged atoms. For, as we have seen, the path the
particles travel before being absorbed is quite comparable
with Ax + A2, so that there is a considerable probability of a

ORIGIN OF SPECTRA 55

particle being absorbed in running through this distance in
which it makes only two ions and when the energy it has
retained is sufficient to make several hundred ions. If all
this energy were spent in ionizing the gas, the number of
ions produced by the absorbed rays would be a very large
multiple of that calculated on the assumption that there is
no absorption. Seeliger's result indicates that it is only five
times as much. The subject is one that would repay further
investigation.

On many theories of the origin of spectra the emission
of series lines is connected with the return of an electron to a
positively charged atom, so that the series lines of the gas
through which the positive rays are passing would not be
excited unless these rays produced some positively charged
atoms in this gas. We see from the preceding considerations
that when a positively electrified particle loses its charge
positively charged atoms are produced in the gas ; when
however, it regains its charge no such atoms need be
produced. Thus, on the theories of the origin ,of spectra
referred to, the positive rays would excite the line spectra of
the gas through which they pass when they lose their charge
but not when they regain it. This might be tested in the
following way : If a pencil of positive rays were sent between
two parallel plates, with a large potential difference between
them, all the positively electrified particles would be driven
against one of the plates, and the beam when it first emerged
from the plates would contain nothing but uncharged particles ;
these would gradually acquire a positive charge, but this
process does not excite the series lines of the gas through
which they are passing, hence the region traversed by the
rays just after they leave the plates ought not to give out the
series lines of the gas.

The light given out by the gas through which the particles

56 RAYS OF POSITIVE ELECTRICITY

pass presumably, since it is a line spectrum, originates from
atoms and not molecules. These atoms cannot be moving with
velocities at all comparable with those of the particles in the
positive rays, for otherwise there would be an appreciable
broadening of these lines. Wien (" Ann. der Phys.," 43,
p. 955) investigated this point for lines given out by mercury
and helium and came to the conclusion that there was no
perceptible broadening. He could have detected easily the
effect if the atoms giving out the light had possessed velocities
of the order they would have acquired by collision with the
positive ray particles provided these collisions had been like
those between elastic spheres. Hence we conclude that those
collisions which result in the absorption of the positive rays
do not split up the molecules of the gas into charged atoms.
This is in accordance with the conclusions we drew (p. 54)
from Seeliger's measurements of the ionization produced by
positive rays.

On the other hand the collisions which result in a loss or
gain of charge by the positive ray particles, where, as we have
seen, the transference of energy from the particles to the
molecules is exceedingly small, not only ionize the gas but
split the molecules up into atoms.

We should expect that the particle would not be able to
lose or gain a charge unless its velocity exceeded a certain
critical value, for either of these charges involves the
tearing of an electron out of an atom or molecule. When the
particle loses its charge the electron is torn from the mole-
cules of the gas through which it is moving, when it regains
its charge the electron has to be torn from the particle.

To tear an electron from an atom or molecule requires
a finite amount of work, and in the case we are considering
this work must be supplied by the moving particle during
a collision with the molecule. Since there is little change of

COLLISIONS BETWEEN ELECTRONS AND ATOMS

direction in the collisions which charge the molecule, the
collisions must be collisions of the particle with an electron,
and not with the part of the molecule which furnishes
any appreciable part of its mass. Now if a mass Mj moving
with a velocity V comes into collisipn with a mass M2 at rest
the maximum amount of kinetic energy which can be

communicated to M2 is

4M1M,

where T is the kinetic energy of Mx before the collision. If
M! is the mass of a particle in the positive rays, M2 that of
an electron, Mj will be large compared with M2, so that the
maximum kinetic energy that can be given to the electron is

4M.? T

MI;

and is thus equal to the kinetic energy of an electron
moving with twice the velocity of the particle. Thus if
the work required to tear an electron from a molecule of
hydrogen is measured by 1 1 volts, which is equivalent to the
kinetic energy of an electron moving with a velocity of 2 X io8
cm./sec., a moving atom or molecule could not under the
most favourable circumstances eject the electron if its
velocity were less than io8 cm./sec. In the preceding
investigation we have supposed that the electron was free;
the result will be modified to some extent if the electron is
bound by forces to the massive part of the atom. Indeed,
if these forces were infinitely strong the effective mass of the
electron might be that of the molecule, and a larger amount
of energy might be transferred from the particle to it and
the molecule ; these collisions would, however, be more akin
to those which produce absorption, than to those which
produce loss or gain of charge.

The case we have considered is that of the loss of charge

58 RA YS OF POSITIVE ELECTRICITY

by the particle when it has to tear from the molecules an
electron to neutralize the charge — the gain of charge will be
affected by similar considerations. Here the molecule has
to tear an electron from the particle, and to do so the
relative velocity of the two must exceed a definite value
depending on the work required to tear an electron from the
particle. In the preceding case the limit depended on the
work required to tear an electron from a molecule of the gas
through which the particle was moving.

We conclude then that a particle will neither lose nor gain
a charge unless its velocity is above a certain limit which
depends both on the nature of the particle and of the
gas through which it is moving. This gives an inferior limit
to the velocity of the rays which undergo transformations
from the charged to the uncharged state. There will also be
a superior limit to the velocity of the particles which pass
from the charged to the uncharged state, for though a particle
might detach an electron, it could not retain it if the relative
velocity of the particle and electron exceeded a certain value.

The ionization we have been considering is that which is
produced by collisions which do not appreciably deflect the
path of the positive rays, for if these rays suffered any
considerable deflections by collisions they could not be
recognized on the photographs. It does not follow that
to ionize by other types of collision the positive particles need
possess velocities approaching the values required when
the collisions are restricted to this particular type. We know
indeed from the experiments of McClelland (" Proc. Camb.
Phil. Soc.," XL, p. 296), Pawlow (« Proc. Roy. Soc.," A. 90, p.
398), v. Bahr and Franck (" Verh. der. Deutsch. Physik Ges.,"
1 6, p. 57) on ionization round positively electrified hot wires
that when all types of collisions are operative positive ions can
ionize a gas when their energy is that due to a fall through a

IONIZATION 59

voltage very small compared with that necessary to give them
velocities comparable with the io8cm./sec., which is the order
of the velocity required by the positive rays.

There must, therefore, be ways other than the ones we
have discussed by which positive particles can produce
ionization, and alternate between the charged and uncharged
state. Let us consider, for example, the loss of charge by a
positively charged particle. This might occur if the particle
in its journey through the gas passed through a molecule of
the gas and captured one of its electrons and carried it away
with it. Again, a neutral particle passing through a molecule
might have one of its own electrons captured and retained
by the molecule, and emerge with one electron less, and
therefore with a positive charge. We observe that in the
first of these cases there is a positive ion produced in the
gas and in the second a negative one, in neither case is a
free electron produced : this distinguishes this process of
ionization from that previously discussed. In this process a
very high velocity of the particle is not necessary : in fact, if
it had sufficient energy to pass through the molecule it would
be more likely to capture one of its electrons if it were moving
slowly.

We could explain in this way the formation of secondaries
by the heavier atoms : the fact that these are exceptional
shows, I think, that this method of ionization is not so
effective as the other. Another reason for this view is that if
the second method took place to any large extent we should
expect to find a considerable number of the particles with a
negative charge. For consider the case when an uncharged
molecule is moving rapidly through other molecules of the
same kind : it is supposed to get its positive charge by a
stationary molecule capturing one of its electrons, but since
the effect depends only on the relative velocity of the two

60 RAYS OF POSITIVE ELECTRICITY

molecules, it is just as likely that the moving particle should
be the one to capture the electron as the stationary molecule,
in which case it would get a negative charge. It is, however,
only special kinds of atoms which give on the positive-ray
photographs any indication of having a negative charge.
Again, if any process of this kind occurred in more than a
small fraction of the collisions we should expect to get far
more ionization by the positive particles than is indicated by
Seeliger's experiments. It must not be forgotten that the
collisions made by the positive particles in their journey
through the gas generate radiations which are able to produce
dissociation.

It is important to point out that the collision which
ionizes a neutral particle and gives it a positive charge must
be a collision with an electron and not with a molecule of the
gas through which the positive rays are passing ; for the mass
of a molecule of the gas is comparable with that of the positive
ray particle, hence a collision between the two would result in
the particle losing an appreciable fraction of its energy and
being deflected through a considerable angle. The appearance
and inclination of the secondary lines show that the particles
suffer little diminution in velocity in these encounters and no
appreciable change in direction, hence we conclude that the
system with which the particle collides must have a much
smaller mass than the particle, i. e. it must be an electron and
not a molecule.

It is to the gain and loss of charge through collision with
the molecules of the gas through which the positive rays are
moving that we ascribe the origin of the lines we have
described on p. 43. We shall call these lines secondary
lines and the parabolic ones primary lines.

The type of ionization we have been considering requires
the particles to have a velocity comparable with io8 cm. /sec. ;

SECONDARIES

the heavier particles could riot, however, acquire a velocity
approaching this under the potential differences which are
usually applied to the tubes used to generate positive rays.
We should, therefore, expect that the parabolas correspond-
ing to the heavier elements would not be accompanied by
secondaries. This absence of the secondaries to the heavier
lines is in general a very marked feature of the photographs.
There are, however, exceptions, e.g. the parabola correspond-
ing to CO is accompanied by a secondary even at very low
pressure ; and Wien has shown that the molecules of oxygen
and nitrogen in the positive rays lose and regain the charges
when the difference of potential is much less than the amount
required to give them a velocity approaching I08cm./sec.

The secondary curves finally join the parabolic arcs pro-
duced by the particles which have been charged during the
whole of their journey. If the junction occurs at a consider-
able distance from the head of the primary, care has to be
taken in some cases to avoid confusing the secondaries with
primaries corresponding to a different value of ejm. Thus, for
example, if the shape of the secondary and primary were
similar to that shown in Fig. 28^, and the point of junction
came off the plate, the appearance on the plate would be that
represented in Fig. 28^, and the secondary might be mistaken

(a)

FIG. 28.

for a primary with a value of elm less than the true value.
If the magnetic field overlapped the electric field on the

RAYS OF POSITIVE ELECTRICITY

camera side of the apparatus, the primary and secondary
might resemble Fig. 29^, and if the right-hand part were off
the plate, the curves would look like Fig. 29^ and the

(a) FIG. 29. (6)

secondary might be mistaken for a primary with a value of
ejm greater than the true value. This possible confusion of a
secondary with a primary line is a point which requires care-
ful attention when the curves produced by the positive rays
are used to identify the gases in the discharge tube ; for this
purpose the primary curves are the only ones that can be
relied upon. The tests for a primary line are (i) that it is
parabolic, (2) that it shows an abrupt increase in intensity at
a point in the same vertical line as the heads of the other
parabolas. The first condition is theoretically sufficient, but
when only short arcs are available it is often difficult, unless
a very high degree of accuracy is obtained in the measurement
of these lines, to tell whether the curve is or is not a parabola.

We shall see that the study of the photographs gives us
further information about the conditions which govern the
loss or gain of a charge by the particles in the positive rays.

An interesting feature of these secondary lines is that
they are generally sharp and well-defined. Even though
the parabolic arc AB which they join may be of con-
siderable length — showing that the velocities of the particles
are spread over a wide range — the secondaries do not
fill up the whole of the region AOB but are concentrated

SECONDARIES 63

along one or more well-defined lines. Most frequently there
is a well-defined line from O to A, the point on the parabola
corresponding to the particles with the greatest velocity ;
sometimes, too, there will be in addition another line running
from O to another point on the parabola as in the photograph
reproduced in Fig. 4, Plate VII. In some cases the secondary
to the end of the parabola is wanting and the secondary line
joins the parabola at another point. This condensation of
the secondaries into lines running to definite points on the
parabola is due, I think, to there being a great condensation
of the particles in the primary rays round certain velocities,
especially round that corresponding to the head of the
parabola. This condensation is apt to be obscured when
photographic plates which are very sensitive to positive rays,
such as Paget or Schumann plates, are used. With these
plates a comparatively small number of particles is able to
produce the maximum effect, and the result is that the
parabolas seem to be of nearly equal intensity along a great
part of their length. When much more insensitive plates
are used the blackening at the head of the parabola is seen
to be in most cases much greater than that at other parts
of the arc.

We sometimes see secondaries going from O to a point on
the parabola corresponding to the hydrogen molecule and
then proceeding, with diminished intensity, up to the parabola
corresponding to the hydrogen atom. This indicates that
some of the particles which start as molecules of hydrogen
split up in their course through the gas into hydrogen atoms.

The view that the secondary lines are connected with
great concentration of the particles around certain velocities
is confirmed by the fact that when the parabolas have a
beaded appearance (see p. 42), and thus indicate considerable
concentration round certain velocities, there are apt to be

64 RA YS OF POSITIVE ELECTRICITY

secondaries running up to the beads in addition to the one to
the head of the parabola.

In some cases where there is a fairly uniform distribution
of velocities among the particles, the secondaries are not con-
centrated along definite lines, but are spread over a consider-
able area. An example of this is shown in Fig. 4, Plate II.

A special type of secondary is shown in Fig. 3, Plate VI.
In this case the magnetic field overlapped the electrostatic, so
that the equation to the secondary corresponding to a particle
with a velocity v will be

y=^l'L+XKv

mv X

where y and x are measured parallel to the displacements due
to the magnetic and electrostatic forces respectively. H is
the magnetic and X the electrostatic force, /' the distance
the magnetic field overlaps the electrostatic, and L the distance
of the strip /' from the photographic plate. These secondaries,
since v varies, form a complex of lines the envelope of which is

This is a parabola and is well marked on the photograph.
The parabola might have been mistaken for one of the
primary ones due to particles with a definite value of ejm ; it
can, however, be distinguished from these by the fact that,
unlike them, it reaches right up to the origin and has no
definite head.

Another point to be noticed is that some kinds of particles
give rise much more easily than others to these secondary
lines. In general the secondaries are much the most con-
spicuous with the lightest particles such as those of H or H2.
These particles are the ones which are moving with the
highest velocity, and in accordance with the reasons given on
p. 57 we should expect that to give rise to secondaries the

SECONDARIES 65

particles must be moving faster than a certain critical
velocity. The velocities of the oxygen atoms are only one
quarter of those of the hydrogen ones, and we can easily
understand that while the speed of the atoms of hydrogen
might be above the critical velocity that of the atoms of oxygen
would be below it, so that we should get hydrogen secondaries
but not oxygen ones.

The critical speed required before a particle could lose its
charge would on the views expressed on pp. 53, 54 depend
mainly upon the gas through which the particles were moving
and so would probably not vary much for the different particles
in one pencil of the positive rays. The velocity required for
a particle to regain a charge depends essentially upon the
ionizing potential of the particle, and so would vary from
particle to particle in the same pencil.

Either loss or gain of charge may give rise to secondaries,
and we have seen how the different types of secondaries may
be distinguished, and that unless the magnetic and electric
fields are coterminous there may be one secondary for the
loss and another for the gain of charge. When the pressure
is high both of these may be detected ; at lower pressures this
is not in general the case, and I am inclined to think that here
the secondaries are all of one type.

This is suggested by the fact that on some plates we find
a straight secondary which stops abruptly after going a
certain distance and is not joined on to any parabola. Such a
plate is represented in Fig. 2, Plate VI. We should get a line
of this kind if the particles, for example, could lose but not gain
a charge, and if they all lost a charge before they had passed
through the electric and magnetic fields. We get ample
evidence from the plates that the limiting speed of the particles
required to produce secondaries varies with the nature of
the particles. Let us take, for example, a very frequent case :
F

66 RA YS OF POSITIVE ELECTRICITY

The plate shows the parabolas corresponding to the hydrogen
atom and molecule, the atoms of carbon and oxygen, and
those corresponding to CO and CO2. Then, if the pressure
is not exceedingly low, we find secondaries corresponding to
Hj and H2, none corresponding to C or to O or to CO2, but a well-
marked one corresponding to CO, although the velocities of
these particles is much lower than those of the atoms of C
and O which do not give secondaries. A similar effect is
shown by the photograph represented in Fig. 2, Plate II.; when
the gas in the discharge tube was exceedingly pure oxygen,
the line a corresponds to the oxygen atom, the line below
it to the oxygen molecule. We see that though the atom line
is very strong it has no secondary, while the line corresponding
to the molecule has a very pronounced one. I have other
photographs where the line corresponding to the oxygen
molecule is by far the strongest line on the plate, and yet
shows no secondary, while the CO line on the same plate
shows a well-marked secondary. Though secondaries to the
CO2 lines are not common they do sometimes occur. Other
things being the same, a low ionization potential ought to
promote the formation of secondaries. It is worthy of notice
that though the line corresponding to the positively electrified
oxygen atom may be free from secondaries, the weaker
line corresponding to negatively charged oxygen atoms shows
a well-developed secondary.^ The loss of charge by a nega-
tively electrified atom merely involves the abstraction from the
atom of the extra electron which gives it the negative charge,
while the loss of charge by a positively electrified atom
involves the abstraction of an electron from the neutral mole-
cule through which the atom is moving ; the two processes are
quite different, and we should expect the loss of the negative
charge to require less energy than that of the positive. The
gain of a negative charge by a neutral atom is accomplished

SECONDARIES 67

by a process very similar to the loss of charge by a positively
electrified one.

Very interesting variations occur in the relative inten-
sities of the secondaries corresponding to the hydrogen
atom and hydrogen molecule respectively. In many cases
the secondaries for the hydrogen molecule are much more
conspicuous than those for the hydrogen atom, indeed on
many photographs the secondaries for the molecule are
quite strong, while those for the atom cannot be detected.
And in others, though some secondaries can be seen reaching
the parabola corresponding to the atom, they are prolongations
of stronger secondaries to the parabola corresponding to
the molecule, and suggest that they arise from particles which
began by being molecules but were dissociated into atoms
in their path through the gas in the electric and magnetic
fields.

Though the secondaries are generally easily distinguish-
able from the primaries there are not infrequently lines on
the plates which require further consideration before their
origin can be determined. Such a
case is represented diagrammatically
in the figure where between the
parabolas corresponding to the
hydrogen atom and molecule there is
a line approximately parabolic and
prolonged backwards until it meets
the vertical line through the origin.
The curved part of this line might
be a primary due to a particle with FlG

a value of m\e between I and 2, the

prolongation being its secondary. If this were so the position
of this line relative to the H1 and H2 lines ought to be
independent of the disposition of the electric and magnetic

Hi;

68 RA YS OF POSITIVE ELECTRICITY

fields. This, however, is not always the case ; for example,
in Fig. 2, Plate VIII. we see on the photograph a line between
those corresponding to H2 and H3. When this photograph
was taken the electrostatic field was short ; on lengthening
the field, leaving other conditions the same, this line between
2 and 3 disappeared and another line appeared in a different
place. Thus, this line cannot represent an element with an
atomic weight between 2 and 3. It is, I think, a secondary
differing from the secondaries we have hitherto considered
by being curved instead of straight. This curvature can be
explained by inequalities in the electric and magnetic fields.
Using the same notation as before, let us suppose that the
particle does not acquire a charge until it has travelled a
distance f in the electric and magnetic fields. The y and x
displacements when the length /of the field is small compared
with the distance L of the photographic plate from these
fields are given by —

L £ [l TT / L e \l ^r i

y — / , Hak ; x = - ~ I , Xds.

v m n v2 mJ £

The secondary on the plate will be the locus of points corre-
sponding to different values of £,. one point on the curve
corresponding to each value of |. We have from the
equations just given —

dy __ _ ILe TT dx _ L£ ^

— jS """" ~~ '^£> "7> ~~ ~~ ~9 ^"f*

d% mv d% mv2

Thus* = ^l.
dx X^

Thus, if the ratio of the magnetic to the electric force is
variable, dy\dx will be variable and the locus will be curved.
The sharpness of the line on the photograph indicates that
the particles which produce it are all moving with the same
velocity, and since from the photographs it is clear that
when this curve joins the primary corresponding to the

SECONDARIES 69

value of m]e the junction must be far from the head of its
parabola, this velocity must be considerably less than the
maximum velocity acquired by these particles in the dis-
charge tube. Secondaries of this type, due to the hydrogen
atom, would always be less deflected than the primary
parabola corresponding to the hydrogen atom, and those
due to the hydrogen molecule less deflected than the
primary of the molecule.

We have assumed throughout that the electrons which
produce the secondaries by neutralizing a positively charged
particle or ionizing a neutral one are not the free electrons,
but those bound up in the molecules of the gas through which
the positive rays are passing. In support of this view the
following considerations may be urged. If the electrons were
free they would be removed by a strong electric field, and
thus the brightness of the secondaries would be diminished
by such a field. I have never been able to obtain any evidence
of such an effect. Again, as these free electrons would have
to be produced by the positive rays, their number would
increase with the number of positive rays passing through
the gas. As the values of A1? A2 (see p. 49) depend upon
the density of the electrons, these values would not be fixed
merely by the pressure and character of the gas through
which the rays passed, the intensity of the stream of positive
rays would have an important influence on their values. The
determination made by Wien of these quantities are quite
inconsistent with this.

In uniform and coterminous electric and magnetic fields
the velocity of a particle is proportional to y\x where y is
the magnetic and x the electric displacement, thus all particles
which have the same yjx have the same velocity. The
straightness of the secondary lines shows that all the particles
which produce them have the same velocity. Since different

70 RA YS OF POSITIVE ELECTRICITY

points on these lines correspond to particles which have
travelled different distances through the gas before losing
their charge, their straightness indicates that there is no con-
siderable loss of velocity by the particles as they pass through
the gas.

By means of the formula (3), p. 21, we can calculate v,
the velocity of the particles which produce the secondaries, for
the hydrogen atom or molecule. The determinations of this
kind which I have made make v for the secondaries for the
atom about 2 x io8 and for the molecule about 1*3 X io8
cm./sec. (

NEGATIVELY CHARGED PARTICLES

We have seen (p. 45) that besides the particles which
carry positive charges of electricity there are others which
carry negative ones. These negatively charged particles
show many analogies with those which produce the secondary
rays we have been considering. Like them they are particles
which have changed their condition after passing through the
cathode. Before they passed through the cathode they were
positively charged, and they owe their velocity to the action
of the electric field in front of the cathode on this charge.
After passing through the cathode they attract first one
electron which neutralizes them, and then a second which
gives them a negative charge. The attraction which brings
in the second electron is one between a neutral particle and
an electron. We may compare it to that due to electrostatic
induction between an electric charge and a neutral body ; the
magnitude of this charge depends on the specific inductive
capacity of the body and vanishes when this is the same as
that of the surrounding medium. It is not surprising, there-
fore, to find that different kinds of atoms and molecules differ
very greatly in their powers of acquiring a negative charge.

NEGATIVELY CHARGED PARTICLES 71

The negative components of the positive rays are, in
comparison with the positive ones, more conspicuous at high
pressures than at low. Thus, for examples at pressures
higher than that used for the photographs reproduced in this
book we often find the molecule of hydrogen with a negative
charge, while at the pressures at which these photographs
were taken the negative molecule cannot be detected though
the negative atom is nearly always present. Again, the line
due to the negative atom of hydrogen is in these photographs
faint compared with that due to the positive; at higher
pressures, however, I have seen the negative line as strong as
the positive.

The electro-chemical properties of the gases play a more
conspicuous part in the occurrence of negative constituents
than in any other feature of the positive rays. Thus, for
example, the atoms of the electro-negative elements oxygen
and chlorine are remarkable for the ease with which they
acquire a negative charge, and though negative charges occur
on atoms of hydrogen and carbon which are not usually
regarded as electro-negative, yet there are many gases, e. g.
helium, nitrogen, neon, argon, krypton, xenon and mercury of
which I have never seen the parabolas corresponding to the
negative atom, though those corresponding to the positive
atoms have been very strong. Again, negatively electrified
molecules, with the exception of those of hydrogen, oxygen
and carbon and these but sparingly, have never been detected
in the positive rays. The only cases of a molecule of a
compound gas occurring with a negative charge which I
have observed are those of radicles such as OH, CH2;
while molecules with positive charges occur readily enough.
The negatives C, CH, 0, OH occurred when the gas in the
discharge tube was hexane.

We can understand why a positively electrified atom or

72 RA YS OF POSITIVE ELECTRICITY

molecule is likely to be much more stable than a .negatively
electrified one. Take the case, for example, of an atom of
hydrogen ; when the atom is negatively charged it contains
two electrons each of which is less firmly held than the single
electron in a neutral atom ; on the other hand the positively
electrified atom does not contain an electron at all. Thus
the negatively electrified atom when exposed to violent
collisions with other atoms and molecules is evidently more
likely to lose its charge than a positively electrified one.
Let us consider for a moment the conditions which deter-
mine whether a neutral atom in a pencil of positive rays
should acquire a positive or a negative charge. It acquires
these charges by collisions with the neutral molecules through
which it is passing. By the collision the previously neutral
positive ray particle acquires a charge of one sign, the
neutral molecule against which it strikes one of the opposite.
The system which gets positively charged loses an electron,
the one which is negatively charged gains one. If one of the
colliding systems is much more easily ionized than the other
we should expect that this would be the one to lose an
electron and acquire the positive charge. Thus, if the gas
through which a neutral oxygen atom were moving were
helium, which has a very high ionizing potential, we should
expect that the oxygen atom would be much less likely to
acquire a negative charge, involving, as this does, the abstract-
tion of an electron from the helium, than it would if it were
moving through a much more easily ionized gas such as
mercury vapour. There is some confirmation of this view,
since Wien ("Ann. der. Phys.," 39, p. 539) noticed that the
presence of mercury vapour increased the number of oxygen
atoms carrying a negative charge ; the effect of mercury
vapour on the negative hydrogen atoms has not, however,
been detected. These considerations suggest that the intensity

\
NEGATIVELY CHARGED PARTICLES 73

of the lines, due to the negatively charged particles, might be
affected to a considerable extent by the presence in the gas
through which they have to pass of gases which are easily
ionized. We have seen that in the case of the loss and gain
of charges by the positively charged particles, it is the process
of getting rid of the charge which produces positive ions
in, and excites the spectrum of, the gas through which the
particles are passing. In the case of the negatively electrified
ones, however, it is the process of gaining the charge that
excites the spectrum of the surrounding gas. Thus, if we
could isolate the light due to a pencil of negatively electrified
and neutral particles, we should not be able to quench it by
driving by means of a strong electric fi^ld the negatively
charged particles out of the gas, leaving the neutral ones
behind.

Though in one sense all the lines on the photographs,
which are due to negatively charged particles, are secondaries,
different parts of them show differences corresponding to
the difference between the primary and secondary positive
lines. Some of the negative lines, like the positive secondaries,
come close up to the origin, while others, like the primary
positives, are finite arcs of parabolas terminating abruptly
when they approach within a certain distance of the vertical
through the undeflected spot. Indeed the lines on the
negative side are sometimes exact reproductions in shape
and size of those on the positive. An example of this is
shown in Fig. i, Plate III. The curve at the top on the right
corresponds to the hydrogen atom with a positive charge, the
lower one on the left to the atom with a negative charge : it
will be seen that every detail in the positive curve is repro-
duced in the negative. This might suggest that the positive
and negative atoms were the two halves of a neutral molecule
which divided after passing through the cathode. Further

74 RAYS OF POSITIVE ELECTRICITY

consideration, however, shows that this view is not tenable at
any rate in the great majority of cases. The heads of the
negative parabolas, like those of the positives, are all on a
vertical line, and the distance of this line from the vertical line
through the origin is about the same as the corresponding
distance for the positive parabolas. It follows from this by
equation (2), p. 21, that the maximum value of the kinetic
energy of the particles is about the same for the negative as
for the positive particles. It is generally a little less, but the
difference is not large. Now, to take a definite case, let us
suppose that the negative hydrogen atom owes its charge to
having been in chemical combination with, say, an atom of
carbon before passing through the cathode, the molecule
of the compound having been positively charged when in
the discharge tube and thus acquiring a high velocity under
the electric field. After passing through the cathode the
molecule^loses its charge, and then dissociates into a positively
charged carbon atom and a negatively charged hydrogen
one. The kinetic energy acquired by the molecule CH, if
it had one charge of electricity, would be measured by V,
the potential difference between the anode and cathode
in the discharge tube. Since the mass of the carbon atom
is twelve times that of the hydrogen one, the kinetic energy
possessed by the latter would be measured by V/I3, so that
if this atom went through the same electric and magnetic
fields as the carbon atom, the horizontal deflection of the
hydrogen atom would be twelve times that of the carbon
one. The photographs show, however, that these deflections
are nearly equal. Thus the view that the negatively charged
atoms arise from the dissociation of rapidly moving neutral
molecules cannot be reconciled with the results of experiment.
The results, as far as they are known, are in accordance
with the view that the negatively charged atom began as

NEGATIVELY CHARGED PARTICLES 75

a positively charged one, and then captured two electrons
in succession, and thus became negatively charged. Even
though a neutral atom or molecule managed to knock an
electron out of a molecule with which it came into collision,
it does not follow that it would be able to capture the electron :
to do this the neutral atom must exert considerable attraction
on the electron. The magnitude of the attraction between
a neutral atom and an electric charge must, if we regard the
atom as made up of electrons and a positive charge, depend
on the freedom with which the electrons can .move under
a force exerted by an electron outside ; if they can move
readily the attraction may be considerable, if, on the other
hand, they are rigidly connected with the atom it will be
very small. A very simple experiment will illustrate this
point. Suppose we have a considerable number of small
compass needles with agate caps placed on a disc which is
suspended from a long string. If we mount the compasses
so that they can turn freely on needle points fixed to the
disc, and then hold a magnet near the disc, the disc will be
strongly attracted by the magnet. If, however, we take the
compasses off the needle points and lay them on the disc the
friction will prevent any motion relative to the disc, and
when the magnet is placed in the same position as before
the attraction will be found to be very much reduced.

Thus we should expect the attraction between a
neutral atom and an electron to be much increased by the
presence in the atom of electrons which can move freely
relatively to the atom. If these freely moving electrons are
those which are near the surface, and which give rise to
the forces which bind the atoms in a molecule together,
we can understand why a neutral molecule should not attract
an electron as vigorously as a neutral atom. For when two
atoms in a molecule are held together by the forces between

76 RA YS OF POSITIVE ELECTRICITY

their electrons, the electrons in each atom will take up
definite positions in the atoms and will resist any displace-
ment. Their mobility will thus be diminished and they will
not exert so much attraction on a charge of electricity outside
the molecule. It is remarkable that, so far as we know,
the atoms of the monatomic gases never occur with a
negative charge in these experiments ; this is consistent
with the preceding theory, for the existence of the molecule
depends on one of its atoms being able to grip one or more
of the electrons of the other, thus one of the atoms must be
able to hold a negative charge.

The properties which prevent an atom in the positive rays
from acquiring a negative charge are operative in the general
case of ionization, produced by such agents as Rontgen rays,
ultra-violet light, etc. For Franck (" Verh. d. Deutsche. Phys.
Gesell.," 12, 613, 1910) has shown that in these cases gases
such as argon and nitrogen, which are never found with
negative charges in the positive rays, exert so little attraction
on the electrons that these remain free after having made
large numbers of collisions with the molecules of the gas.
The circumstances under which the molecule of carbon
acquire a negative charge are of considerable interest. When
the carbon compounds in the discharge tube are such as CH4,
CO or CO2, where there are no bonds between the two
carbon atoms in the molecule we get negatively charged
carbon atoms but no negatively charged molecules. When,
however, we have compounds such as acetylene H — C=C— H,

, , v /

ethylene >C = CS or ethane H —$C — C^—H
R/ ^H H/ \H

where according to the usual interpretation of the constitution
of these substances there are bonds between the carbon atoms
in the molecule, we find molecules as well as atoms of carbon

MULTIPLY CHARGED PARTICLES 77

with a negative charge. This is a very interesting result,
as it shows (i) that there are strong forces between two
carbon atoms in a molecule of the compound, forces strong
enough to keep them together when the compound molecule
is split up ; (2) that the electrons in the constituent atoms of
the carbon molecule have considerable mobility, i. e. that the
pair of carbon atoms are not saturated in the same way that
the pair of atoms in the molecule of nitrogen, for example,
are saturated. These conclusions are in good agreement with
chemical theory. With benzene vapour in the discharge tube
I have found, in addition to negatively charged carbon atoms
and molecules, negatively charged triplets containing three
carbon atoms. I have sometimes thought that in this case I
could see indications of a line corresponding to four carbon
atoms with a negative charge, but the line has been so faint
that I cannot be sufficiently certain of the accuracy of
the measurements to be quite sure that it was due to €4.

ATOMS CARRYING TWO OR MORE POSITIVE
CHARGES

Though the heads of most of the parabolic arcs are
situated in the same vertical' line, in many cases some of the
parabolas, especially those corresponding to the atoms of
oxygen and carbon, are prolonged towards the vertical axis.
The prolongations do not reach right up to this axis, but in
many cases, as in the line a in Fig. 2, Plate II., which is due
to the atom of oxygen, stop after going half-way. These
prolongations of the parabolas are also parabolic and are
continuations of the primary parabola. They are therefore
due to particles which, when they are in the deflecting fields,
have the same value of e\m as the particles which produce
the primary parabolas. The fact that the smallest horizon-
tal deflection of the prolongation is just half that of the

78 RA YS OF POSITIVE ELECTRICITY

corresponding deflection of the primary shows (see p. 21) that
the swiftest of the particles in the prolongation has twice the
kinetic energy of the swiftest in the primary. Thus these
particles when in the electric field in the discharge tube
acquire twice the kinetic energy of the normal particle ; they
must therefore when in the discharge tube have had twice
the normal charge. They must, after passing through the
cathode and before getting into the deflecting fields, have had
their charge reduced to the normal value. For, as we have
seen, the value of ejm in these fields is normal, hence if they
have retained the double charge they must have double the
mass. If, however, they had retained the double charge the
electrostatic deflection would have been normal : for though
the kinetic energy is doubled, which halves the deflection for
normal charge, the charge and therefore the electrostatic
deflection for given kinetic energy is doubled too, and hence
the result would be the normal deflection, while the actual
deflection is only one-half of this. We conclude, therefore,
that the prolongation is due to particles which had a double
charge when in the discharge tube, but which have lost one of
these charges after passing through the cathode.

It is a strong confirmation of this view that when we find
these prolongations we generally find on the same plate para-
bolas with their heads in the normal place giving a value of
elm twice that given by the line with the prolongation ; these
are due to particles which have retained their double charge
after passing through the cathode. And conversely when the
lines corresponding to the double value of e]m are present
we find a tail or prolongation to the line corresponding to
the normal value. This would not necessarily be true at
pressures so low that the particles did not make any collisions
after passing through the face of the cathode, but I have not
been able to reduce the pressure to this point.

MULTIPLY CHARGED PARTICLES 79

The prolongations of the parabolas in some cases extend
much more than half-way to the vertical axis ; this is
especially the case with the parabola produced by the
positively charged atom of mercury. Fig. 3, Plate II.,
shows that even when the electric and magnetic fields are
strong enough to produce several millimetres deflection in the
heads of the parabolas corresponding to the other elements
the head of the mercury parabola is so little deflected that at
first sight it seems to coincide with the origin. When
exceedingly large electric fields are used it can be seen, how-
ever, that the head of the mercury parabola is distinctly
displaced, and on measuring the amount of the deflection it is
found to be one-eighth of the normal displacement of the
heads of the parabolas corresponding to the other elements.

This, as we have seen, implies that the particles which
produce the head of the parabola corresponding to the atom
of mercury must have eight times the maximum amount of
energy possessed by the normal atom ; if the theory given
above is true, this means that some of the mercury atoms
had, before passing through the cathode, eight times the
normal charge, i. e. had lost eight electrons. Eight is a very
large number for an atom to lose, so that if in this case we
can obtain independent evidence of such a loss it will be
a strong confirmation of the theory.

A study of plates taken with large electrostatic deflections
has revealed the existence of seven parabolas due to mercury,
corresponding to the mercury atom with I, 2, 3, 4, 5, 6, 7
charges respectively. The parabola corresponding to eight
charges has not been detected, but as the parabolas in general
get fainter for each additional charge, it is probably on the
plate, although not intense enough to be visible. Fig. 4,
Plate II., taken from a photograph when the gas in the tube
was the residual gas left after exhaustion by the Gaede pump,

8o RA YS OF POSITIVE ELECTRICITY

shows these lines very well. The measurements of mje for
the parabolas on this plate give the following value (m\e is
taken as unity for the atom of hydrogen) —

mje

200 200/1

i 02 200/2

66-3 200/3

50-4 200/4

44 this is not a mercury line but is due to CO2

39-8 200/5

337 200/6

28*6 200/7.

It will be noticed that the heads of the parabolas corre-
sponding to i, 2, 3 ... charges respectively lie on a straight
line passing through the origin. This shows (p. 21) that the
particles which produce these heads are all moving with the
same velocity, and therefore, since each particle is an atom of
mercury, that the kinetic energy of the particles at the heads
of the parabolas is constant. This is in agreement with the
theory, for the heads of all the parabolas are due to particles
which before passing through the cathode had lost eight
electrons. The particles at the head of the parabola corre-
sponding to one charge (mje = 200) has regained seven of
these after passing through the cathode ; the one at the head
of the parabola corresponding to two charges (m/e = 100) has
regained six, and so on. As the charge on these particles
when they were in the discharge tube was eight units in each
case, they would naturally acquire the same amount of
kinetic energy before passing through the cathode.

The question now arises as to how the mercury atom
acquires these very various charges. Can an atom of mercury
when ionized lose any number of electrons from one to

MULTIPLY CHARGED PARTICLES 81

eight, or does it always lose a definite number? Take for
example a mercury atom with five positive charges : has it
got into this condition by losing five charges when it was
ionized, or did it originally lose the maximum number eight
and regain three subsequently ? The photographs suggest, I
think, that the second supposition is the correct one, and that
in the discharge tube there are two, and only two, kinds of
ionization. By one of these the mercury atom loses one
electron, by the other eight. The evidence for this is as
follows : let us suppose for a moment that atoms with any
charges from one to eight were produced by the ioniza-
tion of the atoms of mercury in the discharge tube, and
consider what effect this would have on the parabola
corresponding to the mercury atom with one charge.
This would be due to atoms of the following kinds —

Atoms which had lost ,

(1) 8 electrons in the discharge tube and regained 7 subsequently

(2) 7 » •>•> •>•> ^ » 6 ,,

(3) " ,, », ,, ,, ,, 5 i>

and so on : the last member of the series being atoms which
had lost one electron on ionization and had not regained it.

The parabola seen on the plate would be due to the super-
position of the eight parabolas due to these different types of
atoms. The head of each of these parabolas would be separ-
ated from the head of any of the others : if d were the
horizontal deflection of the one due to the atom which had
only lost one electron in the discharge tube, d/2, d/$, d/4, d/$,
d/6, d/?, d/S would be the horizontal deflection of the heads
of the parabolas due to the atoms which had lost 2, 3, 4, 5, 6,
7, 8 respectively. Thus the resultant parabola would, for the
part which had a horizontal deflection between d/S and d/?,
consist only of the parabola due to atoms of class (i) ; the
part when the horizontal deflection was between d/? and d/6

82 RA YS OF POSITIVE ELECTRICITY

would consist of two parabolas due to the atoms of classes (i)
and (2) ; the part with the horizontal deflection between dj6
and dj$ would be made up of the three parabolas corre-
sponding to the atoms belonging to classes (i), (2), (3), and so
on. Thus at the distance d/7, dj6, d/$, d/4, dj^ dJ2 and dfi
we should expect an abrupt increase in the brightness of the
curve, for at each of these places a new parabola is added
to the old ones ; the intensity of the curve would thus not
vary continuously but would have a beaded appearance.
The abrupt increase in intensity at the distance d is very
marked in the parabola ; it is, however, the only one to be
detected. The intensity of the parabola corresponding to
the atom with one charge is very great, and it might be
thought that the charges in the intensity might escape
detection owing to the breadth of the curve. We may,
however, apply the same reasoning to the parabolas which
correspond to mercury atoms with three or four charges
which are fine and well defined. The intensity of these
curves is, however, perfectly continuous and there are no
signs of the abrupt variations which ought to occur if the
mercury atoms in the discharge tube had charges intermediate
between one and eight. This result suggests that the
ionization is mainly at any rate of two types, in the one type
an atom of mercury loses a single electron, in the other it
loses eight. There would thus seem to be two different
agents producing ionization in the discharge tube.

The maximum number of charges carried by a multiply
charged atom does not seem to be related to any chemical
property of the atom such as its valency, but to depend
mainly on the atomic weight ; thus mercury, the most
massive atom on which observations have been made, can
have as many as eight charges, crypton atomic weight (82)
four or five, argon atomic weight (40) three, neon atomic

MULTIPLY CHARGED PARTICLES 83

weight (20) two, nitrogen atomic weight (14) and oxygen
(16) two, perhaps in rare cases three, helium also occurs with
two charges ; the multiple charge has been found on the
atoms of all elements tested with the very suggestive excep-
tion of hydrogen : no hydrogen atom with more than one
charge has ever been observed, though as the hydrogen lines
occur practically on every plate more observations have been
made on the hydrogen lines than on those of any other
element.

When there are on the plates lines corresponding to atoms
of the same element with one, two, three charges, then the
larger the number of charges the fainter the line. Judging
from the intensity of the lines we should conclude that the
number of multiply charged atoms is only a small fraction of
the number with one charge. The ratio of the number of
atoms which have only one charge to that of those which
have two or more charges is very variable and depends on
conditions which are not yet fully understood. For example,
in the case of the carbon atom this ratio seems to depend to
a very great extent on the type of gaseous carbon compound
in the discharge tube. With some hydrocarbons the doubly
charged carbon atoms are relatively much brighter than with
others. Again, in the case of oxygen I have found that the
purer the oxygen the fainter was the line due to the doubly
charged oxygen atom in comparison with that due to the
atom with only one charge. It would thus seem that atoms
torn from chemical compounds were more likely to have a
double charge than those obtained from a molecule of the
element. Chemical combination cannot, however, be the
only means by which the atoms acquire multiple charges, for
the atoms of the inert monatomic gases, neon, argon and
crypton, are remarkable for the ease with which they acquire
multiple charges.

84 RA YS OF POSITIVE ELECTRICITY

Though double charges occur so frequently on atoms, they
are exceedingly rare on molecules, whether of elementary or
compound gases. They do sometimes occur, as the line
corresponding to mje = 28, which may be due either to a
molecule of nitrogen or of carbon monoxide, has on many
plates a prolongation towards the vertical axis, implying a
double charge on the molecule.

CONCENTRATION OF THE POSITIVE RAYS
ROUND DEFINITE VELOCITIES

The parabolas are not always of even approximately
uniform intensity along their arcs, but sometimes, as in those
represented in Fig. 3, Plate I., Fig. 3, Plate II., Fig. 2,
Plate III., show abrupt increases in intensity at definite points
along the arc. These increases are often comparable with that
which occurs at the head of the parabola. In some cases,
indeed, as in the line for the hydrogen molecule in Fig. i,
Plate III., the second maximum is much greater than the one
at the head of the parabola. The position of the second
maximum is generally connected with that of the first — the
one at the head of the parabola— by the very simple rule
that the electrostatic deflection of the head of the second
maximum is twice that of the head of the parabola. This
means that the kinetic energy of the particles forming the
second maximum is half that of those forming the first.

It is an interesting point that in the great majority of
gases when the photographic plate shows parabolas corre-
sponding to both atoms and molecules, the " beaded "
appearance due to the existence of these maxima is confined
either to the atomic or to the molecular lines, the beading
does not occur on all the lines. If the line due to the atom
of one element is beaded, those on the same plate due to
the atoms of other elements are also beaded, while if the

BEADING 85

line due to one molecule is beaded those due to other mole-
cules are often beaded too. The lines due to the monatomic
elements show beading when either the atomic or molecular
lines of the diatomic gases are beaded, thus the atoms of
the monatomic elements can in this respect behave like
either the atoms or the molecules of a diatomic gas.

We should expect to get a concentration of the positive

rays about particular energies if in front of the cathode there

was a maximum of ionization, not only at the boundary of

the dark space but also at another place P between this

boundary and the surface of the cathode ; then if Vx were the

cathode fall in the dark space, V2 that between the cathode

and P, we should have concentration of the positive rays

about the energies Vj and V2. With curved cathodes where

the cathode rays are brought to a focus it is possible that

such an effect may exist, but the very simple relation that

exists between the energies in the two maxima, viz. that

one is twice the other even though the shape of the cathode

may undergo wide variations, suggests a different explanation.

Let us take first the beading on a line due to a charged

atom. The line due to the hydrogen atom, for example, often

shows an increase of intensity at £, where the energy of the

particles is half that at a the head of the parabola. This

would be the case if the atoms at b were due to the breaking

up of molecules after they had passed through the cathode.

The molecules would have acquired in the discharge tube

energy equal to that possessed by the atoms which strike

the plate at a. When they broke up after getting through the

cathode and before reaching the electric and magnetic fields,

this energy would be divided between the two atoms ; each

atom would have one-half of the energy, one atom would

have the positive charge previously on the molecule, this

atom would strike the photographic plate at fr, the other

86 RA YS OF POSITIVE ELECTRICITY

atom would be electrically neutral and would strike the
plate at the undeflected spot.

As this type of beading is observed on the parabolas due to
the mercury and helium atoms, we must suppose that, although
mercury and helium are monatomic, their positively charged
atoms can unite with a neutral one to form a system sufficiently
stable to hold together while moving through the dark space
in front of the cathode, though not stable enough for an
appreciable number of them to get through the fields of
electric and magnetic force, for if they passed through these
fields lines corresponding to the helium molecule would be
found on the plate. The line corresponding to the mercury
molecule is found occasionally. The maxima on the parabolas
corresponding to molecules could in a similar way be explained
as arising from systems which before reaching the cathode
consisted of a pair of molecules, one singly charged and the
other uncharged, the system breaking up after passing through
the cathode.

They could also be explained if, instead of a two-mole-
cular system breaking up, a molecule was formed by the
union of an atom which had come through the dark space
and acquired the energy due to the fall of potential between
the anode and cathode with an uncharged atom at rest ;
the collision which produced this union being analogous
to that between two equal unelastic bodies where the velocity
of the system after impact is half that of the moving body
before, and the kinetic energy of the system consisting of
the two bodies half that of the moving body before impact.

Though, as far as my experience goes, the energy of the
particles in the second maximum is most frequently one-
half of that in the primary, this is not invariably the case.
I have some plates where the ratio of the energies for the
hydrogen molecule is two-thirds and not one-half. This case

BEADING 87

could be readily explained by the splitting up of a system H3
into a charge molecule H2 and a neutral atom ; a result
supported by the fact that on the plates showing the para-
bolas when this ratio obtains there is a parabola corresponding
to the system H3.

This view of the origin of the beading on the atomic lines
receives great support from some experiments I made when
the gas in the tube was CO. If the molecule of CO were
to split up after passing through the cathode the carbon atom
would have 12/28, and the oxygen one 16/28 of the normal
energy : thus these atoms would appear on the plate with
electrostatic deflection 28/12, and 28/16 of that of the heads
of the parabolas. On the line corresponding to the carbon
atom on some of the plates there was a bead at 2*3 times
the horizontal distance of the head, and on the line corre-
sponding to the oxygen line one at 17 times this distance;
they are thus almost in exactly the positions predicted by
the theory. The beading occurred on both the positive and
negative parabolas for these atoms.

Sometimes the maxima are much closer together than
in either of these cases. I have some plates, for example,
where the ratio of the energies is as 7 to 9. Cases like this
could be explained by a heavy molecule shedding some of
its lighter atoms. Thus, for example, if a molecule CH4 were
to break up after passing through the cathode into CH2 and
H2 there would on the line representing CH2 be a maximum
where the energy equalled 14/16 of that of the primary
compound. I do not think, however, that the maxima which
lie so close together can be explained in this way, for we
find that when the ratio of the energies for one line is 7 : 9
it has the same ratio for the other lines, whereas, if it were
due to the splitting up of molecules we should expect the
ratio to vary with the molecular weight. I think that when

88 RA YS OF POSITIVE ELECTRICITY

the ratio of the energies is so nearly unity as this, the beading
is probably due to some sudden change in the pressure in the
discharge tube producing a sudden change in the potential
difference between anode and cathode, and thus altering the
maximum energy which can be acquired by a charged
particle when it passes through the dark space in front of
the cathode. The lines corresponding to atoms with two
charges sometimes show a second maximum where the
energy is half that corresponding to the primary one. This,
I think, indicates that some of the atoms which when passing
through the electric and magnetic fields have a double charge,
had only one charge when they passed through the dark
space and were under the influence of the electric field in
the discharge. They acquire another charge (i. e. lose another
electron) after passing through the cathode and before entering
the electric and magnetic fields.

ON THE ORIGIN OF THE CHARGED ATOMS
AND MOLECULES IN THE POSITIVE RAYS

The positive rays consist of a great variety of constituents ;
some of these are positively charged atoms, others positively
charged molecules, both of elements and of compounds. We
propose now to consider how it is that some of the carriers
are atoms while others are molecules. In the first place
a study of the photographs, or, what is even better, measure-
ments of the number of particles of different types by the
method described on p. 120 shows that the proportion
between the number of atoms and molecules in the positive
rays is subject to very wide variations, and depends to a very
great extent on such things as the pressure of the gas, the
size and shape of the cathode and its position in the discharge
tube. Examples of this variation in the relative intensities
of the lines, due to the atoms and molecules of hydrogen, are

ATOMS AND MOLECULES 89

shown in Figs. 3 and 4, Plate III. In Fig. 3 the line due to
the hydrogen atom is quite strong, while that due to the
molecule is too faint to be seen in the reproduction of the
photograph ; in Fig. 4, on the other hand, it is the line
due to the molecule which is strong, while that due to the
atom cannot be seen in the figure. As a general rule the
lines due to the molecules are more important relatively to
those due to the atoms the lower the pressure of the gas in
the discharge tube and the greater the potential difference
between the anode and cathode. This effect of pressure
is probably the explanation of why the proportion between

FIG. 31.

the atoms and molecules depends on the position of the
cathode in the discharge tube. If, for example, the cathode is
placed so that the front of the cathode comes inside the neck
of the discharge tube, as in Fig. $ia, the atomic line of
hydrogen is stronger than the molecular ; it is weaker, however,
when the face of the cathode protrudes into the discharge
tube, as in Fig. 31^. The pressure at which the positive
rays are at their best is higher in the first case than in
the second, so that the effect of pressure would be sufficient to
explain this effect. It would also explain why the molecular
lines should be relatively more conspicuous in large tubes
than in small ones. Again, when the discharge tube is, like
a Rontgen-ray tube, provided with an anticathode against
which the cathode rays strike, the proportion between the

90 RA YS OF POSITIVE ELECTRICITY

intensities of the atomic and molecular lines depends on
the position of the anticathode and also upon whether it is
insulated or connected with the anode. The potential differ-
ence in the tube is also affected by these changes. As a
general rule, if the line due to the molecule is stronger than
that due to the atom for one element represented on a
photograph, it will be so for the other elements. Let us
now consider the various agents at work in the discharge
tube in giving a positive charge to the particles which
constitute the positive rays.
These are —

1. High-speed cathode rays.

2. Secondary cathode rays with a much lower speed.

3. The positively electrified particles themselves.

4. The retrograde rays (see p. 134). These carry a

negative charge and have masses comparable with
those of the positively electrified particles.

5. Radiant energy of small wave-length arising from the

impact of the high-speed cathode and positive rays
against the molecules of the gas in the tube, the
walls of the tube and the electrode. These impacts
detach electrons from the molecules, and the falling
into the molecules of electrons to take the place of
those ejected gives rise to radiation which can ionize
the gas.

Let us take these ionizing agents in order and consider
whether they produce charged atoms or charged molecules.

The high-speed cathode rays, since they penetrate into
the atom and come into contact with the individual electrons,
would in general give rise to singly charged systems ; a priori,
we should expect that these systems would be molecules
rather than atoms, except when the electrons which the
cathode rays struck against and ejected were those which

ATOMS AND MOLECULES 91

bound the two atoms in a molecule together. In this case the
disruption of the bond between the atoms might lead to the
disruption of the molecule.

The direct evidence we possess on this point is derived
mainly from observation on the nature of the spectra excited
by cathode rays. As line spectra are usually associated with
atoms, if the cathode rays excite the line spectrum of a gas
through which they are moving, it would be strong evidence
in favour of their power to dissociate a molecule into atoms.
The spectra produced by cathode rays have been investigated
by Wiillner (" Phys. Zeitsch.," I, p. 132, 1899), Lewis (" Astro-
physical Journal," 17, p. 258, 1903), and also by Fulcher (Ibid.,
34,p. 388, 191 1), who comes to the conclusion that the spectrum
of the light produced by cathode rays (i) in nitrogen consists
solely of the negative bands ; (2) in hydrogen consists chiefly
of the compound spectrum together with the main series
lines which are relatively weak ; (3) in oxygen consist of the
negative bands together with the spark lines and series of
triplets. These results agree in general with those obtained by
Wiillner and Lewis. They are consistent with the view that
while the cathode rays do produce some dissociations of the
molecules into atoms, the chief part of the light comes from
the molecules : so that if cathode rays were the only source
of ionization in the discharge tube we should expect that the
number of charged molecules in the positive rays would
exceed greatly the number of charged atoms. We may
remark that the production of " single line spectra " by cathode
rays, when various metallic vapours such as mercury, mag-
nesium and cadmium emit special lines under the impact
of comparatively slow cathode rays is not a case in point, as
these vapours are monatomic, so that there is no question of
dissociation. The bombardment of salts by cathode or
positive rays gives rise to luminosity, and the marked differ-
ences between the nature of the light, in the two cases

92 RA YS OF POSITIVE ELECTRICITY

favour the view that the cathode rays are not efficient in
splitting molecules up into atoms. If, for example, lithium
chloride is bombarded by cathode rays it shines with a blue
phosphorescence and the spectrum is a continuous one ; when
it is bombarded by positive rays the phosphorescence is red
and the spectrum shows the lithium lines.

Since the ionizing power of cathode rays, when their
velocity exceeds a certain value, diminishes rapidly as the
velocity of the rays increases, the secondary cathode rays in
the discharge may produce more ions than the primary
fast rays. We have no reason, however, for believing that they
would be more effective in splitting up molecules into atoms.

We now come to the positively charged particles. The
spectroscopic evidence seems to leave little room for doubt
that these are very effective in producing dissociation of
molecules into atoms, for when the positive rays pass through
a gas they cause it to emit a line spectrum. This is shown
most clearly when the gas through which the electric dis-
charge passes is different from that through which the positive
rays pass after getting through the cathode. This occurs
when the only connection between the region where the
discharge takes place and that where the spectrum is ob-
served is a long, narrow tube through which the positive
rays pass ; we can then have different gases in the two
regions without much mixing, and in this case the spec-
trum shows the lines of each of the gases. The emission
of light by the positive rays will be considered more fully in
a subsequent chapter, but from what we have seen it is
evident that in the positive rays themselves we have the
means of producing the atoms which are observed in the
positive rays. The question arises : Do the positive particles
produce nothing but charged atoms, and have we to attribute
all the positively charged molecules to the cathode rays?
The following experiment suggests, I think, that this is not

ATOMS AND MOLECULES 93

the case, and the cathode rays do not produce directly the
greater part of even the charged molecules. For if the
charged molecules were due entirely to the cathode rays, if we
deflected the cathode rays in the discharge tube to one side
so that they no longer passed through the column of gas just
in front of the hole in the cathode, we should expect to
diminish the number of charged molecules compared with
charged atoms. I have made observations on this point
using the lines due to the atom and molecule of hydrogen for
this purpose. I found that the cathode rays might be deflected
to a considerable extent before any very great diminution in
the intensity of the positive rays set in ; and that as long as
I could observe the rays there was no diminution in the
intensity of the lines due to the molecule as compared with
those due to the atom.

The mechanism by which a molecule is dissociated into
atoms is a subject of great interest and one about which
there is much uncertainty. The most obvious view of the
way the positive particles split a molecule up into atoms is
that the positive particle, by its impact with the molecule,
gives to one of the atoms in the molecule sufficient kinetic
energy to enable it to escape from its companion. The
objections to this explanation are (i) that if the particles
came into collision with masses as great as those of an atom
they would be deflected through an appreciable angle and
would lose a considerable amount of their energy. We have
seen, however, that until the collision occurs which produces
their final absorption they do not suffer any appreciable
deflection or loss of energy by the collisions. Another
objection is that, on this view, the atoms struck by the
particles would, after the collision, have a finite velocity, so
that the Doppler effect would produce a broadening of the
lines in the spectrum of the gas through which the positive
rays pass. Wien looked for this effect but was not able to

94 RA YS OF POSITIVE ELECTRICITY

find it. These results indicate that if the dissociation is
produced by the collisions these must be between the positive
rays and the electrons which bind the atoms together, and
not with the massive parts of the atom.

The difficulties which stand in the way of explaining
dissociation by collisions are not confined to the case of the
positive rays. They exist, as I pointed out many years ago
(" Phil. Mag.," 1 8, p. 233, 1884), m the case of ordinary thermal
dissociation such as that which occurs when iodine vapour is
heated. For when equilibrium is reached the number of
molecules split up per second in unit volume must equal the
number of molecules formed by the re-combination of the
atoms. If m is the number of molecules per unit volume
the number of collisions in unit time per unit volume will be
proportional to m2, and if the dissociation of the molecules is
due to collisions the number of molecules dissociated will
also be proportional to m2. Again, the re-combination of the
atoms results from the collisions' between the atoms, and the
number of such collisions per second in unit volume is pro-
portional to n2, where n is the number of atoms in unit volume.
Hence the number of molecules formed in one second in unit
volume is proportional to n2, and the number split up pro-
portional to m2. When the system is in a steady state these
numbers must be equal, hence m2 must be proportional to «2,
or m proportional to n. We know, however, that m is not
proportional to n but to n2. So that it would seem that in
this case the splitting up of the molecule into atoms is not
due to the knocking of the molecules against each other.
This objection would not apply if dissociation did not take
place throughout the gas but only at the walls of the vessel
in which it is contained. I suggested in the paper referred
to above that the dissociation might be brought about by the
radiant energy which passed through the gas, and whose
quantity and quality is a known function of the temperature.

DISSOCIATION 95

When the dissociation is due to an external agent like this
the number of molecules dissociated in unit volume in unit
time would be proportional to m and not to m2, and when
the steady state was reached we should have m proportional
to #2, which is the relation which does exist between these
quantities. The simplest way of picturing this effect of
radiation is to suppose that some period of the vibrations
of the electrons which bind the atoms in the molecules
together, coincides with the period of the radiation, or when
this is complex of some constituent of it. Then, owing to
resonance these electrons will absorb a considerable amount
of energy, enough it may be to enable them to get free from
the molecules and leave the atoms which they bound together
disconnected. When the radiation is like that of a black
body the energy in the radiation of frequency between n arrd
n + dn> is proportional to

rfidn

where h is Planck's constant, 6 the absolute temperature and
R the gas constant. If n is the frequency of one of the
electrons, w the work required to liberate it, then if we assume
Planck's law w = hn ; and the energy in the radiation in tune
with the binding electrons would thus be proportional to

Hence we should expect that the rate at which the molecules
are split up into atoms would when considered as a function
of the temperature be proportional to

This when the temperature is low enough to make R0 small

96 RA YS OF POSITIVE ELECTRICITY

compared with w would be approximately equal to e ~
and the results of experiments on dissociation are in accord-
ance with this law of variation of temperature. Thus the
view that the dissociation of molecules into atoms is often
produced by the effect of electromagnetic waves receives some
support from the phenomena of thermal dissociation. If we
suppose that the particles in the positive rays are emitting
such waves, not necessarily of a definite period, but covering
a considerable range of periods, then the dissociations which
they produce when they pass through the gas might not be
due to collisions between the molecules and what may be
called the body of the particles in the positive rays, but rather
between the molecules and the electromagnetic field round
the particles.

If radiant energy is an efficient means of dissociation
then the radiations in the discharge tube may be the origin
of some of the atoms which are produced in the positive
rays before they reach the cathode. Radiation analogous
to soft Rontgen radiation, which possesses great powers
of ionization, is a very usual, perhaps an invariable, ac-
companiment of an electric discharge through gases ; the
Entladungstrahlen investigated by Wiedemann and others
form a part of this radiation. It seems not unlikely from
the considerations given above that this radiation may be
able to produce a type of ionization where the molecules are
dissociated into atoms.

The only type of ionizing agent in the list on p. 90
which remains for consideration is the retrograde rays. These
rays are particles similar to those which form the positive
rays but carrying for the most part a negative instead of a
positive charge, and moving in the opposite direction to the
positive rays. As far as ionization and dissociation go, they

SOURCES OF ION1ZATION 97

might be expected to behave in much the same way as the
positive rays.

Let us now consider the places in front of the cathode
where these agents might be expected to be most active.
Let us take first the high-speed cathode rays. These seem
to acquire a high velocity close to the cathode. Such
ionization as they can produce may be expected to occur
from the cathode right up to where they strike against the
walls of the discharge tube. It is not, however, probable
that any large fraction of the ionization in the tube is due
to the direct action of these rays. The amount of ionization
due to such rays has been measured by Glasson (" Phil. Mag.,"
Oct. 1911), who found, as is indicated by theory, that the
number of ions produced by a cathode ray per unit length
of its path varies inversely as the kinetic energy of the ray.
For rays moving with a velocity of 47 X io9 cm./sec. through
air at a pressure of I mm. of mercury he found that 1*5
pairs of ions were produced by each ray in travelling over
I cm. Under the usual conditions for the production of
positive rays the velocity of the high-speed cathode rays is
considerably greater than 5 X io9 cm./sec. This would
reduce the ionization if the pressure remained the same, but
the pressure of the gas in positive ray experiments is generally
less than '01 mm., so that even if we neglect the diminution
in ionization due to increased velocity, a cathode ray in the
positive ray experiments would only produce 1*5 pairs of
ions when it had travelled over a metre, a distance much
greater than the length of the tube. We conclude that the
ionization in the gas is not in the main due to high-speed
cathode rays.

Let us now consider the low-speed cathode rays. The
positive ions from the negative glow, when they get into
the dark space, soon acquire sufficient energy to ionize the
H

98 RA YS OF POSITIVE ELECTRICITY

gas, producing electrons and positive ions. These electrons
will at first move slowly, as they are in the region in the dark
space where the electric field is comparatively weak ; as
their velocity is small they will be efficient ionizers and will
give rise to other electrons ; these will start in a still weaker
field and become still more efficient ionizers, as it is not until
the velocity of the electrons sinks below that due to a fall
through about 200 volts that the ionization due to these
particles increases as their velocity increases. Thus the
number of these slowly moving cathode rays will increase
with great rapidity near the anode end of the dark space,
and the ionization and dissociation, and therefore the positive
rays due to them will be a maximum in this region. As the
positive rays which start from the boundary of the dark space
on the anode side have fallen through the whole potential
difference between the anode and cathode, they will have the
maximum velocity when they pass through the cathode, and
will hit the photographic plate at the heads of the parabolas.
Thus if all the charged molecules in the positive rays were
due to the slow cathode rays, or came out of the negative
glow, we should expect the molecular lines to be short, or, at
any rate, to have a well-marked maximum of intensity at the
head of the parabola.

Let us now consider the effect of the positive rays
themselves. The energy of these when they are near the
negative glow will be small and will increase as they move
towards the cathode, their number too will increase in
consequence of fresh ionization ; thus the ionization due to
the positively charged particles will increase towards the
cathode. The particles produced near the cathode will only
fall through a part of the potential difference between the
anode and the cathode, and the nearer they are to the cathode
when they begin the journey the smaller will be the velocity
when they reach the cathode. Thus among the ions produced

SOURCES OF IONIZAT10N 99

by the positive rays we should expect that the greater
number would have velocities well below the maximum, so
that if these only were taken into account the density of the
parabolas would be small at the head and would increase
towards the part corresponding to smaller velocities.

Let us now consider the retrograde rays. These will not
multiply as they move away from the cathode, though their
energy will increase somewhat as they approach the negative
glow ; as, however, they acquire a high velocity even when
quite close to the cathode we should expect that the ioniza-
tion they produce would be fairly uniform throughout the
dark space with a tendency to increase in the neighbourhood
of the negative glow. The parabolas due to the particles
produced by this type of ionization ought therefore to be
more uniform in intensity than those due to particles produced
by either cathode or positive rays. The ionization due to
radiation would, apart from absorption, be uniform through-
out the dark spaces and would in this respect resemble that
produced by the retrograde rays.

Thus, to sum up, ionization due to cathode rays should
produce parabolas with a maximum of intensity at their
heads ; ionization due to positive rays, parabolas with a
maximum some way from the head ; while ionization in the
dark spaces due to either retrograde rays or radiation ought
to give rise to parabolas of fairly uniform intensity. If we
confine our attention to the intensities at the heads of the
parabolas we eliminate the ionization due to the positive
rays, while we can eliminate that due to the cathode
rays by studying the intensities at some distance from the
heads.

In addition to the positive rays produced in the dark
space we have those produced in the negative glow. Since in
this region the electric force is exceedingly small the particles
will not acquire any appreciable velocity until they emerge
from it into the dark space, so that all particles from the

ioo RA YS OF POSITIVE ELECTRICITY

negative glow will reach the cathode with practically the
same velocity as those which start from the boundary of
the dark space, and will strike the photographic plate close
to the head of the parabolas. The positive rays themselves
will not, while in the negative glow, acquire sufficient energy
to produce ionization, but the cathode rays, the retrograde
rays and the radiant energy may well be able to ionize the
gas in this region. If the great majority of the positive rays
started from the negative glow the intensity at the heads
of the parabolas would be very large compared with that
of the rest of the arcs. As a matter of fact this is very
frequently, though by no means invariably, the case. Thus
in the photograph reproduced in Fig. i, Plate III, the
head of the parabola representing the hydrogen molecule is
exceedingly faint, while there is a great increase in intensity
at the place which would be hit by particles whose kinetic
energy was half that due to a fall through the whole potential
difference between the anode and cathode. This might be
explained by supposing that no charged molecules, but only
charged hydrogen atoms, were produced by the discharge, and
that the charged molecules which gave rise to the parabolas
were formed by one of these charged atoms combining after
it had passed through the cathode with an uncharged atom
of hydrogen.

The great length of the parabolas shows that the particles
which give rise to them, and which are all of the same kind,
have a wide range of velocities. One explanation of this range
is that the particles originate in different parts of the dark space
and so fall through different potential differences and, therefore,
reach the cathode with different velocities. That this is one
reason for the difference in velocities is supported by the
following experiment —

A rod a attached to a glass tube which fitted into a ground-
glass joint at b carried a small metal disc, and by rotating

SOURCES OF IONIZATION /f „,,, IQI,

the tube the disc could either be put on one side out of the
way of the stream of cathode rays coming from the cathode or
else put right in front of that stream and of the opening in
the cathode through which the positive rays passed. The
pressure was such that the disc was well inside the dark
space. Photographs were taken (i) with the disc out of the
way (2) with it right in front of the cathode. When these
were examined it was found that the intensity of the
positive rays with the disc in front was much less than when
the obstruction was removed, and again that the heads of the
parabolas, when the disc was in front, were further away from
the vertical in the proportion of 7 to 5 than when it was away.
This shows that the insertion of the disc had reduced the
maximum kinetic energy of the rays to 5/7 of its normal
value ; this proportion depends on the position of the disc in
the dark space ; the nearer it is to the cathode the greater the
reduction of the maximum energy. No effect is produced
unless the disc is in the dark space. The most natural
explanation of this experiment is that whereas in the normal
case the positive rays are drawn from the region between /
the cathode and g the boundary of the dark space, when the
disc is inserted at d, the supply from dg is cut off, and that
from fd left ; as the potential difference between the cathode
and d is less than that between the cathode and g the
maximum energy of the positive rays is diminished by the
insertion of the disc.

There are other reasons which might be suggested for the
range in velocities. For example, since all the photographs
given in this book were taken when the discharge was
produced by an induction coil, and as the potential difference
between the terminals of this instrument varies from zero to
its maximum value, it might be thought that the particles
with the greater velocity were those produced when the
potential difference due to the coil had its maximum value

F POSITIVE ELECTRICITY

while the lower velocities were produced under the smaller
potential differences. If this were the explanation the
velocities should become constant if a constant potential
difference were maintained between the electrodes, so that if
a large electrostatic induction machine were used instead of
an induction coil the parabolas ought to be reduced to points.
This, however, is not the case.

Another cause which would produce a variation in the
velocity of the particles is the passage backwards and forwards
between the charged and uncharged state, which we have seen
goes on after the particles have passed through the cathode,
and which, presumably, also goes on while the particles are
passing through the dark space on their way to the cathode.
When the particles are without charge they will not be acted
upon by the electric force in the dark space and so when they
reach the cathode their energy will be less than it would have
been if they had been charged for the whole of the time. As
the proportion between the time the particle has a charge
and the time it has not, will vary from particle to particle, the
different particles will reach the cathode with different
velocities. Though an effect of this kind must exist, it is not
sufficient to explain all the variations of velocity in the
particles. It is difficult, for example, to reconcile this explana-
tion with the abrupt way in which the parabolas commence,
when the pressure in the discharge tube is low. The head of
a parabola is caused by the particles which have acquired the
maximum amount of kinetic energy while passing through
the dark space ; this will depend upon the proportions between
the times the particles are charged and uncharged. Suppose
that the thickness of the dark space is comparable with the
lengths ^, A2 discussed on p. 48 ; then there is a finite chance
that a charged particle starting from the boundary of the
dark space may reach the cathode without losing its charge,

SOURCES OF ION1ZATION 103

so that some of the particles will acquire the energy due to
the full fall of potential. The expectation of a particle
passing without loss of charge and having the maximum
energy may not be so great as that for it to have been
without charge for part of its path when the energy it
will have acquired will be less ; there will be thus a certain
energy, or velocity, of the particles for which the expectation
is a maximum and at the point on the parabola corresponding
to this velocity the density of the photograph will be a
maximum. The density, however, will fall away gradually on
either side so that the parabola will not begin abruptly at
the velocity for which the expectation is greatest, unless
that velocity is the maximum due to the fall through the
whole potential difference between anode and cathode. At
low pressures, however, the parabolas commence quite
abruptly and the variation in intensity does not show any
resemblance to that which would be represented by the
ordinate of a probability curve.

Wien compared the energy in the particles as calculated
from their electrostatic deflection by means of equation (2),
p. 21, with the potential difference between the anode and
cathode, the latter being calculated by the method of the
alternative spark gap. He came to the conclusion that the
energy of the particles was only about one-half of that which
they would acquire by falling through the potential difference
between the anode and cathode. This would be the case
if the free path of the particles when charged was equal to
that when it was uncharged, and each of them a small fraction
of the thickness of the dark space.

I tested the relation between the energy of the particles
and the potential fall by a different method, as the method of
the alternative spark gap is not under all conditions a
very satisfactory way of measuring potential differences. The

104

RAYS OF POSITIVE ELECTRICITY

method is shown diagrammatically in Fig. 31 A. C is the
perforated cathode through which the positive rays pass, E
the parallel plates which produce their electrostatic deflection,
and P the photographic plate by which they are detected.
The anode A is also perforated, the perforations of C and A
being in the same straight line. The cathode
rays from C pass through the perforation in
A and then between a pair of parallel plates
EJ, exactly similar in shape, size and
distance, apart to those at E. The cathode
rays then fall on a plate Px covered with
powdered willemite and in such a position
that PnEj. is equal to PE : equal potential
differences were applied to the plates E
and Ex and the electrostatic deflection of
the cathode rays compared with that of
the heads of the parabolas P due to the
positive rays. These two deflections were
found to be very nearly equal. Since
under similar geometrical conditions equal-
ity of electrostatic deflection means equality
of kinetic energy, the kinetic energy of the
cathode rays must be equal to that of the
particles which form the head of the para-
bolas in the positive rays. Now, since the cathode particles
remain charged throughout the whole of their path, and since
the more rapidly moving ones start from the cathode, the energy
in the cathode particles will be that due to the fall of the
atomic charge through the potential difference between the
anode and cathode ; and as we have seen that the energy of
the swiftest positive rays is equal to that of the cathode rays,
this energy must be that due to the fall of the atomic charge
through the potential difference between the electrodes and

FIG. 3 1 A.

SOURCES OF IONIZATION 105

not to half this difference as in Wien's experiments. The
difference in the results is probably due to the difference in the
pressure in the discharge tube. I worked with large vessels
and probably had much lower pressures in the discharge tube
than Wien. It may be pointed out that in a case like that of the
hydrogen molecule shown in Fig. i, Plate III., the particles
which are most prominent are those whose energy is equal to
that due to half the potential between the anode and cathode,
though those which have twice this energy can easily be
detected in the plate. Again, if the conditions are such that
the atomic positive rays in the observation vessel are due to the
splitting up, after passing through the cathode, of molecules
which were charged all the time they were in the discharge
tube, the positive rays being molecules before passing through
the cathode and atoms afterwards, the maximum kinetic
energy would be half that due to the fall through the full
potential difference.

Other observers who have worked at comparatively high
pressures have observed that the energy of the positive rays
is less than that due to the full fall of potential between the
electrodes. Thus, for example, Knipp ("Phil. Mag./' 6, 31,
p. 438, 1916), who produced his discharge by means of small
storage cells so that there could be no ambiguity about the
measurement of the potential difference, found a quite marked
effect of this kind, and we shall see (p. 148) that the
velocity deduced from the Doppler effect of the positive rays
is considerably less than that due to the full fall of potential
between the electrodes. We conclude, then, that with very
low pressures in the discharge tube the charging and dis-
charging of the particles does not play the primary part in
producing the wide range of velocities that exist in the positive
rays, though at fairly high pressures it may possibly produce
an appreciable effect.

io6 RA YS OF POSITIVE ELECTRICITY

Another explanation of the variation in velocity is that it
is due to the collisions between the particles in the positive
rays and the molecules of the gas through which they are
moving. This, however, is open to two serious objections.
The first is that these collisions would produce effects of the
same general character on all the lines, and we should expect
all the lines on a photograph to show a general resemblance
in the way the intensity varied along the parabola. We find,
however, sometimes on the same plate, lines which are quite
short with all the intensity concentrated at the head and
others which are long and of equal intensity throughout. The
second objection is that, as we have seen, the only collision
which a positive ray particle can survive is one that only
produces an inappreciable change in the kinetic energy and
velocity of the particle, collisions which lead to a finite loss
of energy seem always to be accompanied by " absorption "
and to be the death of the positive ray.

The explanation of the range of velocities in these
particles, which seems to agree best with the results of
observation, is that positive rays originate at different places
in the dark space as well as in the negative glow and that
they acquire a larger or smaller amount of energy according
as they start far away from the cathode or near to it. This
explanation would not be valid unless there were finite
differences of potential between different portions of the
dark space. It would not hold, for example, if, as some have
thought, all the fall of potential is concentrated close to the
cathode. There is direct evidence that as the particles
approach the cathode they gain speed, for Strasser (" Ann.
der. Phys.," 31, p. 890, 1910) found that the Doppler effect of
the positive rays due to hydrogen in front of the cathode
increased as the rays approached the cathode ; there was,
however, a well-marked increase in the effect after the

ELECTRIC FORCE IN THE DARK SPACE 107

rays had passed through the cathode, suggesting that there is
at the surface of the cathode a layer in which there is a con-
siderable increase in potential. Direct measurements of the
distribution of potential in the dark space have led to
conflicting results as to the reality of this spring in potential
at the cathode. Aston ("Proc. Roy. Soc.," 84, A. p. 526), who
measured the potential distribution in the dark space in
front of very large plane cathodes, found that the electric
force in the dark space was directly proportional to the
distance from the edge of the negative glow, and that
there was no appreciable spring of potential at the cathode.
On the other hand, Westphal (" Verh. d. Deutsch. Phys. Ge-
sell.," 12, p. 1910) found by two different methods that while
there was considerable electric force in the dark space there
was at the cathode a sudden spring of potential amounting
to from 27 or 70 per cent of the whole cathode fall
in potential. It is probable that these differences can be
explained to a considerable extent by differences in the
pressure of the gas, for the connection between the velocity of
the positive particles and the electric field might be expected
to undergo considerable variations in the neighbourhood of
those pressures at which the dark space is usually studied.
At pressures down to a millimetre or less of mercury the
velocity of a positive ion at a point P is proportional to the
electric force at that point — it does not depend on the previous
history of the ion : the place where it originated, the forces
it has been subject to before reaching P and so on. When,
however, the pressure is very much lower, so that the effects of
collision become inappreciable, all this is changed ; the velocity
of the ion at P is now determined by the condition that its
kinetic energy at P is proportional to the difference of
potential between P and the place where the ion originated —
it can no longer be determined by the value of the electric

io8 RA YS OF POSITIVE ELECTRICITY

field at P, and the differential equations which determine the
distribution of potential will be different in the two cases.
These equations become almost hopelessly complicated when
we take all the different sources of ionization into account
and also pay attention to the effect of the velocity of the
cathode and positive-ray particles on the amount of ionization
they produce.

To illustrate the point we have just been discussing we
shall take the simple case when we only take into account
ionization, such as that produced by radiation, which is
constant throughout the dark space. Let us take the case
when the electrodes are parallel plates whose linear
dimensions are very large compared with the distance be-
tween them, so that all the quantities concerned depend only
on one co-ordinate — the distance from one of the electrodes.
Let x be the co-ordinate of a point measured along an axis at
right angles to the electrodes, m the number of positive
particles, all supposed to be of one kind, per unit volume at
this point, u the velocity of these particles at this point, q the
number of positive or negative particles produced per unit
volume at this point in unit time by the source of ionization,
then if x is measured in the directions in which the positive
particles are moving and we neglect the re-combination of the
ions, we have when things are in a steady state

^(mu) = q. ,

We have supposed q to be independent of x ; hence

mu = qx ....... , . . (i)

if x is measured from the boundary of the negative glow
where u=o. If V is the electric potential at the point x

where n is the number of electrons per unit volume. Now,

ELECTRIC FORCE IN THE DARK SPACE 109

since the velocity of the electrons is enormously greater
than that of the positive particles, unless practically the
whole of the current is carried by the negative particles, and
we shall return to this point later, n will be small compared
with m, and we have approximately

Let us first suppose that the pressure is high enough to
make the velocity of the ion proportional to the electric
force, then

where k is the mobility of the positive ion ; substituting the
values for m and u in equation (i) we have

since dVfdx vanishes when x—o ; thus

dx \ k

or the electric force is proportional to the distance from the
negative glow. This is the result obtained by Aston in the
experiments already quoted. Integrating equation (2) we find

if V is taken as zero at the edge of the negative glow.

Since the velocity of the positive particle at any point is
proportional to the electric force at that point, all the particles
would have the same velocity at the same point even though
they had been produced at different parts of the dark space.
To explain the variation in the velocity of the positive particles
in the positive rays, we must suppose that the pressure is too
low for the particles to acquire a terminal velocity. We shall

1 10 RA YS OF POSITIVE ELECTRICITY

suppose that the particles at any point have the velocity
which they would acquire in passing freely to this point from
the place where they were liberated : u the velocity of a
particle at P will be given by the equation

JM«2 = V*

where M is the mass of the particle, e its electrical charge,
and V the difference of potential between P and Q the place
where the particle was liberated.

•-fir}1

If q particles were produced per second at Q, then if m is the
number of these particles per unit volume when they reach P

mu =

or m =

__
feWp

I'M"/

Now consider a place P at a distance x from the dark
space ; particles will be found there which have been produced
at all places intermediate between the boundary of the dark
space and P. If q$ be the number produced per unit
volume per unit time at a distance | from the boundary, V^
the potential at this place and V^ the potential at P, then the
number of positive particles per unit volume at P due to the
ionization between P and the boundary will be

while if a stream Q flows from the negative glow across unit
area of the boundary per second it will contribute

Q -l

to the density of the positive particles at P. If the number

ELECTRIC FORCE IN THE DARK SPACE in

of positive particles far exceeds the number of negative, then
the number of the positive particles at P is equal to

hence we have

If we assume

this equation may be written
P

n-wt-l ' » ' " * ' * '2/

o «* 5

U T>

where P =

If Q7 is finite we must have

or m= — i. n=—.

Thus, in this case, the potential is proportional to |i where
| is the distance from the junction of the negative glow and
the dark space, the electric force is proportional to £*, and
not to | as in Mr. Aston's experiments, where, however, the
pressure was considerably higher than is usual in experiments
with positive rays.

When m = — i, the value of P becomes infinite ; we may,
however, evade this difficulty as follows. If Q" is the number
of ions produced per second in the dark space

112 RAYS OF POSITIVE ELECTRICITY

and Q" is also infinite, we can, however, without difficulty
show that

Substituting in equation (2) we get

1 l /2o"-4-
y

and the potential at a distance f = Bf t.

Since the ionization is inversely proportional to the distance
from the negative glow most of the ions will be produced
near the boundary of the dark space and will have the
maximum velocity when they reach the cathode. Thus in
this case the heads of the parabolas will be much brighter
than the tails.

If no particles travel in from the negative glow Q'= O,
and instead of the two equations (3) we have the equation

or n = 2 + - m

If m = O, n = 2, which corresponds to uniform ionization
and uniform gradient of electric force in the dark space, and

we find V =

This agrees with the distribution of potential found by
Aston.

Since the ionization is uniform throughout the dark space
the parabolas in this case would be of fairly uniform
intensity.

In these calculations the potential difference considered
is that from the boundary of the dark space to a point in the

CATHODE FALL OF POTENTIAL 113

gas ; if there is a jump V0 in potential at the cathode the
cathode fall of potential, i. e. the potential difference between
the cathode itself and the boundary of the dark space,

where d is the thickness of the dark space.

If the radiation which caused the ionization were excited
by the impact of the positive rays against the cathode, since
qd is the number of positive particles striking in unit time
against the cathode, the energy given to the cathode per unit
time is qd\fe* if V is the cathode fall. If R, the radiant
energy is proportional to this energy, then R will equal
kqdVe, where k is a constant.

But q will be proportional to the amount of R absorbed,
hence we may write q = cRg where o is the density of the
gas and c a constant, characteristic of the gas : from this
equation we have, substituting the value for R,

or for the same gas VdQ = constant ..'.«... (4)
Thus as long as the current through the gas is below the
value at which the potential fall begins to depend on the
current, the thickness of the dark space will be inversely
proportional to the density of the gas ; when, however, the
current gets large and the cathode fall of potential increases
with the current then the dark space will contract as the
current increases. This, as far. as it goes, agrees with experi-
ence, but as radiation cannot be the only source of ionization
we should not expect the relation expressed by (4) to be more
than an approximation.

The question whether V0 is or is not finite will depend
upon the conditions governing the transference of the electric
charges from the gas to the cathode. Eisenman ("Verb.
d. Deutsch. Phys. GeselL," 14, 6, p. 297, 1912), who has

ii4 RAYS OF POSITIVE ELECTRICITY

investigated the distribution of potential in the neighbourhood
of the cathode, finds a jump in the potential at the cathode
which increases as the pressure diminishes.

We may point out in passing that the ionizing effect of
radiation would be to make a self-sustained electric discharge
possible even in an absolute vacuum. For, suppose we have
two electrodes in such a vacuum and that an electron is in
the field, under the electric force it will be driven against the
anode and will give rise to radiation ; this radiation falling
upon the cathode will cause it to give out electrons ; these
will in turn be driven against the anode and will give rise to
radiation which will again eject electrons from the cathode.
Thus the discharge will be maintained when the potential
difference between the electrodes is great enough to give so
much energy to an electron that the radiation it produces
when it strikes against the anode is sufficiently intense to
liberate one electron from the cathode. Thus a body charged
up to more than a certain potential would lose its charge
even if placed in an absolute vacuum. It is interesting to
notice that in a case like this the speed of the electrons
might exceed that due to a fall through the potential
difference between the anode and cathode. For when
electrons are ejected from a surface by radiation they start
with a definite amount of energy, which by Planck's law is
proportional to the frequency of the radiation. Now this
frequency will depend upon the energy possessed by the
electron when it struck against the anode. Thus, suppose
an electron were driven against the anode with the energy
due to the cathode fall. The radiation it would excite would
eject electrons from the cathode, these would start with an
amount of energy equal (say) to E. When they struck against
the anode they would have an amount of energy equal to
E plus that due to the cathode fall ; they would have more

CATHODE DARK SPACE 115

energy than the original electron and thus would give rise
to radiation of a higher frequency. This radiation would eject
electrons from the cathode with initial energy greater than
E, thus the radiation due to these would be of a still higher
frequency and would give still more initial energy to the
particles it ejected. The tuning up of the radiation would
go on until the frequency got so great that the number of
electrons ejected by a given amount of energy in this form
of radiation began to fall off, as there is evidence it does,
with increase of frequency after a critical frequency is
passed.

In a discharge tube under ordinary conditions the chief
source of radiation seems to be the negative glow, little in
comparison seems to come from the dark space. What is the
origin of this difference, and what is the condition which fixes
the limits of the dark space ? I think the answer to this ques-
tion is that in the dark space the electric force is considerable,
while in the negative glow it is inappreciable ; the boundary
of the dark space is fixed by the field of electric force and
is the place where this force vanishes. As the positive ions
move more slowly than the negative ones there must be an
excess of positive electricity around the cathode; this will
make the electric force diminish in intensity as the distance
from the cathode increases. When the intensity of the force
is above a certain value the free electrons are driven away
as fast as they are formed, and there are none left to combine
with the positively charged ions, so that if the reunion of
an electron and an atom is essential for radiation the
existence of the electric force will prevent its formation ;
thus the boundary of the dark space is the surface over
which the electric force is zero. Though in the main there
is little luminosity in the dark space, yet, as for example,
when perforated cathodes are used, bright pencils of light

ii6 RAYS OF POSITIVE ELECTRICITY

may be seen reaching right up to the cathode. The luminosity
of these, like that of the pencil of positive rays, after it has
passed through the cathode is due to the return of an electron
to the positively charged particle, this electron not being,
however, a free electron, but one taken from the molecules of
the gas through which the particles are passing.

We have referred above to the question of the proportion
of current carried respectively by the positively electrified
particles and the electrons. This subject has recently been
investigated by Mr. Aston,1 who measured the proportion
between the quantity of positive electricity passing through
a slit in the cathode and the total current passing through
the discharge tube. By using slits of various areas he showed
that the amount of positive electricity passing through the
slit was proportional to the area of the slit. Then on the
assumption, perhaps open to question, that the positive
electricity passing through the slit was equal in quantity to
that which would strike against an equal area of an unper-
forated electrode, he estimated that in his experiment the
positive particles carried fifty per cent of the current.
If the positive particles carry anything approaching to this
amount the number of positive particles in the dark space
must be very large compared with the number of free
electrons, so that in the equation

_ = _

it is legitimate to neglect, as we have done, n in comparison
with m.

To sum up the results of the preceding considerations,

the range of velocities in the positive particles is evidence

that these are produced to some extent throughout the whole

of the dark space. The concentration of particles about

1 " Proc. Roy. Soc.," 96, p. 200.

CONSECUTIVE FIELDS 117

different velocities which produces the beading of the
parabolas is, however, not due to special foci of production
but to the splitting up of molecules and perhaps also to the
formation of new systems after the particles have passed
through the cathode.

Since the positive particles will not be able to get through
the fine tube in the cathode unless they are moving along the
axis of the tube, it is only those particles which are formed
in the region adjacent to the prolongation of this axis in
the discharge tube which can pass through the cathode.
Those formed in outlying regions would not be moving in
the right direction when they struck the cathode. Thus
to get a copious supply of positive rays it is desirable
to concentrate the discharge as much as possible along the
axis of the tube, and we can understand the great influence
which the shape of the front of the cathode has upon the
brightness and range of velocities in the positive rays.

THE METHOD OF CONSECUTIVE SYSTEMS OF
ELECTRIC AND MAGNETIC FIELDS

A considerable amount of information about the behaviour
of the positive particles can be obtained by an extension
of the method described on p. 45. This extension consists
in having two systems A and B of electric and magnetic
fields placed at some distance apart in the path of the positive
rays, the displacements due to the magnetic and electrostatic
fields are respectively vertical and horizontal. Suppose A
is the system nearest the cathode, and that we take a
photograph which we shall denote by I. with the electric and
magnetic fields at A in action, but those at B out of action,
and compare this with another photograph II. taken with A
still in action and in addition a magnetic field at B. Let us

u8 RAYS OF POSITIVE ELECTRICITY

consider the effect on a line in I. due to a charged atom. If
all the particles producing this line retained their charges
while passing from A to B the line would simply be displaced
vertically ; there would be no resolution of the line ; as far
as the atomic lines are concerned there would be as many
lines in photograph I. as in II. Next, suppose that some
of the particles which were charged while passing through
A lost their charge before getting to B : these will not be
affected by the magnetic field at B, and so photograph II.
will show in addition to the displaced line (a) one (/?) in the
same position as the line in photograph I. Another pos-
sibility is that some of the particles should get another charge
while passing from A to B. These particles would be more
deflected by B than those with one charge and will give
rise to a line y where the vertical displacement is twice that
of a. Thus one line in I. might give rise to three lines in II.
of which the middle one might be expected to be the strongest.
If the original line were due to a doubly charged atom there
again might be three lines, one corresponding to the particle
retaining its charge, another to its losing one charge and the
third to its losing both. In this case the most deflected line
might be expected to be the strongest.

Let us now take the case of a line due to a molecule. Here
the possibilities are greater than for the atomic line, for in
addition to losing its charge the molecule may split up into
atoms between A and B. If some of the molecules were
to split up into two equal atoms the displacement of these
by B would be twice that of the unaltered molecule and
corresponding to one line in I., we should have three lines
in II. with the spacing and intensity similar to those corres-
ponding to an atomic line. If, however, the molecule were
to split up into atoms of different masses, Mx and M2, there
would be one line with a displacement (Mx + M2) JM1

CONSECUTIVE FIELDS 119

times the normal displacement d and another with the
displacement (M1 + M2) /M2 times the normal.

For example, if H3 were to split up into H and H2 then
corresponding to the line H3 on photograph I. there would
on II. be one line whose displacement was 3<5 and another
whose displacement was i*5<5.

If instead of producing the parabolas by A we produce
them by B and take photographs I. and II. with the magnetic
field at A off and on respectively, then corresponding to an
atomic line in I. we miglit have two lines in II., one a displaced
line due to particles which were charged while passing
through A and B and the other an undisplaced line corre-
sponding to particles which were uncharged while passing
through A, but acquired a charge before passing through B.
If the line were due to a doubly charged atom there might
be a third line due to particles which had one charge in A
and acquired another charge before reaching B, the displace-
ment of this would be one-half that of the normal line. Next
consider a line due to a molecule. We should have two lines,
one a corresponding to particles which were charged in
both A and B, another undeflected corresponding to particles
uncharged in A but charged in B : and if two atoms could
combine and form a molecule without suffering appreciable
deflection we might have two other lines due to particles
which were in the atomic state in A but had united to form
a molecule in B. These would be more deflected than the
normal line a which might be expected to be much the
brightest line of the series. As the behaviour of the lines
due to molecules differs from that of a line due to atoms
we can use this method to distinguish between the atomic
and molecular lines.

Another application I have made of this method is to
take a photograph of the parabolas due to B and then apply

120 RA YS OF POSITIVE ELECTRICITY

to A an electrostatic field strong enough to drive all the
particles which were charged while passing through A against
the plates so that the only particles which are recorded on
the photographic plate are those which were uncharged whilst
passing through A but gained a charge before reaching B.
These are but a small fraction of the whole number of particles,
so that the spectrum is very much less intense. Indeed, with
more than two hours' exposure I could only detect the line due
to H and H2, while the photograph without the electrostatic
field had, after an exposure of a few minutes, shown lines
corresponding to H, H2, C, O.

A striking feature of the photograph with the electrostatic
field was the change in the relative intensities of the H and
H2 lines ; with the field on H2 was very much stronger than
H, while without the field there was very little difference.

Though cathode rays may produce some charged atoms
they more frequently produce charged molecules, the chief
source of the charged atoms being positive rays, i. e. rapidly
moving charged molecules or atoms. The view that the
charged atoms and molecules are produced by different agents
helps us to understand the remarkable variations which occur
in the relative intensities of the lines due to the atoms and
molecules of the same element to which we have already
referred.

METHODS FOR MEASURING THE NUMBER OF
THE POSITIVELY ELECTRIFIED PARTICLES

Though the photographic plate furnishes an excellent
means of detecting the existence of positively charged particles
of different kinds it is not suitable for comparing the number
of these particles present in a bundle of positive rays. For
though the intensity of the lines on the photograph will vary

ELECTRICAL METHOD OF COUNTING RAYS 121

with the number of particles, this number will not be the only
factor in the expression for the intensity. As an example,
consider the lines due (i) to the very light particles like the
atoms of hydrogen, and (2) to very heavy ones like the atoms
of mercury. If these particles have acquired the same amount
of energy in the electric field before entering the cathode, the
hydrogen atoms will have a velocity about fourteen times
that of the mercury ones : they might therefore be expected
to penetrate further into the film on the plate and produce a
greater photographic effect than the mercury ones. If this
expectation is realized, and we shall see that it is, it is evident
that the photographic effect cannot be taken as a measure of
the number of positively electrified particles.

A method which does give metrical results is founded on
the following principle. Suppose that we replace the photo-
graphic plate in the preceding method by a metal plate in
which there is a movable parabolic slit, then when this slit is
moved into such a position that it coincides with one of the
parabolas on the photographic plate, positively electrified
particles will pass through the slit ; if these particles are
caught and their total charge measured we shall have a
measureof the number of positively electrified particles of this
kind. Thus if the slit were gradually moved up the plate there
would be no charge coming through it, unless it coincided in
position with one of the parabolas. As one parabola after
another was passed, positive electricity would come abruptly
through the slit, and the amount of the charge would be a
measure of the number of particles passing through the slit.
If instead of moving the parabolic slit we keep the slit fixed
and gradually increase the magnetic field used to deflect the
particles, we shall in this way drive one parabola after
another on to the slit, beginning with the parabola due to the
hydrogen atom and ending with that due to the mercury one

122

RAYS OF POSITIVE ELECTRICITY

and the charges passing the slits will be proportional to the
number of particles.

FIG. 32.

The apparatus used to carry this idea into practice is
represented in Fig. 32. After passing through the electric
and magnetic fields the particles, instead of falling on a
photographic plate, fall on the end of a closed cylindrical
metal box E. In the end of this box a parabolic slit
about i mm. in width is cut, the vertex of the parabola
being the point where the undeflected rays would strike
the box, and the tangent at the vertex the line along
which the particles would be deflected by the magnetic force
alone. This slit is the only entry into a metal box B. In-
side B and immediately behind the slit there is an insulated
long, narrow metal vessel placed so that every particle passing
through the slit falls into this vessel. This vessel is connected
with a Wilson tilted electroscope by which the charge it
receives can be measured.

From the face of the box E a portion was cut away, and
the opening closed by a willemite screen W. The positive

ELECTRICAL METHOD OF COUNTING RAYS 123

rays could be deflected on to this screen and the brightness
of the fluorescence observed ; in this way one can make sure
that the tube is in the proper state for giving positive rays
before attempting to make the measurements.

The impact on the face of the box of the rays which do
not pass through the slit gives rise to the emission of slowly
moving cathode rays ; if precautions are not taken these
diffuse through the slit, enter the Faraday cylinder, and
confuse the measurements. This diffusion can be avoided
by placing a small permanent magnet near the slit. The
force due to this is strong enough to deflect the more mobile
cathode rays without producing any appreciable effect on the
positively charged atoms. The pressure of the gas between
this box and the cathode should be made as small as possible :
the best way of reducing the pressure is to absorb the gas by
means of charcoal cooled with liquid air. This method will
not produce a good vacuum when the gas in the tube is
helium ; with hydrogen, too, the vacuum is not so good as
for heavier gases, for them the pressure can by this means
easily be reduced to 3/1000 of a millimetre of mercury.

The method of observing with this apparatus is as follows :
The positive rays are deflected by a constant electric field of
such a magnitude that the heads of the parabolas are in line
with one end of the slit. The magnetic field is then increased
by small increments and the deflection of the Wilson electro-
scope in ten seconds measured. Unless a parabola comes on
the slit there is practically no deflection ; as soon, however,
as the magnetic force is such that a parabola comes on the
slit, there is a considerable deflection which disappears when
the magnetic force is increased so as to drive the parabola
past the slit. The appearance and disappearance of the
deflection of the electroscope are surprisingly sharp, so that
lines quite near each other can be detected and separated.

124

RA YS OF POSITIVE ELECTRICITY

An example of the results obtained by this method is given
in Fig. 33. The abscissae are the values of the magnetic force
used to deflect the rays, and the ordinates the deflection of
the Wilson electroscope in ten seconds. The gas in the tube
was carbon monoxide.

A comparison of this curve with a photograph of the dis-
charge through the same gas shows many interesting features.
On the photograph the strongest lines are those corresponding

C 0 COj

Carbon Monoxide. 320 Volts.

FIG. 33.

to the atom and molecules of hydrogen. The curve on the
other hand shows that the number of hydrogen particles is
only a small fraction of the number of CO particles. The
extraordinary sensitiveness of the photographic plate for the
hydrogen atom in comparison with that for atoms and mole-
cules of other gases is shown in all the curves taken by this
method. But great as is the discrepancy in the case of the
photographic plate between the effects produced by hydrogen
atoms and an equal number of heavier atoms, it is not nearly

ELECTRICAL METHOD OF COUNTING RAYS 125

so great as it is for a willemite screen : such a screen may
show the hydrogen lines very brightly while the CO line is
hardly visible, when measurements made with the electroscope
in the way just described show that the number of particles
of hydrogen is only a few per cent, of the number of the CO
particles.

It is difficult to get from the photographs any estimate of
the relative amount of the different gases in the discharge tube
when it contains a mixture of several gases ; for example, if
the tube is filled with a mixture of hydrogen and oxygen the
relative quantities of these gases may be varied within wide
limits without producing any very marked effect on the
relative brightness of the hydrogen and oxygen lines in the
photograph. This electroscope method is much more metrical,
as will be seen from Figs. 34 and 35, the first of which
represents the curve when the gas in the tube was a mixture
of one-third hydrogen and two-thirds oxygen, while in the
second the gas was one-third oxygen and two-thirds
hydrogen.

The negatively charged hydrogen atoms seem to have the
same preponderance in their effect on the photographic plate
over other negative atoms as positive hydrogen atoms have
over other positive atoms. Thus on all the plates the line
corresponding to the negatively electrified hydrogen atoms is
well marked, often being comparable with the negatively elec-
trified oxygen atom. With the electroscopic method the
negative hydrogen atom can only just be detected, while the
negatively electrified oxygen atoms produce a large negative
deflection. A curve showing the comparative numbers of
different kinds of negatively electrified atoms is shown in the
curve, Fig. 36 : the gas in the tube was phosgene, COC12 ; the
curve at the top of the figure represents the number of nega-
tively electrified particles, the one at the bottom the positively

126 RA YS OF POSITIVE ELECTRICITY

electrified ones. It will be seen that the negative atoms de-

2. Hy d r o & e n
I Oxy ft e n .

FIG. 35.

tected by the electroscopic method were carbon, oxygen, and
chlorine, and that the chlorine atoms were by far the most

ELECTRICAL METHOD OF COUNTING RAYS 127

numerous. On the photographs taken with this gas the line
due to negatively electrified hydrogen seemed comparable in
intensity with that due to negative chlorine. An interesting
point about the curve representing the distribution of positively

FIG. 36.

electrified atoms is the great variety of atoms and molecules
present in the rays ; thus we find atoms of carbon, oxygen, and
chlorine, and the molecules CO, C12, CC1, and COC12. It will
be noticed that only a small fraction of the current is carried
by free carbon and oxygen atoms, showing that in phosgene

128 RAYS OF POSITIVE ELECTRICITY

the carbon and oxygen atoms are so firmly united that the
greater part of them remain together even when the gas is
dissociated.

Are the atoms from a molecule of a compound gas charged
with electricity of opposite signs ?

The study of the curves obtained by the electroscopic
method throws some light on the electrical states of the two
atoms in a diatomic molecule of an elementary or compound
gas. If we regard the forces which keep the atoms together
as electrical in their origin, the question naturally arises, are
the two atoms in a molecule of hydrogen, for example, charged
one with positive the other with negative electricity ; or in a
molecule of hydrochloric acid gas is the hydrogen atom
positively charged, the chlorine negatively, and if so do the
atoms retain their charges when the molecule is dissociated ?

Let us consider the case of CO for which we have in
Fig- 33 the curve which represents the relative numbers of
the different kinds of positively charged atoms. If the carbon
atom in the molecule were positively, the oxygen atom
negatively electrified, then we should expect that if a molecule
of CO were split into atoms by the impact of a rapidly moving
positively electrified particle, there would be a tendency for
the carbon atoms to have a positive charge and for the oxygen
ones to have a negative, so that in the positive rays we should
expect to find more carbon atoms than oxygen ones. The
curve, Fig. 33, shows that the number of positively electrified
carbon atoms exceeds that of the positively charged oxygen
ones in the proportion of 1 1 to 7. These figures, however,
underrate the number of oxygen atoms which came through
the cathode, for some of them after passing through the
cathode acquired a negative charge. The charges given to the
electroscope show that the proportion between negatively and
positively charged oxygen atoms was as 2 to 7, while the

DISSOCIATION 129

number of carbon atoms which were negatively charged was
very small in comparison with that of the positively charged
atoms. Taking the negative atoms into account as well as
the positive we find that the proportion between the number
of carbon and oxygen atoms passing through the cathode is
as ii to 9 ; the numbers are too nearly equal to allow us to
suppose that after dissociation one of the atoms is positively,
the other negatively charged.

The curve for COC12, Fig. 36, shows that the proportion
of positively electrified chlorine atoms in the positive rays to
the positive CO particles is not very different from the
proportion between the atoms of chlorine and CO to the
normal gas. If the atoms in the molecule COC12 had after dis-
sociation carried electric charges we should have expected the
atoms of the strongly electro-negative element chlorine to have
carried a negative charge and to have been relatively deficient
in the positive rays.

The view that each of the atoms derived from a molecule
of a compound contains as much positive as negative electricity
is supported by considerations drawn from other branches of
physics. If the atoms in a molecule of a gas carried separate
charges so that one kind of atom was positively, another
negatively, charged, then if the gas were dissociated into these
atoms and if the atoms retained their charges the dissociated
gas would be a good conductor of electricity. Now there are
several gases which are dissociated at low temperatures : nickel
carbonyl, for example, is at 100° C. split up into nickel
and CO to a very large extent ; if these atoms were charged
the electrical conductivity of the gas might be expected to
begin to show marked increase at a temperature of about 70° C.
when the dissociation first becomes appreciable. The varia-
tion of the conductivity of nickel carbonyl with temperature
is, however, as Prof. Smith has shown, quite normal, following

130 RAYS OF POSITIVE ELECTRICITY

the same laws as for an undissociated gas. L. Bloch,1 too, has
shown that the dissociation of arseniuretted hydrogen which
also takes place at low temperatures is not accompanied by any
increase in electrical conductivity. He also showed that many
chemical reactions between gases which go on at low tempera-
tures such as the oxidation of nitric oxide, the action of
chlorine on arsenic, the oxidation of ether vapour, have little
or no effect on the conductivity.

Chemical action between gases, unless accompanied by
high temperature, has not been shown conclusively to give
conductivity. The very vigorous combination of hydrogen
and chlorine under sunlight seems to have absolutely no
effect on the electrical conductivity of the mixture, and this
is a strong reason for supposing that the atoms in the
molecules H2 and C12 are not charged.

It is true that chemical action vigorous enough to raise the
gases to a very high temperature, such as, for example, the
combination of hydrogen and oxygen in the oxy-hydrogen
flame, the oxidation in a Bunsen flame, the burning of CO
and so on, make the reacting gases good conductors of electri-
city. This conductivity seems, however, from the result of
recent experiments, to be due to the high temperatures pro-
duced by the chemical action rather than to that action itself.
The conductivity cannot be due to the molecule being dis-
sociated into positively and negatively electrified atoms, for
the determinations of the mobility of the negatively electrified
particles in flames and gases at a very high temperature show
that it is much larger than would be possible if these particles
had masses comparable with that of even the lightest atom.

In considering the ionization of flames we have to separate
two effects —

1 "Annales de Chimie et de Physique," [8] XXII, pp. 370, 441 ; XXIII,
p. 28.

IONIZATION IN FLAMES 131

(a) An effect due to the contact of the flame with hot
bodies. We know that many solids give out electrons when
heated to a high temperature : the oxides of calcium and
barium do this to quite an exceptional extent. Thus when
the flame is in contact with solids, as it is when electrodes are
introduced into the flame or when solid particles are scattered
through it, these being raised to incandescence will emit
electrons which will be scattered through the gas.

(b) We have next to consider the effect produced by the
high temperature of the gas itself apart from the effects pro-
duced by solids. At a temperature of 2500° C. the average
kinetic energy of a molecule due to thermal agitation corre-
sponds to that represented by the fall of the atomic charge
through a potential difference of about one-third of a volt.
To ionize a molecule of a gas by electrons requires the expen-
diture of an amount of energy which varies from gas to gas,
but which is of the order of 10 volts ; this kind of ionization
gives rise to free electrons. For ionization of this type an
atom or molecule is very inefficient compared with an electron,
and we should, from the considerations given on page 57,
expect that to liberate a free electron an atom of hydrogen
would require an amount of energy represented by some
10,000 volts. The number of molecules which even at a
temperature of 2500° C. possess this energy would, if Max-
well's law were to hold, not be more than one in e-8xl°*. We
can, therefore, leave out of consideration this type of ioniza-
tion when considering the effect of collisions. There is,
however, another type of ionization which is much more
probable, when the colliding atom instead of setting the
electron free unites with it and drags it away, thus pro-
ducing a negatively charged ion instead of a free
electron. This method of ionization enables the atom to
utilize its energy to better advantage than when it has to

132 RAYS OF POSITIVE ELECTRICITY

eject an electron by impact. Let us suppose that, under
favourable circumstances, it can effect this ionization when its
energy is that represented by the ionizing potential, say,
10 volts. The number of molecules which at the tempera-
ture of 2500° C. possess not less than this amount of energy

is approximately — /= / e~x^dx or about 8 X io~19 times

the whole number of molecules. If the gas were hydrogen
the number of collisions in a cubic centimetre per second
between these high-speed molecules and the other molecules
would be about 6 X io9. Thus, if every one of the col-
lisions with the high-speed molecules resulted in ionization,
6 X io9 ions would be produced per second per c.c. of gas.
This ionization, though considerable,would not be anything like
sufficient to carry the currents that actually pass through flames.
We conclude that when a molecule is dissociated into
atoms these are uncharged. This might have been expected,
as it requires in general much less energy to dissociate into
uncharged than into charged atoms. Before dissociation,
however, it may be that one of the atoms had one kind of
charge, the other the opposite. There must, however, be a
type of molecule including elementary molecules such as H2
where there is no such distinction between the atoms. I
have, however ("Phil. Mag./' XXVII, p. 757, 1914), given
reasons for thinking that this is not the only type of
compound, there is another type of which water vapour
is a very conspicuous example, where there is such a
separation of electricity inside the molecule that one atom
may be regarded as positively, the other as negatively,
electrified. Perhaps the most direct argument in favour of
this view comes from the study of the specific inductive

POLAR MOLECULES 133

capacities of gases. The measurements made by Baedeker
("Zeits. Physik. Chemie," XXXVI, p. 305) show that if K is
the specific inductive capacity of a gas, K-i for some gases
such as H2O, NH3, and the vapours of the various alcohols, is
far in excess of its value for other gases, and, moreover, that the
variations of K-i with temperature is quite different for the
two types of gases. In the type represented by water-vapour
K-i varies rapidly with the temperature, while in the other
type if the density of the gas is kept constant there is hardly
any variation at all with the temperature. A high value of
K and a rapid variation with temperature would follow if the
molecule possessed a finite electrical moment, such as it
would have if one of its atoms were positively, the other
negatively, electrified. The substances belonging to this type
possess very energetic properties in the liquid state, they
ionize salts dissolved in them, they show the phenomenon
of association, the molecules tending to cling together ; as
the electrical moment gives rise to a very large stray
field, these effects, which would result if the molecules
exerted appreciable action on each other, might have been
anticipated.

From the point of view of the positive rays, the presence
of gases of this type in the discharge tube might be expected
to produce an increase in the negatively electrified constituents
of the rays, since the atoms of the electronegative elements
would, after passing through the cathode, be able to obtain a
negative charge from the molecule of which they formed a
part, and would not have to rely exclusively on obtaining this
charge from the molecules of the gas through which they were
passing. The negative particles obtained in this way would
not possess the kinetic energy due to the full fall of potential
between the anode and cathode. Thus, if there were water-
vapour in the tube the energy in those negatively charged

134 RA YS OF POSITIVE ELECTRICITY

oxygen atoms which owed their charge to the decomposition of
a molecule of water would only be -ff- of the maximum energy ;
while if the negatively electrified oxygen atoms owed their
charge to the decomposition of the molecule of some alcohol of
high molecular weight their energy would be a much smaller
fraction of the maximum energy. The production of negatively
electrified atoms by the decomposition of molecules would thus
not affect the intensity of the heads of the parabolas corre-
sponding to these atoms, they would produce an abrupt increase
in intensity at points on the parabolic arc at a distance from the
head depending on the type of compound from which the
atom was liberated. Some observers — for example, Wien,
Dechend, and Hammer — have observed that the negative
oxygen was more pronounced when water-vapour was ad-
mitted to the tube than when pains were taken to exclude
it, and the suggestion has been made that the negative
constituents are due entirely to this source. I do not think
this position is tenable, as I have found the negative oxygen
exceedingly strong after very elaborate precautions had been
taken to exclude water-vapour, and, moreover, the decom-
position of water-vapour cannot account for the presence
of negatively charged hydrogen atoms, one of the most
prevalent constituents of the stream of particles which form
the positive rays.

RETROGRADE AND ANODE RAYS

The rays we have hitherto been considering consist of
positively charged particles travelling in the direction in which
such particles would be moved by the electric field in the
discharge tube. In addition to these there is another system
of rays travelling in the opposite direction. By far the
larger portion of these rays are cathode rays, i.e. streams of

RETROGRADE RAYS

'35

electrons moving with great velocity, but, as the author
showed long ago,1 these are mixed with rays which are
evidently of a different character, for, unlike the cathode
rays, they are not appreciably deflected when a permanent
magnet is brought near them. It was afterwards shown by
Villard2 and the author3 that some of these new rays were
deflected by strong electric and magnetic fields and that the
direction of the deflection indicated that the particles forming
the rays were charged with positive electricity. The fact that
these rays travel with high velocities away from the cathode
and thus in the opposite direction to the electric forces acting

FIG. 37.

upon them makes their investigation a matter of very consider-
able interest. The apparatus I have used for this purpose is
represented in Fig. 37.

A is a perforated electrode through which the rays pass on
their way to the willemite screen or photographic plate S.
On their journey to S the rays traverse the usual electric and
magnetic fields. B is a plane rectangular electrode at the
other end of the discharge tube : it is carried by a stopper
working in a ground-glass joint and thus can be rotated about
a vertical axis. C is a wire fused in the side of the tube for

1 J. J. Thomson, " Proc. Camb. Phil. Soc.," IX, p. 243.

2 "Comptes Rendus," CXLIII, p. 673, 1906.

8 J J. Thomson, "Phil. Mag.," XIV, p. 359, 1907.

136 RAYS OF POSITIVE ELECTRICITY

use as an auxiliary electrode. D is a side tube in which a
closed glass vessel containing a piece of iron can slide up or
down : this vessel carries a piece of fine metal rod which,
by moving the iron by means of a magnet, can be inserted
in or withdrawn from the line of fire of particles projected
from B. t

When the stopper carrying the electrode B is turned so
that the normal of the plane of the electrode either coincides
with the axis of the hole through A, or makes but a small
angle with it, then if B is made cathode and a discharge sent
through the tube, the cathode rays pass down through the
tube in A and produce vivid phosphorescence on the screen.
In addition to these rays there are others which produce a
phosphoresence different in colour from that due to the
cathode rays and are deflected in the opposite direction by
the electric and the magnetic fields : the amount of electro-
static deflection is about the same as that for the cathode rays
but the magnetic deflection is very much less. It can easily
be shown that these are not ordinary positive rays due to A
becoming cathode through accidental reversals of the coil. For
in the first place they disappear when the electrode B is
twisted round so that a normal to its plane no longer nearly
passes down the tube through A : and secondly the rays per-
sist when A is disconnected from the induction coil and
the auxiliary electrode C used as the anode. Again when the
rod attached to D is put in the line of fire a shadow is thrown
on the phosphorescence on the screen due to these rays. These
rays are strongest when the electrode B is placed so as to be
at right angles to the axis of the tube through A. If the elec-
trode is rotated they diminish rapidly in intensity but can be
detected until the normal to B make an angle of about 15°
with the axis of the tube through A ; they appear in fact to
follow much the same path as the cathode rays from B, for

RETROGRADE RAYS 137

much the same rotation was required to prevent the cathode
rays getting through the tube in A and producing phos-
phorescence on the screen.

These rays get exceedingly feeble when the pressure of the
gas in the discharge tube is very low and they are no longer
observable at pressures when the ordinary positive rays give
quite vigorous effects ; even when most fully developed they
are feeble in comparison with the ordinary positive rays, so
that it is necessary for the tube through A to have a much
wider bore than is required for experiments with positive rays.
As these rays travel in the opposite direction to the positive
rays they are called retro-grade rays.

Using a tube through A about *5 mm. in diameter I ob-
tained a photograph of the retrograde rays which gave the
following results : —

There are in the retrograde rays positively electrified atoms
and molecules of hydrogen and positively electrified atoms of
oxygen : there are also negatively electrified atoms of hydrogen
and oxygen, and with these rays the intensity of the lines
corresponding to the negatively electrified particles is greater
than that of the positively electrified ones ; with the ordinary
positive rays the positive lines are much stronger than the
negative. In the retrograde as well as in the positive rays
there are large numbers of uncharged particles. The photo-
graph taken with the retrograde rays shows that the maximum
velocity of the negatively electrified atom is about the same as
that of the corresponding positively electrified one and differs
but little from the velocity of these atoms in the ordinary
positive rays. This result is suggestive because the electric
field in the tube would accelerate the negatively electrified
retrograde rays and retard the positively electrified one. It
points, I think, to the conclusion that the origin of the retro-
grade rays is analogous to that of the negatively electrified

138 RAYS OF POSITIVE ELECTRICITY

particles which accompany the positive rays, the difference
between them being that the retrograde rays acquire their
negative charge before passing through the cathode, while the
negative constituent of the positive rays do so after passing
through the cathode. We may suppose that the process by
which the retrograde rays are produced is somewhat as follows :
neutral atoms or molecules acquire a negative charge when
they are just in front of the cathode, they are then repelled
from the cathode and driven through the dark space, acquiring
under the electric field in the discharge tube a velocity of the
same order as that acquired by the positively electrified par-
ticles of the positive rays during their approach to the
cathode. Some of these rapidly moving negatively electrified
particles will in their course through the gas come into collision
with the electrons and molecules in the discharge tube ; one
collision will detach an electron leaving the particle in the
neutral condition ; a subsequent one will detach another elec-
tron and leave the particle positively charged. The particles
which have made two collisions form the positively electrified
portion of the retrograde rays, those which have made one
collision the portion which is without charge, and those
which have not made a collision the negatively electrified
portion of these rays.

These retrograde rays are very well developed when a
double cathode of the kind introduced by Goldstein (see p. 5)
is used instead of a flat cathode. If a cathode consisting of
two parallel triangular plates, Fig. 38, is substituted for the
flat cathode B in the apparatus shown in Fig. 37, a plentiful
supply of retrogade rays come from the cathode when it is
turned into a suitable position. By twisting the triangle round
by means of the glass stopper the emission of the rays, both
cathodic and retrogade, can be determined. In this way it
was shown that the maximum emission of cathodic rays is

RETROGRADE RAYS

139

along the line starting from the middle points of the sides.
At the higher pressures this is practically the only direction
in which cathode rays can be detected ; at very low pressures,
however, cathode rays can be detected coming from the corners
of the triangle as well as from the middle points of the sides.
Few, if any, however, are given out in any intermediate direc-
tion. The positively electrified particles stream off at all
pressures from both the corners and middle points of the sides,
but not from the intermediate positions. The most abundant
stream comes, as for the cathode rays, from the middle points of
the sides, but the disproportion between the streams from the

FIG. 38.

corners and from the middle points of the sides is nothing
like so large as for the cathode rays, so that the ratio of positive
to cathode rays is much the greatest at the corners of the
triangle.

A simple method of demonstrating the existence of retro-
grade rays, and also of the places at which the positive rays
originate, is that already described (see p. 15), founded on the
difference between the phosphorescence of lithium chloride
under cathode and positive rays. When lithium chloride is
struck by cathode rays, the phosphorescence is a steely blue
giving a continuous spectrum. When struck by rapidly
moving positively electrified particles the phosphorescence
is a rich deep red, and the red lithium line is very bright in

140 RA YS OF POSITIVE ELECTRICITY

the spectrum. To explore the tube for positive rays a thin
rectangular strip of mica or metal is covered with fused lithium
chloride, the strip is attached to a piece of iron l which rests
on the bottom of the discharge tube. By moving the iron by
means of a magnet the strip can be moved towards the
cathode or away from it. If we start with the mica strip
close to the cathode we find that there is no red light to be
seen on the side of the lithium chloride next the cathode. The
anode side of the chloride is a brilliant red, showing that the
strip is being struck by the positive rays before they reach the
cathode but not by the retrograde ones. If the mica strip is
pulled farther away from the cathode until the distance between
them is about half the thickness of the dark space, red light
appears upon both sides of the strip,showing that now it is struck
by the retrograde as well as by the positive rays. This state
of things continues until the mica reaches the limit of the dark
space and approaches the negative glow ; in this position the
cathode side of the strip is red but the other side is dark, show-
ing that now it is struck only by the retrograde rays. Another
way of making this experiment is to keep the strip fixed at a
distance of about one or two centimetres from the cathode.
Beginning with a fairly high pressure so that the strip is out-
side the dark space, we find that the cathode side of the strip
is red, while the other side is dark ; in this position the strip is
struck only by the retrograde rays. If the pressure is gradually
reduced so that the dark space increases until it reaches just
past the mica, both sides of the strips will now show the red
light, showing that now positive as well as retrograde rays
strike the strip. When the pressure is further reduced until the
dark space is three or four centimetres long, the red light dis-
appears from the cathode side but is very bright on the other.

1 It is better to put the iron in a closed tube and attach the mica strip to the
tube, otherwise so much gas is given out by the iron that it is difficult to reduce
the pressure sufficiently.

RETROGRADE RAYS 141

The reason that the retrograde rays are not observed when
the screen is close to the cathode is due I think to the shadow
cast by the mica on the cathode. The mica stops the positive
rays on their way to the cathode so that the parts in shadow
are not struck by these rays and so cannot be the origin of
retrograde rays, if these are produced in the way we have
described.

This view is confirmed by the following experiments. If
the cathode is placed near the middle of a large bulb and the
mica screen is put a little on one side of the cathode, the red
lithium light can be observed on the side of the screen turned
towards the cathode even when the screen is quite close to the
cathode and the dark space 5 or 6 cm. long.

Again if the cathode stretches across a tube of uniform
bore, and the screen is moved towards the cathode, the shadow
thrown on the cathode becomes much more marked and sud-
denly increases in size at the place where the red light fades
away from the cathode side of the mica strip. The increase in
size is due, I think, to the screen getting positively electrified
when in the region close to the cathode. We know by the dis-
tribution of electric force in the dark space that there is a dense
accumulation of positive electricity just in front of the cathode,
which naturally would charge up an insulator placed within
it. The positively electrified screen repels the positively elec-
trified particles which pass it on their way to the cathode and
deflects them from their course, so that they strike the cathode
beyond the projection on it of the screen. In this way a con-
siderably increased area is screened from the impact of the
positively electrified particles. The portion so screened no
longer emits cathode rays. Thus the region in front of it is
traversed by little if any current and there is consequently
no bombardment of the screen by retrograde rays.

Somewhat similar effects are obtained if the mica screen

142 RA YS OF POSITIVE ELECTRICITY

is replaced by a very fine platinum wire. If this wire is slowly
moved towards the cathode, starting from a place inside the
negative glow, the following effects are observed : almost im-
mediately after entering the dark space the wire becomes red
hot and remains so until it reaches the velvety glow immedi-
ately in front of the cathode (known as Goldstein's first layer).
Here it becomes cold and the shadow which before could
hardly be detected now becomes well marked and much thicker
than the wire. The change takes place very abruptly. In
some cases just before entering this layer the shadow is
reversed, i.e. the projection of the wire on the cathode is now
brighter than the rest of the cathode, indicating, I think, that
the wire when in this position gets negatively electrified
and attracts the positively electrified particles instead of
repelling them.

The retrograde rays are well developed with cathodes made
of wire gauze.

Mr. Orrin H. Smith (" Phys. Review," 7, p. 625, 1916) has
investigated the retrograde rays by a somewhat different
method. The only types of retrograde rays he could detect
were molecules of hydrogen and oxygen : these occurred with
positive and also with negative charges.

ANODE RAYS

The positively charged particles, which we have hitherto
considered, originate in the neighbourhood of the cathode.
Gehrcke and Reichenheim x have discovered rays of positively
charged particles which start from the anode. Their attention
was called to these rays by noticing that a pencil of yellow
light streamed from a point on the anode of a tube with which

1 "Verb, d. Phys. Gesell.," 8, p. 559; 9, pp. 76, 200, 376; 10, p. 217.

ANODE RAYS 143

they were working. It was found that there had been a speck
of sodium chloride at the points on the anode from which the
pencil started. They got these rays developed to a much
greater extent when they used for the anode a piece of platinum
foil with a little pocket in which various salts could be placed,
and which was heated to redness by a battery insulated from
the one used to send the current through the discharge tube.
The current through the tube was produced by a battery

Anocfe

FIG. 39.

giving a potential difference of about 300 volts which, as a
Wehnelt cathode was used, was sufficient to send a very
considerable current through the tube : the pressure in the
tube was very low. The rays were well developed in this tube
when NaCl, LiCl, KCL and the chlorides of Cu, Sr, Ba, In, were
placed in the pocket. The colour of the rays corresponded with
the colour given to flames by the salt. They did not get any
effects when the oxides of calcium or barium were put in the

144 RA YS OF POSITIVE ELECTRICITY

pocket ; these oxides are known when hot to give out large
streams of electrons, and for this reason are used for Wehnelt
cathodes. These rays are apparently only given out by the
salts of the metals and not by the metals themselves ; they
are called Anode Rays.

Gehrcke and Reichenheim arranged a Faraday cylinder so
that the rays could fall into it ; they found that when the rays
entered the cylinder it acquired a strong positive charge.

They subsequently used another form of apparatus
which gave better results than the one just described. The
anode was a rod of salt placed inside a glass tube so that
only the front of it was exposed to the discharge tube ; the
cathode was an aluminium ring encircling the anode, the

FIG. 40.

pressure was reduced to a very small value by the use of carbon
cooled by liquid air. With the discharge from a powerful
induction coil, or still better from a large electrostatic induction
machine, the anode got hot without the aid of an auxiliary
heating current, and a bright stream of rays came from the end
of the salt anode ; the appearance of this beam is represented
in Fig. 40. It was found that a mixture of two or more
salts with powdered graphite gave brighter rays than a simple
salt, the best mixture seemed to be LiBr, Lil, Nal and
graphite. The rays come off at right angles to the surface of
the salt ; thus if the surface is cut off, as in Fig. 41, the rays
come off in the direction AB.

Gehrcke and Reichenheim found that there was a very
considerable difference of potential between the surface of the

ANODE RAYS

anode and a point a centimetre or two away : in some of their
experiments it was as much as 2300 volts. By assuming that
the energy acquired by the rays was due to the fall through
this potential V, and measuring the radius of the circle into
which the rays were bent by a strong magnetic field H, the
values of v and m\e can be determined, for we have

and if r is the radius of the circle into which the rays are bent
by a magnetic force H at right angles to the path

mv'

hence v = j^— and ejin

FIG. 41.

In this way the following values were obtained : —

Salt.

LiCl.

v.
cm. /sec.

Ratio of mass of particles to that
of an atom of hydrogen

X I07 *£* X I03

Li Cl. j;j§ x io7 .°9 x I03

TVT /*"M I*o7 1 *4O o

Na Cl. ^ X io7 .^ x io8

Sr C12. ro8 x io7 '2i x IO3

8-6 - 8-3
14 — ii

21 — 23

90 (if the atom
is doubly charged).

146 RA YS OF POSITIVE ELECTRICITY

The results for Li Cl given in the first line relate to the
brightest part of the rays, those in the second to the least
deflected rays. It would appear from this that the charged
particles are the atoms of the metal in the salt, and that in
the case of strontium they carry a double charge. A very
interesting case of these anode rays is that of a discharge
tube with a constriction in the middle. When two bulbs
A and B, about 10 cm. in diameter, with the anode in A and
the cathode in B, are connected by a narrow tube : then when
the pressure in the tube is very low and a small quantity of
iodine vapour is introduced into it, anode rays start from the
constriction c at the cathode end of the narrow tube and
cathode rays from d, the anode end of this tube. These have
been observed when the gas in the tube was hydrogen, oxygen,
or helium, but not when it was nitrogen. If the connecting
tube were quite straight these anode rays might be the
positive rays corresponding to the cathode d, but as they
appear when the tube is bent this cannot be their origin. It
is especially to be noticed that the anode rays do not appear
unless iodine, bromine, or chlorine is in the tube. This is
perhaps due to the fact that the atoms of these substances
are excellent traps for electrons which unite readily with
halogen atoms. Any positively electrified particles in the
tube will thus have a much better chance to escape being
neutralized by these electrons when these gases are present
than when they are absent : and thus the number of anode
rays will be increased.

The most natural explanation of these rays is that the
hot salts from which they originate act like fused electrolytes,
and that the current through them into the discharge tube is
carried by the ions into which the salts dissociate, the positive
ion, which is a charged atom of the metallic constituent of the
salt, following the current will come to the surface of the glow-

ANODE RAYS 147

ing anode, will get detached from it, and under the influence
of the strong electric field which exists gas close to the anode
will acquire the high velocity characteristic of the anode rays.

Goldstein (Monatsber. d. Berl. Akad., 1876: "Ver. der
Deutsch. Physik Gesellsch.," 20, 123, 1918) and Gouy (" C. R.,"
1909, p. 148) have observed rays proceeding from the anode
in a discharge tube when the anode is in a strong magnetic
field. These rays produce luminosity in the gas, and phos-
phorescence on the walls of the discharge tube.

There is in this case a great fall in potential close to the
anode. We can understand why this should occur, for as
the magnetic field would stop the electrons coming up to the
anode, the only systems available for carrying the current
would be positive ions. These would have to be produced
close to the anode, and this would require a strong electric
field ; unless this was available the current must stop.

The presence of halogens, which, as we have seen, facilitates
the formation of anode rays, also produces a strong field near
the anode ; this, like the effect of the magnet, is probably due
to the withdrawal of electrons from the neighbourhood of the
anode. The magnet effects this by sweeping the electrons to
one side, the halogens by absorbing the electrons. We might,
I think, expect to get rays coming from the anode whenever
the conditions are such that electrons are prevented from
reaching it. An analysis of the anode rays by the methods
used for positive rays might be expected to lead to very
interesting results.

G. P. Thomson ("Proc. Camb. Phil. Soc.," 20, p. 210)
has shown that the anode rays can be analysed and the value
of e\m determined by the photographic method used for
positive rays, and Dempster ("Phys. Review," 2, n, p. 316,
1918) has applied the electrical method (p. 120) for the same
purpose.

148 RAYS OF POSITIVE ELECTRICITY

DOPPLER EFFECT SHOWN BY THE POSITIVE

RAYS

Before the methods described in the earlier part of this
book had been fully developed, Stark 1 had discovered a
property of the positive rays which is of great importance in
connexion with the origin of spectra, and incidentally has led
to results which have confirmed some of those obtained by
the newer methods.

Stark's discovery resulted from the spectroscopic examina-
tion of the light produced by the positive rays passing through
a gas at a pressure comparable with *i mm. of mercury, a
very much higher pressure than that used in the majority of
the experiments when positive rays are studied with the help
of the photographic plate or the willemite screen. The
stream of rays passing through a perforated cathode pro-
duces at these high pressures considerable luminosity in the
gas behind the cathode. Stark examined with a spectro-
scope this luminosity when the gas was hydrogen : (i) when
the line of sight was at right angles to the direction of the
rays ; (2) when the line of sight was approximately in the
direction of the rays. In the first case he found that the
series lines for hydrogen were in their normal positions. In
the second, however, he found that though there were lines in
the normal positions, these lines were broadened out towards
the violet end of the spectrum when the positive particles
were approaching the spectroscope, and towards the red end

1 Stark, "Physik. Zeitschr.," 6, p. 892, 1905. "Ann. d. Phys.," 21, p. $*t,
1906. rt'u

DOPPLER EFFECT 149

when they were receding away from it, indicating that some,
though not all, of the systems emitting these lines were moving
in the direction of the rays with velocities sufficient to give an
appreciable Doppler effect. A closer examination of these
lines brought out some interesting details which are illustrated
in Fig. 2, Plate IV., taken from a photograph by Stark of
the hydrogen line Hy. It will be noticed that though the
displaced line is broadened out into a band, this band does
not begin at the undisplaced position of the line, but is
separated from it by a finite distance. The alteration AA- in
the wave length X of a line given out by a source moving
towards the observer with a velocity v is by Doppler's
principle given by the equation

where c is the velocity of light. In the case of these small
displacements we may take, when we are dealing with one
line in the spectrum, A^- as proportional to the displacement
of the line, and we may use this equation to determine v the
velocity of the particle emitting the line. The fact that the
fine line is displaced into a broad band shows that these
velocities range over somewhat widely separated limits : this
is quite in accordance with the results indicated by the photo-
graphs of the positive rays when deflected by electric and
magnetic forces. We saw that the parabolic arcs were of
considerable length, and therefore were produced by particles
moving with a wide range of velocities. The dark space
between the undisplaced line and the band indicates that the
moving particles do not give out the lines unless the velocity
exceeds a certain value. As this occurs when the spectrum
of the positive rays is observed when the rays are on their
way to the cathode as well as after they have passed through

1 50 RAYS OF POSITIVE ELECTRICITY

the openings in the cathode, it cannot be due to the absorption
of the more slowly moving rays after they pass through the
cathode. According to Stark and Steubing1 this limiting
velocity varies with the different lines of the same element,
increasing as the wave length diminishes. The limiting
velocity given by these observers for the hydrogen lines are
as follows : —

Ha = 1-07 x io7cm. Hj3 = 1*26 x io7cm./sec.

These values are approximately proportional to the square
root of the frequency of the lines. There is some difference
of opinion as to whether this limiting velocity does or does
not depend upon the frequency of the light. Paschen 2 came
to the conclusion that it was the same for all the hydrogen
lines. This velocity is small compared with the average
velocity of the positive rays of hydrogen ; it corresponds to a
fall through a potential difference of less than 100 volts. It is
comparable in value with that which the mercury atom
acquired in many of the experiments represented by the
preceding photographs, when it had possessed one, but only
one, charge throughout its journey through the discharge tube.
The maximum displacement of the line depends to some
extent on the potential difference between the terminals of
the discharge tube ; but it does not increase nearly so quickly
as the square root of that potential difference, as we should
expect if the most rapidly moving particles could give out
the line : the relation between the displacement and the
potential difference is given in the following table due to
Stark and Steubing.3 In this table r is the ratio of the kinetic
energy of a particle moving with a velocity vt calculated by
the Doppler formula (p. 149) from the maximum displacement,

1 "Ann. der Phys.," 28, p. 974. 2 Ibid., 27, p. 599. 3 Ibid., 28, p. 974.

DOPPLER EFFECT 151

to the kinetic energy the particle would possess if it fell when
carrying one charge through the potential difference between
the terminals of the discharge tube.

Potential difference
in Volts.

390

425

555
600

1200
3000
4000
4000
7000

•907

•563

•824

•622

•358
•309
•402
•274

1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2

0.1 0,2 0.3 04 0.5 0,6 0>7 0.$

FIG. 42.

152 RAYS OF POSITIVE ELECTRICITY

Stark indeed suggests that his observations are compatible
with the view that the deflections approach a limit corresponding
to a velocity about 1*5 X io8cm./sec. and do not exceed this
however large the potential difference between the terminals in
the discharge tube may be. The distribution of intensity in the
displaced line is very complicated and seems to be affected by
the purity of the gas as well as by the potential difference
between the terminals in the discharge tube. Paschen l was
the first to observe that there are in some cases two maxima
of intensity in the displaced line, and this has been confirmed
by the experiments of Stark and Steubing 2 and of Strasser.3
The distribution of energy determined by Hartmann's micro-
photometer of the Hy line in very pure hydrogen is shown in
Fig. 42, taken from Strasser's paper. The first peak represents
the intensity of the undeflected line, the other two the intensities
of the deflected. Gehrcke and Reichenheim 4 have suggested
that the atom and the molecule of hydrogen give out the same
line spectrum, and that the most deflected maximum is due to
the atoms, the other to the molecules. If the atom and the
molecule acquired the same kinetic energy by falling through
the potential difference between the terminals of the discharge
tube, the velocity of the atom would be ^2 times that of the
molecule, and Gehrcke and Reichenheim found in the plates
that came under their observation that the - ratio of the dis-
placements of the two maxima was approximately equal to
*J2. This, however, does not seem by any means always to
be the case, as the following table, taken from a paper by
Stark,5 of the results obtained by different observers,
shows.

1 "Ann. der Phys.," 23, p. 247, 1907.

2 Ibid., 28, 978.

3 Ibid., 31, 890, 1910.

4 "Verb. d. Deutsch. Phys. Ges.," 12, p. 414, 1910.
6 Ibid., 12, p. 711, 1910.

DOPPLER EFFECT 153

RATIO OF DISPLACEMENTS OF THE Two MAXIMA.

Observer.

175 Stark and Steubing

1-65 Paschen

1-58 Paschen

1*50 Stark and Steubing

1*63 Paschen

i -45 Strasser

1-40 Strasser

1-37 Stark and Steubing.

The photographs taken of the positive rays under electric
and magnetic forces show also that in certain cases the velo-
cities of the particles are grouped round certain values, for we
find that some of the parabolas have a very decided beaded
appearance : each bead corresponds to a group of particles
moving with pretty nearly the same velocity. An example
of this is shown in Fig. 3, Plate I. The intensity curve
corresponding to the Doppler effect ought to have the same
type of variations in intensity as these parabolas, and a
beaded parabola ought to give rise to a Doppler curve with
as many maxima as there are beads on the parabola.
Sometimes these beads on the parabolas are quite numerous.

It is remarkable that the parabola corresponding to the
atom of hydrogen is often beaded in such a way that the
velocity of the particles producing one bead is to that pro-
ducing the other as *J2 : i. Thus to explain the maxima in
the Doppler curve with displacements in this proportion it is
not necessary to assume that the molecules give out the same
spectrum as the atom. The occurrence of singly charged
atoms of hydrogen with velocities in this proportion of^/2 to
i might be accounted for in some such way as the following :
the atoms with the larger velocity have been charged atoms

154 RAYS OF POSITIVE ELECTRICITY

during the whole of their career ; they were atoms before they
passed through the cathode and continue in this state after
emerging from it ; the atoms with the smaller velocity were
part of a charged molecule before passing through the cathode ;
the molecule would only acquire a velocity i/x/2 that of
the atom. After passing through the cathode and before
being deflected by the electric and magnetic fields this charged
molecule breaks up into two atoms, one with a positive charge
while the other is uncharged.

The Doppler effect we have been considering is that shown
by the "series spectrum" of hydrogen. In addition to this
spectrum, hydrogen gives a second spectrum containing a great
number of lines, and this spectrum is developed, though
not so brightly as the series spectrum, when positive rays pass
through hydrogen. Stark 1 has shown, and his results have
been confirmed by Wilsar,2 that the lines in the second spectrum
of hydrogen do not show the Doppler effect with the positive
rays. We infer from this that the second spectrum of hydrogen
is not due to any of the constituents of the positive rays,
which were present in Stark's or Wilsar's experiments. We
shall return to the question of the origin of the second spectrum
later on.

Another illustration is the case of oxygen. Oxygen gives
a series spectrum, a spark spectrum which has not been
resolved into series, and some banded spectra. All these
spectra are emitted when oxygen positive rays pass through
oxygen, the spark spectrum being the brightest. With oxygen
it is the spark lines that show the Doppler effect. Wilsar
and Paschen could not detect any such effect with the series
lines. Stark, however, who used very large dispersions, found
the effect in some of the lines ; the intensity of the displaced

1 Stark, "Ann. der Phys.," 21, p. 425, 1906.
8 "Ann. der Phys.," 39, p. 1251, 1912.

DOPPLER EFFECT 155

lines was, however, very small compared with that of the
undisplaced lines, while in the spark lines the displaced
intensity1 is quite comparable with the normal intensity.

Nitrogen has a line spectrum which has not been resolved
into series, and some banded spectra. The line spectrum and one
of the banded spectra are found where nitrogen positive rays
go through nitrogen ; the banded spectrum does not show the
Doppler effect. Some of the lines in the line spectrum show
it very distinctly, while it is quite absent from others (Her-
man, Wilsar). A very interesting point about the effect in
nitrogen is that even for those lines which show the effect the
value of zM/A is not constant. Wilsar 2 gives the following
table for the Doppler effect for some of the nitrogen lines : —

Wave Length. A A/A.

5002*9 1 1 -4

4643-4 10-35

4630*9 10-14

4530*3 10-60

3995-2 6-90

Thus the effect for the line 3995*2 is much less than for
any of the others, showing that the velocity of the source of
this line is considerably less than that of the sources of the
others. The different states in which nitrogen occurs in the
positive rays are atoms with two charges, atoms with one
charge, molecules with one charge, and in exceptional cases
atoms with three charges and a tri-atomic molecule with one
charge. If the majority of the lines were given out by the
doubly charged atom, and the line 399 5 '2 by the singly charged
one, we should get relative values of /dA/A, approximately
equal to those in the preceding table.

1 Paschen, " Ann. der Phys.," 23, p. 261, 1907.

2 "Phys. Zeit.," 7, p. 568, 1906.

156 XAYS OF POSITIVE ELECTRICITY

The difference between the spectrum of the gas through
which the rays pass, which does not show the Doppler effect,
and that due to the positive rays themselves which does
show this effect, raises some very interesting and fundamental
questions with regard to the origin of spectra. The point can
perhaps be illustrated most clearly by taking a special case,
that of hydrogen, where, as Stark has shown, the lines of the
second spectrum are found in the spectrum of the gas through
which the rays pass but not in that of the rays themselves.
Since in the positive rays we have both atoms and molecules
of hydrogen changing backwards and forwards between the
charged and the uncharged states we have in the rays all the
forms in which hydrogen exists in the gas through which they
pass, and yet this gas gives the second spectrum while the
positive rays do not. It is however possible and indeed prob-
able that the proportion of molecules to atoms in the positive
rays in Stark's experiments was smaller than in the experi-
ments in which the parabolas due to the positive rays were
photographed. In his experiments the pressure had to be so
high that the path of the positive rays was sufficiently luminous
to allow the spectrum to be photographed, while in my experi-
ments the pressure was so low that the path of the rays was
not appreciably luminous. The proportion of charged mole-
cules to charged atoms in the positive rays increases as the
pressure diminishes ; so that it is to be expected that the
charged atoms in Stark's experiments were more numerous
than the charged molecules, and thus if the second spectrum
were due to the charged molecules it would in the spectrum
of the moving gas be faint compared with that due to the
atoms. The proportion between charged molecules and
charged atoms in the positive rays depends on the pressure
in the discharge tube, the luminosity on the pressure in the
observation chamber, so that if we arrange that the pressure

DOPPLER EFFECT 157

in the discharge tube is low while that in the observation
chamber is high the moving gas should, if this explanation is
correct, show the second spectrum as well as the four-line one.
There is, however, another way in which the second spectrum
might arise. We know that the impact of positive rays against
matter produces streams of slow cathode rays whose energy is
comparable with that due to the fall of the atomic charge
through a potential difference of about 20 volts. Thus the
hydrogen through which the rays pass may be traversed by slow
cathode rays due to the impact of the positive rays against the
hydrogen molecules. Fulcher (" Astrophysical Journ.," 34,
p. 388, 1911) has shown that the second spectrum of hydrogen
is excited readily by slow cathode rays. If the second spectrum
observed in connexion with positive rays arose in this way,
the ratio of the intensity of this spectrum coming from the
positive rays themselves to that coming from the gas through
which these rays are passing would be as the ratio of the
number of particles in the positive rays to the number of
molecules of hydrogen in the gas through which they passed.
As this ratio is exceedingly small, the second spectrum would
arise almost entirely from the gas at rest and so would not
show the Doppler effect.

Passing from the case of hydrogen to that of oxygen or
nitrogen we find in the spectra of those gases lines which do
not show the Doppler effect. Similar considerations to those
given for hydrogen apply here, and again many of the lines
which do not show this effect are those which Fulcher finds
are excited by slow cathode rays. Vegard has, however
(" Ann. der Phys.," 41, p. 625), pointed out that for nitrogen the
lines which do not show the Doppler effect differ materially
from those produced by slow cathode rays, so that the
presumption is that such lines are due to molecules of
nitrogen which may not, at the high pressures required to

158 RAYS OF POSITIVE ELECTRICITY

get sufficient luminosity, form an appreciable portion of the
positive rays.

When there are traces of compound gases in the discharge
tube the spectrum of an element which enters into one of
these compounds may show anomalous Doppler effects. Let
us suppose, for example, that the discharge tube contains
hydrogen mixed with a little hydrochloric acid. Then the
gas which has passed through the cathode may be expected
to contain the following types of hydrogen atoms :

1. Atoms whi-ch were atoms before they passed through
the cathode.

2. Atoms which before passing through the cathode
formed part of a molecule of hydrogen which dissociated into
atoms after passing through the cathode.

3. Atoms which before passing through the cathode
formed part of a molecule of hydrochloric acid which
dissociated into atoms after passing through the cathode.

If the energies of the atom, of the molecule of hydrogen
and of HC1 on reaching the cathode were equal, as we should
expect from the properties of the positive rays, then the
velocities of the three types of hydrogen atoms will be
respectively

Thus an atom which had been dissociated from a molecule
after passing through the cathode would have the same
velocity as the molecule, and the Doppler effects for its
spectrum would be the same as for the spectrum of the mole-
cule, supposing the latter to be capable of giving out a
spectrum. We thus cannot distinguish between molecular
and atomic spectra by the Doppler effect alone. Stark
("Phys. Zeits.," 14, p. 770, 1915) has assigned one of the
series spectra of oxygen to the oxygen molecule. As we have

DOPPLER EFFFCT 159

seen, the argument from the Doppler effect is not conclusive
but the question raises a very interesting point as to what
kind of spectrum is emitted by the molecules which in
many cases form a large part of the positive rays. It is true
that in our experiments on the photography of these rays
the molecules are probably much more numerous than they
are in the experiments which are made on the spectra of the
positive rays, where the pressure is much higher, for the
proportion of molecules to atoms diminishes as the pressure
increases. The molecular spectrum might be brightened by
having the observation chamber at a higher pressure and the
discharge chamber at the same pressure as in the photographic
experiments. It is accepted almost as an axiom in spectro-
scopy that line spectra are due to atoms and band spectra to
molecules, and there is certainly a large volume of evidence
from experiments in support of this view. I do not think
that we could on theoretical grounds exclude the possibility
of a molecule giving a line spectrum. A band spectrum is
usually regarded as being given out by a series of oscillators
which are not all in identical conditions, but exposed to
variations which are spread almost uniformly over a certain
range. Thus, to take a concrete case, we may suppose that,
in consequence of the centrifugal force due to the energy the
molecules possess, the distance between the atoms in a mole-
cule is a function of their energy of rotation. As this energy
is not constant for the various molecules of a gas, but is dis-
tributed according to Maxwell's law of distribution, the dis-
tances between the atoms in a molecule will not be fixed but
will vary continuously within certain limits. If the frequen-
cies of vibration of a molecule depends on the distance
between the atoms, the spectrum given out by the molecules
would be a series of bands whose width would depend on the
extent to which the frequencies are affected by alteration in

i6o RA YS OF POSITIVE ELECTRICITY

the distance between the atoms. If the alteration in frequency
were very small these bands would thin down into lines and
the spectrum would be of the same type as one due to an
atom. The above is merely given as an illustration. I do
not mean to suggest that band spectra arise in this way, in
fact I think it is clear they do not, for if they did, since the
average distance between the atoms in a molecule would be a
function of the temperature, the position of the bands would
vary with the temperature. The fact that in the great majority
of cases there is no evidence of this shows that the kinetic
energy possessed by the molecule has a negligible effect on
the spectrum. But if we can rule out in the consideration of
spectra the effect of the rotational energy of the molecule,
the molecule has as definite a configuration as the atom, and
might be expected to give out as well-defined system of lines.
We cannot, I think, on theoretical considerations, rule out a
line spectrum from a molecule as impossible, whether such
has been observed is another matter. Since by the methods
of positive-ray analysis we can separate the atoms from the
molecules we have the means of separating the two spectra.

G. P. Thomson ("Phil. Mag.," Aug. 1920) has shown that
when the parabola due to the hydrogen molecule in the
positive-ray spectrum is faint compared with that due to the
hydrogen atom, the rays do not show the second spectrum
of hydrogen, and that this spectrum appears when the
parabola due to the molecule is comparable in intensity with
that due to the atom. This points to the hydrogen molecule
as the source of the second spectrum, a conclusion at which
Stark ("Ann. der Physik." 52, p. 221, 1917) had arrived from
the study of the spectrum of the positive column.

There is one type of vibration of an electrical system
which has, I think, not received as much attention as it
deserves in connection with the radiation emitted by luminous

VIBRATIONS OF TUBES OF FORCE 161

gases. The vibrations hitherto considered have been those
of electrical changes, of electrons for visible and ultra-violet
radiations and of positive charges for those in the infra-red.
There is, however, another possible type of vibration which is
not dependent on the motion of electrical charges, but on
the motion of the tubes of force which bind those charges
together.

Suppose, for example, that A and B are two oppositely
charged bodies with their charges held rigidly in a fixed
position. When in equilibrium the lines of force would be
distributed in a definite way which can be deduced from the
laws of Electrostatics. Now let this distribution be suddenly
disturbed by the passage through the field of a very rapidly
moving electric charge. The lines of force will be disturbed
from their equilibrium position where the potential energy is
a minimum, and after the moving charge has passed away
they will vibrate about this position. The possible times of
vibration of such a complex system would probably be very
numerous and would be multiples or sub-multiples of D/c
when D is the distance between A and B and c the velocity
of light. This is on the supposition that there are no bodies
in the neighbourhood of AB which when the electric field is
changing can be set in motion and absorb energy. The wave
length of the vibrations would thus be comparable with D,
the distance between the charges, and if these conditions
applied to atoms and molecules the wave lengths of such
vibrations would be comparable with the diameters of atoms
and molecules, and so would not correspond with visible or
even ultra-violet light. In atoms and molecules, however,
the lines of force do not spread out through an empty field :
the space near the centre of an atom is crowded with electrons
whose free vibrations are exceedingly rapid ; these electrons
will be affected by the lines of force in the vibrating electric
M

1 62 RAYS OF POSITIVE ELECTRICITY

field and will increase the time of its vibrations. In addition
to the electrons near the centre there are others near the
surface whose position relative to the two bodies A and B
may vary from one molecule or atom to the other. These
may be regarded as coupled up with the primary system, the
stiffness of the coupling varying from one molecule or atom
to another. The effect of this coupling on the period of vibra-
tion may be got by the use of the principle that when two
vibrating systems are coupled the quicker vibration of the two
is made quicker and the slower slower. Thus the effect of
the outside electron on the vibrations of the primary system
(including in this the effect produced by the inner electrons)
will be to quicken the vibration (i.e. to shift the corresponding
line to the blue end of the spectrum), if the period of vibra-
tion of the surface electron is longer than that of the primary
system. The amount of the shift will depend upon the firm-
ness of the coupling, and if this varies from atom to atom, or
from molecule to molecule, the spectra corresponding to these
vibrations will be a band with the sharp end at the red end of
the band, this end of the band having the period correspond-
ing to indefinitely small coupling. If the period of the
surface electron is smaller than that of the primary system,
the vibrations of this system will be slowed down by the
coupling and the spectrum would have a band with its sharp
end at the blue end of the band.

The effect of a magnetic field would, on a spectrum pro-
duced in this way, be very much less than on one produced
primarily by the motion of an electron. For though the
magnetic field would modify the time of vibration of the
surface electron and thus affect to a small extent the effect
produced by the coupling, the effect on the vibration of the
whole system would be of the nature of a correction on a
correction, and therefore much smaller than the direct effect

REFLECTION OF POSITIVE RAYS 163

produced on the vibration of the surface electron itself. In
this respect these spectra would behave like " band " spectra,
which are far less susceptible to the action of a magnetic field
than the series spectra. Vibrations of the kind we are con-
sidering might occur in atoms as well as in molecules, their
intensity and character might be expected to depend to a
considerable extent upon whether the atom was charged or
not. The radiation from atoms is not confined to line
spectra, as we know that band spectra are given out even by
a monatomic gas like mercury.

The reflection of hydrogen and, to some extent, of helium
positive rays has been detected by observations of the Doppler
effect (Stark and Steubing, "Ann. der Phys.," 28, p. 995,
1909; Stark, ibid., 42, p. 231, 1913). The reflected rays are
few in number, compared with the incident ones, and their
velocity is very much less. The reflection is more pro-
nounced with slow rays (corresponding to a fall of potential
of 5000 volts or so) than with fast ones. No reflection has
been detected with certainty for positive rays from the heavier
elements, though Stark has looked for it in C, O, Al, S, Cl,
Ar, I, Hg : with the rays from mercury there were indications
of an exceedingly faint reflection, not enough, however, to
establish the result with certainty. It is to be remembered,
however, that the retrograde rays (see p. 134) would give rise
to Doppler effects of the same character as reflected positive
rays. In discussing this subject it is necessary to have clear
ideas as to the meaning we attach to " reflection " : if by
reflection of a positive ray we mean that a ray has rebounded
without any transformation of its electrical condition, it
cannot, I think, be maintained that reflection has been
established.

The lines in the spectrum given out by the positive rays
which show abnormally large Doppler effects, and which are

164 RA YS OF POSITIVE ELECTRICITY

due to atoms with multiple charges, are found in the spectrum
of the stationary gas through which the rays are passing as
well as in that of the positive rays themselves. This is im-
portant in connection with the origin of atoms of this type, as
since no fast cathode rays are passing through this gas, such
rays can not be essential for the production of multiple charges.
Thus positive rays must be able to produce them. The
quicker the rays the more capable they seem of producing
doubly instead of singly charged atoms. Thus in oxygen
Stark ("Ann. der Phys.," 42, p. 163, 1913) found that with
3OOO-volt rays the spectrum due to the singly charged atoms
was vastly brighter than that due to the doubly charged ones,
the latter spectrum increased markedly in intensity when the
voltage- was raised to 7500 volts, while with I5,ooo-volt rays
it was brighter than that due to the singly charged atoms.
Stark has by this method detected atoms of aluminium with
i, 2 and 3 charges, and atoms of mercury with i, 2, 3 and 4.
We saw on p. 80 that the mercury atom occurs in the positive
rays with as many as seven charges.

It would be interesting to compare the ratio of intensities
of the lines in the spectrum of the positive rays due to
the singly and doubly charged atoms respectively with the
same ratio in the spectrum of the gas through which the rays
are passing. Vegard ("Ann. der Phys.," 39, p. in, 1912;
42, p. 625, 1913) has measured the relative intensities in
the two spectra of some hydrogen, oxygen and nitrogen lines,
but his measurements do not include lines due to doubly
charged atoms, there are, of course, none of these in the
hydrogen spectrum ; he finds that there are great variations
in the relative intensities of the lines which show the Doppler
effect and those which do not, when the pressure of the gas or
the potential difference is altered. This is what we should
expect, as we know that the proportion between the different

ORIGIN OF SPECTRA 165

types of particles in the positive rays — atoms, molecules, singly
and doubly charged atoms — is also dependent upon the pres-
sure and the potential difference. Experiments on the spectra
produced by rays whose composition has been determined by
electric and magnetic deflection would probably lead to much
more definite knowledge of the sources of the various lines in
the spectrum.

POLARIZATION OF THE LIGHT FROM
POSITIVE RAYS

Stark and Lunelund ("Ann. der Phys.," 46, p. 68, 1914)
have shown that the light given out by positive hydro-
gen rays is partially polarized. The light which has the
electric force in the direction of motion of the rays being
more intense than the light with the electric force at right
angles to this direction. No polarization was detected in the
light which does not show the Doppler effect and which
comes from the gas through which the positive rays are
passing.

Since close to a positive ray particle there is a strong
magnetic field due to the motion of the electric charge, an
electron returning to the particle will be deflected, and thus
the line joining the centre of the atom to the captured electron
will be more likely to make one angle rather than another with
the direction of motion. The directions of those lines will not
be uniformly distributed, and as the direction of the electric
force emitted by the vibrating electron will depend upon the
direction of the line, we should expect the light emitted by
these moving particles to show polarization.

Stark's experiments have shown that the source of the
series lines is one of the constituents of the positive rays : the
question is, which constituent ? We have seen that in hydrogen,

166 RA YS OF POSITIVE ELECTRICITY

for example, we have positively and negatively charged atoms,
as well as neutral ones : we have also positively charged and
neutral molecules. There is considerable difference of opinion
as to which of these is responsible for the series lines in the
hydrogen spectrum. All theories concur in regarding the
atom and not the molecule as the source of these lines, but
according to Wien's theory the atom radiates when in the
neutral state, while Stark maintains that the radiation is emitted
when the atom has a positive charge : according to his view,
the lines emitted by the neutral atom are far away in the
ultra-violet.

The pressures at which spectroscopic observations have
been made are so high that an atom is continually passing
backwards and forwards between the neutral and charged con-
ditions. It is thus a matter of great difficulty to determine
whether the atom emits the lines in one state or the other,
and there is, I think, at present no experiment which is abso-
lutely decisive between the two views. Thus, for example, it
is found that the Doppler effect is increased when the positive
rays are exposed to an accelerating potential after passing
through the cathode. This, however, does not prove that the
particles are charged when giving out the light, for the particles
which are uncharged at one time have at other times a positive
charge and so would be accelerated.

Perhaps the strongest argument in favour of the radiating
particles being positively charged is that in certain cases, as
Reichenheim has shown, the anode rays (see p. 142) show the
Doppler effect, but even this is not conclusive, as some of the
positively charged particles might have been neutralized after
they had acquired their high velocity under the electric field.

There is another view as to the origin of the radiation
which explains in a simple way some of the characteristic
properties of the Doppler effect : this is that the light is

ORIGIN OF SPECTRA 167

given out by particles which have just been neutralized by
union with an electron. The electron falls into the positively
charged atom and the energy gained by the fall is radiated
away as light. On this view the intensity of the light should
vary with the number of recombinations of positive ions and
electrons. Let n be at any instant the number of neutral
particles per unit volume moving with velocity v, p the
number of positive particles moving with the same velocity,
N the number of electrons per unit volume, whether free or
in the atoms which are in the track of these particles.

Then the number of recombinations per second will be

when/(^) is a function of v which will vanish when v is very
large, for recombination will not take place if the relative
velocity of the positive particle and the electron exceeds a
certain value.

The number of neutral particles ionized per second will be

*NF(v)

where F(z^) is a function of v which vanishes when v is very
small, for if the particle is to be ionized by a collision the
relative velocity of the particle and electron must exceed a
critical value.

When the composition of the beam of positive rays has
become steady the number of ionizations must equal the
number of recombinations, hence

pW(v) = »NF(z;)
(p + *)N/fo)Ffo)

and therefore W + F(V)

Since/^z/)=o when z;=infinity and F(z/)=o when v=o,/(v)F(v)
will have a maximum for a certain value of v which will not
however depend on the potential difference between the
electrodes in the discharge tube. The factor/ + nt the total
number of positive rays charged or neutral whose velocity

i68 RA YS OF POSITIVE ELECTRICITY

isz^, will also be a function of v, and this function will depend
upon the value of E, the potential difference between the
electrodes in the discharge tube, for evidently if E increases,
the value of v for which/ + n is a maximum will increase too.
On the view we are considering, the intensity of the light
showing a Doppler effect corresponding to the value v will be
proportional to the number of recombinations of positive ions
moving with this velocity with electrons. It will thus be
proportional to /N/(z/) which we have seen is equal to

)

The second factor in this expression

f(v) + F(w)

has its maximum value for a value of v which does not depend
upon the potential difference : the other factor (/ + n) does
depend upon this potential difference. Thus the value of v
for which the product of these factors is a maximum will
depend to some extent on E, but since the value of v which
makes one of the factors a maximum is quite independent of
E we should expect that a variation in E would have less
effect on this velocity than on the average velocity of the
particles in the positive rays.

Again since F(z>) vanishes when v is less than a certain
value v0 there will be no light showing a Doppler effect corre-
sponding to a velocity less than v0t thus there will be a dark
space between the original line and the displaced lines. This
also is in accordance with the observations. Since f(v)
vanishes when v is greater than a certain value vt there will
be no Doppler effect showing a greater displacement than
that corresponding to z/. Though this has not perhaps been
absolutely proved there are indications that the Doppler

ORIGIN OF SPECTRA 169

effect cannot be increased beyond a certain definite value,
however large the potential applied to the discharge tube
may be.

When positive rays produced in a gas A pass through a
gas B the spectra of both A and B are given out : Wilsar,
"Phys. Zeitschr.," 12, p. 1091, and Fulcher (ibid. 13, p. 224),
have shown that all the lines of A are displaced while all
those of B are in their normal position. A bibliography of
the Doppler effect in the Positive Rays has been published
by Fulcher, " Jahrb. d. Radioaktivitat," X, p. 82, 1913.

SPECTRA PRODUCED BY BOMBARDMENT WITH
POSITIVE RAYS

The spectra produced when the positive rays strike salts
of the alkali metals are very interesting. The salts give out
the lines of the alkali ; for example, Li Cl give out the red
lithium line and sodium salts the D line. It is remarkable
that the lines due to the metal are more easily excited in the
salts than in the metal itself. Thus if the liquid alloy of
sodium and potassium is bombarded by positive rays the
specks of oxide on the surface glow brightly with the sodium
light while the clean surface remains quite dark. Some
observers have noticed what seems a similar effect with
hydrogen, viz. that the hydrogen lines are more easily excited
in water vapour than in pure hydrogen. The fact that in
the positive ray photographs, the parabolas corresponding to
a certain type of ray, for example the carbon or oxygen atom
with two charges, is more easily developed from compounds
than from the molecules of the gases themselves, is probably
connected with this effect.

The production of spectra by bombardment with cathode
rays has been investigated by Gyllenskold ("Ark. f. Math.

170 RA YS OF POSITIVE ELECTRICITY

Ast. oet. Fys.," 4, No. 33, 1908), and by Stark and Wendt
(" Ann. der Phys.," 38, p. 669, 1912), who have shown that
the colourless salts of the alkalies and alkaline earths and
also of thallium, zinc, and aluminium give out the series lines
of the metal when struck by the positive rays, and that the
lines given out do not depend upon the character of the
salts. According to Stark and Wendt the seat of the emission
is not the surface of the salt itself but a layer of gas, less
than i mm. thick, close to the surface. This is what might
have been expected, for to get a line spectrum we must have
the substance in the gaseous state. This layer is analogous
to the velvety glow which covers the surface of the cathode
where an electric discharge passes through a gas at a low
pressure.

To develop the spectrum of the metal the positive rays
must have more than a certain critical amount of energy
depending on the nature of the salt. The values of V, this
critical energy, measured by the number of volts through
which the atomic charge must fall to acquire it, have been
measured by Stark and Wendt and are given in the following
table :—

Metal.

Salt.

Light given out.

Lithium

chloride

red

600

Lithium

oxide

red

600

Lithium

oxide

A 671

<8oo

Sodium

chloride

yellow light

750

Potassium

chloride and oxide

A 580

<2400

Rubidium

sub-oxide

* 572

<35oo

Caesium

chloride

A 566

<45oo

Magnesium

chloride

A 5l8

<I200

Calcium

fluoride

red violet light

1500

carbonate

red violet light

1500

sulphate

red violet light

1500

oxide

red violet light

1400

Strontium

chloride

A 496

<25<X>

Barium

chloride

* 554-493

<2500

Thallium

sulphate

* 535

4500

Aluminium

oxide

A 396

<45oo

Zinc

oxide

^ 475

<46oo

DISINTEGRATION OF POSITIVE RAYS 171

It must not be supposed that the amounts of energy
given in the last column represent the minimum amount
required to excite the particular kind of light given in the
third column. When energy has to be transferred from a
charged atom to an electron, the latter only receives a very
small fraction of the energy of the atom, thus a very small
fraction of the energy of the positive rays may be transformed
into a kind available for light production.

Gyllenskold observed that in addition to the D lines
sodium chloride gives out a series of bands in the blue, and
Stark and Wendt have shown that for this to occur the
energy of the rays must exceed a critical value which in
most cases is less than that required to excite the line
spectrum.

Ohlon ("Verh. Deutsch. Phys. Gesell.," 20, p. 9, 1918)
found that if the salt was placed in a metal vessel connected
with earth through a galvanometer the positive current
through the galvanometer due to the impact of the positive
rays diminished abruptly when the potential reached the
value at which the line spectrum was emitted; the most
obvious explanation of this is that the conductivity of the
gas round the vessel is suddenly increased.

As the salt has to be vaporized before it can emit the
line spectrum the excitation of these spectra by positive rays
is closely connected with that of "electrical evaporation,"
which is considered in the next paragraph.

DISINTEGRATION OF METALS UNDER THE
ACTION OF POSITIVE RAYS

When positive rays strike against a metallic surface, the
metal disintegrates and forms a deposit on the walls of the
tube surrounding the metal. A well-known instance of

172 RA YS OF POSITIVE ELECTRICITY

this is the " spluttering " of the cathode in a vacuum tube ;
another is observed when working with an apparatus like
that shown in Fig. 14 ; after long use the thin metal tube
which passes through the cathode gets worn away at the end
nearest the discharge tube, as if it had been struck by a
sand blast. Sometimes several millimetres of the tube are
destroyed in this way. An excellent account of the. very
numerous experiments which have been made on the splutter-
ing of the cathode will be found in a report by Kohlschiitter
(" Jahrbuch der Radioaktivitat," July 1912).

The spluttering due to the impact of positive rays is not
confined to metals: Stark and Wendt ("Ann. der Phys.," 38,
p. 921, 1912) found that it occurred in quartz, rock salt, glass
and mica. In all these substances, with the exception of
quartz, long exposure to the positive rays produces a kind
of blistering on the surface which seems to be due to the
positive rays penetrating a finite distance into the substance
and remaining there. This effect was not shown by the
positive rays of the heavy elements such as mercury. The
penetration seems connected with the " hardness " of the sur-
faces struck. Goldsmith (" Phcenix, Phys. Lab. Contrib.,"
No. 26, 1911) found that the positive rays of hydrogen and
helium could pass through plates of mica "002 — '006 mm.
thick.

Rausch v. Traubenberg (" Gottingen Math. Physik.," p. 272,
1914) separated the positive rays by electric and magnetic
fields in the usual way and observed the fluorescence they
produced on a fluorescent screen coated with gold leaf. He
found that the hydrogen atom, the hydrogen molecule, ana
either the oxygen or nitrogen atom or both (the resolution
was not sufficient to separate these lines) penetrated the gold
leaf and produced fluorescence on the screen. The rays lost
their electric charges while passing through the gold leaf.

'SPLUTTERING* OF THE CATHODE 173

The thickness of gold leaf through which the fluorescence
due to the hydrogen atom could be observed was proportional
to the velocity of the atom and was $6'6 x io~6 cm. when
the velocity was 2'6 x io8 cm./sec.

The experiments of Holborn and Austin, Granquist, and
Kohlschutter indicate that with a constant current, w (the loss
of weight in a given time) may be represented by a formula of
the type

w = a - (V - S)

where V is the cathode fall of potential, A the atomic weight
of the metal, n a small positive integer, and a and S quanti-
ties which are much the same for all metals, or at any rate
the metals can be divided into large classes and a and S are
the same for all the metals in one class. For a current of '6
milliamperes, Holborn and Austin found that for all the
metals they tried S was 495 volts. We see that a formula of
this type implies that there is no appreciable spluttering
unless the cathode fall of potential exceeds a definite value S,
and this seems to be verified by experience.

The experiments of Holborn and Austin, Kohlschutter
and others have shown that this expression for the loss of
weight of the cathode fails when V exceeds a certain value ;
for hydrogen this value seems to be so low that the expression
fails before the loss of weight becomes measurable.

The loss of weights of the six metals Al, Fe, Cu, Ft, Ag
Au have been measured by Kohlschutter and Muller (" Zeit.
schr. f. Elektroch.," 12, 365, 1906) and Kohlschutter and
Goldschmidt (ibid. 14, 221, 1908) in the gases H2,* He, N2> O2
and Arg, under as nearly as possible identical electrical con-
ditions. They found that for all gases the amount of weight
lost was in the order in which the metals are written above,
gold always losing the greatest amount and aluminium the

J74 RAYS OF POSITIVE ELECTRICITY

least. For the same metal in different gases the loss of
weight followed the order of the atomic weight of the gases,
the loss in hydrogen being least and that in argon greatest.
This may be connected with the fact that (see p. 82) elements
of high atomic weight acquire multiple charges of electricity
more easily than the lighter elements, and atoms with a
multiple charge have more energy when they strike against
the cathode than those which have only one charge. The
form of the equation for w shows that if instead of con-
sidering the loss of weight we consider the number of atoms
lost by the cathode, the numbers should be in simple pro-
portions for the different metals. This seems to be confirmed
by some experiments of Kohlschiitter on the loss of weight
of Ag, Au, Pt, Pd, Cu, and Ni cathodes in nitrogen for the
same current and cathode fall. He found that for each atom
of Ag detached from a silver cathode one-half an atom of
Au, Cu, Pd, one-third of an atom of Pt, and one one-fourth
of an atom of Ni were detached from cathodes of these
metals. The proportion was, however, not the same in argon
as in nitrogen.

Kohlschiitter and his collaborators (" Zeitschr. f. Elek-
troch.," 12, p. 365, 1906; 14, p. 221, 1908) have compared
the loss of weight of a silver cathode due to spluttering with
the weight of silver deposited in a silver voltameter placed in
series with the discharge tubes. The results are shown in
the following table.

Silver deposited
in voltameter.

2'4
2-05

i '45

Gas in discharge

Loss of weight

tube.

by spluttering.

Hydrogen

•27

Helium

•4

Nitrogen

2-05

Oxygen

Argon

5-2

'SPLUTTERING* OF THE CATHODE 175

To answer the question how many atoms of silver are
detached from the cathode when one positive ray strikes
against it, we should require to know the proportion of
current carried by the positive rays and the cathode rays
respectively. We do not know this, but we have every
reason to believe that the greater part of the current is
carried by the cathode rays ; if this is so, the table shows
that every positive ray which strikes the cathode must on
the average detach a large number of silver atoms from the
cathode. Whatever the proportion between the currents
carried by the positive and negative carriers may be, the
table shows that, at any rate in oxygen and argon, each
positive ray detaches more than one atom of silver. It
may be that the mechanism by which the metal is torn
from the cathode is such that the atoms of the metal are not
liberated separately, but in groups. Thus, to take a purely
mechanical view of the process we may suppose that the
atom struck by a positive ray is driven further into the metal,
that its displacement forces outwards the atoms in a ring
surrounding it, and that these atoms acquire a considerable
part of the energy in the positive ray. On this view the
process would be analogous to that which occurs when a
marble falls upon the surface of water, a crater is formed under
the marble but the rim of the crater moves upwards and
escapes, if the marble has fallen from a considerable height,
from the surface of the liquid in drops. So in the case of
the cathode it may be that it is not the atom struck by the
positive ray which is torn from the metal, but a ring of atoms
in its neighbourhood. It would follow from this that to
disintegrate the cathode the positive ray must possess energy
sufficient to tear away not merely one atom of the metal of
the cathode, but the large number in the ring. The experi-
ments alluded to above have shown that the disintegration of

1 76 RAYS OF POSITIVE ELECTRICITY

the cathode is not apparent unless the energy of the positive
ray is that due to the fall of the atomic charge through a
potential difference of about 500 volts. We can calculate
from the latent heat of evaporation the energy required to
separate one atom from the surface of a metal, and this
comes out less than that due to the fall of the atomic charge
through "7 volts, a minute fraction of the 500 volts necessary
to produce disintegration by positive rays.

There is, I think, something to be said for the view that
the disintegration is effected by radiation produced by the
impact of the positive rays, rather than by an atom acquiring
a high velocity through being struck by a positive ray. I
have shown ("Phil. Mag.," 6, 28, p. 620, 1914) that radiation
analogous to Rontgen radiation of an exceedingly soft type
is produced where positive rays strike against a solid target
so that radiation of the requisite type is available. Now
Lenard and Wolf ("Wied. Ann.," 37, p. 443, 1889); R. v.
Helmholtz and F. Richarz ("Wied. Ann.," 40, p. 187, 1890);
Stark (" Phys. Zeit.,"9, p. 894, 1908) ; Rubens and Ladenburg
("Ber. d. Deutsch. Phys. Ges.," 9, p. 749, 1907) have shown
that disintegration of metals occurs when ultra-violet light
falls upon them.

We should expect that the wave lengths of the vibrations
started by the impact of positive rays would be very much
longer than those started by cathode rays possessing the same
amount of energy. For if these vibrations arise from the motion
of the electric charges carried by the rays, their frequency
will depend on the time the movements of these charges are
affected by an electric field through which they are passing.
This time may be taken as inversely proportional to the
velocity of the moving charges ; thus, from this point of view,
the frequencies of vibrations excited by positive and cathode
rays moving with the same velocity, might be expected to be

DISINTEGRATION OF CATHODE 177

of the same order. If we take this as a rough guide, then
the frequencies of the vibrations excited by 5<DO-volt positive
rays in oxygen would be comparable with those excited by

-?•— volt cathode rays. The wave length of the latter

ID 1700

would, if calculated by Planck's rule, be about 64 X io"4 cm.
and would correspond to the infra-red part of the spectrum.
If the molecules of the metal are arranged in regular order
along a series of space lattices they would have definite times
of vibrations, which might well, from what we know about
absorption, correspond to the infra-red part of the spectrum.
If any of these times of vibration corresponded with that of
the radiation excited by the positive rays, the molecules might
by resonance absorb sufficient energy to be able to escape
from the metal. From this point of view there would be a
lower and a higher limit to the energy of the positive rays
which give 'rise to disintegration : a lower limit where the
vibrations excited would be slower than those within the
compass of the molecules of the metal ; and a higher limit
where the vibrations would be too quick to find a response in
the metal.

KohlschUtter ("Zeitschr. Elektroch.," 18, p. 837, 1912)
considers that the reason why a finite potential fall is required
to effect the disintegration of the electrode, is that the vapour
of the metal escaping from the cathode condenses into dust
and would, unless it possessed more than a certain amount
of energy, be dragged back into the cathode by the electrical
forces in the neighbourhood of the cathode, and in this way
disintegration would be prevented. It must be remembered,
however, that though an increased cathode fall would increase
the energy of the escaping vapour, it would also increase the
forces tending to bring the metallic dust back to the cathode.

1 78 RAYS OF POSITIVE ELECTRICITY

ABSORPTION OF GASES IN THE
DISCHARGE TUBE

The absorption of gases in discharge tubes may arise
from many different causes : thus, for example, in some cases,
notably those where incandescent filaments are used for
cathodes, it is due to chemical action between the metal of
the electrode and the gas. There is, however, a type of
absorption which persists even after the tube has been used
for a long time, and which has been called by Vegard, who
has investigated it, "conservative absorption" which shows
considerable analogy with the disintegration of the cathode.
Vegard ("Phil. Mag.," 6, 18, p. 465, 1909; "Ann. der. Phys.,"
50, p. 769, 1916) finds that this absorption, like disintegration,
does not occur unless the potential fall exceeds a certain
value, about 400 volts for platinum and 320 for gold. These
voltages are about the normal cathode fall for these metals,
so that this kind of absorption only begins when the current
through the tube is large enough to make the cathode fall
abnormal. The order of absorption for different metals seems
roughly, at any rate, to be much the same as that for dis-
integration. This would be the case if the absorption were
produced by the disintegrated metal, which is in a very fine
state of division, and therefore provides a large surface for
absorption.

Dechend and Hammer ("Zeitschr. f. Elektroch.," 17, 235)
allowed the positive rays produced in sulphuretted hydrogen
to pass through a perforated cathode and after deflection by
magnetic and electric fields to fall upon a plate of polished
silver. They could detect the parabolas on the plate, but while
the parabolas due to hydrogen were so faint that they could
only be detected as breath figures, those due to the heavier
atoms, presumably sulphur, had so affected the plate that they

CHEMICAL ANALYSIS BY POSITIVE RAYS 179

could not be removed either by acid or rubbing. The greatest
effect, however, was produced by the undeviated rays. In
addition to the effects produced when the positive rays strike
against a metal plate there is, as Schmidt has shown, a
general oxidation over the surface when the metal is oxidizable
and when the gas surrounding it contains oxygen. The
passage of the positive rays through the oxygen produces
atomic oxygen which is very active chemically and which
attacks the plate. If, on the other hand, an oxidized plate
is placed in hydrogen and exposed to the action of positive
rays the oxide is reduced, the rays produce atomic hydrogen
which acts as a strong reducing agent.

Some of the atoms constituting the positive rays seem to
enter a metal against which they strike, and either combine
with the metal or get absorbed by it. Helium, neon, and
mercury vapour seem especially noticeable in this respect. If
a cathode has once been used for any of these gases, positive
rays corresponding to these elements will be found when the
cathode is used with other gases, and it requires long-continued
discharge and repeated fillings with other gases before they
are eliminated.

A very valuable Bibliography of Researches on Positive
Ra^s'has been published by Fulcher (Smithsonian Miscellane-
ous Collection, 5, p. 295, 1909).

ON THE USE OF THE POSITIVE RAYS FOR
CHEMICAL ANALYSIS

i. We shall now proceed to show how the method of
positive rays supplies us with a very powerful method of
chemical analysis, and how from the study of the positive-
ray photographs we are able to determine the different kinds
of atoms and molecules in the discharge tube. Each kind of

i8o RAYS OF POSITIVE ELECTRICITY

atom or molecule in the discharge tube produces a separate
parabola on the photographic plate, and if we measure these
parabolas then by means of the formula (p. 21)

e
m

we can determine the value of ejm for the particles producing
any parabolas. We know, too, that the charge e is either
the ionic charge whose value on the electrostatic system of
units is 4-8 X icr10, or some multiple of it. We have, too, as
we shall see, the means of determining what this multiple is.
As we can determine the value of e, and since we know by the
measurement of the parabola the value of e\m> we can deduce
the value of m and thus determine the masses of the particles
forming the positive rays. As these particles are the atoms and
molecules of the gases in the discharge tube it is evident that in
this way we can determine the atomic or molecular weight of
the gases in the positive rays. We can thus identify these gases
as far as can be done by the knowledge of their atomic weight.
The study of the photographs gives us in fact the atomic
weights of the various gases in the tube, and thus enables
us to determine the nature of its contents. We can thus
analyse a gas by putting a small quantity of it into a
discharge tube and taking a photograph of the positive rays.
This method of analysis has many advantages. In the first
place, as the pressure is very low, only a very small quantity
of gas is required ; the total amount of gas in the discharge
tube of the size used in my experiments would only occupy
about *oi c.c. at atmospheric pressure, and a constituent
present to the extent of only a very small percentage would
give well-defined parabolas. If there is a new gas in the
tube it is indicated by the presence of a new parabola, but
this parabola does far more than show that something new

CHEMICAL ANALYSIS BY POSITIVE RAYS 181

is present, it tells us what is the atomic weight of the new
constituent. Let us compare for a moment the method with
that of spectrum analysis. We might detect a new gas by
observing an unknown line in the spectrum when the electric
discharge passed through the gas. This observation would,
however, tell us nothing about the nature of the substance
giving the new line, nor, indeed, whether it arose from a
new substance at all : it might be a line given out by a well-
known substance under new electrical conditions. Again,
if a substance is only present to the extent of a few per
cent, it very often happens that its spectrum is completely
swamped by that of the more abundant substance : thus, for
example, in a mixture of helium and hydrogen we cannot
observe the helium lines unless the helium is a considerable
percentage of the mixture.

This is not the case with the positive rays, or at any rate
not to anything like the same extent ; the presence of one per
cent, of helium would be very easily detected by the positive
rays. The method, too, is more sensitive than that of
spectrum analysis. With the apparatus described above the
helium in i c.c. of air, i.e. about 3 X io~6c.c., could be
detected with great ease even when it formed only about one
per cent, of the mixed gases in the tube. No special attention
was paid to making this particular apparatus specially
sensitive. To get the best results, the size of the tube running
through the cathode has to be chosen with reference to the
distance of the photographic plate from the cathode, and
other circumstances ; this was not done in the apparatus
under discussion, nor were the photographic plates used of
any special sensitiveness ; by attention to these points the
sensitiveness of the method could be increased very materially.

Again, the method of the positive rays enables us when we
have found the substance to say whether the molecule is

1 82 RAYS OF POSITIVE ELECTRICITY

monatomic or diatomic ; if it is diatomic we shall have two
new parabolas, one due to the atom and the other to the
molecule ; if the molecule is monatomic there will be only
one parabola unless the particle acquires a double charge :
the presence of this extra charge can be recognized by the
tests previously described. The method of the positive rays
has the advantage of revealing the presence of the molecules
of compound gases as well as the atoms and molecules of
elementary substances. Since different compounds may have
the same molecular weight there is sometimes ambiguity in
interpreting the photographs produced by the positive rays ;
for example, CO2 and N2O produce the same parabolas as
also do CO and N2. In such cases to find out the origin of
such a parabola we must repeat the experiment under different
conditions ; for example, if we put something in the tube
which absorbs CO2 and not N2O, and find that the parabola
disappears, we conclude that it was due to CO2 ; if it does
not disappear it is not due to CO2, but to N2O or some other
compound with the same molecular weight.

The ambiguity as to whether a line with a value of mje
equal say to 8 (mje for the hydrogen atom being taken as
unity) is to be ascribed to an atom with atomic weight 8
carrying a single charge, or to one with an atomic weight 16
carrying two charges, or to one with atomic weight 24 with
three charges may be removed by the considerations given on
page 77. For example, if the particle producing this parabola
A carries a double charge there will be another and more
intense parabola B, for which the value of mfe is twice that for
A ; and the parabola B will have a prolongation towards the
vertical axis, the distance of the head of this prolongation from
the vertical axis being half the distance of the heads of the
normal parabolas. If A represents a particle with a threefold
charge there will be another stronger parabola B for which

CHEMICAL ANALYSIS BY POSITIVE RAYS 183

mje has three times the value corresponding to the parabola
A, and B will have a prolongation towards the vertical
axis extending to one-third of the normal distance.

For the purposes of Chemical Analysis it is not neces-
sary to use the elaborate apparatus with appliances for
measuring the electric and magnetic field. The more
elaborate apparatus is only required when we - require to
know accurately the values of the quantities A and B which
occur in the expression for e\m.

For the determination of the masses of the particles pro-
ducing the different parabolas the measurement of the quan-
tities A and B is unnecessary if we can recognize the particle
which produces any particular parabola. For since A and B
are the same for all the parabolas, then for any two parabolas
we have by the equation on page 180 —

(ejm\ _y?lx±

where (e\m\ (ejm}^ are the values of e\mt for the particles pro-
ducing the parabolas (i) and (2) respectively, (x-^y-^ (#2/2) are
the co-ordinates of any point on the first and second parabolas
respectively.

If the points on the two parabolas have the same values of

x so that xl = X<L then

(e\m\ _y*
(e/m)2 y£

if the charges are the same

As the line corresponding to the atom of hydrogen occurs
on all the plates and can at once be recognized by being the
most deflected line on the plate, the value of (e/m) for the
particles producing any parabola can be at once, by the aid of

1 84 RAYS OF POSITIVE ELECTRICITY

this formula, compared with the value of this quantity for an
atom of hydrogen and the masses of the various particles
thereby determined.

A convenient instrument for making the necessary
measurements is shown in Fig. 14. The plate is inserted in
the holder A. The camera is arranged so that the direction
in which the rays are deflected by the magnetic force alone
(the vertical axis in the preceding figures) is parallel to the
longer side of the photographic plate. The deflection due to
the electrostatic field is at right angles to this and parallel to
the shorter side of the plate. The plate is placed in the holder
so that the. axis of no electrostatic deflection is parallel to,
and that of no magnetic deflection perpendicular to, BB. A
needle NN whose point comes close to the plate is placed in
the carrier C which can move parallel to BB by sliding along
BB, and perpendicular to it by means of the screw S ; the
position of the carrier is read by two verniers, Vj and V2.
There is always a circular patch at the place where the
undeflected particles hit the plate : the zero is at the centre
of the spot. By putting the needle first at the centre of
the spot, then moving the carrier through a certain distance
perpendicular to BB by the screw S, and sliding the carrier
parallel to BB until the needle comes on the parabolas in
turn, the values of y for the different parabolas corresponding
to a constant value of x can be measured.

The equation on page 183 enables us to find the ratio of
the masses of the particles producing the different parabolas.
We can avoid any uncertainty as to the position of the zero
by taking two photographs, the electrostatic field remaining
the same in the two, while the magnetic field in the first
photograph is equal in magnitude but opposite in direction to
that in the second. Thus each kind of particle will now give
two parabolic arcs, as in Fig. 3, Plate 2, and the distance

CHEMICAL ANALYSIS BY POSITIVE RAYS 185

between two points AB situated on the same vertical line will
be twice the vertical deflection due to either magnetic field.
As these arcs are much finer than the central spot, the
distance AB can be measured with greater accuracy than
either deflection separately.

If, as is very often the case, we can recognize two parabolas
as due to atoms, or molecules of known atomic weight, we can
eliminate any uncertainty arising from the position of the
zero without reversing the magnetic field. For if T1? T2, T3
are the vertical displacements corresponding to particles with
charge e and masses mlt m^ m$

- • %-Vsj

2and -

1 3

hence

i , T^T! f i 1 1

= — =- + 3 * ]--= — -= \
m3 *Jini -1 2~ x i W;;z2 v mi*

Since T3 — Tx and T2 — Tx are independent of the
.position of the zero, any indeterminateness in that point will
not affect the values of mz obtained by this equation.

When the values of mlt m2, m3 are so close together that a
horizontal line cuts within the limits of the plate the three
parabolas corresponding to them, it is better to measure the
horizontal rather than the vertical displacements. When
we measure the displacements along a horizontal line, y is
constant, so that from the equation

^2=--?

m A

we get, if x^ x^ xz are the values of x where the horizontal
line cuts the three parabolas,

1 86 RAYS OF POSITIVE ELECTRICITY

The parabolas intersect the horizontal line so that the inter-
cepts on this line are proportional to the masses of the
particles. This gives a very convenient and open scale, but
it is evident from an inspection of the photographs that the
line will only intersect a few parabolas, and these will corre-
spond to particles with not very different atomic weights.
Another disadvantage of this method is that it fails, whereas
in the photographs shown in Fig. 2, Plate i, and Fig. i,
Plate 4, some of the parabolas are very short.

A method which is not open to this objection, and which
gives a more open scale than that obtained by measuring the
vertical deflections on a plate placed at right angles to the
undeflected path of the particles, is to place the plate parallel
to this path, and at right angles to the displacement produced
by the magnetic force.

We see from the equations on page 20 that if y is
the deflection due to the magnet force at a point at a
distance x from the place where the electric and magnetic
fields stop, z the deflection due to the electric field at the
same place.

0^ 2

Thus ^ =

If the photographic plate is placed at right angles to the
axis of y, then y — b&\\ over the plate, and the particles will
intersect the plate in the curves given by the equation

+ X \ = ™

"* ) ' e H"/
The right-hand side is constant when m[e is constant,

CHEMICAL ANALYSIS BY POSITIVE RAYS 187
hence all the particles of the same kind will lie on the
curve z( - •+ x j = a constant.

This is a rectangular hyperbola. Thus when the plate is
placed in this position, the curves registered by the positive
rays will be a series of rectangular hyperbolas and not para-
bolas as in the usual position of the plate. The asymptotes

of these hyperbolas are z = o ; - -\-x=o.

To get the complete hyperbolas all values of v would be
required, but there are no particles in the positive rays with
energy greater than that corresponding to the fall of the
particle through the potential difference between the anode
and the cathode of the discharge tube ; if V is this potential
difference, then the maximum value of J mv* is V>. Hence,
from equation (2) there can be no values of z less than those
given by the equation

This is the equation to a straight line in the plane of xz.
The hyperbolas will only exist on one side of this line. The
"heads" of the hyperbolas will lie on this line just as the
heads of the parabolas in the other method lie on a vertical
line. The intercepts on this line made by the hyperbolas
will by equations (3) and (4) be given by the equations
/ . b h.V.m

~ H ViV>

So that the length of the intercept measured from O, the
point whose co-ordinates are z = o, x = is equal to

\m b f.
VllH/t

188 RA YS OF POSITIVE ELECTRICITY

Thus the distances of the heads of the hyperbolas from O are
proportional to m*y hence if we measure these distances we
can compare, just as on the other method, the atomic weights
corresponding to the various curves. On this method, how-
ever, we have a scale proportional to m* which in some
respects is more convenient than the other scale, which is
proportional to ;«~*. The new method has the disadvantage
that the curves, due to the negatively charged particles,
cannot be obtained without a fresh exposure of the plate.

DISCUSSION OF PHOTOGRAPHS
If we exhaust a tube originally filled with air down to
the lowest pressure compatible with the production of the
positive rays, and take a photograph, we obtain a spectrum
which may be called that of the "residual gas." This gas
consists mainly of hydrogen and carbonic oxide liberated from
the walls of the tube. The spectrum shows the parabolas
due to the atoms and molecules of hydrogen, to the atoms of
carbon and oxygen, to the molecules of CO and CO2 and to
the atom of mercury, the last is due to the mercury vapour
coming from the pump. Unless special precautions are taken
these parabolas occur on all the photographs. It is possible,
however, by maintaining a constant stream of a pure gas
through the tube to reduce the brightness of these lines so
much that they are inconspicuous in comparison with those
due to the pure gas. An example of that is shown in Fig. 2,
Plate II, which represents the photograph obtained when
a stream of pure oxygen was kept running through the tube.
The only parabolas which are strong enough to be seen in
the reproduction are, on the positive side, the one corre-
sponding to the oxygen atom with two charges, that
corresponding to this atom with one charge, and that corre-

DISCUSSION OF PHOTOGRAPHS 189

spending to the oxygen molecule; on the negative side we
have the line corresponding to the oxygen atom with one
negative charge. When hydrocarbons are in the tube we find
in addition to the parabola corresponding to the carbon atom,
those corresponding to the radicles CH, CH2, CH3, showing
that these can have an independent existence; if the hydro-
carbons are complex we get many parabolas corresponding
to more complex combinations of carbon and hydrogen
atoms. We even find some of these occurring on the side of
the photograph corresponding to negative charges. Thus
on a photograph taken when the vapour of hexane was
in the tube were found negative lines corresponding to
C, CH, C2, and C3. By comparing the intensities of the
lines due to the various radicles in the positive ray spectrum
of a hydrocarbon, information might be obtained as to the
constitutional formula by which the molecule could best be
represented.

One point which is brought into prominence by the study
of these photographs is the great number and variety of the
carriers of positive electricity in the electric discharge. Some
of the photographs show more than thirty different parabolas :
many of these correspond to compounds which have not
been detected under other conditions. Thus, taking into
consideration only those parabolas for which m\e is less than
20, we find in addition to the radicles CH, CH2, CH3 already
mentioned, parabolas corresponding to m/e=i? indicating
in some cases the radicle OH, in others the molecule NH3,
others corresponding to m/e—iS, the water molecule, and
others corresponding to m/e= 19, generally I think due to
H3O. Then, again, we find on the photographs, in some
cases, lines due to diatomic molecules of elements which are
usually regarded as monatomic, thus occasionally a line due
to diatomic mercury m\e = 400 is found on the plate. And

190 RA YS OF POSITIVE ELECTRICITY

though diatomic helium would give a parabola mje — 8, which
would be indistinguishable from the very common line due to
the oxygen atom with two charges, I have some photographs
in which the helium parabola m\e = 4 has its head at twice the
normal distance from the vertical, indicating in accordance
with the argument given on p. 85, that before entering the
electric and magnetic fields the atoms producing this parabola
had, during the passage through the discharge tube, been the
constituents of a diatomic molecule of helium.

EXAMINATION OF THE GASES GIVEN

OUT WHEN SOLIDS ARE BOMBARDED BY

CATHODE RAYS

The positive rays supply a very convenient method for
studying the gases given out when minerals or solids of any
kind are bombarded by cathode rays. The apparatus used for
this purpose is shown in Fig. 17, p. 34. G is the vessel in which
the positive rays are produced. A is a vessel communicating
with B by two tubes, one of which BC is a very fine capillary
tube, while the upper one is 5 or 6 millimetres in diameter ;
taps are inserted so that one or both of these tubes may be
closed and the vessels A and G isolated from each other.
The vessel A contains a curved cathode like those used for
Rontgen ray focus tubes, and the cathode rays focus on the
platform on which the substance to be bombarded is placed.
After the solid under examination has been placed on the
platform, the taps between A and B are turned, and A is
exhausted by a Gaede pump until the vacuum is low enough
to give cathode rays. An electric discharge is then sent
through A and the solid on the platform is bombarded. The
result of the bombardment is that in a few seconds so much
gas, mainly CO2 and hydrogen, is driven out of the solid that

GASES GIVEN OFF BY BOMBARDED SOLIDS 191

the pressure gets too high for the cathode rays to be formed.
To lower the pressure a tube containing charcoal cooled by
liquid air is connected with A, the charcoal absorbs the CO2
and enough of the hydrogen to keep the vacuum low enough
to give cathode rays.

To find what gases are given off by the bombardment, the
connection between A and G is cut off while the bombardment
is going on, and after the bombardment is completed a
photograph of the positive rays is taken before the connection
is opened. The taps between A and G are then turned, the
gas from A is allowed to stream through G and another photo-
graph is taken ; the lines in the second photograph which are
not in the first represent the gases which have been liberated
from the solid by the cathode rays. Fig. 2, Plate V, and
Fig. 2, Plate VII, represent two such photographs, the lower
one in Plate V that taken before turning the tap, the upper
one after. In the latter, there are the following lines which do
not occur in the former: (i) a strong line corresponding to
a substance with atomic weight 3 ; (2) one corresponding to
helium, this is generally much fainter than the " 3 " line ; and
(3) lines representing neon with one and two charges. The
amounts of helium and neon are so small that their lines are
often not visible when the discharge in the tube is observed
through a spectroscope. This photograph is typical of what
is observed when substances such as the metals platinum,
palladium, aluminium, copper, zinc, iron, nickel, silver, gold,
lead, graphite and a large number of salts are first
bombarded with cathode rays. The helium line generally
diminishes in brightness after the bombardment has been pro-
longed for some hours, the " 3 " line is, however, much more
persistent and in some cases, for example, that of KHO, the
bombardment may be continued for several weeks without
producing any diminution in the brightness of the " 3 " line.

192 RA YS OF POSITIVE ELECTRICITY

The presence of mercury in the vessel A decreases the
intensity of the " 3," hence, we may conclude, I think, that the
substance which gives the " 3 " line combines with mercury
vapour when an electric discharge passes through a mixture
of the two gases. Another case where the presence of one
gas causes the disappearance of the lines due to another, is
that of oxygen and mercury vapour. The mercury lines are
not seen in the photographs of the positive rays when the
gas in the tube is mainly oxygen, although with most gases
these are about the strongest lines on the plate. The
disappearance of the mercury lines can be accounted for
readily by the combination of the mercury vapour with the
oxygen.

The fact that the brightness of the helium line diminishes
after long bombardment, suggests that helium has been
absorbed by, or accumulated on, the substance bombarded
by the cathode rays. Both helium and neon are present
in the atmosphere, and the positive-ray method is sufficiently
sensitive to detect the helium in a cubic centimetre of air
at standard temperature and pressure, so that if an appreci-
able amount of air were dispersed through the solid it would
account for the presence of the helium and neon lines. The
presence of helium in the air makes it necessary when
investigating the gases given off by solids to take precautions
against the helium being accidentally introduced into the
tube from the atmosphere. It is, for example, necessary to
be very careful in the use of charcoal cooled by liquid air, for
producing the final vacuum. The cooled charcoal only absorbs
a small quantity of the helium in the air, so that this gas is
not removed from the vessel by this method. Fig. 3,
Plate VII, is a photograph when the air from the discharge
tube was exhausted entirely by charcoal ; it will be seen that
the helium line and the "3" line are both strong. It is

GASES FROM BOMBARDED SOLIDS 193

necessary in experiments of this kind to reduce by a mercury
pump the pressure to a fraction of a millimetre of mercury
before applying the cooled charcoal. We can, however, in
experiments, when the helium is liberated by long bombard-
ment, eliminate this source of error, for if the helium and
neon come from the air and not from the solid the quantity
of those gases in the tube would not depend on the duration
of the bombardment. The amount of helium liberated is,
however, not appreciable unless the bombardment is pro-
longed for an hour or so and it increases with the duration
of the bombardment. Thus, if the helium comes from air
the air must have been absorbed by the substance and
liberated from it by the bombardment. To test this point
soluble salts, such as LiCl, NaCl, KC1, KI, RbCl, AgNO3,
were dissolved in water, some of them also in alcohol, and
then evaporated to dryness, the process being in some cases
repeated several times. Even after this treatment they
yielded perceptible amounts of helium, the yield was greatest
from the potassium salts, especially from KI. To test
whether solution and evaporation would get rid of dissolved
helium the following experiment was tried. It is well
known that when the electric discharge passes from
aluminium electrodes through helium the electrodes absorb
some of the helium. A piece of aluminium was divided
into two portions, one half was made into the electrodes
of a vacuum tube filled with helium at the pressure of three
or four millimetres of mercury and a current passed through
the gas for two days. After this treatment the electrodes
were dissolved in hydrochloric acid and the solution evapor-
ated to dryness. The salt thus obtained was then placed
in the positive-ray apparatus, bombarded by cathode rays for
several hours, and a positive-ray photograph of the gas
given off taken : it was found that the helium line was

194 RAYS OF POSITIVE ELECTRICITY

perceptible though faint. The other piece of aluminium which
had not been near helium was then dissolved, evaporated
and bombarded and the photograph taken ; the intensity of
the helium line in this photograph was but little less than
in the other : this experiment shows, I think, that solution
may be relied upon to remove absorbed gas unless the gas
is in some special state which does not occur when the
absorption is due to the use of the metal as an electrode.
The aluminium cathode in the tube used to bombard
the substances with cathode rays might be suspected as a
source of helium. If, however, the helium came from it
the rate of liberation would not depend upon the nature of the
salt bombarded, nor would it make any difference whether
the cathode rays hit the salt or not. As both these con-
ditions have great influence on the liberation of helium we
may regard this source as eliminated, a conclusion confirmed
by the fact that there was no perceptible diminution in the
supply of helium after the cathode had been in almost
continual use for several months.

One feature in the liberation of helium from salts is the
very considerable variation in the amount of the helium set
free by different specimens of the same salt when bombarded
under apparently identical conditions, different salts of the
same metal, too, show considerable differences with respect
to helium production. Thus, I have always obtained more
helium from KI than from KC1. These effects, as well as
the fact that the rate of production falls off after long
bombardment, suggest that the source of the gas is not the
whole mass of the salt, but something which may be described
as an accidental accompaniment. One possible source is a
layer of condensed air over the surface of the salt. We
know that when an electric discharge passes through an
exhausted vessel a considerable amount of gas comes from

GASES FROM BOMBARDED SOLIDS 195

the walls of the vessel unless these have been maintained
for some time at a high temperature, and have also been
subjected to a prolonged bombardment by cathode rays.

It is certain that any solid which has been exposed to the
air gives off when bombarded a considerable quantity of gas,
mainly hydrogen and carbon monoxide ; the hydrogen is
usually ascribed to the water vapour condensed on the sur-
face. We know too little about these layers of condensed
gas to say whether or not they would contain the same
proportion of helium as the free air, or how the amount of
gas on the surface depends on the chemical composition of
the salt. It is consistent with this view that if the salt be
kept in a vacuum the rate of evolution of helium gradually
falls away as the bombardment is prolonged. Another view
worthy of consideration is that in the atoms of the ordinary
elements, and especially, perhaps, in those of the alkali
metals, a process may be at work analogous to that which
causes the expulsion of an a particle from the atom of the
ordinary radioactive elements ; the difference being that in
the case of the elements where radioactivity has not been
detected, the a particle, i.e. the atom of helium, instead of
being projected with the enormous velocity characteristic
of radioactive substances, is projected with so little energy
that it does not wholly escape from the parent atom. It is
loosened, so to speak, by effects which are analogous to those
of radioactivity, and is finally detached when the atom is
exposed to vigorous bombardment by cathode rays. It
would only be a small fraction of the atoms of the element
where the helium had been so loosened as to be able to be
detached by the effect of cathode rays, and when these
atoms are exhausted the supply of helium will cease. The
view that helium can be got from a large number of elements
raises questions of such a fundamental character that few

196 RA YS OF POSITIVE ELECTRICITY

will be prepared to accept it unless every other explanation
has been shown to be untenable. It would strengthen
greatly the proof if we could detect the parts of the atom
which remain after the helium had been given off. I have
made efforts to do this, but have not obtained decisive results.
The difficulties are very considerable. Consider, for example,
the case of lithium : if we took the helium away we should
get a substance with atomic weight 3 ; we do find this sub-
stance when we bombard lithium salts, but we also find it
when we bombard salts which do not contain any lithium.
Then take sodium : the residue after the liberation of helium
would have the atomic weight 23 — 4 = 19. This is the
atomic weight of fluorine, a substance with such energetic
chemical properties that it would enter into chemical com-
bination and so escape detection. Beryllium would seem to
be the most promising, for the atomic weight of the residue
would be 9 -— 4 = 5, and. this would give rise to a new line
which could not be confused with any other. Unfortunately,
though a great deal of helium is given off by minerals like
the beryls which contain beryllium, the beryllium salts which
I have tried give out exceptionally small quantities of
helium.

ON THE NATURE OF X3, THE SUBSTANCE
GIVING THE "3" LINE

When salts and minerals are bombarded we find when we
analyse by the positive rays the gases given off, a line
corresponding to a substance with atomic weight 3 as well
as the helium line. In fact the " 3 " line occurs even more
frequently in such cases than the helium line, for it is rare
to find it absent after any solid has been bombarded by
cathode rays, and it not infrequently occurs when no trace
of the helium line can be detected.

*3 197

Again, though the bombardment of minerals or salts is
necessary for the production of helium, it is not so for that
of X3. Thus, for example, X3 is well developed without
any bombardment by cathode rays when the vapour of
phosphonium iodide, PH3'HI is introduced into the discharge ;
it is generally seen, though the lines are very faint when
ammonia is in the tube. Under some conditions of discharge
it was found when the only gas introduced into the tube
was hydrogen, though this was no doubt contaminated by
gases liberated from the walls of the discharge tube. In
this case it seems to be dependent upon some special type
of discharge, for it is much more frequently absent than
present. On the other hand, when salts and minerals are bom-
barded it is practically always present ; one of the best ways
of producing it being to bombard KOH with cathode rays.
If X8 is not a new substance, it must either consist of three
hydrogen atoms with one charge and be represented by H3,
or it must be an atom of carbon with four charges. Of
course, as a matter of arithmetic, it might be an atom of
beryllium (atomic weight 9) with three charges, or of
magnesium (atomic weight 24) with eight, but explanations
of this type are ruled out by the conditions of the experi-
ment. Hydrogen and carbon, on the other hand, are always
present in the tubes, and so are possible sources of X3. The
view that X3 is a carbon atom with four charges must be
abandoned for the following reasons.

i. We have seen that a line corresponding to an
atom with a multiple charge, is, unless the pressure is ex-
ceedingly low, accompanied by certain peculiarities in the
line corresponding to that atom with one charge. For
example, if there were, as there generally is, a line corre-
sponding to a carbon atom with two charges, the line cor-
responding to the carbon atom with one charge would be

198 RAYS OF POSITIVE ELECTRICITY

prolonged until its extremity was only one-half the normal
distance from the vertical axis ; if there were a line corre-
sponding to a carbon atom with three charges, the ordinary
carbon line would be prolonged until its distance from the
vertical was only one-third of the normal distance; while a
carbon atom with four charges would prolong the ordinary
carbon line to within one-quarter of the distance from the
axis. Again, the greater the charge the less the intensity,
so that a line due to a quadruply charged carbon atom would
be accompanied by a stronger line due to a triply charged
atom, a still stronger one due to a doubly charged atom,
while the normal carbon line would be the strongest of all.
Now we do not find any of these characteristics in the case
of the X3 line; the carbon line is not prolonged to within
one-quarter of the normal distance, and so far from the line
being accompanied by a stronger line due to a doubly
charged carbon atom, in many cases when the " 3 " line is
very strong the line due to the doubly charged atom is not
strong enough to be detected ; indeed, in some cases the
" 3 " line is stronger than the normal carbon line.

Again, since the gas giving the " 3 " line can be detected in
the tube long after the bombardment has ceased, if it were
carbon with four charges some gas must be formed by the
bombardment which gives, when the discharge passes through
it, carbon atoms with four charges. Experiments made with
a great variety of carbon compounds, introduced directly into
the discharge tube, e. g. CH4, CO2, CO, C2H4, C2H2, COC12,
CC14, failed to produce this line, so this view of its origin
must be abandoned.

There is, on the other hand, strong evidence of this line
being due to hydrogen. We have seen that under exceptional
conditions it can be obtained by sending the discharge through
hydrogen without the liberation of gas by bombardment.

Xs 199

The fact that it is produced so easily from phosphonium
iodide is a strong argument in favour of this view. Sal
ammoniac, prepared by allowing streams of hydrochloric acid
gas and ammonia to combine in a vacuum, was found to give
X3 when bombarded. In this case the possibility of this
substance having been absorbed in the salt would seem to be
excluded.

If we test its rate of evolution from bombarded salts,
before and after they have been dissolved and evaporated
again to dryness, we find that, with regard to this effect, salts
may be divided into two classes. In one class of salts, which
includes KI, Li2CO3) KC1, the output of X3 after this treat-
ment is much smaller than it was before. In the other class,
which includes KOH, LiCl, LiOH, CaCl2, the output after
solution is much the same as it was before, and is not
appreciably diminished by numerous repetitions of this
process. The salts of the first class do not contain hydrogen,
while those of the second either contain hydrogen or are very
deliquescent, and thus can absorb water from the atmosphere
on their way to the bombardment chamber after evaporation.
The fact that some salts continue to give supplies of X3 after
repeated solution and evaporation shows, I think, that X3
can be manufactured from substances of definite chemical
composition by bombardment with cathode rays, and the fact
that such salts contain hydrogen either as part of their
constitution, or in water of crystallization, suggests that X3
consists of hydrogen and is represented by the formula H3.
One very remarkable feature is the contrast between the
ease with which this gas is obtained by bombardment and
the difficulty of getting it when the discharge goes through
pure hydrogen. For example, I had a tube containing this
gas which, though many photographs were taken, never
showed a trace of the " 3 " line. As soon, however, as a small

200 RA YS OF POSITIVE ELECTRICITY

piece of mica which was in the tube had been bombarded for
a few minutes, the " 3 " line was developed with great
intensity. The reason for this is not evident ; it is true that
hydrogen is given off by the bombardment, but there was in
the preceding case plenty of hydrogen in the tube before the
bombardment began, so that, unless the hydrogen adhering
to the mica and given out on bombardment is in a peculiar
state, this will not account for it. I have tried to find a
connexion between the rate of evolution and the presence
of trivalent elements in the bombarded solid, but have
not succeeded in finding one. It is quite certain that the
gas is freely given off by KOH, even when considerable
trouble is taken to purify the salt, and this substance does
not contain any trivalent element. A compound containing
a trivalent element might have been expected to contain a
group of three hydrogen atoms and thus facilitate the
appearance of H3. The substance which gives rise to the
" 3 " line has considerable permanence. The gas liberated
by the bombardment can be kept for days before being used
in the discharge tube, and will still, after this interval, give
rise to the " 3 " line. Again, when once the. " 3 " line has been
obtained, the tube continues to give traces of the line after
the active gas has been pumped out and no fresh supply of
this gas has been introduced ; the " 3 " gas must, I think, be
absorbed by the electrodes or condensed on the walls of the
tube, with repeated exhaustions it gets fainter and fainter
and finally disappears.

The presence of mercury vapour in the discharge tubes
diminishes to a very great extent the brightness of the " 3 "
line. This suggests that the substance giving rise to this line
combines under the influence of the electric discharge with
mercury vapour. The evidence is not quite conclusive, as at
very low pressures the presence of mercury vapour has a

X$ 201

considerable effect on the character of the discharge, and this
may have an effect upon the intensity of the line.

The substance giving rise to the " 3 " line, if mixed with
oxygen, gradually disappears if the mixture is exposed to
strong sunlight, or if strong sparks are sent through the
mixture. As X3 is always mixed with a considerable
quantity of hydrogen, a vigorous explosion takes place when
the spark passes through the mixture containing oxygen.
I found, too, that if X3 was placed in a quartz tube containing
copper oxide it disappeared when the tube was raised to a
red heat.

The fact that sparking with oxygen, or heating with
copper oxide, the two most efficient ways of removing
hydrogen, destroys X3, makes the separation of this substance
from the great excess of hydrogen which always accompanies
it, a matter of very considerable difficulty. The most effective
way I know of increasing the percentage of X3, is to first
take out any oxygen and then to put the mixture into a
vessel to which a palladium tube is attached ; when the
palladium is heated to redness the hydrogen diffuses through
it much more rapidly than the X3, though some of this gas
can get through the palladium. The result is that the gas
left behind in the vessel contains a much greater proportion
of X3 than it did before. The preponderance of hydrogen in
the original mixture is, however, so great that even by this
method I have not been able to prepare any sample in which
the hydrogen was not greatly in excess.

Many attempts have been made to obtain spectroscopic
evidence of X3 by putting the mixture containing it in a
quartz tube with tin foil electrodes placed outside the tube.
The spectrum obtained when the discharge passed through
the tube was photographed, but no lines which could be
ascribed to X3 were detected. The first and second spectra

202 RA YS OF POSITIVE ELECTRICITY

of hydrogen were bright, and in spite of efforts to get rid of
mercury vapour the mercury lines were visible. Bombard-
ment by cathode rays is not the only method of obtaining
X3. I heated by an electric current a fine tantalum wire
until it fused, and found that a considerable amount of X3
was given off. Some time ago I found that when the dis-
charge from a Wehnelt cathode was sent through an exhausted
tube X3 was liberated ; later I found that it is not necessary
to send the discharge through the tube, the heating of the
cathode is sufficient to liberate the gas. Again, when
hydrogen, or even air, which has not been specially purified
from hydrogen, is exposed to a rays by streaming past a very
thin-walled tube containing radium emanation, the gas when
examined by the positive rays is found to contain X3. Duane
and Wendt (" Phys. Rev." 10, p. 116, 1917) have shown by a
study of chemical reactions that when hydrogen is exposed to
a rays some modification of it is produced whose properties
differ from those of normal hydrogen.

Summing up the results, we see that X3 can be obtained
by passing the discharge through gases such as phosphonium
iodide, through hydrogen under special conditions of dis-
charge, through hydrogen acted on by a particles ; that when
it is obtained by bombarding a salt a continuous supply
can be obtained when the salt contains hydrogen, while
from salts which do not contain hydrogen the supply is
soon exhausted. And again, that under certain conditions,
such as exposure to bright light or by vigorous sparking, X3
combines with oxygen, and that it is removed by copper
oxide at a red heat. All these results seem to point to
the conclusion that X3 is H3 triatomic hydrogen. At the
same time I do not feel certain that in some cases the " 3 "
line may not arise from another source. My grounds for
this view are : (i) the yield of X3 is exceptionally large from

THE 3-5 LINE 203

certain minerals ; this is what would happen if X3 were a
permanent gas absorbed by the mineral; (2) when these
minerals are bombarded the " 3 " line shows some character-
istics which are not generally present, e.g. the parabolas
are very long, sharp and of very uniform intensity. In the
majority of cases the " 3 " line is rather shorter than, say,
the other hydrogen lines, and the intensity is apt to vary
along the parobola.

It is interesting to find that Fabry from the measurements
of the broadening of the " nebulium " line due to thermal
agitation, came to the conclusion that the source of this line
is an element with atomic weight 3 —

THE ORIGIN OF THE LINE m\e = ^
When examining by the aid of the positive rays the gases
given out when a specimen of fluorspar from Ivigtut in
Greenland was bombarded with cathode rays, I found in
addition to a very strong helium line and a fairly strong " 3 "
line, a line between the two corresponding to an atomic
weight of 3*5. This fluorspar, to which attention was first
called by Thomsen (" Zeits. f. Phys, Chem.," 25, p. 112),
possesses very remarkable properties, it gives off when heated
very large quantities of helium, and when thrown on a heated
plate shines with a bright phosphorescent light. Some other
varieties of fluorspar possess this property, but none, of those
I have tried, to the same extent. I have found the 3*5 line
when some other specimens of fluorspar are bombarded ; it
is not, however, produced by every kind of fluorspar. I have
also found the line when some zircons were bombarded, and
occasionally in air after exposure to a rays. After my
attention had been called to it in this way I examined my
collection of plates to see if any traces of it could be found in
the positive-ray photographs of a very large number of

204 RA YS OF POSITIVE ELECTRICITY

substances. I found that the line could be detected on
several plates, though it was so faint that it would escape
detection unless attention were specially directed to it. The
line is remarkable for the extent to which it is accompanied
by secondaries, and these secondaries are not, as is usual,
confined to well-defined lines running up to a definite point
on the parabola, but spread out like a fan, from one end
of the parabola to the other. Another peculiarity of this
line is that it is a short line with the head of the parabola
much further away from the vertical than the heads of the
other parabolas. Thus the maximum energy possessed by the
particle giving this line is much less than that possessed by
other atoms or molecules in the positive rays. This is what
we might expect, for the very strong secondaries show that
the 3'5 particle very easily loses its charge and so is not
likely to retain it during the whole of its passage through the
dark space, and therefore cannot acquire the energy due to
the cathode fall of potential. A photograph showing this
line is reproduced in Fig. 3, Plate VIII.

It is difficult to account for the line by any known
substances. A lithium atom with two charges would give a
line in the same position, but the occurrence of the line seems
to have no relation to the presence or absence of lithium.

An atom of nitrogen with four charges would give the
3'5 line. The objections to this explanation are :

That it would involve a prolongation of the nitrogen
line towards the axis, so that the head of this line would be
only one quarter of the normal distance from the vertical ;
the nitrogen atom line is often prolonged to within one half
of this distance, and we find, as we should expect, that when
this prolongation occurs the line 7, corresponding to an atom
of nitrogen with two charges, is found on the photographic
plate. When, however, the 3-5 line is found as well as the 7

THE 3-5 LINE 205

there is no increase in the prolongation of the 14 line, nor
is the line 14/3 corresponding to nitrogen with three charges to
be found on the plates.

Again, though nitrogen is, unless special precautions are
taken, nearly always present in the tube, the presence of the
line 3*5 is quite exceptional and does not seem to be con-
nected in any way with the amount of nitrogen in the tube.

The most natural explanation of this line is that it is due
to a new element, and the only reason against accepting this
explanation is that the atomic weight is not a whole number.
There does not at present seem much hope of obtaining
sufficient quantities of this substance from known sources to
give much chance of isolation. The quantity given out even
by the Ivigtut fluorspar is small compared with the amount
of helium given out by those minerals which yield supplies
sufficient for its isolation. Apart from a new element the
only explanation I can think of which is not flatly con-
tradicted by the evidence on the photographs is that it is due
to a doubly charged compound of X3 and He ; both these
gases are present whenever the 3*5 line is visible, and the
3*5 substance is got by bombarding a mineral in which both
helium and hydrogen are present. The complex with one
charge would, if it occurred, produce a line coinciding with
that due to the nitrogen atom with two charges. Such a line
always accompanies the 3*5 line. The existence of X3He
with two charges ought to prolong the line 7, due to the
singly charged complex. I have never observed any such
prolongation of this line, but as the line is always a faint one
this is not quite conclusive. It is in favour of the view that
" 3*5 " is a compound of He and H3 that the space between
the 3*5 lines and both the H3 and the He lines is filled with
faint luminosity, indicating that the 3.5 substance while
passing through the electric fields dissociates into He and H3.

206 RAYS OF POSITIVE ELECTRICITY

There are in addition to the 3-5 line, some other lines
corresponding to smaller atomic weights. There are on several
plates a line for which mje = 1*6, another for which it is
equal to 2*4. These values are independent of the conditions
of the discharge. There are others, such as those described
on page 68, which are affected by such things as the pressure
in the tube, the length of the electric and magnetic fields,
and which are either envelopes (see p. 64), or due to unstable
complexes. The lines r6 and 2^45 do not seem to be depend-
ent, like the other lines, on the presence of gases liberated by
the bombardment of minerals. They occur when, as far as is
known, there is nothing but the ordinary residual gases in the
tube, and they occur most readily when the lines due to the
atom or molecule of hydrogen are very prominent. They are,
I think, most probably due to complexes of hydrogen atoms
with multiple charges. H5 with three and two charges re-
spectively would give lines in approximately the right position,
though if this were the origin of the lines we should expect to
find a line corresponding to H5 with one charge. Figs. I and
2, Plate VIII, show a line for which mje = 5 ; it is, however, ot
rare occurrence. The lines r6 and 2*45 are in nearly every
case exceedingly faint, so that the values of mje are difficult
to determine accurately and cannot be relied upon to much
less than ten per cent. They are sometimes found on the
negative as well as the positive side of the photograph.
There is always a considerable amount of luminosity in
the space between these lines and those due to the atom
and molecule of hydrogen, indicating, I think, that the
substance giving these lines is disintegrating into atoms and
molecules of hydrogen. It would thus appear that hydrogen
has considerable powers of polymerization, forming com-
plexes like H3 and H5. These polymers are formed most
readily when the hydrogen is; absorbed by a solid or con-

GASES CONDENSED ON THE WALLS OF THE TUBE 207

densed on its surface. Of these polymers the evidence from
positive rays shows that H3 is by far the most stable.

The gases liberated when the surfaces of solids are
bombarded by cathode rays furnish a very direct proof that
the surfaces of these solids and the walls of the tube itself
are liable to be coated with layers of gas. These layers play
an important part in positive ray work at very low pressures,
both in the methods which have to be adopted to succeed in
obtaining these pressures and in the interpretation of the
results when the positive ray method is used to analyse a gas.
The amount of gas which adheres to solid surfaces or is diffused
throughout their volumes is exceedingly large, and its removal
is a matter of great difficulty. This is not surprising, for to
separate a molecule from the surface of a solid or liquid
requires the expenditure of a considerable amount of energy
which we can estimate without difficulty if we know the latent
heat of evaporation of the substance. Thus from the latent
heat of steam we find that the work required to separate a
molecule of water from a water surface is that corresponding
to the fall of the atomic charge of electricity through about
half a volt. The tendency to evaporate diminishes very rapidly
as this work increases. Thus if it took twice as much energy
to remove a water molecule from a glass surface as it does
from a water one, the vapour pressure of water vapour over a
film of water one molecule thick on glass at 273° C would be
about that over a free water surface at o° C. So that if the
glass were heated to 273°C the water films would only evaporate
at about the rate ice at o° C would evaporate in a vacuum at
that temperature. If the removal of a molecule required
1*5 volts we should have to heat the surface to 546° C to attain
this rate of evaporation. So that we see that for quite
moderate amounts of adhesion the film may be so firmly held
that it would be practically impossible to liberate it by

208 RA YS OF POSITIVE ELECTRICITY

heating the glass to any temperature below its melting
point.

The work required to remove a molecule with a finite
electrical moment (see page 130) — a polar molecule — will be
greater than that required to remove a non-polar molecule.
Thus since the molecule of H2O is polar, while those of CO,
CO2, N2 are not, we should expect that layers of these gases
would be removed much more easily than the water molecules ;
this is in accordance with experience. The usual experience
when an exhausted bulb is heated to a certain temperature is
that at first a considerable amount of gas is liberated and the
pressure rises, then the rate of liberation of gas slows down and
after a time becomes imperceptible. Though no gas comes
off at this temperature, if the temperature is raised a fresh
supply of gas is liberated, this after a time gives out, and the
tube can remain at a constant pressure at the higher temper-
ature. On increasing the temperature again there will be a
fresh outburst of gas, and so on ; this process goes on certainly
up to any temperature which the glass can stand without
melting. These considerations make us suspect that it is not
possible by heat treatment alone to free the walls of the
discharge tube entirely from gas. This is confirmed by the
fact that when a solid from which all the gas that can be
abstracted by heat treatment has been taken is bombarded
by cathode rays, a plentiful supply of gas is given out. The
gases which survive the heat treatment, and come off under
the bombardment of the cathode rays, i. e. those which are
especially firmly held by the glass, contain a large percentage
of hydrogen. This is what we should expect on the view
that the hydrogen on the glass is in the atomic and not the
molecular condition, and that the uncharged atom of hydrogen
consists of a central positive charge and a single electron :
as there are in this atom two charges, one positive and the

GASES CONDENSED ON SOLIDS 209

other negative, separated by a distance equal to the radius of
the atom, the atom will have a considerable electrical moment
— it will be very polar — and therefore will be difficult to separ-
ate from glass. The work required to remove an atom in the
layer next the gla^s will be far greater than that required to
remove an atom in the layers piled on the top of this layer; so
that it seems probable that after heat treatment the solids will
be left with a single layer of hydrogen atom spread over the
surface. To remove this the surface must be bombarded with
cathode or positive rays. Mr. Langmuir, who has made many
interesting investigations on the layers of gases condensed
on solids, gives reasons for thinking that in the layer next the
solid the gaseous atoms or molecules are packed as closely
as possible together, and that the number per sq. cm. of
surface may amount to IO15. To remove this number by
cathode rays, assuming that each atom requires one cathode
ray particle for its removal, would involve the reception
by each square centimetre of surface of about 4-8 x io5
electrostatic units of electricity.

The existence of a highly compressed layer of hydrogen
atoms over the surface of solids would explain the very
interesting fact that H3 is produced so much more readily by
bombarding solids than in any other way, for on the surface
of these solids we have the atoms in an ideal condition for
combination, packed so close together that they are almost in
contact, and at the same time ionized and liberated by the
action of cathode rays.

Though we cannot expect to get rid of layers of hydrogen
atoms by heat treatment we may expect to be able to do so
by longrcontinued bombardment with cathode rays ; the
maintenance of the vacuum in a Coolridge tube in constant
use is a proof that this is the case. Though we ought in this
way to be able to eliminate the hydrogen from the walls of

210 RAYS OF POSITIVE ELECTRICITY

the tube, yet I have never yet been able to eliminate the
lines corresponding to the atom and molecule of hydrogen
from the positive ray photographs. This is usually ascribed
to the hydrocarbon vapours given off by wax used to join up
the glass bulb to the metal parts of the apparatus or to the
grease used to lubricate the taps. No doubt each of these is
a source of vapours containing hydrogen, but I am not
satisfied that it is the only source. I have replaced the wax
joint by one in which a layer of copper deposited on the glass
was soldered to the metal, and liquid air traps were placed
between all the taps and the bulb : even with these precautions
the hydrogen lines were quite bright on the photographs.
Another interesting thing about these lines is that even when
a bulb has been running for a long time so that its walls have
had a long exposure to cathode rays, the introduction of a
little mercury vapour produces a remarkable increase in the
brightness of the hydrogen lines ; some of this is due to an
increase in the current, but this is only part of the reason, for
the increase in the hydrogen lines is much greater than in the
other lines. I am inclined to think that not only can a hydrogen
atom cling to a mercury surface, but that it can also cling to
an atom of mercury vapour* The union of the two need not
necessarily be of the type of the ordinary valency compounds,
where there is a transference of electrons from one atom in
the molecule to another. The hydrogen atom may be held
to the molecule, in the same way as it is against the mercury
surface, i.e. by the forces between the electrostatic doublet
formed by the hydrogen atom and the electrons in the
mercury atom. Compounds of this type where there is no
transference of electrons from one atom to another would
have properties quite different from those possessed by a
compound represented by the same chemical formula, but be-
tween whose atoms a redistribution of electrons had occurred.

MERCURY AND HYDROGEN 211

Thus consider the case of the combination of an atom of
chlorine and an atom of hydrogen : if there is no transference
of electrons we have a neutral chlorine atom with seven
electrons in its outer shell, attached to a neutral hydrogen
atom ; the chlorine atom can receive an additional electron
without its electrons becoming unstable, and its attraction for
the hydrogen atom will be increased thereby, thus the molecule
of this compound can receive a negative charge. Again, if
the chlorine atom instead of gaining an electron lost one
or more its attraction for the hydrogen atom would increase,
and thus this molecule could receive a multiple charge as
readily as an atom of chlorine itself. Let us now consider
the union of the same atoms when an electron has gone
from the hydrogen atom to the chlorine, making up the
number of electrons in the outer layer of its atom to eight.
The chlorine atom as a whole has got a negative charge
equal to unity, while the hydrogen atom has a unit posi-
tive charge. The chlorine atom having eight electrons in
its outer layer cannot receive another electron, so that the
molecule will not get negatively electrified, while if the
chlorine atom were to lose two electrons it would become
positively charged and repel the positively charged hydrogen
atom. Thus the molecule would break up — so that this type
of molecule, unlike the former, could not receive a multiple
charge. The work required to dissociate the second type of
molecule might be expected to be much greater than that
required for the first type, so that the second type would be
much more stable than the first : compounds of the first type
might form an intermediate stage between the valency com-
pound and the separate atoms of which it is composed.

212 RAYS OF POSITIVE ELECTRICITY

THE LINES DUE TO NEON

Sir James Dewar was kind enough to supply me with
samples of the gases obtained from the residues of liquid
air ; when the treatment of these residues had been such as
to retain the lighter constituents of the atmosphere, the
photographs showed a line corresponding to helium, strong
lines corresponding to neon with both single and double
charges ; and, in addition, a line corresponding to an element
with an atomic weight 22, and also a line corresponding
to this element with a double charge. A molecule of CO2
with a double charge would give the line 22, but this cannot
be its origin, as the CO2 can be removed from the gas
without diminishing the intensity of the line. This line is
much fainter than the neon line, so that in the atmosphere
the quantity of the gas which is the source of the line must
be small compared with the quantity of neon.

The compound NeH2 would have the required mass, but
the fact that the origin of the line can carry a double charge,
as is shown by the presence of the line mje = 11, is strong
evidence that it is due to an element and not a compound.
The atoms of the elements other than hydrogen all occur
with double charges, and though the occurrence of a molecule
of a compound with a double charge is not unknown, it is
very exceptional.

Mr. Aston made many attempts at the Cavendish La-
boratory to separate the new gas from neon, which has an
atomic weight of 22. The first method he tried was to
fractionate a mixture of the two gases by means of their
absorption by coco-nut charcoal cooled by liquid air, the
absorption of the heavier gas being expected to be greater
than that of the lighter.

THE NEON LINES 213

No appreciable effect, however, was produced by this frac-
tionation ; indeed, from an investigation by Lindemann and
Aston, "Phil. Mag." [6], 37, p. 523, 1919, it would seem that the
effect which was to be anticipated was smaller than could have
been detected by Aston's experiments. Another method used
by Aston was to allow the mixture to diffuse through a porous
substance, like the stem of a clay tobacco-pipe, when the
lighter constituent would get through a little more rapidly
than the heavier one ; he designed an automatic apparatus
in which the diffusion went on uninterruptedly, but no decisive
results were obtained.

Mr. Aston has recently attacked the problem by quite a
different method, based on the following considerations : —
If the neon in the atmosphere contains two different
constituents, then the atomic weight 2O'2, determined by
the measurement of the density of the gas, will not be the
atomic weight of either constituent, but a mean value
depending on the proportion in which the constituents are
present. The measurement of the positive-ray photographs
enable us to determine the atomic weight of the substance
giving rise to any particular line, and if these measurements
can be made with such accuracy as to enable us to say that
neither of the lines has an atomic weight which corresponds
to that of the atmospheric neon, this would prove that the neon
in the atmosphere is a mixture. The evidence would be
still stronger if the atomic weight of the mixture 20*2 agreed
with the mean of the atomic weights of the constituents,
the proportion between the constituents being determined
from the relative intensities of the lines on the photographic
plate.

Mr. Aston determined, by the focus method described
on page 36, the atomic weights of the substances producing
the neon line, and its companion the line for which mje = 22.

2i4 RAYS OF POSITIVE ELECTRICITY

The focus method is an interpolation method where the
atomic weight of a substance producing a line is determined
by comparing the position of the line relative to lines due to
substances of known atomic weight. The way in which this
is done will be understood by considering the following
example. When the gas in the discharge tube is the residual
gas left after the tube has been exhausted, the positive-ray
spectrum shows a group of five lines due to C(i2), CH(i3),
CH2or N(i4), CH3(i5), CH4 or O(i6) : those form the five
bands a, /?, y, <5, e. The edges of the bands are well
defined, and the distance between two of the edges can be
measured with a high degree of accuracy. The measurements
showed that for this group the distances between corre-
sponding edges of adjacent bands was constant throughout
the group. As the adjacent bands are due to particles whose
atomic weights differ by unity, we may conclude that in
this part of the photograph the relation between the position
of the edge of the band and the atomic weight is a linear
one. When neon is put into the tube, there are in addition
to the five bands already mentioned two new ones a, b.
The edge of a is very accurately two units away from the
corresponding edge of the band C(i2), while the edge of b
is one unit away from the same edge ; hence we conclude
that a is due to a substance for which mfe = 10, and b
to one for which mje= n. The lines a, b are the lines
corresponding to doubly charged particles, the singly
charged particles giving the neon line and its companion.
Hence we conclude that the atomic weight of the particles
are respectively 20 and 22. Mr. Aston regards his
measurements as being accurate to a small fraction of
one per cent, and that a particle with the atomic weight
20*2, that usually assigned to neon, could not possibly be
the origin of either of these lines. Thus on this view

THE NEON LINES 215

the neon of the atmosphere is a mixture of two substances,
one having an atomic weight of 20 and the other of 22 ;
the mixture containing nine parts of the former to one of the
latter, so that its density is 20*2. Those proportions are not
incompatible with the intensity of their lines in the positive-
ray photographs. Since no difference either in the spectrum
or in the chemical properties can be detected between these
substances they are called isotopes. Examples of such
isotopes had previously been observed in the products of
radioactive transformations, such as radio-lead and thorium,
which have different atomic weights, and which are supposed
to be inseparable from each other by any chemical
process.

As far as the evidence from the positive rays goes, the
proof that the substance 22 is a separate element and not a
hydride of 20 is not absolutely conclusive. This evidence is
based on the occurrence of the line n as well as of 22,
showing that the particle producing the line can carry a double
charge. The occurrence of a molecule with a double charge
is not unknown, though it is exceedingly rare, while with the
exception of hydrogen most atoms can carry a double charge.
The fact that the atomic weight of ordinary neon is not affected
by sending powerful electrical discharges through it is against
the view that 22 is a hydride, for neon does not combine
under ordinary circumstances with hydrogen, and if as might
be expected the discharge dissociated the hydride into Ne
and H2 the hydride would not be reformed and the density
of the " neon " would approach 20.

On the view that the atoms of the different chemical
elements are built up of the same constituents, say atoms of
hydrogen and helium, the atom of 22 would be that of 20
with the addition of a molecule of hydrogen, in this sense it
might be called a compound of 20 and hydrogen, but whereas

216 RAYS OF POSITIVE ELECTRICITY

in ordinary chemical compounds the atoms of the different
elements are separated by distances comparable with io~8cm.
in " 22 " the 20 and H2 are only separated by a very minute
fraction of that distance.

DETERMINATION OF ATOMIC WEIGHTS BY
THE POSITIVE RAYS

In addition to its use for the detection of new substances
the method of positive rays furnishes, when it can be used, a
method for determining the atomic weight of the elements
which possesses great advantages over all other methods,
inasmuch as the presence of impurities does not produce any
effect on the result. We have seen (see p. 183) that by the
measurement of the positive-ray parabolas we can compare
the atomic weight of the particles producing the lines. WThen
three lines are near together, then, as was shown on p. 185, we
can find with great accuracy the atomic weight of the carriers
of one of the lines in terms of the atomic weights of the
carriers of the other two. Thus, for example, the lines due
to carbon, nitrogen and oxygen come near together, and
assuming the weights of carbon and oxygen we can deduce
that of nitrogen. A few years ago I measured a considerable
number of plates with this object, and found that the atomic
weight of nitrogen was 14 to an accuracy of one part in a
thousand. This is a point of some interest, because nitrogen
and beryllium are, among the elements with atomic weights
less than forty, the only exceptions to the rule that the
remainder when the atomic weight is divided by 4 is either
nothing or 3, and it was important to prove that 14 was the
atomic weight of a single element and not the mean of the
atomic weight of two elements which could not be separated

DETERMINATION OF ATOMIC WEIGHTS 217

by chemical methods. The result of the positive-ray determina-
tion shows that nitrogen is a genuine exception to the law.
I have made similar determinations for beryllium ; the beryl-
lium line is very difficult to obtain on positive-ray photographs,
but by bombarding some beryls with cathode rays I obtained
a faint line between the line 8, corresponding to oxygen with
two charges, and the carbon line 12 ; the line was not sharp
enough to obtain very accurate measurements, but the atomic
weight was certainly nearer to 9 than to any other integer.
It would seem from this that beryllium, like nitrogen, is a
genuine exception to the law just quoted, beryllium giving a
remainder I and nitrogen a remainder 2.

Mr. Aston has applied his focus method to determine the
atomic weight of most of the elements which can be obtained
in the gaseous state, linking the atomic weight of one element
with that of another by the method outlined on page 214.
The results he obtains differ in some cases very materially
from those hitherto accepted, the atomic weights (O = 16)
being much nearer to integral values than the earlier values.
This is especially marked in the case of chlorine. With
chlorine in the tube no line was found in the position corre-
sponding to 35*4, the accepted value, but lines were found
corresponding to atomic weights 35, 36, 37, 38 (Plate
IX).

On Mr. Aston's view 35 is the atomic weight of one form
of chlorine, 36 the hydride of this form HC1, while 37 is not
H2C1 but an isotope of chlorine of atomic weight 37 ; the
ordinary chlorine whose atomic weight has been determined
by the chemists he regards as a mixture of two isotopes, one
having the atomic weight 35, and the other the atomic weight
37. The ground for supposing that 37 is an isotope, and not
the hydride C1H2, is that the line 18*5, which corresponds to
a particle with an atomic weight 37 with a double charge, is

2i8 RAYS OF POSITIVE ELECTRICITY

found on the plate, and as a general rule it is atoms and not
molecules which carry a double charge. Though this rule is
generally true, there are exceptions to it, and the occurrence
of the double charge cannot be regarded as conclusive
evidence of the atomic character of the origin of the line. As
a matter of fact, the line 18, which would correspond to the
hydride C1H with a double charge, is found on the plate, but
as this might also arise from water vapour H2O we cannot
draw any conclusion as to whether 36 can carry a double
charge. Again Mr. Aston has found that lines corresponding
to atomic weights 35 and 37 are found on the negative side
of the plate. This again is a presumption that these lines
correspond to atoms and not to molecules, as atoms occur
more frequently than molecules with a negative charge. The
fact that no line occurs on the positive-ray photographs
corresponding to an atomic weight 35*46 is a very interesting
and important fact, for the atomic weight of chlorine was
supposed to be known with an accuracy of one part in a
thousand by determinations separated by long intervals of
time, and therefore made with samples of chlorine, presumably
obtained from very different sources and localities ; these
determinations were made by many different methods and
included comparisons of the density of gaseous chlorine with
that of a standard gas. The case of chlorine is exceptionally
interesting, because it seems to be the one which promises to
give the best chance of success in demonstrating by direct
methods the existence of isotopes. This is due, firstly, to the
energetic properties of chlorine, and, secondly, to the fact that
each of the constituents is present in comparable proportions ;
for if the atomic weight 35-5 is due to a mixture of 35 and 37,
ordinary chlorine must contain about 70 per cent, of the
lighter and 30 of the heavier element.

It is easy to exaggerate the similarity of isotopes and the

DETERMINATION OF ATOMIC WEIGHTS 219

theoretical difficulties in the way of their separation. Many
of the chemical effects due to an element, and especially to
one in the gaseous state, must be influenced by its mass, the
velocity of chemical reactions is a case in point. Let us, for
example, suppose that we have a mixture of two gaseous
isotopes of HC1 of molecular weights 36 and 38 flowing through
a tube lined with some substance which combines with both
of them. The average velocity of the molecules of the lighter
constituent would be greater than that of the heavier one, the
number of collisions made by the molecules of the lighter
constituents with the walls of the tube in a given time would
for the same number of molecules be greater than for those
made by the heavier ones. The absorption of the lighter
constituents will be greater than that of the heavier one, so
that after passing through the tube the proportion of the
heavier constituent will increase and the composition of the
mixture will be changed. A simple calculation will show that
if a litre of the gas is reduced by absorption to about I c.c.
the density of the residue will be greater by about *2 per cent
than that of the original gas, to produce an increase of density
of i% the absorption would have to go on until the litre was
was reduced to about io~12 c.c. A change of '2 per cent in
the density could be determined with certainty. This
experiment would be an easier one than the attempt to
separate the two constituents of neon by diffusion through
porous tubes.

I made myself, a few years ago, a number of experiments
with the object of seeing whether I could get any evidence
that " ordinary " chlorine is a mixture of different substances,
as this seems the most natural explanation of the anomaly in
its atomic weight. The resolution with the apparatus I used
was not sufficient to enable me to find the atomic weight to an
accuracy of more than one cent, so it was not possible to settle

220 RA YS OF POSITIVE ELECTRICITY

the question by the measurement of the atomic weight of the
element giving the chlorine line. I observed, however, in the
neighbourhood of this line a number of other lines; I attributed
these lines, however, to hydrides, and not to isotopes, because
though they were fairly strong on the side of the photograph
corresponding to positively charged particles on the side
corresponding to the negatively charged particles, I could
only see one line, and that a very strong one, on the negative
side. If the difference of density is due to the presence of an
isotope of atomic weight 37, the amount of this isotope must
be about one-third of that of the lighter constituent, and on
my photographs the intensity of the main negative line was
so great that I thought a line with one-third of this intensity
could not escape detection. I found too large differences in
the relative intensities of the lines due to particles with positive
charges ; this is not what we should expect if they were due to
isotopes possessing identical chemical properties and present
in invariable proportions.

Mr. Aston's experiments show conclusively that one of
the constituents of Cl has the atomic weight 35 ; direct
evidence that the substance responsible for the line 37 is an
isotope of this constituent is very desirable.

In addition to his experiments on chlorine Mr. Aston has
examined the atomic weights of most of the elements which
can conveniently be studied by the positive-ray method ; these
include H, He, C, N, O, Ne, Cl, A, Kr, Xe, Hg. He concludes
from these experiments that the atoms of the first five
elements are all of one kind, while the atoms of the others
are of two or more different kinds, the atomic weights of the
different atoms differing in most cases by two units, though
in the case of Krypton there is one that only differs by
one unit from its nearest neighbour. The results of the
experiments are given in the following table : —

DETERMINATION OF ATOMIC WEIGHTS 221

ement.

Accepted
atomic

Minimum
number

Atomic weight of isotopes
in order of intensity.

weight.

of isotopes.

I -008

1-008

3'99

I0'9

II, IO

I 2 '00

14-60

1 6-00

19-00

20'2

20,22 (21)

28-3

28, 29, (30)

31-04

32-06

35 '46

35, 37, (39)

39'9

(2)

40 (36)

74-96

79-92

79,81

82-92

84, 86, 82, 83, 80, 78

126-92

127

130-2

(129, 132, 131, 134, 136)

200-6

(5)

(197-200, 202, 204).

The figures enclosed in brackets are provisional.

It will be noticed that within the accuracy of the experi-
ments, which was estimated to be about one part in a
thousand, all the atomic weights determined by the positive
ray method are integers, a most interesting and important
result, involving as it does the conclusion that measure-
ments which were regarded as the most trustworthy in the
whole range of chemistry have given results which are only
the roughest approximation to the truth.

Lithium, atomic weight 7, has been shown by G. P.
Thomson and Aston (Nature, Feb. 24, 1921) to have an
element with atomic weight 6 as a companion, while Dempster

222 RAYS OF POSITIVE ELECTRICITY

has shown that magnesium atomic weight 24 has elements
26, 29 as companions (Proc. Nat. Ac., Wash, 7, p. 45, 1921).
The relative intensities of the lithium lines 6 and 7 have been
found both by Thomson and Dempster to be very variable.

The method of analysing a gas by superposed magnetic
and electric fields can be applied to cases other than those in
which the ions are positive rays streaming through a hole
in the cathode. It can be applied, for example, to investi-
gate the nature of the ions in the electric arc, in the positive
column of a discharge through a gas at low pressure, the ions
produced in flames, and so on. In these cases the ions
have not in general sufficient energy to affect a photographic
plate, so that it is necessary to accelerate them before they
reach the plate. To do this the ions are produced in a vessel
A, which is connected by a very narrow channel with another
vessel B, in which a high vacuum is maintained. The gases
from A rush through the channel into B, when they at once
pass through two parallel pieces of wire gauze, between
which there is a potential difference of several thousand volts
obtained by connecting them with the poles of a small
Wimshurst electrical machine, a spark a few millimetres long
passing between the poles. The field between the gauzes
accelerates the ions of one sign and gives them energy enough
to affect the photographic plate which they reach after passing
through the usual electric and magnetic fields.

The method of positive rays enables us to apply searching
tests to theories of the constitution of the atom and the
structure of molecules. Thus for example on one theory
the atoms of the elements are made up of electrons and one
positive charge, the positive charge being at the centre and
the electrons distributed around it. The negative electricity
on the electrons is equal in magnitude to the positive
electricity on the positive charge. The atoms of the different

ATOMIC STRUCTURE 223

elements contain different numbers of electrons, thus the atom
of hydrogen is supposed to possess one electron, the helium
atom two, the lithium atom three, and so on, the number of
electrons in the atom being equal to the atomic number of the
element.

The arrangement of the electrons in the atom is determined
by the condition that each electron is in equilibrium under the
forces acting upon it. These forces are the mutual repulsion
of the electrons and the force exerted by the positive charge.
The latter force, though following the inverse square law at
distances which are either very large or very small compared
with the radius of the atom, is supposed at distances which
are comparable with this radius to follow a more complicated
law and to change at certain distances from attraction to re-
pulsion, and at others from repulsion to attraction as the
distance diminishes. At the places where the force changes
from attraction to repulsion a single electron would be in
stable equilibrium under the action of the positive
charge.

The most obvious arrangement for a number of electrons
would be a symmetrical distribution over the surface of a
sphere with its centre at the positive discharge. If there
were a considerable number of electrons this arrangement
would bring them near together, and, in consequence of their
mutual repulsions, there would be a tendency for the con-
figuration to become unstable ; this tendency would increase
rapidly as the number of electrons increased. Whatever be
the law of force between the positive charge and an electron,
there will be a limit to n, the number of electrons which can
be in stable equilibrium on the surface of a sphere with a
positive change ne at the centre, there will thus be a limit
to the number of electrons which can form the outer layer
of an atom. If the law of force between a positive charge

224 RA YS OF POSITIVE ELECTRICITY

and an electron is -^ g, it can be proved that eight is

the maximum number of electrons which can be in stable
equilibrium on the surface of the outer layer. Thus if an
atom contained nine electrons, they could not all be on the
surface of a sphere, eight would be on such a surface and one
outside. Similarly, if there were ten electrons, eight would
be on the surface of a sphere and two outside ; with eleven
electrons there would be three outside, and so on ; with six-
teen electrons there would be eight outside — this is the
maximum that can be on one layer, so that a seventeen
electron atom would have two layers of eight electrons each
and one electron outside, the number outside being the
same as the nine or the one electron atom. Thus the
one, nine, and seventeen electron atoms have this in common,
that the outermost layer contains one electron ; similarly each
of the two, ten, and eighteen electron atoms have two elec-
trons outside ; the three, eleven, and nineteen electron atoms
will each have three electrons outside, and so on. Thus,
as the number of electrons in the atoms of the element
increases, i. e. as the atomic weight increases, the number of
electrons in the outer layer will recur periodically, and any
property which depends on this number, such as the valency
of the element, will recur periodically also. Thus we get in
this way an explanation of Mendeleef's Periodic Law, the
elements in the same group having the same number of
electrons in the outer layer.
Thus the atoms of the group

H, Li, Na, K,

are supposed to have one electron in the outer layer ;
those in the groups

Be, Mg, Ca,
Bo, Al,

ATOMIC STRUCTURE 225

C, Si,

N, P,

O, S, Sc,

Fl, Cl, Br,

Ne, Arg,
two, three, four, five, six, seven and eight respectively.

Let us consider the bearing of this on the existence of
multiply charged positive ions, i.e. ions which have lost
more than one electron. Those electrons will have come
from the outer layer, as the electrons in this layer are much
more easily detached from the atom than those in the inner
layers; thus we should not expect to find atoms carrying
multiple charges unless there were more electrons than one
in the outer layer. Thus if this theory is true we should
expect to find the atoms of the elements of the first group
characterized by their inability to receive a double charge ;
this is a striking feature of the hydrogen atom. Mr. G. P.
Thomson has got by the anode ray method positive ray
photographs of the lines corresponding to lithium, sodium
and potassium, but has not detected the existence of double
charges on the atoms of any of these elements.

Thus, as far as it goes, the evidence from multiply charged
atoms in the positive rays is consistent with this theory.

Let us next consider the question of negatively charged
atoms ; these are atoms which have received an additional
electron. On this theory, however, eight is the maximum
number of electrons that can be on stable equilibrium on the
outer layer, hence atoms like those of neon and argon which
have already eight electrons in the outer layer have no room
for more electrons and hence cannot receive a negative charge ;
the atoms of the elements in the other groups might be
expected to get negatively charged. The positive ray
photographs never give any indications of the lines due to

226 RAYS OF POSITIVE ELECTRICITY

atoms of the inert gases with a negative charge — these results
are in accordance with the theory, as is also the existence of
negative charges on the atoms of hydrogen, carbon, oxygen,
fluorine and chlorine. It is remarkable, however, that we have
no evidence of the existence of negatively charged nitrogen
atoms. This is, however, explained, and affords a remarkable
confirmation of the theory, by a calculation of the work
required to detach the additional electron from negatively
electrified atoms of hydrogen, carbon, nitrogen, and oxygen.

E b
If the law of force is -^ — —^ where E is the atomic number,,

the work required to detach an electron from the negatively
electrified atoms is given in the following table :

e2
Hydrogen = '125—

e*
Carbon — '39'-. for three of the electrons.

= '034— for the other two.

e*
Nitrogen = '0037—

Oxygen = -033^

e is the charge on an electron and r the distance of an
electron from the centre of the atom. Thus the work required
to remove the additional electron from nitrogen, i. e. to remove
its negative charge, is only about one-tenth of that required
to remove the charge from atoms of carbon and oxygen. As
the nitrogen atom loses a negative charge so easily, we should
not expect to find it with this charge in the positive rays.
It must be remembered, too, that the negatively electrified
atoms which produce an effect on the photographic plate
have received their charge after passing through the cathode,,
and have had to snatch the electron from some other atom L

STRUCTURE OF MOLECULES 227

thus the atom has not only to be able to find room for an
electron, it has to be able to snatch it from a rival.

Let us now turn from the consideration of atoms to that
of molecules. On the theory we are considering, in the mole-
cules of compounds such as HC1, H2O, H3N, H4C, the
electrons from the hydrogen atoms have been transferred to
the more electronegative atoms, making the total number of
electrons in the outer shells of these atoms up to eight ; thus
Cl, whose outer shell contains normally seven electrons, receives
one electron ; O, whose outer shell had six, two, and so on.
Thus the electrons are arranged in' sets of eight around the
more electronegative atoms ; as eight is the maximum number
of electrons which can exist in stable equilibrium in an outer
layer these layers are already saturated and cannot receive
an additional electron ; but if the molecule is to get negatively
electrified it must receive an additional electron, hence we
should not expect a molecule of this type to occur with a
negative charge — this is in accordance with the results of
positive ray analysis. Again, in molecules which are not
saturated, such as HO, NH2, CH, CH2, the number of
electrons round the electronegative element is less than eight,
they can therefore receive an additional electron and so acquire
a negative charge — we find again that negatively electrified
molecules of this type do occur among the positive rays.

Take now the question of a double charge on molecules of
saturated compounds. In these the electropositive atom is
positively electrified because it has lost its electrons, the electro-
negative atom is negatively electrified because it is surrounded
by more electrons than are required to neutralize its central
charge : the cohesion of the atom is in part due to this
separation of its electrical charges. Now the positive charge
on the molecule must be due to the ejection of electrons ; if,
as in this case, the electrons are concentrated on the negatively

228 RA YS OF POSITIVE ELECTRICITY

charged part of the molecule the ejection of electrons must
diminish the charge on the negatively charged atom, and
thus diminish the attraction between the atoms and therefore
the stability of the system. Thus the positive electrification
of molecules of this type would tend to disrupt the molecule.
For example, if the molecule of HC1 were to possess a double
charge, the chlorine atom must have lost two negative charges,
it had only an excess of one to begin with, so that it would
be positively charged and repel instead of attracting the
hydrogen atom.

The molecules where one of the atoms can be regarded as
positively, the other as negatively electrified are of the polar
type discussed on page 133. There are others which have
not this polar quality, and to which the preceding reasoning
does not apply ; it is in accordance with this that we also find
a few molecules, CO is one, which occurs with double charges,
among the positive rays.

We have seen that unsaturated radicles such as CH2, OH
occur with negative charges in the positive rays ; these are
not found outside discharge tubes in a free state. There
are other molecules, however, of which O2 is the most con-
spicuous example which can exist in the free state and yet
can occur with a negative charge among the positive rays.
This is consistent with the theory, since in the molecule of O2
the twelve disposable electrons are supposed to be arranged
in two octets, each atom of oxygen being surrounded by an
octet of electrons, the two octets are supposed to have four
electrons in common, so that together they accommodate
twelve electrons. If, however, the octets were placed so that
they had three electrons in common they could accommodate
thirteen electrons, and the molecule would then have a unit
negative charge. If the octets were arranged so that they
had two instead of four electrons in common they could

STRUCTURE OF MOLECULES 229

accommodate fourteen electrons, which would give the molecule
a double negative charge. Thus we see that in certain types of
molecules the electrons may readjust themselves, so as to
make room for more electrons and thus enable the molecule
to acquire a negative charge. This readjustment is possible
for all molecules whose structural formulae when represented
by the usual chemical notation contain double bonds.

The ability of molecules to receive a negative charge is of
great importance in connexion with the mobility of the ions
produced in gases by the action of Rontgen-rays. These rays,
when they fall on the molecules of a gas, ionize it by ejecting
electrons from the molecules, thus producing in the gas
electrons and positively electrified molecules. Thus the
negative ions, or at any rate the great majority of them,
start as electrons, and while in this state their mobility will
be much greater than that of the positive ions. If the
molecules of the gas are unable to receive a negative charge
the electrons will remain free and the negative ions will retain
their high mobility. If, on the other hand, the electrons can
attach themselves to molecules, the mobility of the negative
carriers will fall and will become comparable with that of the
positive ones. Franck and Hertz have shown that in argon
and nitrogen, whose molecules cannot receive a negative
charge, the mobility of the negative ions is enormously
greater than that of the positive, but that the introduction
of a very small quantity of oxygen, whose molecule can
receive a negative charge, reduces the mobility of the
negative ion almost to that of the positive.

On the view we have taken of the arrangement of the
electrons in the atom the valency of a charged atom should
be different from that of an uncharged one. Thus, on this
theory, the outer layer of the chlorine atom contains seven
electrons, and as eight is the limiting number which it can

230 RAYS OF POSITIVE ELECTRICITY

hold in stable equilibrium it cannot take an electron from
more than one hydrogen atom, so that the compound HCi
would be saturated. If, however, the chlorine atom were posi-
tively electrified it would have only six electrons in the outer
layer, it would therefore have room for two more electrons so
that the positively charged compound H2C1 would be possible.
On the other hand if the chlorine atom were negatively elec-
trified it would have eight electrons on its outer layer and would
not be able to find room for another. Again, the outer layer
of an uncharged atom of oxygen contains six electrons : it can
therefore accommodate the electrons from two, but not from
more than two, hydrogen atoms. The positively electrified
atom of oxygen has, however, only five electrons in its outer
layer, and can therefore accommodate the electrons from three
atoms of hydrogen forming the compound H3O. This has the
molecular weight 19, and a line corresponding to this molecular
weight is very frequently found on positive ray photographs
under circumstances which preclude the presence of fluorine,
which would give a line in the same position.

Again, we might expect that the inert gases might be able
to form compounds if they were positively electrified. For a
positively electrified atom of neon would only have seven
electrons in the outer layer, and thus would be able to accom-
modate an electron from an atom of hydrogen and form the
compound NeH. The compounds formed by electrified atoms
of the inert gases would, I think, be an interesting subject for
investigation. In this connexion it may be remarked that the
helium parabola in the positive ray photographs sometimes
shows an abrupt increase in intensity at a place twice as far
from the vertical as the head of the parabola ; showing (see
p. 151) that two helium atoms have combined to form a
molecule which broke up after passing through the cathode.

Another subject on which the Positive Rays may, I think,

STRUCTURE OF MOLECULES 231

be expected to throw light is that of the structure of the
molecule. For, as we have seen, when a compound gas
is in the discharge tube there are among the positive
rays not only the individual atoms which went to make
•up the molecule, but also unsaturated combinations of these
atoms, the proportions in which these combinations are
present yield information about the configuration of the
molecule. To illustrate this by a definite example let us
take the case of C2H2C12, if the molecule is represented by

H /H

/>C = CC then when it is split up in the discharge
CK XC1

vessel we should expect to get the radicle CHC1 in much
larger quantities than either CH2 or CC12. If, however,

H\ /CI

the molecule is represented by /C = C\ we should

R/ XC1

on the other hand expect the combinations CH2, CC12 to be
more plentiful than CHC1. To determine questions of this
kind it is necessary to use a metrical method such as
that described on p. 120, which was introduced for this
purpose. Investigations of this kind are now, after interrup-
tion by the war, in progress at the Cavendish Laboratory.
The curves given on pp. 124, 126 illustrate the kind of
information that such experiments can give. The curve
for COC12, p. 127, shows that the number of its undis-
sociated molecules in the positive rays is small compared
with the number of some of its constituents such as Cl and
CO, while the curve for CO, p. 124, shows that for this gas
the number of undissociated molecules is very much larger
than the number of any of the products of dissociation.
Thus COC12 is very much more easily dissociated than CO,
but the dissociation in the main consists in the tearing away
of the chlorine atoms, leaving the CO intact, for we find that

232 RA YS OF POSITIVE ELECTRICITY

the number of C or O atoms is small compared with the
numbers of either CO or Cl. It is interesting to note that
there are comparatively few particles of the type COC1 with
only one chlorine atom detached, in the great majority of
cases both chlorine atoms have been removed. The chlorine
atoms are thus much more easily detached from the molecule
than the oxygen ones.

Investigations on the positive rays from a compound
CR^RoRa, where Rlf R2, R3, R4 are monovalent atoms or
radicles, would enable us to compare the strengths of the
bonds uniting the different radicles to the central carbon
atom. Thus, for example, if R4 were much more rigidly
attached than any of the others there would be a pronounced
absence of the combination CRJR2R3 in the positive rays.

These, however, are only a few of the questions which
could be attacked by this method ; it gives us, for example,
the means of testing whether, as is generally believed, a
multiple bond between carbon atoms is an especially weak
part of the molecule, for if this is so then with acetylene in
the discharge tube there should be a much larger number of
CH radicles in the positive rays than of either C or H. The
decomposition of the molecule of an elementary gas into
atoms is another problem which could be studied in this way,
and it would be very interesting to see whether the proportion
between the numbers of molecules and atoms of a gas in the
positive rays depends upon the nature of the gas. It would
seem from the curves given on pp. 124, 126, comparing
those for hydrogen and oxygen with that for CO, that the
proportions of molecules of undissociated CO in the positive
rays to the number of atoms of O and C, which are the
results of dissociation, is at any rate in some cases higher
than the proportion of molecules to atoms of oxygen when
the discharge passes through this gas, and thus that the

STRUCTURE OF MOLECULES 233

bonds between the different atoms in CO are stronger than
those between the atoms in O2.

The apparent absence of any influence of the valency of
an element on the amount of charge carried by its atom in
the positive rays has already been noticed. It may, to a
large extent, be due to the method by which the atoms in
the positive rays are obtained from the molecules in which
they occur. If we take the view that the atoms in the
molecule are bounded together by the attractions exerted by
electrons on their positive charges ; that, for example, the
molecule of hydrogen may be represented by the diagram
below, where A and B are positively charged atoms and C
and D electrons.

- C

A+ + B

-D

It is evident that a very effective way of decomposing the
molecule would be to neutralize the positive charge by
exposing it to a stream of electrons possessing sufficient
energy to enable them to approach the atoms close enough
to neutralize their charges. If this were the method by
which the atoms were detached from the molecule they
would, when set free, be uncharged whatever may have been,
the charge they possessed in the original molecule. Any
charge they might acquire subsequently would be due to
ionization by collisions, and would depend on other con-
siderations besides valency. Now the main seat of the
production of the particles in the positive rays is the negative
glow, a region swarming with electrons, and therefore one
in which the kind of dissociation we have been considering
would be especially likely to occur. It must be remembered^
too, that it requires much less work to dissociate a molecule
into uncharged atoms than into charged ions, so that an

234 RAYS OF POSITIVE ELECTRICITY

atom would be unlikely to retain in the free state the charge
it possessed in the molecule.

The type of dissociation which a priori would be most
likely to show the influence of valency is that due to colli-
sions when one of the atoms in the molecule is hit directly
by a particle of the positive rays. If the carbon atom, for
example, in marsh gas suffered a direct hit by a positive-
ray particle, it might acquire sufficient energy to escape from
the hydrogen atoms and from the electrons which bound
these to itself, it would then escape with four positive
charges. The best chance of getting in the positive particles
evidence of charges corresponding to the valency charges
would be under conditions in which the ionization by the
impact of positive rays becomes comparable with that of the
ionization by electrons in the negative glow. This would be
the case if we studied the nature of the particle produced
in a vessel B by positive rays coming from another vessel A,
the gases in A and B being different so as to prevent
confusion between the primary and secondary particles.

The positive rays thus seem to promise to furnish a
method of investigating the structure of the molecule, a
subject certainly of no less importance than that of the
structure of the atom.

PLATE I.

FIG. i.

FIG. 2.

FIG. 3.

FIG. 4.

PLATE II.

FIG. 2.

FIG. 3.

FIG. i.

FIG. 4.

PLATE III.

FIG. i.

FIG. 2.

FIG. 3.

FIG. 4.

PLATE IV.

FIG. i.

FIG. 4.

FIG. 3.

PLATE VI.

FIG. i.

FIG. 2.

FIG. 3.

FIG. 4.

PLATE VII.

FIG. i.

FIG. 2.

FIG. 3.

FIG. 4.

PLATE VIII.

FIG. i.

FIG. 2.

FIG. 3.

PLATE IX.

|:::

-80
-83

1-44

fe— 130
1— ,31

B1 K,

• — 20

•_«

£-37 «.

I Hg |-3?

:te~35

** 63 m,*

-«

— 52

INDEX

ABSORPTION of gases in discharge tubes,
178

of positive rays, 49, 51

Analysis, chemical, by positive rays, 179

Ancde rays, 134, 142

•--• analysis of, by photographic

method, 147, 225

Argon, unable to receive negative
charge, 76, 229

Aston, 9, 32, 36, 107, 116, 212, 213,
214, 217, 218, 220

and Lindeman, 213

and G. P. Thomson, 221

Atomic weight of beryllium, 217

of chlorine, 217

of nitrogen, 216

weights, determination of, by posi-
tive rays, 216

Atoms, arrangement of electrons in, 224

and molecules, relative brightness

of lines due, 67, 88

multiply charged, 77, 227

negatively charged, 27, 70

Austin and Holborn, 173

Baedeker, 133

Baerwald, 13

v. Bahv and Franck, 58

Beading on photographs of positive

rays, 63

Beryllium, atomic weight of, 217
Bibliography of Doppler effect, 169

of positive rays, 179

Bloch, L., 130

Bombardment by cathode rays, gases

given out by, 190

by positive rays, spectra due to, 169

Bonds, effect of double, on negative

charge, 76

•Carbon monoxide, positive rays in, 124
Cathode dark space, fall of potential in,
109, 113

rays, 12

spectra due to, 91

shadows on, 141

splattering of, 171

I Cathodes, hollow and double, 5

j Chemical action, effect on ionization, 130

: analysis by positive rays, 179

i Chlorine, atomic weight of, 217

! Consecutive electric and magnetic fields,

45> H7

Crookes, 9

Current carried by positive and nega-
tive particles respectively, 116

Dark space, distribution of potential in,
107, 109

boundary of, 115

Davis and Horton, 14

Dechend and Hammer, 134, 178

Dempster, 36, 40, 147, 221

Dewar, 212

Disintegration of metals by positive
rays, 171

by radiation, 176

j Doppler, bibliography of, 169

! effect, 93, 106, 148

! Double cathodes, 5, 138

Doubly charged molecules and atoms,
77, 227

Duane and Wendt, 202

Eisenmann, 113

Electrical methods of measuring posi-
tive rays, 120

Electric fields, method of consecutive,
117

Electrons, disposition of, in atom, 224
| Electrostatic deflection of positive rays,

elniy methods for determining, 21, 35,

36, 40, 120

Envelope of secondaries, 64

Fabry, 203

Fluorspar gives line for which mjc =

3'5> 203

Focus method for measuring e/rtt, 36
Force between positive charge and

electron, 224
Franck, 76
and v. Bahr, 58

235

236

RA YS OF POSITIVE ELECTRICITY

Franck and Hertz, 229

Fuchtbauer, 13

Fulcher, 91, 157, 169, 179

Gases condensed on glass surfaces, 207
Gehrcke and Reichenheim, 142 et seq.,

152

Glasson, 97

Glimme and Konisberger, 48

Goldschmidt and Kohlschuiter, 173

Goldsmith, 172

Goldstein, I, 5, 7, 142, 147

Goldstein's layer, 142

Gouy, 147

Granquist, 173

Gryllenskold, 169

H3, 191,

Hammer and Dechend, 134, 178

Helium, diatomic, ico

- given out by bombardment, 191

- positive rays in, 6

R. v. Helmholtz and Richarz, 176

Hermann, 155

Hertz and Franck, 229

Hexane, negatively charged particles in,

Holborn and Austin, 173
Horton and Davis, 14
Hydrogen and oxygen, positive rays in

mixture of, 126

- given off by bombardment, 191

- presence in discharge tube, 210

- second spectrum of, 154, 160

lonization by cathode rays, 90, 97

- by positive rays, 54, 56

- by radiation, 94
Isotopes, 215, 217
Iviglut-fluorspar from, source of 3'5

line, 203

Knipp, 36, 105

Kohlschiitter, 172, 173, 174, 177

- and Goidschmidt, 173

- and Muller, 173
Konigsberger and Glimme, 48

- and Kutschewski, 48, 50
Kunz, 7

Ladenburg and Rubens, 176
Langmuir, 209
Lenard and Wolff, 176
Lewis, 91

Lindernan and Aston, 213
Lithium chloride used to detect positive
rays, 15, 92

- isotope of, 221
Lunelund and Stark, 165

Magnetic deflection of positive rays, 16-

fields, use of consecutive, 45, 117

production of anode rays, 147

m\C) substance for which it is equal to-

3-5, 203; i '6 and 2-4, 206
McClelland, 58
Mendeleef's law, 224
Mercury atoms, diatomic, 189

effect of, on H3, 192

multiple charge on, So

Molecules and atoms, relative intensiiies-

of lines due to, 67, 88

doubly charged, 77, 227

negatively charged, 47, 71

Muller and Kohlschiitter, 173
Multiply charged atoms, 77, 227

Nebulium, 203

Negatively charged rays, 27, 70, 227

Neon, and its isotope, 212 et seq.

positive rays in, 6

Nickel carbonyl, dissociation of, 129
Nitrogen, atomic weight of, 216

absence of negatively charged

atoms, 226

Ohlon, 171

Orange, 7

Oxygen and hydrogen, positive rays in

mixture of, 126
compound H3O, 189,,

230
molecule with negative charge, 47,,

Parabolas on photographic plate, .21

Parabolic envelope of secondaries, 64

Paschen, 152

Pawlow, 58

Penetration of metals by positive rays,

172

Perforated cathodes, 9
Phosgene gas, positive rays in, 127
Phosphonium iodide a source of H3, 197
Phosphorescence produced by cathode

and positive rays, 3
Phosphorescent screens, 4
Photographic plates for positive rays, 4

measurement of, 34

Polarization of light from positive rays,

165

Polar molecules, 132
Positive rays, apparatus for studying,
25, 29, 35, 36, 40, 120

bibliography of, 179

disintegration of metals by, 171

electrical method of measuring, 1 20

loss and gain of charge by, 45, 49

INDEX

237

Positive penetration of metals by, 172
rays, used for chemical analysis,

179 et seq.
Potential, distribution of, in dark space,

107, 109

Primary lines, 60
tests of, 62

Radicles, negatively charged, 71
Reflection of positive rays, 163
Reichenheim, 166

and Gehrcke, 142, 152

Retrograde rays, 96, 134
Richarz and R. v. Helmholtz, 176
Rubens and Ladenburg, 176

Sandwich cathodes, 5
Saxen, 13
Schmidt, 179
Schumann plates, 5
Secondaries, 42, 59, 60, 67

envelope of, 64

Seeliger, 54

Shadow on cathodes, 141

Smith, O. H., 142

Prof., 129

Specific inductive capacity, 133
Spectra of multiply charged atoms., 164
Spluttering of cathode, 171
-Stark, 148, 152, 154, 156, 158, 163,
164, 1 66

Stark and Lunelund, 165
— — and Steubing, 150

and Wendt, 170, 172

Steubing and Stark, 150

Thomsen, 203

Thomson, G. P., 147, 160, 225

and Aston, 221

v. Traubenberg, 172

Ultra-violet light, disintegration by, 176

Valency, 224, 229

of charged atoms, 229

Vegard, 157, 164, 178

Velocity of positive rays, 21, ico

Villard, 135

Wehnelt, 14

cathodes, 35

liberate II3, 202

Wendt and Stark, 170, 172

and Duane, 202

Wien, 16, 22, 27, 48, 52, 61, 72, 103,

134, 166
Willemite, 4
Wilsar, 154, 169
Wolf and Lenard, 175
WUilner, 91

Zinc blende, 4

PRIMTSD IN GREAT BRITAIN P.Y
RICHARD CLAY & SONS, LIMITED,

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APR 1 2 1966

LD 21-100m-8,'34

THE UNIVERSITY OF CALIFORNIA LIBRARY

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