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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 

ix 



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 X 3 , 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 







_, 




u 

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 x y 
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 

dx 



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 xo. 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 

mV dx~~Jo e ' *' 
hence if y is the deflection when x = I 

mV = \' o {\* a eKd X }d X .. 

Integrating by parts we have 

/i n 

eUdx I xeHdx 
o Jo 



or writing A for 



e f 

o 



mv 



i8 



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 
xa 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 




N 



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 

PN 



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



DE 



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, ^ = v z -^ approximately, 
and hence 

*%-* 

t 

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 xl 



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) 

mv 



(2 



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 x t 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 



22 



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 A 2 /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. Wien 1 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. 

S 




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 io 7 . 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 



M 




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 P x P 2 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 io 4 , for that 
of band b> 5 X io 3 . 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 io 4 , 5 x io 3 , 
2*5 X io 3 , 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 io 4 , thus these negative 
particles are not cathode rays for which ejm is 17 X io 7 , 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 io 4 , that for 
* the other 5 X io 3 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 

o 

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 G A slides in a vertical box B 

made of thin metal ; this 



" -- "Tl"""^ 
) \ 




FIG. 16. 



at 



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 



34 



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 



35 



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 Knipp 1 
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., 



ASTON 3 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 

(p 2 = C 
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 S 1; S 2 , to produce the fine pencil 



ASTOATS FOCUS METHOD 



39 



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 J 2 , 5 cm. 
long and 2'8 mm. apart, which can be maintained at any 
required difference of potential. K x and K 2 are the diaphragms, 
Kj is fixed and K 2 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 

m 

7 



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 ejmv 2 . If the energy of the 
particles is due to the fall of the charge through a potential 
difference V 

%mv 2 = 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 b 1 ', 
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 N 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 ^ = A 2 = 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 A x and A 2 . 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, 

E 



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 
A x , A 2 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 A x , A 2 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~ 5 cm., 
and that of an atom of hydrogen through molecules of 
hydrogen about 2X io~ 5 cm. 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 ^ + A 2 (see p. 49) ; taking the values of Aj + A 2 found 
by Wien we find that at a pressure of yJ- F of a mm. 

AJ + A 2 = 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 A x + A 2 , 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 



57 



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 M 2 at rest 
the maximum amount of kinetic energy which can be 



communicated to M 2 is 



4M 1 M, 



.T 



where T is the kinetic energy of M x before the collision. If 
M! is the mass of a particle in the positive rays, M 2 that of 
an electron, Mj will be large compared with M 2 , 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 io 8 
cm./sec., a moving atom or molecule could not under the 
most favourable circumstances eject the electron if its 
velocity were less than io 8 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 io 8 cm./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 io 8 cm. /sec. ; 



SECONDARIES 



61 



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 I0 8 cm./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 



62 



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 +X K v 

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 H 2 . 
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 CO 2 . Then, if the pressure 
is not exceedingly low, we find secondaries corresponding to 
Hj and H 2 , none corresponding to C or to O or to CO 2 , 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 
CO 2 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 H 1 and H 2 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 H 2 and H 3 . 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 v 2 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% mv 2 

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 A 1? A 2 (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 io 8 and for the molecule about 1*3 X io 8 
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, CH 2 ; 
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 CH 4 , 
CO or CO 2 , 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 CO 2 

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 V x were the 

cathode fall in the dark space, V 2 that between the cathode 

and P, we should have concentration of the positive rays 

about the energies Vj and V 2 . 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 H 3 
into a charge molecule H 2 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 H 3 . 

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 CH 4 were 
to break up after passing through the cathode into CH 2 and 
H 2 there would on the line representing CH 2 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 m 2 , and if the dissociation of the molecules is 
due to collisions the number of molecules dissociated will 
also be proportional to m 2 . 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 n 2 , 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 n 2 , and the number split up pro- 
portional to m 2 . When the system is in a steady state these 
numbers must be equal, hence m 2 must be proportional to 2 , 
or m proportional to n. We know, however, that m is not 
proportional to n but to n 2 . 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 m 2 , 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 io 9 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 io 9 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 ^, A 2 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 P x 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 E x 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 



dx 

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



or 

since dVfdx vanishes when xo ; 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 = 



Q 



If Q 7 is finite we must have 



A 

or m= i. n=. 

3 

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 / 2 o"-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 V 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 V 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, M x and M 2 , there 
would be one line with a displacement (M x + M 2 ) JM 1 



CONSECUTIVE FIELDS 119 

times the normal displacement d and another with the 
displacement (M 1 + M 2 ) /M 2 times the normal. 

For example, if H 3 were to split up into H and H 2 then 
corresponding to the line H 3 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 H 2 , while the photograph without the electrostatic 
field had, after an exposure of a few minutes, shown lines 
corresponding to H, H 2 , C, O. 

A striking feature of the photograph with the electrostatic 
field was the change in the relative intensities of the H and 
H 2 lines ; with the field on H 2 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 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, COC1 2 ; 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 . 



30 



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, C1 2 , CC1, and COC1 2 . 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 COC1 2 , 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 COC1 2 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 H 2 and C1 2 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 io 9 . Thus, if every one of the col- 
lisions with the high-speed molecules resulted in ionization, 
6 X io 9 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 H 2 
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 H 2 O, NH 3 , 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 
Villard 2 and the author 3 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 



2V 




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 I0 7 ** X I0 3 



Li Cl. j;j x io 7 .9 x I0 3 

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

Na Cl. ^ X io 7 .^ x io 8 

Sr C1 2 . ro8 x io 7 '2i x IO 3 

L 



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 Steubing 1 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 io 7 cm. Hj3 = 1*26 x io 7 cm./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 v t 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 



r\ 



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 io 8 cm./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 
intensity 1 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 

v, 



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/ + n t 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 



) 

J 



N 



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 v there will be no light showing a Doppler effect corre- 
sponding to a velocity less than v 0t 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 v t 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. 


v. 


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 io 8 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 H 2 ,* He, N 2> O 2 
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. 

T2 



2' 4 
2-05 

i '45 



Gas in discharge 


Loss of weight 


tube. 


by spluttering. 


Hydrogen 


27 


Helium 


4 


Nitrogen 


2-05 


Oxygen 


47 


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 7 6 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 icr 10 , 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~ 6 c.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, CO 2 and N 2 O produce the same parabolas as 
also do CO and N 2 . 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 CO 2 and not N 2 O, and find that the parabola 
disappears, we conclude that it was due to CO 2 ; if it does 
not disappear it is not due to CO 2 , but to N 2 O 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\m t 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 x l = 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 V 2 . 
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 T 1? T 2 , T 3 
are the vertical displacements corresponding to particles with 
charge e and masses m lt m^ m$ 

- %-Vsj 



2and - 

1 3 



hence 



i , T^T! f i 1 1 

= =- + 3 * ]--= -= \ 
m 3 *Jini - 1 2~ x i W;;z 2 v m i* 



Since T 3 T x and T 2 T x are independent of the 
.position of the zero, any indeterminateness in that point will 
not affect the values of m z obtained by this equation. 

When the values of m lt m 2 , m 3 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^ x z are the values of x where the horizontal 
line cuts the three parabolas, 



or 



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. 



He 



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 

" 



+ 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 CO 2 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, CH 2 , CH 3 , 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, C 2 , and C 3 . 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, CH 2 , CH 3 already 
mentioned, parabolas corresponding to m/e=i? indicating 
in some cases the radicle OH, in others the molecule NH 3 , 
others corresponding to m/eiS, the water molecule, and 
others corresponding to m/e= 19, generally I think due to 
H 3 O. 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 CO 2 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 CO 2 
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, AgNO 3 , 
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 X 3 , 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 X 3 . Thus, for example, X 3 is well developed without 
any bombardment by cathode rays when the vapour of 
phosphonium iodide, PH 3 '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 X 8 is not a new substance, it must either consist of three 
hydrogen atoms with one charge and be represented by H 3 , 
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 X 3 . The 
view that X 3 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 X 3 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. CH 4 , CO 2 , CO, C 2 H 4 , C 2 H 2 , COC1 2 , 
CC1 4 , 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. 



X s 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 
X 3 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, Li 2 CO 3) KC1, the output of X 3 after this treat- 
ment is much smaller than it was before. In the other class, 
which includes KOH, LiCl, LiOH, CaCl 2 , 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 X 3 after 
repeated solution and evaporation shows, I think, that X 3 
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 X 3 
consists of hydrogen and is represented by the formula H 3 . 
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 H 3 . 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 X 3 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 X 3 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 X 3 , 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 X 3 , 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 X 3 , 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 X 3 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 X 3 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 X 3 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 
X 3 . I heated by an electric current a fine tantalum wire 
until it fused, and found that a considerable amount of X 3 
was given off. Some time ago I found that when the dis- 
charge from a Wehnelt cathode was sent through an exhausted 
tube X 3 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 X 3 . 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 X 3 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, X 3 
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 X 3 is H 3 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 X 3 is exceptionally large from 



THE 3-5 LINE 203 

certain minerals ; this is what would happen if X 3 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 X 3 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 X 3 He 
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 H 3 that the space between 
the 3*5 lines and both the H 3 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 H 3 . 



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. H 5 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 H 5 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 H 3 and H 5 . 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 H 3 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 273C 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 H 2 O is polar, while those of CO, 
CO 2 , N 2 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 IO 15 . 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 io 5 
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 H 3 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 CO 2 
with a double charge would give the line 22, but this cannot 
be its origin, as the CO 2 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 NeH 2 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. 



2i 4 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), 
CH 2 or N(i4), CH 3 (i5), CH 4 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 H 2 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~ 8 cm. 
in " 22 " the 20 and H 2 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. W T hen 
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 
H 2 C1 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 C1H 2 , 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 H 2 O 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. 




H 


I -008 


I 


1-008 


He 


3'99 


I 


4 


B 


I0'9 


2 


II, IO 


C 


I 2 '00 


I 


12 


N 


14-60 


I 


H 


O 


1 6-00 


I 


16 


F 


19-00 


I 


19 


Ne 


20'2 


2 


20,22 (21) 


Si 


28-3 


2 


28, 29, (30) 


P 


31-04 


I 


31 


s 


32-06 


I 


32 


Cl 


35 '46 


2 


35, 37, (39) 


Ar 


39'9 


(2) 


40 (36) 


As 


74-96 


I 


75 


Br 


79-92 


2 


79,81 


Kr 


82-92 


6 


84, 86, 82, 83, 80, 78 


I 


126-92 


I 


127 


Xe 


130-2 


5 


(129, 132, 131, 134, 136) 


Hg 


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 : 

e 2 
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, H 2 O, H 3 N, H 4 C, 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, NH 2 , CH, CH 2 , 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 CH 2 , 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 O 2 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 O 2 
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 H 2 C1 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 H 3 O. 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 C 2 H 2 C1 2 , if the molecule is represented by 

H /H 

/>C = CC then when it is split up in the discharge 
CK X C1 

vessel we should expect to get the radicle CHC1 in much 
larger quantities than either CH 2 or CC1 2 . If, however, 

H \ / CI 

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

R/ X C1 

on the other hand expect the combinations CH 2 , CC1 2 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 COC1 2 , 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 COC1 2 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 R lf R 2 , R 3 , R 4 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 R 4 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 O 2 . 

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. 



|::: 



X 



| 



:H 



| V 



-80 
-83 



1-44 



fe 130 
1 ,31 



B 1 K, 



20 



_ 

m 

- 37 . 



I Hg |-3? 

: te~35 

** 63 m,* 



- 



I 



52 



" 






P 





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, 

19 

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 



2 3 6 



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 



H 3 , 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, 

71 

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 H 3 , 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 H 3 O, 189,, 

230 
molecule with negative charge, 47,, 

71 

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 H 3 , 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, 3 6 , 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 II 3 , 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 



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