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Published in connexion with the Science 
Exhibit arranged by a Committee of the Royal 
Society in the Pavilion of His Majesty's Govern- 
ment at the British Empire Exhibition, 1925. 

Sold by A. and F. Denny, Ltd., 163A, Strand, 
London, W.C.2. 


The armorial bearings granted to the Royal Society by Charles II 
are described in the second Charter in the following words : 

" These following blazons of honour, that is to say, in the dexter 
corner of a silver shield our three Lions of England, and for crest a helm 
adorned with a crown studded with florets, surmounted by an eagle 
of proper colour holding in one foot a shield charged with our lions ; 
Supporters, two white hounds gorged with crowns ; to be borne, ex- 
hibited, and possessed for ever " (Translation.) 





Radiation. By SIR OLIVER LODGE, F.E.S. 1 

The Electron. By SIR JOSEPH THOMSON, O.M., F.R.S. ... 21 

X-Rays and Crystal Structure. By SIR WILLIAM BRAGG, 

K.B.E., F.R.S 24 

Electricity and Matter. By SIR ERNEST RUTHERFORD, 

O.M., F.R.S 30 

Atoms and Isotopes. By DR. F. W. ASTON, F.R.S. ... 37 

Verification of the Theory of Relativity. By SIR FRANK 

DYSON, F.R.S 48 

The Interior of a Star. By PROF. A. S. EDDINGTON, F.R.S. 52 

The Origins of Wireless. By SIR RICHARD GLAZEBROOK, 

K.C.B., F.R.S 62 

Thermionic Valves. By PROF. J. A. FLEMING, F.R.S. ... 67 

The Origin of Spectra. By PROF. A. FOWLER, F.R.S. ... 72 

The Circulation of the Atmosphere. By SIR NAPIER 

SHAW, F.R.S 80 

The Water in the Atmosphere. By DR. G. C. SIMPSON, 

F.R.S 88 

Radiation and the Atmosphere. By F. J. W. WHIPPLE ... 07 

Atmospheric Electricity. By DR. C. CHREE, F.R.S. ... 100 

Darwinism. By C. TATE REGAN, F.R.S 112 

Insect Mimicry and the Darwinian Theory of Natural 

Selection. By PROF. E. B. POULTON, F.R.S 117 

Life in the Sea. By DR. E. J. ALLEN, F.R.S 124 


F.R.S 128 

The Human Brain. By PROF. G. ELLIOT SMITH, F.R.S.... 135 

The Circulation of the Blood. By PROF. E. H. STARLING, 

F.R.S. ... 139 



Muscular Work. By PROFS. A. V. HILL, F.R.S., AND 


The Biological Action of Light. By PROF. D. T. HARRIS. . . 1 17 

The Origin of the Seed-Plants. By DR. D. II. SCOTT, 

F.R.S 150 


Introduction ... ... ... Mil 

Chart showing Range of Electro-magnetic Wave-* ... K>2 

The Atom Ki3 

Gamma Rays ... ... ... ... ... ... 173 

X-Rays 174 

Ultra- Violet Rays 17<> 

Visible Rays 178 

Infra-Red Rays I'M 

Short Hertzian Waves M>2 

Wireless Waves ... ... ... ... ... ... M>3 

Slow Oscillations 201 

Geophysics 203 

Zoology 212 

Botany 217 

Physiology... ... ... ... ... ... ... 223 

Scientific Films 230 

Index to Exhibitors 231 

Index to Exhibiting Firms 232 



TIE book constitutes a second edition of the Handbook to the 
Royal Society's exhibit in the Pavilion of His Majesty's 
Government at the British Km pi re Exhibition in 1924. 
Certain of the articles included in the earlier volume have been 
omitted, while others related more closely to the exhibits in their 
altered arrangement have been added. The original articles 
have been revised by the authors. 

The first section contains the revised series of articles by 
well-known authors, and is intended to give some indication of 
the state of Science at the time of the Exhibition ; the second 
section is the Handbook to the exhibits, numbered to correspond 
with cards displayed on the benches and in the cases. 

The .British Empire Exhibition Committee of the Royal 
Society desire to express their indebtedness to the authors who 
have contributed articles and to the various bodies and persons 
who have kindly given permission to incorporate in these articles 
material appearing in their publications. 



Jt is now generally accepted that the material universe 
is composed of atoms, and most people now know (though no 
one knew a quarter of a century ago) that the atoms are built 
up of electric units, and that the different atoms, the atoms of 
the chemical elements, only differ in the number and arrange- 
ment of the electric particles. It is also known that these 
electric particles are of two kinds, and two only : the positive 
kind, which is now usually called a proton ; and the negative 
kind, which for some thirty years has been called an electron. 
Some idea of the size of these particles has also been obtained. 
They are the smallest things at present known ; and they are 
the basis of all electrical manifestation. In fact, they constitute 
what we call electricity. 

The first and most obvious property of these particles is 
difference of sign, so that they tend to attract and to neutralise 
each other, and thus in combination to form a neutral (that 
is a non-electrified) atom. One of each constitutes the simplest 
atom possible, which is the familiar atom of hydrogen. It 
may be supposed that two of each would constitute the next 
atom, the atom of helium ; and so on up to 92, constituting 
ninety-two chemical elements, which differ in no other way 
than in the number and distribution of these ultimate par- 
ticles. The word " ultimate " may be premature, and may 
have to be modified ; for at one time \v$ used to speak of 
" the ultimate atom " meaning the atom of matter. We 
now know that the atom of matter is a complex thing, built 
.up of two simple units, which separately are the positive and 


the negative electric charges. The atom of matter has a 
constitution therefore a structure which is being ascertained. 
Whether the electric units have a structure, and what that 
structure is, is .at present unknown. We must deal with them 
as if they were the ultimate units at any rate, for a time. 

The way in which the units are arranged inside the atom 
may be the subject of legitimate difference of opinion. But 
that electricity consists of these two units, in enormous 
numbers, and that groupings of them constitute the atoms, 
no one doubts. The atoms arrange themselves in molecules 
according to certain laws, which are studied in chemistry : 
and thus all material bodies are constructed. In other words, 
everything we see or handle, and all the worlds in space, are 
composed of two kinds of electricity that is, of two apparently 
fundamental things with opposite properties and at first 
sight of nothing else. 

But the two kinds of electricity attract each other ; each 
unit is very highly electrified, and the attraction between them 
is very forcible. The attraction of an electron for a proton, 
say, 4 diameters apart is comparable to the weight of a couple 
of pounds. The first question which arises is, what keeps 
them from clashing into each other ? We know of another 
set of bodies which attract each other but do not clash, 
namely, the planets and satellites in the heavens. The earth 
does not fall into the sun, though it is attracted by a tremendous 
force, comparable to a thousand million million tons ; the 
moon does not fall upon the earth. What keeps them apart 
is their motion. Pull or attraction is satisfied when the smaller 
body revolves round the other, in a definite orbit at a definite 
pace ; and the rate of motion is adapted to the orbit, or the 
orbit is adapted to the rate of motion. If the moon were 
nearer it would have to revolve faster : the farther a planet 
is off, the slower it goes. 

Something the same sort seems to be happening inside the 
atom : the attracting particles may revolve round each other 
and thus keep at a fixed distance apart. That is what by 
most people is believed to happen : though there is another 
way of keeping bodies apart, analogous to springs or elastic 
cushions ; and this more static method of contemplation is 
favoured by some, though springs require much more explana- 
tion and are less satisfactory as a separating cause, and certainly 
less simple, than iftere motion. Moreover, the analogy of the 
heavenly bodies is not to be despised ; and it is tempting to 
apply the laws of astronomy to the electric particles inside the 


Indeed, it turns out that the analogy of the solar system 
is remarkably borne out by facts. The proton and the electron, 
though electrically equal and opposite, are not equal in all 
respects : the proton is more than 1,800 times as massive 
or heavy as an electron. The earth is 80 times as massive or 
heavy as the moon. Hence, just as the earth stays fairly still 
while the moon revolves round it, so in the atom of hydrogen 
the proton can stay fairly still, as a centre, while the electron 
revolves round it. 

In the atoms of the other and heavier chemical atoms this 
is further accentuated. The weight of the nucleus has long 
been known under the name " atomic weight " and this atomic 
weight is many thousands of times greater, even up to nearly 
half a million times greater, than the weight of any of the 
satellite electrons. Consequently, the analogy of a nearly 
stationary central body like the sun, and a number of much 
lighter planets circulating round it, is a very natural and 
satisfactory one. This is the theory of the constitution of 
matter which at present holds the field or rather it is the 
outline and first beginning of that theory. We may well pause 
to express wonder and admiration at the simplicity of the 
material elements of the cosmos, and at the elaboration of all 
the beauty and majesty and complexity of the material 
universe out of two fundamental units. 

But is there nothing else ? Still keeping strictly to physics 
(and not touching on the other things which we know to 
exist but which transcend physics, such as life and mind and 
consciousness), is there no third thing in the physical scheme 
other than the two electrical units ? If not, it may well be 
wondered, not so much how the attracting units keep apart, as 
how it is that they attract each other at all : and still 
more how it is that we have learnt anything about either them 
or their attraction. 

There certainly is a third thing ; and, without the slightest 
controversy, that third thing is radiation radiation and all 
that it implies. The most usual view of radiation is that it 
consists of waves in a connecting medium, commonly called 
" the ether," and that this same unique and only ether is 
responsible for gravitative attraction, for electric and magnetic 
attractions, and for cohesion : that is, for all the forces which 
tend to bring bodies together, while motion tends to keep them 
apart. This, however, is a fact which ma/ be expressed by 
different people in different ways : there are some who do not 
care to use the term " ether," and are not sure of the nature of 
the waves, but no one can doubt the fact of radiation. 


Again we make appeal to the heavenly bodies. All the 
large bodies are furiously radiating ; they are what we call 
" hot/' and it is through their radiation that we are aware 
of their existence. They are extruding or leaking energy at 
a tremendous rate, and in the eye we have a receiving instru- 
ment which responds to that energy ; so that althoiigh some 
of these hot bodies or suns are many million of million miles 
away, they still appeal to us, and give us information about 
themselves, through the fraction of their radiation which falls 
upon our optical receiving instruments. This is a kind of 
wireless telegraphy to which the human race has always been 

The very fact that bodies separated in space attract each 
other shows that there must be some intervening mechanism 
or substance to account for what are called their fields of force, 
and this is clinched by finding that the intervening space is 
full of radiation. It is through and by means of radiation 
that we are acquainted with all the other worlds in the 
universe : it is by the same channel that we are aware of all the 
objects in the neighbourhood -all that we do not touch : it 
is by means of radiation that we see the landscape and all the 
beauties of Nature. Our sense of music depends upon the 
vibrations of the air ; while everyone agrees that the sense 
of colour, and of vision generally, is wholly dependent upon 
radiation. We apprehend matter through our sense of touch, 
our muscular sense : we apprehend radiation through our 
optical sense. Thus, the physical universe contains not only 
protons and electrons but also a connecting link, which for 
brevity is best called the ether ; while radiation is most con- 
veniently thought of as a perturbation, or vibration or tremor 
or quiver, in the ether, excited somehow by a corresponding 
movement in the protons or electrons themselves. 


Returning once more to the larger heavenly bodies the 
sun and other stars it is obvious that they are radiating ; but 
it is not obvious why they are pouring out their energy like 
that, nor is it obvious whence they derive that energy. So 
long as we study pure astronomy on the large scale there is 110 
explanation. It ftiight be that occasionally there was a clash in 
the heavens two bodies colliding, and that this would account 
for some of the energy. Such collision may indeed, possibly, 
account for the occasional appearance of a newjstar that 


is, for an extra and unexpected splash of radiation. But 
collisions are infrequent, and ideas of that sort are quite 
insufficient to explain the steady glow of the sun and the other 
stars. There must be some source of energy in the units of 
which they are composed that is, in the atoms. We must 
return to the atom and see if we can find it. 

At first sight the idea of the atom that we have so far 
formulated contains no clue. A central nucleus with electrons 
revolving round it appears as a simple, quiescent, regular 
system, from which radiation is no more to be expected than 
from the earth and moon. If you want to excite ripples on a 
calm sheet of water you must make a splash -say, by throwing 
a stone into it ; the ripples will then spread out from the 
centre of disturbance. If you want to make a bell vibrate, 
it is no use swinging it gently; you must hit it. To excite 
radiation, some kind of sudden motion, something analogous 
to a blow or a collision or a fall, is necessary something like 
a projectile striking a target, some sudden or violent disturb- 
ance. Otherwise, things will go on placidly, like the steady 
motion of the planets, or like well-oiled machinery. If the 
electrons in the atoms kept to their respective orbits there 
would be no radiation. As a matter of fact, atoms do radiate 
when treated vigorously ; in a vacuum tube it is possible to 
fling electrons against a target, arid they thus produce X-rays. 
That is the simplest way of producing radiation the way that 
is best understood and it gave us a hint. Prof. N. Bohr, of 
Copenhagen, had the genius to surmise that although a hydrogen 
atom consisted of a single electron revolving in an orbit round 
a proton, yet that there was more than one orbit in which it 
might revolve, and that occasionally it dropped from one 
possible orbit to another. This would be regarded as a sudden 
unexplained catastrophic movement, analogous, perhaps, to 
the striking of an electron against a target, except that there 
was no target. Hence the process is not quite easy to imagine, 
and there is no analogy to it in the heavenly bodies. The 
planet Mars does not suddenly drop to the orbit of the earth. 
Nor have we any reason to suppose that only a selection of 
planetary orbits are possible. If there is any reason for 
Bode's law it is unknown. There is something that goes on 
in the atom for which we have no large scale analogy ; but if 
only a selection of orbits round an atomic nucleus is possible, 
and if a drop from one possible orbit to another did occur, 
there would certainly be radiation of energy, because the drop 
would generate a surplus of energy, which would have to be 
emitted somehow. 


Assuming that atomic radiation is thus generated, the 
kind of radiation could be calculated. Given a regular suc- 
cession of possible orbits, a certain kind of radiation would be 
produced by dropping from any one to any other. A small 
drop would give comparatively long-wave or low frequency 
radiation : a big drop into a region of high speed would give 
high-frequency short-wave radiation. Each drop would give 
a definite wave-length, and these wave-lengths could be 
analysed by the spectroscope that is, by the lines in the 
spectrum ; for, in the spectrum, each line, or each position, 
corresponds to a definite wave-length. Bohr made the 
necessary calculation. The actual radiation was analysed 
and was found exactly to correspond with Bohr's theory. 
Moreover, the theory enabled other wave-lengths to be pre- 
dicted, and these were looked for, and in due course found. 
So far as the atom of hydrogen is concerned, the dropping 
from one of the alternative orbits to another accounts for every 
line and every series of lines which hydrogen is able to emit ; 
and not only accounts for them roughly or approximately 
but also with accurate and, indeed, astronomical precision. 
Nor are only the relative positions of the lines given ; the abso- 
lute wave-length can be calculated when we know the elec- 
trical attraction between the electron and the proton. 

Since that great discovery the theory has been applied 
to other atoms, and, although naturally it becomes more 
complex, the growing completeness with which most of the 
phenomena are explained has caused it to be almost universally 
accepted. The cause of radiation is not so fully understood 
as we hope to understand it in the next ten or twenty years ; 
but it is undoubtedly due to sudden movements sudden 
readjustments of the constituent electrons in an atom. 

Thus the physical universe consists not only of electrons 
and protons in regular grouping and steady orbital movement, 
but also in sudden changes or rearrangements of that 
movement, and consequent excitation or perturbation of the 
universal connecting medium in which the movements occur. 

The physical universe consists, therefore, of three things, 
however they be constituted and further explained : 

(1) The positive electric charge. 

(2) The negative electric charge. 

(3) The radiatign due to sudden movements and rearrange- 

ments among those charged particles.' 

It is by this third or apparently supplementary, subsidiary 
or accidental, consequence of the spasmodic behaviour of the 


other two that we know anything about them and about the 
other worlds in space which they constitute. The radiation 
of the sun and stars is explicable and has to be explained 
in terms of these atomic convulsions. 

The atoms and worlds are like each other in many respects. 
Looking at the midnight sky, we see discontinuity, portions of 
matter scattered about with large spaces between : looking 
with the mind's eye at the atom, we see the same thing. All 
the objects we touch and handle are constructed after some- 
what the same fashion as the midnight sky ; except that 
whereas the astronomical bodies are controlled wholly by 
gravitation, and their large scale motion gives them no radiating 
power of their own, the atoms are controlled mainly by 
electrical forces. Their particles are in close touch with the 
ether, and they are subject to perturbations, which in a more 
or less known and explicable manner disturb the medium, 
as a struck bell disturbs the air, so that they emit the radiation 
that we call light. The astronomy of the atom is like the 
astronomy of the heavens, but is more fundamental and 
contains more surprising facts. 

The simplicity with which results are secured in Nature 
is marvellous. The rotation of the earth, with its atmosphere, 
gives us day and night and the beauties of dawn and sunset. 
The earth's revolution round the sun gives iis the year, and a 
slight tilt in the earth's axis of rotation gives us all the beauty 
of the seasons. So, also, the fact that in an atom there is more 
than one possible orbit for each electron, and that occasionally 
electrons are able to jump from one to another, gives us the 
whole of what we study under the immense science of radiation, 
and through our sense of vision gives us colour, luminosity, 
apprehension of distance, and perception of the multiplicity 
of worlds far away in the depths of space. 


Now let us begin to consider in detail what we have learnt 
about this great branch of science. A certain kind of ether 
wave has been employed by the human race, and animals 
too, from time immemorial, since in the eye we possess a 
sensitive receiving instrument which has enabled us to detect 
ether waves of extremely short wave-length ; but only within 
the last century have we known that the physical basis of light 
was a succession of waves in the ether, of a length which could 
be measured, and that the weaMi and variety of colours 


were due to different wave-lengths or frequencies of vibration. 
It is well known that waves which produce the sensation of 
red are the longest of those that can be called light, that those 
which stimulate the sense of violet are the shortest, and that 
the waves exciting the sense of green are intermediate. Not 
that the waves have any colour attached to them ; colour 
seems to be wholly an interpretation of the mind. Physically 
there is nothing but different frequencies of vibration, over a 
definite range of wave-length. We can analyse any beam 
of light into its constituents ; for when the waves are bent 
out of their course by a prism, the different wave-lengths are 
differently bent, and accordingly are sorted out in regular 
gradation, in what is called a spectrum " continuous " if 
waves of all lengths are present, a " discontinuous " or " line 
spectrum " if there is only a clearly marked and definite selec- 
tion of wave-lengths to be separated out. 

The visible spectrum is but a small portion of the whole : 
beyond its violet end is a great range of still more rapid vibra- 
tions which are called " ultra-violet " ; and beyond that 
again come the X-rays emitted by vacuum-tubes, and the 
gamma-rays emitted by radium. All these affect certain 
chemicals and therefore are able to be photographed. The 
ultra-violet spectrum extends a very long way. At the oppo- 
site or infra-red end of the spectrum arc waves which do not 
excite the sense of sight, and cannot easily be photographed, 
but they are able to generate heat when their energy is absorbed 
by matter. By these waves the earth is kept warm. Far 
below them again we have the waves discovered by Hertz, 
which may vary in length from a few centimetres to several 
miles : these are used in radio-telegraphy. There appears 
to be no limit to either the length or the smalmess of ether 
waves ; and the remarkable thing is that they all travel at 
precisely the same pace. We have every reason to believe 
that the wave- velocity in free ether that is, in space empty 
of matter is always identically the same, namely, 300,000 
kilometres or 186,000 miles a second. 

Matter only obstructs ether waves : it may absorb some 
and transmit others, and the selective absorption thus exer- 
cised depends on the nature of the matter. Thus, a line 
spectrum may consist either of bright lines, when only a selec- 
tion of waves is emitted ; or it may consist of dark lines 
when a selection irom an otherwise continuous series of waves 
is absorbed, by passing through a partially transparent 
medium. This is called an " absorption spectrum," and all 
matter is partially absorbent. 


A transparent laminated structure is often able to reflect 
a little light from every one of its concealed internal surfaces 
or strata : and if the strata are equidistant, one below another 
with perfect regularity, then, in the light reflected by each, 
one single wave-length (corresponding to the distance of the 
layers slantingly apart) can be reinforced by co-operation ; 
thus giving a remarkably pure colour at any given angle of 
incidence, and a different pure colour at every other angle. 
To this is due the variegated and changing colour of opals 
and of some crystals like chlorate of potash. Any thin plate 
like a film of oil or a soap-bubble can give colours, by 
reflection from its two surfaces, but a series of slightly reflect- 
ing layers, regularly arranged, can give a much purer and more 
beautiful set of colours. Insects' wings are well known to 
exhibit this iridescent effect. 

The co-operation of small fractions of waves reflected from 
a regular series accounts for many important phenomena. 
It can be lightly illustrated by the sound or echo from equally 
spaced railings : the echo of a tap on the pavement can be 
a sort of whistle. In the case of light this same effect has 
important consequences. A spectrum can be formed, not 
only by bending rays of light, but also by reflecting them from 
a discontinuous reflector like a grating or finely ruled or 
laminated surfaces ; and by using the ultra-fine natural 
molecular discontinuities in a crystal, it has been possible to 
form a spectrum even of X-rays. By this means an extra- 
ordinary amount has been ascertained about the arrangement 
of atoms in a crystal, and even about the structure of the 
atoms themselves. A recent illuminating, though elementary, 
book by Sir William -Bragg entitled, " On the Nature of Things/' 
gives an admirably clear exposition of the results of this refined 
analytical process. 


Emission and absorption are mainly atomic properties ; 
and an atom is able to absorb much the same radiation as it 
can emit. The real emitter is now known to be the electric 
charges of which the atom is composed. Emission takes 
place whenever the constitution of an atom is shocked : it 
may be shocked by collision or external impact, or it may 
experience a sort of internal catastrophe like an earthquake 
subsidence. Whenever an electron drops nearer the nucleus 
of an atom, radiation is emitted. When radiation is absorbed, 
the electrons are jerked back again, or sometimes jerked out 
(B 31-2285)Q B 


of the atom altogether-- --a process which is called " ionisa- 
ticm." The ionising })ower of a beam of light depends on its 
wave-length : and the shorter the wave the more effective 
it is in the ionising process. We are beginning to think that 
photographic effects are accomplished primarily by the 
ionising power of the radiation : and it may be the ionising 
power of visible light which excites the retina of the eye. 

Before Clerk Maxwell we did not know what light waves 
consisted of : we now know that every electric' oscillation 
emits some waves, but that to get waves of anv power the 
oscillations must be very rapid. A discharging capacity, 
such as a leyden-jar, can easily give vibrations at the rate of 
a million a second, and therefore can emit waves ^00 metres 

All ether waves can be polarized -that is, made to vibrate 
in a definite fashion. A vertical aerial will generate electrical 
vibrations in a vertical plane, the magnetic vibrations being 
at right-angles or horizontal. Such radiation is said to be 
polarized. Tf we had a large number of radiators inclined at 
all angles, the radiation would not be polarized, but would 
be analogous to common light. By the properties of crystals, 
by reflection, and by other methods, if is possible to restore 
regularity to common light ; either separating the* horizontal 
and vertical components, and sending them along different 
paths, or else transmitting one and suppressing the other. 
The polarization of light can thus be illustrated, not only by 
Hertz's waves, but quite easily by visible light, and also 
(though not so easily) by X-rays. 

When two sets of similar waves are superposed, the energy 
is apt to be distributed in a regular pattern, showing places of 
extra density and of diminished or zero intensity. These 
patterns are spoken of as " interference bands "' or 4k inter- 
ference effects." There is no destruction of energy, but only 
a redistribution ; so that a receiving screen is bright and dark 
alternately, the bright places being extra bright to compensate 
for the darkness. A similar phenomenon in sound is known 
as " beats." , |p 

When light waves curl round an obstacle, or penetrate the 
meshes of a canvas or grid, somewhat similar effects are pro- 
duced, which are said to be due to diffraction. For these 
effects to be well marked, the obstacles have to be comparable 
to the wave-length in size. If the obstacles are big they only 
cast shadows ; but if the obstacle has a sharp clear edge, like 
a knife-blade, and if the source of light is a point or thin 
wire, and not a luminous surface, the shadow will be fringed 


with light and dark bands, differing in breadth according to 
the size of the obstacle compared with the wave-length, and 
therefore showing colours when white light is employed. 
A series of narrow obstacles like a wire grating can cast an 
extraordinarily variegated and rather beautifully patterned 
shadow, exhibiting the effects of diffraction in a market I way. 
Some natural objects which are ribbed in a regular manner, 
like some shells, can imitate the effect of gratings : and the 
familiar colours of these objects are due to diffraction, or the 
curling of waves round small or narrow obstacles which either 
obstruct or reflect them. 

Thus the phenomena familiar in optics are : - 

(1) RcJlwtioHi as when waves rebound from an opaque 


(2) Refraction* as when they encounter a partially trans- 

parent obstacle and have their direction of advance 
changed. Dispersion or spectrum analysis follows, 
since the different waves are bent differently. 
The whole theory of optical instruments, such as 
telescopes and microscopes and cameras, fall 
under these two heads. 

(3) Diffraction occurs when the waves encounter a sharp 

edge or a regular series of obstacles, and curl round 

(1) Polarization is the term applied when the vibrations 
are regularised, so that the oscillations occur 
either in straight lines or circles or ellipses, and 
continue the same without irregularity. 

(5) Absorption, is experienced when wave energy is 
quenched and converted into the agitated mole- 
cular motion we call heat. 

(()) lonisalion. is a remarkable phenomenon observed 
when the waves partially discompose or interfere 
with the structure of an atom, a subject which is 
also known as photo-electricity. 

(7) Interference, or conflict of waves, occurs when a beam 
of light is separated into two parts, and reunited : 
or when radiation travels by two different paths to 
the same spot, the paths being of unequal length. 
Interference of waves that tfave travelled by 
different paths is considered by some wireless 
operators to be responsible for the curious effect 
they stigmatise as " fading." 

(B 34/2285)Q B 2 


All these properties can be illustrated by waves of every 
length, though very different kinds of apparatus have to be 
employed ; just as in engineering, different apparatus has 
to be used for dealing with, say, grains of wheat or shot on 
one hand, and girders, bridges, and mountain masses on the 



Radio-active substances not only emit waves but also 
particles electrified particles. Such particles are also 
emitted from an electrified negative surface in a vacuum, 
when they are called " cathode rays/' also from some illu- 
minated surfaces, when they are called kt photo-electricity,' ' 
and from heated bodies, when they are called 4 ' thermionic 
emission." A continuous supply of electrons can be obtained 
by keeping a hot wire supplied with negative electricity : 
these are the electrons made use of in vacuum valves, con- 
stituting an extremely sensitive and rectifying relay-- 4 ' recti- 
fying " because the freed particles inside the bulb only convey 
negative electricity, not positive ; and very perfect as a relay 
because of the extreme mobility and docility of the highly 
charged and exceedingly light particles. 

About every one of the subjects thus mentioned in this 
cursory summary, an immense amount might be said. Radia- 
tion is a vast subject, covering nearly all the connexion between 
ether and matter : it is impossible to do justice to it in a general 
introduction. One can only take a bird's-eye view of the 
territory, and get people to realise what a wealth of information 
has been obtained by the explorers who for the last hundred 
years have examined and erected beautiful structures upon 
what had previously been an unknown continent. 

Why is so much importance attached to radiation ? 
Because it is the best-known and longest-studied link between 
matter and ether, and involves the only property we are 
acquainted with that affects the unmodified great mass of ether 
alone. Electricity and magnetism are associated with the 
modifications or singularities called electrons : most phenomena 
are, connected still more directly with matter. Radiation, 
however, though oxcited by an accelerated electron, is after- 
wards let loose in the ether of space, and travels as a definite 
thing at a measurable and constant pace a pace independent 
of everything so long as the ether is free, unmodified, and 


unloaded by matter. Hence radiation lias much to teach us, 
and we have much to learn concerning its nature. 

A strange thing is that radiation is showing signs of be- 
coming atomic or discontinuous. The corpuscular theory of 
radiation is by no means so dead as we thought it was. Some 
radiation is certainly corpuscular ; and even the ethereal kind 
shows indications, which may be and probably are misleading, 
that it is spotty, or locally concentrated into points, as if the 
wave-front consisted of detached specks or patches, thus 
suggesting that the ether may be fibrous in structure, and that 
a wave runs along line of electric force, as the genius of 
Faraday surmised might be possible. A recent modification 
of that view is that a wave- front mav be accompanied by 
concentrated vorte c rings or other projectiles -somewhat as 
the spreading-out jet from a iirc engine, when used for 
dispersing a crowd, might be accompanied by solid pellets 
capable of hurting individuals, instead of only administering 
a broadcast wetting. 

A speckled or discontinuous character for a wave-front 
must be regarded as highly improbable : the remarkable thing 
is that any facts should haw suggested such an. idea. What 
seems quite certain is that the characteristic or specific radia- 
tion appropriate to each kind of atom is always omitted or 
absorbed in jerks, not continuously- --the structure of the atom 
being such as to forbid its interaction with the ether except 
as the result, or at least as the accompaniment, of an internal 
convulsion. A definite experiment has proved that the smooth 
motion of matter as a whole has 110 grip on the ether. There 
are only a certain number of convulsions possible among the 
electronic constitutents of an atom, and each of them is 
associated with a certain definite frequency of ether waves, 
which if of the right kind are then freely either emitted or 

Apart from sudden jerks or internal convulsions, an atom 
behaves like a rigid body, and is unable either to impart or 
receive energy, or to interact with the ether, by mere motion 
to and fro. This kind of molecular motion constitutes tem- 
perature, and radiation is only emitted by a hot body when 
atoms are jostled together as must occasionally happen 
even at low temperatures in accordance with the laws of 
probability. Such indirect or temperature radiation is most 
perfectly exhibited by assemblages of atcfrns to which the 
term " black " can be applied : it contains waves of all lengths, 
though in very varying proportions, the most prevalent 


wave-length depending on the absolute temperature of the 
body, and the total amount of radiation being proportional 
to the fourth power of that temperature. By analysing the 
distribution of energy in a continuous spectrum of this kind 
the temperature of the source can be inferred ; and it is in this 
way that the temperature of the sun has been ascertained. 
Its most prevalent wave-length comes in the yellow of the 
visible spectrum ; whereas for bodies at lower or furnace 
temperatures the maximum is down in the infra-red. 

AVhatever be the truth in this matter, a discussion on 
radiation will continue for a long time, and the outcome 
cannot fail to yield a much closer insight into the connexion 
between ether and matter a problem of the highest physical 
and philosophic interest, which may have consequences of 
the utmost importance to humanity. 


The radiation we have hitherto mainly dealt with has been 
the radiation spontaneously emitted by atoms of matter ; 
some of it corpuscular, like alpha rays and beta rays, which 
consist of material particles shot out with great velocity, 
but most of it consisting of ether waves, likewise emitted 
spontaneously by atoms when they are jostled or flung against 
each other or otherwise violently perturbed. None of this 
radiation can really be produced artificially : it all depends 
on the properties of the atoms themselves. All we can do 
is to subject them to such conditions that they can exercise 
their powers which for the most part is done by utilising 
the energy of their molecular or chemical combinations, 
whereby they are thrown into the random agitation that we 
call heat or temperature ; that is, by making them move 
vigorously among themselves, so as to bring about the necessary 
collisions. Even this process, however, fails to excite or 
increase the corpuscular radiation from radio-active substances. 
That emerges spontaneously from sufficiently complex atoms 
at its own time and rate, and seems at present not to be subject 
to our control ; for the process is not hastened by thermal 
agitation, nor is it slowed down by extreme cold. 

The nearest approach to direct and purposeful production 
of short-wave radiation is achieved by the cathode rays in 
a vacuum-tube, which can be excited electrically by familiar 


means. For when a current is driven through a vacuum- 
tube from a source at high potential, high-speed electrons 
are the carriers of the current ; and when these are suddenly 
stopped by an obstacle or target in their path, they generate 
by their stoppage the high-frequency or short-wave radiation 
called X-rays. 

Some years before X-rays were discovered, however, 
Hertz found out how to generate long ether waves on somewhat 
the same plan, by utilising the crowd of electrons which are 
loose in a metallic conductor. When a current is established 
in a straight wire of finite length, its loose electrons are set 
moving, somewhat after the fashion of the cathode rays in 
a vacuum ; and when they reach the ends of the wire they 
cannot proceed farther, but are reflected, surging back again, 
what is left of their energy being reflected at the other end of 
the wire. In so far as they thus oscillate to and fro in the 
length of the wire, their oscillations generate a wave of length 
comparable to twice the length of the wire : much as the air 
in an open organ pipe, surging up and down in the pipe, gene- 
rates sound waves twice the length of the pipe. But both 
the organ pipe arid the electric wire require maintenance, 
for the oscillations damp out very rapidly. By giving some 
capacity to the ends of the wire, by means of terminal knobs 
or plates, the wave can be lengthened and the succession a 
little prolonged. By coiling the middle of the wire into a 
close spiral, the effective inertia of the particles can be 
increased (like using a heavy pendulum instead of a light 
one), and therey the oscillations can be a good deal prolonged, 
so that a single stimulus can generate a train of waves, with 
a definite frequency or tune. 

A similar arrangement at a distant station immersed in the 
stream of waves (which as they spread out in all directions 
must sooner or later reach it) can be set oscillating by them 
in the same way and with the same frequency ; thus constitut- 
ing a tuned receiver, in which the electrons are made to surge 
by the impact or influence or inductive effect of the waves. 
The experiment of the tuned or synchronised leyden-jars, 
each with a carefully closed circuit (first described in Nature 
for February 20, 1890), lies at the basis of all tuned wireless, 
and is the foundation of the wave-meters used in connexion 
with it. In this arrangement one of the jars responds by over- 
flow or side-spark to the synchronous oscillations of the other, 
or indeed to waves from a distant station if they are exactly 
of the right frequency. 


It lias long been known that one conductor could in- 
ductively affect another when they were fairly near each other, 
an oscillating current artificially produced in one setting up 
a secondary or induced current in the other ; but until the 
time of Hertz it was not known that actual waves would break 
off from an oscillating current at a certain distance and travel 
out independently into space with the speed of light. Even 
though, in accordance with the theory of (lerk Maxwell, 
some such effect was anticipated by Fitzftcrald and others, 
it was not known that these waves could be detected by a 
similar conductor -detected even when it was at a considerable 
distance from the source. Hertz found that the excited or 
responsive surgings were strong enough to produce little sparks 
or scintilla), and these little sparks constituted his detector. 
Soon afterwards the coherer was applied instead of the small 
spark gap, thereby giving a much more sensitive means of 
detection ; and FitzGerald began to find that a coherer acted 
to some extent as a rectifier or valve, transmitting oscillations 
of one sign more easily than the other, so that a sensitive 
galvanometer could be used even without a battery. But 
very soon it was found that crystals achieved this rectification 
much better than metals ; and presently Fleming found that 
the electrons in a vacuum could achieve the purpose still 
better than a crystal, or at any rate with clearer knowledge 
of what was happening. Thus the vacuum valve began, was 
converted into a magnifier by Lee de Forest, and modern 
wireless telephony became possible. 

The electrical oscillations themselves, being of the order 
of a million a second, are far too rapid for any instrument to 
detect. But when they are rectified, so that only the positives 
or only the negatives are transmitted, a group of high-frequency 
waves becomes a single stimulus ; and an ordered succession 
of such groups may follow each other at intervals correspond- 
ing to a hundred or a thousand rectified groups a second, 
thus bringing them easily within the range of sound, and 
enabling a telephone and a human ear to respond. 


All this may be considered now well known ; but, inasmuch 
as it has all happened within the lifetime of most of us, a 
short summary lik# the above is not inappropriate. For our 
present purpose the interest of these long -wave phenomena, 
which can be produced and detected under complete control, 


lies in the information they give and the analogies they show 
about the other older, more intangible and obscure, processes 
which are responsible for the generation and detection of the 
short ether waves that we call light, or radiant heat, or ultra- 
violet, or X-rays. A single Hertz radiator, or wireless aerial, 
vibrates at a given frequency and emits waves of one wave- 
length one tone or one colour, as it were. But a sending 
station could be imagined in which were, a large number of 
differently attuned aerials, from which there woidd emerge 
radiation not with a single wave-length, like a definite colour, 
but more like a multiplicity or mixture of colours analogous 
to white light. Different receiving stations might be also 
imagined, scattered about in different places, each one of 
which would respond only to a definite frequency of vibration : 
one to long waves, another to short waves, with others of 
intermediate character. Each of these stations would thus 
take up or absorb some of the energy appropriate to 
its particular frequency or wave-length : we should have 
what is called '' selective absorption/' If there were a 
group or interposed screen consisting of a large number 
of such stations attuned to- the same wave-length, that 
particular portion of energy would be removed from any 
train of waves which passed the screen of receivers, so 
that the waves would go on without that particular wave- 
length, a process which is analogous to the production of 
colour by selective absorption. White light going through a 
certain kind of glass might leave behind it, absorbed in the 
glass, the waves corresponding to green and blue, so that what 
was transmitted was only the red ; the effect on a receiving 
eye would then be a red sensation, and the glass would be 
called red glass. 

A whole set of phenomena of this kind are, and long have 
been, familiar in optics. A source of coloured light emits 
only one or a few wave-lengths ; and those same wave-lengths 
it is able to absorb, for they correspond to the frequency to 
which its atoms happen to be attuned. The processes of 
selective radiation and absorption in optics are quite analogous 
to the employment of a number of different signalling stations, 
each with its own wave-length, and to a still larger number 
of receiving stations, each of which can be attuned to one 
or other of the wave-lengths. In optics we have to choose 
the selectors by employing the appropriate chemical substance. 
We know that certain dyes will absorb some colours and 
transmit or reflect others : we know the properties of certain 


pigments, vermilion, gamboge, indigo, and the rest : and 
when we want to deal with the w r aves which excite a certain 
colour sensation, we use the appropriate pigment. We have 
no other means of control ; we must depend on the properties 
of the specific atoms or molecules to give the desired result. 

When we. work with long waves on the large scale, the 
whole process is better understood and more within our 
control. We can adapt a receiving station to respond to 
any of the others by merely turning a handle that is, by 
altering the electric capacity of the receiver, or altering its 
magnetic inertia or inductance that is, by changing either 
its electric or its magnetic property, or both. Thus we can 
detect waves over a considerable range, picking out the station 
we want to hear : just as a painter picks out a pigment and 
puts it in a certain place where he wants you to see it. The 
different pigments in a picture are like different receiving 
stations, each attuned to its own wave-length ; and the white 
light which illuminates the picture conies from a source 
emitting all those wave-lengths and many others. One might 
illuminate the picture by a single wave-length only say, by 
red light. Then the red pigments in the picture would all 
respond, while the others would remain dark. If the incident 
light is now changed to, say, green, another set of pigments 
that is, another set of signalling stations would respond : 
and the red ones would remain dark and ineffective. 

Long-wave stations, like the Eiffel Tower or Ohelmsf ord , 
might be said to be far down in the infra-red. The visible 
portion of the spectrum of wireless stations may be said to 
lie in the range permitted for broadcasting purposes. The 
whole " octave " corresponding to a visible spectrum is by 
no means yet consumed ; or rather the red end of it is used 
up by the Navy and other official stations. Broadcasting 
is at present confined between the limits of wave-length 
from 500 to 300 metres, which corresponds in a spectrum to a 
range from orange to blue. The civil list of broadcasting 
stations begins in the yellow with Aberdeen 495, continues 
through Swansea 485, Birmingham 475, Belfast 435, Glasgow 
420, etc., etc., and finishes in the blue with Stoke 306, and 
Sheffield 301. London, 2 LO, comes in the green, with the 
wave-length 365. Some amateurs are beginning to get good 
and distant results by exploitation of the short waves 
analogous to ult* a- violet 200 or even 50 metres. The 
analogous w r ave-lengths in actual light are also usually 
expressed in metres, but in metres divided by 10 l which 


are technically known as Angstrom units of length, and are 
comparable in size to the diameter of atoms. The wave- 
length of yellow light is about 5,800 of such units. 

That must suffice to show how closely the analogy can 
be worked between the apparently quite different phenomena 
of long-wave and short-wave radiation ; they both consist 
of waves in the ether obeying the same laws, travelling at the 
same rate, and causing appropriately attuned mechanism 
to respond. In the eye we have such a mechanism concealed 
in the retina, on a minute scale ; and we have reason to say 
that the normal eye has three kinds of receiving instruments 
or particles, one responding to a certain shade of red, another 
to a certain shade of green, and another to a kind of violet. 
Those three constitute our primary colour sensations, and 
of them in different proportions all our sense of colour is 

The receiving stations in the retina are almost certainly 
atomic, different atoms responding to the different waves. 
How, when they respond, they manage to stimulate the 
nerves that is to say, what is the nature of the apparatus 
which converts the energy of ether waves into the energy 
of a nerve stimulus is not yet fully known. We know how 
it is done on the large scale : we use rectifying valves or 
crystals and a telephone, and thereby convert the ether vibra- 
tions into sound vibrations suitable for stimulating the human 
ear. Somehow, on a minute and very perfect scale, some- 
thing corresponding to this is automatically done in the eye. 
It is suspected that it must be done photo-electrically, for 
it is known that certain atoms respond to radiation of a given 
wave-length by ejecting an electron, and ejecting it with 
such energy that it may well be conceived of as exciting a 

Whether this is the explanation of vision or not, it must 
be manifest that the intelligent production and detection of 
long-wave radiation, by apparatus humanly constructed and 
understood, is likely to throw a flood of light on the other 
processes which men and animals have always made use of 
without in the least understanding what is happening. The 
physiological instruments are extraordinarily efficient, with 
many more complicated peculiarities than our crude physical 
instruments possess, and yet they act in complete obedience 
to the laws of physics and chemistry. The physiological 
mechanism makes full use of these laws, even the most recently 
discovered and most intricate of them, but in many ways its 


behaviour supplements and transcends all known laws under 
the mysterious influence of life. 

It is indeed by life that all our instruments are constructed 
those in the laboratory as well as those in the organism ; 
and the construction of laboratory instruments involves not 
only life but intelligence ; they are consciously adapted to 
their purpose. The instruments in the organism are quite 
unconsciously constructed by the organism, in ways we have 
scarcely begun to understand : but whether design and 
consciousness of some higher kind are involved in those too 
(perhaps in some indirect and prearranged manner) is a 
matter on which everyone is not agreed, and on which our 
views may become clearer and more certain as time goes on 
and knowledge increases. To some minds the analogy will 
seem helpful and stimulating : to others it may seem mistaken 
and misleading. 

The Royal Society is concerned with the advancement 
of Natural Knowledge. All knowledge is natural in the long 
run ; but all knowledge is very far from being attained. 
In order to progress we must limit ourselves or those who 
pursue natural knowledge feel that they must limit them- 
selves to that which we at present know or can soon hope 
to know ; realising, however, that there is still an infinity 
which we do not know, and showing wisdom in not denying 
that which at present lies outside our intellectual ken. For 
that region lies open to faith and to the higher imagination 
possessed by poets and seers, who exercise a faculty which, 
though it goes beyond the intellect, is still truly among the 
functions and privileges of man. 



Electrons are particles of exceedingly small mass carrying a 
charge of negative electricity ; there is only one kind of electron, 
for all electrons have the same mass and carry the same electric 
charge. Until the discovery in 1897 of the electron, the smallest 
mass known to science was that of an atom of the lightest element, 
hydrogen, but the mass of this atom is 1,800 times greater than 
that of an electron. The mass of an electron is by far the smallest 
of all known masses. The electrons are the bricks which build 
up the atom ; an atom of hydrogen contains one electron, an atom 
of oxygen eight, one of lead about one hundred, and so on. 
Differences in the number and arrangements, of the electrons in 
the atom are supposed to account for the difference in the pro- 
perties of the atoms of the different chemical elements. 

Although the electron is by far the, commonest and most 
widely distributed thing known, it was not discovered until 1897, 
and then in what may be called a highly specialised region. It had 
been shown by Plucker in 1859 that when an electric current 
passes through a gas at a low pressure, the glass tube in which the 
gas is contained phosphoresces in the neighbourhood of the cathode. 
The phosphorescence is due to something travelling in straight 
lines from the cathode, for if an obstacle such as a piece* of glass 
rod is placed between the cathode and the walls of the tube, a 
shadow of the obstacle is thrown on the wall. The nature of the 
" cathode rays/' as the agents which produce this phosphorescence 
are called, was the subject of a long controversy. One view was 
that they arose from waves in the ether, another that they were 
due to negatively electrified particles. In support of the latter 
view were the facts that negative electricity travelled along the 
direction of the rays, and that the rays were deflected by a magnet ; 
but against the view was the fact that the rays^ould pass through 
thin sheets of metal, such as gold foil. If these rays are electrified 
bodies, it is possible by certain methods to determine their mass, 


velocity and electric charge, and when this was done it was found 
that the carriers were not atoms or molecules, but something 
almost infinitesimal in comparison. 

The mass and velocity of the electron were determined by 
measuring the deflexions they experienced when acted on by 
electric and magnetic forces. Suppose the electron started off 
horizontally in a discharge tube. It it were not acted on by any 
forces it would strike the glass wall of the tube at a point opposite 
its starting point, and the place of impact would be marked by a 
patch of phosphorescence. If, however, it is acted on by a 
constant electric force X, acting vertically downwards, the 
electron w r ould have a downward 1 acceleration XC/M (if c is its 
electric charge and M its mass), and would fall just as a rifle 
bullet shot off horizontally would fall under gravity. The vertical 
fall of the bullet is <j l z /2v 2 , where / is the horizontal distance 
passed over, v the velocity of projection, and y is the acceleration 
due to gravity. Putting Xe/Ht, the acceleration of the electron, 
instead of y, we see that the distance fallen through by the electron 
will be X(e/nt)l~/ ( 2r~. Hence the electron acted on by the electric 
force will hit the glass at a point which is this distance below its 
original destination. Jf li is the deflexion 

we can measure //, X, and I, and hence from this equation we can 
get e/i)iv z . 

If instead of acting on the electron by an electric force we 
act on it by a magnetic one, at right angles to the direction of 
motion, the force on the electron due to the magnet is Jlev, where 
// is the strength of the magnetic field ; hence the acceleration is 
HevjtH, and is at right angles to the path of the particle and in the 
direction of the magnetic force. Now suppose we let both the 
electric and magnetic forces act simultaneously, and let one act 
upwards and the other downwards. We can adjust the two forces 
until the accelerations due to them just balance, and the electron 
will then move as if neither electric nor magnetic force acted on it, 
and the phosphorescence will occur on the tube at the same point 
as that affected by the electron before either electric or magnetic 
force was introduced. For the accelerations to be equal 

Xe Hev 

= -- or o = X H. 
M m 

We can measure both X and //, and thus determine v. We 
have previously determined ejmv 2 , so that when we know ?; 
we can find ejm. This was found to be equal to 1 -8 x 10 7 . 

Now if E is the charge of electricity carried by the hydrogen 
atom in the electrolysis of solutions, and M the mass of that atom, 
EjM can be determined by measuring the quantity of hydrogen 
liberated when a known quantity of electricity passes through an 


aqueous solution. This was done long ago, and the result was 
that EjM ~ 1C 4 . Special investigations have shown that r, the 
charge on the electron, is equal to E, the charge on the hydrogen 
ion ; hence since c - E ami <'>nt ==1-8 K) 7 , while E tn == 1C) 4 , 
m _. A// 1, 800, or the mass of an electron is only 1 1,S(!O of that 
of an atom of hydrogen. 

Experiments wen* made on cathode rays produced with 
electrodes made of various metals and with different gases in the 
tube, but the mass of the electron and the charge of electricity it 
carried were found to be the same whatever might be the nature 
of the metal or the gas. The velocity of the rays, which was 
always very high -many thousand miles per second varied with 
the potential difference between the electrodes. This high 
velocity makes the energy of an electron, in spite of its small 
mass, enormously greater than the energy of the ordinary mole- 
cules of a gas. Thus a comparatively slow electron moving with 
a speed of 30,000 miles per second has 250,000 times the average 
energy of a molecule of a gas at ordinary temperatures. It is 
this comparatively enormous energy which makes the detection and 
study of electrons easier than that of ordinary molecules. 

When once electrons had been detected they were found to be 
very widely distributed. Thus it was found that they were given 
off by hot wires, and the hot-wire valves now so largely used in 
wireless telegraphy and for many other purposes work entirely 
by electrons ; electrons are also given off by bodies when struck 
by ultra-violet light or by llontgen rays. Radio-active bodies 
give off very high-speed electrons moving very nearly as fast as 
light. But whatever may be the means used to liberate them, or 
the source from which they corne, the electrons themselves are 
always found to be the same. 




There is ;i .strong analogy between the use of X-rays in the 
invest i Cation of crystal structure and the employment of light in 
conjunction with a diffraction grating. There is, however, a 
very great difference in scale, for the X-ray waves are ten thousand 
times shorter than those of light. The ordinary diffraction grating 
consists of a <heet of metal or glass on which parallel lines are 
ruled, say, 20,(!<;o to the inch. When a ray of homogeneous light 
is directed upon such a grating, diffracted pencils of light leave 
the grating in various directions according to well-known rules. 
That which i> least diffracted we call the effect of the first order, 
and the others are of the second order, the third order and so on. 
The angles which these diffracted pencils make with the original 
rays are determined by two factors namely, the wave-length 
of the light and the spacing of the lines on the grating, and it is 
possible, given a wave-length, to find the spacing by measuring 
the " angle- uf diffraction." Such a measurement is very exact. 

There is another well-known grating effect which is sometimes 
made use of. The relative intensities of the different orders, but 
not their angles of diffraction, depend upon the dimensions and 
form of the groove which the ruling diamond makes on the plate. 
Sometimes one spectrum is intensified by some particular charac- 
teristic in the forms of the grooves. It might be possible to work 
back from observations of the relative intensities to a determina- 
tion of the shape of the groove. 

When we turn to X-rays, we find the analogue of the light waves 
in the waves of the X-rays and the analogue of the grating in the 
ordered arrangement of the crystal. If X-rays are allowed to fall 
upon a crystal, dirlracted pencils may be emitted, and the angle 
which the diffracted pencil makes with the original ray depends 
upon the wave-length of the X-rays and the spacings of the crystal. 


So far the diffraction of X-rays resembles the diffraction of light 
by a grating. There is, however, an additional effect in that 
the direction of the original rays has to be related to the lie of the 
crystal planes in different ways before any diffraction takes place 
at all. In the actual experiment the crystal is rotated about some 
important axis until the diffracted pencil of rays flashes out 
and the jingle of diffraction is then observed just as in the case of 


The chemical molecule consists of a certain number of atoms 
arranged in an ordered way. When the molecules are built into a 
crystal they tend to an arrangement which has a higher symmetry 
than the molecule itself possesses. We may say that Nature puts 
together two, three, four or even more molecules in such a way 
as to make for higher symmetry. In this way she makes a unit 
of pattern. 

The units of pattern are distributed on the lines and planes of a 
lattice, each unit having exactly the same form, composition 
and outlook as every other unit ; the whole structure of the crystal 
is an orderly arrangement of these units. They may be con- 
sidered to lie on various planes or sheets within the crystal just 
as the rows of trees in an orchard may be considered to He in 
various rows, and each sheet corresponds to a line in the light- 
grating. The X-rays give the distance between sheet and sheet. 
They may do this for sheets drawn in various ways, and hence it is 
possible to determine the arrangement of the units in the crystal 
that is to say, the form and dimensions of the tk cell " occupied 
by one unit. This measurement corresponds to the determination 
of the spacing between two lines in the diffraction grating. 

A further step which the X-rays can take is the determination 
of the relative intensities of the various orders of the diffracted 
rays. This information leads, if it can be interpreted, to a know- 
ledge of the mutual arrangement of the molecules in the crystal. 
The operation which is always carried out in the case of X-rays, 
so far as present experience will allow, is analogous to a deter- 
mination of the form of the grooves of the diffraction grating by 
measuring the relative intensities of the various orders of diffracted 
light. Two stages may be distinguished in this process. One of 
them comparatively easy ; the other of difficulty, and sometimes 
of very great difficulty. 

The outer form of a crystal depends upon the internal arrange- 
ment of the atoms and molecules, and the group employed by 
Nature is in general the chemical molecule. It is possible to 
distinguish thirty-two different classes, each characterised by its 
own special form. Mathematical crystallography has carried the 
possibilities of classification further than the outward form can 
reveal. Taking any group of atoms, it has been shown that in 

(B 34/2285)Q c 



each class having its special external characteristics there are 
several ways of arranging the groups so as to give the same outward 
appearance. These different methods of arrangement are nearly 
always distinguishable from one another by their action on the 
X-rays. There are in all 230 of them. An early result of the 
X-ray analysis is, therefore, the determination of the special 
arrangement of the molecules within the crystal. 

The next stage, the more difficult one, is a determination of 
the full linear and angular relations between the position of the 
molecules and the atoms within the molecules. The aids to this- 
determination are relative measurements of intensities of diffrac- 
tion as revealed by X-rays, to which must be added all that may 
be known of the chemical, physical and mechanical characteristics 
of the atoms and the molecules. In some simple cases the analysis 
may be said to be already complete, but in the hundreds of 
thousands of known crystals there is, of course, an immense 
field still to be covered. Models can be constructed, based on the 
results already obtained, which illustrate the structure of many 
crystalline substances. 


It will now be convenient to refer to some of the principles of 
structure which X-ray analysis has already revealed. It is clear 
that such principles are worthy of careful investigation, for they 
may throw light on chemical and physical actions and also help in 
further determinations of crystal structure. There is in the first 
place a broad division into different methods of combination 
between the atoms. Three types can be recognised. The first 
of these is illustrated by such crystal structures as rock-salt, 
fluorspar, calcite and so forth. The structure depends mainly 
upon a group formed in obedience to laws of electrostatic action. 
If, for example, we take the case of rock-salt, the chlorine atom 
has taken away from the sodium atom one electron which it has 
incorporated into its own structure. According to the modern 
views of atomic constitution the chlorine atom, which has seven 
electrons in its outermost electron shell, is eager to complete the 
shell completion implying the presence of eight electrons in that 
shell. Neon, which has eight, already seems to show by its 
unwillingness to enter into chemical combination in other words, 
by its unwillingness to give, take or share electrons that there is 
something which makes the eight a satisfactory and complete 
number. Sodium has the completed outer shell of eight and one 
which is the beginning of a new shell external to the old. It 
appears to have a poor hold on this odd electron, so that chlorine- 
easily removes it. In consequence the chlorine becomes a 
negatively charged body, and the sodium a positively charged 


Each positive surrounds itself with as many negatives as 
possible, and each negative with as many positives as possible. 
The cubic structure of rock-salt is obtained in this way, each atom 
having six neighbours of opposite sign. In fluorspar, where the 
calcium atom has been robbed of its two extra electrons by two 
fluorine atoms, each of which takes one, Nature has found a 
structure in which the positively charged calcium is surrounded 
by eight negative fluorines, and the fluorines by four calciums. 
In Iceland spar the same principle governs the structure. Each 
calcium atom is surrounded by six OO 3 groups, and each CO 8 group 
by six calciums. The structure is not, however, so regular as in 
rock-salt because the CO 3 group is not spherical in form and 
characteristics. There is a large class of crystals built on the same 

There is a certain indefiniteness about the molecule because 
a positive can be associated with any one of the six negative- 
neighbours which it possesses. It is notable that, in the calcite> 
the CO 3 group must have such a degree of symmetry as is repre- 
sented by the properties of an equilateral triangle. If it is turned! 
round 120 in its own plane, it has the same appearance as before* 
This implies that the three oxygen atoms are all alike in their 
relations to one another, and to the other atoms of the crystal. 
However the crystal may come to pieces under chemical action, 
a compoiind of calcium atoms and (JO 3 groups is not a mixture 
of 00 2 and CaO. It is supposed that the carbon atom is stripped 
of all the four electrons which it normally possesses in its outer 
shell, while the calcium atom loses the two electrons which it has 
outside its completed shell. Each of the oxygen atoms takes 
two of the six electrons thus set free. Consequently each carbon 
atom has a quadruple positive charge, each oxygen a double 
negative charge, and the calcium a double positive charge. 

A second method of combination is to be found in the diamond. 
The carbon atoms, of which alone it consists, are so arranged that 
each carbon has four neighbours arranged about it in tetrahedral 
fashion. Each shares two electrons with each of its neighbours, 
and in this w r ay covers itself with the desired shell of eight electrons. 
It appears that the sharing produces a very strong bonding ; the 
diamond is the hardest of known substances. 

In graphite there are sheets of atoms tied tightly to one another 
by the sharing bonds of the diamond, but these sheets are separated 
from one another by a considerable interval. In this way may be 
explained the slipperiness of graphite and its usefulness as a lubri- 
cant, because in the first place the layers slip on one another easily, 
the bonds that tie them together being weak, and, in the second 
place, the atoms in each layer hold tightly together. There are 
indications that these tight bonds are much less affected by 
temperature than bonds of a looser type. For example, the 
co-efficient of expansion with heat of the diamond is far less than 

(B 34-2285)q c 2 


the average expansion of graphite, hut the expansion of graphite 
takes place almost entirely through increased separation of the 


There is yet a third method of combination, in general much 
weaker than the other two. When molecules, as in organic 
crystals, are built together into a structure, the forces that bind 
together molecule and molecule may be comparatively weak. The 
separate molecules are not positive or negative to each other, nor 
do they share electrons, but no doubt there are stray fields, perhaps 
electric, perhaps magnetic, at different points on their surfaces 
which cause the molecules to be joined on to one another like the 
girders of an iron bridge. The crystal structure is very empty ; 
it is like lace- work in space. We get the first hint of this likeness 
in the diamond, where the empty spaces are big enough to accom- 
modate as many more carbon atoms as a diamond already 
contains. The root principle seems to be that the carbon atom, 
when sharing electrons, gathers round it four neighbours more or 
less at the corners of a tetrahedron. If these points of attachment 
are spaced, so to speak, over the surface of the carbon atom, it is 
easy to understand how it comes about that these open structures 
can be formed. 

Tn the diamond crystal the structure shows two types of 
arrangement which form the basis of two of the great groups of 
organic substances. There is in the first place the hexagonal ring, 
which appears to be capable of separate existence in unchanged 
form and dimension, and when fringed with various atoms or 
radicles, to form the innumerable members of the aromatic series. 
The double and treble rings are found in naphthalene and anthra- 
cene respectively, and the structure of these crystals, as revealed 
by X-rays, shows that the ring is the same in all respects as in the 

There is also to be found in the diamond an arrangement 
of long chains, which may have any length, of carbon atoms. 
These chains, when fringed along their length by hydrogen atoms 
and finished off at each end with various groups of atoms, such 
as the methyl group (CH 3 ), the carboxyl group (COOH), the 
hydroxyl group (OH), and so on, form the well-known chain 
compounds of organic chemistry. Measurements of the lengths 
of these chains have recently been made very exactly by the 
X-ray methods in a number of cases, and it appears that the 
arrangement is, as the models show, just the same as is found in 
the diamond. The essential feature is that any two carbon atoms 
are joined on to a third at points on the surface of the latter, which 
are, at least, close to tetrahedral points. 



The application of the X-rays to the crystal analysis of metals 
has shown very remarkable results, which will probably receive 
great extension in the future. Many of the metals aluminium, 
silver, copper and gold, for example are of a structure which 
implies the simplest form of close packing of spherical atoms. 
These plates are those in which the packing is most dense. A mass 
of crystals is stronger than a single crystal because the planes of 
weakness lie in all directions. An admixture of a certain number 
of foreign atoms causes a distortion of the structure, which 
diminishes the possibility of slip, and thus the hardening effect of 
an alloy is explained. 

In the case of steel it seems likely that the carbon atoms do 
not replace iron atoms, as, for example, tin atoms replace those of 
copper in the formation of bronze ; they appear to fit into the 
interstices of the structure. The structural nature of the various 
crystals which form in alloys, as, for example, cementite in steel and 
inter-metallic compounds in other alloys, have also been the subject 
of investigation. 

Among the many other developments to which X-ray analysis 
is leading, one more may be mentioned, It now seems possible 
from a knowledge of the structure of the atoms which compose the 
crystal, to calculate the effects upon electro-magnetic waves, such 
as those of light, on their way through it. A beginning in this 
respect has been made with the measurements of the refraction 
indices of calcite and aragonite. 




The discovery by Sir J. J. Thomson in 18 C .)7 of the individual 
existence of the negative electron of small mass, and the proof 
that it was a component of all the atoms of matter, was an event 
of extraordinary significance to science, not only for the light 
which it threw on the nature of electricity, but also for the prgmiso 
it gave of methods of direct attack on the problem of the structure 
of the atom. This discovery of the electron, coupled with the 
recognition of the atomic nature of electricity, has created a 
veritable revolution in our ideas of atoms. 

It was soon recognised that the negative electron of small 
mass was an actual disembodied atom of electricity, and that its 
apparent mass was electrical in origin. Sir J. J. Thomson had 
shown so early as 1881 that a charged body in motion behaved 
as if it had an additional electric mass clue to its motion. The 
moving charge generates a magnetic field in the space surrounding 
it, resulting in an increase of energy of the moving system which 
is equivalent to the effect produced by an increase of the mass of 
the body. 

Since there must always be electric mass associated with the 
movement of electric charges, it is natural to suppose that the mass 
of the electron is entirely electrical in origin, and no advantage is 
gained by supposing that any other type of mass exists. If the 
atom is a purely electrical structure, the mass of the atom itself 
must be due to the resultant of the electric masses of the charged 
particles which make up its structure. As only a small fraction of 
the mass of an atom can be ascribed to the negative electrons 
contained in it, the main part is due to the positively charged 
units of its structure. 

* Abstracted from the Kelvin Lecture delivered before the Institution 
of Electrical Engineers on May 18, 1922. 



One of the main difficulties in our attack on the question of 
atomic constitution has lain in the uncertainty of the nature of 
positive electricity. The evidence as a whole supports the idea 
that the nucleus of the hydrogen atom, i.e., a positively charged 
atom of hydrogen, is the positive electron. No evidence has been 
obtained of the existence of a positively charged unit of mass less 
than that of the hydrogen nucleus, either in vacuum tubes or in 
the transformation of the radio-active atoms, where the processes 
occurring are very fundamental in character. 

It might a priori have been anticipated that the positive elec- 
tron should be the counterpart of the negative electron and have 
the same small mass. There is, however, not the slightest evidence 
of the existence of such a counterpart. On the views outlined, the 
positive and negative electrons both consist of the fundamental 
unit of charge, but the mass of the positive is about 1,800 times that 
of the negative. This difference in the mass of the two electrons 
seems a fundamental fact of Nature, and, indeed, is essential 
for the existence of atoms as we know them. The unsymmetrical 
distribution of positive and negative electricity that is charac- 
teristic of all atoms is a consequence of this wide difference in the 
mass of the ultimate electrons which compose their structure. 
No explanation can be offered at the moment why such a difference 
should exist between positive and negative electricity. 

Since it may be argued that a positive unit of electricity asso- 
ciated with a much smaller mass than the hydrogen nucleus may 
yet be discovered, it may be desirable not to prejudge the question 
by calling the hydrogen nucleus the positive electron. For this 
reason, and also for brevity, it has been proposed that the name 
"proton"' should be given to the unit of positive electricity 
associated in the free state with a mass about that of the hydrogen 
nucleus. In the following, the term " electron " will be applied 
only to the well-known negative unit of electricity of very small 

On the classical electrical theory, the mass of the electron can 
be accounted for by supposing that the negative electricity is dis- 
tributed on a spherical surface of radius about 1 x 10' 18 cm. 
This is merely an estimate, but probably gives the right order 
of magnitude of the dimensions. 

The greater mass of the proton is to be explained by supposing 
that the distribution of electricity is much more concentrated for 
the proton than for the electron. Supposing the shape spherical, 
the radius of the proton should be only 1/1800 of that of the 
electron. If this be so, the proton has the smallest dimensions of 
any particle known to us. It is admittedly very difficult to give 
any convincing proof in support of this contention, but at the 
same time there is no evidence against it. 



Progress during the last twenty years of our ideas on the 
structure of atoms has depended mainly on a clearer understanding 
of the relative part played by positive and negative electricity 
in atomic structure. It is now generally accepted that the atom 
is an electrical system and that the atoms of all the elements 
have a similar type of structure. 

The nuclear theory of atomic constitution has been found to be 
of extraordinary value in offering an explanation of the funda- 
mental facts that have come to light, and is now generally employed 
in all detailed theories of atomic constitution. At the centre of each 
atom is a massive positively charged nucleus of dimensions minute 
compared with the diameter of the atom. This nucleus is sur- 
rounded by a distribution of negative electrons which extend to 
a distance, and occupy rather than fill a region of diameter about 
2 x 10' 8 cm. Apart from the mass of the atom, which resides 
mainly in the nucleus, tho number and distribution of the outer 
electrons, on which the ordinary physical and chemical properties 
of tho atom depend, are controlled by the magnitude of the nuclear 
charge. The position and motions of the external electrons are 
only slightly affected by the mass of the nucleus. 

According to this view of the atom, the problem of its con- 
stitution naturally falls into two parts -first, the distribution 
and mode of motion of the outer electrons, and secondly, the 
structure of the nucleus and the magnitude of the resultant positive 
charge carried by it. In a neutral atom the number of external 
electrons is obviously equal in number to the units of positive 
(resultant) charge on the nucleus. 

The general conception of the nuclear atom arose from the need 
of explanation of the very large deflexions experienced by swift 
particles thrown off by radio-active substances, known as a- and 
p- particles, in passing through the atoms of matter. A study 
of the number of a- particles scattered through different angles 
showed that there must be a very intense electric field within the 
atom, and gave us a method of estimating the magnitude of the 
charge on the nucleus. Similarly, the scattering of X-rays by the 
outer electrons provided us with an estimate of the number of 
these electrons in the atom, and the two methods gave concordant 
values. The next great advance we owe to the experiments of 
Moseley on the X-ray spectra of the elements. He showed that 
his experiments received a simple explanation if the nuclear charge 
varied by one unit in passing from one atom to the next. In 
addition, it was deduced that the actual magnitude of the nuclear 
charge of an atom in fundamental units is equal to the atomic or 
ordinal number when the elements are arranged in order of 
increasing atomic weight. On this view, the nuclear charge of 
hydrogen is 1, of helium 2, lithium 3, and so on up to the heaviest 


element uranium, of charge 92. Between these limits, with few 
exceptions, all nuclear charges are represented by known elements. 
This relation, found by Moseley, between the atoms of the 
elements, is of unexpected simplicity and of extraordinary interest. 
The properties of an atom are defined by a whole number which 
varies by unity in passing from one atom to the next. This atomic 
number represents not only the ordinal number of the elements, but 
also the magnitude of the charge of the nucleus and the number of 
outer electrons. The atomic weight of an element is not nearly so 
fundamental a property of the atom as its nuclear charge, for its 
weight depends upon the inner structure of the nucleus, which may 
be different for atoms of the same nuclear charge. 


The most definite information we have of the structure of the 
nucleus of an atom has been obtained from a study of the modes of 
disintegration of the radio-active atoms. Tn the great majority of 
cases the atom breaks up with the expulsion of a single a-particle* 
which represents the doubly charged nucleus of the helium atom ; 
in other cases a swift (3-ray or electron is liberated. There can be 
no doubt that these particles are liberated from the nuclei of the 
radio-active atoms. This is clearly shown by the variation of the 
atomic numbers (the figures enclosed by the circles) of the suc- 
cessive elements in the long series of transformations of uranium 
and thorium (Fig. 1). The expulsion of an a-particle lowers the 

.,. . Ur.I i* UrX x Ur.X 2 Ionium Radium Email. ) 

Atm.Wt.238 234 234 234 23O 2Z6 Z22 ^/ 

C84} (?> (g> _ _ _ _ 
LA Kad.B IUd.C fcad.D Kad.E Kad.F Lead 

218 214 214 210 210 210 206 

Uranium-radium series 
FIG. 1. 

nuclear charge of the atom by two units and its mass by four, 
while the expulsion of an electron raises its charge by one. On 
this simple basis we can at once deduce the atomic number and, 
consequently, the general chemical properties of the long scries 
of radio-active elements. In this way we can understand at 
once the appearance in the radio-active series of isotopes, i.e., 
elements of the same nuclear charge but different atomic masses. 
The existence of isotopic elements was first brought to light 
from a study of the radio-active elements. For example, radium- B 9 
radium-Z) and the end product, uranium-lead, are isotopes of lead 
of nuclear charge 82, but of masses 214, 210, and 206 respectively. 


As regards ordinary chemical and physical properties, they are 
indistinguishable from one another, differing only in properties that 
depend oil the nucleus, namely, atomic mass and radio-activity. 
For example, radium- B and radium-Z) both emit (3-rays, but with 
different velocities, while their average life is widely different . 
Uranium-lead, 011 the other hand, is 11011- radio- active. Many 
similar examples can be taken from the thorium and actinium 
series of elements. These illustrations show clearly that elements 
may have almost identical physical and chemical properties, and 
yet differ markedly in the mass and structure of their nuclei. 

From the radio-active evidence, it seems clear that the nuclear 
structure contains both helium nuclei and electrons. In the 
uranium-radium series of transformations, eight helium nuclei are 
emitted and six electrons, and it is natural to suppose that the 
helium nuclei and electrons that are ejected act as units of the 
nuclear structure. It is clear from these results that the nuclear 
charge of an element is the excess of the positive charges in the 
nucleus over the negative. Tt is a striking fact that no protons 
(H nuclei) appear to be emitted in any of the radio-active trans- 
formations, but only helium nuclei and electrons. 

Some very definite and important information on the structure 
of nuclei has been obtained by Aston in his experiments to show 
the existence of isotopes in the ordinary stable elements by the 
well-known positive- ray method. He found that a number of the 
elements were simple and contained no isotopes. Examples of 
such fc * pure " elements are carbon, nitrogen, oxygen and fluorine. 
It is significant that the atomic weights of these elements are nearly 
whole numbers in terms of O = 16 ; on the other hand, elements 
such as neon, chlorine, krypton, and many others, consisted of 
mixtures of two or more isotopes of different atomic masses. 
Aston found that within the limit of error about 1 in 1,000 the 
atomic weights of these isotopes were whole numbers on the 
oxygen scale. This is a very important result, and suggests that 
the nuclei of elements are built up by the addition of protons, 
of mass nearly one, in the nuclear combination. 


There seems to be no doubt that the nucleus of an atom is held 
together by very powerful forces, and that we can only hope to 
effect its disintegration by very concentrated sources of energy 
applied directly to the nucleus. The most concentrated source 
of energy known to us is the swift oc-particle expelled from radium 
or thorium. It is liberated with a velocity of 10,000 miles per 
second, and has so much energy that it produces an easily visible 
flash on striking a zinc sulphide crystal. Its speed is twenty 
thousand times greater than that of a swift rifle bullet, and, mass 
for mass, its energy of motion is four hundred million times greater. 


A stream of a-particles is therefore made to bombard the atoms 
of the material under examination. On account of the minute size 
of the nucleus, we can expect an oc-particle only occasionally to get 
near enough to the nucleus to effect its disintegration, and this 
method should be more likely to succeed with a light atom, in 
which the repulsive force of the nucleus would not be so great as 
that of a heavy atom with high nuclear charge. 

The first observation indicating the disruption of the nitrogen 
nucleus was made some years ago. When a-particles were passed 
through oxygen or carbon-dioxide, a few particles of long range 
were observed. These appeared to be H-nuclei set free from 
hydrogen in the radio-active source, which, on account of their 
small mass, would be expected to have a greater range than the 
a-particles liberating them. When, however, dry air or nitrogen 
is submitted to such a bombardment, the number of long-range 
particles is three or four times as numerous, and they have a 
greater average range. These behaved in all respects as H-nuclei, 
and it was concluded that they arose from disruption of the 
nitrogen nuclei. 

Using improved apparatus, it was possible to show that similar 
long-range particles were liberated from boron, fluorine, sodium, 
aluminium and phosphorus. The range of the particles is in all 
cases greater than that of the H-particles liberated from free 
hydrogen atoms under similar conditions. For example, using 
radium-0 as a source of X-rays, the range of the H-nuclei is about 
28 cm. Under similar conditions, the range of the particles from 
nitrogen is 40 cm., while the range of particles from aluminium is 
as much as 90 cm. 

Tt is thus natural to conclude that " protons " have been 
ejected from the nuclei of certain light elements by the action of 
the a-particles. It is significant that no protons are liberated 
from carbon (12) and oxygen (16), the atomic weights of which 
are given by \n< where n is a whole number. Protons are only 
observed from elements of which the atomic weights are expressed 
by 4w + a, where a is 2 or 3.* These results suggest that the 
elements are, in the main, built up of helium nuclei of mass 4, 
and protons. The a-particle is unable to liberate a proton from 
elements like carbon and oxygen, which are built up entirely of 
helium nuclei as secondary units, probably because the helium 
nucleus is too stable to be broken up by the swiftest a-particle 
available. It should be borne in mind, however, that this dis- 
integration phenomenon effected by a-particles is on an exceed- 
ingly minute scale. Only two protons are liberated from 
aluminium for a million a-particles traversing it. 

* Later experiments have shown that all the elements from boron to 
potassium are disintegrated by bombardment with a-particles, with the 
two exceptions of carbon and oxygen. 



From the radio-active evidence, we know that the nuclei of 
heavy atoms are built up, in part at least, of helium nuclei and 
electrons, while it also seems clear that the proton can be released 
from the nuclei of certain light atoms. It is, however, very 
natural to suppose that the helium nucleus which carries two- 
positive charges is a secondary building unit, composed of a close 
combination of protons and electrons, namely, 4 protons and 2 

From the point of view of simplicity, such a conception has- 
much in its favour, although it seems at the moment impossible 
to prove its correctness. If, however, we take this structure of the 
helium nucleus as a working hypothesis, certain very important 
consequences follow. 

Taking the mass of the oxygen atom as 16 (tho standard which 
is usually adopted in atomic weight determinations), the helium 
atom has a mass very nearly 4-000, while the hydrogen atom 
has a mass 1 -0077. The mass of the helium atom is thus con- 
siderably less than that of four free H-nuclei. Disregarding the 
small mass of electrons, in the formation of 1 gram of helium 
from hydrogen there would be a loss of mass of 7 -7 milligrams. 

It is now generally accepted that if the formation of a complex 
system is accompanied by the radiation of energy, a reduction of 
its mass occurs, which can be calculated. In the formation of 
1 gram of helium from hydrogen an enormous amount of energy 
is set free ; the energy radiated in forming one single atom of 
helium is equivalent to the energy carried by three or four swift 
oc-particles from radium. On this view we can at once under- 
stand why it should be impossible to break up the helium nucleus 
by a collision with an a-particle. In fact, the helium atom should 
be by far the most stable of all the complex atoms. 

Most workers on the problem of atomic constitution now take 
as a working hypothesis that the atoms of matter are purely 
electrical structures, and that ultimately it is hoped to explain 
all the properties of atoms as a result of certain combinations of 
the two fundamental units of positive and negative electricity, 
the proton and electron. Some of the more successful methods 
of attack that have been made on this most difficult of problems 
have been indicated. During recent years unexpectedly rapid 
%advances have been made in our knowledge, but we have only 
made a beginning in the attack on a very great and intricate 


By DK. F. W. ASTON, F.K.S. 

That matter is discontinuous and consists of discrete particles 
is by no means obvious to the senses. The surfaces of clean 
liquids even under the most powerful microscope appear perfectly 
smooth, coherent and continuous. The merest trace of a soluble 
dye will colour millions of times its volume of water. It is not 
surprising therefore that, in the past, there have arisen schools 
who believed that matter was quite continuous and infinitely 

The upholders of this view said that if you took a piece of 
material lead, for instance and went on cutting it into smaller 
and smaller fragments with a sufficiently sharp knife, you could 
go on indefinitely. The opposing school argued that at some stage 
in the operations, either the. act of section would become impossible 
or the result would be lead no longer. 

The accuracy of modern knowledge is such that we can carry 
out, indirectly at least, the experiment suggested by the old. 
philosophers right up to the stage when the second school is proved 
correct, and the ultimate atom of lead is reached. For con- 
venience, we can start with a standard decimetre cube of lead 
weighing 11 -37 kilograms, and the operation of section will consist 
of three cuts at right angles to each other, dividing the original 
cube into eight similar bodies, each of half the linear dimensions 
and one-eighth the weight. Thus the first cube will have 5 cm. 
sides and weigh 1*42 kilograms, the second will weigh 178 gm., 
the fourth 2-78 gm., and so on. Diminution in the series is very 
rapid, and the result of the ninth operation is a quantity of lead 
just weighable on the ordinary chemical balance. The last 
operation possible, without breaking up the lead atom, is the 

* Abstracted from lectures delivered before the Franklin Institute, 
Philadelphia, on March t-10, 1922. Additional data on Isotopes added in 


twenty- eighth. The twenty-sixth cube contains 64 atoms, the 
size, distance apart and general arrangement of which can be 
represented with considerable accuracy, thanks to the exact 
knowledge derived from research on X-rays and specific heats. 
The following table shows at what stages certain analytical 
methods break down. The great superiority of the microscope is a 
noteworthy point. 

Cube. Side in cm. Mass in gra. Limiting Analytical Method. 

9 0-0195 8 5 x 10 ~ 5 Ordinary Chemical Balance. 

14 (M Xl0~ 4 2 -58x10 ~ 9 Quartz Micro-balance. 

15 3 -05 X 10 -* 3 -22 x 10 ~ 10 Spectrum Analysis (Na lines). 
18 3-8 Xl0~" 5 6-25 x 10 ~ 13 Ordinary Microscope. 

24 6-0 Xl0~ 7 2-38xlO~~ 18 Ultra-Microscope. 

28 3-7 Xl0~ 8 5-13X10" 22 

Atom 3-0 XlO~ 8 3 -44x10 "2* Radio-activity. 

Just as any vivid notion of the size of the cubes passes out of 
our power at about the twelfth the limiting size of a dark object 
visible to the unaided eye so when one considers the figures 
expressing the number of atoms in any ordinary mass of material, 
the mind is staggered by their immensity. Thus if we slice the 
original decimetre cube into square plates one atom thick, 
the area of these plates will total one and one- quarter square miles. 
If we cut these plates into strings of atoms spaced apart as they 
are in the solid, these decimetre strings put end-to-end will reach 
6*3 million million miles, the distance light will travel in a year, 
a quarter of the distance to the nearest fixed star. If the atoms 
are spaced but one millimetre apart, the string will be three and 
a half million times longer yet, spanning the whole universe. 

From the above statements it would, at first sight, appear 
absurd to hope to obtain effects from single atoms, yet this can 
now be done in several ways, and indeed it is largely due to the 
results of such experiments that the figures can be stated with so 
much confidence. Detection of an individual is only feasible 
in the case of an atom moving with an enormous velocity, when 
its energy is quite appreciable, although its mass is so minute. 
The charged helium atom shot out by radio-active substances in 
the form of an a- ray possesses so much energy that the splash of 
light caused by its impact against a fluorescent screen can be 
visibly detected ; the ionisatiori caused by its passage through a 
suitable gas can be measured on a sensitive electrometer, and, 
in the beautiful experiments of Prof. C. T. R. Wilson, its path in 
air can be both seen and photographed by means of the con- 
densation of water drops upon the atomic wreckage it leaves 
behind it. 


In the first complete Atomic Theory put forward by Dalton 
in 1803, one of the postulates states that : " Atoms of the same 


element are similar to one another and equal in weight/' Of 
course, if we take this as a definition of the word " element " 
it becomes a truism, but, on the other hand, what Dalton probably 
meant by an element, and what we understand by the word to-day, 
is a substance such as hydrogen, oxygen, chlorine, or lead, which 
has unique chemical properties, and cannot be resolved into more 
elementary constituents by any known chemical process. For 
many of the well-known elements Dalton's postulate still appears 
to be strictly true, but for others, probably the majority, it needs 
some modification. 

The idea that atoms of the same element are all identical in 
weight could not be challenged by ordinary chemical methods, for 
the atoms are by definition chemically identical, and numerical 
ratios were only to be obtained in such methods by the use of 
quantities of the element containing countless myriads of atoms. 

There are two ways by which the identity of the weights of the 
atoms forming an element can be tested. One is by the direct 
comparison of the weights of individual atoms : the other is 
by obtaining samples of the element from different sources or by 
different processes, samples which, although perfectly pure, do not 
give the same chemical atomic weight. It was by the second and 
less direct of these methods that it was first shown that substances 
could exist which, though chemically identical, had different 
atomic weights. To these the name " isotopes " was applied by 
Prof. F. Soddy. 


In the absence of the special radio-active evidence which can 
be used in special cases such as that of lead, the presence of isotopes 
among the inactive elements can only be detected by the direct 
measurement of the masses of individual atoms. This can be 
done by the analysis of positive rays. 

The condition for the development of these rays is, briefly, 
ionisation at low pressure in a strong electric field. lonisation, 
which may be due to collisions or radiation, means in its simplest 
case the detachment of one electron from a neutral atom. The 
two resulting fragments carry charges of electricity of equal 
quantity but of opposite sign. The negatively-charged one is the 
electron, the atomic unit of negative electricity itself, and is the 
same whatever the atom ionised. It is extremely light, and there- 
fore in the strong electric field rapidly attains a high velocity and 
becomes a cathode ray. The remaining fragment is clearly 
dependent on the nature of the atom ionised. It is immensely 
more massive than the electron, for the mass of the lightest atom, 
that of hydrogen, is about 1,800 times that of the electron, and 
so will attain a much lower velocity under the action of the electric 
field. However, if the field is strong and the pressure so low 


that it does not collide with other atoms too frequently, it will 
ultimately attain a high speed in a direction opposite to that of the 
detached electron, and become a " positive ray." 

Positive rays can be formed from molecules as well as atoms, so 
any measurement of their mass will give us direct information 
as to the masses of atoms of elements and molecules of com- 
pounds : moreover, this information will refer to the atoms or 
molecules ittdicidunlty, not. as in chemistry, to the mean of an 
immense aggregate. It is on this account that the accurate 
analysis of positive rays is of such importance. 

In order to investigate and analyse them it is necessary to 
obtain intense beams of the rays. This can bo done in several 
ways. The one most generally available is by the use of the 
discharge in gases at low pressure. 

The comparatively dimly lit space in a discharge tube between 
the cathode and the bright tfw negative glow " is named after its 
discoverer the " Crookes' dark space." lonisation is going on at 
all points throughout the dark space, and it roaches a very high 
intensity in the negative glow. This ionisatioii is probably caused 
for the most part by electrons liberated from the surface of the 
cathode (cathode rays). These, when they reach a speed sufficient 
to ionise by collision, liberate more free electrons, which in their 
turn become ionising agents, so that the intensity of ionisatioii 
from this cause will tend to increase as we move away from the 
cathode. The liberation of the original electrons from the surface 
of the cathode is generally regarded as due to the impact of the 
positive ions (positive rays) generated in the negative glow and 
the dark space. In addition to cathode ray ionisatioii the positive 
rays travelling towards the cathode are themselves capable of 
ionising the gas, and radiation may also play an important part 
in the same process. 

The surface of the cathode will therefore be under a continuous 
hail of positively charged particles. Their masses may be expected 
to vary from that of the lightest atom to that of the heaviest 
molecule capable of existence in the discharge tube, and their 
energies from an indefinitely small value to a maximum expressed 
by the product of the charge they carry multiplied by the total 
potential applied to the electrodes. If the cathode be pierced, 
the rays pass through the aperture and form a stream, 
heterogeneous both in mass and velocity, which can be subjected 
to examination and analysis. 


In Sir J. J. Thomson's t% parabola " method of analysis of 
positive rays, the particles, after reaching the surface of the cathode, 
enter a long and very fine metal tube. By this means a narrow 
beam of rays is produced, which is passed through electric and 
magnetic fields causing deflexions at right angles to each other, 


and finally falls upon a screen of fluorescent material or a photo- 
graphic plate. It can then be shown that if the mass of any 
particle is m and its charge e, when both fields are on together, 
the locus of impact of all particles of the same e/m, but varying 
velocity, will be a parabola. Since e must be the electronic 
charge, or a simple multiple of it, measurement of the relative 
positions of the parabolas on the plate enables us to calculate 
the relative masses of the particles producing them that is, the 
masses of the individual atoms. The fact that the streaks were 
definite, sharp parabolas, and not mere blurs, constituted the first 
direct proof that atoms of the same element were, even approxi- 
mately, of equal mass. 

Many gases were examined by this method, and some remark- 
able compounds, such as H 3 , discovered by its means. When in 
1912 neon was introduced into the discharge tube, it was observed 
to exhibit an interesting peculiarity. Whereas all elements 
previously examined gave single, or apparently single, parabolas, 
that given by neon was definitely double. The brighter curve 
corresponded roughly to an atomic weight of 20, the fainter 
companion to one of 22, the atomic weight of neon being 20.20. 
Sir J. J. Thomson was of the opinion that line 22 could not be 
attributed to any compound, but that it represented a hitherto 
unknown elementary constituent of neon. This agreed very 
well with the idea of isotopes which had just been promulgated, 
so that it was of great importance to investigate the point as 
fully as possible. 

The first line of attack was an attempt at separation by 
fractional distillation over charcoal cooled with liquid air, but 
even after many thousands of operations the result was entirely 
negative. The second method employed was that of fractional 
diffusion through pipeclay, which after months of arduous work 
gave a small, but definite positive indication of separation. A 
difference of about 0-7 per cent, between the densities of the 
heaviest and lightest fractions was obtained. It therefore seemed 
probable that neon was a mixture of isotopes. 


By the time that research on the subject was resumed in 1919, 
the existence of isotopes among the products of radio-activity had 
been put beyond all reasonable doubt by the work on the atomic 
weight of lead. This fact automatically increased both the value of 
the evidence of the complex nature of neon and the urgency of its 
definite confirmation. It was realised that separation could only 
be very partial at the best, and that the most satisfactory proof 
would be afforded by measurements of atomic weight by the 
method of positive rays. These would have to be so accurate 
as to prove beyond dispute that the accepted atomic weight 
lay between the real atomic weights of the constituents, but 
corresponded with neither of them. 

(B 34/2285)Q D 


The parabola method of analysis was not sufficient for this, 
but the required accuracy was achieved by a new arrangement. 
The rays, after arriving at the cathode face, are made to pass 
through two very narrow parallel slits of special construction, and 
the resulting thin ribbon of rays is spread out into an electric 
spectrum by means of parallel charged plates. After emerging 
from the electric field, a group of the rays is selected by means of 
a diaphragm, and made to pass between the parallel poles of a 
magnet. By this means the rays are brought to a focus, though 
spread out spectrum fashion, on a photographic plate. 

Thus the instrument is a close analogue of the ordinary spectro- 
graph, and gives a " spectrum " which, however, depends upon 
mass ; it was therefore called a " mass-spectrograph " and the 
spectrum it produces a " mass-spectrum." 

The measurements of mass thus made are not absolute, but 
relative to lines which correspond to known masses. Such lines 
due to hydrogen, carbon, oxygen and their compounds are 
generally present as impurities or purposely added, for pure 
gases are not suitable for the smooth working of the discharge 

It must be remembered that the ratio of mass to charge is the 
Teal quantity measured by the position of the lines. Many of the 
particles are capable of carrying more than one charge. A particle 
carrying two charges will appear as having half its real mass, one 
carrying three charges as if its mass was one- third, and so on. 
Lines due to these are called lines of the second and third order. 
Lines of high order are particularly valuable in extending the 
reference scale. 

When neon was introduced into the apparatus, four new lines 
made their appearance at 10, 11, 20 and 22. The first pair are 
second order lines and are fainter than the other two, and a series 
of consistent measurements showed that- to within about one part 
in a thousand the atomic weights of the isotopes composing neon 
are 20 and 22 respectively. Ten per cent, of the latter would 
bring the mean atomic weight to the accepted value of 20 *20, and 
the relative intensity of the lines agrees well with this proportion. 
The isotopic constitution of neon seems therefore settled beyond all 

The element chlorine was naturally the next to be analysed, 
and the explanation of its fractional atomic weight was obvious 
from the first plate taken. Its mass spectrum is characterised by 
four strong first order lines at 35, 36, 37, 38, with fainter ones at 
39, 40. There is no sign whatever of any line at 35*46. The 
simplest explanation of the group is to suppose that the lines 35 
and 37 are due to the isotopic chlorines and lines 36 and 33 to 
their corresponding hydrochloric acids. The elementary nature of 
lines 35 and 37 is also indicated by the second order lines at 17-5, 
18*5, and also, when phosgene was used, by the appearance of 
lines at 63, 65, due to COC1 85 and COC1 87 . 



Later it was found possible to obtain the spectrum of negatively- 
charged rays. These rays are formed by a normal positively- 
charged ray picking up two electrons. On the negative spectrum 
of chlorine only two lines, 35 and 37, can be seen, so that the lines 
at 36 and 38 cannot be due to isotopes of the clement. These 
results go to show that chlorine is a complex element, and that 
its principal isotopes are of atomic weight 35 and 37. 

The method of positive ray analysis having been applied so 
successfully to neon and chlorine, other elements were quickly 
submitted to its searching investigation. About half turned out 
to be mixtures and sonic arc very complex. Thus krypton has 
six, tin at least seven, and xenon possibly nine constituents. 
It was also demonstrated definitely that hydrogen is a simple 
element and that its chemical atomic weight, 1 -0077, is the true 
weight of its atom (see p. 36). Positive rays of the metallic 
elements cannot, in general, be obtained by the discharge tube 
method, but require special devices. Thus the isotopic nature of 
lithium was first demonstrated by the use of anode rays derived 
from anodes containing salts of that metal, and since then, all the 
other alkali metals have been successfully analysed. 

A powerful and ingenious method of generating positive rays 
of metallic elements has been used with great success by Dempster 
at Chicago. He employs the element in the metallic state, and 
ionises its vapour by means of a subsidiary beam of cathode rays. 
The ions so produced are allowed to fall through a definite potential, 
and, being therefore of constant energy, can be analysed by the 
use of a magnetic field alone. By this arrangement Dempster 
discovered the three isotopes of magnesium, and has since analysed 
zinc and calcium. 

By a special arrangement called the method of accelerated 
anode rays, it was found possible in 1 923 to extend mass spectrum 
analysis to a large number of metallic elements, so that more than 
half the known elements have now been analysed. 

A complete list of the isotopes of the non-radio-active elements 
so far discovered is given in the following table : 

Table of Elements and Isotopes. 




number of 

Mass-numbers of 
Isotopes in 
Order of Intensity. 






He . . 



















11, 10 
















(B 34/2285)Q 

D 2 



Table of Elements and Isotopes continued. 




number of 

Mass-numbers of 
Isotopes in 
Order of Intensity. 




















24, 25, 26 










28, 29, 30 















35, 37 










39, 41 





40, 44 






























56, 54 










58, 60 

Cu . . 




63, 65 





64, 66, 68, 70 





69, 71 





74, 72, 70 






Se .... 




80, 78, 76, 82, 77, 74 










84, 86, 82, 83, 80, 78 





85, 87 





88, 86 









3, (4) 

90, 91, 92, (96) 

Ag .... 




107, 109 





114, 112, 110, 113, 

111, 116 









7, (8) 

120, 118, 116, 124 

119, 117, 122, (121) 





121, 123 





128, 130, 126 









7, (9) 

129, 132, 131, 134, 

136, 128, 130, (126), 




















140, 142 









3, (4) 

142, 144, 146, (145) 





(197-200), 202, 204, 






(Numbers in brackets are provisional only.) 



By far the most important general result of these investiga- 
tions is that, with the exception of hydrogen, the weights of the 
atoms of all the elements measured, and therefore almost certainly 
of all elements, are whole numbers to the accuracy of experiment. 
With the mass-spectrograph, this accuracy is generally one part 
in a thousand. Of course, the error expressed in fractions of a unit 
increases with the weight measured, but with the lighter elements 
the divergence from the whole number rule is extremely small. 

This enables the most sweeping simplifications to be made in 
our ideas of mass. The original hypothesis of Prout, put forward 
in 1815, that all atoms were themselves built of atoms of " protyle," 
a hypothetical element which he tried to identify with hydrogen, is 
now re-established, with the modification that the primordial 
atoms are of two kinds : Protons and electrons, the atoms of 
positive and negative electricity. The atom, as conceived by 
Sir Ernest Rutherford, consists essentially of a positively-charged 
central nucleus around which are set planetary electrons at dis- 
tances great compared with the dimensions of the nucleus itself. 

The chemical properties of an element depend solely on its 
atomic number, which is the charge on its nucleus expressed in 
terms of the unit charge, e. A neutral atom of an clement of 
atomic number N has a nucleus consisting of K + N protons and 
K electrons and around this nucleus are set N electrons. The 
total number of protons in the atom K + N is called its mass- 
number. The weight of an electron on the scale we are using is 
'0005, so that it may be neglected. The weight of this atom will 
therefore be K + N, so that if no restrictions are placed on the 
value of K, any number of isotopes are possible. 

A statistical study of the results given above shows that the 
natural restrictions can be stated in the form of rules as follows : 

(a) In the Nucleus of an Atom there is never less than One 
Electron to every Two Protons. There is no known exception to 
this law. It is the expression of the fact that if an element has an 
atomic number N, the atomic weight of its lightest isotope cannot 
be less than 2N. True atomic weights corresponding exactly to 
2N are known in the majority of the lighter elements up to argon 
(A 38 ). Among the heavier elements the difference between the 
weight of the lightest isotope and the value 2N tends to increase 
with the atomic weight ; in the cases of mercury it amounts to 
37 units. 

(b) The Number of Isotopes of an Element and their Range of 
Atomic Weight appear to have Definite Limits. So far the element 
with the largest number of isotopes determined with certainty is 
xenon, with seven, but the majority of complex elements ha\e only 
two each. The maximum difference between the lightest and 
heaviest isotope of the same element so far determined is 8 units 


in the cases of krypton and xenon. The greatest proportional 
difference, calculated on the lighter weight, is recorded in the case 
of lithium, where it amounts to one-sixth. It is about one-tenth 
in the case of boron, neon, argon and krypton. 

(c) The Number of Electrons in the Nucleus tends to be Even. 
This rule expresses the fact that in the majority of cases, even 
atomic number is associated with even atomic weight and odd with 
odd. If we consider the three groups of elements, the halogens, 
the inert gases and the alkali metals, this tendency is very strongly 
marked. Of the halogens odd atomic numbers all 6 atomic 
weights are odd. Of the inert gases-- even atomic numbers 
18 ( + 2 ?) are even and 3 odd. Of the alkali metals odd atomic 
numbers 7 are odd and 1 even. In the cases of elements of other 
groups the preponderance, though not so large, is still very marked 
and beryllium and nitrogen are the only elements yet discqvered 
to consist entirely of atoms the nuclei of which contain an odd 
number of electrons. 

In consequence of the whole-number rule there is now no 
logical difficulty in regarding protons and electrons as the bricks 
out of which atoms have been constructed. An atom of atomic 
weight m is turned into one of atomic weight m -f 1 by the 
addition of a proton plus an electron. If both enter the nucleus, 
the new element will be an isotope of the old one, for the nuclear 
charge has not been altered. On the other hand, if the proton 
alone enters the nucleus and the electron remains outside, an 
element of next higher atomic number will be formed. If both 
these new configurations are possible, they will represent elements 
of the same atomic weight, but with different chemical properties. 
Such elements are called " isobarcs " and aro actually known 
e.g., the principal constituents of argon and calcium. 

The case of the element hydrogen is unique ; its atom appears 
to consist of a single proton as nucleus with one planetary electron. 
It is the only atom in which the nucleus is not composed of a 
number of protons packed exceedingly closely together. Theory 
indicates that when such close packing takes place the effective 
mass will be reduced, so that when four protons are packed together 
with two electrons to form the helium nucleus, this will have a 
weight rather less than four times that of the hydrogen nucleus, 
which is actually the case. It has long been known that the 
chemical atomic weight of hydrogen was greater than one-quarter 
of that of helium, but so long as fractional weights were general 
there was no particular need to explain this fact, nor could any 
definite conclusions be drawn from it. The results obtained by 
means of the mass-spectrograph remove all doubt on this point, and 
no matter whether the explanation is to be ascribed to packing or 
not, we may consider it absolutely certain that if hydrogen is 
transformed into helium a certain quantity of mass must be 
annihilated in the process. 


The theory of relativity indicates that mass and energy are 
interchangeable and that in C.G.S. units a mass m at rest may be 
expressed as a quantity of energy we 8 , where c is the velocity of 
light. Even in the case of the smallest mass this energy is 
enormous. If instead of considering single atoms we deal with 
quantities of matter in ordinary experience, the figures for the 
energy become prodigious. Take the case of 1 gram-atom of 
hydrogen that is to say, the quantity of hydrogen in 9 c.c. of 
water. If this is entirely transformed into helium the energy 
liberated will be 0-0077 x 9 x 10 20 = 6-93 x 10 18 ergs. Ex- 
pressed in terms of heat this is 1 -66 x 10 11 calories or, in terms of 
work, 200,000 kilowatt hours. The transmutation of the hydrogen 
from 1 pint of water would liberate sufficient energy to drive the 
Maurelania across the Atlantic and back at full speed. 

Should the research worker of the future discover some means 
of releasing this energy in a form which can be employed, the 
human race will have at its command powers beyond the dreams of 
scientific fiction ; but the remote possibility must always be con- 
sidered that the energy once liberated will be completely un- 
controllable and by its intense violence detonate all neighbouring 
substances. In this event the whole of the hydrogen on the earth 
might be transformed at once and the success of the experiment 
published at large to the universe as a new star. 


By SIR FRANK DYSON, F.R.S., Astronomer Royal. 

In order to explain the transmission of the undulations of 
light across space, the existence of a medium called " ether " 
was assumed. This was supposed to possess properties such as 
rigidity and elasticity similar to those of matter. When it was 
found that electro-magnetic oscillations (such as we now have 
in radio-telegraphy) were transmitted with the same velocity 
as light, the same all-pervading medium was naturally taken as 
their home. 

Many noteworthy attempts have been made to determine by 
optical and electrical means the movement of the earth through this 
medium. They all gave negative results, and in explanation 
Einstein put forward in 1905 the restricted theory of relativity. 
This theory reviewed our fundamental ideas of time and space ; 
it denied the existence of absolute space and absolute time, but 
regarded these as dependent on the observer. Einstein showed 
that a simple relationship held between the measures of space 
and time made by two observers moving uniformly with respect 
to each other. This theory was in harmony with the experi- 
mental results which had failed to discover the motion of the earth 
through the ether, and also accounted for the change of mass 
found by experiment in particles moving with very great velocities. 

In 1908 Einstein's theory was put in a clearer light by Min- 
kowski, who introduced the idea of the continuum. Events take 
place in a four-dimensional continuum of space and time and not 
in a three-dimensional space and a wholly independent one- 
dimensional time. The relationship between the space and time 
of two observers moving relatively to one another was shown to be 
analogous to a rotation of axes in ordinary Euclidian geometry. 



So far, the theory of relativity had applied only to systems in 
uniform motion relatively to one another. Could it be extended 
to systems in which there is accelerated motion ? In Newtonian 
dynamics acceleration is attributed to force. Centrifugal force is 
regarded as a fictitious kind of force attributable to the rotation 
of the system of reference, but " gravitational " force as some- 
thing inherent in matter. Is it possible to explain the latter by 
the properties of the continuum ? By an extraordinarily brilliant 
piece of mathematical analysis, Einstein was led to formulate 
in 1915 a law of gravitation. In the neighbourhood of matter 
the geometry of the continuum differed slightly from that of 
Euclid. It is not possible to visualise this, but it is analogous 
to the difference, in two-dimensional geometry, between the 
surface of a large sphere and a plane. The non-Euclidian pro- 
praties of the continuum manifest themselves as a field of force. 
This can be illustrated in principle by the deflexion of path under- 
gone by a pedestrian who tries to walk in a straight course over 
the slope of a hill. The deflexion is due to the geometrical pro- 
perties of the slope which may be regarded as a non-Euclidian 
space of two dimensions. 

Einstein's law of gravitation, though entirely different from 
Newton's in mathematical form as in the ideas from which it 
arose, gives results almost identical with those of Newton. This 
is its first merit, for Newton's law of the inverse square has been 
found suflicient to explain in great detail the movements of sun, 
moon and planets, precession of the equinoxes, the tides, the figure 
of the earth and many other phenomena. To the first order, then, 
Einstein's law gives results identical with those of Newton. But 
there is one phenomenon which has puzzled astronomers since the 
time of Leverrier. The planet Mercury moves round the sun in 
an orbit which is, to a first approximation, an ellipse. But closer 
study shows that the position of this ellipse undergoes a change in 
the course of time, so that the point at which Mercury is nearest 
the sun (its perihelion) is not fixed, but is slowly revolving. The 
greater part of this revolution is duly explained by the attraction 
of the other planets, but a part is left over-- only 40 seconds of arc 
a century which had not been satisfactorily accounted for, 
although numerous hypotheses had been framed. Einstein's law 
of gravitation took this discrepancy in its stride and accounted 
for it exactly. 


This was an achievement which greatly enhanced the proba- 
bility of Einstein's law being correct. He accordingly examined 
it to see if there were other phenomena which would follow from 
his law, but were not given by that of Newton. He found two. 
'The first of these relates to the bending of light. If light in its 


journey to the earth from* a star passes near the sun, it will be 
slightly deflected in its course, just as a particle of matter would 
be. He gave the exact amount of this deflexion, which is greater 
the nearer the light passes by the sun. This prediction was 
verified at the total eclipse of the sun on May 29th, 1919. British 
expeditions were sent to Brazil and to the west coast of Africa 
to photograph the eclipsed sun. Seven photographs were taken 
which showed a number of stars. The observers in Brazil waited 
for two months, when they were able to photograph the same stars 
just before sunrise, and the photographs were brought home and 
carefully measured. It was found that the relative position of the 
stars had been slightly changed in accordance with Einstein's 

The differences in the relative positions of the stars are, of 
course, not visible to the eye, as they are very minute. The largest 
displacement is only one-third of the diameter of the star's image 
shown on the photograph. 

The predicted amount of the bending of the light by the sun's 
gravitation for the stars shown on one of the photographs taken 
is compared in the following table with the amount actually 
observed : 

Predicted. Observed. 

0-32* 0-2(T 

0-33 0-32 

0-40 0-5G 

0-53 0-54 

0-75 0-84 

0-85 0-97 

0-88 1-02 

The observers in Africa were not so fortunate in weather con- 
ditions as those in Brazil, but they nevertheless succeeded in 
verifying Einstein's prediction. At the total solar eclipse of 1922 
these results were confirmed by Canadian and Australian and still 
more by American astronomers. 


Another test which Einstein proposed for the verification of his 
theory is a slight displacement in position of the lines in the solar 
spectrum. The exact position of a line in a spectrum may be con- 
sidered as measuring the time of some particular vibration in the 
atoms of the substance the light of which is being analysed. 
According to the theory of relativity, the time of vibration of an. 
atom in the sun will be lengthened slightly by the effect of gravi- 
tation. If, then, the position of the iron lines in the solar spectrum, 
due to iron vapour, for example, are compared with the position, 
of those arising from the light of an electric arc with iron poles, 
they should be found to be shifted very slightly towards the red. 
end of the spectrum. 


The verification of this consequence of the theory of relativity 
was a matter of considerable difficulty, because there are many 
causes which produce slight displacements in spectral lines. Of 
these the effects due to possible movements of the solar gases 
were the most difficult to eliminate. Motion effects due to the 
sun's rotation and to the earth's rotation and varying distance 
from the sun are well understood, and could readily be allowed for. 
There was, however, a puzzling difference in the displacement in 
different parts of the sun's disc, the observed shift of the lines 
increasing from the centre towards the solar limb, where it was 
found to be in excess of Einstein's prediction. To determine the 
cause of this involved measuring the shift in light coming from the 
hidden face of the sun, as reflected to us by the planet Venus 
when near superior conjunction (behind the sun). 

In addition to effects due to motion, the positions of spectrum 
lines also depend to a small extent on the pressure and on the 
electrical conditions of the gas from which the light comes, and 
on the effects of anomalous refraction if the gases have an appreci- 
able density. This complicated problem was attacked by several 

Mr. J. Evershed, the Director of the Indian Observatory at 
Kodaikanal, found that the lines in the solar spectrum did, in 
fact, show a displacement, and he came to the conclusion that this 
displacement was for the greater part that predicted by Einstein, 
the disturbing effects, due to pressure, &c., being negligible 
according to his researches. His conclusion has since been con- 
firmed in a very complete manner by Dr. St. John, of the Mount 
Wilson Observatory, who has not only verified the relativity 
prediction, but has given an explanation of some shifts of the 
lines in excess of the Einstein effect. These residual effects had 
also been noticed by Evershed. 



On December 13, 1920, the angular diameter of a star was 
measured for the first time in history with an apparatus devised 
by Prof. A. A. Michelson. Hitherto every star had appeared as 
a mere point of light, and no test had been able to differentiate 
it from a geometrical point. But on that eventful evening a 
20-ft. interferometer constructed at the Mount Wilson Observatory 
was turned on the star Betelguese, and the measurement revealed 
that this star had a disc one-twentieth of a second of arc in 
diameter about the size of a halfpenny 50 miles away. The 
distance of Betelgeuse is known roughly (unfortunately it cannot 
be found so accurately as the distance of many stars), so that wo 
can convert this apparent size into approximate actual size. 
Betelgeuse is not less than 200 million miles in diameter. The 
orbit of the earth could be placed entirely inside it. 

The stars are thus not limited to objects of comparatively 
small bulk like the sun ; there are among them individuals truly 
gigantic in comparison. We can add another step to the astro- 
nomical multiplication table - a million earths make one sun ; 
ten million suns make one Betelgeuse. This is a comparison of 
volume, not of amount of material. It leaves open the question 
whether in order to obtain one of these giants we should take the 
material of ten million suns rolled into one, or whether we should 
take the material of the sun and inflate it to ten million times 
its present size. 

There is no doubt that the latter answer is nearer the truth. 
Betelgeuse contains more matter than the sun (perhaps fifty times 
as much) ; but in the main its vast bulk is due to the diffuseness 

* Abstracted from a discourse delivered before the Royal Institution on 
February 23, 1923. 


with which this material is spread out. It is a great balloon of 
low density, much more tenuous than air, whereas in the sun the 
material is compressed to a density greater than water. 

Whether a star is one of these balloon-like bodies or whether 
it is dense like the sun depends on the stage of its life at which we 
catch it. It is natural to think that the stars gradually condense 
out of diffuse material, so that they become denser and denser 
as their life history proceeds. We can now see in the heavens 
samples of every stage in the development of a star. The majority 
of those seen with the naked eye are in the early diffuse state ; 
that is not because these young stars are really more numerous, 
but because their great bulk renders them brighter and more 
conspicuous. What I shall have to say about the inside of a star 
refers chiefly to the young diffuse stars -the " giant stars " as 
they are called. The reason is that we understand much more 
about the properties of matter when it is in the condition of a 
perfect gas than when it is condensed ; although the difficulties 
of treating a dense star like the sun are not insuperable, we have 
naturally made the most progress with the easier problem of giant 


We only observe the physical conditions of the surface of a 
star, and at first it might seem impossible to learn anything about 
the conditions in the interior. Consider for example, the question 
of temperature. The nature of the light received from Betelgeuse 
teaches us that the temperature is 3,000 C. not an extravagantly 
high temperature judged even by terrestrial standards. But this 
refers, of course, to the layer near the surface from which the 
observed light is corning ; it is just the marginal temperature of 
the furnace, affording no idea of the terrific heat within. 

The internal temperature of a star depends on the particular 
star considered, but it is generally .from 2 to 20 million degrees at 
the centre. Do not imagine that this is a degree of heat so vast 
that ordinary conceptions of temperature have broken down. 
These temperatures are to be taken quite literally. Temperature 
is a mode of describing the speed of motion of the ultimate particles 
of matter. In a mass of helium at ordinary temperatures the 
average speed of the atoms is rather less than 1 mile per second ; 
at 4 million degrees it is 100 miles per second. This is a high 
speed, but not a speed to feel uncomfortable over. In the 
laboratory physicists experiment with atoms of helium, the 
a-particles from radio-active substances, moving at 100,000 miles 
a second. The physicists are rather disappointed with the jog- 
trot atoms in the stars. 


We must imagine, then, a typical giant star as a mass of 
material with average density about that of air swollen to at least 


a thousand times the bulk of the sun. The atoms of which it 
consists are rushing in all directions with speeds up to 100 miles 
a second, continually colliding and changing their courses. Each 
atom is being continually pulled inwards by the gravitation of the 
whole mass, and as continually boosted out again by collision 
with atoms below. The energy of this atomic motion, which we 
may term " material heat," constitutes a great store of heat con- 
tained in the star ; but this is only part of the store. The star 
contains a store of another kind of heat, " ethereal heat," or 
ether- waves like those which bring to us the sun's heat across 
90 million miles of vacant space. These waves also are hastening 
in all directions inside the star. They are encaged by the material, 
which prevents them leaking into outer space except at a slow 
rate. An ether- wave making for freedom is caught and absorbed 
by an atom, flung out in a new direction, and passed from atom 
to atom ; it may thread the maze for hundreds of years until by 
accident it finds itself at the star's surface, free now to travel 
through space indefinitely, or until it ultimately reaches some 
distant world, and perchance entering the eye of an astronomer 
makes known to him that a star is shining. 

The possession of this double store of heat is a condition which 
we do not encounter in any of the hot bodies more familiar to 
us. In the hot bodies of the laboratory the heat is almost entirely 
in the material form, the ethereal portion being insignificant. 
In the giant stars the heat is divided between the two forms in 
roughly equal amounts. Can we not imagine a third condition 
in which the heat is almost wholly ethereal, the material portion 
being insignificant ? We can imagine it, no doubt ; but the 
interesting, and perhaps significant, thing is that we do not find 
it in Nature. 


You have heard of the pressure of light that light actually 
has mass and weight and momentum, and exerts a minute pressure 
on any object which obstructs it. A beam of light or ether 
waves is like a wind, a very minute wind as a rule ; but the 
intense ethereal energy inside the star makes a strong wind. 
This wind distends the star. It bears to some extent the weight 
of the layers overhead, leaving less for the elasticity of the gas to 
bear. That of course has to be taken into account in our calcula- 
tion of the internal temperatures making them lower than the 
older theory supposed. 

Just as ether and matter share the heat-energy between them, 
so' the ethereal wind and the material elasticity share the burden 
of supporting the weight of the layers above, and we are able to 
calculate the proportions in which they share it. To a first 
approximation the same proportion holds throughout nearly the 
whole interior, and the proportion depends only on the total 


mass of the star, not on the density or even on the chemical 
composition of the material. Moreover, in order to make this 
calculation we do not need any astronomical knowledge ; all the 
constants in the formula have been determined by the physicist 
in his laboratory. 

Let us imagine a physicist on a cloud-bound planet, who has 
never heard tell of the stars, setting to work to make these calcula- 
tions for globes of gas of various dimensions. Let him start with 
*i globe containing 10 gm., then 100 gm., 1,000 gm., and so on. 
The globes mount up in size rather rapidly. No. 1 is about the 
weight of a letter ; No. 5, a man ; No. 8, an airship ; No. 10, an 
ocean liner ; after that comparisons are difficult to find. The 
following table gives part of his results : 

No. of Globe. Ethereal Pressure. Material Pressure. 

30 0-00000016 0-99999984 

31 0-000016 0-999984 

32 0-0016 0-9984 

33 0-106 0-894 

34 0-570 0-430 

35 0-850 0-150 

36 0-951 0-049 

37 0-984 0-016 

38 0-9951 0-0049 

39 0-9984 0-0016 

40 0-99951 0-00049 

The early part of the table would consist of long strings of 
O's and 9's. For the 33rd, 31th and 35th globes the table gets 
interesting ; and then lapses back into 9's and O's again. Re- 
garded as a tussle between ether and matter to control the situa- 
tion, the contest is too one-sided to be interesting, except just 
from Nos. 33 to 35, where something more exciting may be 

Now let us draw aside the veil of cloud behind which our 
physicist has been working, and let him look up into the skies. 
He will find there a thousand million globes of gas all of mass 
between the 33rd and 35th globes. The lightest known star comes 
just below the 33rd globe ; the heaviest known star is just beyond 
the 35th globe. The vast majority are between Nos. 33 and 34, 
just where the ethereal pressure begins to be an important factor 
in the situation. 

It is a remarkable fact that the matter of the universe has 
aggregated primarily into units of nearly constant mass. The 
stars differ from one another in brightness, density, temperature, 
etc., very widely ; but they all contain roughly the same amount 
of material. With a few exceptions they range from half to five 
times the mass of the sun. There can no longer be serious doubt 
as to the general cause of this, although the details of the explana- 
tion may be difficult. Gravitation is the force which condenses 
matter ; it would if unresisted draw more and more matter 


together, building globes of enormous size. Against this, ethereal 
pressure is the main disruptive force (doubtless assisted by the 
centrifugal force of the star's rotation) : its function is to prevent 
the accumulation of large masses. 

This resistance, as we see, only begins to be serious when the 
mass has already nearly reached the 33rd globe, and if indeed it 
is efficacious, it will stop the accumulation before the 35th globe 
is reached, because by then it has practically completely ousted 
its more passive partner (material pressure). We do not need 
to know exactly how strong the resistance must be in order to 
prevent the accumulation, because when once the resistance 
begins to be appreciable it increases very rapidly, and will very 
soon reach whatever strength is required. All over the universe 
the masses of the stars bear witness that the gravitational aggrega- 
tion proceeded just to the point at which the opposing force was 
called into play and became too strong for it. 


It was shown by Homer Lane in 1870 that as a gaseous star 
contracts its temperature will rise. Betelgeuse is typical of the 
first stage when the temperature has risen just far enough for the 
star to be luminous. It will go on. contracting arid becoming 
hotter, its light changing from red to yellow and then to white. 
When the condensation has proceeded far enough, the material, 
if it behaves like terrestrial substances, will become too dense to 
follow the laws of a perfect gas. A different law then begins to 
take control. The rise of temperature becomes less rapid, is 
checked, and finally the temperature falls. We can calculate 
that the greatest temperature is reached at a density of about one- 
quarter to one- third of that of water. The sun is denser than 
water, so that, according to this theory, it must have passed the 
summit and is in the stage of falling temperature. 

So long as the temperature is rising, the brightness of the star 
scarcely changes. It is becoming hotter but smaller. Calculation 
shows that the increased output of light and heat per square 
metre of surface, and the decreased area of the surface, very 
nearly counteract one another, so that the total output remains 
fairly steady. But on the downward path the falling temperature 
and diminishing surface both reduce the light, which falls off 
rapidly between the successive stages or types which we recognise. 
That is entirely in accordance with what is observed to happen. 

Taking any level of temperature, a star will, in its life-history, 
pass through it twice, once ascending and once descending. In 
the main we have been in the habit of classifying stars according 
to their surface temperature, because it is on this that the spectral 
characteristics of the light, its colour and the chemical elements 
revealed, chiefly depend. But that classification mixes together 


stars from an early ascending stage and those from a later descend- 
ing stage. For example, a star like Betelgeuse, just beginning 
its career, is put in the same class with a dense red star, which 
has run its course and reached its second childhood. They are 
both red stars of low temperature, and that was good enough for 
the early attempts at classification. Sir Norman Lockyer always 
stoutly maintained the existence of the ascending and descending 
series, but he was almost alone among spectroscopists in this. 
He did not actually succeed in separating the ascending and 
descending stars, though sometimes he came very near to the 
right criterion. We owe to Russell and to Hertzsprung the actual 
separation. They discovered it not by spectroscopy, but by 
measuring the absolute brightness of stars ; the greater brightness 
of the ascending stars, due to their large bulk, easily distinguishes 
them from the descending stars, at any rate in the low temperature 
groups. At the highest temperatures the two series merge into 
one another. 

Eecently, however, some new results have confronted us 
which seem to call for a considerable modification of the theory 
of stellar evolution just described. It appears that even when 
the density of a star is greater than water, the material does not 
cease to behave as a perfect gas, The reason is that at the very 
high temperature in the interior, the atoms are broken up (highly 
ionised) ; the remnants occupy much less volume than terrestrial 
atoms and can be compressed much closer together. Both theory 
and observation indicate that in some of the stars we are dealing 
with material denser than anything observed on the earth, which 
is nevertheless perfectly compressible like an ideal gas. 


We have hitherto pictured the inside of a star as a hurly-burly 
of atoms and ether waves. We must now introduce a third 
population to join in the dance. There are vast numbers of free 
electrons unattached units of negative electricity. More 
numerous than the atoms, the electrons dash about with a hundred- 
fold higher velocity, corresponding to their small mass, which is 
only about -r^Vo f a hydrogen atom. These electrons have come 
out of the atoms, having broken loose at the high temperature 
here involved ; and in a typical star a large proportion of them 
must have become free. 

This condition solves for us our chief difficulty as to the 
molecular weight of stellar material, which we must know in 
order to perform our calculations as to the state of the star. At 
first sight it might seem hopeless to arrive at the molecular weight 
without knowing the elements which constitute the bulk of the 
material. But suppose first that the temperature is so high that 
all the satellite electrons, which are believed to be revolving about 

(B 34/2285)Q B 


the composite central nucleus of any atom, have broken away. 
An atom of sodium will have separated into 12 particles, viz., 
11 electrons and 1 mutilated atom ; its atomic weight 23 is divided 
between 12 independent particles, so that the average weight of 
each is 23/12 = 1-92. Next take iron; the atomic weight 56 
is divided between 27 particles : average 2-07. For tin we have 
119 divided by 51 : average 2-34. For uranium 238 divided by 
93 : average 2 56. It scarcely matters what element we take ; 
the average weight of the ultimate particles (which is what we 
mean by the molecular weight) is always somewhere about 2. 
If only the stars were a bit hotter than they actually are it would 
make our task very easy. Unfortunately they are not hot enough 
to give complete separation, and the actual degree of separation 
will depend on the temperature of the star, thus introducing a 
difficult complication. Generally at least half the electrons are 
detached, and the molecular weight must be taken between 
2 and 3. 


We pictured a physicist on a cloud-bound planet who was able 
from laboratory data to predict how large would be the masses 
into which the material of the universe must aggregate. Let us 
now set him a harder task. We inform him that we have observed 
these masses of gas, and, choosing one equal, say, to his 34th 
sphere, we ask him to predict how brightly it will shine. As 
already mentioned, the star keeps practically the same brightness 
so long as it is a perfect gas ascending in temperature ; so it 
should not be necessary to give the physicist any data except the 
precise mass. To use the same plan as before, we imagine a series 
of lamps of 10 candle-power, 100 candle-power, 1 ,000 candle-power, 
and so on ; and his task is to pick out which lamp in this series 
corresponds approximately to the star. I believe that it is now 
possible for him to perform this task, and to pick out (correctly) 
the 31st lamp. For this purpose, however, it is not enough that 
he should know all about the heat stored in the interior of the 
star ; the brightness of the star depends on the rate at which the 
ether waves are leaking out, and that introduces a new subject 
the obstructive power of the material atoms which dam back the 
radiant flow. 

Another name for this obstructive power is opacity. A sub- 
stance which strongly obstructs the passage of light and heat 
waves is said to be opaque. The rising temperature towards the 
centre of the star urges the heat to flow outwards to the lower 
temperature level ; the opacity of the material hinders this flow. 
The struggle between these two factors decides how much light 
and heat will flow out. We have calculated the internal tempera- 
ture distribution, so that we know all about the first factor : if, 
then, we can observe the outward flow which occurs, that should 


settle the value of the second factor the opacity. The outward 
flow is capable of observation, because it constitutes the heat 
and light sent to us by the star. 

One of the troubles of astronomy is that our information about 
the stars is so scattered. We know the mass of one star very 
accurately, but we do not know its absolute brightness ; we know 
the brightness of another, but not its mass ; for a third we may 
have an accurate knowledge of the density, but nothing else. 
For Sirius, Procyoii and a Ceiitauri our knowledge is fairly com- 
plete and accurate ; but none of these are giant stars in the state 
of a perfect gas and they are therefore useless for the present 
discussion. Within the last year or so, however, we have been 
so fortunate as to obtain complete and very accurate information 
for one of the giant stars, Capella. This is another of the benefits 
which astronomy has derived from Prof. Michelson's interfero- 
meter method of observation. The brighter component of Capella 
(which is a double star) has a mass 4-2 times that of the sun, and 
a luminosity 1 60 times greater. We can use these facts to calculate 
the opacity of Capella, and it turns out to be 150 in C.G.S. units. 
To illustrate the meaning of this, let us enter Capella and find 
a region where the density is that of the terrestrial atmosphere 
we are accustomed to ; a slab of this gas only 6 inches thick 
would form an almost opaque screen. Only l/20th of the radiant 
energy falling on one side would get through to the other, the 
rest being absorbed by the gas. 


It seems at first surprising that 6 inches of gas should stop 
the ether waves so effectually, but we might have anticipated 
something like this from general physical knowledge. We give 
different names to ether waves according to their wave-lengths. 
The longest are the Hertzian waves used in wireless telegraphy ; 
then come the invisible heat waves, then light waves, then photo- 
graphic or ultra-violet waves. Beyond these we have X-rays, 
and finally the shortest of all- -the /-rays emitted by radio-active 
substances. Where in this series are we to place the ether waves 
in the interior of a star, which constitute its ethereal heat ? It 
is solely a question of temperature, and the ether waves at stellar 
temperatures are those which we call X-rays more precisely, 
they are very " soft " X-rays. Now X-rays, and soft X-rays 
especially, are strongly absorbed by all substances, and the 
opacity which we have found in Capella is of the same order of 
magnitude as the opacity of terrestrial substances to X-rays 
measured in the laboratory. 

The physicist in the laboratory, investigating the opacity of 
substances to soft X-rays, has a big advantage, however, because 
he can vary the material experimented on, whereas we have to be 
content with the material, whatever it is, composing the stars. 

(B 34/2285)Q - E 2 


The physicist is also interested in finding how the absorption 
changes for different wave-lengths. We can follow him in this, 
and even do better than he, because he is restricted by certain 
practical difficulties to a narrow range of wave-length, whereas 
we can explore a range of wave-length covering a ratio of at least 
10 to 1 by using stars of different temperatures. It is true that 
our results are not yet very accurate ; we have only one star, 
Capella, for which a really good determination is possible, but for 
other stars rough values can be found. The terrestrial results 
indicate an extremely rapid change of absorption for slight 
alterations of wave-length ; the astronomical results, on the 
contrary, give a nearly steady absorption-coefficient. We cannot 
yet detect certainly whether it increases or decreases with wave- 
length, but, at any rate, there is no very rapid change. This 
profound discrepancy between astronomical and laboratory results 
leads us to inquire more deeply into the theory of absorption 
in a star. 

It is now generally agreed that when ether-waves fall on an 
atom they are not absorbed continuously. The atom lies quiet, 
waiting its chance, and then suddenly swallows a whole mouthful 
at once. The waves are done up in bundles called " quanta," and 
the atom has no option but to swallow the whole bundle or leave 
it alone. Generally the mouthful is too big for the atom's diges- 
tion, but the atom does not stop to consider that. It falls a 
victim to its own greed in short, it bursts. One of its satellite 
electrons shoots away at high speed, carrying off the surplus 
energy which the atom was unable to hold. The bursting could 
not continue indefinitely unless there were some counter-process 
of repair. The ejected electrons travel about, meeting other 
atoms ; after a time a burst atom meets a loose electron under 
suitable conditions, and induces it to stay and heal the breach. 
The atom is now repaired and ready for another mouthful as soon 
as it gets the chance. 

From this cause a big difference arises between absorption of 
X-rays in the laboratory and in the stars. In the laboratory the 
atoms are fed very slowly ; the X-ray bundles which they feed 
on can only be produced by us in small quantities. Long before 
the atom has the chance of a second bite it is repaired and ready 
for it. But in the stars the intensity of the X-rays is enormous ; 
the atoms are gorged and cannot take advantage of their abundant 
chances. The consumption of food by the hungry hunter is 
limited by his skill in trapping it ; the consumption by the 
prosperous profiteer is limited by the strength of his digestion. 
Laboratory experiments test the atom's skill in catching food ; 
stellar experiments test how quickly it recovers from a meal and 
is ready for another. This accounts for the different opacity 
of stars, as compared with the opacity of terrestrial substances 
to ether waves. 


Some of the leading factors participating in the problem of the 
interior of a star have now been discussed, and it is clear that 
many varied interests are involved. The partial results already 
attained, however, correspond well enough with what is observed 
to encourage us to think we have begun at the right end in dis- 
entangling the difficulties, and we do not anywhere come against 
difficulties which appear likely to be insuperable. The fact is that 
gaseous matter at very high temperature is the simplest kind 
of substance for a mathematical physicist to treat. To understand 
all that is going on in the material of a piece of wood is a really 
difficult problem, almost beyond the aspirations of present-day 
science ; but it does not seem too sanguine to hope that in a not 
too distant future we shall be able to understand fully so simple a 
thing as a star. 



In seeking the originators of radio-communication, the men 
who discovered electricity and investigated its fundamental pro- 
perties are apt to be overshadowed by those who are concerned 
rather with the development of the art as we know it to-day. 
Many would be content to mention the names of Hertz, who in 1887 
first produced and measured the wireless waves predicted twenty 
years earlier by Clerk Maxwell ; of Lodge, who a year later showed 
at the Royal Institution some of its effects ; of Marconi, whose 
inventions have done so much to forward its practical use ; and of 
Fleming, who first investigated the properties of the rectifying 

These are great names in the growth of radio-communication, 
but tribute should also be paid to those who made this growth 
possible. To find them we must go back many years centuries 
in some cases to investigators who, driven by their love of dis- 
covery and impelled by their thirst to know, sought, not to discover 
wireless telegraphy, but to improve our knowledge of Nature and 
to bring under the realm of law and order some of the strange 
happenings which their searches led them to note. 

Amber, found chiefly on the shores of the Baltic Sea, was much 
sought for in early days ; recently an interesting dissertation on 
the trade routes of the ancient world has been written based on 
the dispersion of amber. About 600 B.C. it had reached Asia Minor, 
and Thales of Miletus is said to have been the first to observe its 
property of attracting light bodies to itself when rubbed. Thus 
we derive the name " electricity " (rjXeKrpov = amber). 

It was probably at a later date than this that the curious 
property of a stone found in Magnesia was first noted, at any rate 
in the Western world, when it was observed that if freely suspended 
it always set itself in a definite direction. It gained the name of 
the leading stone or loadstone, and this property formed the basis 


of the science of magnetism. Tradition tells us that the Chinese 
knew of this property centuries earlier. 

Modern knowledge both of electricity and magnetism dates from 
Dr. Gilbert, of Colchester, Physician to Queen Elizabeth, who in 
1600 published his great and interesting work " De Magnete." 
Gilbert studied only electricity produced by friction ; the electric 
current was still unknown, and for nearly 200 years remained 
unknown until Galvani, at Bologna in 1786, observed the con- 
vulsive shock produced in a frog's leg -at first when it was con- 
nected to a frictional electrical machine, and then later wher 
two dissimilar metals, iron and copper, were placed in contact 
with nerve and muscle respectively and were then made to touch, 
His observations were continued and extended by Volta at Pavia 
who showed in 1800 that the electricity originated at the contacl 
of the metals. This led him to the discovery of the voltaic pile 
and the construction of an electric battery. 

Various workers from Gilbert onwards had surmised that ther* 
must be some relation between electricity and magnetism. Th< 
verification of this was due to Oersted, Professor at Copenhagen 
who in 1820 showed that a wire carrying a current held near s 
magnet caused the magnet to move. Oersted's great discovery was 
at once repeated by Ampere in Paris, and he, by the aid of a fe\\ 
brilliant fundamental experiments, discovered the laws whicli 
govern the mutual reaction between a current and a magnet, 
About the same time Faraday, at the Royal Institution in London, 
pursued the matter still further, and laid the foundations of the 
science of electro-magnetism, the basis of all electro- technical 
applications of to-day. 

Meanwhile in Germany G. S. Ohm was at work. Volta had 
shown in 1800 that electrical force or " electro-motive force " was 
produced in his battery, and that when the two metals which 
constitute its two poles are joined by a wire, a current of electricity 
flows round the circuit. It was left for Ohm to state the relation 
between the current, the electro- motive force or electrical 
pressure producing it and the resistance of the circuit. Mean- 
while, iti America, Joseph Henry had during the same period 
discovered for himself many of the fundamental laws of electro- 

These men, scattered throughout many lands, yet inspired b} 
the same end the improvement of natural knowledge were the 
founders of modern electricity. When, therefore, some sixty yean 
ago, a body of English men of science, led by Lord Kelvin, realised 
that the time had come to consolidate their knowledge into f 
system of accurate measurement, they found that new ideas 
needed definition, new units and standards required names, and 
with one consent they agreed to give to these standards the names 
of the great men whose labours through the centuries had wrested 
from Nature the secrets of electricity and magnetism. Thus we 


have the ohms and volts, amperes, henrys and farads which now 
form part of our daily language. 

On the work of the men whose names are thus commemorated 
is based the discoveries of the brilliant experimenters who have 
made it possible to girdle the earth with a wireless chain depending 
on two or at most three great stations. 

Foremost among those associated with modern developments is 
Clerk Maxwell, who in 1865 read before the Royal Society his paper 
on " The Equations of the Electro-Magnetic Field." It was an 
attempt, which has stood the test of time, to apply mathematical 
reasoning to those principles, enunciated by Faraday, on which 
the construction of generators and motors, transformers and, 
indeed, practically all electric machinery, is based. This reasoning 
led him to the result that the effect of changes in an electric current 
in a conducting wire would be propagated through space with a 
speed depending on the two constants, inductive capacity and 
magnetic permeability, which define the electric and magnetic 
conditions of the medium surrounding the wire. The values of 
these constants for air can be found from electrical considerations, 
and hence the velocity with which electro-magnetic disturbances 
are propagated can be calculated . To quote Maxwell's words :- - 

" We now proceed to investigate whether these properties of 
that which constitutes the electro-magnetic field, deduced from 
electro-magnetic phenomena alone, are sufficient to explain the 
propagation of light through the same substance," and his con- 
clusion is : " The agreement of the results seems to show that 
light and magnetism are affections of the same substance, and that 
light is an electro-magnetic disturbance propagated through the 
field according to electro-magnetic laws." 

Maxwell found that when the calculations were made, the 
resulting value for the velocity was approximately equal to the 
velocity of light. The work was extended in his " Treatise on 
Electricity and Magnetism," published in 1873. The values of 
the velocity of light and the velocity of propagation of electro- 
magnetic waves were not known then with present-day accuracy, 
and he concludes that they are quantities of the same order of 
magnitude. Present-day figures show that they are identical, 
and the electro-magnetic theory of light is universally accepted. 
Nor was the result true only for propagation through air or inter- 
stellar space ; such observations as were then available showed 
that, in all probability, it held for all transparent media, though 
there were discrepancies, known now to be due to dispersion, 
which required explanation. But there was a wide gap between 
this theoretical deduction of Maxwell and the wireless telegraphy 
of to-day, which needed many more investigations in " pure " 
science before the bridge was complete. No one had received 
electro-magnetic vibrations at any rate, to his certain knowledge. 
The method of generating them and the means for measuring 
them were still to come. 


For the former we have to go back to a remarkable paper of 
1853 by Lord Kelvin. Helmholtz seems to have been the first 
to conceive that the discharge of a condenser through a wire might 
consist of a forward and backward motion of electricity between 
the coatings a series of currents in opposite directions. Lord 
Kelvin took up the question mathematically and investigated the 
phenomena. He showed that, under certain conditions, there 
would be oscillations of periodic time 27rvLC, where L is the 
inductance of the coil, and C the capacity of the condenser. 
These oscillations must, according to the theory, give rise to 
waves travelling out into space with the electro-magnetic velocity. 
Fitzgerald had predicted in 1 883 that they might be produced by 
utilising the oscillatory discharge of a leyden jar, and Sir Oliver 
Lodge in 1887 produced and detected them. For their detection 
the principle of resonance was employed. Any mechanical 
system free to vibrate has its own period of oscillation, and the 
application to it of a series of small impulses at intervals coincident 
with the free period of the system results in a disturbance of large 
amplitude. So, too, an electric system having capacity and 
inductance has its own period of electrical oscillation, and, if this 
coincides with the period of incoming electrical waves, electrical 
disturbances of a magnitude which can be detected by our appara- 
tus are set up. It is necessary that the receiver and the trans- 
mitter should be in tune. Lodge made use of this principle, and, 
by receiving the waves on wires adjusted to resonance with his 
leyden jar and coil, was able to detect them, David Hughes, 
working in the early 'eighties, had already detected such oscilla- 
tions, but was discouraged from pursuing the subject. 

In 1879, in consequence of the offer of a prize by the Berlin 
Academy, the attention of Heinrich Hertz, then a student under 
Helmholtz, was attracted to the problem of electric oscillations and 
their detection. He came to the conclusion that with the means 
of observation then at his disposal " any decided effect could 
scarcely be hoped for, but only an action lying just within the 
limits of observation." The investigation was laid aside, only 
to be revived in 1886 by a chance observation of the effect of 
resonance in two circuits which happened to be in tune, and his 
realisation of the fact that herein lay the means of solution of his 
problem. His paper " On Very Eapid Electric Oscillations " 
appeared in 1887, and from this experiment came verification of 
Maxwell's theory, the basis of all our knowledge of wireless. 
Fitzgerald directed the attention of English physicists to the 
work at the British Association meeting in 1888, and Lodge 
exhibited many of the effects of the waves at the Roval Institution 
in 1889. 

The investigations which led to such brilliant results were 
inspired by the desire for knowledge ; the idea of their practical 
application was entirely absent. Signalling by wireless waves 


was not foreshadowed until Crookes suggested it in 1892, and in 
1893 Lodge heard of Branly's coherer and applied it to the rectifi- 
cation and reception of wireless waves. From this started the 
investigations of many of those whose names as pioneers are 
familiar to all. But another discovery in pure science was 
necessary to complete the work. 

Edison had shown in 1883 that if an insulated electrode is 
inserted in an ordinary glow lamp, there is a current of negative 
electricity from the filament to the electrode, and Fleming made 
some observations about this date on the Edison effect. In 1904 
he applied them to produce a valve rectifier for high-frequency 
oscillations by connecting one pole of his receiving circuit to an 
insulated plate or cylinder within a carbon lamp, of which the 
negative electrode formed the other pole of the receiving circuit. 

Dr. Lee de Forest improved this oscillation valve a little later, 
making it an amplifier as well as a rectifier by placing between the 
filament and the plate or cylinder a grid of metal wire connected 
to an external source of electromotive force. There is ordinarily 
a current of negative electricity passing from the filament to the 
plate the plate current it is called- - through the interstices of the 
grid. By varying the potential of the grid this current can be 
varied, and the conditions can be so adjusted that small changes 
in the potential of the grid will produce large changes in the plate 
current. The grid is connected to one pole of the circuit receiving 
the incoming waves, and the small variations of potential which 
they produce thus give rise to large variations of the plate current. 
These can be made to actuate a telephone and thus to produce 
audible sounds. By placing a number of valves in series, very 
large amplifications are possible. 

The other uses of the valve are numerous. It is employed as a 
transmitter for wireless work, while it finds many applications 
as a source, or rather regulator, of vibrations of comparatively 
short period. The Post Office has used it as an amplifier of .speech, 
while Mr. F. E. Smith has applied it as a source of sound in con- 
nection with the measurement of audibility. 

The whole of this arose from Edison's observation of the dis- 
charge of negative electricity from the heated filament, but its 
development may be said to have been dependent on another and 
more fundamental discovery about 1897- that of the existence of 
the electron, which we owe to Sir J. J. Thomson. 

Before the introduction of the oscillation or thermionic valve, 
as it is sometimes termed, radio-communication was in practice 
confined to telegraphy. Signals were sent out and received which 
were interpreted by the use of the Morse code. The advent of 
the thermionic valve has made wireless telephony, with its recent 
remarkable development in the form of broadcasting, a practical 
proposition and a factor of interest in the lives of innumerable 


By Prof. J. A. FLEMING, F.R.S., Professor of Electrical Engineering 
in the University of London. 


The history and development of the thermionic valve is a 
striking example of the important industrial applications that 
sometimes follow from discoveries which take place in the course 
of purely scientific researches. In 1883 Edison sealed a metal 
plate into the ordinary electric incandescent lamp between the 
legs of its carbon filament. This plate was carried on a wire 
sealed through the wall of the glass bulb. He noticed that when 
the filament was made incandescent by a direct current sent 
through it, simultaneously a small electric current could be 
detected in a circuit between the positive terminal of the filament 
and the wire carrying the metal plates ; on the other hand, no 
current could be detected between the negative end of the filament 
and the plate. This phenomenon was called the " Edison effect," 
but no explanation of it was given by its discoverer, nor was any 
practical use made of it at the time. 

Investigations on the nature of the Edison effect undertaken 
by the writer in 1883 and onwards showed that the effect was 
connected with the projection in straight lines of particles from the 
filament ; further, these projected particles carried a charge of 
negative electricity and could convey negative electricity from 
the filament to the plate, but not in the opposite direction. A 
further step in advance was made about 1897 by Sir Joseph 
Thomson, who showed that the chemical atoms of matter, which 
at the time were thought to be incapable of being divided, con- 
tained still smaller atoms of electricity, now called electrons. 
Soon afterwards, it was ascertained that the incandescent filament 
of the ordinary electric lamp is a prolific and continuous source 
of electrons, which are sent out in all directions. 

* Abstracted from Pamphlet No. 2, British Science Guild Publicity 


About 1897 the application by Senatore Marconi of Hertzian 
electric waves for the purposes of wireless telegraphy began to 
create public interest. For detecting these waves he first used 
his improved form of the coherer of Branly and Sir Oliver Lodge. 
It was, however, rather capricious and somewhat difficult to 
manage, and Marconi replaced it by his magnetic detector in 


In Marconi's system of wireless telegraphy, the electric waves 
are generated by creating powerful vibratory currents of electricity 
in an aerial wire. The electric oscillations in this wire produce in 
surrounding space an electric wave which travels outwards with 
the speed of light, viz., 186,000 miles per second. When these 
waves cut across another similar wire, a receiving aerial, they 
create in it feeble electric vibrations of the same type. 

Now, if means could be found of converting the very rapid 
alternating movements of electricity in the receiving circuits 
into a uniform motion of electricity in one direction, it would 
then be possible to detect them, and therefore the electric waves, 
by the use of the telephone or galvanometer as in ordinary tele- 
graphy, without the use of a coherer. The vibrations of electricity 
in wireless telegraph aerials are, however, very rapid, even up 
to a million per second, and none of the devices for " rectifying " 
or converting slow alternating electric currents into direct currents 
are of any use. The Edison effect, however, seemed to offer a 
solution of this difficulty, and in 1904 the writer found that if a 
metal cylinder, carried on a wire sealed through the bulb, was 
placed around the filament inside the vacuous bulb of an electric 
lamp, the appliance could ik rectify " and therefore detect by the 
aid of a telephone or galvanometer these feeble high frequency 
oscillations. They cannot directly affect a telephone because 
of their rapid reversals of directions, but the instrument above 
described acts as a valve when placed in the path of the oscillations 
and converts them into motions of electricity in one direction, in 
virtue of the fact that negative electrons are passing in the vacuous 
space only from the filament to the surrounding metal cylinder. 

The apparatus was therefore termed an oscillation valve, and 
afterwards a thermionic valve. Later, in 1909, tungsten was used 
as the material for the filament in place of carbon, as it withstands 
a higher temperature and emits more electrons. This two- 
electrode or hot and cold electrode thermionic valve was soon 
extensively adopted as a means of rectifying and detecting electric 
oscillations and detecting wireless waves. 

In the spark system of wireless telegraphy then exclusively 
used, the waves come in little groups of 20 or 30 with longer 
intervals of time between the groups. The Fleming valve rectifies 


the groups of oscillations produced in the receiving aerial into 
short gushes of electricity in one direction, and when these are 
passed through a telephone they give rise to a more or less musical 
sound which can be cut up by a key in the transmitter into the 
dot and dash or short and long sounds of the Morse code. 


In 1907 an addition was made to the Fleming oscillation valve 
by Dr. Lee de Forest, in the United States. After he had 
become acquainted with the Fleming valve, Dr. de Forest intro- 
duced into a low vacuum valve a grid or zig-zag of wire between 
the filament and the plate. This started a new line of development, 
and it was found that, if a cylinder of metal gauze or spiral of 
wire was introduced into the hard or high-vacuum Fleming valve 
between the cylinder and the filament, it enabled the device to 
act as an amplifier of oscillations, as well as a detector, so that 
very feeble high frequency oscillations could be magnified five 
or ten times by its aid. This suggestion was developed practically 
by the resources of the Western Electric and General Electric 
Companies of America. 

This modified form, then, became known as a three-electrode 
valve, and is sometimes called for shortness a triode, or other trade 
names. To employ it as an amplifier, a high-tension battery 
giving, say, 40 to 140 volts, is connected with its negative pole 
joined to the filament and its positive pole to the plate. A torrent 
of electrons is then forced from the filament, through the holes in 
the grid or gauze, to the plate. If, however, a feeble electrification 
is given to the grid, positive or negative, it increases or decreases 
this electric current. The grid potential electrification can be 
obtained from any two points on a circuit in which a feeble high- 
frequency current flows, and the variation of the plate current 
of the valve will follow the variations of its grid potential. A 
number of such valves can be used in series and inter-connected 
by suitable induction coils or transformers, and the plate current 
variations in one valve be made to create changes of grid potential 
in the next valve. By a series of such coupled amplifying valves, 
feeble electric oscillations can be magnified in any required pro- 
portion. It is the invention of this detector, comprising a series 
of amplifying valves, which has given us a detector of electric 
oscillations so enormously sensitive that has enabled us to signal 
half round the world. 


The thermionic valve, in its two- and three-electrode forms, 
possesses the power not only of rectifying and detecting electric 
oscillations, but also of creating so-called continuous or undamped 
oscillations. This discovery at once rendered possible radio- 


telephony on a large practical scale, whereas it had previously 
only been an occasional feat of experts. The proper coupling 
through a transformer of the grid and plate circuits results in the 
production in these circuits of self-sustained oscillations by energy 
drawn from the plate circuit. 

During and since the War, improvements have continually 
been made in the construction of large generating valves. Beginning 
originally with very small powers of a few watts in valves with 
bulbs like incandescent lamps, very large valves in glass bulbs,the 
size and shape of Eugby footballs, yielding an output of 6 or 7 
kilowatts, are now made. Valves of 10-20 kilowatts output or 
more have been made with bulbs of silica. The most recent 
advance in this direction has come to us from the United States. 
A method of making high power valves with bulbs partly of glass 
and partly of copper has been developed by the Western Electric 
Company of America, based on the fact that a copper tube with 
a sharp edge can be welded to a glass tube. In large valves a 
source of trouble is the heating of the metal cylinder by the bom- 
bardment of the electrons. In the metal bulb valves the copper 
part forms also the anode cylinder, and it can be kept cool by 
immersion in water. 

Large generating valves of 10 to 100 kilowatts have been made 
in this manner, and the General Electric Company of America are 
said to be preparing a thermionic generating valve of the two- 
electrode or Fleming type with an output of 1,000 kilowatts or 
1,300 horse-power. If this can be done, large thermionic valves 
will replace high frequency alternators entirely in long distance 
wireless stations. Already Marconi's Wireless Telegraph Com- 
pany have a valve panel of 56 large glass valves in their Carnarvon 
lladio Station with which communication is made direct to 
Australia. The present public wireless telephone broadcasting 
stations in Great Britain employ large valve generators in their 
transmission plant. 


The improvements made in the construction of the thermionic 
valve and the close study of its action imposed by the necessity 
for developing wireless telegraphy and telephony during the War 
have given us an extraordinarily sensitive and easily managed 
detector of electric waves, and the advent of wireless telephone 
broadcasting has created a novel trade in the manufacture of these 
valves for generating, amplifying and detecting electric waves. 

In the receiving valve most commonly used, a straight filament 
of tungsten, or thoriated tungsten, or else platinum-iridium, 
coated with oxides of barium and strontium, is used. This is 
surrounded by a spiral wire forming the grid and by a nickel or 


molybdenum cylinder forming the plate. The ends of the fila- 
ment, grid and plate, are connected to pins on a cap, so that the 
valve can fit into a socket like an electric lamp. 

In modern wireless telegraph receivers, one or more valves 
are used to amplify the oscillations ; one to detect, and one or 
more to amplify the rectified currents. Valves of this type were 
made to the number of three or four million during the War 
(1914-1918) and are manufactured now by the hundred thousand 
per annum for broadcasting purposes. 


An additional service the thermionic valve renders is as a per- 
fect telephone relay or repeater. Telephone electric speech 
currents are enfeebled by flowing along a telephone wire, and for 
long distance working very thick and therefore costly wires were 
required. Thermionic amplifiers can, however, be inserted in the 
line to re-enforce the currents. 

By the use of these repeaters, telephonic speech is now trans- 
mitted right across the Continent of America (4,000 miles), and 
they are now much used by the British Post Office. For shorter 
distances a great economy in copper can be obtained by their use. 
In short, the thermionic valve has effected a revolution in ordinary 
telephony just as it has made possible wireless telephony. 


By Prof. A. FOWLER, F.R.S., Yarrow Research Professor of the 

Royal Society. 

Spectra are of two kinds, band spectra and line spectra. Band 
spectra are very complex and originate in molecules. Line spectra 
are of varying degrees of complexity and have their origin in 
atoms. All compounds which can be excited to luminosity with- 
out decomposition give rise to band spectra, but the application 
of sufficient energy results in the appearance of the spectra of the 
component elements. Similarly, an element which gives a band 
spectrum in its molecular form will yield a line spectrum when the 
energy which excites it to luminosity is capable of dissociating 
the molecules into their constituent atoms. For example, the 
band spectrum of oxygen or nitrogen may be produced by the 
passage through the gas of uncondensed discharges from an 
induction coil, and the line spectrum by the, passage of the more 
intense condensed discharges. 

Some of the earlier workers in spectroscopy were urged on by 
the idea that a spectrum must provide a clue to the structure of 
the atoms or molecules which produce it, and probably also to the 
mechanism of radiation. The majority of spectra, however, 
are exceedingly complex, and it was evident that the first step 
towards the elucidation of these problems was to discover the 
laws governing the distribution of the lines or bands, so that the 
essential features of a spectrum might be expressed in a simplified 
form. Theories of the origin of spectra, and of the constitution of 
atoms, are thus largely based upon investigations of regularities 
in the arrangement of spectral lines and bands. 

In the discussion of such regularities, the position of a line is 
most usefully indicated by its " wave-number," or number of 
waves per centimetre. Thus, if A. be the wave-length in vacuo, 

o o 

expressed in Angstrom units (1 Angstrom unit = 10~ 8 cm.), 
the wave-number, denoted by v, is given by 10 8 /A. These wave- 
numbers are strictly proportional to the oscillation frequencies. 



The simplest of all line spectra is that of hydrogen. In the 
most familiar part of this spectrum, beginning with a line in the 
red, the lines follow each other with gradually diminishing in- 
tensities and at gradually diminishing distances from each other, 
so that they approach a definite limit in the near ultra-violet. 
Lines arranged in this manner constitute a " series " and it was 
discovered by Balmer in 1885 that the lines of the hydrogen 
series could be included in the simple formula 

A -,= 3646 -14 w/(w a 4), 

where m takes successive integer values ranging from 3 to infinity. 
In terms of wave-numbers, the formula becomes 

*' = 27419-6 109678 -3/w*. 

There is another series of hydrogen lines in the extreme ultra- 
violet, called the Lymaii series from the name of its discoverer, 
and others in the infra-red, each of which is generally similar in 
structure to the Balmer series. The entire spectrum is accurately 
represented by the simple formula 

-H 1 .- 1 -.) 

V>/*, 2 in* I 

where m > nij, and R 109678-3. This number was found by 
Rydbcrg to appear in the formulae for other spectra, and is called 
the Rydborg constant. In the formula for hydrogen, the Lymaii 
series is given by putting in l = 1, in 2, 3, 4 . . . ; the Balmer 
series when in l -= 2, M 3, 4, 5 ... ; and similarly for the 
other series in the infra-red. 


Series which are of generally similar character to those of 
hydrogen were found by Rydberg, and by Kayser and Runge, 
to occur in the spectra of other elements. In the general case, 
several associated and overlapping series occur in the same 
spectrum, and are distinguished by the names principal, sharp, 
diffuse, fundamental, and super-fundamental series. Each of 
such series may be represented approximately by Rydberg's 


. + <".)' (m + f. 

where m t and m are integers, and /A I , /x are constants special to 
each series. More exact representations of the series are given 
by including correcting terms in the denominators. 

The formula representing a series thus consists of two parts, 
the first of which indicates the end, or " limit " of the series, 

(B 34/2285)Q F 


while the second is a variable part dependent upon a sequence 
of integers. The position of an actual spectral line consequently 
appears as the difference of two terms, one of which is the limit 
of the series to which it belongs. The word " term " has thus 
come to have a special meaning in spectroscopy ; it signifies a 
wave-number which does not represent a spectral line in itself, 
but only when combined with another wave-number. The limit 
of a series is a term of one of the other series. The " combina- 
tion principle " of Ritz expresses the fact that, with certain 
restrictions, terms from any one series may be combined with 
terms from other series to produce spectral lines. 

Series, and their constituent terms, are now usually represented 
by abbreviated notations, of which the following are typical : 

Principal series ... ... ... ... Is mp 

Sharp series ... ... ... ... Ipms 

Diffuse series ... ... ... ... Ip tnd 

Fundamental series 2dmf 

In each case the term on the left represents the limit of the 
series, and that on the right the sequence of variable parts corre- 
sponding to successive values of m. It should be further observed 
that series may consist of singlets, doublets, or triplets. In a 
singlet system all the terms have but one value. In a doublet 
or triplet system, the s term has a single value, but other terms 
have two or three values respectively. A triplet system is in- 
variably accompanied by a singlet system. 

Elements of Group I in the Periodic Table of Elements, 
including the alkali metals, give doublet series, those of Group II 
triplets, and those of Group III doublets. Recent investigations 
have shown still greater complexities in the terms relating to the 
later groups of elements, but even and odd multiplicities have 
been found to alternate throughout all the successive groups. 
The general increase in complexity of spectrum in passing 
to the higher groups of elements will be sufficiently indicated by 
the following abbreviated table : 

I. II. III. IV. V. 

Singlets. Singlets. 

Doublets. Doublets. Doublets. 

Triplets. Triplets. 

Quartets. Quartets. 



In the more complex systems, the s term is always single 
and the p term always triple, but the complexity increases in 
the other terms to a maximum indicated by the name of the 
system. The combination of terms to produce spectral lines 
is subject to certain selection rules which have been formulated 


by the assignment of <k azimuthal quantum numbers " to each 
type of term (1 for ,<?, 2 for p, and so on), and of " inner quantum 
numbers " to the individual terms. If the combination of terms 
were unrestricted, spectra would be far more complicated than 
those actually observed. 

The various combinations of terms represented by spectral 
lines show characteristic resolutions when the source is placed 
in a strong magnetic field. In a recent remarkable general isation, 
Lande has shown how these " Zeeman patterns " may be calcu- 
lated from the characteristic quantum numbers of the terms 
involved ; or, conversely, how the type of combination may be 
deduced from the pattern observed. 


In view of the results of the analysis of spectra, it is clear 
that a successful theory must first account for the terms which 
give rise to the spectral lines by their combination. The terms 
have, in fact, a more immediate physical significance than the 
lines themselves. In the now well-known theory of Bohr, following 
Rutherford's conception of atomic structure, an atom is supposed 
to consist of a heavy positively charged nucleus, with a number of 
electrons circulating round it. In the normal state, the atom is 
neutral, and the number of external electrons is equal to the 
number of units of positive charge of the nucleus. When the atom 
is unexcited, the electrons may be regarded as traversing orbits 
more or less similar to those of planets or comets travelling around 
the sun, and obeying similar laws, with the difference that in the 
case of atoms the controlling forces are electrical. 

The nature of the theory may be best indicated by reference 
to hydrogen, which has the simplest possible structure, each atom 
consisting of a positive nucleus of unit mass and unit positive 
charge, and a single electron. When the atom is disturbed, the 
electron may temporarily traverse a larger orbit, but it is not free 
to occupy any orbit whatsoever, but only those in which the energy 
has definite values determined by the Quantum Theory. When 
the electron traverses one of these orbits, there is no radiation, and 
the atom is said to be in a lk stationary " or non-radiating state. 
A spectrum line is produced when the electron returns to a smaller 
permissible orbit. Only one line is produced in a single transition, 
and the actual spectrum of many lines represents the integrated 
effect of a large number of transitions between the different 
permissible stationary states. The energy radiated during a 
transition is always a single quantum e = kv, where h is Planck's 
quantum of action, and v is the frequency of the radiation. The 
frequency of the emission, and therefore the position of the 
corresponding spectral line, is thus dependent upon the difference 
(B 34/2285)Q F 2 


of energies of the initial and final orbits. Exactly what happens 
during a transition is not yet understood. The " terms " of 
the spectra which have already been considered are proportional 
to the energies in the corresponding stationary states. 

On this theory, Bohr has obtained a formula for the hydrogen 
spectrum which is identical with that derived from observations, 
within the limits of accuracy with which the quantum of action 
and the charge of the electron have been determined. The 
theory has also been extended, with remarkable success, to the 
explanation of the complex structure of the spectral lines, and 
of the effects of external electrical and magnetic fields. 

Atoms of elements other than hydrogen are more complex 
in structure. A helium atom has a nucleus of mass 4 and positive 
charge 2 units, and two external electrons. The atom of lithium 
has three external electrons, and a nucleus with a treble positive 
charge, and so on throughout the table of the elements, the 
nuclear charge being equal to the " atomic number " of the 
element. The spectra, however, do not simply increase in com- 
plexity with increase of atomic number, but become more complex 
from group to group of the periodic table. The structure of the 
spectrum thus depends on the number of electrons in the outer- 
most group, and not upon the total number. The spectrum is, 
in fact, considered to be produced by a single one of the external 
electrons, interacting with the rest of the atom, which, as a whole, 
will have a single positive charge. Apart from a small effect, 
due to the mass of the nucleus, the series constant for all elements 
is, therefore, the same as that for hydrogen. Owing, however, 
to the presence of one or more electrons in the atomic residue, 
the possible stationary states are more numerous than in the case 
of hydrogen, so that several series occur in the same spectrum. 
The theory, however, is not sufficiently developed to permit the 
actual calculation of the positions of spectral lines other than 
those arising from a nucleus and a single external electron. 


Spectra produced in the electric arc and the electric spark, 
though mostly showing some lines in common, usually exhibit 
important differences. Lines which are intensified, or only 
appear, in the spark are called " enhanced lines." Similar 
differences also occur in the spectra of gases as the energy which 
excites them to luminosity is increased. 

The distinction between arc and spark spectra has become 
more definite in the light of Bohr's theory. Lines which occur 
in the arc, excluding those which are stronger in the spark, form 
series which are characterised by the Rydberg constant R, as 
already explained, and are attributed to neutral atoms. Enhanced 


lines, on the other hand, form series which involve the constant 
4/2, and are attributed to ionised atoms ; that is, to atoms which 
have lost an electron. The simplest example is afforded by 
ionised helium. When a helium atom has lost an electron, it 
resembles the hydrogen atom, except that the nucleus has a 
greater mass and has a double positive charge. The spectrum is 
correspondingly similar to, but not identical with, that of hydrogen ; 
it may be represented by the simple formula 

where /2 He is slightly larger than /2 H on account of the greater mass 
of the nucleus as compared with that of the hydrogen atom. This 
theoretical prediction agrees completely with the actual observa- 
tions of the spectrum ; the important line of ionised helium at 
A4.686, for example, is the first member of the series given by 
putting nil = 3. 

The spark spectra of elements other than helium show series 
which differ from those of arc spectra only in having a four-fold 
value of the series constant ; they are also explained in a general 
way by an extension of the theory similar to that made in the case 
of arc spectra. It should be noted that in accordance with the 
so-called " displacement law/' the spark spectrum of an element 
is of the same type as the arc spectrum of the element which 
precedes it in atomic number. 


Bohr's theory further indicates that atoms which have lost 
two electrons, or are doubly-ionised, may be expected to yield 
series which are characterised by the series constant 9/2. Trebly- 
ionised atoms would give series for which the constant would be 
16/2, and so on. Series with 9/2 for the constant have, in fact, 
been established for aluminium by Paschen, whilst both 9/2 and 
16/2 series have been traced by Fowler in silicon by the action' of 
strong discharges through silicon fluoride. The chief lines of 
highly-ionised atoms are of necessity in the extreme ultra-violet, 
and can only be observed by the use of the vacuum spectrograph. 
The lines which appear within the ordinary range of observation 
belong to secondary series, but are, nevertheless, sometimes well 

Important contributions to the theory of spectra have also 
been made by investigations of resonance and ionisatiori potentials. 
In these experiments a gas or vapour is bombarded by electrons, 
the speed of which can be regulated by an adjustable electric 
field. Energy from an impacting electron is thus transferred to 
the atom, and is subsequently radiated on the return of the atom 


to its normal state. The energy required to develop certain 
spectral lines, or the complete spectrum, has thus been directly 
measured and has been found to be in agreement with that 
deduced from Bohr's theory. 

The observational evidence is thus entirely consistent with 
Bohr's theory, and continued researches on spectra in the direc- 
tions outlined may be expected to aid in the development of the 
theory, and in the deduction of the normal structure of the atoms 
of additional elements. 


The appearance of a band series is in several respects very 
different from that of a line series. The constituent lines are 
usually much more numerous and much closer together ; further, 
although there is a certain resemblance to line series in the crowding 
together of the lines towards a * k limit," series lines invariably fade 
out before the limit is reached, whilst band lines often reach the 
limit, and may even have their maximum of intensity in its 
neighbourhood, so that the resulting " heads " are frequently very 
conspicuous features of the spectrum. Again, the law according 
to which the lines are arranged is quite different from that of the 
lines series, being of the form v = A -f Bm 4- ^w a , where A, 
B and C are constants and m is the number of the line in the series 
reckoned from any convenient starting point. The arrangement 
of the heads relative to one another may also be expressed by a 
formula of this type, which is the analytical representation of a 

Experimental evidence has persistently related band spectra to 
molecules, but only in recent years has any detailed theoretical 
explanation been achieved. The quantum principles which have 
proved so strikingly successful in the elucidation of the problems of 
atomic radiation are no less applicable to molecules. But the 
case is considerably more complex, for in addition to the move- 
ments of the electrons within the molecule, we have to take into 
account the vibrations of the atomic nuclei and the rotation of the 
molecule as a whole. The total energy (Wi) associated with all 
these motions is characteristic of the particular state of the 
molecule at the moment, and if a change occurs in one or more 
of them, the total energy will assume a new value (W 2 )- Then, 
exactly as for line spectra, it is found that the frequency emitted 
(considering the case of a decrease of energy, i.e., of radiation) 
is given by 

It is the greater variety of possible states of a molecule as compared 
with an atom which gives rise to the greater complexity of a band 
spectrum as compared with a line spectrum. 


The present position is that, while the Quantum Theory has 
succeeded in accounting for all the main features of band structure, 
as, for example, the parabolic law mentioned above, there are 
many details as yet unexplained. Even the simplest bands 
hitherto studied, those due to helium, present problems of this 
kind, and in the case of the more massive and complex molecules, 
many difficulties present themselves. But it seems probable that 
these very discrepancies will, in the light of further study, provide 
the material for valuable extensions of our theoretical knowledge. 




Recent progress in our comprehension of the circulation of the 
atmosphere derives largely from the law of relation between the 
velocity of air in steady motion and the distribution of pressure in 
any horizontal surface. If one looks through the meteorological 
literature of forty years ago, one can scarcely fail to be impressed 
with the notion that the writers always had in mind the conditions 
of starting and stopping, and thought little about the long stretches 
of the travel of the air. These stretches represent neither starting 
nor stopping, but steady or persistent motion under balanced forces, 
provided that we are permitted to include among the forces the 
effect of the rotation of the earth, which can be neither avoided 
nor ignored in any general atmospheric question. Yet it is no 
exaggeration to say that, with the motion of the atmosphere, 
starting and stopping are of no greater importance than they are 
in the passages of ocean-going steamers or non-stop trains. 

We know that at the surface of the earth, from which most of 
our experience of weather is derived, there never is and never can 
be the steady motion which represents the balance between the 
gradient of pressure and the rotation of the earth, because the 
friction between the moving air and the earth or sea is always 
dissipating the energy of motion in eddies and ultimately in heat. 
To compensate for that loss and keep the motion steady, some 
force driving the air along its path would be required, I ut it is 
not forthcoming. However, in the free air above the suiface at 
the height of a kilometre or two, say, 5,000 ft., we need not think 
any longer about the disturbances due to the surface, and there, 
provided we are not directly involved in the convolutions of a 
cyclone, we may rely upon what is called " the geostrophic wind," 


that is to say, a wind along the lines of equal pressure (isobars), 
with velocity inversely proportional to the distance between 
consecutive isobaric lines, as a valid normal representation of the 
actual wind. 

The consequence of this recognition of a simple dynamical 
relation between undisturbed wind and pressure-distribution is- 
that a " geostrophic scale " always lies on the modern meteoro- 
logist's working chart, and when he wants to know the effective 
wind, disregarding the surface-friction, he lays his geostrophic 
scale across his isobars and reads off the result in metres per second 
or miles per hour as he pleases, or as it pleased the person who 
made the scale. 


The general introduction of the use of pilot-balloons for deter- 
mining the motion of the free air brings at one and the same time 
confirmation of the general principle and challenge of the indi- 
vidual facts. The assumption of the relation brings the winds of 
the upper air within the possibility of mathematical calculation 
in a manner which surprises all who take up the questions treated. 

From this position, which is a very natural extension of Buys 
Ballot's law, it follows immediately that, if by any means we can 
determine the distribution of pressure at any level in the atmo- 
sphere, we can determine the horizontal velocity of the wind at 
that level by the simple process of laying a properly graduated 
scale across the isobars which represent the distribution of pressure. 


The barometric equation of Laplace gives us the means of 
calculating the pressure to be deducted from the value at the surface 
for any step of height when we know the temperature of the air at 
each level. Such a determination of pressure in the upper levels, 
without observations carried out on the spot ad hoc, may be 
regarded as being beyond the discretion of a cautious meteorolo- 
gist when he is dealing with the distribution of pressure of to-day 
or yesterday, as represented on a " synchronous " chart of actual 
pressure and temperature at a definite epoch, with all the pecu- 
liarities of the meteorological situation and its local incidents ; 
but it is not at all out of the question when we come to deal with 
average conditions representing the mean result of observations 
for an individual month extending over so long a series of years 
that the transient local conditions are merged in the general 

The reason for supposing that we can calculate the distribution 
of pressure at an upper level from the observed pressures at the 
surface, with sufficient accuracy for general purposes, is derived 


from an entirely unexpected result obtained from records of 
registering instruments sent up on balloons, known as " sounding 
balloons." They carry instruments but no passengers. Things 
are so arranged that after an upward journey of from 6 to 10 miles 
or even more, a balloon bursts and comes down with the instrument. 
The instrument and its precious record reach the earth undamaged 
within about two hours of the start and within a hundred miles or 
so of the starting point. 

Records obtained in this way, in Europe to begin with, and then 
in America, over the Atlantic Ocean, the Greenland seas, the 
Antarctic, the Victoria Nyanza, the Dutch East Indies and Aus- 
tralia, disclose the remarkable fact that, provided the temperature 
at the surface is the highest of the record, the rate of fall of tem- 
perature with height is the same in any part of the world. There 
are a good many occasions, particularly in the winter of the locality 
where the sounding is made, when the surface is colder than the 
air in the layers immediately above it, and then there is no satis- 
factory starting point for calculating pressure in the upper air. 
Even on these occasions the regime of the fall of temperature with 
height according to the numerical rule asserts itself when a certain 
height has been attained ; but that does not help us in the calcu- 
lation of pressures in the upper levels because the starting point 
at the surface is off the line by an altogether unknown amount. 

Confining ourselves to summer- temperatures, therefore, in 
which that difficulty does not arise, the pressures in the upper 
levels have been calculated with some assurance that we are at 
least within the range of probability, and on this basis the distri- 
bution of pressure over the northern hemisphere in July has been 
calculated for 2 km., 4 km., 6 km., 8 km., and 10 km. The results 
are represented upon maps or models for the corresponding levels. 


The distribution of pressure being known, we can calculate the 
normal atmospheric circulation at the surface and at the different 
levels. It is very complicated at the surface, but becomes much 
simplified at 2 km. At higher levels the regime is clear ; it is a 
circulation from west to east round the pole, not quite along circles 
of latitude because there is some distortion of shape consequent 
upon the transitions from land- areas to sea- areas and vice versa. 
The circumpolar circulation, west to east, extends to about latitude 
30 ; there it falls off very rapidly, and along the equator and 
intertropical belt, there is a circulation in the opposite sense, from 
east to west. 

The two circulations at any level are not altogether inde- 
pendent : there are limited regions, along the latitude 30, around 


which apparently air may pass from the equatorial circulation to 
the polar circulation and vice versa. These localities seem to 
supply moving belts which carry, or gear with, the east to west 
circulation on the southern side and the west to east circulation 
on the northern side. 

The final conclusion has been reached by Mr. A. W. Lee that, 
so far as the polar circulation is concerned, and apart from the 
local disturbances due to coast-lines, the successive layers of our 
atmosphere are rotating " like a solid." The west to east velocity 
represents a travel a little faster than that of the solid earth, the 
values of the angular velocities of the successive shells in July 
being 1 -()3o> at 4 km. and 6 km. and 1 -05w at 8 km., where w 
is the angular velocity of rotation of the earth. The corresponding 
velocity for January, as determined from a chart of isobars by 
Teisserenc de Bort, is I '08^. The intertropical circulation at 
high levels has an angular velocity of 0-92o, which represents a 
motion relative to the earth from east to west of 0-08<o, but at 
lower levels the relative motion westward is much less. These 
figures mean that a shell, from 4 km. to 6 km. high, makes a 
complete rotation with regard to the earth from west to east in 33 
days in summer and 12 days in winter. Another higher shell at 
8 km. takes only 20 days to complete a spin even in the summer. 
On the other hand, the higher air over the equator gets round the 
earth the opposite way in 12 days. 


This simple regime, so easily imagined and remembered, does 
not, however, reach the surface. There we find a complication 
which only a carefully constructed may) can represent. If we seek 
for an explanation of that complication we must remember that, 
whereas during the day, when the surface is solarised, the layer 
of earth and sea is receiving heat from the sun, the opposite 
is the caso at night, and long before night in the regions of long 
shadows. There the surface is losing heat, and as an inevitable 
consequence air runs down the shaded hills more and more as the 
shadows lengthen and deepen. How much air runs down and how 
fast it runs we do not know, but we know that the flow must be 
there and huge pools of cold air must accumulate in the lower 
levels. Moreover, the process is irreversible ; cooled air must 
stick to the ground, warmed air cannot. Hence we may regard 
the shadowed hills as pouring an immense volume of air on the 
lower regions, and thereby spoiling the simplicity of the distri- 
bution of pressure at the surface, and consequently that of the 
general circulation of the atmosphere in the lower layers. 

There can scarcely be any question that the descent of cold 
air in this fashion- ex presses- itself 'in .the play of the general circu- 
lation as local modifications of the distribution of pressure at the 


surface and the formation of seasonal anticyclones. The con- 
verse process, the ascent of warm air, is another story and a much 
more complicated one. The descending air, which of necessity 
clings to the hill-side, can always take advantage of the cooling 
of the ground, and is thereby helped all the way ; it goes down 
headlong like an avalanche ; but to climb, air has to leave the 
ground and make its way through the layers above with only the 
trifling assistance which it can obtain from the absorption of 
radiation by the water vapour which it carries. While rising it 
is subject to the automatic reduction of its temperature conse- 
quent upon the reduction of its pressure, if no heat is supplied 
to it. It loses heat at the rate of 10. for 100 metres, while the 
temperature of the environment falls off with height generally only 
1C. for 200 metres. The transparent air through which we see 
the sun and stars looks perfectly similar and homogeneous, and 
is all called simply air ; but it is really stratified by its temperature 
into layers which are quite impervious to air rising from below, 
unless the rising air has the temperature necessary to furnish the 
key to get through. 

Facilis descends Averni, 
Noctes atque dies patet atra janua Ditis; 
JSed revocare gradum suporasque ovadero ad auras ; 
Hoc opus hie labor est. 

Yet the air as we know it manages the ascent quite easily by 
an ingenious trick. It climbs to higher things on stepping-stones 
of its own dead water vapour. Jt loads itself with moisture. As 
it rises it cools, and, if it is fortunate enough to pass the dew point, 
it condenses part of its moisture and takes over the heat of vapori- 
sation previously latent, but now set free. Fortified therewith, 
it passes on its victorious way upwards, 'sometimes with a rush 
great enough to carry up huge hailstones, until it meets its match 
in an environment that has less lapse of temperature with height 
than the rising air itself, in spite of its propensity to appropriate 
the latent heat of its accompanying water. 

At and above a height varying with latitude from some 8 kilo- 
metres at a pole to 17 kilometres at the equator, there is a layer 
of air called the " stratosphere," where there is no fall of tempera- 
ture with height. That layer even the wettest atmosphere can 
never penetrate ; it cannot be overcome either by opus or by 
labor it is just impossible and impassable. 

However, in the " troposphere," the region that lies between 
the ground and the stratosphere, all kinds of enterprises are 
possible to rising air fortified with a sufficient supply of water 
vapour : Clouds, rain, hail, snow, thunder, lightning and nearly 
all the other incidents of weather. 

These striking phenomena are most notable characteristics of 
the local disturbances of the general circulation which are called 
cyclones or cyclonic depressions. This has been recognised for a 


long time, and many meteorologists have thought that the cyclones 
really derive their energy from the convection of wet, warm air. 
Certain it is that if a rapid vertical ascent of air took place within 
the normal circulation, the circulation would be disturbed ; 
the only question is by how much. It is also certain that if the 
layers of air in the middle atmosphere were traversed by air 
coming from below and passing out above, the rising air would 
carry with it from the middle layers more than its own mass, 
and the result of the eviction would necessarily be such a circu- 
lation as we may associate with a cyclone freed from the friction 
of the surface. But, as yet, we cannot speak with certainty as 
to the extent to which the origin or maintenance of the energy of a 
cyclone is due to the travel of air upward through the layers with 
the aid of the condensation of water vapour. By means of 
diagrams which represent the thermodynamic changes in dry and 
saturated air under continuous reduction of pressure, and also 
the conditions of the environment, we are now able to estimate 
numerically the amount of energy which is available for these 
dynamical operations. 

The new school of meteorologists in Norway traces the origin 
of cyclones to the mutual action of two currents of air across a 
surface of discontinuity and regards the accompanying weather as 
incidents of no dynamical importance ; a vortex as only the 
transient final form of wave-motion. On the other hand, a 
Japanese student of the Imperial College has recently shown that 
a vortex with its recognised distribution of pressure, travelling 
along at the height of a kilometre, would produce automatically 
in the layers near the surface the phenomena upon which the 
Norwegian school bases its conclusions. 


The measurements of the direction and velocity of the air at 
different levels obtained by the observation of pilot balloons 
afford information of the kinematic structure of the atmosphere 
in clear weather. The information can be supplemented by 
observations of the motion of clouds of various forms and 
sometimes by observation of pressure and temperature in 

To gain a general idea of the kinematic structure, the results 
of soundings with pilot balloons have been combined in various 
ways. One of the most instructive methods is to set out the 
direction and velocity at different levels on glass plates, using one 
glass surface for each level. The plates are put together to form 
a block, in which the motion at the various levels is easily apparent. 

Models of this kind, in which the results for different stations 
are combined with a map of the distribution of pressure at the 


surface of the same time, show very clearly the complexity of the 
dynamical problem of the motion of the atmosphere at any 
moment. They remind us that the actual motion depends partly 
on the conditions of general and local circumstances, represented 
by the distribution of pressure, and partly on minor local dis- 
turbances not otherwise represented, which for the moment must 
be regarded as accidental disturbances of dynamical or, perhaps, 
of thermodynarnical origin. Hypothetical combinations can be 
represented in like manner. 

Something is already known about the dynamical effects of 
the inequalities of land-surface, but scarcely anything about the 
thermodynamical effects, and it would be well if these could be 
reduced to the simplest form. Hence the most inviting line of 
approach to a solution of the kinematic problem is by way of 
observation of pilot balloons at sea, where the surface conditions 
are free from many of the complexities of land-areas. 


The general meteorological problem would be brought within 
more manageable limits if we were able to analyse the complex 
motion of the air into combinations of motion of recognised types 
even for a few occasions. Certain combinations, not very well 
denned, however, are familiar in meteorological literature, such 
as translation and vortical rotation, secondary vortical rotation 
within a larger vortical rotation and wave motion combined with 
translation, but none of these ideal structures can be recognised 
in ordinary weather maps, nor are they likely ever to be recognised 
prima facie in a surface map, because the friction of the surface 
air affects the motion in every combination. 

In meteorological practice certain features are selected as 
suggesting certain ideal combinations the consequences of which 
are traced as a basis of forecasting. Finality cannot bo reached 
until the existence of these ideal combinations can be demonstrated 
by reference to the actual atmospheric structure. On a few 
occasions, with suitable correction of the surface observations, the 
analysis can be exhibited, and meanwhile, to afford a clue to possible 
analysis, the study of ideal combinations on the scale of the 
weather map may be pursued by the construction of synthetic 


It is acknowledged by all, however, that the phenomena of 
the atmosphere represent the working of an exceedingly complex 
air-engine or steam-engine, and that the ultimate explanation of 
the local disturbances, as of the normal circulation, must be looked 
for in the quantitative relationships of all the physical quantities 


inherent in the atmosphere. Gravity, heat, work, temperature 
wind- velocity, solar radiation, terrestrial radiation, vapour- 
pressure, all are associated and all will have to be combined when 
the explanation conies to be worked out. If that be accepted, it 
is just as important for meteorologists to provide themselves with 
units of measurement on a systematic plan as it was for electrical 
workers fifty years ago. When we have to combine tempera- 
ture with pressure in a formula, the measurement of temperature 
as a number of degrees from the freezing point of water has to be 
changed, whether the operator is aware of the fact or not. When 
we have to deal with the intricate relations of heat and work in 
the atmosphere, to have to introduce a factor A or J, the very defini- 
tion of which is uncertain, is adding to the inevitable opus and 

Hence one of the first steps in the explanation of the circulation 
of the atmosphere, when it comes to be written, will be the setting 
out of the measurements involved in systematic units ; and 
therefore, as it will certainly be indispensable in the end when the 
work is done, so it will make things easier as the work proceeds. 
Thus we build our representation of the present state of knowledge 
of the circulation of the atmosphere, and the means of extending 
it, upon the foundation of meteorological quantities expressed 
in systematic units. 


By Dr. G. C. SIMPSON, F.R.S., Director of the Meteorological 


The dictionary definition of " saturation " is " the state of a 
body when quite filled with another," and it is usual to think of 
saturated air as air which is full of water vapour to such an extent 
that further water cannot be added without condensation taking 
place. This, however, is a wrong conception, for there is no limit to 
the amount of water vapour which air can contain at any tempera- 
ture, provided that it is perfectly pure, except that ultimately the 
molecules of vapour will be so near together that there will be no 
distinction between vapour and liquid. 

We will describe air as saturated when the water vapour it 
contains is in equilibrium with a flat surface of pure water at the 
same temperature. This will define the saturation pressure at 
each temperature, and relative humidities will be given as per- 
centages of this saturation pressure. 

It is well known that water can be cooled below its freezing 
point without becoming ice, and therefore water and ice may exist 
side by side over a large range of temperature. But the vapour 
pressure which is in equilibrium with ice at a given temperature is 
lower than that which is in equilibrium with super-cooled water 
at the same temperature ; that is, air is in equilibrium with ice at a 
relative humidity below 100 per cent. Thus, according to our 
definition of relative humidity, the water vapour in air may be in 
equilibrium with water over a large range of relative humidities 
according to the physical state of the water present. 


It was in 1880 that Aitken first showed that condensation does 
not necessarily take place in air when its temperature is lowered 

* Abstracted from Supplement to Nature of April 14, 1923. 


below that at which the water vapour it contains is sufficient to 
saturate it (dew point). He expanded carefully filtered air and 
found that 110 fog formed even when there was considerable super- 
saturation. Aitkeri concluded " that vapour molecules in the 
atmosphere do not combine with each other, that before con- 
densation can take place there must be some solid or liquid nucleus 
on which the vapour molecules can combine, and that the dust in 
the atmosphere forms the nuclei on which the water vapour 
molecules condense." 

Aitken invented a most ingenious instrument, easy to work and 
very transportable, by means of which it is possible to count the 
number of nuclei present in the air. Tests made with this instru- 
ment show that nowhere is air free from nuclei. Their number is 
seldom less than 100 per c.c., while in most country places the 
nuclei rise to thousands, and in cities such as London and Paris the 
number may be so great as 100,000 to 150,000 per c.c. 

The general explanation of these observations is as follows. If 
there were no dust particles present the drops of water would have 
to be built up from aggregates of water molecules ; if a few 
molecules met together by chance, they would form so small a drop 
that it could not exist unless there was large supersaturation. If, 
however, there were dust particles present, the molecules of water 
would be deposited on them, and the radii of the initial drops 
would be so large that little supersaturation would be required to 
maintain them. 

This explanation appeared to satisfy everyone for a long time. 
In 1912, however, Wigand found that even when he created large 
clouds of dust he could not find any increase in the number of 
nuclei in his condensation apparatus. Apparently Aitken's 
instrument does not measure the number of dust particles present, 
but the number of hygroscopic particles, and meteorologists are 
now of opinion that condensation commences on these hygroscopic 

Kohlcr, working in Norway, is tempted to contend that sea-salt 
provides these particles. It is, however, not necessary to go so far 
as this, for there are many other sources of hygroscopic substances. 
Lenard and Ramsauer have shown that sunlight probably only 
the ultra-violet part acts on the oxygen, nitrogen and water 
vapour of the atmosphere, producing very hygroscopic substances. 

Large quantities of material capable of becoming condensation 
nuclei, however, are produced by all processes of combustion. Thus 
the household fires and factory chimneys of centres of industry pro- 
duce vast quantities of nucleus-forming material, chief of which is 
sulphurous oxide. This, when illuminated by sunlight in the 
atmosphere, is a very hygroscopic substance capable of causing 
condensation in unsaturated air. It is estimated that in England 
something like 5,000 tons of sulphur are burnt each day in coal 
fires, giving enough sulphur products to pollute the atmosphere 
(B 34/2285)Q ' G 


from Land's End to John o' Groats. Other products of combus- 
tion are also hygroscopic ; thus it is not surprising that air from 
large industrial centres contains enormous quantities of nuclei. 

When the temperature of the air is below the freezing point, 
it is inconceivable, owing to the small amount of vapour present, 
that condensation will take place by the fortuitous meeting of 
molecules ; some kind of nuclei therefore will be necessary. 

When sledging in the Antarctic with Captain Scott in 1911 we 
became enveloped in a fog during sunshine. On the fog opposite 
the sun we saw a white bow in the position usually occupied by a 
rainbow. This phenomenon can only be explained on the assump- 
tion that the fog was composed of small spheres. But the 
temperature was 29C. (~21F.) and almost a dead calm existed 
at the time ; hence these drops could not have formed at a high 
temperature and then been super-cooled. The only explanation 
appears to be that in the clear air of the Antarctic, where there are 
no " dust " particles suitable for condensation available, there are 
plenty of hygroscopic molecules of some sort. 


Perfectly pure air is almost completely transparent to visual 
light waves, and if the air were always pure we should see distant 
objects through air almost as clearly as through a vacuum. But 
there are always more or less particles of foreign matter present. 
The action of these particles is two-fold : first, they reduce the 
amount of light reaching the eye from distant objects ; and, 
secondly, in the daytime they scatter the general light of the sky and 
so send to the eye extraneous light which reduces the contrast 
between distant light arid dark objects on which visibility depends. 
Generally this foreign matter consists of a mixture of solid ponder- 
able particles and hygroscopic molecules. In perfectly dry air 
the latter would be practically invisible, but when loaded with 
water in a humid atmosphere they add to the obscurity of the 

Haze is due to this kind of obscurity, and varies in intensity 
from the slight obscurity of polar regions, which depends almost 
entirely on the hygroscopic particles, to the dense obscurity of a 
dust storm in tropical regions, which is due almost entirely to solid 


If the temperature falls below the dew point, the hygroscopic 
particles are sufficiently large to form excellent nuclei for condensa- 
tion, and relatively large amounts of water arc deposited for small 
falls of temperature. Heal condensation has now commenced, and 
the obscurity changes from that of haze to that of mist. The whole 
process of the formation of haze and mist is continuous, but they 
are fundamentally different, for haze owes its origin to foreign 


matter and the small amount of water associated with hygroscopic 
nuclei, while mist is due to an actual precipitation of water from 
vapour to liquid. 


There is no fundamental difference between mist and fog : in 
most cases fog is only a dense mist, and the density at which mist 
becomes fog is a matter of definition. It is now the practice of the 
London Meteorological Office to limit fog to the obscurity in which 
objects at 1 kilometre are not visible. 

When mist and fog are formed in fairly clear air they are white. 
On the other hand, if the air contains a large quantity of impurities, 
such as carbon particles from imperfect combustion, the mist 
particles absorb the impurities and become themselves dark- 
coloured. In this way are formed those dense fogs in London which 
aro likened to pea soup. It was originally thought that the 
density of a London fog was due to the fact that the smoke of the 
city provided an unusually large number of nuclei on which 
condensation could take place, thus offering a temptation to the 
air to deposit its moisture which it could not resist. As a matter 
of fact, there are always sufficient nuclei in the purest air in 
England to allow of the formation of fog whenever the meteoro- 
logical conditions are suitable. 

The relationship between smoke and fog is peculiar, and may be 
said to be accidental. The meteorological conditions which are 
necessary for the formation of fog are such that, while they last, 
smoke cannot get away either vertically or horizontally from the 
place of its origin. Thus during a fog practically all the smoke 
which London makes is kept over it and within a few hundred feet 
of the ground. This smoke, combined with the deposited water, 
can, as we all know, produce such an obscurity that midday is as 
dark as midnight. The total abolition of smoke from London 
would not reduce the occasions on which mist and fog occur, but 
many fogs would remain mists, and we should never have a 
" London particular." The fogs of London are caused almost 
entirely by loss of heat from the lower layers of the atmosphere 
into a clear sky above. The air radiates its heat, its temperature 
falls, and condensation takes place. Other methods of fog 
formation, such as the mixing of warm and cold air, are of secondary 
importance and never give rise to more than patchy local mists or 
light fogs. 


When air not saturated rises in the atmosphere its temperature 
is reduced by about 1C. for every 100 metres of ascent. When the 
ascent is carried far enough the dew point is reached, after which 
any further rise will cause condensation on the nuclei present. As 
the ascent is carried beyond the point of condensation, more and 
more water is deposited, with a consequent increase in the size of 

(B 34/2285)Q o 2 


the drops. This is the manner in which clouds are forined, and 
there are very good reasons for saying that it is the only way. 
Thus there is a fundamental difference between the method of 
formation of clouds and fogs : fogs form without any ascent of the 
air, while clouds are never formed without it. Thus it is not 
correct to describe clouds as fogs of the upper atmosphere. 

The very sharp line of demarcation between the air under a cloud 
and the cloud itself needs explanation. The hygroscopic nuclei 
collect more and more water around them as they rise with the 
ascending current, owing to the increase in relative humidity. But 
when saturation is reached they are still very small, and produce 
little obscurity in the air. Such drops, however, need only 1 per 
cent, supersaturation to grow. Moreover, they are unstable, for 
as they grow they need less supersaturation. Thus, as soon as the 
air is sufficiently supersaturated to be in equilibrium with the 
nuclei, the slightest further rise causes the drops to grow very 
rapidly to the size in which they are in equilibrium with saturated 
air. The height at which this change occurs is the height of the 
base of the cloud. 


When bodies fall through a resisting medium, such as air, they 
more or less quickly reach such a velocity that the resistance of the 
air equals the pull of gravity, after which they fall with a constant 
velocity, which is different according to the density and shape of the 
falling bodies. 

Experiments have been made to determine the rate of fall of 
water drops through air at atmospheric pressure, and they show that 
the small drops in clouds would fall only at the rate of a little over a 
centimetre a second. As the drops get larger the rate of fall tends 
to a constant value of about 8 metres a second, while drops half a 
centimetre in diameter have the most rapid fall. Larger drops fall 
more slowly, for, instead of retaining the shape of spheres, they 
become flattened out, thus presenting an increased resistance to the 
air through which they are falling. When the size of the drop is 
such that, if it were not flattened it would have a diameter of about 
half a centimetre, the drop becomes very unstable, and all drops 
larger than this quickly break up into a number of smaller drops, 
which of course fall more slowly. This means that raindrops can 
never fall through air at a greater velocity than 8 metres a second. 
Small drops fall slower than this, and large drops flatten out as 
soon as they are falling at 8 metres a second, and then soon break 
up into smaller drops. 

W. H. Dines has found that in Europe the quantity of vapour in 
air is always very small. If the whole water vapour in the 
atmosphere on an average summer day were precipitated, it would 
only give a total rainfall of 0-80 in. The greatest amount ever 
measured on a summer day in Europe would only give 1-5 in. of 
rain, and, of course, the quantity is much less in winter. Kainfall 


of several inches of rain in the course of an hour or so such as 
occurs in the tropics is due to ascending currents which carry with 
them their own water vapour to supply the rainfall. Ascending 
currents up to many metres a second are possible, and do occur 
in the atmosphere. Let us think of air rising at about 10 cm. per 
second, which is the order of the upward velocity of the air in 
depressions. At a certain height cloud particles form as already 
described. These have a radius of about O'OOl cm. and fall 
relatively to the air at 1'3 cm. per sec. ; hence they are carried 
upwards with the air, but the base of the cloud remains at the 
same height because new cloud particles are constantly being 
formed at that height. As the air rises the cloud particles grow 
in size, because water is being condensed on them, and they lag 
more and more behind the air. Drops with a radius of 0*002 cm. are 
falling as rapidly as the air is rising, and therefore remain stationary, 
while drops of 0*007 cm. are falling at the rate of 1 metre a second, 
and therefore fall through the rising air and appear at the earth's 
surface as rain. This process will continue so long as the ascending 
currents continue, and in this way we get the continuous steady 
rain with which we are so familiar in this country. 

When the upward velocity of the ascending air becomes greater 
than 8 metres a second, no water can fall through the ascending air 
for the reason already explained. All water condensed in such an 
upward current and it will be a very large amount is carried 
upwards until the upward air velocity falls below 8 metres a 
second, as it is bound to do at some height owing to lateral 
spreading out. Here water accumulates in large amounts. It is 
the sudden cessation of the upward velocity in such an ascending 
current which gives rise to the so-called cloud-bursts, for when the 
sustaining current stops, the accumulated water falls just as 
though the cloud had literally burst. *. * *' 

The accumulated water while it is suspended in the air is 
constantly going through the process of coalescing into large drops, 
which at once become deformed and broken up again into small 
drops. Every time a drop breaks there is a separation of electricity 
and this is probably the chief source of electricity in a thunder- 
storm. This explains why thunderstorms are associated with 
heavy rainfall and do not occur in polar regions, where there is no 


Let us consider a region in the atmosphere through which there 
is an ascending current of air. The air is supposed to have a 
temperature of 20 C.. and a relative humidity of about 50 per 
cent, at the ground. As the air rises, at first its temperature is 
reduced by 1 0. for each 100 metres of ascent. Hence, by the time 
it has risen 1,000 metres its temperature will have been reduced 
to 10C., and it will have reached its dew point. Here the cloud 


level begins. As it rises still farther its temperature continues to 
decrease, but not so rapidly as before, because the condensation 
of water vapour releases the latent heat of vaporisation. It 
reaches 0C. at a height of 3,000 metres. Hence the region between 
1,000 and 3,000 metres contains only drops of water. As the air 
rises above 3,000 metres the temperature falls still lower, but the 
water particles do not freeze at once, they remain super-cooled. 
We may assume that at 20 C., which is reached at about 
6,000 metres, the super-cooled drops solidify and the remaining 
part of the cloud above this level is composed of snow alone. 

There will not be a sharp division between the region of super- 
cooled water and the region of snow. For a certain distance ice 
crystals and super-cooled water will be mixed together. Such 
conditions are very unstable, the ice particles grow rapidly, and, 
if the ascending current is not too largo, they will commence to fall. 
The ice-particle has, however, to fall through 3,000 metres of 
super-cooled water drops, and in doing so it grows appreciably. 
As each super-cooled water particle -strikes the ice it solidifies, and 
also imprisons a certain amount of air, so that by the time the ice 
particle reaches the bottom of the super-cooled region, it is simply 
a ball of soft white ice. 

If the descent through the super-cooled region has been fairly 
rapid, the temperature of the ice ball will be considerably below the 
freezing point when it arrives in the region where the temperature is 
OC. and the cloud particles are not super-cooled. As it continues 
its way downwards it receives a considerable addition of water : in 
the first place by direct deposition, because it is colder than the air ; 
and, secondly, by collision with the water particles. This water 
covers the surface of the cold ice ball with a uniform layer of liquid 
which quickly freezes into clear solid ice, with little or no imprisoned 
air. Finally, the ice escapes from the bottom of the cloud, and 
falls to the ground as a hailstone. 

When hailstones are split open to show their internal structure 
we can nearly always see the inner soft white mass of ice which was 
collected while the stones were in the super-cooled region, sur- 
rounded by a layer of clear transparent ice formed by the freezing 
of the water deposited when the stone was passing through the non- 
super-cooled region. 

Hailstones are formed only during thunderstorms, when violent 
ascending currents of air occur. Thus, while the hailstone is 
growing in size, its rate of fall may well be less than the upward 
velocity of the air. All the time, however, it will be moving 
relatively to the air, and its effective height of fall will be great. 
This would enable it to collect water, and so would account for the 
large size often attained by hailstones. Moreover, such an 
ascending current is not steady. Just as there are gusts and lulls 
in horizontal winds, so there are increases and decreases in the 
velocity of ascending currents. Thus a hailstone which has 

SNOW. 95 

penetrated into the lower part of the cloud might be blown 
upwards and so go through the whole process again. In this case 
we should have a layer of white ice deposited around the clear 
layor, around which again there would be another layer of clear 
ice. This process might be repeated indefinitely, giving several 
concentric layers of clear and white ice, and a broken stone would 
have the appearance of an onion. Such cases are not at all 

For the formation of hailstones two conditions must be fulfilled : 

(a) The clouds must extend through a great vertical height so 

that the three regions of water particles, super-cooled 
particles, and snow are extensive and well developed. 

(b) There must be violent ascending currents, otherwise the 

stones would fall too rapidly to grow to a large size. 

These conditions are best fulfilled in warm regions, for there 
violent ascending currents are most easily developed, and the con- 
densation starts at a relatively high temperature, so giving regions 
of water particles and super-cooled water particles of great depth. 
These are the reasons why hailstones only occur during the summer 
in temperate regions, and why the most violent hailstorms and the 
largest hailstones are found in tropical regions. 


The hailstone receives its coat of clear transparent ice in the 
region between the bottom of the cloud layer and the zero iso- 
thermal. If this region is much reduced, as when the temperature 
at the ground is low, the hailstones are relatively small, and consist 
only of the soft white balls appearing in the centre of the more 
complete hailstones. Falls of soft hail of this nature are quite 
common in the winter in Europe. The temperature of the ascend- 
ing current is so low that the freezing point is reached almost 
at the bottom of the cloud, so the hail falls almost immediately out 
of the region of super-cooled water particles, and has no opportunity 
for building up a layer of transparent ice. 


Snow which forms over an ascending air current in which water 
particles first form will probably have solidified cloud particles 
for nuclei. Whatever the nuclei may be, as soon as the initial 
crystals are formed, further condensation takes place exactly as in 
the precipitation of water, but the vapour condenses directly 
into the solid state without first going through the liquid state. 
The crystals of water are hexagonal prisms. Having once started, 
the crystals may grow either along their central axis, giving rise 
to long thin prisms, or along their six axes to form hexagonal plates, 


showing all the wonderful shapes that this form of crystallisation 
can take. 

In cold regions the crystals are small, because there is little 
water vapour present from which they can grow. In the Antarctic 
during the winter, when the temperature was always near or below 
0F., only the smallest crystals were seen, so small that they were 
almost like dust. 

When crystals form at temperatures near the freezing point 
they grow to their largest size. When the air is full of large 
crystals frequent collisions take place. The crystals become inter- 
locked and bundles of many separate crystals are formed ; these 
produce the ordinary snowflakes which, on account of their size 
and weight, fall relatively rapidly. It is to these latter that the 
term snow should be applied. 



In a general way, we all realize that the earth is only habitable 
because of the heat received by radiation from the sun. How 
that heat is distributed and ultimately lost is one of the funda- 
mental problems of meteorology. 

For our knowledge of the quantity and quality of the energy 
in the sun's rays on the confines of the earth's atmosphere, we are 
indebted to Langley, Abbot and Fowle, workers attached to the 
Smithsonian Institution of Washington. By means of the 
spectrobolometer, an instrument in which the principles of the 
spectroscope and the thermopile are combined, Abbot and Fowle 
determine the energy in each part of the spectrum of the sunshine 
that reaches their observatory. This being done several times 
in the course of the day, both when the sun is low so that the 
heat flow is considerably obstructed by the atmosphere, and when 
the sun is high so that the obstruction is as little as possible, the 
observers are in a position to estimate the energy in the 
unobstructed radiation. 

In the first instance it was tacitly assumed that the strength of 
the solar heat stream would not vary, and the term " solar con- 
stant " came into use to denote the rate of flow of thermal energy 
outside the atmosphere, but at the average distance of the earth 
from the sun. There is, however, strong if not conclusive evidence 
that the " solar constant " varies from day to day, the sun being 
in fact a variable star. The average value of the 4t solar constant " 
is such that, in the course of 24 hours, the earth receives enough 
heat to raise the temperature of a layer of water a metre deep by 
7C., to melt a layer of ice 9 cm. deep, or to evaporate a layer of 
water 12 mm. deep. 

It is instructive to notice that a body like a meteor, with no 
atmosphere, but so small that its temperature was equalised by 
conduction, would have a temperature of about 10C. At such 


a temperature the outward radiation would just balajice the radia- 
tion received from the sun. In the case of the earth, conduction 
is ineffective, but the movements of the atmosphere and the 
oceans tend to equalise temperature, and the average temperature 
of the air near the ground does not differ greatly from that of the 
hypothetical meteor. 

The observations from which the value of the u solar constant " 
is deduced show, also, how much of the radiant energy is absorbed 
in the atmosphere or dispersed by it. When the air is free from 
haze (solid particles), and also from mist or cloud (water in the 
liquid or solid state), there is little absorption, but much of the 
incoming light is dispersed by diffraction. The light of shortest 
wave-length is most liable to diffraction ; the diffracted light 
gives us the blue sky and the blue of the distant landscape seen 
through air illuminated by sunshine. The longer waves are not 
diffracted so much ; the setting sun seen through a great thickness 
of clear air is yellow or orange, and only the red rays survive in 
light which passes from a low sun to the clouds and then to earth. 

The scattered radiation also plays its part in warming the 
earth, though half of it passes away into space and is lost. 

In contrast with the transparency of the atmosphere to the 
short-wave radiation which we call light is its absorption of the 
long heat waves. The stopping power is mainly due to the presence 
of water vapour in the atmosphere, though the carbon dioxide 
helps. Water vapour acts like the glass firescreen, which lets us 
see the fire but cuts off the heat. Thus the heat which the earth's 
surface loses by radiation is absorbed by the air. Some of the heat 
comes back to earth as radiation, some is sent on its way into outer 

Here we have the explanation of some very remarkable facts. 
Ft is well known that it is colder at such heights as have been 
reached by mountaineers and airmen than on the ground, but 
no speculator had suggested that the cooling with increase of 
height would stop short at a certain level until that was revealed 
by the instruments sent up on small free balloons. In our 
latitude, the limiting temperature is reached at a height of about 
six miles ; over the equator the limit is found at about nine miles : 
near the poles at four. The temperatures recorded 10 miles up 
over the equator, averaging 80 C., are the lowest that have been 
found under natural conditions in the atmosphere. Over the 
poles the average is higher, about 45C. 

Exactly why this happens has not been determined, but the 
point of greatest importance is, no doubt, the excess of water 
vapour in the tropical air. The air of the polar regions holds 
very little water vapoui^-perhaps enough to make half an inch of 
rain. Near the equator there may be six times as much. If the 
polar regions be thought of as a glass-house with one thickness of 
glass, the tropics have six thicknesses. In spite of the higher 
temperature inside the well-protected house, less heat escapes from 


it to the outermost glass surface. In spite of the higher tempera- 
ture at sea level in the tropics, less heat escapes to outer space 
than from the polar regions. The superfluous heat from the tropics 
is conveyed by the winds, mostly by the upper winds, to the parts 
of the globe where escape by radiation is possible. 

Thus the curious distribution of temperature in the atmosphere, 
coldest on the ground near the poles, coldest at considerable 
heights over the equator, is mainly due to the way in which radia- 
tion is affected by tho presence of water vapour. 


By DR. C. CHREE, F.R.S. 

According to current ideas, the earth carries a negative charge 
of electricity of some 700,000 coulombs, while the loss of charge 
represents a current of some 1,000 amperes. Thus the capital 
represents only some 12 minutes' expenditure, and yet bankruptcy 
does not ensue. It is thus clear that the relations between the 
loss and the accumulated charge must be widely different from 
what is supposed, or some unknown active source of replenishment 
must exist. 

On investigation it will be found that our information is very 
incomplete, both as regards the charge and the air-earth current 
which represents the loss. Our information as to the latter is 
particularly meagre. There is, however, a consensus among 
observers that an air-earth current is constantly flowing, and that 
in fine weather it almost invariably represents a transfer of positive 
electricity downwards (or negative electricity upwards). The most 
exact measurements of its intensity are probably those made by 
Prof. C. T. R. Wilson, whose mean value, 2 x 10~ 16 amp. per cm.*, 
has been used in the estimate given above. 

The charge o- per cm. 2 is not measured directly, but is calcu- 
lated from the observed value of the potential gradient i.e., the 
surface value of the vertical electric force. In fine weather the 
potential normally rises as we ascend. In the above estimate 
the mean value P of the potential gradient at ground level was 
assumed to be 150 volts per metre (I -6 v./cm.). This gives in 
electrostatic units (E.S.U.) 

-4 TTO- = P = (I -5)/300 = 0-005. 

Taking the earth's radius as 64 x 10 7 cm., the whole charge on 
the earth =0-005 x 4 x 10 17 = 2 x_10", or 67 x 10 4 coulombs. 

To get a really good value for P, we ought to have results 
from numerous well-distributed stations, and at each station 
observations are wanted for the whole year. Systematic observa- 
tions have, however, been taken at only a few stations, and at 


most of these the numerical measures obtained may give relative 
values of the potential gradient at different times satisfactorily 
enough, but not the absolute values. Trees, buildings or other 
objects not insulated from the earth, lower the potential in their 
neighbourhood. Thus the observed values are apt to under- 
estimate the true potential gradient unless the place of observation 
has been carefully chosen, and precautions have been taken to 
eliminate the disturbing effects due to the presence of instruments 
and observers. 


Potential gradient, in addition to incessant irregular fluctua- 
tions, has large diurnal and annual variations. Thus a few stray 
observations at a particular spot may give a very erroneous idea 
as to the mean annual value. Even at the best equipped stations, 
with continuous registration, information as to the true mean 
annual value is hard to come by. Little uncertainty exists on 
fine days free from large rapid oscillations, but it is otherwise 
during heavy rain, and especially during thunderstorms. We 
have then to do with large rapid oscillations, recorded by an 
instrument which is not dead-beat, the indications of which may 
be prejudiced by the direct action of rain. 

In view of the difficulty of dealing satisfactorily with highly 
disturbed days, few stations include these when calculating diurnal 
or annual variations. The practice at Kew Observatory is to 
deduce diurnal inequalities and mean monthly values from hourly 
measurements on ten quiet days a month, the results thus repre- 
senting fine weather conditions. Measurements are also made 
on all days when possible at 3 h., 9 h., 15 h. and 21 h. The mean 
from these four hours, when a large number of days is included, 
makes a close approach to the more complete 24-hour mean. 
Thus an idea may be obtained of the effect on the yearly mean 
of deriving it from the selected quiet days by comparing this 
mean with that derived from the above specified 4 hours on un- 
restricted days. The following results were obtained for the year 
1922 : - 

Mean from hourly measurements on selected quiet days, 
318 v./m. ; mean from measurements at 3 h., 9 h., 15 h., 21 h., 
omitting negative potentials, 293 v. /m. ; mean from measurements 
at 3 h., 9 h., 15 h., 21 h., on all days of complete trace, 271 v./m. 

The quiet day mean would thus appear to be in excess of the 
mean that would be derived if all days were included. 

The extent to which negative potential gradient prevails 
may be gauged by the fact that at Kew during 1923 a fairly 
representative year there were approximately 350 hours of 
negative potential, distributed among 190 days, the longest 
duration on any one day being 12-5 hours. Fewer days are 
free from negative potential at Eskdalemuir than at Kew, but 
in this respect Kew is likely to be the more representative station. 



The following are some of the mean annual values actually 
obtained, most, if not all, from selected days : 

Station. Period. v./m. 

Karasjok (Lapland) 1904 ... 139 

Eskdalemuir 1912-22 ... 256 

Potsdam 1904 ... 242 

Kew .... 1913-23 ... 333 

Kremsmunster (Austria) 1902 ... 98 

Tortosa (Spain) 1922 ... 76 

Cape Evans (Antarctic) 1911-12 ... 87 

The values at Eskdalemuir, Potsdam and Kew are much in 
excess of those at stations in both higher and lower altitudes. 
Defective methods or apparatus are more likely to lead to low 
than to high values. On the other hand, atmospheric pollution 
has been found to raise the potential gradient. At Kew, when 
the selected days for 1921 and 1922 were divided into two equal 
groups, representing greater and less atmospheric pollution, the 
mean potential gradients derived from the two groups were 
respectively 343 v./m. and 252 v./m. A calculation based 011 the 
impurities measiired in the two groups of days led to the conclusion 
that the mean value for the two years in the total absence of 
pollution would have been only 156 v./m., as against the actually 
observed value of 298 v./m. As the greater portion of the earth 
is still free from smoke pollution, such as occurs in Britain and 
Germany, we seem justified in believing that the ideal clean day 
value for Kew represents average conditions much better than 
does the value actually observed. 

As the greater portion of the earth's surface is sea, the earth's 
charge will naturally depend largely on the potential gradient at 
sea. According to observations taken on the Atlantic, Pacific 
and Indian Oceans between 1915 and 1921 on board the Carnegie, 
the surveying ship of the Carnegie Institution of Washington, the 
mean value at sea is more like 120 v./m. than 15(3 v./m. But 
the reduction of sea observations presents special difficulties, 
and much more numerous and more widely distributed observa- 
tions are necessary before a satisfactory final result can be obtained. 

The annual variation of potential gradient is usually large, as 
may be seen from the following results, which are in volts per 
metre, and refer to the same years as the mean annual values 
already given :--- 







Cape Evans . . 


















Cape Evans 











If we regard summer and winter as each composed of four 
months, either May to August or November to February, we 
obtain the following seasonal means :- 

Wi ntcr. Sum nier. 

v./m. v./m. 

Eskdalemuir .. 327 .. . 190 

Row 450 .. . 217 

Tortosa 98 .... 60 

Cape Evans 78 .... 95 

At the European stations, as seems generally true of the northern 
hemisphere, the winter mean is markedly the higher. At Cape 
Evans, however, it is the lower. 

In discussing this apparently anomalous result, the observer, 
Dr. G. C. Simpson, remarked that the potential gradient had been 
found to be highest during summer in four previous Antarctic 
expeditions, and the same conclusion had been reached at two 
out of five stations in lower southern latitudes. If the results 
observed in the Antarctic are representative of the southern 
hemisphere, the maximum of potential gradient occurs syn- 
chronously in the two hemispheres. A result so remarkable, and 
if true so important, calls for investigations at a much larger 
number of southern stations. The smaller difference between 
summer and winter in the Antarctic results is also exhibited in 
the results obtained by the Carnegie at sea. This suggests that the 
larger difference observed at European stations may arise from 
the increased impurity of the atmosphere due to smoke in winter. 
The application, however, of a correction for pollution, while 
reducing substantially both the winter and the summer mean 
values of potential gradient at Kew, left their ratio little affected. 


Potential gradient, besides a large annual variation, has also in 
general a large diurnal variation. A full discussion of the diurnal 
variation of potential gradient is out of the question. The extent 
to which it varies, even within the narrow limits of the British 
Isles, may be seen on comparing the Kew and Eskdalemuir curves 
in Fig. 1. Both sets refer to quiet days free from negative 
potential, and both refer to Greenwich mean time, which is in 
advance of local mean time by 1 minute at Kew and 13 minutes 
at Eskdaiemuir. The results are in each case derived from 31 
years, 9 being common to the two stations. In addition to the 
whole year, winter (November to February) and summer (May to 
August) are separately represented. At Kew a double oscillation 
with two distinct maxima and minima is prominent the whole year 
round, especially in summer. At Eskdalemuir the secondary 
maximum in the forenoon is distinctly visible in winter, but in 
summer it is represented merely by a slackening in the fall to 
the conspicuous minimum near noon. The tendency for the 



interval between the forenoon and afternoon maxima to lengthen 
as the day lengthens is prominent at Kew, and recognisable at 

FIG. 1. 

The double oscillation is unusually prominent at Kew, but is 
recognisable at most land stations ; and most of these agree with 
Kew in having a prominent minimum in the early morning. 

In a recent discussion, Dr. S. J. Mauchly, of the Carnegie 
Institution of Washington, has supported the view that the diurnal 
variation of potential gradient at sea follows not local time but 
Greenwich time i.e., that either daily extreme tends to occur 
simultaneously at all parts of the ocean. If this is true, even of 
parts of the sea remote from land, it is certainly a very unique 


The conductivity of the atmosphere and the elements on which 
it depends, namely, the number of free positive and negative ions, 
and their mobility, have undergone a good deal of investigation 011 
the Continent, but the diurnal and annual variations of these 
elements and of the air-earth current are still very imperfectly 
ascertained. In Britain the only systematic observations have 
been those made at Kew Observatory on the air-earth current 
and the ionic charges associated with the more mobile ions. 
These refer to only one hour of the day, 3 p.m. 

The air- earth current varies directly as the potential gradient 
and as the conductivity. The potential gradient at Kew at 3 p.m. 


is 2 -36 times as large in " winter " as in " summer " ; but, taking 
a mean from the five years 1919-1923, the winter value of the air 
earth current has been only of the summer value. The con- 
ductivity must thus be three times as great in summer as in winter 
This is partly explained by the fact that the number of the more 
mobile ions, both positive and negative, in summer exceeds the 
corresponding number in winter in the ratio 5:3. It results 
to an even greater extent from the reduced mobility of the ions in 
winter, which is due at least partly to the greater impurity of the 
atmosphere and the consequent loading of the ions. 


Precipitation (rain, snow, etc.) is unquestionably an important 
vehicle for the interchange of electrical charge between the earth 
and its atmosphere. The most complete investigation of this 
subject hitherto made seems to be that carried out at Simla in 
1908 by Dr. G. 0. Simpson. He measured the amount of rain 
that fell and the charge brought down by it every two minutes. 
The sign as well as the amount of the charge was determined. For 
an aggregate fall of 76 -3 cm. of rain per square centimetre of earth's 
surface, there was a total of 22-3 E.S.U. positive, and 7-6 E.S.U. 
negative electricity. Thus the mean charge per centimetre cube 
of rain was -M>19 3 E.S.U., or 64 x 10~ u coulomb. 

The total charge + 14-7 E.S.U. thus observed is sufficient to 
maintain for a whole year u current of 1 -5 x 10~ lft ampere per 
cm. 2 . This is very similar to the supposed mean value of the fine 
weather air-earth current. It is, however, in the same direction 
as that current, so its existence only doubles the prevailing 

At Simla 75 per cent, of the electricity brought down by rain 
was positive, and the sign was positive during 71 per cent, of the 
time when sensibly- charged rain fell. The proportion of positive 
charges increased as the rain was heavier. There were ninety- 
seven 2-niiuute intervals during which rain fell at a rate of not less 
than -56 mm. per minute, and during every one of these the sign 
was positive. On the other hand, of the 2-minute intervals during 
which rain fell at a less rate than -07 mm. per minute, 47 per 
cent, gave the negative sign, and, of the total charge recorded in 
these light rains, 54 per cent, was negative. 

In general the charge per centimetre cube of rain was greater 
in light than in heavy rain. Of the 34 occasions when the charge 
exceeded 6 E.S.U. per centimetre cube, 20 gave a negative sign 
(18 of these occasions, however, were on one day) ; and while the 
highest value per centimetre cube for positive was 8 E.S.U., 
15 higher values than this ranging from 9 to 20 E.S.U. per 
centimetre cube were recorded for negative charge. 

Most of the rainfall recorded at Simla would be considered 
exceptionally heavy in temperate climates. Thus it obviously 

(B 34/2285)Q ' H 


would not be safe to infer that the predominance of positive rain 
experienced at Simla was true everywhere and at all times. 
Several other observers have, however, obtained a balance of 
positive charge with rain elsewhere. On 33 occasions the current 
measured by Dr. Simpson exceeded 3,000 x 10" 18 amp. per cm. 2 , 
or 1,500 times the supposed mean fine weather current, the largest 
observed value being 10,000 x 10 ' 18 amp. 


Another vehicle for the exchange of charges between the earth 
and the atmosphere, the importance of which is even more difficult 
to assess, is lightning. For the investigation of thunderstorm 
phenomena we are mainly indebted to Prof. C. T. R. Wilson. 
His method is based on the fact that the charge on a plate area 
at zero potential which can be treated as part of the earth's surface 
is proportional to the potential gradient at the surface. The 
change in the charge of the plate during the passage of a lightning 
stroke supplies a measure of the change in the potential gradient 
brought about by the discharge. Prof. Wilson's measurements 
refer to the integral of the changes during a time which to ordinary 
ideas is very short, but still may be long from the point of view 
of those interested in wireless atmospherics. 

The significant phenomenon accompanying a flash of lightning 
is the change of potential gradient. The change of potential 
observed by Prof. Wilson was positive for 528 out of 864 flashes 
i.e., in 61 per cent, of the total. 

Prof. Wilson's conception of an ordinary thundercloud is that 
it consists normally of an upper and a lower part. The charges in 
the two parts are of opposite sign, but there is no necessary 
relation between the two amounts. In most cases Prof. Wilson 
believes the upper charge to be positive, say, Q 2 , and the lower to 
be negative Qt. Suppose these charges to be concentrated at 
points in the same vertical line, at heights H 2 and H! above the 
ground, supposed horizontal. To calculate the resulting potential 
we may regard the charge induced in the earth as replaced by 
the image charges Q 2 and 4- Qi, at depths H 2 and H x respec- 
tively. The (downwardly directed) force at ground level is then 
easily found to be 

2Q 2 H 2 (H^ + L 2 )~ f - 2Q 1 H 1 (H* + L a )~* 
where L is the horizontal distance of the point at which the force 
is being measured from the vertical line on which the charges are 
supposed located. 

The force at the ground immediately under the charges is thus 

2(Q 1 H 1 ~ I Q 2 H 2 *), while at a distance L, large compared 

with H 2> it is 2(Q 2 H a - QJSOL"" 1 . 

As H 2 exceeds H lf unless Q 8 and Q t are widely different, the 
force in the case supposed is downwards (as in fine weather) at a 


considerable horizontal distance, but upwards immediately under 
the thundercloud. At some intermediate distance the force 
vanishes. An upwardly directed force means a negative potential 
gradient, and a positive charge at the earth's surface. Thus, if a 
discharge passed in this case between the ground and the lower 
cloud, it would naturally carry a positive charge from the ground 
to the cloud. This, it will be remembered, is what Prof. Wilson 
observed in 61 cases out of 100. 

A lightning stroke might, of course, produce only a partial 
discharge of a cloud, but Prof. Wilson's observations fit in best 
with the view that the discharge is usually complete, or nearly so. 
When discharges were at a considerable distance, the potential 
changes observed followed in a general way the law of the inverse 
cube of the distance, which is that given by the above formula 
on the hypothesis of total discharge. 

The observations really gave the product Q H. The values 
obtained for Q, on the hypothesis that H was 2 km., averaged about 
20 coulombs. Immediately on the discharge, regeneration took 
place of the charge in the thundercloud, the initial rate of regenera- 
tion being very rapid. In fact, at the initial rate, the charge in 
the average case would have been replenished in about 7 seconds. 
Supposing 20 coulombs in the thundercloud, this would imply 
a current of some 3 amperes. The rate of replenishment fell off 
in reality very rapidly as the time increased, following an approxi- 
mately exponential law. 

For a spark to pass in air at ordinary atmospheric pressure, a, 
voltage of 30,000 v./cm. is required. This voltage would be found 
at the surface of an isolated sphere of 250 metres radius charged 
with 20 coulombs. The presence of the earth, and the bipolarity 
of the cloud, will naturally have a large effect on the voltage at the 
surface of, say, the lower cloud, so that the above is likely to be a 
considerable under-estimate of the volume discharged by a 
lightning stroke. The changes in the potential gradient accom- 
panying a lightning stroke are, according to Prof. Wilson, of the 
order of 1,000 v./m. when the distance of the flash is 10 km., 
and might be as high as 100,000 v./m. when a flash takes place 
right overhead. 


An " upper conducting layer " plays a prominent part in 
Prof. Wilson's theoretical views. In fine weather the potential 
gradient falls off rapidly as we go up, and observations point to the 
conclusion that even at 10 km. it is practically negligible. There 
can thus be little further increase of potential at greater heights, 
and Prof. Wilson regards 10 6 volts as a liberal estimate for the 
higher regions of the atmosphere. Assuming considerable con- 
ductivity, we may consider the upper atmosphere as approxi- 
mately at one potential. If, then, we have a thundercloud of 
(B 34/2285)Q H 2 


which the upper charge is positive, the potential at the top of that 
cloud, which according to Wilson is of the order 10 9 volts, is very 
high compared with that of the conducting layer. Lightning 
may pass between the ground and the lower or the upper cloud, 
between the lower and the upper cloud, or possibly upward from 
the upper cloud. In any case a considerable electrical current 
may pass between the upper cloud and the conducting layer. 
If the upper cloud is charged positively, the current will carry a 
positive charge to the conducting layer, and thus tend to make 
up for the downwardly directed fine weather current and the 
charge brought down by rain. 

This explanation would obviously be insufficient if lightning 
were a rare phenomenon. While, however, a thunderstorm is a 
comparatively rare event at any one station in temperate latitudes, 
the number of thunderstorms per diem experienced by the earth 
as a whole must be large. Even in such a limited area as France, 
there are comparatively few days in the year where thunder is 
nowhere reported. The explanation would thus seem a possible 
one, but a great deal more information is required as to lightning 
charges in different parts of the world before a conclusion would 
be justified. 

The existence of an upper conducting layer or, as it is some- 
times called, a Heaviside layer - -is also of interest in connexion 
with wireless. It has been invoked, especially by Prof. W. H. 
Eccles, in explanation of the surprisingly great distances to which 
wireless signals are transmitted. The most direct evidence we 
have of the existence of such a layer is afforded by auroral and 
magnetic phenomena. The co-existence of magnetic storms and 
brilliant aurora is conclusive evidence that aurora is an electrical 
phenomenon. As Prof. Stormer's measurements have shown, 
the lower edge of visible aurora is usually at a height of about 
100 km. Electrical currents at that height, to produce the large 
effects seen in magnetic storms, must be of large size, and the 
conductivity of the space where these currents occur must thus 
be high, unless we postulate a quite incredible expenditure of 
energy. Wireless experts seem in. favour of a smaller height 
than 100 km. for the conducting layer, and Prof. Wilson's argu- 
ments would seem to fit in better with a lesser height. Various 
observers, moreover, have thought they have seen aurora at 
much smaller heights, but the recent precise measurements made 
in Norway tend to show that under normal conditions, aurora at 
heights below 85 km. must be very rare. 

The auroral layer is certainly not a thin one, as Prof. Stormer 
has measured aurora at heights lip to 650 km. It is also not certain 
that the electrical currents which produce the ordinary daily 
changes in terrestrial magnetism are at the same height as those 
which manifest themselves in aurora and magnetic storms. But, 
if not certain, it is at least probable that the heights are the same. 


Otherwise it would be difficult to explain the fact that the regular 
diurnal variation increases with magnetic disturbance, and that 
this increase is particularly prominent in high latitudes where 
aurora prevails. 

An increase in the amplitude of the regular diurnal magnetic 
changes is a prominent feature of sunspot maximum, while aurora 
and magnetic disturbance tend to a minimum near sunspot 
minimum. The natural inference is that the conductivity of 
the upper atmosphere increases with sulispot frequency and with 
magnetic disturbance. It would thus not be at all surprising 
if wireless phenomena showed an eleven-year period, and exhibited 
some dependence on aurora and magnetic disturbance. If any 
relation of the kind exists, it is in high magnetic latitudes that we 
should naturally look for it. 

In view of the large changes of voltage accompanying even 
distant lightning strokes, it is natural to regard lightning as the 
probable source of wireless " atmospherics." But there are also 
rapid changes at least of the magnetic- field during aurora. 

The electrical currents visible at night as aurora doubtless 
exist, though invisible, during the day. But in Britain magnetic 
disturbance is normally greatest during the evening hours when 
aurora is visible. Also it is greater in the equinoctial than the 
summer months. Thus any disturbances associated with the 
electrical phenomena visible as aurora would naturally in Britain 
be greatest after dark and in the equinoctial months. They should 
also be markedly less numerous and more poorly developed near 
sunspot minimum than at other times. Adequate statistics as to 
the diurnal and seasonal variation of ** atmospherics " would be 
hard to come by. The problem is complicated by the fact that 
the development of atmospherics at any given instant varies with 
the orientation of the receiving aerial. The observations carried 
out by Mr. 11. A. Watson Watt for a year at Aldershot suggested 
a maximum of frequency in June (naturally a very quiet month 
magnetically) and a minimum in March. Mr. Watt's observations 
were, however, confined to 7 h., 13 h., and 16 h., all hours remote 
from the hour of maximum auroral frequency and magnetic 
disturbance. The diurnal and annual variations indicated by 
Mr. Watt's figures are by no means so prominent as we should 
expect if lightning flashes were the sole cause of atmospherics. 
Observations for several years from a variety of stations, covering 
adequately both day and night, would appear desirable. 

In a later paper, Mr. Watt and Prof. E. V. Appleton have 
investigated the nature of " atmospherics." They found two 
principal classes which they describe as " aperiodic " and " quasi- 
periodic." These were about equally numerous. The aperiodic 
group, showing a growth and a decay period usually of similar 
length, had durations varying from 0-0001 to 0-055 sec., the 
average being 0-004 sec., but the most common only 0-001 sec. 
The quasi-periodic group, consisting usually of one complete 


but unsymmetrical oscillation, were much less variable in length, 
having on the average a duration of 0-002 sec. The average 
amplitude of the change of field was about 0-13 v./m. in both 
groups. But, whereas there was no marked unbalanced transfer 
of electricity in the quasi-periodic group, seven out of eight of the 
aperiodic type carried positive electricity from the earth. This, 
it will be remembered, is also the direction which Prof. Wilson 
found most prevalent in lightning flashes. 


Little has been said here as to the numerous theories that have 
been advanced to account for the earth's charge, but reference 
may be made to one of them, namely, that the charge is due to 
a very penetrating radiation reaching the earth from outside, 
probably from the sun. An apparently fatal objection to this has 
been the absence of the ionisation naturally expected to accom- 
pany such a radiation. A way out of this difficulty has been 
suggested in a recent paper by Prof. W. F. G. Swann. His 
theoretical investigations lead him to the conclusion that a 
corpuscle travelling with a velocity which fell short of the velocity 
of light by 45 metres per second or less would not ionise the air it 
passed through. In the absence of positive knowledge, Prof. 
Swann was obliged to make several assumptions in his calcula- 
tions, and whether these will recommend themselves to experts 
in the subject appears very doubtful. 


The references we have made to atmospheric pollution, 
thunderstorms and wireless will have shown that atmospheric 
electricity has practical as well as theoretical interests. But there 
are two still more practical aspects which may be just glanced at. 
The first is the possibility that the existence of the potential 
gradient, entailing under normal conditions a big difference of 
potential between the earth's surface and the atmosphere at 
comparatively small heights, may be utilised practically as a source 
of mechanical power. The second is the possible utility of the 
potential gradient as a stimulus to the growth of plants. The 
suggestion of Lemstrom that the potential gradient influences 
growth has led to experiments in electroculture. Experiments 
have been made in England and elsewhere to see how crops 
growing under zero potential gradient compare with those grown 
in the ordinary earth's field. No very decisive results seem 
to have been obtained. It should, however, be remembered that 
the potential gradient above tree tops will ordinarily be very 
large as compared with the potential gradient at ground level. 
Thus atmospheric electricity might be of importance for sylvi- 
culture, without being of importance for ordinary farm crops. 


A second point, which may be of significance in connexion with 
the high potentials used in electro-culture, is that when Nature 
applies high potentials and large air-earth currents to grass and 
ordinary crops, it is usually to the accompaniment of a liberal 
supply of moisture. 

It is hoped that the above slight discussion of some of the 
phenomena of atmospheric electricity will bring home to the 
reader the numerous directions in which patient, intelligent, 
observational work is urgently wanted. 




Although many different and conflicting opinions are held as 
to the causes and methods of evolution, it cannot be too strongly 
emphasised that the evidence in favour of evolution, or descent 
with modification, is unassailable. Darwin showed that the facts 
of structure, classification, development, geographical distribution 
and geological succession of animals and plants were inexplicable 
unless there had been evolution. He gave a new impulse and new 
ideas to work in these branches, all of which had confirmed his 
conclusion, so that at the present day we are justified in saying 
that evolution is established. Organic evolution, indeed, is just 
as well established as the movement of the earth round the sun ; 
it is an inference from many kinds of evidence, all leading to the 
same conclusion. This comparison is not without interest, for it 
may be recalled that, a few hundred years ago, the geocentric 
theory of the universe was generally accepted, and that when the 
idea that the earth moved round the sun was first put forward, 
.it was opposed for very much the same reasons that the idea of 
evolution was opposed later on. 

Before Darwin's time naturalists had made great progress in the 
classification of organisms according to their structure, arranging 
them in groups within groups, always striving towards a Natural 
System, expressing the true affinities of one form to another. 
Darwin pointed out that the Natural System is founded on descent 
with modification, that the characters considered to show true 
affinity between species are those inherited from a common 
ancestor, and that all true classification is genealogical. 

Thus the expression of relationships has become the primary 
aim of classification, and the method of study is an attempt 
to estimate the meaning of resemblances and differences in 
structure to what extent these may be due to the nearness or 


remoteness of a common ancestor, to what extent to similarities 
or dissimilarities of habits and environment. 

Related forms, although very dissimilar when adult, are often 
quite alike in the early stages of their development. The study of 
embryology has led to the formulation of the statement that the 
development of the individual tends to repeat the history of the 
race, the adult stage of the ancestral form being represented at an 
earlier stage in its modified descendants. The value of this 
generalisation has sometimes been disputed, but its truth is well 
shown by such series as the fossil corals from successive 
horizons studied by Mr. R. G . Carruthers. I n these the young stages 
of the form found in one horizon arc structurally similar to the 
adults of that found in the earlier horizon immediately below it. 
Darwin insisted on the imperfection of the geological record, 
and pointed out that we could not expect to find in the rocks a 
large number of transitional forms connecting the past and 
present species of a group. Although great progress has been 
made in palaeontology, we still know but a minute fraction of the 
vast number of forms of life that must have inhabited the 
earth in former times. The value of the evidence from palaeonto- 
logy lies not in its completeness, but in the fact that it all points 
in the same direction. In recent years lineages, or lines of descent, 
have been established ; for example, the ancestry of the elephant 
has been traced back to a primitive hoofed animal, very similar 
in appearance to a tapir, found in the upper Eocene beds of Egypt. 
Also many connecting forms have been discovered ; for example, 
extinct types of man, with skulls possessing ape-like features 
that have been lost in modern man. 

The Mammals of Australia may be used to illustrate how 
evolution throws light on the facts of geographical distribution. 
The Monotrernes, the Duck-billed Platypus and the Spiny Ant- 
eaters, found in Australia, Tasmania and New Guinea, are the only 
survivors of a primitive group that appears to have become 
extinct elsewhere millions of 'years ago. In other parts of the 
world the placental Mammals are dominant, but these, except for 
a dog introduced by man, and a few kinds of small rodents, which 
appear to have made their way over the sea on floating timber, 
have never reached Australia, which was evidently cut off^by 
sea from the rest of the world before they developed. The 
Marsupials, a much more advanced type than the Monotremes, 
but ancient, and in some ways more primitive than the placental 
Mammals, have, in the absence of these, developed in the Australian 
region into a number of different types, Kangaroos, Thylacines, 
Bandicoots, Wombats, etc., which have doubtless caused the 
extinction of the less highly organised Monotremes, with the 
exception of the Platypus, which is aquatic, and so occupies a place 
that no Marsupial seems to have tried to fill, and the Anteater, 
which is protected by- its covering of hedgehog-like spines, and 
by its burrowing habits. 



Darwin, after patiently accumulating the evidence and con- 
sidering it for many years, put forward the theory that evolution 
has been effected chiefly through the natural selection of slight, 
favourable variations, aided in an important manner by the 
inherited effects of use and disuse, and in an unimportant manner, 
in relation to adaptive structures, by the direct action of the 
environment and by variations which seem to us in our ignorance 
to arise spontaneously. The foundation of the Natural Selection 
Theory is that animals and plants are eminently variable, and 
that in every generation many more are born than can possibly 
come to maturity ; consequently it was suggested that in the 
struggle for existence, those best fitted to survive would do so, and 
would leave descendants more or less like themselves, the fittest 
of whom would again be selected. 

Darwin considered that natural selection was the most 
important, but not the only means of modification. To-day 
biologists tend to split up into schools, one of which claims that 
natural selection alone has been operative, whereas others deny 
the importance of natural selection ; of these, one attributes most 
to the inherited effects of use and disuse and of the direct action 
of the environment, and another denies that these effects are 
inherited and relies on those variations which, as Darwin said, 
seem to us in our ignorance to arise spontaneously. 

Biology is now so vast a subject that one man cannot hope to 
become an authority on more than one branch of it ; all biologists 
are specialists, and their outlook on evolution seems to be to a 
considerable extent determined by the nature of their special 

In his essay on mimicry, Prof. Poulton shows how well 
the facts may be explained by the Theory of Natural Selection. 
Dr. Gahan's work on mimicry in beetles is also most instructive. 
He states that mimicry occurs only in day-flying beetles, those 
that fly by night being generally protectively coloured, resembling 
the objects on which they rest during the day. He contrasts the 
uniformity of one family of beetles with the diversity of another. 
The Lycidae, easily recognised by their form and colour, are 
protected by a distasteful secretion, and wherever they occur 
are mimicked by day-flying beetles of other families and by other 
insects, which thus get the advantage of being mistaken for them. 
The Cerambycida) are not protected by a distasteful secretion, and 
exhibit the greatest diversity, mimicking other insects ; even 
closely related forms are quite dissimilar, some resembling stinging 
insects, such as wasps, others noxious beetles, etc. Here we have 
a condition of affairs that cannot possibly be explained as acci- 
dental, nor as the result of use and disuse or of the direct action of 
the environment ; but it could have been brought about by 
natural selection. 


Let us next consider the Dodo, a flightless bird that formerly 
inhabited the island of Mauritius. It was about as large as a 
goose, a heavy, clumsy bird, with very small wings and with the tail 
represented by a little tuft of feathers. Structurally it shows 
relationship to the pigeons, and there can be little doubt that its 
ancestors were pigeon-like birds that flew to Mauritius. Here 
they found that food was plentiful and that there were no carni- 
vorous animals to attack them, and in course of time their 
descendants lost the power of flight and grew larger, whilst their 
wings and tail decreased in size. About 300 years ago Europeans 
introduced pigs and dogs into Mauritius, and the Dodo, unable to 
cope with these new invaders, soon became extinct. 

If the evolution of the Dodo can be explained in terms of the 
Natural Selection Theory, it must be admitted that the explanation 
is not very convincing, and this example is given here as typical 
of many that lead to the conclusion that the effects of use and 
disuse are inherited and are of importance in evolution. The 
opponents of this conclusion demand experimental proof, but it is 
obvious that the attempt to repeat the operations of Nature might 
involve an experiment that would last for thousands of years ; 
nevertheless, there are signs that experimental proof may yet be 

The non-adaptive spontaneous variations, which Darwin con- 
sidered might be of some importance in specific differentiation, if 
their unknown cause were sufficiently widespread, have come into 
prominence in recent years, as they have been especially studied 
by those engaged in experimental breeding. New varieties, 
differing from the normal in some well-marked character, suddenly 
make their appearance ; if these are crossed with the parent form, 
and the hybrids so obtained are bred together, the new character 
appears intact in a certain proportion of the next generation. 
In experimental breeding it has been found possible to form new 
combinations of characters that are inherited in this definite 
manner, so that striking novelties have been produced. But now 
it seems to be admitted that this work does not throw much light 
on evolution, that the production of these novelties has no relation 
to the origin of species. Indeed, there is strong reason for believing 
that in Nature the first step in the origin of a new species is not the 
appearance of a new character, but the formation of a community 
with new habits, or in a new or a restricted environment. 

The present writer's attitude towards these matters may 
be illustrated by an example from his own work. Shad are fishes 
of the herring family that enter rivers to breed. The Twaite 
Shad (Alosa jinta) of the Atlantic coasts of Europe is represented 
in Killarney by a form that differs from it in its smaller size, deeper 
body, and more numerous gill- rakers projections from the gill- 
arches that intercept the food. The Killarney Shad never leaves 
the lake and must have evolved from Twaite Shad that formed a 
lacustrine colony, probably founded by young that preferred 


staying in the lake to going to the sea. We have good reason to 
suppose that this evolution has taken place since the Glacial 
Period say, within the last 100,000 years for the presence of a 
Char* in Killarney proves that in Glacial times the sea off the 
coast of Kerry was cold, and the Twaite Shad and its allies are 
fishes of the Mediterranean and the warmer parts of the North 
Atlantic. Also, it is a legitimate inference that the increased 
number of gill-rakers in the Killarney Shad is related to a diet 
of minute Crustacea, and that its form and size may be related 
to its restricted environment and changed habits. Here, then, we 
see what has happened, and where and when it has happened ; 
we have some evidence as to why ; the problem that remains to be 
solved is, how ? Whether that or similar problems could be 
solved by experimental attempts to repeat the operations of 
Nature is an open question, but such experiments would at 
least be on the right lines. 

* Char are Salmonoicl fishes of the Arctic- Ocean ; they breed in fresh 
water and often form permanent colonies in lakes. 


By Prof. E. B. POULTON, F.R.S. 

The superficial resemblances between insects constantly 
attracted the attention of the older naturalists, as we realise 
from the names they gave when they called certain moths " bee- 
like," ki wasp-like," etc. It was the same with resemblance to 
surroundings. The fine old naturalist, W. J. Burchell, writing 
more than a hundred years ago of his travels in the interior of 
'South Africa, described a grasshopper which exactly resembled a 
stone, and also fleshy plants of the Karoo which were hidden in 
the same way. Ho fully recognised the benefit conferred by this 
likeness, but held the common belief of his day that insect and 
plant came into existence exactly as we see them, and that their 
resemblances were part of " the harmony with which they have 
been adapted by the Creator to each other and to the situations in 
which they^are found." 

The appearance of Darwin's " Origin of Species " in 1859 
brought clear evidence that animals and plants had reached their 
present state by a process of evolution, and that the main motive 
cause had been Natural Selection or the Survival of the Fittest, 
acting upon hereditary variations. One of the first problems 
to which these principles came to be applied was insect mimicry 
and the protective resemblances or concealing colours and shapes 
of insects both resemblances so widespread and evident in Nature 
that the failure to explain them on Darwinian principles would 
have meant the breakdown of the principles themselves. Insect 
mimicry and insect concealment became test problems. If 
produced by Natural Selection, these resemblances must be bene- 
ficial and must have been attained by transition from different 
.and less beneficial stages. 


It is necessary at the outset to clear away certain miscon- 
ceptions which here arise from the word '* mimicry," used in 
ordinary speech to signify conscious imitation. As used technically 
for these deceptive superficial resemblances, conscious imitation 
is out of the question. No insect " by taking thought " can effect 
any change in its own appearance. Mimicry is akin to protective 
resemblance, and is sometimes employed to include the latter ; 
but it is convenient to keep the two separate because they lead 
to such different kinds of appearance. In mimicry, an insect 
resembles another, the model, which possesses some special 
defence, such as a sting, an unpleasant taste or smell, etc., and 
advertises its powers by conspicuous warning colours. The mimic 
therefore becomes itself conspicuous. In protective resemblance, on 
the contrary, an insect resembles something, such as earth or 
bark, of no interest to its enemies, and in resembling it becomes 


Returning to the relation between Mimicry and Natural 
Selection, it must have been obvious to naturalists for many 
years that the resemblance of a stingless moth or ily to a bee or 
wasp is likely to be advantageous. It was otherwise with the 
likeness between many butterflies and day-flying moths collected 
by H. W. Bates in the Amazon Valley striking likenesses of 
colour and pattern in each locality, all changing together, " as 
it were with the touch of an enchanter's wand/' in passing from 
one area to another. The late Dr. F. D. Godman has told us 
that Bates did not solve this problem in the tropics. The solution 
came after studying his collection at home and reflecting on his 
memories of the living insects. In November, 1861, just two years 
after the publication of the ik Origin of Species," he read before 
the Linneaii Society his classical paper in which it was shown 
that the imitated butterflies or k4 models " were specially abundant, 
conspicuous and slow-flying, and belonged to groups with these 
characteristics, while the 4i mimics " members of several widely 
separated groups had departed from the colours and patterns 
still borne by their no n- mime tic allies. 

A few years later, in 1866, A. R. Wallace showed that Bates's 
interpretation was valid for the Tropical East ; and, again in a 
few years, Roland Trimen proved that it held in Africa. All 
three memoirs were published in the " Transactions " of the 
Linnean Society, and the last mentioned, appearing in 1870, 
showed for the first time that three Swallowtail butterflies with 
entirely different patterns, and without " tails " to the hind 
wings, were the females of a fourth Swallowtail (Papilio 
dardanus) which bore " tails " t and was of a still more 
divergent pattern. All four had been described as different 


species. Trinien further showed that in Madagascar a closely 
allied male had a tailed female very like itself, and that on the 
mainland of Africa the " tails " had been lost and different 
patterns gained by the females in mimicry of three different 
species of the tailless group DanainaB which provides the chief 
models for mimicry in the East, and is closely allied to the chief 
models of tropical America. 

Trimen's conclusions were received with incredulity, and 
indeed contempt by some of the older naturalists ; but he lived 
to see them everywhere accepted, and put beyond the possibility of 
doubt by breeding the males and all three female forms in one 
family produced by a known female parent. 


Bates, in the great paper already referred to, directed attention 
to the fact that butterflies belonging to the, groups which supply 
the models also mimic each other, and this puzzled him. The 
interpretation came in 1878, when Fritz Miiller, a German natur- 
alist living iu Brazil and deeply influenced by the " Origin of 
Species " and by correspondence with Darwin, brought forward 
the theory which has since been known as Mullerian mimicry. 
He then showed the advantage of a resemblance between un- 
palatable conspicuous insects, because it reduces the number of 
warning patterns which must be learnt by enemies and the number 
of injuries inflicted in learning them. 

Batesian mimicry is like the action of a struggling unscrupulous 
firm which imitates the trade-mark or advertisement of a success- 
ful house. Mullerian mimicry is like the action of a group of 
powerful firms which become still better known at ti lessened cost 
by combined advertisement. 

The decision, whether any mimic is Batesian or Mullerian, is in 
some instances easy, in others difficult, and opinions are divided as 
to the relative importance of the two theories. When the allies 
of a mimetic species are well concealed by protective resemblance, 
then the mimic is most probably Batesian ; when the allies are 
specially protected and warningly-coloured, then the mimic is 
probably Mullerian, and has merely exchanged one warning 
pattern for another. Again, a large proportion of mimetic species 
are, like Papilio dardanus, only mimetic in the female sex, the 
original pattern of the female being retained by the male. Here 
the appearance of the male, and especially of its under-surface 
pattern, helps us to decide whether the female is a Batesian or a 
Mullerian mimic. 

The prevalence of mimicry in the female was explained by 
A. R. Wallace by the greater needs of that sex their greater 
weight and slower flight, and the necessity for them to alight and 
lay their eggs. Another co-operating explanation was suggested 
much later, namely, that the females are more variable than the 


oaales, and thus produce the requisite changes of pattern more 


Butterflies and day-flying moths are especially suitable for 
the study of mimicry, because the resemblances are chiefly shown 
in the colours and patterns on the broad surface of their wings, both 
colours and patterns being very variable, and thus affording 
material for rapid change by the operation of Natural Selection. 
But the same phenomena are conspicuous in other groups of 
insects, such as the beetles. We find among the beetles, as among 
the butterflies and moths, that the models belong to the distasteful 
waniingly coloured groups, and that members of these tend to 
mimic each other, as well as to be mimicked by species of less 
powerful groups. 

There can be little doubt that Mendelian heredity has been of 
great importance in the origin of mimicry, diminishing the 
'" swamping effect of intercrossing " between the parent form 
and the incipient mimic and between the different fully formed 
mimics belonging to one species, as in P. dardanus. There is also 
a considerable body of facts which suggests that Mendelian 
heredity does actually operate in this and other mimetic species, 
the most complete evidence being that obtained by Mr. J. C. F. 
Fryer by his breeding experiments on Papilio polytes. This species, 
in Ceylon, where the experiments were conducted, has three 
forms of female, one like the male and two resembling other 
Swallowtails which belong to a distasteful group. The Mendelian 
relationship was found to exist between these three females. 


It is in the facts of geographical distribution that we find the 
most conclusive evidence of the production of mimicry by Natural 
Selection. Two Danaine butterflies in America belong to an Old 
World group, and are evidently recent invaders by way of the 
north. In temperate North America they have met the natives 
of the Northern Belt, among them the White Admirals, allied to 
our own well-known species L. sybilla. If, therefore, mimetic 
resemblance is, as some have supposed, the common result of 
common causes associated with locality, the invader ought to have 
come to resemble the ancient resident, but as a matter of fact the 
resident has lost its original pattern and mimics the invader. 
The change, although immense, so far as the pattern is concerned, 
is so superficial and recent that the early stages are entirely un- 
affected, and the mimic can interbreed with another unchanged 
North American White Admiral, producing an intermediate 
hybrid an experiment successfully carried out by Mr. W. L. W. 


The PseudacrsGas of Tropical Africa, nearly all of them mimetic 
in both sexes, are butterflies closely allied to the White Admirals. 
A species in Uganda, P. eurytiis, is a very complicated example of 
mimicry, for this single species includes in the same locality three 
different forms, two with sexes alike, mimicking two Acrseine 
species with sexes alike, one with sexes different, mimicking the 
corresponding sexes of another Acrseine. The fact that all these 
mimics are one species was proved by breeding by Dr. G. D. H. 
Carpenter. Now in Uganda intermediates between these three 
forms of enrytus are rare, whereas on some of the islands in Lake 
Victoria they are very common, and Dr. Carpenter, who proved 
this fact, found that on these same islands the models are, for 
some unknown reason, rarer than their mimics. The facts suggest 
strongly that there is more severe extinction of intermediates in the 
presence of abundant models, but less severe when models are few. 


Further convincing evidence of the production of mimetic 
likeness by the operation of Natural Selection is provided by a com- 
parison of the different methods by which a resemblance to 
formidable insects, such as wasps and ants, has been attained. 
The variations which offer the possible beginnings of such a 
likeness are determined by the present constitution of each species, 
and this again has been determined by its past history. Great 
differences in the method of resemblance are therefore to be ex- 
pected and are found. In some flies the slender " waist " of a wasp 
is represented by an actual narrowing of the body ; in certain 
beetles by a patch of white which " paints out " the superfluous 
thickness a device very elaborately carried out in the young 
stages of an African long- horned grasshopper, which lives among 
green leaves and has the un-antlike parts of its body coloured 
green, the antlike parts black. 

The likeness requires astonishing readjustments when the 
mimicking animal is widely different from its model. Thus many 
small spiders mimic ants ; but spiders are not insects, having no 
antennae, having eight legs instead of six, and the body divided 
into two sections instead of three. A North American spider 
observed by Dr. and Mrs. Peckham got over these difficulties by 
holding up one pair of legs to represent antennae and by developing 
a groove across one of the body sections, making it look like two. 


Mimetic likeness to be efficient nearly always demands appro- 
priate movements, and these are often the most important part of 

(B 34/2285) Q i 


the likeness, and sometimes the probable starting-point. Thus 
beetles which in the cabinet do not at all closely resemble a wasp 
may be convincingly wasplike in the rapidity and jerkiness of 
their movements. This is true of our British wasp-beetle and of a 
rather similar Brazilian species of which Burchell wrote on his 
South American journey nearly 100 years ago : "It runs rapidly 
like an ichneumon or wasp, of which it has the appearance." 
A note by the same naturalist on a small Brazilian spider suggests 
that the first stage of mimicry was produced in this way : " Black 
. . . . runs and seems like an ant with large extended jaws." 
Now this spider does not belong to a group known to include 
antlike species, and BurchelPs observation suggests, as Mr. R. I. 
Pocock has pointed out, " that the perfect imitation in shape, 
as well as in movement, seen in many species was started in forms 
of an appropriate size and colour by the mimicry of movement 


Of all the methods by which both mimicry and protective 
resemblance are produced, the most remarkable and the most 
convincing as evidence for the operation of Natural Selection 
is that followed by tropical American insects allied to the Cicadas 
and our too well-known Greenfly. It is only because these insects 
- the Menibracidse are ail of them small that the examples are 
unfamiliar and the lessons they teach unknown to many who are 
interested in the subject. The body of these little insects is 
shaped much like that of the Greenfly, but it is completely hidden 
when looked at from above by a covering shield, which is developed 
from the body-ring behind the head and grows backwards. 
Therefore, when the insect is concealed by resembling some object 
such as a thorn or when it mimics an ant, the deceptive likeness 
to be of any use must appear in the covering shield, and not in the 
hidden body ; and this is exactly what has happened. 

The criticism has been urged by Jacobi that these insects, when 
disturbed, leap like their relatives, the Froghoppers, and therefore 
the mimicry of an ant is meaningless. This is a good example 
of the kind of objection often raised against the theory of mimicry. 
But, if a hopping insect comes to resemble an ant, it still stands to 
gain by keeping its older means of escape when the newer one is. 
seen through, or when it is attacked by the enemies of ants. 


Another objection often brought forward, especially against the 
theory of mimicry as applied to butterflies, is the assertion that 
these insects are "rarely attacked, if at all, by birds the only 


enemies which are believed to cause first the growth, and then the 
maintenance, of a deceptive resemblance to the model, by destroy- 
ing on the average more of the less like and fewer of the more like 
in each generation. The critics have especially relied upon the in- 
sufficiency of direct evidence of such attacks, and the almost 
complete absence of butterfly remains from the stomachs of an 
immense number of American birds which were examined in 
order to determine the nature of their food. 

In reply to the former objection, Dr. G. A. K. Marshall collected 
and published in 1901) all the observations recorded up to that 
date, and proved that the evidence was much stronger than had 
been supposed. Furthermore, attention having been thus directed 
to the subject, many naturalists, especially Mr. 0. F. M. Swyn- 
nerton, Dr. Gr. D. H. Carpenter, and Mr. W. A. Lamborn, made 
a special study of the relation between birds and butterflies in 
various parts of Africa, and soon produced abundant positive 
evidence. Mr. Swynnertoii and Mr. Lamborn also demonstrated 
the frequent presence of. birds' beak-marks upon the wings of 
butterflies, marks which afford the strongest circumstantial 
evidence of attack. Beautiful examples of these impressions of 
beaks on Fijian butterflies have still more recently been received 
from Mr. H. W. Simmoiids. 

As regards the objection founded on American birds, Mr. 
Swynnerton has proved, and Mr. Lamborn has confirmed, that 
the digestion of birds is remarkably rapid, and that a butterfly 
is quickly reduced to a condition in which it can only be recognised 
by means of the compound microscope. 

The facts of mimicry and protective resemblances are now 
patent to all, and no valid interpretation of these facts except 
that which is based on the theory of Natural Selection has ever 
been oiler ed. 

(34/2285)Q i 2 


By DR. E. J. ALLEN, F.R.S.. Director of the Marine Biological 
Laboratory, Plymouth. 

For the biologist there has always been a peculiar fascination 
in the study of life in the sea. There are good reasons for thinking 
that it was in the sea that life on our earth had its origin. It is 
there that we see life in its most perfect grace of form and move- 
ment, with the simplest adaptations of means to ends, and with the 
least interference from the ravages of modern man. Much effort 
has been directed in recent times to the scientific study of the 
relations which exist between the living organisms of the sea and 
their physical surroundings, as well as to the study of the inter- 
relations of these plants and animals among themselves. As on 
the land, so in the sea, the life of the animals is directly or indirectly 
dependent upon plant-life ; and indeed it is of the essential nature 
of an animal that it derives its nourishment from previously 
formed organic or living material from the body of a plant or of 
another animal already in existence. The plants of the sea, on 
the other hand, obtain their food directly from dissolved inorganic 
or non-living materials which they build up into living substance 
under the influence of sunlight, the sunlight supplying the power 
or energy required for the building process. 

The plants of the sea which are most generally known are the 
brown and red sea-weeds, but these do not extend to any great 
depth, as they are all plants which flourish only when fixed to some 
solid object on the sea-floor. They therefore die out when the 
depth of water exceeds about 15 fathoms, as below that depth 
sufficient light cannot penetrate to enable them to grow. These 
brown and red sea-weeds are thus confined to a comparatively 
narrow belt or zone around the coasts, and furnish only a small 
fraction of the whole vegetable food-supply of the sea. The main 
supply is provided by vast multitudes of minute, microscopic 
plants, which may be thought of ^is floating like a fine, brownish- 
green dust in the surface-waters of the ocean. The great efficiency 


of this arrangement will at once he realised, for these innumerable, 
fine particles of living plant-suhstance offer the greatest possible 
surface for gathering from the surrounding sea-water the food- 
materials they require, as well as for receiving the light from the 
sun. It permits of the maximum quantity of living plant material 
being built up, so long as the light of the sun continues. By far 
the greater part of these, floating plants live in the top J5 or 20 
fathoms of water, but they may be found in smaller numbers to a 
depth of 100 fathoms. 

Intermingled with the minute plants and feeding on them are 
myriads of small animals, of which the larger number belong to 
the group of Crustacea, the group to which the shrimps and prawns, 
and the crabs and lobsters, belong. Jelly-fishes, transparent 
worms, young molluscs, as well as the glass-clear eggs and the 
minute larvae and young stages of fishes, form part of this great 
assemblage of microscopic floating life, which, animals and plants 
together, is known by the general name of " plankton." A 
curious and interesting feature is that the animals of the plankton 
undergo daily vertical migrations, being found nearer the bottom 
of the sea during the daylight hours and ascending towards the 
surface waters during the night. Thus during the day the plants 
hold the field in the surface waters, and are busy manufacturing 
food substance under the influence of sunlight. During the night 
the plankton animals ascend from the bottom layers to utilise 
the food which has been produced. 

The plankton animals become the food of two other classes 
of marine animals, which eat them with avidity. These are (1) 
the freely swimming or pelagic fishes, such as the herring, the 
mackerel and the pilchard, and (2) the bottom-living animals 
(benthos), which include many kinds of small crustaceans, creeping 
and burrowing shell-fish or molluscs, marine worms, and many 
fixed animals, growing on stones and shells, such as corals, sea- 
anemones and their allies. 

The bottom-living animals, especially the Crustacea, molluscs 
and worms, to which reference has just been made, are the principal 
food of those fishes for the capture of which our great trawl 
fisheries are organised : fishes such as the plaice, the sole, the had- 
dock, the cod and the ray, and their interest and importance is 
therefore great. Their habits are very varied and they possess 
many elaborate mechanisms and devices for the capture of their 
minute floating food. Quantitative estimates have shown 
clearly that the abundance of some of these bottom-living animals 
on a particular fishing bank may vary greatly from year to year, 
and such variations without doubt exert a considerable influence 
on the abundance of fish in the locality. 

We have already seen that the pelagic fishes, such as herring, 
mackerel and pilchard, feed directly on the animals of the plankton: 
Another large class of fishes are found, which support themselves 
by feeding on these pelagic fishes, as well as upon those fishes 


which feed on the bottom-living animals. In this class are the 
hake, the tunny, the tnrbot, the dogfishes, and the sharks. Seals 
and some whales also consume large quantities of pelagic fish. 
It will be seen, however, that with these animals, also, the food- 
chain leads ILS back ultimately to the minute plants building up 
their food-substance under the influence of sunlight in the surface 

In describing the distribution of marine animal and plant life, 
it is usual to distinguish a number of zones or regions each with its 
particular group of species. We have first the tidal zone, where 
the conditions of life are severe and exacting. Only such 
organisms can survive here as are specially adapted to withstand 
the battering of the waves, and the withdrawal of the water when 
the tide recedes, with the consequent exposure to great extremes 
of temperature. 

Beyond this tidal zone is the zone of the brown and red sea- 
weeds, where wave action is still an important factor and the 
amount of light reaching the bottom is sufficient for vigorous 
plant growth. These sea- weeds afford food and shelter for a large 
number of animals specially adapted to live under the conditions 
existing in the zone. 

In still deeper water, extending from about 15 fathoms to 
100 fathoms, is a third region, which comprises the greater part of 
the " continental shelf," the broad area which borders the con- 
tinental land masses and over which the sea is comparatively 
shallow. The seaward edge of this continental shelf is abrupt 
and depths increase rapidly from 100 to 1,000 fathoms, which is 
already the region of the deep sea. 

It is the large region of the continental shelf, between 15 and 
100 fathoms, that supports life in most abundance and variety, 
and it is here that the great commercial fisheries are carried tm. 
The conditions of life remain fairly uniform. The action of the 
waves, however great at the surface, has but slight effect on the 
bottom ; the movement of the water due to tidal and other 
currents, though persistent, is not violent, and changes of tem- 
perature are limited in extent and take place slowly. The water is 
well provided with the food substances necessary for plant growth, 
these substances being brought into it by the rivers and other 
drainage water from the land, some of them also, especially the 
phosphates, being obtained by solution from the sea-floor. The 
two factors influencing growth which are most subject to change 
from season to season and from year to year are (1) the amount 
of light which enters the water, and (2) the movements of the 
water masses due to the set of the great ocean currents. 

It is well known that, notwithstanding the comparative 
uniformity of the physical conditions in this region, its living 
organisms, and particularly the fishes, are subject to extensive 
fluctuations, some years giving an abundant yield, whilst in 


other years the harvest is poor. Much recent research has been 
directed to an examination of the extent and of the causes of the 
variations in the quantities of fish present on the fishing grounds, 
and it is becoming increasingly clear that a great deal depends 
upon the success or failure in any year of the eggs and young 
brood. A successful brood- year results in a good fishery four 
or five years later, and this success may persist for several further 
years as the fishes continue to grow. 

The age of any particular fish can now be determined with 
considerable certainty by an examination of the markings on 
the scales, which give a record of the fish's growth, or by an 
examination of the seasonal rings in the otoliths or ear-stones 
small calcareous bodies found in connexion with the ear. It is 
therefore possible in the case of a shoal of fish to discover in what 
year or years the individual fishes were born. In this way we can 
follow the fishes of a successful brood- year for quite a number of 
years and determine the extent to which they influence the 
fishery. One of tho most striking suggestions which has been 
made to account for the great differences in the survival of the 
brood in different years is that, in a successful year, the spring 
crop of minute planktonic food on which the young fish larvae 
feed has been produced and is present in the water at the time 
when the mouths of the little fishes open and they are ready to 
begin to feed. A year would be unsuccessful if the production of 
the small food organisms were delayed beyond the time when the 
fishes were ready to feed. Other factors which would influence 
the success or failure of the brood are the set of the current, which 
might drift the eggs and larvae into unsuitable situations, and the 
presence in exceptional abundance of such enemies of the eggs 
and larvae as devour them. 

In the last of the regions we are considering the deep sea 
the conditions under which the bottom-living organisms exist 
are remarkably uniform and in many ways not too favourable for 
animal life. Yet in the greatest depths which have been sounded, 
depths as great as 5,000 fathoms (between 5 and 6 miles), living 
animals are still found. The temperature at these depths 
approaches the freezing-point of water and is practically constant. 
The pressure is very high, but, being the same within and without 
the bodies of the animals, produces no injurious effects. Any 
movement of the water will be at the most an. exceedingly slow 
current creeping along the sea-floor. No sunlight can penetrate 
to such depths, and the only light is that produced by the phos- 
phorescent organs of the animals themselves. The food on which 
these deep-sea creatures live must be derived from the bodies of 
animals living in the layers above them, and through a chain or 
succession of food organisms must depend ultimately, as in other 
regions of the sea, upon the microscopic plant life which flourishes 
in the simlit surface layers. 



There are many structures in the human body not specially 
adapted for present use, which can only be satisfactorily explained 
by supposing that man is descended from animals which once lived 
in trees. The structures are so numerous and so striking that a 
few years ago Prof. F. Wood Jones wrote a whole book about them 
entitled " Arboreal Man.' 7 It has indeed been said that, if there 
had been no trees in the world, there could never have been man 
in his present form. 

If this inference be correct, we naturally look to the apes and 
monkeys of the present day as affording the best idea of the ances- 
tors which we suppose to have existed. We thus, on strictly 
scientific grounds, recognise a relationship between man and apes, 
which has long been fancifully imagined by speculators who have 
merely been familiar with their external appearance. Science, 
however, would not admit that any of the existing apes are the 
unaltered descendants of those which, ages ago, gave rise to man. 
Just as man has gradually become a perfect biped, adapted for 
an easy upright gait when walking on the ground, so the apes have 
acquired an increasingly effective adaptation for swinging about 
in trees. Just as man has lost the power of his jaws in proportion 
as his hands have become more mobile, so the apes have acquired 
more powerful jaws and teeth both to increase the efficiency of 
their feeding and to improve their means of offence and defence. 
Science, indeed, points to a remote common ancestor of man and 
apes which might, by changes in two divergent directions, become 
either one or the other. This ancestor, of course, would be 
popularly described as an ape if it happened to be still living, 
but it would be very different from any modern ape. It would be 
less forbidding in aspect more lifco the comparatively fascinating 
baby of the modern ape. 


Unfortunately, of animals which formerly lived in the world, 
we scarcely ever find more than the hard parts. Of ancestral apes 
and man we can only expect to discover the bones and teeth. 
The nature of the soft parts, therefore, can only be inferred from 
the shape and markings of the bones. In fact, in searching for 
ancestors, the skeleton alone concerns us. 


Very little is actually known about the ancestral apes. The 
oldest remains are some comparatively small lower jaws, with 
feeble canine (or corner) teeth, from Egypt. The next in 
antiquity are jaws and teeth of gibbons and of apes as large 
as a chimpanzee from central Europe, France, and Spain. Of the 
skeleton of these only one thigh-bone has been discovered. Jaws 
and teeth of more numerous species of apes occur in India. An 
imperfect skull of a very young individual with milk tooth has 
also boon found in a limestone of unknown ago at Taungs in 
Bechuanalaiid, South Africa. The bony face and teeth of this 
specimen, however, liffor in no essential respects from those of 
a young modern ape, and the cast of its brain-cavity is too 
much crushed for satisfactory comparison. 


The skeleton in both apes and man happens to be very charac- 
teristic and it is easy to distinguish the former from the latter. 
First, in all the apes the brain-case is comparatively small, and 
the face very large, often prominent ; in modern man the brain- 
case is large and beautifully domed, while the face is comparatively 
small. Secondly, most of the apes have relatively large and 
prominent bony brow-ridges when they are full-grown ; modern 
man lacks such brow-ridges. Thirdly, in all the apes the bony 
chin is receding, and the canine (or corner) teeth are relatively 
large arid interlocking, as in a dog or cat ; in modern man the 
bony chin is a little prominent at its lower edge, and the canine 
teeth are neither large nor interlocking they are in an even series 
with the rest of the teeth. Fourthly, in all the apes the backbone 
is nearly straight it is so even in the gibbons, which can run 
swiftly on their hind limbs ; in modern man the backbone has a 
beautiful S- shaped curvature to produce the elasticity which is 
needed for a comfortable upright gait. Fifthly, in all existing 
apes, at least, the arms are relatively much longer than in man, 
and the great toe is as well adapted for grasping as the thumb. 
Sixthly, in the apes, as a rule, the thigh-bone is somewhat arched, 
and the shin-bone comparatively short and stout, in adaptation 
to the crouching gait ; in upstanding modern man the thigh-bone 
is nearly straight. 


If the theory of man's origin in an ape-like ancestor is well 
founded, the older the human skeletons that we find buried in the 
earth, the more closely they should approach ape-skeletons in the 
distinctive features just enumerated. Among the fossils there 
should indeed be " missing links." The study of other fossil 
animals leads us to suppose that we shall not find a single graduated 
series of missing links, but a multitude of forms of approach of 
the human frame to the ape-condition. We must infer, in short, 
that existing modern man is the triumphant survivor of many 
tentative advances towards a being with an overgrown and 
elaborate brain which should dominate and increasingly control 
the rest of Nature. 


The great difficulty is, that very few remains of man's ancestors 
have been discovered which date back before the time when he 
had so far progressed as to acquire ideas of a future life and hence to 
bury his dead in security. Before that time, human remains 
could be preserved only when an individual happened to fall into 
a hole or into a river or lake where the body could be covered up 
with sand or gravel or mud. Hitherto, the remains of not more 
than three such accidents have been discovered, and even in these 
cases only fragments of the skeletons have been preserved, so that 
we have very little material for the investigation of the subject. 

The first of the discoveries of early man just mentioned was 
made by Prof. Eugene Dubois, in 1892, in an old river deposit in 
Java, which also contained the remains of extinct kinds of elephant 
and rhinoceros and other animals closely related to those still 
living in the East Indies. The principal fragment recovered is 
the top of a skull as large as that of a small man, with immense 
ape-like bony brow-ridges instead of the usual human forehead, 
but with impressions of a brain which is said to have been 
essentially human. Two associated teeth are riot completely 
human, but in some ways resemble those of the little gibbon which 
still lives in the forests of Java. A thigh-bone, which is rather 
disappointing as being affected by disease at the upper end, is 
as straight as that of a man or a gibbon, and implies the possibility 
of an upright gait. If all these remains belong to one individual, 
as seems most probable, they represent either an ancestral man 
who approached the apes in his brow-ridges and teeth, or a 
gigantic gibbon which had an unusually enlarged brain. In 
any case, the being was well named Pithecanthropus the 6< ape 
man " by Prof. Dubois, who was justified in claiming it as one 
of the " missing links." The specimens are now in the Teyler 
Museum at Haarlem, in Holland. 


The second discovery, which seems to date back to the time 
before man buried his dead, was made in a thick bed of sand 


deposited by a river at Mauer, near Heidelberg, in Germany, and 
was described by the late Prof. Otto Schoetensack in 1907. It 
consists solely of a lower jaw, which was found in association with 
the bones and teeth of elephant, rhinoceros, hippopotamus, and 
other animals which are known to have lived in Europe at the 
beginning of the Pleistocene period of geologists. The jaw is 
astonishingly large and massive, and, though essentially human, 
it differs from every known human jaw in the backward slope of 
the bony chin, which in this respect approaches that of the ape. At 
the same time it contains typically human teeth in even series, 
without any enlargement or prominence of the canines. The fossil 
thus seems to represent an extinct species of man, Homo heidel- 
benjcnsis, who still retained the retreating bony chin. It is now 
in the Geological Museum of the University of Heidelberg. 


The third very early accident to a primitive human being was 
revealed in an old river-gravel at Piltdown, Sussex, by the late 
Mr. Charles Dawson in 1912. This gravel occurs in the Wealden 
country about midway between the southern chalk downs and 
the sandstone ridge on which ( -rowborough is situated. It con- 
tains many water worn flints which were derived from the chalk, 
and as no river in that part of Sussex could now bring them to the 
Piltdown district, it evidently dates back to a time when the local 
topography was entirely different from that of the present. 
It also contains fragments of elephant, hippopotamus, and other 
animals, which show that it dates back at least to the beginning 
of the Pleistocene period. 

The only fragments of a human skeleton hitherto discovered 
are the greater part of a skull and nearly half of a lower jaw with 
two molars and a canine tooth. The skull agrees with that in 
some existing low races of men in being remarkably thick, but it is 
unique in having a very fine spongy texture, which would make it 
highly resistant to blows. It is as destitute of bony brow-ridges 
as the skull of modern man, with a good forehead, but the crown 
of the head is less domed than usual, and the hinder or occipital 
part remarkably low and broad. The brain must have been 
essentially human, and it is distinctly larger than the smallest 
human brain of the present day ; but it exhibits some pecuilarities 
that are more suggestive of the ape pattern than any other human 
brain hitherto studied. The whole skull, indeed, is curious, and 
must have belonged to a human creature very different from 
modern man. The lower jaw is comparatively weak, but it is so 
much elongated that it implies a relatively large face. Its re- 
treating bony chin is shaped almost exactly like that of an ape ; 
it is much more ape-like than that of Homo heidelbergensis. The 
molar teeth, though essentially human, are unusually large and 


elongated ; and the much-enlarged canine tooth is so worn as to 
show that it completely interlocked with the upper canine, as in 
an ape. This canine tooth, however, differs in shape from that of 
any known ape, and agrees best with the temporary (or milk) 
canine of modern man. It is certainly a tooth of the permanent 
series, and therefore represents the first or preliminary stage in 
the making of a typical human dentition. 

Piltdown man indeed belongs to the real dawn of the human 
race, and has been appropriately named Eoanthropus, or kt dawn 
man." The original specimens, with fragments of a second skull 
and molar tooth discovered by Mr. Dawson in another locality 
near Piltdown, are now in the Geological Department of the British 
Museum (Natural History). 


The earliest form of man in Europe who was intentionally 
buried in security after death, is now known by several more or 
less nearly complete skeletons from the caves and rock shelters of 
France and Belgium. The first skeleton, of which only the top 
of the skull and a few other fragments were rescued from destruc- 
tion, was found in the Neanderthal (or valley of the Neander) 
near Diisseldorf in Germany. The best French skeletons were 
found with flint implements of the peculiar pattern which is met 
with in the cave of Le Moustier in the Dordogne. The race repre- 
sented is therefore commonly known as that of Neanderthal or 
Mousterian man. 

The finest skeleton of Neanderthal man, which was described 
by Prof. Marcellin Boule and is now in the National Museum of 
Natural History at Paris, was found in 1908 in a small cave near 
La Chapelle-aux-Saints in the Correze in south-west France. 
The circumstances showed that it had been intentionally buried, 
while the associated flint implements and remains of woolly 
rhinoceros, reindeer, hyaena, and other animals proved its geological 
age. A leg of a bison, which must have been covered with flesh 
when it was buried, seems to have been placed there as food for 
the deceased in a future life. The skull is relatively the largest 
ever seen in healthy man, and the brain-case, which is curiously 
depressed and expanded behind, is larger than that of the average 
modern European. The brain, however, may have been inferior 
in quality. There are strongly inflated bony brow-ridges, as in 
an ape ; and the face also slightly approaches that of an ape in 
being relatively large and in having no depression in the bony 
cheek beneath the eye. The mouth is also very large, but the 
teeth are in all respects typically human, and the bony chin only 
differs from that of modern man in being sharply truncated, not 
prominent near the lower edge* The backbone is remarkably 
stout, about 2 inches shorter than usual, and the man must have 


been of short stature. The arm is relatively long, and the two 
bones of the forearm are much arched, thus again retaining 
marked traces of an ape ancestry. The thigh bone is stouter and 
more bent than in ordinary man, and the shin bone is comparatively 
short and stout. While essentially human, therefore, Neanderthal 
man had probably a slouching rather than an upright gait, and his 
heavy face would give him a bestial aspect. 


We may, then, pause to remark that the earliest known fossil 
remains of man approach the hypothetical ape ancestor in at least 
two distinct ways. In one case there are no bony brow-ridges, but 
an ape-like jaw ; in the other case, there are great bony brow- 
ridges, but a typically human jaw. So far as the scanty evidence 
goes, it fulfils our expectation of finding more than one kind of 
46 missing link." 

In Western Europe there is still no indication of typically 
modern man having lived with the immature grades of humanity 
just described. All statements to the contrary are based on 
modern burials which have been wrongly interpreted. In the 
metropolis of early man in central France, however, typically 
human skeletons are found in deposits in the rock shelters and 
caves which are immediately above those containing the remains 
or handiwork of Neanderthal man. As no skeletons of an inter- 
mediate race have been discovered, it may therefore be inferred 
that modern man originated elsewhere and appeared as an 
immigrant in this part of the world. Indeed all our present 
knowledge suggests that the successive phases of dawning humanity 
were passed through somewhere in the East, probably in South 
Central Asia. In that case, the periodical westward migration of 
peoples which is so familiar a feature of historic times must have 
begun in remote prehistoric antiquity. 

One reason for suspecting that South Central Asia may have 
been the original home of man is that, just before his beginnings, 
a very varied assemblage of great apes lived in the forests of 
northern India. They are unfortunately known only from a few 
scattered teeth and fragments of jaws found in the deposits of 
Miocene age which now form the Siwalik Hills, so that we have 
very little information about them ; but no such series of great 
apes has hitherto been discovered elsewhere. Now, at the 
beginning of the Miocene period, the Himalayan Mountains did not 
exist, and (as the late Joseph Barrell first suggested) it may have 
been during the uplift of this mountain, range at the end of the 
period that primitive man came into being. As the land rose, the 
temperature would be lowered, and some of the apes which had 
previously lived in the warm foit?st would be trapped to the north 
of the raised area. As 'comparatively dry plains would there take 


the place of forests, and as the apes could no longer migrate 
southwards, those that survived must have become adapted for 
living on the ground, and acquired carnivorous instead of frugi- 
vorous habits. By continued development of the brain and 
increase in bodily size, such ground apes would tend to become 


It has long been generally recognised that the lowest races of 
men in the present-day world are the blacks who inhabit Australia 
and those who, until lately, survived in Tasmania. They have 
often been regarded as closely related to the Neanderthal man who 
disappeared so long ago from Europe ; but the discoveries of 
skeletons in France, already mentioned, show that the two races 
are entirely different. The remote lands of the southern hemis- 
sphere have always been the refuges in which old types of life have 
survived long after they became out-of-date and displaced in the 
more progressive northern hemisphere. There is, however, still 
no evidence of the Neanderthal or any earlier race in the south, 
and the Australians and Tasmanians are probably the survivors 
of the true men of later Pleistocene times. Their immediate 
ancestors seem to have had a much wider range in the southern 
hemisphere at a recent period, for an Australoid skull is known 
from a rock-shelter at Wadjak in Java, and another skull, asso- 
ciated with parts of the skeleton, which seems to have similar 
relationships, was found in 1921 buried in a cave in Northern 
Rhodesia. The Rhodesian skull, however, is unique in having 
the most inflated bony brow r -ridges and the largest face ever seen 
in man. At first sight, these features seem more ape-like even 
than the corresponding parts of the European Neanderthal man ; 
but more careful examination shows that the face is not enlarged 
on the ape model the enlargement is not in the middle of the 
face, as in the ape, but round the edge- and the only known 
specimen which approaches the Rhodesian in the depth and extent 
of the bone below the nostril is a fossil Australian skull from 
Talgai in Queensland. In the characters of his brows and face, 
therefore, Rhodesian man probably exhibits merely a modern 
reversion to an ancient human type. 

It may be added that man does not appear to have reached the 
American continent until much more recent times, for none of the 
fossil remains hitherto found in that region of the world differ 
essentially from the corresponding parts of the skeleton of the 
existing American Indians. 

For further details, with illustrations and references to litera- 
ture,* see the English translation of M. Boule's " Les Hommes 
Fossiles " (1923) ; W. J. Sollas, " Ancient Hunters," third edition 
(1924) ; A. Keith, " The Antiquity of Man," second edition 
(1925) ; also " A Guide to the Fossil Remains of Man," published 
by the Trustees of the British Museum. 



The human brain is the instrument of the high powers of 
intelligence that distinguish man from all other living creatures. 
In animals endowed with the power of voluntary movement, 
which necessarily involves the ability to choose between cqn- 
flicting impulses, the fundamental condition of progress is the 
attainment of quickness of appropriate response. The evolution 
of the nervous system is the means employed to enable increasingly, 
complex and more completely adapted muscular actions to be 
performed with promptitude and precision. 

Mammals differ from all other living creatures in having a true 
neopallium, a development of a region of the cortex of the brain 
in which sensory impulses from all parts of the body meet, react 
on each other, and directly influence the mechanism that initiates 
movements ; this is an instrument of almost unlimited poten- 
tialities for the cultivation of skilled movements of increasing 
degrees of complexity and adaptation to diverse circumstances. 
Tn man these potentialities achieve their highest expression. 

The human cerebral cortex provides the vital mechanism that 
can be fashioned by education to initiate and control an almost 
endless variety and complexity of muscular actions. It is able 
to perform these functions in virtue of the fulmess of the informa- 
tion it obtains from a variety of sense-organs and the efficiency 
of the amazing machinery in the central nervous system for in- 
tegrating the effects of these afferent currents and for controlling 
increasingly complex combinations of groups of muscles. But 
even more important is the ability of the neopallium, by some 
means which is quite unknown, to record the results of past 
experience and to put the influence of such knowledge at the 
service of the muscular system. This not only provides the 
means whereby behaviour can be modified in the light of know- 
ledge, but also enables a^high degree of automatism to be acquired 


by training, which is perhaps the most essential factor in the 
attainment of high degrees of skill. The acquisition of these 
extensive powers plays a fundamental part in the development 
of the physiological dispositions which are expressed in intellectual 
operations. In the evolution of man the attainment of in- 
creasingly skilled movement involved the growth of mind. 

Upon the lateral aspect of the cerebral hemisphere in most of 
the apes there is a furrow which was supposed to be so peculiarly 
distinctive of these Primates that it was labelled the AffenspaUe 
or ape-fissure. More than twenty years ago its presence was 
demonstrated in the human brain, and as its own name was clearly 
inappropriate, the new designation, sulcutt lunalus, in reference 
to the semi-lunar form it usually assumes, was given to it. The 
identification of this furrow was followed by the measurement of 
the extent of the area striata, the cortical area responsible for its 
presence, in which the optic radiations end ; this led to the 
discovery that the visual receptive territory is just as extensive in 
the brains of many monkeys, even small macaques, as it is in those 
of men. The investigation showed the important part played 
by the early cultivation of vision as the dominant sense in man's 
ancestors, and pointed to the necessity for a detailed study of 
how and why this particular trend in evolution should have led 
to results of such vast significance as the emergence of the human 

Man has emerged as the result of the continuous exploitation, 
throughout the Tertiary period, of the vast possibilities which 
the reliance upon vision as the guiding sense created for a mammal 
that had not lost the plasticity of its hands by too early specialisa- 
tion. Under the guidance of vision the hands were able to acquire 
skill in action, and incidentally to become the instruments of an 
increasingly sensitive tactile discrimination, which again reacted 
upon the motor mechanisms and made possible the attainment of 
yet higher degrees of muscular skill. But this in turn reacted 
upon the control of ocular movements and prepared the way for 
the acquisition of stereoscopic vision and a fuller understanding 
of the world and the nature of the things and activities in it. 
For the cultivation of manual dexterity was effected by means of 
the development of certain cortical mechanisms ; and the facility 
in the performance of skilled movements once acquired was not a 
monopoly of the hands, but was at the service of all muscles. 
Skilful use of the hands was impossible without the appropriate 
posturing of the whole body. High co-ordination of hand move- 
ments arid high co-ordination of movements localised elsewhere 
in the body must go together. The sudden extension of the range 
of conjugate movements of the eyes and the attainment of more 
precise and effective convergence were results that accrued from 
this fuller cultivation of muscular skill. They were brought about 
as the result of the expansion of the prefrontal cortex, which 


provided the controlling instrument, and also by the building up 
in the midbrain of the mechanism for automatically regulating 
the complex co-ordinations necessary to move the two eyes in 
association in any direction. 

The attainment of stereoscopic vision enormously enhanced 
the value of the information acquired by the eyes. The develop- 
ment of maculce lutew made possible the fuller appreciation of 
the details, tho texture and the colour, of objects seen ; and in 
association with the increased precision of muscular control, 
enabled the eyes to follow the outlines of objects and appreciate 
better their exact size, shape and position in space. But this 
completer vision of objects in the outside world stimulated a 
curiosity to examine and handle them, and so led to a yet further 
cultivation of skill in movement and an enhancement of tactile 
discrimination. This higher skill was attainable because the 
powers of stereoscopic vision conferred more accurate control on 
the hands than was possible before it was at their service. 

Thus the fuller cultivation of the results of the visual powers 
provides a new stimulus and new means for enhancing vision 
itself, and this cycle of developmental changes was repeated again 
and again in the history of the Primates, at each stage leading 
to a further enhancement of muscular skill and visual acuity. 

Et is of fundamental importance to remember that one result 
of this continued handling of objects is the attainment of a fuller 
understanding of the nature of the objects seen and of the forces 
that are operating. The closer correlation of the information 
gained by vision and touch played a leading part in the cultivation 
of an appreciation of form, which represents the germ of the 
aesthetic sense. There also emerged the aptitudes to estimate 
weight and to discriminate between textures. 

When these had attained such a degree of exactitude that it 
became possible for the individual to distinguish sharply one 
object from another and to appreciate its physical properties 
and understand something of its significance, the time had arrived 
when the process of naming it acquired a definite biological value. 
Man's ancestors were already provided with the muscular instru- 
ments for speech and the ability to use them for the emission 
of a variety of signals, mainly in the nature of cries to express 
emotional states. Hence, long before the need made itself felt 
for an instrument to express the names of objects, it was already 
in being ; and all that required to be done was to devise the 
necessary vocal symbolism to express the visual experience to 
give a name to an object seen. Moreover, long before the dis- 
covery of articulate speech, the ancestors of modern man were 
conveying information of an intellectual kind one to another 
through the visual appreciation of the meaning of gestures and 
facial expressions. With the introduction of an auditory sym- 
bolism, man became able to convey this information in a manner 
more precise and more capable of intellectual elaboration. 

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Thus the acquisition of speech was based primarily upon the 
fuller understanding of the world around the ancestors of men 
and the need for names as a sort of shorthand concisely to express 
the various attributes of a single object and other more complex 
states of consciousness ; but it involved the seeing eye and the 
understanding ear and the highly skilled muscular act involved 
in phonation and articulation. In other words, while the ex- 
pansion of most cortical areas is essential for the interpretation 
of experience, the special development of territories in the neigh- 
bourhood of the areas concerned with the reception of acoustic 
and visual impulses, and with the control of the musculature of 
the head and neck, should be expected. 

If the brain of man's nearest relative, the gorilla, be compared 
with the human brain, it will bo found that the enormous increase 
in the cortical territories of the latter affects chiefly three areas, 
the parietal region (especially that part of it known as the supra- 
marginal and angular convolutions), the prefrontal region, and the 
inferior part of the temporal area. These areas are concerned 
respectively with the comprehension of speech, with muscular 
skill, and with speech, and are the areas that reach their full 
development last in the human child. They were the most 
defective parts of the brains the forms and proportions of which 
can be inferred from the moulds of the brain- cases of Pithe- 
canthropus and Eoanthropus, 

Appreciation of the nature of the objects and events happening 
in the outside world are dependent upon certain cortical develop- 
ments which did not occur until man's immediate ancestors 
were assuming human qualities. The attainment of the realisa- 
tion of space and time, and the faculty of recognising objects by 
their shape, colour, size, and texture, marked the transformation 
of the ape into a man. For the ability to appreciate these things 
made it useful for him to devise names for things, and so initiated 
the development and use of language, with all that language 
implies in vastly increased capacity for thinking in symbols of 
value to himself and intelligible to others. 

When man began to examine the objects around him, he 
did not neglect the study of himself. The knowledge he accu- 
mulated of the world included a knowledge of his own body and 
the estimation of the aesthetic qualities of his fellows, for vision 
came to acquire an increasing influence in his selection of sexual 
mates ; and it is possible that in the human family, Darwin's 
claim for sexual selection may find much ampler confirmation 
than most biologists are inclined to attach to it for other organisms. 
No one can question the appeal of physical beauty to mankind, 
and it is difficult to believe that an attraction so universal and 
deep-seated could possibly have been devoid of effect in the process 
of transmuting the uncouth form of an ape into the graceful 
figure of a human being. 


By Prof. E. H. STARLING, F.R.S. 

Before Harvey's time, anatomists, by dissection of the bodies 
of man and animals, had shown that the heart in the c-hest is 
connected by tubes to all parts of the body, and they had described 
the structure of the heart. Although it was known that these 
tubes and the heart contained blood, which in the living body was 
in motion, no clear idea was held as to the function of the heart 
until the demonstration by William Harvey in 1616 that the blood 
was in continual circulation throughout the body, the circulation 
being maintained by contractions of the muscular wall of the heart. 

The heart is a hollow organ which presents four cavities two 
thin- walled, the auricles, and two thick- walled, the ventricles. 
There is no communication in the heart itself between the right 
and left sides. The orifices between the auricles and ventricles 
are provided with valves which allow the blood to pass only from 
auricles to ventricles. From the ventricles also lead off two large 
tubes, the pulmonary artery from the right ventricle, and the 
aorta from the left ventricle. The orifices of these two tubes 
(arteries) are provided with very perfect valves, which prevent 
the regurgitation of blood from the arteries into the heart, but 
present 110 resistance to the flow of blood from the heart into the 
arteries. The heart can be considered as a hollow muscle. When 
the muscle contracts all the cavities of the heart become smaller, 
so that the contents of the ventricles are forced through the valves 
into the arteries. 

The circulation is really a double one, and could be maintained 
if the right and left sides of the heart were separated into two 
distinct organs. From the left ventricle the blood passes through 
the aorta and then by large arteries to all the tissues of the body, 
where it flows through fine hair-like vessels, the capillaries, which 
permit the passage of material through their walls and so allow 
the tissues of the body to take up oxygen and food from the blood, 
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The capillaries of the tissues lead into thiii-walied tubes, the veins ; 
and these gradually run together until they form two large veins 
called the superior and the inferior venae cavae, which enter the right 
auricle. The blood going to the tissues is bright red in colour 
(arterial blood) ; that coining from the tissues has lost a large part 
of its oxygen and is bluish in colour (venous blood). From the 
right auricle the blood passes into the right ventricle, and when 
this contracts is forced into the pulmonary artery and through 
the capillaries of the lungs. These capillary vessels form a fine- 
meshed network, which surrounds all the air vesicles, and are 
separated from the air in these vesicles only by a microscopic 
layer of colls. The blood therefore loses the carbonic acid collected 
in the tissues, which is the result of tissue activity, and takes up 
oxygon from the air, so that it leaves the vesicles bright red in 
colour, being once more arterial blood. From the lung capillaries 
the blood is collected into four thin-walled tubes, the pulmonary 
veins, which enter the left auricle, and passes from the left auricle 
into the left ventricle, to recommence its circuit through the body 
when the ventricle contracts. 

In order that a town may be supplied with water for its 
domestic purposes, it is necessary to maintain a head of pressure 
in the water mains. In the same way, it is necessary to maintain 
a pressure in the large arteries, in order that blood may be supplied 
to the different tissues according to their needs. The arterial 
blood pressure was first mentioned by the Rev. Stephen Hales, 
who was also the first to point out its importance. In man, we 
find that in the large arteries the blood pressure varies at each 
heart-beat between 80 and 120 millimetres of mercury i.e., we 
have a head of pressure of about one-seventh of an atmosphere. 

In order to maintain this pressure there must be a resistance to 
the free outflow of blood from the great arteries, and to this end 
the muscular walls of the arterioles are kept by the central nervous 
system in a constant state of partial contraction, so that they only 
allow the escape of blood with difficulty. The arterial blood 
pressure, on which the circulation to the tissues depends, is there- 
fore determined by two factors (1) the amount of blood pumped 
out into the arteries, and (2) the resistance to the escape of blood 
afforded by the contraction of the small arterioles. With this 
head of pressure in the arteries, the central nervous system, the 
master tissue of the body, can always receive a sufficient supply of 
blood with its contained oxygen. If by any means there is a 
diminution of the arterial pressure, the central nervous system at 
once takes measures to raise the pressure again by constricting 
the arterioles of all other parts of the body, so as to maintain the 
blood pressure and therewith the circulation through the brain. 

The central nervous system controls not only the condition of 
the arterioles, but also the rate, and force of contraction of the 
muscle which forms the heart pump. Thus the heart may be 


quickened or slowed as the result of emotions. In the same way, 
the blood vessels may be constricted or dilated the pallor or 
flushing produced by different emotions is familiar to everyone. 
The brain may also affect the circulation indirectly. One of the 
main factors in increasing the return of blood to the heart is the 
contraction of the voluntary muscles, which press on the blood in 
the veins and send it on towards the heart. In this way, by 
increasing the inflow into the heart, muscular exercise increases 
the output from the heart, and therefore tends to cause a rise of 
blood pressure ; the increased inflow of blood may also reflexly 
affect the heart centres and cause a quickening of the pulse. 

It is important to note that the rhythmic contraction of the 
heart, which continues from the formation of the heart in the 
developing child until the death of the individual, is dependent 
on the heart itself. The heart of a cold-blooded animal cut out 
of the body will beat for hours or even days. The hoart of the 
warm-blooded animal, if supplied with oxygenated blood, can be 
made to beat for many hours after being cut out of the body. 


By Prof. A. V. HILL, F.U.S. 

Bodily movement is an apparently simple phenomenon and 
its characteristics can be measured in absolute physical units. 
For example, the work done by a contracting muscle can be 
measured in ergs and as accurately as the work done by a steam 
engine. The mystery of how the muscle fibre performs its 
important and easily recognised function has long appealed to those 
who desired to study living response in a form approachable by 
the methods of exact science. Moreover, there was the hope that 
once a beginning had been made, the results and generalisation 
attained by studying muscle might be found to apply to all other 
forms of living tissue, and so a way be found of bridging the gap 
between biology and physics. 

The subject is full of great names, Helmholtz, Fick, Blix, and 
many others. Of British workers, Gaskell's and Mines' researches 
on the isolated heart have proved the fundamental basis of modern 
cardiology. W. M. Fletcher perceived the extreme importance 
of the fact that oxygen delays the onset of, or abolishes, fatigue in 
an isolated muscle ; W. M. Fletcher and F. G. Hopkins, working 
together, recognised that lactic acid was the key to this phenomenon 
a.nd that the concentration of it in a muscle is high in states such 
as fatigue and rigor mortis. From these original sources has arisen 
a network of investigations, illuminating many branches of 
physiology and throwing sidelights on several aspects of medicine 
and everyday life. 

When a muscle contracts, lactic acid is liberated, in amount 
proportional to the strength of the contraction : when it relaxes 
lactic acid is neutralised : while the contraction is upheld, a 
balance is maintained between continual production and continual 
neutralisation. During the succeeding ten minutes, restoration 
occurs : the lactic acid is slowly removed in the " recovery 


mechanism " of the muscle, but only if oxygen be present ; 
it is restored to its previous state, the necessary energy being 
provided by the oxidation of a fraction of the lactic acid. The 
system is analogous to an electrical accumulator, together with 
a motor and a dynamo ; the accumulator can rapidly provide 
mechanical work when needed, but must be slowly recharged 
afterwards by the use of energy required to drive the dynamo. 
Oxygen in muscle is used only in recovery from previous exertion, 
even during exercise, each element of the oxygen consumption 
is used in recovery from a previous element of effort. 

The lactic acid arises from and is restored to glycogen, a 
body peculiarly important in connexion with modern work on 
carbohydrate metabolism, insulin and diabetes ; indeed, the 
fact that, in muscle, glycogen breaks clown into lactic acid more 
readily than does glucose, suggests that a study of the chemical 
nature of glycogen, and its reactions in the muscle mechanism, 
may find some strange and interesting application to the problems 
of human diabetes. 

The onset of fatigue is due to the accumulation of lactic acid 
in the muscle : the limits of violent effort are set by the maximum 
amount of acid which the body can tolerate. During exercise, 
and in the earlier stages of recovery, the acid passes into the blood ; 
part of its oxidative removal may even occur in distant portions 
of the body. The laboured respiration accompanying, and 
following, active exertion is due to acid in the bloodstream, 
affecting the respiratory centre in the brain. The laboured 
respiration occurring even after moderate exercise, in some forms 
of cardiac or other disease, is due to acid appearing in the blood 
as the result of an imperfect mechanism for its oxidative removal. 
The changes occurring in the blood, as the result of exercise or of 
oxygen want, in respect of its combinations with oxygen and 
carbon dioxide, are partly due to lactic acid. 

It would seem a far cry from the obscure labours of physiologists 
to the making of records on the athletic track. Yet a study of the 
oxygen intake and the carbon dioxide output in man, during and 
after violent exertion, together with the results of recent work 
on the dynamics, thermodynamics and chemistry of isolated 
muscle, have shown otherwise. A man's capacity for muscular 
effort is limited by precise and clearly defined factors, depending 
upon his supply and utilisation of oxygen, his economy in move- 
ment, his efficiency in recovery, and the maximum amount of 
lactic acid to which his body will submit. The general type 
of relation existing between the distance (in a flat race) and the 
speed at which it can be run, can be predicted on simple physio- 
logical grounds. It has been the writer's good fortune, though 
himself an inconsiderable performer, to have had for many years 
a close personal acquaintance with athletes, and it has been recently 
almost a daily pleasure and excitement to find some phenomenon. 


known to runners, turning up again in another form in the physio- 
logical laboratory. Mountaineers, airmen, students of human 
movement in industry and everyday life, will find the same. 

We must recall, however, that it was the pure science which 
found the path and built the bridge, and we are really only at the 
beginning of our knowledge of muscle. The adventurer's instinct 
is still needed : it is necessary to explore as well as to exploit. 

The recovery process, capable of completion only in the 
presence of oxygen, still goes on in part, even in its absence : the 
details of the process are unknown. The course and magnitude 
of the liberation of energy associated with all phases of con- 
traction and recovery have been described, and it remains for the 
chemist to fit his details into the thermodynamic picture. Again, 
there are many curious and complex effects connected with the 
actual shortening process itself : changes in energy liberated, 
changes in work done, changes in mechanical efficiency. There 
are the physico-chemical factors underlying the power a muscle 
possesses of using oxygen : there are the highly specific actions 
of certain drugs upon its mechanism. But behind them all 
remains the mystery the solution of which still so far away 
offers so fruitful a field of understanding men's bodies ; the 
mystery of the little fibre, about 5 ^ of an inch thick, designed 
and constructed in a material not unlike egg-white, growing, 
feeding, repairing itself, and exhibiting in its function so 
admirable a simplicity, an efficiency, and a directness of apparent 

By Prof. E. P. GATHCART, F.R.S. 

Although man is no machine in the ordinary sense of the term, 
he, just like the locomotive, must be supplied with fuel for the 
production of work. In the case of the locomotive, however, the 
fuel is only required for the performance of work, whereas in the 
case of man the fuel food not only supplies the necessary energy 
for the performance of work, but it also serves for the repair of the 
wear and tear of the tissues and for growth. Before a correct 
assessment can be made of the amount of food required, it is 
necessary to determine the amount of energy expended. 

This question of the best and most accurate method for the 
determination of the energy expended is no new one. The first 
experiments, which changed the whole outlook on the problem, 
were made almost at the same time, about 1780, by Crawford in 
Glasgow and Lavoisier in Paris. Crawford, indeed, claimed priority, 
but his insight into the problem was not so fundamental as that 
of his French rival. The method, then adopted is in essentials the 
one we use to-day. It consists in the estimation, either directly 


or indirectly, of the amount of heat lost. The thermal unit or 
calorie is chosen because eventually all the potential energy of the 
food consumed or the tissues oxidised is reduced to heat, and, 
further, the external work done can also be measured in heat 
units. Thus by the use of the calorie we obtain a common factor 
for the statement of energy problems. The calorie used to-day 
by physiologists throughout the world is the large or kilo calorie 
which represents the amount of heat required to raise the tem- 
perature of one kilogram of water through 1 degree Centigrade. 
The amount of external work done is calculated in kilogram- 
metres i.e., the amount of energy expended in raising one kilo- 
gram, vertically through one metre distance. Largely from the 
pioneer work of Joule, of Manchester, we know that approximately 
427 kilogram- metres equal one large calorie. 

The amount of energy, calculated as calories, contained in the 
food consumed is .determined by burning a weighed amount of the 
food material, under very definite conditions, in an apparatus called 
a bomb calorimeter, and measuring the amount of heat liberated. 
A great many of the earlier determinations of the calorie values 
of foods were made by Frankland, of Birmingham, in 1866. 

The estimation of the energy lost by the living organism or man 
may be carried out either directly or indirectly. In the direct 
method, which was the one originally adopted, the amount of heat 
given off by an animal was measured by noting the increase in 
temperature of the cold water or ice which surrounded the chamber 
in which the animal was placed. The modern development of 
this method has been for the most part confined to America, and 
is associated with the names of Atwater, Rosa, Benedict and 

The indirect method, which is much easier and less expensive, 
has been developed chiefly in Britain and in Germany. Although 
a somewhat similar method was adopted by Smith, in London, so 
long ago as 1859, Zuntz, of Berlin, was the first to devise a trust- 
worthy and portable apparatus, but it had the drawback of being 
heavy and cumbrous. The method introduced by J. S. Haldana 
and C. Gordon Douglas, of Oxford, is infinitely better, being both 
accurate and easy to use. In this indirect method, the changes 
in the composition of the inspired and expired air are measured. 
The subject, with his nose " clipped," breathes through a special 
mouthpiece (or a special facepiece) equipped with two one-way 
valves, an inspiratory and an expiratory. The mouthpiece on 
the expiratory side connects by means of rubber tubing with an 
airtight bag which serves to collect all the expired air for the period 
of the experiment. The amount of air expired is measured by 
passing it through a meter and a sample of this air is taken for 

The analysis shows, as compared with the composition of the 
normal inspired atmospheric air, an increase in the content of 


carbon dioxide and a diminution in the amount of oxygen. The 
alteration is due to the combustion of the various foodstuffs in the 
tissues. The amount of oxygen used is multiplied by a factor, 
which is obtained from the ratio of the amount of carbon dioxide 
breathed out to the amount of oxygen utilised, and the result is 
a statement of the energy expended in calories. The assumption 
is made that the amount of oxygen utilised is a direct index of the 
amount of material burnt in the tissues. That this assumption is 
correct is shown by the fact that from a series of double estima- 
tions made by the direct and indirect methods, the two varied 
in their final result by less than 1 per cent. 

As the apparatus can be utilised for the determination of the 
energy expenditure of mobile subjects, it is possible to examine 
and compare the energy expended by a great variety of workers. 
It is possible to assess and compare the cost of work, for example, 
of such very diverse occupations as postmen, riveters, tailors, 
clerks, etc. 

Tf, however, it is desired to study more particularly the various 
phases of energy expenditure, it is customary to make the subjects 
perform given amounts of work on special types of apparatus 
known as ergo meters (work- measuring machines). These machines 
are of different types ; some are used for the investigation of work 
performed by the arms either of rotary or lever movement, whereas 
others have been devised, such as the " walking " platform and the 
bicycle ergometer, for the study of the cost of movement of the leg 
muscles. As a general rule, the work done consists in rotating a 
wheel against resistance which can be varied within wide limits. 
Under these conditions, it is possible to carry out very accurate 
determinations of the actual amount of work a man can perform 
in the course of an hour, or eight hours, or any other given unit 
of time. 

A number of investigations have been made of the energy 
expenditure in various occupations by this method of indirect 
calorimetry, but a great deal of work must yet be done before 
anything like final conclusions can be drawn regarding the average 
energy expenditure of any class of workers. In spite of the 
importance of this subject, which is in reality the basis for the 
study of the nutrition of man, Britain possesses no institute or 
laboratory specifically devoted to the study of the nutrition of 
man, although it has at least two institutes for the study of the 
nutrition of farm animals. 


By Prof. D. T. HARRIS. 

The distribution of solar energy over the British Empire shows 
immensely wide variations, and inhabitants of the large cities of 
Britain probably receive the smallest share. It, is only in recent 
years that the London child sufferer from tubercular joint disease 
has had the chance to enjoy sun baths. The pioneer work of Sir 
Henry (ran vain at Alton and Hayling Island has demonstrated 
conclusively the curative action of the sun's rays in bone and joint 
disease of tubercular origin. The wonderful results of Dr. Rollier 
in Ley sin, in the Swiss Alps, show on a more extensive scale the 
beneficial effects of insolation at high altitudes. To Dr. C. W. 
Saleeby is due the credit for bringing this powerful agent to the 
notice of the English-speaking public. It was through his untiring 
efforts that the Medical Research Council appointed a committee 
to investigate the biological action of light, under the chairman- 
ship of the late Sir William Bayliss, who was the first to write 
an authoritative account of this youthful and difficult subject 
(* k Principles of General Physiology." Longmans). 

The investigation of the mode of action of an agent like light, 
to which wo and our ancestors through the ages have grown so 
accustomed, presents unusual difficulties ; we are apt to accept it 
as an unanalysable fact. No one will question the existence of the 
stimulating effect of the morning sun, but to determine the tissue 
on which it Jicts and its mode of action, whether chemical or 
electrical, is a problem demanding the co-operation of physiology, 
chemistry and physics. 

The physicist continues to make his valuable contributions. 
The colours of the rainbow, which represent the visible part of the 
spectrum, are now known to be only a very short link in a huge 
electromagnetic spectrum connecting the immense waves of 


wireless telegraphy, on one hand, with the extremely small X-ray 
waves, on the other. All these waves travel at about the same 
speed, eight times round the earth in one second. On the red 
side of the visible spectrum we pass into the region of dark heat 
rays, including those emitted from a hot flat iron. These, though 
invisible, can be appreciated by the heat receptors in the skin 
of the cheek. Dark-heat, or infra-red rays, constitute about half 
the energy we receive from the sun. On the more active violet 
side of the visible spectrum the waves are only half the length of 
the red waves, and as we pass beyond the faintly visible violet 
we come to a chemically active region called the ultra-violet, 
where the waves average only half the length of the violet. These 
ultra-violet rays are proportionally few in ordinary daylight ; 
they are absorbed by window glass, and so cannot enter a house 
with closed windows. They are also reduced in intensity by 
absorption in the atmosphere, and hence are more abundant at 
high altitudes, as in the Alps. 

It has been supposed that man through the long ages has 
become immune to the visible rays of the sun. It is only when 
the infra-red rays become excessive that he seeks the shade, and 
in this way he also escapes from the destructive action of a too 
powerful dose of ultra-violet light. It is the infra- red which are the 
potent rays in the causation of sunstroke, whilst the ultra-violet 
cause sunburn. The latter may be easily demonstrated by ex- 
posing an area of skin for five minutes to the ultra-violet light 
obtained from an artificial source rich in ultra-violet rays e.g., 
the mercury vapour lamp in a quartz tube ; the other rays can be 
filtered off, the heat rays by a water cell and visible rays by a cobalt- 
quartz plate. 

This powerful action on the human skin is of great interest. 
After the sunburn subsides the majority of people develop pig- 
ment. If now the same region of skin be exposed to ultra-violet 
light, it will be found that a burn does not appear on the pig- 
mented skin, but only on the neighbouring unpigmented skin ; 
protection has therefore been conferred by the development of 
pigment. This experiment suggests the mode of evolution of 
the pigmented races of the tropics. How the pigment actually 
works, especially in view of the fact that a black body is a better 
absorber of heat than a white body, is a problem under investiga- 

Another effect of ultra-violet light and one which has been 
definitely proved is its destructive action on bacteria, and this 
has been applied commercially to the sterilisation of water by 
passing the water in thin sheets over quartz cylinders in which 
are placed large mercury vapour lamps. 

Ultra- violet light appears to play a very important part in 
the growth and development of tjie young child, and may prove 
to be one of the chief agents in the prevention of the bony deform- 


ities known as rickets. The results of some experiments seem to 
point to the conclusion that ultra-violet acting alone is a more 
powerful agent than when acting in the presence of the visible 
light. Indeed, the writer found that the stimulant action of 
ultra- violet light on the total chemical changes in the body could 
be annulled by the addition of visible light ; a similar antagonising 
action of the visible light was found on the tonic effect of ultra- 
violet on the isolated stomach kept alive with oxygenated Ringer's 

As only a short comparatively unexplored region exists between 
ultra- violet radiation and X-rays in the electro- magnetic spectrum, 
it is probable that many of the problems of the biological action of 
these two types of radiation may be solved simultaneously. Two 
outstanding differences, however, exist between them. The ultra- 
violet rays produce their effect in a few minutes, and their direct 
action is entirely superficial, while X-radiation sometimes takes 
weeks to reveal its effects, and it penetrates deeply into the tissues 
and is only stopped by dense structures like bone. The rays from 
radium produce effects on the tissues very similar to those of 

May we hope that the investigation of these artificial radia- 
tions in the laboratory will yield the secrets of the beneficial action 
of sunlight, and that man's activities will be directed to the 
removal from our atmosphere of the suspended matter which at 
present cuts off the health-promoting (ultra-violet ?) rays. It is a 
matter for some regret that the Empire on which the sun never sets 
has not developed great institutes for the open-air treatment with 
sun baths of the young victims of the darkness of our large cities. 


By Dr. D. H. SCOTT, F.R.S. 

The seed-plants are, and (as geological history shows) have 
long been, the dominant sub-kingdom of the plant world. They 
include all Phanerogams, the Gymnosperms (such as Conifers and 
Cycads), as well as the true flowering plants or Angiospenns. 
These two divisions are widely different, both in their characters 
and in their geological record. The Angiosperms, in fact, are the 
youngest, while the Gymnosperms are among the oldest of the 
groups which constitute the vegetation of the land. 

The Angiosperms alone (among living plants) bear " flowers " 
in the natural and usual sense of the word ; they have their seeds 
enclosed in an ovary or seed-vessel, and are fertilised through the 
mediation of a stigma and conducting tissue. The Gymnosperms, 
as their name implies, have naked seeds, commonly borne on a 
cone ; the ovule, or young seed, is fertilised directly, itself 
receiving the pollen, without the intervention of any accessory 
organs. It would take too long to enter into the further funda- 
mental distinctions which render the two classes so profoundly 

In the present article we are only concerned with the history of 
the older seed-plants, the Gynmosperrns. The relation between 
them and the Angiosperms is another story, which we shall not 
touch oil here. 


All seed-plants alike are reproduced (apart from mere vegeta- 
tive propagation) by means of complex bodies, the seeds, often 
of large size, always composed of various tissues, and usually 
containing an embryo. The spore-plants or Cryptogams, on the 
other hand, have extremely simple reproductive bodies, the spores, 
always minute, and usually consisting of a single cell. A fern, 


such as the bracken, is a good representative of the higher spore- 
plants. Here all the spores are of one kind. When sown, the 
germinating spore gives rise, not to a fern, but to an independent 
little organism, the prothallus, on which the sexual organs are 
borne. Fertilisation is accomplished, in the presence of sufficient 
water, by the actively- moving male cells (spermatozoids) and the 
fertilised egg grows up into a new fern-plant, which produces 
spores, and so the cycle is completed. 

This is a simple example. Tf we take a more advanced Vascular 
Cryptogam, such as a water- fern or a Selayinella. we find that 
sexual differentiation begins to show itself in the spores them- 
selves, which are of two kinds, microspores and megaspores. The 
little spores, on germination, produce the free- swimming sper- 
matozoids, and that is all ; there is no prothallus worth men- 
tioning. The large spores, on the other hand, develop a fairly 
massive prothallus (though it remains mostly enclosed within the 
megaspore-wall) ; this prothallus bears the female organs and 
ultimately serves to nourish the growing embryo. 

Now this fc * heterosporous " condition, only met with in the 
highest spore-plants, is no doubt to some extent an approach 
towards the reproductive methods of the seed-plants, especially 
the Gymnosperrns. The pollen-grains are clearly the same thing 
as the microspores. The female prothallus (endosperm) of a fir 
tree or a Cycad is quite comparable to that of a water-fern or 
Selayinella. The great difference is that in the seed-plant the 
rnegaspore (embryo-sac) is permanently retained within the 
sporangium, while the latter becomes enveloped by a new organ, 
the highly developed integument, or testa, constituting the seed- 
coat. The pollen (microspores) is received, and usually fertilisa- 
tion is effected, while the ovule or young seed is still borne on the 
parent plant ; as a rule the seed is not shed until the embryo 
within it is well developed. 

it is evident, from what has been said, that a certain relation 
in 'the reproductive processes between the seed- plants and the 
highest spore-plants can be traced. Botanists, since these 
relations were first established by Hofmeister in the middle of the 
last century, have come to believe that the Gymnosperms (and 
ultimately the seed-plants generally) were descended from hetero- 
sporous Vascular Cryptogams. Opinion, however, has been much 
divided as to the special group or groups which played the role 
of ancestors. While the general relations seem clear, there is 
no sign, among living plants, of any transition from spore- plants 
to seed-plants beyond the one important fact that in the Cycads 
and the Maidenhair tree, fertilisation is still carried out in the 
Cryptogamic manner, by means of active spermatozoids. 


The question thus arises, what light does our knowledge 
of extinct plants throw on tl\e problem of the origin of the 


Spermophytes ? Palaeobotanists, like other botanists, have held 
very diverse views, but for the last twenty years there has been a 
strong tendency to trace a connexion between the seed-plants 
and the ferns, through a group of Palaeozoic seed-bearing plants, 
with a fern-like habit. 

The existence of such a group was first realized in the year 
1903. Up to that time it was commonly estimated that almost 
exactly half the species of known Carboniferous plants were ferns. 
Everyone knows of the fine " fern fronds " preserved as impressions 
in the coal measures. Most botanists never doubted that these 
familiar fossils were really the relics of true ferns. Sir Joseph 
Hooker, in 1848, expressed his conviction that the Carboniferous 
genus Pecopteris was " the fossil representative, if not congener, 
of the modern Pteris (bracken). It is not improbable that there 
are other genera of living ferns fossilised in the shales of the coal 

Doubts, however, began to arise so early as 1883, when the 
Austrian palaeobotanist, Stur, pointed out that a considerable 
number of the Carboniferous fronds had never been found with 
fern- fructifications, and therefore, could not have been ferns. 
Stur's scepticism, though well justified, did little to shake the 
belief of palaeobotanists in general. 

From the anatomical side, evidence soon began to accumulate 
showing that some of the fern-like plants of the coal period could 
scarcely have been ferns in the strict sense of the word. Williamson 
first showed that in two genera with the foliage of Sphenopteris 
(quite like a fern), the structure in certain points approached that 
of a Cycad. He fully realised the significance of his discovery. 
In other genera also it was soon found that the foliage of ferns 
co-existed with certain anatomical characters of Gymnosperms. 

In 1897 the German botanist, Potonie, suggested the founda- 
tion of a group, Cycadofilices, to embrace such intermediate or 
indeterminate forms, and his proposal found wide acceptance. 
So far, however, all was still in doubt, for we could not be certain 
whether these Cycad-ferns were seed-bearing plants or only 
Cryptogamic ferns simulating the structure of a higher class. 


The first definite evidence came from Prof. F. W. Oliver, who 
in 1903 identified the seed of Lyginopteris oldhamia, one of the 
plants in which Williamson had demonstrated a Cycad-like 
anatomy. Oliver found that the seed Lagenostoma lomaxi, 
already known to Williamson, bore on its husk or cupule peculiar 
glands, identical with those on the stem and leaf of the Lyginopteris 
with which the seeds were associated, but unknown in any other 
fossil plant. The internal structure showed further points of 
agreement, and the evidence, though indirect, has been generally 

8EED-FEUXS. 15$ 

accepted as conclusive. It was further confirmed by the subse- 
quent discovery of very similar seeds borne on branched stalks 
resembling the naked rachis of a Sphenopteris frond. 

The seeds in question are highly organised and show important 
points of agreement with those of living Gymnosperms of the Cycad 
family. As the result of Oliver's discovery, the name " Pterido- 
sperms " was coined, to apply to those Cycadofilices in which 
there was evidence for reproduction by seeds. 

More obvious proof of the existence of " seed -ferns " was soon 
forthcoming. Dr. Kidstoii at once stepped into the field, and in 
J904- showed us great seeds, of the size of a filbert, borne oil 
fronds with the well-known leaflets of Nenropteris heterophylla. 
Then America made her contribution, for in the same year, Dr. 
.David White, of Washington, proved that the maidenhair-like 
fronds of Aneimites fertilis and other species, of Millstone Grit 
age, bore among their leaflets numbers of little winged seeds. 
Then, a year later, the distinguished French palseobotanist, 
Graiid'Eury, discovered fronds of Pccopteris pluckeneti studded 
all over with hundreds of seeds, winged like those of the Aneiniites. 
This was the most startling revelation of all, for up to that time 
nobody had doubted that such species of Pecopteria were true 
ferns. Other cases of actual continuity between seed and frond 
have since been recorded ; there is further a considerable amount 
of indirect evidence, from intimate and exclusive association, 
indicating that the seed habit was widely spread among the so- 
called Carboniferous ferns. In. fact, it is now generally recognised 
that an actual majority of these fern-like plants were not true 
ferns at all. but Pteridos perms. 

In all known cases it appears that the seeds of the Pterido- 
sperms wore borne on the frond itself, either on unaltered parts, 
showing the vegetative leaflets, or on special pinna) reduced to 
a more or less denuded rachis. The occurrence of seeds on an 
ordinary leaf, otherwise little or not at all modified, is without 
direct parallel among living plants. The nearest analogy is to be 
found in the female Cycas, where the seeds are borne on leaf-like 
carpels which grow out from the main stem, just as the leaves 
themselves do. In the way the seeds were borne, the Pterido- 
sperms appear to have been the simplest seed-plants known, though 
the seeds themselves were very far from simple. 


The discovery of the existence in Palaeozoic times of an 
extensive class of seed-bearing plants, in some respects primitive 
and externally altogether similar to ferns, naturally had a great 
influence on evolutionary speculation. It was thought by some 
botanists that we had at last tracked down the actual Crypto- 
gamic stock from which the seed-plants were descended. The 
(B 34-2285)Q L 


Pteridosperms were described as " Ferns which had become Spermo- 
phytes." The present writer at one time maintained that the 
fern phylum had been the source from which the groat majority, 
if not the whole, of the seed-plants was derived. The Pterido- 
sperms were regarded as the most primitive and the most ancient 
of the Spermophytes, and through them the Phanerogams 
generally were traced back to the fern stock. 

Some botanists, however, were more cautious. The gravest 
warning came from Dr. Kidston, who wrote, in 1906 : <fc The 
Cycadofilices [Pteridosperms] are undoubtedly the oldest group 
of c fern-like ' plants of which we have fossil evidence. 
It seems therefore to be highly improbable that the Cycadofilices 
could have descended from plants to which the name ' fern ' 
as understood in recent botany could be applied. What the 
progenitors of the Cycadofilices were, for the present remains 
unknown." Dr. Kidston's words, as it now appears, put the 
question in its true light. But for many botanists the temptation 
to hail a great phylogenetic discovery was too strong. The belief 
in the fern ancestry of the Pteridosperms, and through them of 
the other seed-plants, undoubtedly became prevalent. 

There was much excuse for this idea. Attractive in itself, as 
offering a solution of one of the greatest problems of plant- 
evolution, it further seemed very natural in the light of our new 
knowledge. We had been accustomed to believe that half the 
Carboniferous plants were ferns. Then we had discovered that 
many, probably most, of these " Carboniferous ferns " bore seeds. 
They were " ferns which had become Spermophytes." Yet surely 
they were ferns after all - they were so like them. We should 
have reflected, with Brutus, " That every like is not the same." 


Let us look a little more closely into this likeness between 
ferns and k * seed- ferns." Th resemblance in habit is undoubtedly 
striking in the highest degree. But it goes too far. An agree- 
ment so exact as to have led a botanist like Sir Joseph Hooker to 
refer to the bracken genus a plant now known to have been a 
Pteridosperm (Alethopteris decurretis, named Pecopteris hetemphylla 
in Hooker's time) must indicate a very near relationship, if it can 
be trusted to prove any relationship at all. Yet there cannot, 
from the nature of the case, be any -near affinity, for the Pterido- 
sperms bore highly organised seeds, rivalling the most complex 
seeds of later times, while the ferns are spore-plants, pure and 
simple. Thus there is in any case such a tremendous gap that 
similarity of habit ceases to be any evidence of affinity. 

That habit is illusory as a guide to relationship is a fact familar 
to all botanists. We need only < recall such obvious examples as 
the resemblance between a Cactus and a succulent Hupliorbia, 


between a horsetail, a Casuarina and an Ephedra, or between a 
water-lily and a frogbit. The external likeness between ferns 
and Pteridospcrms may be no more significant. The Car- 
boniferous flora grew under peculiar conditions, and it may well 
be that similarity of habit among plants of that period is simply 
the expression of a like reaction to a special environment. The 
fern-habit, now for the most part restricted to one group of 
Cryptogam ic plants, was in those days much more generally 
found appropriate to the prevailing conditions. 

The theory of a direct relationship between ferns and " seed- 
ferns " has further been supported by arguments drawn from the 
anatomical structure. Various analogies have been traced 
between the structure of certain Pteridosperms and that of some 
recent ferns. Such comparisons are fallacious, for it is inconceiv- 
able that Cryptogamic ferns now living should show any demon- 
strable affinity with a long- extinct race of Palaeozoic seed-plants. 
The resemblances which have been found are no doubt analogies 
and nothing more. 


The only sound structural evidence must clearly bo sought 
from the comparison of k " seed- ferns " with their contemporaries 
among the true ferns of Palaeozoic age. Such a comparison 
proves to be by no means favourable to the hypothesis of a direct 
connexion. Some of the " seed-ferns " have a verv simple 
anatomy, and so have some of the true ferns of the same period. 
But the simplest representatives of the two groups are not in the 
least like each other. Further, if we compare the more complex 
forms, we find that anatomical advance in the ferns and their 
seed-bearing contemporaries followed very different lines. Neither 
do we meet with any approximation in structure between the 
two, if we trace back both groups to older rocks, such as the 
Lower Carboniferous. 

We still have but little detailed knowledge of the pollen- 
bearing organs of the Pteridosperms. Jt is often extremely 
difficult to distinguish between the pollen-sac of a seed-plant and 
the asexual sporangium of one of the higher spore-plants. Where, 
as in the Pteridosperms, the pollen-sacs were borne on the frond 
and not on a specialised sporophyll (stamen) the distinction may 
almost disappear. We are scarcely yet in a position to compare 
the true ferns with the tk seed-ferns " in this respect. Tt may, 
however, be pointed out that in the one case in which the pollen- 
sacs of an undoubted Pteridosperm are adequately known 
(Crpssotheca htininf/hausi, the male fructification referred to 
Lytjinopteris), Dr. Kidston found that they were bilocular, a con- 
dition not met with among the sporangia of the ferns. 

When we come to the seeds, till resemblance to or even analogy 
with the reproductive- organs of true ferns vanishes. Broadly 


speaking, the seeds of the Pt endosperms, many of which have 
been most thoroughly investigated, were on the highest level of 
complexity ; they show no closer relation to a fern- sporangium, 
than does the seed of a living Cycad or the Maidenhair tree. In 
fact, it would scarcely be too much to say that, in the Pterido- 
sperms and other Carboniferous Spermophytes, the seed reached 
its zenith of elaboration, later changes having been largely in the 
direction of simplification ; only a few living plants, chiefly those 
just mentioned, have retained the old complex Palaeozoic type of 


While we believed that the Pteridosperms had once been 
ferns, we were under the necessity of deriving their seeds from 
Cryptogamic spore-sacs, such as ferns possess. Many ingenious 
hypotheses were framed in order to explain how so profound a 
transformation might have been effected. But they remained 
hypotheses and nothing more. No basis of fact could bo found, 
for nothing in the least suggesting an intermediate stage is known. 
It would be difficult to produce any vegetable object less like a 
fern-sporangium than many of the seeds referred to Pterido- 

On the whole of the evidence, one must conclude that there are 
no sufficient grounds for deriving the so-called " seed-ferns " 
from the true ferns. The two phyla appear to have run on 
independent and parallel lines. Possibly they may ultimately 
be found to converge, if we can ever trace them back far enough, 
in some common initial group of primitive land-plants. Tho true 
inference from the mixed characters of the Pteridosperms would 
seem to be, not that the seed plants are descended from ferns, 
but that they, or at least some of them, once passed through a 
fern- like phase, just as some of the Euphorbias are now passing 
through a cactus-like phase. 



Once more, therefore, we are left without any satisfactory 
theory of the origin of the Spermophyta. We may, however, 
briefly recall what is known of their early history, in order to see 
more clearly how the matter stands. In. Upper Carboniferous 
times, besides the Pteridosperms, there was the great Oordaitean 
family of fine forest trees, with tall branched trunks, long 
simple leaves, and complex male and female cones or catkins. 
The seeds, like those of the Pteridosperms, were of the Cycad 
type. The Cordaiteans, while totally different from the <k seed- 
ferns " in habit, show certain points in common with them, 
notably in the organisation of thp seeds, and also in some 
anatomical details. 


When we go back to the Lower Carboniferous, we find little 
trace of the Cordaiteans, but another family of great trees was 
flourishing. The well-known fossil trunk set up in the garden 
of the British Museum (Natural History) belongs to one of them, 
the Craigleith tree, Pitys withatn/i. The Pity* trees had in some 
respects a peculiar anatomical structure : their foliage, as Dr. 
Gordon has recently shown, consisted of small simple leaves, 
something like stout pine-needles, but more complex in internal 
structure. It was thus quite different from the leafage either of 
the Cordaiteans or the Pteridospcrms. Dr. (Jordon thinks he 
can trace some affinity between Pity$ and the puzzle-monkeys 
(Araucarians) among recent Conifers. Unfortunately, nothing is 
yet known of the fructification, but the whole vegetative organisa- 
tion is that of advanced (Tymnosperms. Thus, in Lower Car- 
boniferous times, the seed- plants were represented by at least 
two perfectly distinct groups, the fern-like Pteridosperms and the 
Araucaria-like Pity* family. 

Descending from the Carboniferous to the Upper Devonian 
we have found, until lately, little evidence of the presence of 
Pteridosperms. Quite recently, however, it has been announced 
from America that a whole forest of Pteridospermous trees, of 
Upper Devonian age, existed at (Jilboa, in the State of New York. 
The remains of the forest have long been known, but it is only 
within the last year or so that the seeds borne on the fern-like 
fronds have been recognised. Further details of this striking 
discovery will be a, waited with interest. Otherwise the most 
remarkable point is the occurrence of highly organised Uymno- 
>sperinous stems, notably the genus Callixylou. This appears to 
have belonged to the Pity* group ; the wood shows an excep- 
tionally beautiful structure, comparable to that of the more 
advanced Conifers among Jiving plants. In this case no seeds 
are known, but on anatomical evidence it seems clear that in 
Upper Devonian days, Gynmosperms had already attained a high 
grade of organisation. 

When we get back still further, to the Early (Middle and Lower) 
Devonian, the period of the oldest known land flora, we find no 
conclusive evidence for the existence either of Pteridosperms or 
ferns. Fossils resembling the naked rachis of a fern-frond are 
known, but they cannot be certainly distinguished from the 
branched, undifferentiated thallus which in those days was the 
form assumed by many of the archaic vascular plants. The 
extraordinarily simple organisation of some of these early types 
has been fully revealed by recent work, especially that of Kidston 
and Lang. 

Side by side with this primitive vegetation, however, plants 

of a much higher grade occur. The most famous of these is 

Hugh Miller's " cone-bearing tree," which he discovered some 

eighty years ago, in the Middle, Old Red Sandstone of Cromarty, 

(B 34-2285)ci M 


Miller inferred its affinities from the structure of the wood. This 
has recently been re-investigated by Kidston and Lang. It is a 
highly organised type of wood, differing in some respects from 
that of known Gyrnnosperms, but, in the opinion of the present 
writer, more like a Gymnosperm than anything else. Thus it is 
possible, though not yet proved, that seed-plants may have already 
existed in the earliest land-flora of which we have any knowledge, 
and contemporary with the simplest land-plants, of a thalloid, 
Alga-like habit. 

On the existing evidence we have no right to assume that the 
ferns preceded the seed-plants in their appearance on the earth. 
It may be that the phylum of the Spermophyta is as old as any 
known line of vascular Cryptogams. We may still hold to the 
belief that the seed-plants must have been derived from some race 
of heterosporous spore-plants. But what these supposed ancestors 
actually were is totally unknown. We may feel fairly sure that 
the progenitors of the Spermophyta belonged to a primitive stock, 
wholly unlike any of the higher Cryptogamic families, with which 
they have hitherto been compared. 

[In preparing this article some use has been made, with the 
sanction of the University College of Wales, of the writer's paper 
u On the Origin of the Seed- plants," published in k{ Abcrystwyth 
Studies," Vol. IV., 1922. The article cited deals with the question 
from a somewhat more technical point of view.] 






r 1T\HE Science Exhibition forms part of the scheme for Govein- 
I nient ])articipation in the British Empire. Exhibition, 
and has been organised, with funds provided through the 
Department of Overseas Trade, by the Royal Society. 

On the invitation of the Inter-Departmental Committee 
responsible for Government participation, the Council of the 
Royal Society appointed a Britisli Empire Exhibition Committee, 
under the Chairmanship (11*25) of Mr. F. E. Smith, C.B.E., 
F.R.S., to arrange the Exhibition. This committee is con- 
stituted as follows : 

MR. F. E. SMITH. C.B.E., K.R.S., Chairman. 

SIR UKIIHKRT JACKSON, K.B.E., F.R.S., Vice-Chairman. 

SIR OLIVKU LOIHJE, D.Sc.. Sc.D., LL.D., F.R.S., Vice- 


MR. C. V. BOYS. F.R.S. 

MR. J. \V. EVANS, C.B.E., D.Sc., LL.B., F.R.S. 
MR. D. T. HARRIS, M.B., B.S., Ch.B. 
MAJOR E. O. HEXRICJ, R.E. (retired). 
COL. H. (r. LYONS, R.E. (retired), D.Sc., Sc.D., F.R.S. 
MR. C. TATE RE<;AX, M.A., F.R.S. 
MR. A. B. REXDLE, M.A., D.Sc., F.R.S. 
MR. U. C. SIMPSON, D.Sc., F.R.S. 
PROF. J. F. THORPE, C.B.E., D.Sc., F.R.S. 
Mu. W. J. U. WOOLCOCK, C.B.E. 
MR. T. MARTIN, M.Sc., Secretary. 

In many cases the exhibits are shown by the scientific men 
actually engaged in the work, and are supplemented by instru- 
ment loaned by some of the leading firms of scientific instrument 
makers. The majority of the exhibits are working demonstrations. 
Benches, fitted with gas, water and electricity, are provided, and 
a staff of scientific assistants are in attendance throughout 
the Exhibition. (The Demonstration benches by Messrs. BAIRD 

The arrangement of the principal section of the physical 
exhibits is based on the extended spectrum of electro-magnetic 

M 3 


oscillations and radiations. The experiments illustrate the 
phenomena met with in the different regions of the spectrum, 
from gamma rays and X-rays at one end, through the visible 
region, to wireless waves and slow oscillations at the other. As 
a key to these exhibits a large chart of the spectrum (described 
below) has been specially prepared and is displayed in the galleries. 

The exhibit dealing with Solar and Terrestrial Kadiation 
constitutes a link between the purely physical and the geophysical 
exhibits. These are arranged to illustrate recent British con- 
tributions to the sciences of Meteorology, Terrestrial Magnetism, 
Atmospheric Electricity and Seismology. 

Biological science is represented by three groups of exhibits 
dealing respectively with Zoology, Botany and Physiology. 
The Zoological section illustrates principally recent research 
bearing on the theory of evolution. The botanical exhibits 
include illustrations of the preservation of colour in plants for 
exhibition purposes and experiments in plant physiology. In 
Physiology the theme is the application of physical methods and 
appliances in this science. 


The chart is designed to show, in a manner which appeals 
to the eye, the whole range of radiations which are now known 
to be electro-magnetic. According to classical theory, all the 
types of radiation mentioned on the chart are propagated as strains 
in the ether of space with a constant velocity which is approxi- 
mately equal to 30,000,000,000 centimetres per second. There 
is every reason to believe that the radiations shown on the 
chart, from the shortest gamma-ray having a wave-length of 
only 0' 000 000 0002 centimetre up to wireless waves having a 
wave-length of 500,000 centimetres, travel in space with this fixed 
velocity, and that although their effects are very diverse, the 
only fundamental difference between the many types of radiation 
studied lies in the wave-length. 

The whole range of radiations is divided for convenience into 
groups. Within any group the radiations are similar in their 
characteristic effects and properties, and the same methods 
may be employed to detect and produce a wave of any length 
within a group. The radiations of each group overlap to some 
extent, so that by changing our methods we may generate or detect 
all the waves shown on the chart, discarding one set of methods 
when we have succeeded in producing, say, the shortest wave of 
the group of radiations next above in the scale of wave-lengths. 
As an example of this, we may generate a radiation of wave- 

THE ATOM. 163 

length 0-02 centimetre either by the use of an Hertzian oscillator 
or by employing a source of heat radiation. Although the two 
sources of radiation are very different, the radiation itself is 
identical in all its properties. 

Since a chart in which the lengths of the lines are made pro- 
portional to the wave-length would circle the earth very many 
times, a scale of octaves has been chosen, an octave implying that 
the numerical value of the wave-length has been doubled. The 
chart shows about 60 such octaves, and there is an unbroken 
series of radiations increasing in wave-length from gamma-rays 
to X-rays, through the ultra-violet and the visible rays to the 
infra-red, so to the Hertzian and wireless waves, finally ending 
with the very long waves corresponding to relatively slow electrical 
oscillations. The wave-lengths are given in centimetres and in 
Angstrom units. One Angstrom unit is equal to 0-00000001 
centimetre. It is only recently that radiations have been dis- 
covered corresponding to all the wave-lengths shown in the chart. 
Scientists are now able to generate and detect ethereal (or electro- 
magnetic) waves in any part of the spectrum. When one considers 
the vast range of wave-lengths covered by the chart, this is in 
itself a most remarkable achievement. 


1. Sir Joseph Thomson, O.M., F.R.S. The Atom. 

The Electron. 

(a) Apparatus by which the existence of electrons was detected 
and their mass and velocity measured. 

(6) The modified type of Perrin tube used by Sir Joseph 
Thomson in 1897 to show that, when a magnet was used to deflect 
the cathode rays, the negative electrification followed exactly the 
same course as the rays which produced the fluorescence on the 

2. Sir Ernest Rutherford, O.M., F.R.S. 

Some Aspects of Radio-Activity and their Bearing on 
Atomic Structure. 

(A) The Production of Kadium Emanation. 

The radium atom transforms with the emission of an a-particle 
into the atom of a gaseous substance, the radium emanation. 
The atom of the radium emanation is much more unstable than 
that of radium, so that the volume of the emanation in equilibrium 
with 1 gm. of radium is very small, only 0-6 cubic millimetre. A 
sufficient quantity of the emanation* has been collected and isolated 
to enable its chemical and physical properties to be examined, and 


Th6 Atom. to show that, apart from its radio-active properties, it behaves 

like the inert gases, helium, neon, argon, etc. Its atomic weight 
is 222, and its boiling point is -65 3 ( 1 . at atmospheric pressure. 
(At the low pressures at which it is generally used, its boiling point 
is about- 150 0.) 

(B) The Discovery of the Nature of the ^-Particle. 

In the earlier history of radio-activity, little attention xvas- 
given to the study of the //-rays, and their importance was not 
generally recognised. Thus some years elapsed before the nature 
of the rays was disclosed. It \\as suggested by various workers 
that the //-rays might consist of positively charged bodies projected 
with great speed. They should therefore be deviated in magnetic 
and electrostatic lields. The earlier attempts to show this devia- 
tion were unsuccessful owing to the extreme smallness of the cO'< ct. 
In 1002, however, Rutherford was able to show that the (/-rays 
were deflected in a magnetic field in the opposite sense to the 
cathode or /^-rays. Later he succeeded in measuring the deflexion 
by an electrostatic field, and also in showing that the (/-particles 
carried a positive charge, by collecting the charge of a large number 
of //-particles in a Faraday cylinder. Combining the magnetic 
deflexion, which measures wi^e, with the electrostatic deflexion, 
which measures tnr'^f^ lie obtained r, the velocity of the particle, 
and f Hi, the ratio of its charge to its mass. The magnitude of 
the value of f in indicated that, if the //-particle consisted of any 
known kind of matter, it must either be hydrogen or helium, 
and the observed production of helium by radium and its emana- 
tion lent weight to the latter suggestion. This was confirmed 
by direct experiment by Rutherford and Koyds, who showed that 
accumulated //-particles formed a gas which gave the spectrum of 

(C) The Counting of tt- Particles. 

There are two direct methods of detecting a single //-particle, 
the electrical method and the scintillation method. 

The electrical method depends on the principle of ionisation 
by collision. In the first form of //-ray counter, duo to Rutherford 
and Geiger, the pressure of the gas in an ionisation chamber and 
the voltage between the electrodes were adjusted so that any ions 
produced in the gas were multiplied several thousand times by 
collision. The magnification was so great that the entrance of 
a single a-particlc into the chamber produced a measurable 

A counter devised by Geiger has the advantage that the gas 
in the counter may be at atmospheric pressure and that even 
greater magnification is obtained. 

When a screen of phosphorescent zinc sulphide is exposed 
to a-rays, a luminosity is produced which, examined under a 
microscope, is found to cdiisist of scintillating points of light, 
which come and go with great rapidity. Each scintillation 


corresponds to the impact of an a-partiele on a zinc sulphide The Atom* 
crystal. On a uniform screen every a-particle produces a visible 
scintillation, so that we have an extremely simple method of 
counting a-particles. As zinc sulphide gives only a weak general 
luminosity when exposed to ft- or -y-rays, the counting of a-particles 
by this method is, up to a certain limit, independent of ft- and 
>-ray effect. This gives the scintillation method a great advantage 
over the electrical method. 

The screen is made by dusting a thin layer of small zinc sulphide 
crystals on a cover-slip moistened with a trace of oil or adhesive 
material. The observation of the scintillations is carried out in 
a darkened room. 

The Strinir Electrometer by the CAMBRIDGE TXSTRUMKXT 
CO., LTD. 

DEMONSTRATION of scintillations. 

(I)) The Skittering of ^-Particles. 

The a-partiele travels through matter in general in a straight 
line, its energy of motion being so great that intense forces are 
necessary to deflect it ; but occasionally an a-particle suffers a 
deflexion through a large angle in an encounter with a single 
atom. In order to account for these large deflexions Rutherford 
put forward, in 191 1, the theory of the nuclear constitution of the 
atom. On this theory, the whole of the positive charge associated 
with the atom is concentrated in a minute, but heavy nucleus, 
while the negative charge is made up of electrons distributed over 
a space surrounding the nucleus comparable with the size of the 
atom, as usually understood. Owing to its large positive charge 
the nucleus of a heavy atom, like gold, is surrounded by an intense 
electric field, and if an a-particle enters this field it will be deflected 
from its straight path. Assuming that the electric force around 
the nucleus varies inversely as the square of the distance, Ruther- 
ford obtained the relations connecting the fraction of a-particles 
scattered through any angle with the charge on the nucleus and 
the velocity of the a-particle. 

These relations were tested by Geiger and Marsden in an 
extensive scries of experiments. Their results were in remarkable 
agreement with Rutherford's calculations and proved conclusively 
the truth of the nuclear theory. 

Later, Chadwick was able to show by accurate measurements 
of the scattering of a-particles that the charge on the nuclei of 
platinum, silver, and copper was given in electronic units by the 
atomic number of the element, and that the force at moderate 
distances from the nucleus varied Inversely as the square of the 


The Atom. . (E) The Artificial Disintegration of Elements. 

The first evidence of the artificial disintegration of an element 
was obtained by Rutherford in 1919. He found that when swift 
a-particles pass through dry air or nitrogen, a few long-range 
particles are produced which can be detected by their scintillations 
on a zinc sulphide screen. He concluded that some of the nitrogen 
atoms were disintegrated by the close collision with an a-particle, 
with the liberation of a hydrogen nucleus at a high speed. Later, 
Rutherford and Ohadwick found that these particles have a greater 
range than the swift H nuclei set in motion by the collision of an 
^/-particle with a hydrogen atom. For example, using radium-C as a 
source of a -rays, no H nuclei from hydrogen can be detected after 
passing through absorbing screens of aluminium or mica of stopping 
power equivalent to 30 cm. of air, while the maximum range of 
the particles from nitrogen corresponds to 40 cm. of air. This 
shows at once that the emission of particles from nitrogen cannot 
possibly be ascribed to the presence of free hydrogen or of hydrogen 
in combination as a contamination. 

This observation gave a simple* method of testing whether 
other elements besides nitrogen emitted long-range particles when 
bombarded by rt -particles. If the scintillations are counted for 
absorptions greater than 30 cm. of air, the results are quite in- 
dependent of the presence of hydrogen as an impurity in the 
substance under examination, in this way, definite proof of the 
disintegration of boron, nitrogen, fluorine, sodium, aluminium, 
and phosphorus was obtained. 

In further experiments it was found that neon, magnesium, 
silicon, sulphur, chlorine, argon, and potassium are also dis- 
integrated by the impact of swift a-particlcs. The H particles 
liberated from these elements are of shorter range and smaller 
in number than those ejected under similar conditions from the 
elements previously mentioned. Thus, with the two exceptions 
of carbon and oxygen, all the elements from boron to potassium 
have been disintegrated by bombardment with a-particles. 

Recently, Blackett has photographed by the Wilson method 
the tracks in nitrogen of about 300,000 a-particles. Among these 
he found eight tracks which show the event of the disintegration 
of a nitrogen nucleus by an impinging a-particlc. The striking 
point of these photographs is that such an a-ray track divides 
into two branches only. One branch is a fine straight track, 
along which the ionisation is distinctly less than along an a-ray 
track. This must be due to a particle of small charge and high 
velocity, and it is the path of the ejected H particle. The second 
arm of the fork is a short track similar in appearance to the track 
of a nitrogen nucleus, such as has been obtained in a normal clastic 
collision. This is the path of the nitrogen nucleus after disintegra- 
tion. There is no sign of a third arm to correspond to the track 
of the a-particle itself after the collision. It is concluded, therefore, 
that the a-particle docs not escape, but that in ejecting the H 
particle from the nitrogen nucleus, the a-particle is itself bound 
to the nucleus. Of the nature of the residual nucleus little can 
be said without further data. 


<F) The Beta and Gamma Rays from Radio-active Sub- Th Atom* 

If a radio-active source is placed in a uniform magnetic field, 
the #-rays which are ejected from it in directions at right angles 
to the lines of force travel in circles and leave a trace on a photo- 
graphic plate. From the radius of the circle, the velocity of a 
homogeneous group of /3-rays can be determined. The #-rays 
emitted are found to consist of a number of such groups of electrons 
of homogeneous velocity, characteristic of the element in question, 
together with electrons of continuously varying velocity. The 
latter probably include the electrons shot out from the nucleus in 
the disintegration. The homogeneous groups have been shown 
by Ellis to consist of electrons ejected from the K, L, M, etc., 
levels of the atom by the y-rays, and since the ejection is governed 
by the law of photo-electric action, the wave-lengths of the lines 
in the y-rays spectrum emitted by a disintegrating atom can be 
found. In the case of radium B, there is agreement between the 
wave-lengths so determined and those found by Rutherford and 
Andrade directly by the crystal method, but the #-ray method is 
applicable to much shorter wave-lengths. The existence of 
difference-relations between the quantum energies of the various 
groups of y-rays emitted from radium B and radium C makes the 
applicability of quantum dynamics to the nucleus very probable, 
and a scheme of nuclear levels concerned with the emission of 
y-rays can in each case be set up, in the same way that from a 
knowledge of the optical and X-ray wave -lengths the level structure 
of the atom may be determined. 

<G) The Work of Moseley on the Atomic Number of the 


Moseley found that the X-ray spectra of the elements depended 
on the square of a number which increased by unity in passing 
from one element to the next of higher atomic weight. This 
number was not exactly equal to the atomic number (i.e., the 
number of the element when all the elements are arranged in order 
of increasing atomic weight), but was equal to X-'/, where N is 
the atomic number and ft a constant for the series. In order 
to obtain perfect regularity in the X-ray spectra, it was necessary 
to leave four places for unknown elements corresponding to atomic 
numbers 43, 61, 72 (the recently discovered hafnium), and 75, 
and to adjust the places of the elements A, Co, and Te, where the 
order of atomic weights dashed with the order of chemical pro- 
perties. Then, in every case, from Al, for which X was assumed 
to be 13, to Au, N -= 79, the X-ray spectra of an element were 
defined by the number assigned to it. On the nuclear theory of 
atomic structure this characteristic number must be closely con- 
nected with the charge on the nucleus, and Moseley concluded 
that the number gave in fundamental units the actual value of 
this charge. 

In this way Moseley determined the number and order of the 
elements. * 


Tht Atom. 3. Mr. D. R. Hartree. 

Models Illustrating Atomic Structure. 

The present idea of the nature ami structure of atoms, which 
is based mainly on the work of .1. J. Thomson, Rutherford and 
Bohr, is roughly as follows : An atom consists of a very small 
positively charged nucleus, which is responsible for most of the 
mass of the atom, surrounded by a number of electrons, of which 
some at least are at distances from the nucleus of the same order 
of magnitude as the radii of atoms deduced from other evidence 
(about 10 cm.), the linear dimensions of the nucleus and individual 
electrons being about 50,000 times smaller ; thus the atom is an 
exceedingly empty and open structure (an exhibit of a diagram- 
matic model of a neon atom illustrates this). Also the charge 
on the nucleus (measured in such units that the charge on the 
electron is unity), which is equal to the number of electrons in the 
neutral atom, is also equal to the atomic number, or number 
associated with the atom when all the elements are arranged in 
order by means of their chemical properties and X-ray spectra, 
as in the periodic table, and numbered consecutively from I for 

The electrons are supposed to be moving in orbits about Uie 
nucleus, so that in some ways the atom resembles the solar svslcm. 
But there are several important differences between the solar 
system and the atom, apart from the difference of scale. 

(1) Tn the solar system the forces are gravitational in origin, 
and for any planet the attraction of the sun is large compared to 
that of the other planets, so that to a first approximation the mutual 
forces between the planets need not be taken into account in 
calculating the orbit of any one of them. In an atom, however, 
the fields are electrical, and also the mutual forces of the electrons 
are by no means negligible compared to the force Jjctween the 
nucleus and any one of them. 

(2) In the solar system the orbits of the planets all lie nearly 
in the same plane, those of the major planets are nearly circular, 
and each lies wholly outside those inside it. Tn an atom the 
orbit of each electron may perhaps be thought of as roughly plane, 
but the planes of different orbits have different orientations in space, 
the orbits are often not nearly circular but more nearly elliptical, 
and different orbits may interpenetrate (somewhat as the orbits 
of the periodic comets in the solar system penetrate into the 
regions of the orbits of the planets). 

(3) Lastly, and most important, the possible states of motion 
of the solar system form a continuous set of states, so that if the 
system is disturbed, by however little, there is always a possible 
(icw state of motion, different from the initial state, which it can 
take up when the disturbance is removed. The possible states of 
motion of an atom form a discontinuous set of discrete states, 
which are known as the stationary states, so that if an atom is 
disturbed, it either changes over to some other one of the definite 
separate stationary states, or 1 it returns exactly to its initial state 
when the disturbing influence is removed. 

THK ATOM. 169 

When an atom changes from one stationary state to another The Atom*. 
with emission or absorption of light (in the general sense of electro- 
magnetic radiation e.r/., X-rays, ultra- violet and visible light, 
" radiant heat "), there is a close relation between the frequency of 
the light emitted or absorbed and the difference of energy of the 
stationary states. From the idea of stationary states and this 
relation alone, many of the phenomena of spectra can be explained. 

For an atom consisting of a nucleus and one electron it is 
possible to specify the conditions for the stationary states in a 
mathematical form, and work out their energies and so the spectrum 
arising from transitions between them ; the agreement with the 
observed spectra,, even down to the minutest details, is almost 
perfect. Kor atoms consisting of a nucleus and more than one 
electron, the mutual interactions of the various electrons com- 
plicate the problem so much that it is not yet possible to specify 
the conditions for a stationary state, and even if they could be 
specified, the working out of the energies of the stationary states 
would probably be very difficult. But it is possible to make some 
simplificat'onx and obtain approximate results without going so 
far that, they lose their significance. 

Instead of treating the atom, as a whole, as a system the station- 
ary states of which are to be found, we think of each individual orbit 
as a stationary state in the field of the nucleus and electrons in 
other orbits ; and after some further approximations it is possible 
to state the conditions for a single orbit to be a stationary state. 
It appeal's, then, that each orbit can be specified approximately 
by two whole* numbers, which are written // and A 1 and called 
''quantum numbers"; the orbit so specified is usually referred 
to as an n\ orbit. The precise meaning of these numbers cannot 
be explained simply and shortly, but speaking very roughly, 
n-k is a measure of the radial motion of the electron in its orbit 
(i.e., motion in towards and out from the nucleus) and / is a 
measure of the angular motion (/>., motion round the nucleus) ; for 
example, in a circular orbit there is no radial motion, and // and 
k are equal. It is possible also to calculate approximately the 
sizes and shapes of orbits ; the models of atoms exhibited have 
been made from such data. 

The electrons occur in groups of orbits with the same quantum 
numbers ; if there are enough electrons, there are two in the 
smallest orbits with n 1 , eight in the next largest with u 2 ; 
the numbers in the groups with higher values of n are different for 
different atoms. The distribution of electrons among the orbits 
with the same value of n but different values of k, and the orienta- 
tions of the different orbits, are still uncertain. 

It will be seen from the models that lithium with three electrons 
has the completed group of two small l^ orbits and one electron 
in a very much larger orbit, which is also rather loosely bound; 
and sodium with 11 electrons has 2 and 8 respectively in the 
completed groups with n 1 and 2 respectively, and again one in 
a very much larger and rather loosely bound orbit. The outermost 
electrons are mainly responsible for the chemical properties of the 
atoms, and the 1, 2 and 3 outer and rather loosely bound electrons 
of the atoms of sodium, maznesium and aluminium, the orbits of 


The Atom. which are shown in the models, are responsible for the single, double 

arid triple electro-positive valencies of these atoms. The similarity 
of structure of lithium and sodium atoms is an example of the 
way in which present theories of atomic structure are explaining 
the regularities of the periodic system of the elements. 


4. Prof. W. L. Bragg, F.R.ti., and Mr. 1). R. Hartree. 
The Crystalline Structure of Rocksalt. 

The model represents atoms of sodium and chlorine placed in 
the relative positions which they occupy in a crystal of sodium 
chloride, and with the form of the electronic orbits shown approxi- 
mately by the curves. The whole model is constructed to a scale 

of 10 cm. to the Angstrom unit, or a thousand million to one, in 
order that the relative dimensions of the spaces between the 
atoms and the atomic structures themselves may be appreciated. 
The sodium atom is the smaller and the chlorine atom the larger 
in the model. It is not possible to show all the inner electronic 
orbits in every case, as some of these are too small. 


5. Prof. C. T. R. Wilson, F.R.8. 

(loud Method of Studying the Tracks of Ionising Particles. 

Atoms or molecules from which electrons have been ejected 
(positive ions), and atoms or molecules to which such ejected 
electrons have attached themselves (negative ions), may be made 
individually visible by causing water to condense upon them. 
In a dust-free moist gas, in which a suitable degree of super- 
saturation is brought about by sudden expansion, water condenses 
on any ions which may be present and on them alone. The drops 
of water condensed on the ions may be photographed immediately 
after their formation. 

In its passage through a gas, an ionising particle (an a- or 
^-particle emitted by a radio-active atom, or an electron ejected 
by X-radiation or otherwise from an ordinary atom) passes through 
a large number of atoms under conditions such that an electron 
is ejected. The positive and negative ions which are thus pro- 
duced are left as a trail along the track of the ionising particle. 
This trail of ions may be made visible as a cloud of water-drops 
and photographed. 

THE ATOM. 171 

If two simultaneous photographs are taken from different The Atom* 
directions, a three-dimensional study of the tracks may be made, 
stereoscopically or otherwise. 

The Wilson Cloud Expansion Apparatus, to the modified 
design of Shimizu, by the CAMBRIDGE INSTRUMENT CO., 


6. Clarendon Laboratory, University Museum, Oxford. 
(Prof. F. A. Lindemann, F.R.8., Mr. T. C. 
Keeley, and Mr. E. Tiolton Kitty.) 

A Method of making audible the Movement of a- and 
^-Particles in an Electric Field. 

Radio-active substances send out a-, p- and y-rays. a- rays 
are simply helium atoms which have lost two of their four electrons 
and are travelling at a very high speed (10-20,000 km. per sec.), 
/^-rays arc electrons moving at yet higher speeds (up to 295,000 
km. per sec.). Owing to the very much larger mass of the a- 
particles compared with the /^-particles (7,000 to 1), both of them, 
in spite of the difference in speed, possess much the same energy. 

When one of these very rapidly moving particles strikes an atom, 
it usually breaks off one or more of the outer electrons, thus leaving 
the mutilated atom (or ion) with a positive charge. In an electric 
field these ions move, thus transferring charge and thereby forming 
an electric current. If the field is strong enough, the velocity of 
the ions rises to such a high value that they themselves can break 
electrons off other atoms, thus forming new ions which in their 
turn repeat the process. In this experiment, so powerful a field 
is maintained between the two electrodes, that the ions formed 
by one single a- or 0- particle produce sufficient new ions to give a 
momentary current, which, when amplified, works a loud speaker. 

Owing to their large size and comparatively low speed, a- 
particles can scarcely pass a molecule of air without ionising it, and 
are therefore braked and brought to a stop at a fairly definite range 
(3-8 cm.). They are absorbed by all but the thinnest sheets of 
light material, such as aluminium or paper. The smaller and more 
rapid /3-particles have much larger and less well defined ranges 
owing to the different stopping powers which different parts of the 
molecules of air possess. For the same reason they can pass 
through comparatively thick sheets of material. 

The loud speaker by MESSRS. S. G. BROWN, LTD. ; the 
amplifier by MESSRS. H. W. SULLIVAN, LTD. ; the valves 
and H.T. batteries used in this and other exhibits from the Clarendon 
CO., LTD., and MESSRS. SIEMENS, BROS. & CO., LTD., respec- 



iheAtoin. 7. Prof. J. Joly, F.R.S. 

Photographs of Pleochroic Haloes. 

Pleochroie haloes appear in certain rock minerals (notably 
in brown mica) as minute circular markings. Under favourable 
conditions a central particle will be noticed. This contains radio- 
active elements, the a -radiations of which, by altering chemically 
the containing mineral, give rise to the haloes. The haloes are, in 
fact, spherical objects. 

The outside dimensions of the halo depend on the penetration 
of the swiftest a -ray projected from the nucleus. Accordingly, 
it is easy to determine by measurement whether uranium or 
thorium is the parent radio-active element responsible for the halo. 
Haloes often show inner structure consisting of concentric rings. 
These represent spherical surfaces developed around the nucleus 
where the alteration (by ionisation) of the containing mineral 
attains special intensity. 

The exhibit shows haloes due to uranium and to thorium. 
They are, of course, highly magnified. A uranium halo is 0-033 
mm. in radial dimensions. A thorium halo measures 0-041 mm. 
.Different stages of development are shown in the case of uranium 
haloes. The oldest haloes often show '" reversal " corresponding 
to solarisation in over-exposed photographs. All haloes are very 
old and have been formed by the rt-radiation from the nucleus 
extending over a period of not less than 50 million years. The 
younger rocks do not show haloes. 

Three of the photographs are of haloes of unknown origin. 
Their measurements do not agree with the a -radiations of any 
known element. 

(See Phil Trans. R.S., Vol. 217, pp. 51-79, and Proc. R.S., 
A. Vol. 102, W23.) 

8. Sir Joseph Thomson, O.M., F.R.8. 
Positive Rays. 

Photographs of positive ray spectra, illustrating the positive 
ray method of chemical analysis. The line due to H 3 is to be 
seen on some of the plates, and that due to the isotope of neon 
on others. 

9. Dr. F. W. Aston, F.R.8. 

Isotopes and the Mass-Spectrograph. 

Isotopes are elements having the same chemical properties 
but different atomic weights. They were first shown to exist 
among the products of radio-active disintegration by Soddy. 
This exhibit illustrates progress in the discovery of isotopes among 
the more common elements^ This was first suggested by the two 
parabolas given by the element neon when subjected to Sir Joseph 



Thomson's positive-ray analysis. They are clearly seen in the 
enlargement shown. This result indicated that neon had atoms 
of two different weights, 20 and 22, but the accuracy of the method 
was not sufficient to prove this conclusively. Further evidence in 
favour of this view was afforded by the partial separation by diffu- 
sion of the constituents of neon by Aston in 191*5, which was 
demonstrated by means of the special microbalancc exhibited. 

The matter was finally set-tied by the mass-spectrograph. This 
instrument, a model of which is exhibited, was devised by Aston 
in 1010. In it positive rays of atoms and molecules give separate 
focussed lines on a photographic plate, by which means the iceights 
ofindiridnalat'tniH crnt be determined witluin accuracy of 1 part in 1,000. 
Chlorine (At. Wt. 3. r r4tt) was shown to be composed of two isotopes, 
3f> and *57, and other elements were proved even more complex. 
Thus krypton has six isotopes, 78, SO, 82, 83, 84, 86. Mass-spectra 
obtained with these elements are shown. No less than 56 of the 
known elements have been successfully analysed by means of this 
instrument, and all have been shown to consist of atoms having 
whole number ireiyht*. This removes the last difficulty in accepting 
the conclusion that the atoms of all elements are themselves built 
of atoms of electricity. 

The Mass-Spcctrograph by Messrs. ADAM HILGER, LTD. 

The Atom* 

i. E.A.Owen}. 


10. National Pht/xical laboratory. 
Penetration of Metals by y-rays. 

Among the radiations emitted by radium is a group of rays, 
called -y-rays, which arc X-rays possessing on the average a much 
shorter wave-length than those emitted by an X-ray bulb. As a 
consequence, the -y-rays possess the power of penetrating solids 
in an even more marked degree. The effect cannot be shown by 
fluorescent screens as with X-rays, but use is made of another 
property of these radiations namely, that of ionising (or rendering 
conducting) the air through which they pass. Thus the charged 
leaf of a gold-leaf electroscope becomes discharged when the rays 
pass into the electroscope. 

In the exhibit a specimen of radium is placed in the centre of 
a massive lead block provided with a small hole so that a beam of 
y-rays falls on an electroscope. Blocks of lead, iron or brass, each 
1 in. thick, which can be interposed in the beam, only partially cut 
down the rate of fall of the electroscope leaf, showing that an 
appreciable fraction of the radiation is able to pass completely 
through the block. 

The radium provided by MESSRS. WATSON & SONS (ELEC- 
TRO-MEDICAL), LTD., representing the RADIUM BELGE 





X-rays. H. Mr. F. D. Edwards. 

Electrical Discharge through a Large Tube during Reduc- 
tion from Atmospheric Pressure to High Vacuum. 

This demonstration illustrates, on a large scale, the very beauti- 
ful effects obtained when a current of electricity at high potential 
is passed through air and other gases at low pressure. By gradual 
exhaustion of the tube to the point where X-rays are produced, 
and finally to u hardness " or non- conductivity of the tube, the 
phenomena observed by Crookes and others, whose investigations 
led to the discovery of the rays by Rontgen, are shown. 

The apparatus used includes : (1) Discharge tube 4 ft. 6 in. 
long with sealed-in electrodes ; (2) induction coil and mercury 
interrupter ; (3) 2-stage rotary oil pump and motor arranged as 
" backing " pump for a small mercury vapour diffusion pump 
(4) simple measuring instruments. 

The experiment is started with the air in the tube at atmo- 
spheric pressure, when the high tension discharge sparks across 
the 10 in. gap between the terminals on the induction coil. As 
the pressure in the tube is reduced, the resistance of the remaining 
air falls, and vivid lightning-like discharges start from the electrodes. 
On further pumping a succession of phenomena arc noticed in the 
tube : 

Low pressure arc --heating effect deviation by magnet. 
The negative glow fluorescence. 
The Faraday dark space. 
Cathode glow. 
Crookes dark space. 

Cathode ray.s phosphoresce rice magnet ic deviation 
heating effect. 
' k Hardness " of tube sparking at induction coil. 

A number of smaller hermetically sealed tubes, illustrating 
experiments carried out by Sir William Crookes and others, are 
shown working. 

(1) De la Rive's apparatus, showing the rotation of a 
luminous arc round a vertical electro-magnet. 

(2) Crookes' tubes. (a) An aluminium cross placed in 
the cathode stream casts a shadow on the tube wall. 
'* Fatigue " of the glass is shown by removing the cross, when 
the glass phosphoresces most brightly where previously 
protected by the cross. 

(b) A vane wheel is set in motion by the vigorous bombard* 
ment of the electron stream. The motion is reversed by inter- 
changing cathode and anode. 



(c) A narrow pencil of cathode " rays " grazes a vertical X-rays, 
screen. The approach of a magnet causes the phosphorescent 
track to move up or down according to the nature of the pole 

A water cooled X-ray tube is arranged to show the modern 
development of these appliances, and their application. 

Apparatus by MESSRS. W. EDWARDS & CO. 

1 2. National Physical Laboratory (Dr. G. W. C. Kaye). 

Transparency of Materials to X-rays. 

As is well known, the X-rays possess the power of penetrating 
solids to an extent which depends on the thickness and density. 
A working exhibit shows visually the relative transparencies of 
some half-dozen elements ranging from carbon (graphite, density 
1-6) to lead (density 11-4). X-rays are capable of exciting a 
fluorescent screen, and the density of the shadow cast by each 
element on such a screen serves as a measure of the absorption. 

A number of X-ray photographs are also shown which illustrate 
the varying transparency of different materials to X-rays. 


13. Rontcjen Society. 
Historic X-ray Tubes. 

1 and 3. Crookes' tubes, with lattice screen. Both made in 1879. 

5. Pear shape tube as used by Rontgen. 

12 and 13. Jackson's first focus tubes. Made 1896. 

15. Tube with platinum anticathode and auxiliary anode. 

19. Campbell Swiiiton's first heavy anode tube, with penny 
as anode. 

22. Tube (Jackson type) with lengthened electrodes to prevent 
sparking over. 

24. Tube with adjustable electrodes to vary hardness. 

34. American tube with automatic regulator. 

57. Lodge's metal tube. 
Coolidge X-ray tube. 

The tubes are selected from the collection of the Rontgen 
Society at the Science Museum. They are exhibited, with the 
kind permission of the Society, by arrangement with the Director 
of the Science Museum. 


x-Rays. 14 sir William Bragg, K.B.E., F.R.S. 

The Analysis of Crystal Structure by X-rays. 

The exhibit illustrates the application of X-rays to the analysis 
of crystal structure. 

The whole exhibit comprises : 

(1) Two forms of spectrometer, one adapted for measurement by 
ionisation methods, the other by photographic methods. 

(a) The Bragg Ionisation Spectrometer. 

In this instrument the intensity of the rays reflected at a crystal 
face is measured with an ionisation chamber which is capable of 
rotation about the axis of the spectrometer. 

(6) The Muller Spectrogmph. 

The X-rays after reflection by the crystals fall in this case on a 
photographic plate. This gives a permanent record of all the 
possible reflections from the crystal planes. A knowledge of the 
positions and intensities of the reflected beams, which is supplied 
by the above methods of measurement, enables the relative positions 
of the atoms in the crystal to be determined. 

(2) Some of the photographs obtained by the use of the latter 

(3) Models representing the arrangement of the atoms in crystals 
of (a) diamond, (6) naphthalene, (r) long-chain molecules. 

The Bragg Ionisation Spectrometer by MESSRS. W. G. PYB 
& CO., the Muller Spectrograph by MESSRS. ADAM HILGER, 
LTD. X-ray tube as used for the investigation of crystal structures 
provided by Dr. 0. Shearer. 


Ultra-Violet 15. Clarendon Laboratory, University Museum, 
Rays - Oxford (Prof. F. ' A. Lindenwnn, F.R.S., 

Mr. T. C. Keeley and Mr. E. Bolton King). 

Schumann X-Rays. 

The energy of electrons ejected from metals under the influence 
of radiation is equal initially to the energy of the quantum of the 
radiation concerned. This quantum is simply proportional to the 
frequency, so that red light ejects comparatively slow electrons, 
whereas violet light ejects electrons of twice the energy, or *J% 
times the velocity. If the metal exposed to the light is insulated, 
on losing electrons it acquires a positive charge which attracts the 
electrons and tends to slow them up, and ultimately, when the 
charge is high enough, prevents them from escaping altogether. 
Hence an insulated piece of metal will acquire a charge proportional 
to the frequency of the incident light. 


In this experiment, it is shown that an insulated electrode Ultra^Violet 
connected to an electrometer acquires a charge of some three volts 
when exposed to visible light. When exposed to radiation produced 
by stopping electrons the velocity of which corresponds to a 
potential drop of some hundred volts, the electrode charges up 
to a potential of the same order as that producing the radiation. 
Under the influence of X-rays, the electrode charges up to thousands 
of volts, and the radiation shown in this experiment is therefore 
of a frequency intermediate between X-rays and the ultra-violet. 
This is the region of the spectrum which in ill-informed circles is 
said to include the " death-ray." It will be noted that the whole 
of the experiment has to be carried out in a vacuum, as the slightest 
trace of air entirely absorbs radiation of this frequency. 


16. National Physical Laboratory (Dr. G. W . C. Kaye). 

Fluorescence by Ultra-Violet Light. 

Ultra-violet light possesses the property of causing certain 
materials to fluoresce brilliantly. In the exhibit, the light from a 
quartz mercury -vapour lamp is passed through a sheet of Wood's 
glass which removes the visible radiation. The residual ultra- 
violet light is received by a variety of fluorescent materials. 

The late Sir William Abney's fluorescent tubes exhibited by 
Prof. A. Fowler, F.R.S. 


17. Sir Herbert Jackson, K.H.E., F.R.S. 

Focus of Ultra-Violet Radiations shown on Phosphorescent 

The rays from a condensed spark between aluminium electrodes 
are caused to converge by a quartz lens. A focus of the rays in the 
visible spectrum is obtained on the screen at about two feet from 
the lens. This represents the focus of radiations of wave-lengths 

from about 7000 A.U. to 4000 A.U. By moving the screen nearer 
to the lens a smaller focussed image of the spark is obtained at a 
distance of about 8 in. from the lens. This shows the focus of rays 

of short wave-length, mainly 1860-1850 A.U. On one of the screens 
(zinc silicate) the image is green : on the other screen (zinc 
phosphate) the imago is red. These colours represent the phos- 
phorescent response of the materials of the screen to short invisible 
wave-lengths. The materials of the screen are chosen to give only 
very slight phosphorescent response to the radiations intermediate 
between visible violet and the wave-lengths given above. To such 
wave-lengths the thinnest film of glass is entirely opaque. 


(B 34/2285)Q N 2 



Visible 18. Pro/. L. R. Wilberforce. 

Apparatus for Studying the Motion of Waves. 

The main interest of the experiments shown is in the illustrations 
they furnish of problems in the wave theory of light and of other 
electro-magnetic radiations. 

The waves are produced on the surface of water in a shallow 
trough by dippers of appropriate form attached to an electrically 
maintained tuning-fork. The trough has a glass bottom, a converg- 
ing beam of light is sent upwards through it, passed through a 
convex lens and reflected by a mirror so that an image of the liquid 
surface is formed on a screen. The light is intermittently trans- 
mitted by a slotted disc coupled to a phonic wheel which is driven 
by the current supplied to the fork. The coupling is given by the 
fluid friction of oil between two coaxial cylinders, and its effect is to 
damp out irregularities of motion in the phonic wheel and to give 
the disc a uniform rotation slower than that of the wheel. The 
difference of speed, initially due to air friction, can be increased at 
will be producing a suitable magnetic field to be traversed by the 

The effect of the intermittent illumination is that the waves 
appear stroboscopically to have a motion so slow that their details 
can be readily studied. The speed of this apparent motion can be 
increased if desired by the action of the magnetic field. By the use 
of double dippers the phenomena of interference are shown. The 
formation of shadows, reflection, the production of stationary waves 
and the passage of waves through apertures greater or smaller than 
the wave-length, can be studied by the use of movable partitions. 
The fusion of secondary wavelets into a wave -front and the action 
of the diffraction grating can be illustrated by comb-shaped dippers. 
The convergence of waves to a focus can be produced by a concave 
dipper. The refraction of waves due to change of velocity can be 
demonstrated by producing suitable local variations of depth. 


19. National Physical Laboratory (Mr. J. Guild). 
Projected Spectrum. 

A continuous spectrum, produced by refraction by a prism 
of calcite, is projected on a screen. This is intended to show 

(a) The properties of the visible spectrum : 

(i) The variation of the refractive index of transparent 
material with the wave-length of the radiation is exemplified 
by the fact of the formation of the spectrum instead of a white 
image of the slit. 


(ii) The variation of colour of the sensation produced by Visible 
radiation of different wave-lengths is shown by the sequence Rays. 
red, orange, yellow, green, green-blue, blue, violet, in descend- 
ing order of wave-length. 

(iii) The origin of the colours of materials is shown to be 
due to the fact that the light leaving the coloured material is 
relatively deficient in some parts of the spectrum. By 
interposing various coloured glasses, etc., in the path of the 
beam, the effect on the spectrum which gives rise to the colour 
can be observed. 

(iv) The action of a diffraction grating is illustrated by 
crossing the prism with a coarse grating. The separation of 
the various orders of the diffracted images are seen to depend 
on the wave-length, being approximately twice as great for the 
red end of the visible spectrum as for the blue end. This 
dependence of separation 011 wave-length is the basis of one 
method of measuring wave-length. 

(b) The existence of radiation in parts of the spectrum outside 
the region which gives rise to the sensation of light : 

(i) By inserting a screen coated with zinc sulphide, a 
material which has the property of glowing with a green light 
when radiation of shorter wave-length than visible light falls 
011 it, the presence of such radiation beyond the violet end of 
the visible range (the <k ultra-violet " region), is demonstrated. 

(ii) By means of a thermopile and galvanometer it is shown 
that the energy of radiation 011 being absorbed heats the 
absorbing material. The relative amounts of energy in 
different regions of the spectrum are shown to vary con- 
siderably, and the existence of radiation beyond the red end 
of the spectrum (the " infra-red " region), is demonstrated. 

The Moll Thermopile and Galvanometer by the CAMBRIDGE 
INSTRUMENT CO., LTD., the phosphorescent screen prepared 
by Mr. T. Haigh. 


20. ClaretidoH, Laboratory, University Museum, 
Oxford (Prof. F. A. Lindemann, F.R.S., Mr. 
T. C. Keeley and Mr. E. Bolton King). 

The emission of electrons under the influence of light, the so- 
called photo-electric effect, can be applied to a large number of 
scientific and industrial uses which involve comparison and measure- 
ment of light intensities. It has been shown that the current from 
a properly designed cell is propogional to the light intensity over 
a large range, and the sensitivity of the cell is of the same order 


Visible as that of the eye. The metal used in the cell depends on the wave- 

Bays. length of the light to be measured, the alkali metals being usually 

employed, as they are most sensitive in the neighbourhood of the 

visible spectrum ; the most effective wave-length varies from about 

"3400 A.U. for sodium to 5500 A.U. for caesium. The cell is usually 
filled with an inert gas such as neon, at a small pressure, and an 
accelerating potential applied, so that the current is magnified by the 
ions formed by collision between the accelerated electrons and the 
molecules of the gas. 

In the experiment a photo-electric cell with a suitable accelerat- 
ing potential is connected to a galvanometer. When the light is 
switched on, a current flows through the galvanometer proportional 
to the intensity of illumination. This may be checked by inserting 
between the lamp and the cell a piece of metal gauze which cuts off 
50 per cent, of the light. 


21. Royal Observatory, Edinburgh. 
Photograms of Stellar Spectra. 

These prints are autographic reproductions from spectra taken 
with a prismatic camera -prism, 12 angle, telescope, photo-visual 
object glass of 6 in. aperture and 100 in. focal length. The spectra 
between C and K are about 1 in. in length. The reproduction is 
made by passing the spectrum through a Koch photo-electric 
photometer constructed for the purpose at the Observatory. 
They are chiefly designed for measuring the partition of energy 
throughout the spectrum, for the purpose of reading stellar tem- 
peratures. The measures are made by a method described in 
Monthly Notices Roy. Ast. Soc. 85 (1925), p. 211, which eliminates 
the photographic peculiarities of the plate and other adventitious 

The stars represented are : 

Orion is BO a Cassiopeiae KO 

y Ursae Major is AO Dracoiiis K5 

Aurigae F5 # Pegasi M3 

a Aurigae GO 

The progressive entry and disappearance of the hydrogen scries 
will be noticed, followed by the increasing prominence of metallic 
lines, especially the II and K lines of calcium, and the G band, and 
finally the titanium oxide bands, with a general absorption which 
leaves virtuallv the whole radiation in the red. 


22. National Physical Laboratory (Mr. T. H. visible 

Colour Sensitivity of Photo-Electric Cells. 

Before comparing the candle powers of electric lamps photo- 
electrically it is essential that they should operate at the same 
colour distribution, and in order to obtain the full accuracy of the 
photo-electric method, the colour-matching must be done by a 
method more sensitive than the visual one. A method of colour 
matching has been developed by the staff of the Research Labora- 
tories of the General Electric Company, and is being adopted at the 
National Physical Laboratory as a preliminary to the accurate 
standardisation of electric lamps. A rubidium and a sodium cell 
are connected in series and the photo-electric currents balanced 
against each other when the two cells are exposed to the illumina- 
tion of the same electric lamp. If the temperature of the lamp is 
raised the sodium cell becomes relatively more sensitive and vice 
versa. By this method lamps can be colour-matched to within 
1 K. of their equivalent temperature and within ()] per cent, 

Dolezalek Eleetometer by the CAMBRIDGE INSTRUMENT 
CO., LTD. ; Photo-Electric *Cells by the GENERAL ELECTRIC 
CO., LTD. 


23. Prof. F. Norton, F.R.S., and Dr. Ann C. Davies 

The Excitation of the Spectra of Gases by Electron 

The apparatus consists of a small glass vessel supported between 
the poles of an electromagnet. The vessel contains the gas to be 
experimented upon at a low pressure, usually a few tenths of a 
millimetre. Sealed into the vessel are two parallel, lime-coated 
platinum filaments which are heated electrically, one at a time, and 
supply the electrons for the bombardment of the gas. A short 
distance beneath the two filaments a circular grid of fine mesh 
platinum gauze is situated, while a circular disc of platinum is 
placed parallel to and concentric with the grid at a distance of 
about 1-5 cm. from it. 

The electrons from the filament are accelerated towards the 
grid by means of a potential difference supplied by a battery con- 
nected to these two electrodes outside the apparatus. Some of 
the electrons pass through the interstices of the gauze into the 
space between the grid and the disc, where they collide with gas 
atoms. The production of luminosity results from the recovery 
of the atoms after such impacts. The magnetic field in which the 
apparatus is situated concentrates the luminosity into a bright 


Visible central column between the grid and the disc, and by directing the 

slit of the spectroscope towards this column, and watching for 
changes in the spectrum of the luminosity as the potential difference 
applied between the filament and the grid is gradually increased, 
the voltages necessary for the excitation of different groups of lines 
can be determined. For such an investigation, the grid and the 
disc would be connected together outside the apparatus. 

By employing a second battery so as to accelerate or to retard 
the electrons as they travel between the grid and the disc, the 
luminous column can be made to exhibit marked differences of 
colour along its length because of the different values of the electron 
energy at different distances below the grid. 

Spectroscope by MESSRS. ADAM HILGER, LTD. ; laboratory 
stand by MESSRS. W. G. PYE & CO. ; electrical instruments by 


24. Dr. W. E. Curtis. 

Origin of Spectra. 

When a luminous source is examined with the aid of a spectro- 
scope, which analyses the light into its constituent colours, spreading 
it out into a so-called spectrum, the latter is usually found to be 
more or less complex. Each colour manifests itself as a line in a 
particular position in the spectrum, and there are often hundreds of 
such lines. But the complexity is to some extent relieved by the 
fact that each substance participating in the emission of light gives 
a set of lines which are in perfectly definite positions, so that after 
acquiring sufficient experience of spectra, we can utilise measure- 
ments of the positions of the lines for obtaining information as to 
the chemical composition of the luminous source. Hence the 
original designation of this science, kw spectrum analysis. 1 ' But 
while it is true that a particular line can always be attributed to a 
particular substance and no other, yet one substance may give rise 
to several totally distinct groups of lines, according to the circum- 
stances in which it is rendered luminous. Thus, for example, an 
element may emit a different spectrum according to whether it is 
vapourised in a flame or in an electric arc, or again, supposing it 
to be a gas and to be enclosed in a tube through which a high tension 
electric discharge is passed, the spectrum may be radically altered 
by altering the intensity of the discharge. 

As regards the precise nature of the emission process, although 
the last few years have seen great advances in our knowledge, it is 
as yet far from complete. In particular, it is unable to suggest 
any simple mental picture of the phenomenon ; there is not even 
any familiar analogy which would help to render an explanation 
intelligible. All that one can say is that the emission of a spectrum 
line appears to be the result of an abrupt change in the configura- 
tion of the atom or molecule. In the former case this change 


consists simply in the transference of an electron from one orbit Visible 
to another. In the latter case the change affects not only the Rays. 
electrons but also the speed of rotation of the molecule and the 
magnitude of the vibrations of the atomic nuclei from which it is 
built up. The molecule therefore gives a more complex spectrum 
than the atom ; there are many more lines, and further, they are 
arranged in a different manner. We may thus broadly divide 
spectra into two classes so-called line spectra, which are due to 
atoms, and band xpectra, which are due to molecules. Several of 
the points touched on above are illustrated in the demonstration 
of the line and band spectra of nitrogen. 

Line and Band Spectra of Nitrogen. 

The discharge tube contains nitrogen at low pressure ; this 
is made to glow by passing through it an electric current from an 
induction coil. By connecting a condenser in circuit a much more 
powerful discharge may be produced. It will be seen that 

(1) The <k uncondenscd " discharge gives a glow of reddish 
colour, which when analysed by the spectroscope is found to 
consist of a number of regularly spaced " bands " (which are 
actually composed of numerous lines very close together) 
in the red, yellow and green, and several in the blue, much 
farther apart. 

(2) The condensed discharge presents quite a different 
appearance and is bluish in colour. The spectroscope shows 
that this is because the bands have disappeared and have been 
replaced by lines which exhibit no obvious regularity of 

The explanation is as follows : 

In (1) the nitrogen is in its normal state i.e., the atoms are 
associated in pairs or molecules, and a molecule always gives 
a band spectrum. 

In (2) the powerful discharge has broken up these molecules 
into their component atoms, and an atom necessarily gives a 
line spectrum. 

Induction coil by the COX CAVKND1SH ELECTRICAL 
CO. (1924), LTD* ; spectroscope by MESSRS. ADAM 


25. Prof. A. Voider, F.R.S. 
Types of Spectra (Visible Rays). 

(i) Band Spectrum. 

The appearance of a band spectrum is illustrated by the bands of 
calcium fluoride, as produced in an arc between commercial " flame 
carbons " which are charged with this compound. It should be 


Visible noted that the calcium fluoride is partially dissociated in the arc, 

so that lines originating in atoms of calcium appear in addition to 
bands due to molecules of the undissociated compound. 

(ii) Line Spectrum. 

(a) Lines of iron appearing in the spectrum of an arc between 
two rods of iron. 

(6) Lines of helium and neon appearing in the spectrum of the 
electric discharge through " vacuum tubes " containing these gases 
at low pressures. 

(iii) The Solar Spectrum. 

The spectrum of sunlight consists of dark lines (the " Fraunhofer 
lines ") on a background of continuous spectrum. These lines are 
due to absorption by the luminous gases and metallic vapours 
surrounding the bright interior of the sun, which, if it could be 
observed alone, would yield a continuous spectrum. The dark 
lines occupy the same positions as the bright lines emitted by the 
gases and vapours themselves. The experimental arrangement 
shows the coincidence of the dark " D " lines of the solar spectrum 
with the bright yellow lines of a sodium flame, from which it is 
deduced that sodium is a constituent of the sun's atmosphere. 

Apparatus for demonstrating the spectra of helium and neon 


26. Dominion Astropliysical Observatory, British 
Columbia (Dr. J. S. Plaskett, F.R.S.j. 

Astronomical Photographs, 
(i) Star Cluster in Herculis. 

Enlarged 4-5 diameters from the original negative, which was 
exposed for 60 minutes on Seed 30 plate on the 72-in. reflector. 
Distance of cluster, some 30,000 light-years. 

(ii) Ring Nebula in Lyra. 

Enlargement 8 diameters from original negative which was 
exposed for 25 minutes on Seed 23 plate on 72-in. reflector. Pro- 
bable distance of Nebula, 800 light-years. 

(iii) Absorption Spectra of Q-Type. 

Examples of spectra, enlarged 14 diameters, of the 0-type stars, 
the hottest, brightest and most massive of the stars. Example of 
spectral subdivisions according to classification of H. H. Plaskett. 

(iv) Emission, Wolf Rayet Spectra of Q-Type. 

Examples of spectra, enlarged 14 diameters, of the Wolf Rayet 
stars arranged in order of decreasing excitation. 

27. Mr. F. Twyman, F.R.8. 

Michelson Interferometer ( Wave-Length Measurement). 

The Michelson interferometer illustrates the principle of the 
apparatus whereby Michelson carried out, in 1892 and 1893, his 
measurements of the wave-length of the cadmium radiation in terms 
of the metre (Trav. et Mem. dn Bureau International des Poids et 
Mesures, 11, 1895) ; and the optical system is very much the same 
as that used by Michelson and Morley in their classical attempt to 
find an effect of the earth's velocity on the velocity of light. 

The instrument is arranged to give a demonstration of the inter- 
ference ring system seen with a neon lamp. When one ring is 
replaced by another of the same colour, it shows that the moveable 
mirror has traversed a distance of approximately 00001 in. 

It is obvious, therefore, that by counting the rings, one can 
effect any desired change of position of the moveable mirror. 
Michelson elaborated, however, an ingenious routine whereby the 
bands only required to be counted over a length of 10 cm. The 
principles of his method, particulars of which are too long to be given 
here, will be found in his book fcfc Light Waves and their Uses." 


28. Mr. William Gamble. 

The Lippmaim Interference Process of Colour Photography. 

The Lippmami process, invented by the late Prof. Gabriel 
Lippmann, of Paris, is based on the theory first propounded by 
Zcnkcr of stationary light waves in the phenomena of interference. 
He obtained within the thickness of the photographic film a record 
of colour waves in the form of a series of extremely thin layers of 
silver deposit, separated by equally thin layers of no deposit 
merely the clear layers of gelatine in which the silver had been 
emulsified. When these layers are looked at in light falling 011 them 
at a suitable angle, they show colours just as in a soap bubble, 
owing to the light reflected from one layer interfering with that 
reflected from the next. 


(v) Spectra of 5 1267 Variable Spectrum. Visible 

Spectra, enlarged 10 diameters, of this remarkable star 
showing changes of spectrum on 4 different dates. Type B3e on 
October 9th to A2ep on March 25th, and back to B5e on April 10th. 
Spectra of P Cygni and a Cygni above and below. 

(vi) Q-Type Spectra in the Ultra-violet. 

Spectra obtained with Hilger ultra-violet spectrograph attached 
to 72-in. reflector, enlarged 12 diameters. Emmission spectra 
arranged in order of excitation. 


Visible To produce photographs in colour Lippmann prepared plates 

Rays. with an extremely thin film of silver emulsion and backed them in 

the dark slide of the camera with a film of mercury which acted 
as a mirror, causing the light coming through the lens and passing 
through the transparent film of the plate to be reflected back on 
itself. The effect was that where the reflected ray and the incident 
ray reinforced one another at the points at which the crests of both 
their waves coincided, there would be the greatest amount of light 
action, and where they opposed one another there would be dark- 
ness consequently 110 photographic action. The result was that 
the film on the plate consisted of light and dark layers separated by 
distances proportional to their wave-lengths. For example, the 
parts of the film thus acted upon, corresponding to the red parts 
of the object photographed, being formed by the red rays, would 
have their layers farther apart than those corresponding to the 
violet parts formed by the shorter waves, other colours being 
rendered by intermediate layers. The fact that such layers actually 
existed has been proved by cutting a section of the film and 
reproducing it by photomicrography, thus distinctly showing the 
layers in dark bands. Looked at by transmitted light the plates 
have no definite colours, being grey like an ordinary photographic 
negative, but with a reddish tinge, but when examined under light 
falling at a suitable angle, and with a black background under the 
plate, the natural colours of the object are seen most brilliantly, 
as the specimens show. 

The examples shown by Mr. (Gamble include one, a reproduc- 
tion of an old print, prepared by Prof. (T. Lippmann. Others have 
been kindly lent by Prof. A. Fowler, K.R.S., Mr. T. Khein berg and 
Mr. E. Senior. 

29. Prof. A. 0. Rank! ne. 

(A) DiffractioTi of Light : Effects in the. Shadow of a 
Spherical Obstacle. 

In the centre of the shadow of a truly spherical object formed 
by a point source of light, a luminous spot is seen. This is due to 
the fact the light waves curling round the edges of the sphere reach 
the central spot by equal paths, and are thus in a condition to 
reinforce one another, so that the combined effect is great. Else- 
where in the shadow the paths are not equal for all the diffracted 
waves, and mutual interference produces practical extinction. 

If the source is not strictly a point, as is impossible in practice, 
each point of the source produces a corresponding luminous spot 
in the shadow, with the result that there appears in the shadow 
an image of the whole source having the same shape as the source, 
but inverted. This effect was first noticed by Prof. A. W. Porter. 
The spherical obstacle thus behaves like a lens, the relation between 
the size of the image and tho size of the source being the same as for 
a lens. The amount of light available is, of course, very small. 


The exhibit demonstrates the effects for sources of various shapes Visible 
such as a triangle, a square and a half-moon, and the images are Rays. 
seen on a ground glass screen. Photographs taken with the 
opaque ball mounted on a thin sheet of mica as an equivalent 
lens are also shown. The spherical object used is a ball such as is 
incorporated in ball-bearings, these balls being remarkably spherical. 
True sphericity is an essential condition for the success of the 


(B) Interference of Light : Newton's Rings. 

Newton noticed rings like those in the exhibit, before the wave 
theory of light, upon which their explanation depends, came to bo 
generally accepted. The rings become visible when the interface 
between, a flat glass plate and a slightly curved plate in contact 
with it is viewed by reflected (or transmitted) parallel light. The 
essential point is that the two surfaces should make very small 
angles with one another, so that they gradually separate as one passes 
away from the point of contact. The light undergoes partial 
reflection at both surfaces, and the light reaching the eye from a 
given point of incidence traverses different distances according to 
the amount of separation of the surfaces at the point in question. 
If this path difference is such that the waves are out of step by half 
a wave-length (or any odd number of half wave-lengths) destructive 
interference takes place, and the light of that particular wave-length 
is extinguished. Where, on the other hand, the difference of path 
is an even number of half wave-lengths, maximum light is seen. 
Thus one gets successive rings of luminosity and darkness in any 
particular colour ; but since different colours have different wave- 
lengths, extinction does not take place in the same places for all. 
Viewed in white light coloured rings are therefore seen. 

The effects are more striking if a single colour (or wave-length) 
is used. In the exhibit the source of light is the practically mono- 
chromatic radiation emitted by a sodium salt in a Bunsen flame. 
Many more rings are seen under these conditions than in white 

An additional exhibit consists of two strips of plate glass clamped 
together at one end and separated by a very thin distance piece 
at the other. The interference fringes, which arise in the same way 
as already indicated, are in this case practically straight, extending 
across the strips. The necessity for such small distances of separa- 
tion is due to the minute length of the light waves, about 0-000059 
cm. for the sodium light. 



Visible 30. National Physical Laboratory (Mr. J. S. Clark). 
Diffraction Gratings. 

A diffraction grating consists of a very large number of equidis- 
tant and parallel straight lines, ruled by means of a diamond on a 
suitable plate of material, which may be glass, speculum, or gold, 
etc., according to the purpose for which the grating is intended. 
The ruling is accomplished by means of a very precise dividing 
engine, and the success of the grating depends on the parallelism 
and accuracy of spacing of the lines, and on the precision of form 
of the surface on which the lines (or grooves) are ruled. 

The diffraction grating is used for analysing radiation (heat 
rays of long wave-length, visible and ultra-violet rays). Like 
a prism, it has the effect of separating the various wave-lengths 
falling upon it, the shorter being reflected at different angles from 
the longer, the whole of the reflected (or diffracted) band of wave- 
lengths constituting a spectrum. 

(A) Plane Grating for General Work throughout the 
Visible and Ultra- Violet Regions of the Spectrum. 

14,400 lines to the inch, ruled on a plane speculum blank. 

The form of the ruled groove is such that all the lower orders 
(1st to 4th) of spectra are fairly bright for general use, but the 3rd 
order on one side of the normal is specially bright, so that the 
grating may be used in that order to obtain accurate wave-lengths 
using a known comparison spectrum. 

(B) Concave Grating for General Work. 

Similar to (A), but ruled on a concave speculum blank of 
3 metres radius of curvature. The concave form does not require 
a lens to focus the spectra. 

(C) Special Grating ruled on Gilt Brass, for Infra-red 

A plane grating of 2,400 lines to the inch ruled in gold deposited 
on a flat brass plate. The form of the ruled grooves is such that 
practically the whole of the original surface is removed by ruling, 
and the major portion of the incident light falling normally on the 
grating is diffracted into a limited region at an angle of about 15 
on one side of the normal, corresponding to the 5th order spectrum 
of wave-length 5461 A.U., or the 1st order of wave-length 2-7 /x. 

(D) Concave Grating of 28,800 lines to the inch. 

Ruled on a concave speculum plate of 1-5 metres radius of 
curvature, with twice the normal number of lines per inch. For 
use when high resolving powrr is required, and for the study of^the 
shorter wave-lengths. 


(E) Concave Grating of 2 metres Radius of Curvature. Visible 


Gratings of this size and type are also ruled on blanks of 1 and 

1 -5 metres radius of curvature. 

The gratings are ruled at the NATIONAL PHYSICAL 
LABORATORY on the ruling engine constructed by the late 
Lord Blythswood, using blanks supplied by MESSRS. ADAM 

31. Prof. Frederic J. Cheshire, C.B.E. 

Double Refraction. 

Many transparent bodies, more especially crystalline ones, have 
what may be looked upon as optical grain, and as a consequence the 
ether waves which constitute light are transmitted by them more 
easily in some directions than in others. Double refraction results ; 
that is, a beam of plane-polarised white light passing into such a 
crystal, in general, breaks up into two beams, which travel with 
different velocities, and in different directions, the vibrations in one 
beam taking place in a direction at right angles to those in the other. 
If these two beams be ultimately brought together again with their 
vibrations parallel to one another as they are by an anatyser 
interference takes place, with the result that certain wave-lengths, 
or colours, are eliminated. Jf the linear retardation of one beam 
with respect to the other be equal to r, then between crossed nicols 
a series of wave-lengths equal in succession to 2r/l, 2r/3, 2r/5, etc., 
will be eliminated, whilst between parallel nicols the series eliminated 
will be 2r/2, 2r/4 ? 2/-/0, etc. 

In the examination of thin rock sections in polarised light, the 
different minerals which occur in the section have practically the 
same thickness, but the orientation of the direction of the grain and 
the refractive powers of the different minerals differ ; hence the 
resultant retardations are different and the colours resulting 

(A) Spectro-Polariscope. 

An apparatus for showing what happens to light in its passage 
through a bi-refracting substance, such as a cleavage lamina of 
selenite or mica. The apparatus is essentially a combination of a 
spectroscope and a polariscope. Light, after passing through a bi- 
refracting lamina between two nicol prisms, as in the ordinary 
polariscope, is decomposed spectroscopically by a direct-vision 
prism, so that a spectrum is projected upon the retina instead of an 
image of the object itself. In this way, the series of wave-lengths 
eliminated by interference, as explained above, can be seen as dark 
absorption bands occurring at intervals from one end of the spectrum 
to the other. The use of a double-image prism, correctly oriented 
as an analyser, gives two partially overlapping, vertically superposed 


Visible spectra corresponding to those given by crossed and parallel 

nicols respectively. Thus the dark bands in one spectrum occur 
opposite to bright bands in the other and, when the analyser is 
rotated through 90, the dark and bright bands in each of the spectra 
exchange places. 

The apparatus shown consists of a microscope stand, with a 
sub-stage polariser and a slit in place of the usual objective. This 
slit is collimated by a lens in the draw tube, above which is mounted 
a direct-vision prism, and above this again a double-image prism 
giving a small angular separation of the two images. The bi- 
rcfracting lamina is placed close against the slit. By this arrange- 
ment very sharp pictures can be obtained from rough cleavage 
lamina directly. Optical surfaces are no longer necessary. 


(B) Projection Polariscope. 

Simple apparatus for demonstrating the application of polarised 
light to the differentiation and identification of minerals in sections 
of rocks, etc. The apparatus consists essentially of a low power 
projection microscope in which the object is illuminated with plane 
polarised light a nicol prism polariser being mounted between 
the light source and the condensing lens, and the projection lens 
being combined with a second nicol prism to act as a polariser. 
Maximum brightness of a large angular field of view is secured by 
bringing the whole of the light which passes through the object to 
a focus in the projection lens itself. In this way a small arc 
can be made to give a picture large and bright enough for class 


32. Sir Herbert , Jackson, K.B.E., F.R.S., and 
Mr. W. D. Haigh. 

Selective Absorption in the Visible Spectrum. 

The example shown is a striking instance of selective absorption 
by a special glass rich in didymium oxides. 

The spectroscope by MESSRS. ADAM HiLGER, LTD. 



33. Mr. F. Tivuman, F.R.S. Infra-Red 

y Rays. 

Infra-Ked Spectrometer (Wave-length Measurement). 

The instrument is used for measuring the wave-lengths of radia- 
tions in the infra-red. The radiation, which enters the instrument 
by a fine slit, as in an ordinary spectroscope, is dispersed into a 
spectrum by means of a prism of rocksalt, the spectrum being 
focussed 011 a thermopile. A galvanometer connected to the 
thermopile shows the intensity of the part of the spectrum under 
examination. The spectrum can be traversed across the thermopile 
by means of a fine screw, and the whole spectrum thus explored 

from 5,000 to 100,000 A.U. (the visible part of the spectrum 


terminates at about 8,000 A.U.). A bunsen burner emits a 

strong infra-red radiation in the neighbourhood of 44,000 A.U.. 
and this can be demonstrated on the instrument exhibited. 

A photograph in the near infra-red, taken by the late Sir William 
Abney, kindly lent by Prof. A. Fowler, F.R.S. 


34. Clarendon Laboratory, University Museum, 
Oxford (Prof. F. A. Lindemann, F.R.S., Mr. T. 
C. Keeley and Mr. E. Bolton King.). 

Photo-electric Effect in the Infra-Red. The Caesium Cell. 

Caesium, which is characterised by the fact that its atom has 
one electron on an extremely eccentric orbit, is photo -electrically 
sensitive even 111 the infra-red, since this electron can be ejected 
from its orbit by the comparatively small quantum corresponding 
to low frequency radiation. Ebonite is transparent in the infra-red^ 
and in this experiment the photo-electric current produced by 
radiation, from which all the visible light has been cut off by a thin 
sheet of ebonite, is shown by the charge it gives to an electrometer. 
The charge can leak away from the electrometer through a high 
resistance, and the potential of the electrometer at which this 
leak balances the inflowing electricity is a measure of the magnitude 
of this current. 


(B 34/2285)Q 




34A. Sir Robert Robertson, F.R.S., and Dr. J. J. Fox. 

Selective Absorption in the Infra-Red. 

Many substances exhibit characteristic selective absorption 
in the near infra-red region of the spectrum (up to about 16ji). 
Carbon dioxide gas, for example, possesses infra-red absorption 
bands. The diagram shows the percentage transmission of carbon 
dioxide gas at H atmospheres pressure, drawn from observations 
made by Sir R. Robertson, F.R.S., and Dr. J. J. Fox. The abscissa? 
represent wave-lengths, and the ordinates percentage transmission. 
The deep depressions in the curve of transmission are the bands, 
which are clearly seen at 11,000, 20,000, 27,700 and 43,200 A.U. 





35. Mr. F. E. Smith, F.R.S. 

The Production of very short Hertzian Waves and Demon- 
stration of their Heating Effect. 


36. Sir William Bragg, K.B.E., F.R.S. 

Lindman's Apparatus designed to illustrate the Mechanism 
of Optical Activity (Rotatory Polarization) by Means of 
Electromagnetic Waves : the Apparatus can also be 
used to demonstrate other Polarization Phenomena. 

It is known that optical activity is due to a special arrangement 
of the atoms in the molecule or the crystal. There is a screw which 
may either be right- or left-handed. In quartz and certain other 
crystals there are actually continuous spirals, running parallel 
to the crystal axis : in other substances the spirals are broken into 
fragments, and a very common arrangement is that of four carbon 
atoms placed at the four corners of a parallelopiped, the corners 
being chosen so that no two lie one side. Four such corners 
form, so to speak, one turn of a screw which may be either right- or 
left-handed. It is the screw-like arrangement which causes the 
rotation of the plane of polarisation of light when passing through 
the substance. 

Lindman has illustrated these effects by the use of electromag- 
netic waves, which, though very short of their kind, are many 
thousands of times longer than the waves of light. The screw 
arrang ment that gives optical activity is replaced by actual metal 
spirals or by arrangements of metal spheres. 




37. National Physical Laboratory (Mr. D. W. Dye). 
Determination of Frequency and Wave Form. 

The measurement of frequency is of fundamental importance 
in wireless telegraphy. There are many methods by which this 
measurement can be made, and these are of two kinds: (1) abso- 
lute measurements, (2) secondary measurements. The absolute 
measurements are those in which the wireless frequency is referred 
more or less directly to a standard of time. The secondary methods 
of measurement include all those instruments and devices known 
as wave -meters. 

Of the primary or absolute methods of measurements, those 
which have been developed in recent years to a high degree of 
precision may be termed harmonic methods. The principle under- 
lying a harmonic method of measuring frequency is to correlate 
the unknown wireless frequency with a known higher or lower 
frequency in such a manner that the ratio of the two frequencies 
is an integer. If the integer is known -it may be such a number 
as 12, 50, etc. then the wireless frequency under measurement is 

A primary method of this kind consists in superposing upon 
a time trace of known telephonic frequency, a displacement pro- 
duced by the wireless frequency. The frequency of the wireless 
source is then smoothly and accurately adjusted until its value 
is an integral multiple of the telephonic frequency time trace. If 
the time trace takes the form of a circle or ellipse of considerable 
magnitude, Avhilst at the same time the wireless frequency is caused 
to produce a circular displacement of the agent producing the time 
trace, the result will be a closed looped figure when the two fre- 
quencies are harmonic to one another. By counting round or 
photographing the loops the wireless frequency becomes determined. 

In the apparatus shown, a very steadily vibrating tuning-fork, 
having a frequency of 1,OOO vibrations per second, produces, by the 
aid of a valve amplifier, a circularly rotating ray in a cathode ray 
tube. This circle forms the time base. The source of wireless 
frequency oscillations is caused to operate on the ray in such a 
manner that, in the absence of the large circular movement, a 
small circular movement of the ray is produced at the wireless 
frequency. By carefully adjusting the wireless frequency, the light 
spot in performing its looped journey round the time circle can 
be caused to arrive at exactly the point from which it started. 
When this occurs the light spot continually retraces the same path, 
forming a stationary looped figure. If the two circular motions 
of the light spot are in the same direction, the ratio of the frequencies 
is equal to the number of loops minus one. 

The setting of the wireless frequency to a known value consists 
therefore in producing a stationary looped pattern, counting the 
number of loops and adding one. In the case, for example, of a 
tuning-fork frequency of 1,000 cycles per second and a number of 
internal loops of 24, the wireless ffequency is exactly 25,000 cycles 
per second. 

(B 34-2285)Q o 2 , 


Wireless By suitable means the time trace can be made a long ellipse. 

Wftves. If the wireless displacement is adjusted to be a straight line perpen- 

dicular to the long axis of the ellipse, the wave form can be shown 
and examined. 

Apparatus used in this and other exhibits by the National 
Physical Laboratory in the Wireless group by the following 
firms : 


Unipivot Galvanometers, CAMBRIDGE INSTRUMENT 
CO., LTD. 

Condensers, DUBILIER CONDENSER CO. (1921), LTD. 

38. Prof. R. WhidAinyton. F.R.S. 
Standing Electric Waves on "Wires. 

Any wireless aerial provides an example of the production of a 
stationary electric wave, but in the usual case only part of one 
wave is formed. 

The best known arrangement for producing a series of such 
waves was due to Lecher. A pair of parallel wires were stretched 
taut and made to oscillate by association with a neighbouring closed 
circuit excited by spark gap and transformer. The presented 
arrangement is simply Lecher's system making use of continuous 
oscillations sustained by valves. The lengths of the wire have 
been suitably chosen for resonance and the standing waves are 
detected by bridging across the wires with a small flash lamp. The 
wave-length is about 1 metre. 


39. National Physical Laboratory (Dr. R. L. Smith- 
Wireless Waves. Directional Effects. 

The electric and magnetic fields accompanying a simple wireless 
wave are at right angles to each other and also perpendicular to 
the direction of travel of the wave. If, therefore, the direction of 
both the electric and the magnetic fields can be ascertained, the 
direction in which the waye is travelling becomes known. In 
the case of the transmission of waves for short distances over 


the earth's surface, the electric field is nearly vertical and the Wireless 
magnetic field nearly horizontal. Making use of these facts the Waves* 
practical types of wireless direction-finders ascertain the direction 
of the source of transmitted waves by finding the direction of the 
magnetic field in a horizontal plane. 

If a closed coil is placed with its plane vertical iri the path of the 
waves, an electromotive force will be induced therein, the magnitude 
of which is directly proportional to the sine of the angle between 
the magnetic field and the plane of the coil. Thus when the coil 
is parallel to the magnetic field the induced E.M.F. is zero, and 
when it is perpendicular to the field the resulting E.M.F. is a 

The apparatus demonstrates the principle of such a rotating 
closed coil direction-finder. On the right is shown a small valve 
oscillator generating oscillations at a frequency typical of that 
employed at a modern wireless transmitting station. At the left 
is a vertical closed coil which can be rotated about a vertical axis. 
When the coil is set so that its plane is perpendicular to the magnetic 
field from the oscillator, the current resulting from the E.M.F. 
induced in the coil causes a movement on the galvanometer used for 
detecting this current. As the coil is rotated the reading on the 
galvanometer steadily decreases, until it becomes zero when the 
coil has been turned through a right angle and its plane is parallel 
to the magnetic field. 

In the practical use of a direction-finder, the magnitude of the 
current in the coil is indicated by the intensity of the signal heard 
in telephone receivers. It is then found possible to determine the 
position of the coil in which the signal passes through its zero 
intensity much more accurately than the position in which the 
signal is a maximum. Operating on this principle, such apparatus 
enables the direction of incoming wireless signals to be determined 
to within about 1 J under the most favourable conditions, and 
such direction-finders are of considerable use as aids to modern 
navigation both on sea and in the air. 


40. National Physical Laboratory (Dr. R. L. Smith- 
Wireless Waves : Heating Effect. 

The heating effect of the currents induced in a coil by a 
wireless wave is demonstrated by the inclusion of a small glow lamp 
in the circuit of a small receiving coil. When this coil is placed 
in the field of the wireless valve oscillator, the resulting current 
causes the filament of the lamp to be heated. If this current is 
sufficient the filament becomes incandescent, first, at a red and then 
at a white heat, the magnitude of the current being approximately 
indicated by the temperature of the filament. 



Wiretesj 41. National Physical Laboratory (Dr. R. L. Smith- 
Ww ~ \Rose). 

Wireless Waves : Rectification by Crystals. 

The electric and magnetic fields of a wireless wave are of an 
oscillatory or alternating nature, and the resultant current induced 
in any wireless receiver is exactly similar. Such alternating currents 
are unable to cause a deflexion on the simple type of direct current 
galvanometer. When, however, the receiving circuit includes a 
crystal detector, the current flowing is partially rectified, and this 
unidirectional component may be detected by the galvanometer. 

In the apparatus shown the presence of the alternating E.M.F. 
induced in the receiving coil is made known by connecting the 
coil to a crystal detector and a direct current galvanometer. A 
deflexion on the galvanometer is obtained, the magnitude of which 
bears a simple relation to the current received in the coil. When the 
crystal is short-circuited by a switch, the galvanometer deflexion 
disappears, due to the inability of the alternating current in the 
coil to operate this galvanometer. 


42. National Physical Laboratory (Mr. D. W. Dye). 
Wireless Wave-meter showing Resonance. 

The arrangement consists of a simple oscillatory circuit con- 
taining an inductance and a variable condenser. The inductance 
coil of the wave- meter can be coupled to a valve oscillator. When 
the variable condenser is adjusted to cause the frequency of the 
circuit of the wave-meter to resonate to that of the source, a large 
current is induced in the circuit and a considerable voltage occurs 
at the terminals of the condenser and the inductance. 

This condition of resonance may be shown in a variety of ways. 

1. Heating of a wire in series in the oscillatory circuit. This 
may take the following forms : 

(a) Hot wire milliammeter. 

(6) Heater, ther mo junction combination and a galvano- 

(c) A small incandescent lamp. 

2. The voltage rise on the terminals of the condenser may be 
shown by the following means : 

(a) Electrostatic voltmeter or electrometer. 
(6) Vacuum tube. 

(c) Crystal or valve tectifier and galvanometer. 

(d) A thermionic amplifier and galvanometer. 


3. An independent detecting circuit may be used coupled to Wireless 
the inductance coil of the wave-meter circuit. This method pos- Waves. 
sesses several advantages over the other two methods and is to be 
strongly recommended where possible. The advantages are 

(i) The damping of the wave-meter can usually be kept 
smaller and hence the tuning is sharper. 

(ii) Freedom to choose the proportions of the independent 
circuit to suit the detecting device used. 

(iii) The detecting device may be changed from one kind 
to another to suit requirements or the convenience of what is 
available without altering the calibration of the wave- meter. 

The disadvantage of the method is that slightly more energy 
is consumed from the source for a given detector when this detector 
is that best suited to the wave-meter when used in methods 1 or 2. 
This, however, is of small consequence in most cases. 


43. National Physical Laboratory (Mr. D. W. Dye). 
Delineation of Damped Radio Waves. 

If a resonant electric circuit has a current suddenly started in ifc 
arid is then left free to oscillate, the resulting oscillations will 
gradually die down to zero. There may be only a few oscillations 
or there may be a few hundred before they are reduced sensibly to 
zero. The rate at which the decay of amplitude occurs depends 
upon the resistance of the circuit in relation to the inductance and 
the capacity. Examples of damped oscillations are those provided 
by spark and buzzer excitation of a circuit. These methods, how- 
ever, whilst very commonly used in practice, arc riot sufficiently 
steady to show on the screen of an oscillograph, where it is necessary 
to repeat several hundred successive trains of such oscillations at 
exactly equal intervals of time. 

Tn order to show the oscillations satisfactorily on. the elliptical 
time trace given by the oscillograph, it is necessary to give impacts 
or electrical blows to the oscillating circuit at such a rate as to be 
'equal to a multiple of the frequency of the tuning- fork providing the 
time trace. The tuning-fork itself cannot be used directly to 
deliver the electrical blow to the oscillatory circuit, since the 
former has a current wave which is smooth and free from dis- 

An arrangement known as a mult i- vibrator is therefore employed. 
The action of this is somewhat complicated and difficult to follow 
and will not be described here. It will suffice to state that it 
supplies to an inducing coil a small current, the wave form of 
which possesses an almost perfect discontinuity. The current 
changes almost instantaneously from one value to another, thus 
producing what amounts to an electrical blow in the resonant 
circuit coupled to the inducing coil. The rate at which the blows 
are produced can be varied within wide limits by adjustment of 
the variable condensers of the mult i- vibrator. 


Wifeless I n the present arrangement the frequency of the multi- vibrator 

Waves. is adjusted to 1,000 cycles per second, and this frequency is held 

exactly constant by the help of a small voltage obtained from the 
tuning-fork. The multi-vibrator delivers two electrical blows to 
the oscillatory circuit at each alternation of its operation. There 
will therefore be two trains of damped oscillations started in the 
oscillatory circuit at each revolution of the light spot of the cathode- 
ray tube. Each of the two damped trains of waves will commence 
at an invariable point on the time trace. The resulting wave 
traces will therefore remain stationary on the screen. 

If the multi-vibrator is freed from the control of the tuning-fork, 
and is then adjusted in frequency to be very slightly different from 
that of the fork, the resulting trains of damped waves will progress 
or retrogress round the ellipse. The peculiar phenomena occurring 
in two coupled resonant circuits of slightly different frequencies 
can also be well shown. 


44. National Physical Laboratory (Mr. D. W . Dye). 
Interference between Two Radio Frequency Waves. 

The cathode ray tube is arranged to show wave form at radio 
frequency. (See Exhibit 37.) 

Two wave forms from two independent valve oscillators are 
simultaneously projected upon the screen of the oscillograph. If 
the two frequencies are adjusted so that each is an integral multiple 
of the tuning-fork frequency, they will interfere with a difference 
frequency which is also an integral multiple of the fork frequency. 
The pattern produced will therefore be stationary and hence visible. 
By making the amplitudes of the two waves equal, the combined 
wave will swell up to a maximum and then die clown to zero as the 
waves first reinforce and then oppose one another. If the two 
waves are unequal, there is a swelling-up and dying-down of the 
wave of a kind very similar to that produced by modulation at the 
frequency equal to the difference between the two radio frequencies. 


45. Research Laboratories of the General Electric 

Company, Ltd. 
Anode of Valves Eed Hot by Bombardment of Electrons. 

Bombardment of the anode of a two-electrode valve by electrons 
is shown. The number of electrons can be controlled by the filament 

Two valves are shown, the first a valve of normal design with 
nickel anode, the second with an anode consisting of a disc of 
tungsten which can be heated to much higher temperatures than 
the nickel anode. 



46. Research Laboratories of the General Electric Wireless 
Company, Ltd. Waves ' 

Change in Temperature Distribution along an Emitting 

Filament (Illustration of Thermionic Emission). 

A long thin filament surrounded by an anode of grid form. The 
filament is heated by direct current and is normally of uniform 
temperature. When a D.C. voltage is applied between filament 
and anode, and an emission current is taken from the filament, the 
temperature at the negative end increases visibly and at the positive 
end decreases. 


47. Research Laboratories of the General Electric 

Company, Ltd. 
A Method of Reducing Temperature Change resulting from 

Thermionic Emission. 

The effect shown in Exhibit 46 may have serious consequences 
on the life of a filament. 

A rectifier is shown rectifying alternating current and the 
filament is heated from the same source, but the phase of the 
filament current can be changed by means of a phase-shifting trans- 
former. When the filament current is in phase with the anode 
volts, one end of the filament is always negative when emission 
current is flowing and consequently is overheated. When the phase 
Js shifted 90 D this overheating disappears, for now each end of the 
filament is equally negative and positive when the emission current 
is flowing. On shifting the phase another 90 the other end of the 
filament becomes overheated. 


48. Research Laboratories of the General Electric 

Company, Ltd. 
Wehnelt Cathode (Illustration of Thermionic Emission). 

(a) The thermionic valve depends on the fact that at high 
temperatures electrons are emitted from all substances. A tungsten 
filament is shown mounted in vacuum surrounded by a metal anode. 
A high voltage is applied between the filament and anode sufficient 
to drag across all electrons emitted from the cathode. 

The electron current to the anode is indicated by a milliammeter 
and its variation with filament temperature can be observed. 

(b] A modern development of the original Wehnelt cathode is 
shown. A cylinder of nickel, coated with mixed oxides of the 
alkaline earths, is heated to a dull red by radiation from an enclosed 
tungsten filament. 



Wireless 49. Research Laboratories of the General Electric 
Wav8S * Company. Ltd. 

Characteristic of Valve. 

The static characteristic of a three electrode valve is shown 
by means of a cathode ray oscillograph. The filament current can 
be varied and the consequent changes of characteristic observed. 


50. Dr. J. A. Fleming, F.R.S. 

The Origin and Development of the Thermionic Valve in 
Wireless Telegraphy and Telephony. 

The thermionic valve has been developed in the last twenty 
years by the work of numerous inventors out of a pioneer inven- 
tion made in 1904 by Fleming, which was the first technical appli- 
cation of purely scientific researches on the emission of negative 
electricity from hot bodies. The thermionic valve has given us 
the most sensitive appliance yet discovered for the detection of 
electric waves and also one of great utility for generating the waves 
used in wireless telegraphy and telephony. 

It consists essentially of a form of incandescent electric lamp 
in which a wire of some material, generally tungsten, is rendered 
incandescent by an electric current, this wire being enclosed in a 
glass or silica bulb from which the air is most completely exhausted. 
Around the filament is fixed a cylinder of metal called the anode, 
which is connected to the wire sealed through the bulb wall. When 
the filament is hot it emits torrents of electrons or particles of 
negative electricity, and these are caught on the anode. In this 
simple form it is called a i; rectifying valve " because it has the 
property of permitting (negative) electricity to be conveyed only in 
one direction through the vacuous space from filament to anode. 
Hence it is used to convert alternating or oscillating electric currents 
into direct or unidirectional currents. It was first introduced in 
this form as a rectifier and detector of the feeble electric oscillations 
in the receiving circuit of a wireless telegraph apparatus. It is now 
manufactured in very large sizes, as shown in the exhibit, for the 
purpose of rectifying large electric currents. 

An improvement was made in it in 1907 by Lee de Forest by the 
introduction of a grid or zig-zag of wire between the anode and 
filament. This grid now takes the form of a spiral of wire or a 
cylinder of metallic gauze. In this so-called three-electrode form, 
the valve can be used to amplify or magnify electric currents and 
also to generate electric oscillations. Samples of large generating 
valves are shown in the exhibit. 

Another important improvement was the introduction of metallic 
thorium into the filament, which enables it to give a larger electron 
emission at a dull red heat. These dull-emitter valves are now used 
as receiving valves and are made in very small sizes. 


The exhibit comprises examples of modern two-electrode (Flem- Wireless I 
ing) or rectifying valves, of three-electrode generating and amplify- 
ing valves and of dull emitter detecting valves made and contributed 
also speciments of Fleming's original rectifying valves, which 
are the progenitors of all the rest. 

51. Prof. R. Whiddington, F.R.8. 

The Ultra-micrometer. A New Electrical Device for 
Minute Measurement. 

The ultra- micrometer was devised in 1919-20 (Philosophical 
Magazine, 1920), and represents one of the many possible applica- 
tions of the thermionic valve to physical measurement. 

The principle involved is extremely simple. When the distance 
between the plates of a parallel plate condenser is slightly altered 
its capacity changes in a way readily calculable, the capacity change 
being the greater the closer the plates. This change can be indicated 
in a variety of ways, but in the present case is observed by noting 
the change in frequency of the high frequency oscillations main- 
tained by a three-electrode valve in an inductance circuit containing 
the condenser. 

In the particular apparatus exhibited, the condenser is linked 
to a spherical ball bearing, so that variations of the diameter of the 
ball in different directions may be observed. A null method is 
employed, so that by restoration of the original frequency of 
oscillation the diameter variation may be computed. The manner 
in which the restoration is here effected is to apply a small bending 
moment to the stiff bar carrying the movable condenser plate by 
depressing one end of another much lighter bar through an observed 
amount. From the observed depression required to restore the 
original frequency the diameter change can be calculated. 

While the instrument shown is intended only to indicate changes 
of about one-tenth millionth of an inch, it has been found possible 
under favourable conditions, with an apparatus of different design, 
to detect changes of so little as 1/200 millionth of an inch. 



- D U T)ii0\ 


52. National Physical Laboratory (Mr. D. W. Dye). slow 

Slow Oscillations. 

The apparatus exhibited consists of (a) a generator of electric 
currents of audio-frequency by uieans of a triode valve, and (6) a 
resonant circuit and various indicating devices. Current from this 


glow source flows through an inductive coil. The magnetic field pro- 

Oscillations. duced by the coil can be used to demonstrate various phenomena 

as follows : 

(A) Resonance and Selective Absorption. 

The coil of the generator induces into a resonant circuit con- 
sisting of an inductance coil to the terminals of which a condenser 
is connected. When the condenser is adjusted to that value at 
which 4rr 2 n 2 LC = 1, where n is the frequency of the oscillation and 
L is the value of the inductance, a large resonant current occurs in 
the circuit. This condition of resonance may be demonstrated 
by various means. 

If the current wave in the inducing coil of the oscillator possesses 
harmonics, the resonant circuit will only respond to that component 
of the wave to which it is tuned and so selectively absorbs energy 
of this frequency. The frequency may be that of the fundamental 
or it may be any one of the harmonics. 

(B) Heating Effects. 

A small lamp connected to a coil of a few turns of wire can be 
caused to glow when brought near the inducing coil of the oscillator. 

(C) Directive Effects. 

When the coil and attached lamp or a telephone are turned so 
that the plane of the coil is in a position of zero mutual inductance 
with respect to the inducing coil, no electro-motive force will be 
induced therein and the lamp will not glow. When a telephone 
is used as an indicator of this condition, the position of zero mutual 
inductance can be observed with great precision. 

(D) Acoustical Effects. 

W T hen a coil having a telephone receiver connected to its 
terminals is brought near the inducing coil of the generator, a loud 
sound results. The pitch and character of this sound depend 
upon the frequency and wave -form of the current traversing the 
inducing coil. 

When the current is of a sinusoidal wave-form, the note pro- 
duced is a pure tone, but when harmonics are present in the source 
of current the note assumes a character which can be approximated 
to various sounds. The presence of numerous high harmonics' or 
overtones gives the sound a strident character. 

By the use of a second independent audio-frequency generator, 
various combined musical notes may be produced such as the 
octave, major and minor third, fourth, fifth, sixth, etc. These are 
produced by inducing currents from each source into the telephone 
and adjusting the frequencies so that the ratio of them is a simple 
fraction such as 2 : 1 for the octave, 3 : 2 for the major fifth, etc. 
When the two frequencies are adjusted to be nearly equal the 
phenomena of beats is observed. If the intensities of the currents 
from the two sources are made equal, the sound produced falls to 
silence and swells up again in a slow rhythmic manner. 



These effects may be shown visually by means of the Low-Hilger 
audiometer, which can conveniently be used to measure the 
frequency and show the wave-shape of sounds produced either by 
the telephone as used in the above experiments or by speaking, 
singing, etc.. in front of the trumpet of the instrument. 

Audio-frequency oscillators by MESSRS. H. W. SULLIVAN, 
LTD. ; Low-Hilger Audiometer by MESSRS. ADAM HILGER, 
LTD. ; Condensers by MESSRS. H. TINSLEY & CO. ; Loud 
speaker by MESSRS. S. G. BROWN, LTD. 



53. Research Laboratories of the General Electric 
Company, Ltd. 

Thermionic Rectifier. 

An experimental type of high power thermionic rectifier for use 
on a 30,000?;. supply is shown. The anode is of copper and is 
cooled by circulating water through the water jacket (shown 
separately). The electron emission from the filament is more than 
five amperes. 

54. Research Laboratories of the General Electric 
Company, Ltd. 

Conversion of Alternating Current to Direct Current by 
Means of the Thermionic Rectifier. 

The cathode ray oscillograph shows an alternating current 
passing through a resistance. A thermionic rectifier is then 
inserted in the circuit and the suppression of half the wave is 
shown. A smoothing network of condensers and inductances is 
then inserted in addition and almost steady direct current obtained. 



101 Sir Napier Shaw, F.R.S. 

Solar and Terrestrial Radiation. 

Among the natural effects of radiation with wave-length between 
one ten-thousandth and a few hundredths of a millimetre may be 
classed the whole sequence of weather. The ultimate sources of 
all atmospheric disturbances are solar and terrestrial radiation, the 
effects of both of which are largely contingent upon the condition 
of the atmosphere in regard to radiation and absorption. 



Geophysics* Included in the exhibits are therefore, first, a diagram illustrat- 

ing the distribution of energy according to wave-length in a beam of 
sunlight outside the atmosphere, and in the long wave radiation 
from a black body at a temperature of 287. 

Secondly, a map showing the positions of stations where solar 
radiation has been measured and the results obtained. 

Thirdly, a map showing the rate at which energy would escape 
by radiation through transparent atmosphere from black bodies 
in different parts of the world according to the mean temperature 
of the air at the surface in the month of July. 

Fourthly, instruments designed by Prof. H. L. Callendar, 
C.B.E., F.R.S., to measure direct solar radiation and sky radiation, 
and an instrument designed by Mr. W. H. Dines, F.R.S., to com- 
pare the radiation from the sky with that from a grass meadow. 

Fifthly, a table of the results of observations of radiation 
obtained in England, day by day, during 1924, with summaries 
for weeks and for the quarters of the " May Year/ 1 

102. Sir Napier Shaw, F.R.S. 

Meteorology : the General Circulation of the Atmosphere 
and its Local Disturbances. 

The direct expression of solar and terrestrial radiation is the 
general circulation of the atmosphere with its local disturbances of 
a transient character, the study of which constitutes the science of 
meteorology. Chief among the results of the general circulation 
of the atmosphere is the supply of water, the most important of all 
considerations of the well-being of any community. Nearly all 
parts of the habitable world are dependent upon rainfall ; a few 
only depend entirely upon irrigation. Evaporation, on the other 
hand, takes away vast quantities of water from plants, soil, rivers 
and lakes. This perpetual conflict is illustrated by two maps of the 
world, one showing the normal amount of rainfall in the year and 
the other showing the loss of water by evaporation ; both are 
expressed in millimetres of water. 

The contrast between the effectiveness of the methods of 
observation of the two elements respectively is brought out by the 
irregularities of the measures of evaporation as compared with the 
orderly lines obtained from readings of the rain-gauge. The proper 
form of gauge for evaporation and the proper exposure have still 
to be found. Illustrations of the form which is most approved in 
this country and of other forms used elsewhere are associated with 
the maps. 

The exhibit, which is designed to illustrate the dynamical 
processes of the general circulation and its changes, starts from 
barometers and thermometers for use at ground-level stations. 
Thence it leads by way of special instruments adapted to measure 
the temperature and pressure in the upper atmosphere even at such 
great heights as 20 kilometres, or 12 miles, up to a representation of 
the general circulation of the atmosphere derived from the co- 
ordination of the results obtained. 


The representation includes, first, a model of the distribution of Geophysics. 
temperature at different levels, with the mode of incorporation of 
cyclones and anti-cyclones in the general circulation. Secondly, a 
scheme of normal distribution of pressure and associated winds at 
heights of 4 kilometres, 6 kilometres, and 10 kilometres set out as 
isobaric lines on concentric hemispherical globes. Geostrophic 
scales are exhibited by which the wind velocity corresponding with 
the distribution of pressure can be ascertained, and a rough drawing 
of the circumpolar circulation derived from the measurements 
with the scales, or from numerical calculation. 

The local disturbances, which are common in temperate latitudes 
as cyclonic depressions in a great variety of forms, are presented in 
the examination and analysis of a particular depression of 
September 10th, 1903, by which the apparent complication of the 
record of an anemometer, after making allowance for the friction 
of the ground, is traced to the combined motion of spinning, 
according to a certain law, and travelling at the same time with an 
ascertained velocity. 

The complexity of local disturbances, including the winds, dis- 
closed bv observations of pilot-balloons, is represented in five 
models composed of glass-plates, two for Great Britain, one for 
Bermuda, one for Jamaica, together with one which suggests the 
gradual changes from a N.E. wind near the surface to a S.W. wind 
at 1,500 metres. 

A collection of diagrams illustrates the thermal and dynamical 
effects of the heat which we derive from the sun, and the actual 
conditions of air shown by the observations of temperature at 
successive heights in relation to the properties of air elicited by 
physical experiments and mathematical study. Special forms of 
diagram for eliciting these relations are displayed from which the 
energy of saturated air or of dry air in an environment defined by 
observation can be estimated. The eventual outcome of the 
arrangement is an " indicator-diagram " for the atmosphere, 
regarded as a steam-engine. 

All the instruments are graduated systematically and the 
diagrams are adjusted in close relation with the C.G.S. system. 

The apparatus for the "sounding" of the upper air is from 
designs by Mr. W. H. Dines, F.R.S. The millibar barometers, 
thermometers for air and earth and the dial gauge for evaporation 
are by MESSRS. NEGRETTI & ZAMBRA ; the special 
logarithmic paper and scales by MESSRS. W. F. STANLEY & 
CO., LTD. 

103. Capt. C. J. P. Cave. 
Cloud Photographs. 

The collection of cloud photographs is designed to show certain 

typical cloud formations. Upper clouds are represented by various 

forms of cirrus, clouds varying much in appearance ; cirro-cumulus 

and alto-cumulus ordinarily have either waved or tessellated struc- 

; ture. Some of the photographs in the collection showing cloud 


Geophysics* structure indicate the rapid way changes take place in the finer 

structure of upper clouds. At all levels, clouds may show a lenticular 
shape ; examples are given from cirro-cumulus at great heights 
to strato-cumulus in the lower levels of the atmosphere. The 
upper lenticular clouds are often wavy at the edges. The tendency 
of clouds to be arranged in bands is very evident in the upper clouds, 
but examples are given of cumulus arranged in parallel lines, and 
of the extreme case of roll-cumulus. In the same collection with 
low clouds will be found examples of fogs. In the same frame 
is a photograph of the whole sky on one plate taken with a special 

Photographs of clouds taken from aeroplanes are mostly of low 
clouds of the strato-cumulus variety, the upper surfaces of which 
resemble the tops of fogs as seen from hills ; one remarkable 
example shows a cumulus cloud rising through a level sheet of cloud. 
Examples of cumulus clouds show small clouds of this type as well 
as the towering clouds with false cirrus at the top, which mark 
showers and thunderstorms. With these is a photograph of a 
rainbow, showing supernumerary bows and the outer bow, and the 
darkness of the sky outside the primary. A few examples of light- 
ning are given, showing also the spectrum of lightning, and the way 
flashes follow each other in nearly identical paths, as revealed by 
photographs taken with a moving camera. 

Diagrams are shown in which the heights of different forma of 
cloud are indicated ; these are mean heights as observed in England ; 
the actual heights vary very much. 

104. Dr. C. Chree, F.R.S., and Mr. C. S. Wright. 

Terrestrial Magnetism. 

As a principal object of the exhibition is to show the advance of 
science, a brief reference is needed to the state of our knowledge 
30 years ago. The increased amplitude of the regular diurnal 
variation of the magnetic elements with increase of sunspots, 
discovered towards the middle of last century, was generally 
accepted by magneticians, who also generally believed in a tendency 
to an increased number of large disturbances i.e., so-called magnetic 
storms near sunspot maximum. A claim that magnetic storms 
tended to recur after an interval corresponding to the sun's rotation 
1 a phenomenon suggestive of direct solar action had been advanced 
by Broun, but had passed into oblivion. On the other hand. 
Lord Kelvin had demonstrated it was thought conclusively by 
all who assumed that the sun must act, if at all, as a distant magnet 
that for the sun to produce a magnetic storm on the earth was a 
physical impossibility. It was supposed that irregular phenomena 
such as magnetic storms, and the regular diurnal changes such as 
the usual swing to the west of the compass needle in the forenoon, 
were quite independent. While recognising that their assumptions 
were hypothetical, Riicker and Thorpe, in reducing the field observa- 
tions for their great magnetic survey for the epoch 1891, assumed 
that the regular diurnal variation, when referred to local time, 


was identical for all places in Britain, and was the same on quiet Geophysics, 
and disturbed days, and that superposed on this were disturbances, 
which at any instant were the same all over the British Isles. 

A series of Kow curves, declination (D), horizontal force (H) and 
vertical force (V) for 1894, a highly disturbed year, illustrate 
the usual phenomena of magnetic storms. Of the four storms 
selected three had " sudden commencements " (Sc's.). An Sc. is 
not instantaneous, but occupies several minutes. Tt is specially 
prominent in H where it normally consists of a rise (movement 
up the sheet), sometimes preceded by a small rapid fall. The 
large movements may follow the Sc. immediately, or only after 
several hours. Sc's. occur simultaneously, or very nearly so, all 
over the world. During a magnetic storm the D and H traces 
may have several large oscillations on both sides of the normal, 
but as a general rule H is finally left depressed. The shape of the 
V trace is more dependent on the time of day. In almost every 
storm the trace has a humped appearance (force above normal) in 
the afternoon, a cup appearance (depression) in the early morning, 
but one of these features may be lacking if the storm is a short 

A storm larger than any experienced in 1894 possibly the largest 
storm of the last fifty years occurred in May, 1921. Tt lasted 
for several days. Its most disturbed part is represented by Kcw D, 
H and V traces. The very rapid oscillations shown during the 
clay on all the Kcw V traces are mainly due to disturbance from 
the local electric railways. Fortunately, the largest natural 
movements occurred in the night hours when trains were not 

The points of agreement and difference between simultaneous 
magnetic disturbance in different parts of Britain arc illustrated 
by copies of declination curves obtained in 1923 from three or 
more of the following places : Ke\v, bottom and surface of Sand- 
well Park Colliery near Birmingham, Eskdalemuir and Lerwick. 
The ordinate scale was only about half as open at Lerwick as at 
the other stations. Corresponding points, distinguished by letters, 
are easily recognised, especially at the more southern stations. 
But the amplitude of the movements, instead of being uniform, 
as Riicker and Thorpe supposed, increases rather rapidly as we go 
north. This, it may be added, seems almost invariably the case in 

The tendency for a series of magnetic storms to follow at 
intervals of about twenty-seven days, which Broun had observed, 
was rediscovered independently from a study of Greenwich curves 
by Mr. W. Maunder, who supposed the cause to be a jet-like 
discharge of some kind from a sunspot area, the repetitions arising 
from the persistence of the sunspot for several solar revolutions. 
According to modern physical theories such as that of Birkeland, 
the discharge takes the form of ions emanating from the sun. 

Of interest in this connexion is the diagram from Greenwich 
Observatory, showing for an 11 -year period the distribution of 
sunspots in latitude, and their comparative number, together 
with the mean areas of sunspots, and the variation which proceeds 
pari passu in the amplitude of th3 diurnal range of the magnetic 
(B 34-2285)Q p 


Geophysics* Views analogous to Mr. Maunder* s led the late Father A. L. 

Cortie, of Stonyhurst, to connect individual storms with individual 
sun-spots, and to trace their contemporaneous development. 
His views and investigations are illustrated in a large frame, 
which shows the Stonyhurst records of a number of storms with 
the state of the sun's surface at the time. 

The 27-day interval is not confined to magnetic storms. It 
can be traced even in quiet conditions. There is, in short, a very 
sensible correlation between the magnetic character, whether 
disturbed or quiet, of two days which are twenty-seven days apart. 
This is illustrated by a number of diagrams in which the magnetic 
state of the day is measured by the international character figure 
assigned at De Bilt, or by the daily range of one of the magnetic 
elements. One of the diagrams shows that the 27 -day interval 
also manifested itself in the auroras recorded in 1911 by the Scott 
Antarctic Expedition. There is not, as yet, universal agreement as 
to the precise nature of the connexion between the sun and the 
magnetic phenomena on the earth, but the views entertained prior 
to Lord Kelvin's criticisms are again in the ascendant. 

At ordinary stations, while small magnetic disturbances are the 
rule rather than the exception, large disturbances are rare, and 
when they occur, they are large everywhere, and usually last for 
a number of hours. Also there are many days, especially in years 
with few sunspots, free from any but trifling irregularities. But 
in high magnetic latitudes really quiet conditions are very rare, 
and there are often active disturbances lasting for an hour or two 
which are represented only by comparatively trifling disturbances 
in low latitudes. This phenomenon is illustrated by copies of 
simultaneous records obtained during 1911 and 1912 at the base 
station of the Scott Antarctic Expedition and at various observa- 
tories, ranging from Mauritius in the south to Sitka (Alaska) in the 
north. After making due allowance for the greater sensitiveness 
of the magnetographs at some of the stations, e.g., Buitenzorg, in 
Java, it was found that these short disturbances tended to be 
simultaneously Large in the Antarctic and the Arctic, while com- 
paratively small near the equator. 

With the object of following more minutely the sequence in 
time of magnetic changes, arrangements were made in connexion 
with recent Antarctic Expeditions for obtaining " quick run " 
curves with a very open time scale on certain occasions. These are 
obtained by rotating the drum on which the photographic paper is 
wound at a much higher rate than usual. Examples of quick 
run curves with interesting magnetic changes are shown. 

During 1912 expeditions led by the late Captain Scott and by 
Sir Douglas Mawson had magnetographs running simultaneously 
at stations on opposite sides of the south magnetic pole. Diagrams 
show the diurnal variations of the magnetic elements at the two 
stations. These can be interpreted as oscillations in the position 
of the magnetic pole, V rising and H falling as the pole moves 
towards a station. In this way, two independent estimates were 
formed of the average daily motion of the magnetic pole for groups 
of days, the different groups representing different degrees of 
magnetic disturbance. The large increase in the calculated daily 


travel of the magnetic pole, as we pass from the very quiet to the Geophysics- 
less quiet days, is conspicuous. This shows how wide of the mark 
was the view that the regular diurnal variation is independent of 
disturbance. But the influence of disturbance on the regular 
diurnal variation is much larger in the Antarctic than in Britain. 

The magnetic elements are in a continual state of change. The 
changes of declination and inclination in the London area have 
been observed at one station or another for nearly 350 years. 
They are jointly illustrated in a diagram, declination east and west 
being measured in the horizontal direction, and inclination in the 
vertical direction. When first observed, the compass needle in 
London pointed to the east of north. Gradually it pointed more 
and more westerly, until about 1818, when it pointed 24 J west 
of north. Since then westerly declination has steadily declined. 
Inclination when first observed was nearly 5 greater than it now 
is, and it increased to a maximum about 200 years ago. Thereafter 
it declined steadily until a few years ago. Of late years it has been 
nearly stationary. 

Another diagram shows declination changes separately. The 
more recent years' results, which are derived from Kew, are shown 
in more detail. The change has accelerated of late years and is 
now at the rate of about 1' per month, a rate which was not 
approached during the nineteenth century. 

105. Dr. G. Ghree, F.R.S., and Mr. C. S. Wright. 

Atmospheric Electricity. 

The phenomena of atmospheric electricity vary greatly accord- 
ing to tho weather. In fine weather electric potential as a rule is 
higher in the air than on tho earth i.e., the potential gradient is 
positive. During rain, the potential gradient usually alternates 
between positive and negative. During a thunderstorm, every 
lightning flash is accompanied by a large sudden change of potential. 
During fog especially dirty fog potential gradient at Kew is 
usually very high. 

A series of corresponding Greenwich and Kew electrograms 
shows the extent to which the phenomena agree at comparatively 
near stations. During the great thunderstorm of July 9-10, 
1923, lightning was almost incessant for several hours in the London 
area, and the movements of the light spot across the photographic 
paper were often too fast to leave a clear trace. 

Eskdalemuir, in Dumfriesshire, is so distant that the weather 
there usually differs considerably from that at Kew. Thus no 
comparison is made between simultaneous Kew and Eskdalemuir 
electrograms, but traces are shown representative of corre- 
sponding weather conditions. 

The Eskdalemuir electrograph is less sensitive and uses a 
wider photographic sheet than the instrument at Kew ; but even 
the Eskdalemuir trace is incapable of showing changes of more 
than a few thousand volts jper metre in the potential gradient, and 

(B 34/2285)Q " p 2 


Geophysics. thus cannot show the true nature of the enormous and extremely 

rapid changes of potential which occur when a lightning flash 
occurs within a few miles. The apparatus of Prof. C. T. 11. Wilson, 
of which a photograph is exhibited, is designed for this purpose, 
and copies of some of the records obtained with it are also shown. 

In the elect rograms representative of fine weather, a more or 
less regular diurnal variation of potential is recognisable. To 
show its real nature, it is necessary to combine hourly measure- 
ments from a number of days. The diurnal variations thus 
resulting at Kew and Eskdalcmuir are illustrated by diagrams. 
The variations are shown separately for the midwinter months 
(November to February) combined, and the midsummer months 
(May to August) combined, as well as for the whole year. The 
Kew type of diurnal variation has a well- marked double oscillation, 
with maxima in the evening and in the late morning hours, and 
minima in the early morning and early afternoon. At Eskdnlemuir a 
double daily oscillation is fairly recognisable in winter, but in 
summer only a vestige of it is left. 

The diurnal variation of potential gradient at Kew is compared 
in another diagram with that of atmospheric pollution, as measured 
by Dr. Owens' pollution recorder, which functions near the electro- 
graph. There is a close general resemblance between the two 
diurnal variations. There is much less dirt in the air in summer 
than in winter, and the potential gradient is also much lower in 
the former season than in the latter. As, however, one of the 
diagrams shows, the annual variation of potential gradient is little, 
if at all, more conspicuous at Kew than it is at Eskdalemuir, in the 
open country. 

Amongst other important atmospheric electricity elements arc 
the ions positive and negative always present in the atmosphere, 
and the (small) vertical electrical current, always passing between 
the upper atmosphere and the earth. Diagrams are exhibited 
showing the annual variation of these elements at Kew. 

106. Prof. H. H. Turner, F.R.S., and Mr. J. J. 

(A) The Milne-Shaw Seismograph. 

This is a new type of earthquake recorder with high magnifica- 
tion and electro-magnetic damping. The magnification is approxi- 
mately forty times greater than in the standard Milne instrument, 
while in practice the sensitivity to tilt is from ten to twenty times 
greater, according to the pendulum period adopted. 

The general principle of the apparatus is to multiply the move- 
ments of a short horizontal pendulum by reflecting a beam of light 
from a pivoted lens of half-metre focus. The pendulum carries a 
weight of 1 Ib. together with an electrolytic copper damping vane, 
floating between the poles ofefour tungsten steel magnets, whereby 
any degree of damping is obtained and the pendulum brought to 


rest^after each excursion. The outer end of the boom, is coupled Geophysics. 
to and rotates the mirror ; by this means 500 multiplications of 
the motion may be readily obtained, but in practice 150 to 250 
are found to be more suitable. 

Special calibrating and adjusting devices were necessary with 
such high magnification. This point has received special attention ; 
tilts of 1/100 of a second of arc (one inch in 300 miles) can be applied 
and registered by the image of a spider line on a distant scale. All 
such operations are performed and the scales read at a distance from 
the pendulum, so that there are no movements of the observer to 
confuse the issue. Adjustments to the recording light-spot are 
made by means of a long flexible cable, and artificial deflexions for 
testing purposes by a solenoid. 

The record is timed by an electromagnetic shutter contained 
within the lamp, which may be illuminated by electric current, gas 
or oil. 

The Wk Mi hie -Shaw " seismograph is particularly suited for 
measuring not only earthquake movement, but also small changes 
of level, sucli as deflexion of the coast due to tidal load. 

Pendulum periods of GO to 90 seconds can easily be obtained. 
When set up to 00 seconds, with a multiplying ratio 250 : 1, a 
deflexion of 1 mm. of the light spot is produced by so small a tilt as 
1 inch in 8,000 miles, or 0-0004 seconds of arc. 

Arrangements 1'ave been made for showing the seismograph 
in operation. 

DEMONSTRATION. The instrument is housed in a separate 
chamber and may be seen on application to a demonstrator. 

(B) Seismological Maps. 

Maps of the world are exhibited (a) showing the position of 
stations possessing seismographs which send records to head- 
quarters (at Oxford), or have done so in times past (e.g., those in 
Russia) ; and (6) showing localities where earthquakes have 
occurred in the years 1913-1919, as determined at Shide or Oxford. 
These earthquake centres will be seen to lie upon two or three well- 
defined lines on the earth's surface, which have some interesting 
geographical relationships. A set of publications giving details of 
earthquake records 1913-1919 is also exhibited. 

(C) The Location of Earthquake Centres. 

A globe used by Prof. Milne for determining the position of 
earthquake centres by measurement of distances from the observing 
stations is exhibited, and, if possible, another globe of more modern 
construction, showing the great increase in number of observing 
stations since Milne's time. 



Zoology. 201. Pro/. E. B. Poulton, F.R.S. 

Mimicry in an African Swallowtail Butterfly. 

The females of an African Swallowtail Butterfly, Papilio 
dardanus, provide the most interesting and elaborate example of 
mimicry yet known. 

In Madagascar, the Comoro Islands, Abyssinia and Somaliland, 
this species is represented by closely allied forms in which both 
sexes are alike and non-mimetic. In other localities on the main- 
land of Africa the females of Papilio dardanus mimic butterflies of 
a very different group, the Danainae ; in these both sexes are alike, 
they are unpalatable, and they are mimicked wherever they occur 
by other butterflies and by moths. 

Three different forms of female of Papilio dardanus mimic 
three very distinct Danaine patterns ; two of these patterns are 
modified in passing from one area to another, and in the same 
regions the mimetic females are correspondingly altered to match 
them. In Uganda and West Africa a fourth form of female of Papilio 
dardanus occurs ; this mimics an entirely different pattern, that of 
two species of the Acraeinae, a group of unpalatable butterflies 
remote from the Danainae, but like them mimicked in various parts 
of the world. 

Most swallowtail butterflies have long " tails " to the hind 
wings. These are present in the male of Papilio dardanus, but the 
mimicking female forms of this species have become tailless, like 
their " models." Vestiges of tails occur, however, in some 
transitional females, and occasionally in the mimetic form that has 
the most primitive pattern-, that is, the one most like the male 
pattern. Pockets for the tails are found in the female chrysalis, 
but tails are not developed in them. 

In the exhibit the Danaine and Acraeine * ; models " may be 
distinguished by their red-printed labels and by their position 
above their mimics. 

The arrangement is geographical. Non-mimetic forms from 
Madagascar and Somaliland are followed by primitive females from 
Nairobi, exhibiting some of the steps by which the mimetic forms 
arose. Then come the fully formed mimics with their models, first 
along the east coast of Africa from north to south, next Uganda, 
where all four mimetic females and their models occur together, 
finally the west coast. The map shows the localities in which the 
specimens were taken. 

Experimental breeding has shown that all the mimetic forms 
may be bred from females of the same or of a different form, and 
that the same brood may contain two or three different mimics, 
as well as the non-mimetic males. 


202. British Museum (Natural History) : Dr. C. J. zoology. 

Mimicry in Beetles. 

(i) The Lycidae and their Mimics. 

The beetles of the family Lycidae are conspicuously coloured, 
fly slowly, and secrete distasteful liquids. They show little 
diversity in form and, in the same region, but little in colour, most 
of the Asiatic species being red and the South American ones black 
and yellow. Thus they are easily recognized and it is established 
that they are singularly free from the attacks of insectivorous 
animals. Wherever they occur they are mimicked by day-flying 
beetles of other families, and sometimes also by day-flying moths 
and other insects. It is of interest to note that mimicry is unknown 
in beetles that fly by night ; as a rule these have a coloration that 
harmonizes with the object ground, tree trunk, or leaf, etc. on 
which they rest during the day. 

(ii) The Cerambycidae and their Models. 

The beetles of the family Cerambycidae are not protected by a 
distasteful secretion. Many of them look quite unlike their nearest 
allies, but show a close resemblance in form, size and coloration to 
unrelated insects, either noxious beetles of other families, or 
stinging insects such as wasps. The diversity of this family, when 
contrasted with the uniformity of the Lycidae, is most striking. 

203. British Museum (Natural History). 

Biological Eesults of the " Terra Nova " Expedition. 

The "Terra Nova" Expedition (1910-1913), under the com- 
mand of the late Captain R. F. Scott, R.N., C.V.O., was thoroughly 
equipped for scientific work in both men and material. On the 
outward and homeward voyages from England to New Zealand, 
fine meshed tow-nets were put overboard whenever possible, and 
hauls with the trawl were made off Rio Janeiro and near the Falk- 
lands. In the winter cruise (July to October, 1911) to the north 
of New Zealand, hauls made with trawl and dredge at depths of 
15 to 300 fathoms revealed a bottom-fauna of extraordinary variety, 
including a great number of forms new to science. Samples of 
plankton and of muds and oozes were obtained between New 
Zealand and McMurdo Sound, and the results of trawling in the Ross 
Sea much increased our knowledge of the Antarctic marine fauna. 
The work on shore included the study of birds and of parasitic 
worms ; especially notable was the hazardous winter journey to 
Cape Crozier, to secure eggs with early embryos of the Emperor 

The exhibit includes charts showing the stations where specimens 
were obtained either by tow-netting or by trawling, and a selected 
set of the scientific reports in which the collections are described. 


Zoology. 204. British Museum (Natural History) : Mr. C. 
Tate Regan, F.R.S. 

Evolution in Fishes. 

(A) The Protractile Mouth of Epibulus. 

Cheiliniis and Epibulus are two genera of Wrasses from the 
tropical Indo-Pacific ; they are closely related, agreeing in form, 
scaling, structure of the fins, dentition, etc. Cheilinm has the 
mouth moderately protractile, but in Epibulus it is extremely 
protractile. There can be no doubt that the remarkable structural 
modifications connected with the great protractility of the mouth 
inEpibulush&vc been derived from the normal arrangement found 
in Cheilinus. It is of great interest that this marked adaptive 
change has taken place whilst the fish has remained a Cheilinus 
in other characters. It is probable that Epibulus has evolved from 
a Cheilinus that formed the habit of catching its prey by a sudden 
protrusion of the mouth. 

(B) The Killarney Shad (Alosa finta killarncsis). 

The Twaite vShad ( Alosa f into) of the Atlantic coast of Europe 
enters rivers in the spring to breed ; the young fish remain about 
a year in the rivers or estuaries and then make for the sea, first 
returning to breed when they are about a foot long. In Killarney 
there is a Shad which lives in the lakes throughout its life and differs 
from the Twaite Shad only in its smaller size (maximum length 
about 9 in.), deeper body and more numerous gill-rakers. The 
Killarney Shad has evidently evolved from the Twaite Shad, young 
of which remained in the lakes instead of going to the sea, and 
founded a lacustrine colony. The increased number of gill-rakers 
in the Killarney Shad is doubtless related to a life-long diet of 
minute Crustacea ; these form the sustenance of the Twaite Shad 
during the first year of its life, but in the sea it feeds on larger 
Crustacea, small fishes, etc. 

205. British Museum (Natural History) : Dr. W. D. 

Continuous Evolution, with Recapitulation, in Carboni- 
ferous Corals. 

In corals the polyp an animal very similar to a sea-anemone 
is attached to the inside of a cup-like structure formed of the hard 
coral substance ; the inner surface of this cup bears a number of 
ridges the septa that project inwards towards the centre 

The diagrams exhibited represent enlarged transverse sections 
of the " cup " of some simple corals found in the Carboniferous 
Limestone of Scotland, studied by Mr. R. G. Carruthers. The 
five different forms represented are found in five successive 
horizons, and it is inferred tihat each is the direct descendant of 
the earlier one below it. It will be seen that the septa, which at 


first mect in the middle of the cup, become reduced, and in the Zoology. 
latest form are quite short. In the three lower diagrams the change 
in shape of the f ossula a space due to the shortness of the septum 
at the bottom of each diagram should also be noted. 

The smaller diagrams represent the younger stages of the corals 
they accompany ; it will be noticed that each closely resembles 
the adult of the form below. This is an example of recapitulation, 
the adult stage of an organism being repeated as a young stage of 
its descendant. 

206. British Museum (Natural History). 
Piltdown Man and Rhodenian Man. 

The skull discovered in a river gravel at Piltdown in Sussex 
is the oldest human skull known ; it was described in 1913 by the 
late Mr. Charles Dawson and Sir Arthur Smith Woodward, F.H.S. 
The skull is chiefly remarkable for the great thickness of the bone, 
but the lower jaw is almost precisely that of an ape, with a very 
retreating chin and with the lower canine teeth interlocking with 
the upper canines in the true ape-fashion. 

The skull found in 1921 in a cave at the Broken Hill Mine, 
Northern Rhodesia, is noteworthy for the immense size of the 
bony face, surmounted by brow ridges stronger than in any other 
human skull and recalling those of a gorilla. 

The exhibits include casts of these two skulls, made and exhibited 
by Mr. F. O. Barlow, and, for comparison, skulls of a modern man 
and of an ape. 

207. Prof. J. P. Hill, F.R.S. 

The Egg-Laying Mammals. 

The Monotremcs, or egg-laying mammals of Australia, Tasmania 
and New Guinea, are by far the most primitive living mammals, 
and constitute a connecting link with our reptilian ancestors. 
The Platypus (Ornitharhynchns) has a soft furry coat, webbed feet, 
and a snout like a duck's bill ; it inhabits pools and quiet reaches 
of rivers, and makes a long burrow in the bank, with side chambers 
in which the eggs are laid. The Spiny Ant-eaters (Echidna and 
Tachyglossus) are terrestrial animals protected by sharp spines. 

The eggs generally number two in Ornithorhymhus ; they 
are oval, about two-thirds of an inch long, and provided with a 
tough shell ; when laid, the mother lies on or round them to keep 
them warm. In Echidna the single egg is incubated in a pouch on 
the abdomen of the mother. 

In their development the Monotremes show a remarkable 
mixture of reptilian and mammalian features, and thus afford 
strong evidence of the derivation of mammals from reptilian 
ancestors. The earliest developmental process, segmentation 
or cleavage, undergone before tho egg is laid, is of the partial type 
characteristic of reptiles. During incubation the embryo lives on 


Zoology* nutritive material contained in the yolk-sac and breatheg by means 

of a membranous sac rich in blood vessels applied to the inside of 
the porous egg-shell. At hatching, the shell is broken by means 
of a conical projection on the snout, aided by an " egg-tooth " 
just behind the middle of the upper lip. So far the development 
has been essentially reptilian in character, but the newly hatched 
Monotreme, although it exhibits reptilian features such as an up- 
turned nose or " shell- breaker," an egg-tooth, and a remnant of the 
yolk-sac, is mammalian in its general form and structure. The 
helpless, naked little creature (about two-thirds of an inch long in 
Ornithvrhynchus, half an inch in Echidna) subsists on milk secreted 
by the mammary glands of the mother, and gradually increases in 
size and grows more like its parents. 

208. Dr. F. A. E. Crew. 

Sex-Reversal in the Common Fowl. 

Assumption by old hens of male plumage, spurs, etc., is known 
to be associated with disease of the ovaries. About forty old hens 
that showed signs of such disease were observed for two years ; at 
the end of this time all had assumed the male plumage to a greater 
or less extent. Most remained sexually indifferent, but one, a 
Buff Orpington, paid ardent attention to hens and crowed lustily ; it 
was mated with a virginal hen of the same race, which three months 
later produced fertile eggs from which two chicks were hatched. 
These chicks grew up ; they were quite good Buff Orpingtons, one 
a cock, the other a hen ; chickens have been obtained from them. 

Examination of the hen that had behaved as a cock showed 
that the ovaries were destroyed by tubercular disease, and that small 
testes were present. Examination of other birds showed that they 
were at different stages in sex-reversal. The formation of the 
spermatic tissue appears to be accomplished by a thickening of 
the epithelium covering the surface of the ovary, a proliferation 
inwards from this epithelium of columns of cells, and the enlarge- 
ment of these cells to form tubules of a testicular character. 

The exhibit consists of photographs of hens and photo-micro- 
graphs of the reproductive glands. 

209. Prof. R. C. Punnett, F.R.S. 

Synthesis of a White Breed of Fowls from Two Coloured 

A cross between a Silver Campine cock and a Chamois Campine 
hen gives *' ghost barred" white birds of both sexes. Such F.I 
birds bred together give rise to an F.2 generation comprising five 
different sorts of birds, namely, ghost-barred whites similar to the 
F.I birds, silver and chamois birds similar to the two original 
parental varieties, gold Campine and pure white. Certain of these 
whites are fixed and may be used to produce a pure white breed 
of Campinep. r 

BOTANY. 217 

210. JBritish Museum (Natural History). Zoology* 

Evolution of the Elephant. 

The earliest member of the Proboscidea is Moeritherium, dis- 
covered by the late Dr. C. W. Andrews, F.R.S., in the Upper 
Eocene beds of the Fay urn in Egypt ; this was an animal about 
the size of a tapir, with a skull very like that of other primitive 
hoofed animals. 

Palaeomastodon, from the lower Oligocene of the Fayum, also 
discovered by Dr. Andrews, was larger than Moeritherium and in 
other characters approached the Miocene Tetrabelodon, which was 
very similar to an elephant, but had a long lower jaw with a pair of 
incisors at the end, used for grubbing in the earth. The later changes 
leading to the modern elephants were the reduction of the chin, loss 
of the lower incisors, and increase in size and complication of the 

Casts of skulls of Moeritherium and Palaeomastodon, made and 
exhibited by Mr. F. 0. Barlow, may be compared with the skull 
of an Indian elephant. The increase in size, the development of 
the tusks, the increase in number of the ridges on the molar teeth, 
and the shifting back of the nasal opening (corresponding to the 
development of the trunk), are features to which attention may be 


211. Department of Botany, British Museum (Dr. Botany. 
A. B. Rendle, F.R.S., and Mr. R. UO. Good). 

Colour in Plants : its Preservation or Replacement for 
Purposes of Exhibition. 

Two objects have to be achieved in the successful preservation 
of botanical specimens for exhibition purposes namely, the preser- 
vation of the original form and the preservation or reproduction of 
the original colour. Last year's exhibit dealt more particularly with 
the former of these : this year's exhibit is designed especially to 
illustrate the latter. 

The colours of healthy plants living under normal conditions 
are due to the presence within the living tissue of certain highly 
coloured organic pigments. These pigments are of six kinds and 
are found either included in colour-bearing granules (chromoplasts) 
within the protpolasm of the cell or in solution in the liquid contents 
of the cell (cell-sap). Under certain conditions a degree of colour 
may be due to substances other than true pigments, as is often the 
case with plants which contain tannins, but such substances are 
not considered in this exhibit. The pigments illustrated all play 
a definite r61e in the physiology of the plant and are all more or 
less essential to the performance 'of its vital processes. 


Botany. The substances concerned fall under the following six heads : 

(i) Chlorophylls. 

Green chromoplast pigments found in almost every part of the 
plant but most abundantly in the leaf. The green pigments of the 
leaf consist of two closely related and similar complex carbon 
compounds containing a small proportion of the element mag- 
nesium and having the chemical formula) C-^H^O^Mg and 
C 55 H 70 6 N 4 Mg. It is interesting to note that there is a considerable 
similarity in structure and reactions between chlorophyll and 
haemoglobin, the red colouring matter of mammalian blood. To 
the plant chlorophyll is of vital necessity, since it is only when this 
substance is present that the organism is able to manufacture its 
own food out of inorganic materials. 

(ii) Carotins. 

Yellow, orange or red chromoplast-pigments. These are most 
of ten found associated with chlorophyll in the green leaf, but they 
also occur alone in many plant-organs, to which they frequently 
give a bright colour, e.g., the root of the carrot and many yellow 
flowers and fruits. There are three principal carotin pigments : 
carotin itself, xanthophyll and f ucoxauthin. The first is a hydro- 
carbon with the formula 0| H 56 . The second, usually associated 
with the first, has the formula C 40 H 56 2 , and the third, which is 
only found in the brown seaweeds, has the constitution C 40 H. 54 O 6 . 

(iii) Anthocyans. 

Blue, purple or crimson cell-sap pigments. Very widely dis- 
tributed in plants and the cause of nearly all the blue, purple and 
crimson colours in flowers, fruits, leaves and stems. They are 
often abundant in young unfolding plant-organs and in leaves and 
stems at the end of the growing season (autumn-tints). They occur 
chiefly as the glucosides of certain complex aromatic compounds 
derived from benzo-pyrilium. 

(iv) Anihoxanthins (Flavones). 

Yellow cell-sap pigments. These are of common occurrence in 
many plant-organs, especially flowers, but usually in such dilute 
solutions that their colour is not striking. When present in 
greater abundance, they give brilliant colours as is the case in the 
yellow varieties of the garden snapdragon. The yellow dyes of 
Dyer's weed and Dyer's greenweed are also due to anthoxanthins. 
They are found as the glucosides of certain complex aromatic 
carbon compounds derived from flavone and xanthone. 

(v) Phycoerythrin. 

A red chromoplast-pigmeiit. It is found almost exclusively in 
the red seaweeds, associated with chlorophyll, and gives their 
characteristic colour. Its chemical constitution is uncertain, but 
it is possibly a colloidal substance allied to the proteins. 

(vi) Phycocyan. 

A blue-purple chromoplast-pigment. This also is found 
associated with chlorophyll in certain seaweeds but is much less 
common than phycoerythrfn. It is said to have a similar con- 

BOTANY. 219 

Tfce general form of botanical specimens can be preserved in Botany. 
two ways either by drying in air or sand, or by immersion in 
preservative liquids, of which the most convenient for ordinary use 
are. formalin (4 per cent, solution of formaldehyde) or colourless 
commercial spirit. In this latter " wet " method, loss of colour 
is usually rapid and there is no satisfactory way of preventing it. In 
the " dry " method a considerable degree of colour -retention can 
be achieved by suitable means. 

Colour loss may be due to two processes : (a) Upon the death 
of coloured plant -tissues the contained pigments are soon decom- 
posed by the action of substances, called enzymes, in the plants 
themselves. This action is facilitated by the presence of moisture. 
(b) In the absence of enzyme action the pigments are more slowly 
decomposed by the action of light, and, more rarely, of air. These 
processes affect different specimens in very different ways. The 
colours of some arc extremely fugitive, while in others they are 
almost permanent, even in formalin or alcohol. 

J3y rapid drying under warm conditions either by pressure 
between layers of absorptive material or in warm, dry, silver-sand- 
enzyme action is partially or entirely prevented, and the specimens 
retain their natural colour for a period depending upon the resist- 
ance of these pigments to the action of light and air. In certain 
cases this period may be almost indefinite. A series of specimens 
is shown to illustrate colour-retention under dry conditions. 

In the " wet " method of preservation enzyme action is not so 
easily prevented, light cannot be so completely excluded, and the 
pigments are often affected by the liquid media used. la these 
circumstances loss of colour occurs rapidly and a more perfect pre- 
servation of form is achieved at the sacrifice of colour. Some pig- 
ments seem to be but little affected if formalin is used, but these are 
few. In some cases the action of the preserving fluid can bo 
prevented by first dipping the specimen into melted gelatine. This 
is then allowed to harden, and the specimen becomes covered by a 
thin, translucent, impervious coating. One section of the exhibit 
illustrates the effect of " wet " preservation upon natural colour. 

As an alternative to their preservation, the natural colours of 
plants can often be imitated and replaced by chemical means. 
Such a method is particularly effective and valuable in the case of 
the green pigments (chlorophylls). The process is performed in 
the following way: A saturated solution of copper acetate in 
glacial acetic acid is diluted to four times its original volume. It 
is then brought to the boil and the specimen is immersed in it. The 
period of immersion varies with the nature of the subject from a few 
minutes to half an hour in the case of very fleshy plants. During 
the immersion a complex chemical change takes place, in the course 
of which the magnesium in the chlorophyll is replaced by copper 
and a copper-chlorophyll compound is formed. This compound 
has the same colour as the original chlorophyll and is not decom- 
posed by the action of formalin or light. Specimens treated by 
this " greening " process can afterwards be preserved either in 
formalin or by drying in the usual way. 

Certain of the other plant-pigments can be imitated by means of 
suitably coloured dyes. This mothod is particularly effective in 


Bot&ny* the case of seaweeds. The specimens so stained can be preserved 

either dry or in a fluid which does not dissolve the dye. The last 
section of the exhibit illustrates these " greening " and dyeing 

212. Mr. H. Hamshaw Thomas. 

The Earliest Known Fruit-Bearing Plants and theit 
Relation to the Problem of the Evolution of the Flowering 

The origin and early evolution of the flowering plants is one o f 
the most baffling problems of botany. Plants of this type form 
the greater part of the present vegetation of the world and provide 
the main source of vegetable food for man and animals. They are 
extremely varied in character and more than 135,000 species have 
been recognised. Their flowers exhibit innumerable forms, but 
they all agree in the possession of ovaries which give rise to fruits 
containing seeds, an din having stamens of a characteristic form in 
which pollen-grains are produced. 

The record of the rocks seems to show that this class originated 
at a relatively late period in the earth's history, and that they 
quickly spread over the whole world, evolving myriads of new 
types with great rapidity when compared with other groups of 
plants. Charles Darwin regarded this as one of the most difficult 
evolutionary problems which had to be solved, and referred to it 
as " an abominable mystery." 

In spite of a long-continued and careful search in the rocks, 
no definite traces of flowering plants have been found in the strata 
of pre-Cretaceous age, while many of the leaves of the earliest 
fossil flowering plants are not far different from the leaves 
of their modern representatives. The theory of evolution indicates 
that they must have sprung from some pre-existing group, but 
the practical difficulty is to find evidence of the existence of such a 
group. The exhibitor has recently found in the Middle Jurassic 
(the age of many giant fossil reptiles) rocks, exposed on the shore 
near Scarborough, in Yorkshire, the remains of a new group of 
plants which may be regarded as allied to the hypothetical plant - 
type from which our flowering plants have evolved. 

This group which is named the Caytoniales included plants 
which possessed fruits containing seeds, and stamens containing 
pollen-grains, which are very similar to those characterising the 
modern flowering plants, but they had no definite flowers. Their 
leaves were also simpler in structure than the leaves of most 
modern trees. Evidence has been obtained pointing to the 
existence of this group of plants in Greenland at a still earlier date 
(Rhaetic age), and probably it was very widely spread over the 
world at this period. 

Although it may not be possible to regard the Caytoniales as 
the direct ancestors of the flowering plants, yet their characters 
and their wide distribution point to the existence of an important 
plant-type, which possessed the most essential organs which 

BOTANY. 221 

go to form a flower, at a period prior to the appearance of the Botany* 
first definite flowering plants. This discovery may lead to the 
gradual elucidation of the great problem. 

Apart from their theoretical aspect these plants possess some 
interest, for they are the earliest known types in which were formed 
small fleshy fruits containing hard seeds, somewhat like those ,of 
the currant grape. 

The exhibit illustrates the problem referred to above. It 
includes an autograph letter on the subject written to Darwin by 
the Marquis de Saporta, one of the most eminent palaeobotanists 
of his day ; diagrams showing the rapid increase in the numbers 
of fossil flowering plants in the successive periods after their 
appearance ; and specimens showing the great similarity in form 
between early fossil leaves of flowering plants and those of to- 
day. Specimens of fruits and seeds of the Caytoniales from York- 
shire are shown, together with drawings and photographs illustrating 
their structure. A piece of rock containing parts of the stamens 
is shown, and a photograph and drawings of the winged pollen- 
grains which they produced. Specimens of the leaves which probably 
belonged to these plants are also exhibited. 

213. Mr. H. Hamshaiv Tliomas. 

Photographs illustrating the Use of Aerial Photography in 
the Study of Vegetation. 

Aerial photography provides valuable assistance in the study 
of the distribution of vegetation and of its relation to the soil and 
topography. Vegetation characterised by different plant-types 
can generally be clearly recognised in photographs taken from 
above, and its features and extent can be noted in a way which 
could scarcely be excelled by many months of survey -work on the 
ground. This method of work is of special interest and importance 
in the study of the development of a plant-covering on the newly 
exposed sand or mud of coastal lands. 

The examples exhibited include part of an aerial survey of 
Blakeney Point on the Norfolk coast, in which the distribution of 
the plants on the sand-dunes, shingle-bank, and mud-flats can be 
seen ; photographs taken at various points on the ground are added 
for comparison. 

Other examples show aerial views of different vegetation-types, 
and illustrate the way in which the distribution of desert scrub, 
and of woodlands, etc., can be determined. The distribution of 
riverside woods or thickets in the Jordan valley is shown by three 
views taken in different directions. Other photographs show 
European woodlands and fields in winter and summer. 


Botany. 214. Pro/. V. H. Blacfonan, F.R.S. 
Plant Physiology. 

(A) Apparatus for Determining the Eate of Assimilation 
of Green Leaves. 

The leaf is placed in a glass jar containing air enriched with 
carbon-dioxide, the amount of this gas absorbed being determined 
by the change in colour of a solution. By means of the watfcr 
vacuum-pump and mercury valves the air is made to circulate 
through the apparatus ; in its passage it bubbles through the 
coloured solution and returns again to the jar. As assimilation 
proceeds the carbon-dioxide decreases in amount and the colour 
of the dye changes from yellowish- blue to purple. From this 
change in colour the amount of the gas absorbed by the leaf can be 

(B) Apparatus to Determine the Amount of Water lost 
by a Potted Plant in the Process of .Transpiration. 

The plant is placed on one pan of a balance and counterpoised 
by weights in the other pan. As the plant loses water the pan 
supporting it rises and so makes electrical contact between two 
wires and the mercury in the cup attached to the pan. An electrical 
circuit is thus closed, as a result of which the magnet pulls away 
the cup through which the slow stream of water from the reservoir 
has been flowing to waste. The water now drops into the pot, 
and the left-hand pan gradually falls again. This brings about 
the rise of the right-hand pan, resulting in the closing of another 
electrical circuit seen on that side of the balance ; the cup is then 
pushed forward again and the water once more runs to waste. 
The cycle ia started again by the further loss of water from the 

Every time the left-hand electrical circuit is closed, a current is 
sent through the magnet actuating the recording pan and a mark 
is made on the revolving drum. A continuous record of water 
lost by the plant can thus be obtained for a period of twenty-four 
hours without any attention, and at the same time the moisture 
of the soil is kept nearly constant. 

(C) Apparatus to Kecord Electrically the Rate of Growth 
of a Plant. 

The top of the stem is attached by a thread to a spring under 
slight tension. Elongation of the stem allows the spring to rise 
and to make electrical contact above. The current passing causes 
the toothed rod bearing the spring to rise about 1/200 in. As a 
result the spring is again extended and contact is broken. Each 
time contact is made a record appea v s on the revolving drum. 




215. Prof. P. Groom, F.R.S. 

Cultures of Fungi causing Decay of Timber in Houses. 

The cultures include Merulius lacrymans, Merulius corium, 
Merulins tremellosus, Coniophora c:rebelto, Polyporus destructor and 

Specimens of wood from houses attacked by some of ^hese species 
arc shown. 

Some of the fungi (e.g., Coniophora cerebella) can attack only wood 
that is thoroughly damp, as they cannot manufacture water suffici- 
ent for their growth ; others (e.g., Merulius lacrymans), when once 
established, can attack the driest wood, as they can render this 
moist by means of the water that they produce and excrete. All 
the fungi exhibited grow inside timber, but only certain of them 
(e.g., Meruli'iui lacrynums, Coniophora cerebella) can also advance 
over or through other materials (e.g., brick walls), and can thus 
cause rapid destruction of the woodwork of a building. 



216. Dr. K. D. Adrian, F.R.S. 

Electrical Apparatus for Research in Physiology. 

Electrical measurements play a large part in physiology, for 
muc.h of our knowledge about the tissues concerned with movement 
(the muscles and the nervous system) is derived from observations 
of the electrical effects which accompany activity. The currents 
which have to be measured are small and art 1 of very brief duration ; 
for example, the tw action current " which accompanies a nervous 
impulse docs not last for more than a few thousandths of a second. 

Special instruments are needed to give a faithful record of 
these brief currents, arid one of the most successful is Einthoven's 
string galvanometer. In this, the moving part is a silvered glass 
thread stretched in a magnetic field and so light in weight that it 
can follow the most rapid changes with a minimum of distortion. 
It is shown arranged for recording photographically the currents 
produced by the beating heart, the limbs of the subject acting 
as leads from the heart to the galvanometer. The study of these 
records in patients with heart disease has been of great importance 
to the physician. Photographs are also shown of the action current 
in a nerve and of those developed in the muscles of the forearm 
when the hand is tightly clenched. 

Electric currents are also used by the physiologist for stimulating 
muscles and nerves to activity, and instruments of great precision 
are needed to control the duration of the stimulus or to send in two 
stimuli in rapid succession. The mechanical contact breaker, 
designed by Keith Lucas, strikes open two switches at an interval 
which can be varied from 1/5000 to 1/50 sec. It is used for deter- 
mining the " chronaxie " and the " refractory period " of nerves 
and muscles. 





Physiology. Another field in which electrical methods have come f to play a 

large part is in the determination of the acidity or hydrogen-ion 
concentration (P H ) of the various fluids of the body. The smallest 
change in reaction may have far-reaching effects on the organism, 
and the most delicate method of measuring such changes is by 
means of the hydrogen electrode. The E.M.F. from the electrode 
is balanced against a potentiometer and standard cell. 

The Electro-cardiograph and other apparatus by tl>o 


217. Prof. E. P. Cathcart, F.R.S. 

The Measurement of the Energy expended during Move- 

(A) The Bicycle Ergoinetor. 

This apparatus permits of the accurate investigation of the 
energy of expenditure during bicycling, and more particularly the 
determination of the mechanical efficiency with which the work is 
done. It consists of an ordinary bicycle frame carrying a rear 
wheel which is in. the form of a heavy copper disc. This disc, when 
the subject pedals, is made to rotate between the poles of a powerful 
electro-magnet. By varying the current in the magnet, varying 
resistance to the passage of the disc through the field may be induced. 
The amount of work done in kilogrammetres ean be determined by 
a simple calculation. 

(B) Bomb Calorimeter. 

This is the apparatus which is used to determine the heat (or 
caloric) value of different foodstuffs. The foodstuff, squeezed into 
pellet form, is placed in the bomb, and the bomb, after closure, is 
filled with oxygen under pressure. The bomb is next placed into a 
known volume of water, the temperature of which is very carefully 
determined. When the water temperature is constant, the pellet 
of food material in the bomb is fired by means of electricity and is 
rapidly burnt in the oxygen-rich atmosphere. When it burns, the 
heat given off is taken up by the water, with a rise in temperature 
as a result. The amount of the rise is carefully determined, and 
from this and the volume of water warmed, the heat of combustion 
of the foodstuff may be determined. 

(C) Douglas Bag. 

This is the apparatus used for the indirect determination of the 
energy expenditure of a human being during the performance of 
any kind of work, When the subject, carrying the bag on his back, 
is using the mouthpiece, all his expired air is collected in the bag 
during any determined period. The duration of the experiment 
depends on (i) the size of the bag and (ii) the nature of the work 


(D) Haldane Gas Analysis Apparatus. Physiology. 

A sample of the expired air is collected from the Douglas bag, 
and its content in carbon dioxide and oxygen is determined by 
analysis in this apparatus. A measured amount of air is taken into 
the graduated burette. It is then passed into a solution of potas- 
sium hydroxide to remove the carbon-dioxide, and the resultant 
diminution in volume is measured in the burette. It is next passed 
into an alkaline pyrogallate solution to remove the oxygen and is 
again measured. In this way the carbon -dioxide and oxygen 
content of the sample taken are determined. As the total volume 
of air expired is known, it is easy to calculate the total amount of 
carbon-dioxide expired and of oxygen used during a given period. 

Apparatus in (B), (C) and (D) by MESSRS. BAIRD AND 


218. Prof. D. T. Harris. 

The Physiological Action of Light. 

An assembly of apparatus for the determination of the action of 
light upon the affinity of blood for oxygen is shown. The two 
containers are bottles of identical size and shape made of quartz, 
because this material is transparent to almost all types of radiations, 
including the ultra-violet. One quartz bottle contains a measured 
quantity of blood and the second bottle contains exactly the same 
volume of a saline solution ; the latter bottle is the control bottle and 
is subjected to precisely the same conditions as the bottle con- 
taining the blood. These two bottles are kept at a constant 
temperature of 37 0. in an electrically-controlled thermostat ; 
in this way the effect of heat can be eliminated. The two bottles 
are connected by very thick india-rubber tubing to a U-tube and a 
gradiiated burette arranged according to the compensating device 
of Dr. J. S. Haldane, F.R.S. 

The mercury- vapour lamp is used as a source of light and the 
radiations transmitted through its quartz bulb pass through the 
quartz window in the side of the bath ; the heat rays are trapped 
by the distilled water of the bath and thus only the light rays 
(visible and ultra-violet) reach the blood. Continuous agitation 
of the bottles by means of an electro -motor causes the blood to 
be spread out in a thin film, thus allowing a larger surface of blood 
to be presented to the light and to enable gas exchange between the 
blood and air in the bottle to occur more rapidly. 

The bottle is first covered with a dark opaque jacket and fresh 
blood introduced and suitably*diluted. Time is given for the 


Physiology. attainment of equilibrium between the oxygen in the blo<yl and the 

oxygen in the air above it and the graduated burette is then read 
off. The jacket is now removed, the light switched on, and the 
change of reading due to the light is observed. Finally, the light 
is switched off, the dark jacket replaced, and return to the original 
state occurs. 


219. Dr. E. H. J. Schuster. 

A New Respiration Pump. 

The object of this pump is to keep animals alive by artificial 
respiration while under experiment. It can be used successfully 
with an animal, the brain of which has been removed and even the 
whole head cut off. The pump is provided with two barrels each 
connected with two nozzles ; by means of mechanically operated 
valves, each barrel discharges air through one nozzle and takes it 
in through the other. In the simplest application of the pump, the 
delivery nozzle of one barrel and the suction nozzle of the other 
are connected by rubber tubing with the windpipe of the animal, 
the other two nozzles being left open. On the delivery stroke, air 
is blown by one barrel into the lungs of the animal, while the other 
barrel empties itself into the surrounding atmosphere. On the 
suction stroke, clean air is drawn into the first barrel, and used air 
from the lungs of the animal into the second. Respiration pumps 
with two barrels working on this general principle have been used 
successfully for many years. 

The chief novelty of the pump exhibited consists in the fact 
that it is possible to vary the distance of the crank pin from the 
centre, and so the amount of air delivered at each stroke, while the 
pump is running. This amount can thus be accurately adjusted to 
the needs of the experiment. Another novelty is the provision of a 
simple attachment by which the pump may be converted into 
a perfusion pump that is to say, used to maintain a circulation of 
blood or other fluid through an organ which has been removed 
from the body of an animal. The organ can be kept alive in this 
way for many hours, and experiments made on it for example, by 
introducing various drugs their effects may be studied. Finally, 
the pump is by far the most compact yet made. 




220. Mr. N. K. Adam. Physiology. 

Apparatus for the Study of Thin Surface Films 

on Water. 

A development of the apparatus used by Langmuir (1917), who 
first applied this method to the study of surface films. The 
essentials are : (1) a shallow trough filled with water ; (2) scale for 
measuring length of the films ; (3) glass strips for sweeping the 
surface clean from impurities ; (4) a special balance for measuring 
the force of compression on a float bounding one end of the films ; 
and (5) air jets directed on the gaps between the ends of the float 
and sides of the trough, to prevent escape of the films. 

In use, the surface is first swept clean, and a known quantity of 
the substance to be investigated immediately put on. Measure- 
ments of the areas occupied under various compressions are made, 
at different temperatures. The structure of the film is deduced 
from the compression-area curves. 

The films of fatty substances on water are found to be always 
one molecule thick. There are two main types, the condensed and 
the expanded films. Condensed films pass into expanded films 
when the temperature is raised above a definite point. In. the 
condensed films the molecules are as closely packed as in solids or 
liquids, and the films may be either solid or liquid. The molecules 
are arranged vertically, perpendicular to the water surface. Their 
shapes may be found from the films, to some extent, and are in 
good agreement with the structural formulae of organic chemistry ; 
the dimensions found agree with the results of X-ray analysis of 
crystals. In the expanded films the molecules are not closely 

While the principal application of this method is to the problem 
of exploring the fields of force of molecules, these films are the 
simplest kind of membrane. Leathes has found that the influence 
of lecithin and cholesterol on these films is closely parallel to their 
action on red blood corpuscles. (Jortcr (J . Expl. Me.d., vol. 41, 
p. 4, 1925) has employed this method for estimating very small 
amounts of fat. 


221. Dr. H. Hartridge and Mr. F. J. W. Eoughton. 

Apparatus used for Measuring the Velocity of Kapid 
Chemical Reactions. 

In order to study the velocity of reaction between any two 
fluids, the latter are placed in separate bottles and are driven from 
these through leads, which deliver by means of fine jets into a 
small mixing chamber. Here the two fluids meet one another, mix 
(in less than one thousandth of a second) and then travel with 
uniform velocity down an observation tube fixed to the mixing 
chamber. Observation at different points of the latter, by means 
of the spectroscope (or by other suitable physical methods) gives 
the chemical composition of the fluid at different intervals of time 


Physiology. after mixture. The time can be calculated at once f romp a know- 

ledge of the bore of the tube and the volume of fluid passing in a 
given time along the observation tube. In this way the time course 
of suitable chemical reactions can b? readily followed. The 
apparatus has so far been chiefly used for measuring the rate at 
which oxygen combines with, and dissociates from, the blood 
pigment haemoglobin. 

The method would appear to be applicable to the study of 
many other chemical reactions and physico-chemical processes* 
Reactions half completed in times varying from , O ' ff?5 to 5 seconds 
have been satisfactorily investigated by its use. 


222. Dr. H. flartridge. 
The Keversion Spectroscope. 

This instrument is used for determining accurately the position 
of absorption bands in the visible spectrum. The absorption 
spectrum of the solution under examination is duplicated, one 
spectrum being reversed and situated above the other. By means 
of a suitable mechanism, one of the spectra can be shifted bodily 
so that different parts of the spectra can be made to coincide in 

When absorption spoctra are being investigated, corresponding 
bands are brought into accurate alignment. When two pigments 
with overlapping bands are present in a solution, it is possible to 
calculate the relative proportions of the two pigments present, if 
the position of the resultant absorption band has been accurately 
measured as above. The apparatus was originally designed for 
estimating the ratio of oxygen to carbon monoxide in blood, but 
has since been applied to a number of other purposes. 


223. Dr. //. Harlridye and Mr. C. F. Williams. 
A Self-contained Microscope Illuminator. 

In this illuminator, a small electric lamp contained in a metal 
box forms the source of light. For diffusing the light, a slab of opal 
glass is mounted in the 1 id of the box. An iris diaphragm is mounted 
above this for controlling the area of illumination. The box is 
attached to the tail-piece of the microscope in place of the mirror. 
A substage condenser focusses an image of the aperture in the iris 
diaphragm on to the specimen, which lies on the stage. The 
principal advantage of the illuminator is that when once it has been 
adjusted to the optical axis of the microscope, it remains in adjust- 
ment no matter where the microscope is placed or at what angle it 
is tilted. 



224s. ^Portraits of Eminent Biologists. 

Photographs are exhibited of : 

Charles Darwin (1809-1882). 
Thomas Henry Huxley (1825-1895). 
Alfred Russel Wallace (1823-1913). 
Francis Galton (1822-1911). 
Charles Lyell (1797-1875). 
Joseph Dalton Hooker (1817-1911). 
William Harvey (1578-1657). 
Walter Holbrook Gaskell (1847-1914). 
Tx>rd Lister (1827-1912). 

225. Charter Book of the Royal Society. 
Facsimile Reproduction of the Signatures. 

" Soon after the incorporation of the Society a folio volume 
was prepared of leaves of finest vellum. It is bound in crimson 
velvet with gilt clasps and corners, having on one side a gift plate 
bearing the shield of the Society and on the other the eagle crest. 
Into this volume the Charters were transcribed, and it is thus 
known as the ' Charter- book.' After the Charters and Statutes 
follow the signatures of the Fellows, commencing with that of the 
King, and on the same page, those of the Duke of York (afterwards 
James 11.), George (Prince of Denmark and Consort of Queen Anne) 
and ' Rupert, Fellow.' In the Journal- book, under date January 
I Ith, 1664/5, it is recorded that l the Charter-book of the Society 
was produced wherein His Majesty had written himself CHARLES 
R., FOUNDER : and His Highness the Duke of York, James, 
Fellow ; the Duke of Albemarle also having entered his name at 
the same time.' Pcpys relates that being at Whitehall, k 1 saw the 
Royal Society, bring their new book wherein is nobly writ their 
Charters and Laws, and comes to be signed by the Duke as a Fellow 
and all the Fellows' hands are to be entered there, and lie as a 
monument ; and the King hath put his with the word Founder.' 
Prince Rupert, who was elected in March, 1664, took much interest 
in some branches of Science, and in tho work of the Society. Prince 
George, on November 30th, 1704, * was unanimously chosen a 
member of the Society,' and, on December 13th following, wrote 
his name in the book. After the Royal signatures come the auto- 
graphs of the Fellows who have been admitted from that date down 
to the present day. At the time of his admission each Fellow first 
signs his name in the Charter-book beneath the declaration that he 
will endeavour to promote the good of the Society and obey its 
rules, and he then shakes hands with the President, who declares 
him to be a duly elected Fellow of the Society." 

From the Record of the Royal Society of London, 1912. 

The volume exhibited contains^ series of facsimile reproductions 
of the pages bearing the signatures. 



A programme of scientific films is shown in the exhibition 
galleries, with a miniature projector, the Kodascope, using safety 
film. Certain of the films have been specially taken ; others have 
been copied from existing films of standard size, by arrangement 
with the owners, as under : 

" X-Rays. A Film showing the Principles of X-Rays and th^ 
Essential Operations in the Manufacture of Coolidgo 
Tubes for X-Ray Work." British Thomson-Houston 
Co., Ltd. 

" Wireless Direction-Finding for the Air Services." Marconi's 
Wireless Telegraph Co., Ltd. 

" The European Corn Borer." Provincial Government 
of Ontario, Canada. 

A number of films of scientific interest by the General Electric 
Co., Ltd., Messrs. Kodak, Ltd., and others. 

Examples are also shown of the work of Mr. W. Heape, F.R.S., 
and Mr. H. B. Grylls on high-speed cinematography. The originals 
of these films have been taken on the Heape and (irylls Rapid 
Cinema Machine. 

The machine is capable of taking stereoscopic pairs of photo- 
graphs at any rate from 500 to 5,000 per second, on standard 
film. At full speed, the exposure of each portion of film is 7 5 5 0() 

The principle employed is that the portion of film being exposed 
and the lens forming the image move together, so that there is no 
motion of the image relative to the film. Exposure is made 
through a narrow slit of variable width. 

Those films which have been reduced from the standard size 
have been copied by MESSRS. KODAK, LTD., who have also 
kindly provided the Kodascope projector. 



(The. Numbers refer to page*). 


Adam, N. K. 227 

Adrian, Dr. K. I). . 223 

Aston, Dr. F. W. . . . 172 

Blackmail, Prof. V. H. 222 

.Botany, Department of, British 

Museum 217 

Bragg, Prof. W. L. 170 

Bragg, Sir William . 176, 192 

British Museum, Department of 

Botany . 217 

British Museum (Natural His- 
tory) 213, 214, 215, 217 
British Thomson -Houston Co., 

Ltd . 230 

Cathcart, Prof. K. P 224 

Cave, Capt. C. J. P. . . 205 

Cheshire, Prof. F. J. . 189 

Chree, Dr. C. . . 200, 209 

Clarendon Laboratory 171, 170, 179 


Clark, J. S. . 1K8 

Crew, Dr. F. A. K. 210 

Curtis, Dr. W. K. 182 

Da vies, Dr. Ann C. 181 

Dominion Astrophysical Obser- 
vatory, British Columbia 184 
Dye, D. W. .. . 193, 197, 198, 201 
Edwards, F. D. 174 

Fleming, Dr. J. A. 200 

Fowler, Prof. A. 183 

Fox, Dr. J. J. . . 192 

Gahan, Dr. C. J. ... 213 

Gamble, W 185 

General Electric Company, Ltd. 230 
General Klectric Company, Ltd., 
Research Laboratories' 198, 199, 
200, 203 

Good, K. D'O. . 217 

Groom, Prof. P. . .... 223 

Grylls, H. B 230 

Guild, J. .... 178 

Haigh, W. D. . .... 190 

Harris, Prof. D. T. . 225 

Harrison, T. H. . . . 181 

Hartree, D. R 168, 170 

Hartridge, Dr. H. .. 227, 228 

Heape, W. i 230 

Hill, Prof. J. P. 215 

Horton, Prof. F. 181 

Jackson, Sir Herbert ... 177, 190 

Joly, Prof. J 172 

Kaye, Dr. G. W. C 175, 17? 

Keeley, T. C 171, 176, 1T9,191 

(B 34/2285)Q 


King, E. B. . .. 171, 176, 179, 191 
Kodak, Ltd. ... . 230 

Laboratories, Research, General 

Klectric Company, Ltd J98, 

199, 200, 203 

Laboratory, Clarendon 171, 176, 
179, 191 
Laboratory, National Physical 

173, 175, 177, 178, 181, 188, 

193, 194, 195, 196, 197, 198, 


Lang, Dr. W. 1). 214 

Lindemann, Prof. K. A. 171, 176, 
179, 191 
Marconi's Wireless Telegraph 

Co., Ltd 230 

National Physical Laboratory 

173, 175, 177, 178, 181, 188, 

193, 194, 195, 196, 197, 198, 


Observatory, Dominion Astro- 
physical, British Columbia . 184 
Observatory* Royal, Kdin burgh 180 
Owen, Dr. K. A/ . 173 

Plaskett, Dr. J. S. ... 184 

Poulton, Prof. K B. . . . 212 
Provincial Government of 

Ontario 230 

Punnett, Prof. R. C. . .216 

Rankine, Prof. A. 186 

Regan, C. Tate . . . . 214 

Reiivlle, Dr. A. B. . .. 217 

Robertson, Sir Robert .... .192 

Rontgen Society 175 

Roughton, F. J. W. . 227 

Royal Observatory, Edinburgh 180 
Rutherford, Sir Ernest . . 163 

Schuster, Dr. E. H. J .226 

Shaw, J. J 210 

Shaw, Sir Napier .. . 203, 204 

Smith, F. E 192 

Smith- Rose, Dr. R. L. 

194, 195, 196 

Thomas, H. Hamshaw 220, 221 
Thomson, Sir Joseph .*.. 163, 172 

Turner, Prof. H. H 210 

Twyman, F. . . 185,191 

Whiddington, Prof. R. 194, 201 

Wilberforce, Prof. L. R. ... 178 

Williams, C. F 228 

Wilson, Prof. C. T. R 170 

Wright, C. S. .. . :... 206, 209 



(The Number A refer to pages.) 


Baird and Tatlock (London), Ltd., 14, Cross Street, Hatton Garden, 

K.C.I 101, 225, 226 

British Thomson- Houston Co., Ltd., Crown House, Aldwych, W.C.2 201, 230 
S. G. Brown, Ltd., Victoria Road, North Acton, W.3 .. . 171, 203 

Cambridge Instrument Co., Ltd., 45, Grosvenor Place, R.W.I, 

165, 171, 179, 181, 194 

Cox-Cavendish Electrical Co. (1924), Ltd., 105, Great Portland Street, 

W.I 183 

Dubilier Condenser Co. (1921), Ltd., Ducon Works, Victoria Road, 

W.3 194 

Edison Swan Electric Co., Ltd., 123, Queen Victoria Street, E.C.4 .... 201 

W. Edwards and Co., 8A, Allendale Road, S.E.5 175 

General Electric Co., Ltd., Magnet House, Kingsway, W.C.2 .... 181, 194, 230 

Adam Hilger, Ltd., 75A, Camdcn Road, N.W.I . 173, 176, 182, 183, 184, 185, 

189, 190, 191, 203 

Kodak, Ltd., Kingsway, W.C.2 230 

M.O. Valve Co., Ltd., Brook Green, W.6 201 

Marconi's Wireless Telegraph Co., Ltd., Marconi House, Strand, W.C.2 230 
Metropolitan-Vickers Electrical Co., Ltd., 4, Central Buildings, S.W.I 171 

Mullard Radio Valve Co., Ltd., 45, Nightingale Lane, S.W.12 201 

Negretti and Zarnbra, 38, Holborn Viaduct, E.C.I 205 

W. G. Pye and Co., Grarita Works, Cambridge 176, 182 

Siemens Bros, and Co., Ltd., Caxton House, Tothill Street, S.W.I .... 171 

W. F. Stanley and Co., Ltd., 286, High Holborn, W.C.I 205 

H. W. Sullivan, Ltd., 368, Winchester House, E.C.2 171,203 

H. Tinsley and Co., Werndee Hall, Stanger Road, S.E.25 .... 194, 203 

Watson and Sons (Electro- Medical), Ltd., 43, Parker Street, Kings- 
way, W.C.2 173 

White Electrical Instrument Co., Ltd., 2, Gloucester Street, E.C.I .... 182 

P 81 18/cc H & S, Ltd. Gp. 34