[Illustration]
CONCERNING THE
NATURE OF THINGS
_By_
SIR WILLIAM BRAGG
K. B. E., D.Sc., F. R.S.
_Illustrated with Many
Diagrams and Plates_
[Illustration]
HARPER & BROTHERS, PUBLISHERS
NEW YORK AND LONDON
MCMXXV
CONCERNING THE
NATURE OF THINGS
Copyright, 1925
By Harper & Brothers
Printed in the U. S. A.
CONTENTS
CHAPTER PAGE
Introduction xii
Preface to the English Edition xv
I. The Atoms of which Things are Made 1
II. The Nature of Gases 46
III. The Nature of Liquids 90
IV. The Nature of Crystals: Diamond 126
V. The Nature of Crystals: Ice and Snow 172
VI. The Nature of Crystals: Metals 215
Note 250
LIST OF PLATES
PLATE _Facing page_
I. (A) MODEL OF BISMUTH CRYSTAL.
(B) THE SHAKING SAND BOX 16
II. SHIMIZU-WILSON HAY TRACK APPARATUS 26
III. ALPHA RAY TRACKS 30
IV. (A) BAR MAGNETS ON SPIRAL SPRINGS.
(B) MODELS OF ATOMS WITH
ELECTRONS 34
V. (A) FLOATING MAGNETS. (B) CRYSTALS
IN TUBE CONTAINING
EMANATION 42
VI. (A) TUNING FORK OVER JAR. (B) FOG
APPARATUS 60
VII. CIGARETTE SMOKE 66
VIII. EXPERIMENT SHOWING THE PRESSURE
OF THE ATMOSPHERE 72
IX. CAVITATION CAUSING EROSION OF PROPELLER
BLADES 98
X. (A) LARGE DROP OF ORTHOTOLUIDINE.
(B) ONE SOAP BUBBLE INSIDE
ANOTHER 104
XI. (A) CIRCLES CLEARED BY MINUTE
DROPS OF OIL. (B) THE CAMPHOR
BOAT 112
XII. (A) STORMY WATER, (B) OIL STILLING
THE STORM, (C) GRAPE IN
SODA WATER 118
XIII. CRYSTALLINE FORMS 134
XIV. (A) DIAMOND MODEL, (B) PLANE LATTICE
IN THE DESIGN OF A WALL-PAPER 148
XV. THE CULLINAN DIAMOND 156
XVI. (A) LAYERS OF THE GRAPHITE CRYSTAL.
(B) POSSIBLE FORMS OF
THE BENZENE RING 160
XVII. SNOW CRYSTALS OF VARIOUS FORMS 172
XVIII. MORE SNOW CRYSTALS 174
XIX. A HALO AND MOCK SUNS 176
XX. (A) GLACIER ICE. (B) MODELS OF ICE
STRUCTURE 188
XXI. (A) MODEL OF PENTANE, (B) X-RAY
SPECTRUM OF A HYDROCARBON.
(C) MODEL OF ROCK SALT 206
XXII. TWO PHOTOGRAPHS OF ALUMINIUM 216
XXIII. CRYSTAL GRAINS IN A SAMPLE OF STEEL 218
XXIV. ILLUSTRATING “CLOSE PACKING” 220
XXV. (A) CUBIC PACKING, (B) HEXAGONAL
PACKING 222
XXVI. THE YIELDING OF ALUMINIUM UNDER
STRAIN 230
XXVII. DAMASCUS BLADES 236
XXVIII. CEMENTITE CRYSTALS 238
XXIX. (A) CEMENTITE CRYSTALS BEING
BROKEN UP AND ROUNDED OFF.
(B) A NEEDLE SCRATCH IN A
POLISHED PIECE OF SPECULUM
METAL 240
XXX. EFFECT ON SPECULUM METAL OF RUBBING
(WITH FINE EMERY) AND
POLISHING (WITH ROUGED
LEATHER) 242
XXXI. A CONTINUATION OF PLATE XXX 244
XXXII. EXPERIMENT SHOWING EFFECT OF
TEMPERATURE ON ELECTRICAL
RESISTANCE 246
LIST OF FIGURES
PAGE PAGE PAGE PAGE
Figure 1 10 Figure 1A 18 Figure 2 28 Figure 3 35
Figure 4 57 Figure 5 68 Figure 6 69 Figure 7 70
Figure 8 73 Figure 9 74 Figure 9A 75 Figure 10 78
Figure 11 79 Figure 12 79 Figure 13 93 Figure 14 96
Figure 15 100 Figure 16 101 Figure 17 104 Figure 18 104
Figure 19 106 Figure 20 108 Figure 21 109 Figure 21A 109
Figure 22 117 Figure 23 122 Figure 24 123 Figure 25 124
Figure 26 133 Figure 27 134 Figure 27A 136 Figure 28 139
Figure 29 146 Figure 30 147 Figure 31 148 Figure 32 150
Figure 33 152 Figure 34 159 Figure 35 160 Figure 36 162
Figure 37 162 Figure 38 167 Figure 39 168 Figure 40 170
Figure 41 176 Figure 42 176 Figure 43 179 Figure 44 181
Figure 45 183 Figure 45A 184 Figure 46 189 Figure 47 191
Figure 48 192 Figure 49 196 Figure 50 205 Figure 51 208
Figure 52 209 Figure 53 219 Figure 54 222 Figure 55 225
Figure 56 226 Figure 57 229
INTRODUCTION
IT may be well to explain to American readers for whom this special
edition is published that the Juvenile Lectures of the Royal
Institution were first instituted in 1825. At a general meeting of
the members in December of that year, the Managers reported “that
Mr. Millington had engaged to deliver a set of Lectures on Natural
Philosophy, suited to a Juvenile Auditory, during the Christmas recess,
to be continued and concluded in those of Easter and Whitsuntide.”
For some reason the actual delivery of the course was commenced on
Christmas, 1826, by J. Wallis on “Astronomy.”
In February, 1826, the Managers reported that “they had consulted with
Mr. Faraday on the subject of engaging him to take part in the Juvenile
Lectures proposed to be given during the Christmas and Easter recesses.
The Managers found it would be exceedingly inconvenient to withdraw
Mr. Faraday from his experimental investigations to conduct such
lectures.” Faraday, however, found time in subsequent years to give
no less than nineteen courses of Christmas Lectures. He was extremely
interested in the importance of Science Lectures to young people, and
his Juvenile Lectures were masterpieces of exposition.
Tyndall gave a number of these courses, and others were given by Dewar,
Ball, Fleming, S. P. Thompson, Lodge, Boys, and other well-known
lecturers.
It has become a tradition in the Royal Institution that the lectures
shall be fully illustrated by experiments, and it has generally been
the case that both lectures and experiments have been to some degree
novel, in both the matter and the manner of their exposition. The
audience consists not only of the juveniles for whom the lectures are
primarily intended, but also of seniors who are interested in the
attempts to explain new points or show novel illustrations.
I have written this in the hope that those in America who do me the
honor of reading this book will sympathize in the problem of exposition
which I have tried to solve. The English edition contains a preface
which will be found reproduced in the following pages, and will help
further to explain the immediate purpose of the lectures.
W. B.
London, January, 1925.
PREFACE TO THE ENGLISH EDITION
IT was my endeavor at the Christmas Lectures given at the Royal
Institution in 1923-24 to describe certain features of the recent
discoveries in physical science. Many of the facts that have come to
light might well be the subject of “Lectures adapted to a Juvenile
Auditory,” and would be at the same time interesting and helpful;
interesting because they display a beautiful order in the fundamental
arrangement of Nature, and helpful because they have given us light on
many old questions, and will surely help us with many that are new.
I was aware of two special difficulties. The first was the difficulty
of understanding the minuteness of the scale on which the action and
properties of the atoms must be represented; but, after all, this was
only a difficulty due to unfamiliarity, and would come to a timely end.
The other was the difficulty of grasping arrangements in space. There
are some who think that this difficulty is incurable, and that it is
due to the want of some special capacity, which only a few possess. I
am persuaded that this is not the case: we should have nearly as much
difficulty in grasping events in two dimensions as in three were it
not that we can so easily illustrate our two dimensional thoughts by
pencil and paper. If one can turn over a model in one’s hand, an idea
can be seized in a mere fraction of the time that is required to read
about it, and a still smaller fraction of the time that is required to
prepare the description.
Perhaps some of the readers of this book will be sufficiently
interested to make models of the few crystal structures that are
mentioned in it, and may even go on to other structures that are
described in larger books or in original papers.
I have added somewhat to the lectures as originally given. The
additions are intended to make the treatment of the subject a little
more complete: they were not very suitable for consideration at the
lectures, but are perhaps permissible in the book because the reader
can omit them if he desires or read them more than once, or consider
them with a model in his hand.
At the end of the book there is a short note on the making of models.
CONCERNING THE
NATURE OF THINGS
CONCERNING THE NATURE
OF THINGS
CHAPTER I
_The Atoms of Which Things Are Made_
NEARLY two thousand years ago, Lucretius, the famous Latin poet, wrote
his treatise, _De rerum natura_--concerning the nature of things.
He maintained the view that air and earth and water and everything else
were composed of innumerable small bodies or corpuscles, individually
too small to be seen, and all in rapid motion. He tried to show that
these suppositions were enough to explain the properties of material
things. He was not himself the originator of all the ideas which he
set forth in his poem; he was the writer who would explain the views
which were held by a certain school, and which he himself believed
to be true. There was a rival set of views, according to which,
however closely things were looked into, there would be no evidence
of structure: however the water in a bowl, let us say, was subdivided
into drops and then again into smaller drops and so on and on, the
minutest portion would still be like the original bowl of water in all
its properties. On the view of Lucretius, if subdivision were carried
out sufficiently, one would come at last to the individual corpuscles
or _atoms_, the word atom being taken in its original sense,
something which _cannot be cut_.
There is a mighty difference between the two views. On the one, there
is nothing to be gained by looking into the structure of substances
more closely, for however far we go we come to nothing new. On the
other view, the nature of things as we know them will depend on the
properties of these atoms of which they are composed, and it will be
very interesting and important to find out, if we can, what the atoms
are like. The latter view turns out to be far nearer the truth than the
former; and for that all may be grateful who love to inquire into the
ways of Nature.
Lucretius had no conception, however, of atomic theories as they stand
now. He did not realize that the atoms can be divided into so many
different kinds, and that all the atoms of one kind are alike. That
idea is comparatively new: it was explained with great clearness by
John Dalton at the beginning of the nineteenth century. It has rendered
possible the great advances that chemistry has made in modern times
and all the other sciences which depend on chemistry in any degree.
It is easy to see why the newer idea has made everything so much
simpler. It is because we have to deal with a limited number of sorts
only, not with a vast number of different individuals. We should be
in despair if we were compelled to study a multitude of different
atoms in the composition of a piece of copper, let us say; but when
we discover that there is only one kind of atom in a piece of pure
copper, and in the whole world not many different kinds, we may feel
full of enthusiasm and hope in pressing forward to the study of their
properties, and of the laws of their combinations. For, of course, it
is in their combinations that their importance lies. The atoms may be
compared to the letters of the alphabet, which can be put together in
innumerable ways to form words. So the atoms are combined in equal
variety to form what are called molecules. We may even push the analogy
a little further and say that the association of words into sentences
and passages conveying meanings of every kind is like the combination
of molecules of all kinds and in all proportions to form structures and
materials that have an infinite variety of appearances and properties
and can carry what we speak of as life.
The atomic theory of Lucretius did not contain, therefore, the
essential idea which was necessary for further growth and progress.
It withered away, and the very atom came to be used in a vague,
incorrect fashion as meaning merely something very small: as sometimes
in Shakespeare’s plays, for instance. In another and very different
application of “atomic” theory Lucretius was strangely successful. He
had the idea that disease was disseminated by minute particles. At the
time of the Renaissance Fracastoro was inspired by the atomic theory of
infection as he read it in the poem of Lucretius; but after his day the
secret of bacteriology was again covered up until it was laid bare by
Pasteur.[1]
Let us think of Nature as a builder, making all that we see out of
atoms of a limited number of kinds; just as the builder of a house
constructs it out of so many different kinds of things--bricks, slates,
planks, panes of glass, and so on. There are only about ninety sorts of
atoms, and of these a considerable number are only used occasionally.
It is very wonderful that all the things in the world and in the
universe, as far as we know it, are made of so few elements. The
universe is so rich in its variety, the earth and all that rests on
it and grows on it, the waters of the seas, the air and the clouds,
all living things that move in earth or sea or air, our bodies and
every different part of our bodies, the sun and moon and the stars,
every single thing is made up of these few kinds of atoms. Yes, one
might say, that is so: but if the builder is given bricks and mortar
and iron girders he will build you an infinite variety of buildings,
palaces or cottages or bridges; why may not Nature do something
like that? But one has to think that when a builder sets out to make
a structure he has a plan which has cost thought to devise, and he
gives instructions to his workmen who are to carry out his wishes,
and so the structure grows. We see him walking about with his plans
in his hand. But the plans of the structures of Nature are locked up
in the atoms themselves. They are full of wonder and mystery, because
from them alone and from what they contain grows the infinite variety
of the world. How they came to be such treasure-houses we are not
asking now. We ask ourselves what these atoms are like: we have been
asking the question ever since their exceeding importance began to be
realized more than a hundred years ago. Have they size and form and
other characteristics such as are possessed by bodies with which we are
familiar? We must look into these points.
But first let us realize that in the last twenty-five years or
so we have been given, so to speak, new eyes. The discoveries of
radioactivity and of X-rays have changed the whole situation: which is
indeed the reason for the choice of the subject of these lectures. We
can now understand so many things that were dim before; and we see a
wonderful new world opening out before us, waiting to be explored. I
do not think it is very difficult to reach it or to walk about in it.
In fact, the new knowledge, like all sudden revelations of the truth,
lights up the ground over which we have been traveling and makes things
easy that were difficult before. It is true that the new lines of
advance now open lead the way to fresh difficulties: but therein lies
the whole interest and spirit of research. We will try to take the
first steps into the new country so that we may share in the knowledge
that has already come, and comes in faster every day.
We go back to our questions about the atoms. Before the new period
set in remarkably accurate answers had already been given to some of
them, at least. In this theater of the Royal Institution, Lord Kelvin
gave several addresses which dealt with the properties of atoms, and
especially with their sizes. By several most ingenious and indirect
devices he arrived at conclusions which we are now able to test by
accurate methods; and we find that he was remarkably close to the
truth. It was, of course, far more difficult to say what was the size
of any particular atom than it was to say how much larger one atom was
than another. For instance, the sizes of the atoms of potassium and
carbon could be roughly compared by taking into account the relative
weights of equal volumes of the solid potassium metal and of diamond,
which is a form of pure carbon. Potassium is lighter than water, the
diamond is three and a half times as heavy. We know from chemical
observations that the individual potassium atom is rather more than
three times as heavy as the carbon atom. If we suppose that the packing
of the atoms in the two cases is the same (as a matter of fact, we now
know that it is only approximately so) we must conclude that the atoms
in the metal potassium are much larger than the carbon atoms in the
diamond, because, though heavier individually, they pack so as to make
a lighter material.
To make a reasonable estimate of the actual size of any one atom is a
much more difficult matter, but all the four lines of reasoning which
Kelvin employed led him to very nearly the same result. “The atoms or
molecules of ordinary matter must be something like the 1/10,000,000th
or from the 1/10,000,000th to the 1/100,000,000th of a centimeter in
diameter.”[2] Our new methods tell us that the diameter of the carbon
atom in diamond is 1.54 hundred-millionths of a centimeter, and that of
the atom in the metal potassium is 4.50 hundred-millionths. We see that
Lord Kelvin’s estimate was wonderfully near the truth, considering the
indirect and inexact methods which alone were at his disposal.
In Fig. 1 are shown sections of certain atoms on a scale of fifty
millions to one. The inserted figures give in each case the distance,
in hundred-millionths of a centimeter, between the centers of two
neighboring atoms in the pure substance. For example, the distance
between two carbon atoms in the diamond is 1.54 hundred-millionths of
a centimeter. In the case of oxygen the diameter has been calculated
from the structure of crystals in which oxygen occurs. If the lecture
room of the Royal Institution were magnified as much as the atoms of
Fig. 1, its height would be greater than the distance from the earth to
the moon. We need some such comparison as this to make us realize the
excessive smallness of the things of which we are talking.
[Illustration: Fig. 1.--Sections of some common atoms, in
hundred-millionths of a centimeter. In reference to bismuth see below
(p. 13) and Plate I a.]
At the same time, we must keep in mind that they are not negligible
because they are small: they are the actual elements of construction of
the world and of the universe, and their size has nothing to do with
their importance. But their smallness accounts readily for the case
with which we all overlook them, and for the difficulty we have in
examining them when at last we have realized what they mean to us. The
value of the new methods of which I propose to speak lies in the fact
that they enable us to deal with them, although they are so small.
We have now answered in a way the question as to the size of the atoms;
but when we go further and ask ourselves about the shape we are not so
successful.
The chemist, whose science is immediately concerned with the
combinations of atoms, has rarely found it necessary to discuss their
shapes, and gives them no particular forms in his diagrams. That does
not mean that the shapes are unimportant, but rather that the older
methods could not define them. There is one sense, however, in which
the chemist pays much attention to form. The atoms in a compound
are arranged in some fashion or other which is important to the
combination. If one could see it and sketch it, one would be obliged to
show it in perspective. In the science of organic chemistry especially
it is found to be necessary to imagine such arrangements in space. It
is not enough to represent them on the flat with no perspective at all;
in fact, it is obvious that any flat design must be imperfect in any
sort of chemical picture. We are, unfortunately, compelled to use the
flat for our drawings; solid models in space are costly to make, while
paper and pencil are cheap. It is curious to reflect what a handicap
this technical difficulty puts on the proper development of a very
important matter. Now, when we come to prescribe the arrangements of
the atom to its neighbors, and to say that if one neighbor lies in this
direction, another must lie in that, we are, in effect, giving shape
to our atoms; at any rate, it is all the shaping that can be done for
the present. We cannot do more until we know more about the internal
structure of the atom: what its parts are, and how they are disposed to
one another.
In the newer work, as we shall see, the arrangement of the atoms
is much more closely examined, and for the first time their actual
distances apart are measured. We find it absolutely necessary to make
models because we do not see with sufficient clearness if we are
content to draw on paper. We represent our atoms as round balls, and we
find that we are able to represent most of our discoveries in this way.
This really means that when an atom has several neighbors of the same
kind it is equally distant from them all; and this is actually the
case. Nevertheless, there are exceptions, as in the crystal of pure
bismuth, where each atom has six neighbors and three of them are closer
than the other three. We have to make a ball with three flats on it for
use in constructing the bismuth model (Plate I a).
Let us now ask ourselves what binds the atoms together into the
various combinations and structures. Like our builder, we have got
in our materials--the bricks, slates, beams, and so on; we have our
various kinds of atoms. If we look round for mortar and nails we find
we have none. Nature does not allow the use of any new material as a
cement. The atoms cling together of themselves. The chemist tells us
that they must be presented to one another under proper conditions,
some of which are very odd; but the combination does take place, and
there is something in the atoms themselves which maintains it when the
conditions are satisfied. The whole of chemistry is concerned with the
nature of these conditions and their results.
The atoms seem to cling to one another in some such way as two magnets
do, when opposite poles are presented to each other; or two charges of
electricity of opposite nature. In fact, there is no doubt that both
magnetic and electric attractions are at work. We are not entirely
ignorant of their mode of action, but we know much more about the rules
of combination--that is to say, about the facts of chemistry--than we
do about the details of the attractions. However, we need not trouble
ourselves about these matters for the present; we have merely to
realize that there are forces drawing atoms together.
We may now ask why, if there are such forces, the atoms do not all join
together into one solid mass? Why are there any gases or even liquids?
How is it that there are any atoms at all which do not link up with
their neighbors? What prevents the earth from falling into the sun and
the final solidification of the entire universe?
The earth does not fall into the sun because it is in motion round
the sun, or, to be more correct, because the two bodies are moving
round each other. It is motion that keeps them apart; and when we look
closely into the matter we find that motion plays a part of first
importance in all that we see, because it sets itself against the
binding forces that would join atoms together in one lump. In a gas,
motion has the upper hand; the atoms are moving so fast that they
have no time to enter into any sort of combination with each other:
occasionally atom must meet atom and, so to speak, each hold out vain
hands to the other, but the pace is too great and, in a moment, they
are far away from each other again. Even in a liquid where there is
more combination and atoms are in contact with each other all the time,
the motion is so great that no junction is permanent.
In a solid the relative importance of the attractive forces and the
motion undergoes another change: the former now holds sway, so that
the atoms and the molecules are locked in their places. Even in the
solid, however, the atoms are never perfectly still; at the least
they vibrate and quiver about average positions, just as the parts
of an iron bridge quiver when a train goes over it. It is difficult
to realize that the atoms and molecules of substances which appear
to be perfectly at rest--the table, a piece of paper, the water in a
glass--are all in motion. Yet many of the older philosophers grasped
the fact. For example, Hooke, an English physicist of the seventeenth
century, explains by a clear analogy the difference which he supposed
to exist between the solid and the liquid form: ascribing it to a
movement of the atoms which was greater in the liquid than in the
solid state. “First,” he says, “what is the cause of fluidness? This
I conceive to be nothing else but a very brisk and vehement agitation
of the parts of a body (as I have elsewhere made probable); the parts
of a body are thereby made so loose from one another that they easily
move any way, and become fluid. That I may explain this a little by
a gross similitude, let us suppose a dish of sand set upon some body
that is very much agitated, and shaken with some quick and strong
vibrating motion, as on a millstone turn’d round upon the under stone
very violently whilst it is empty; or on a very stiff drum-head, which
is vehemently or very nimbly beaten with the drumsticks. By this means
the sand in the dish, which before lay like a dull and unactive body,
becomes a perfect fluid; and ye can no sooner make a hole in it with
your finger, but it is immediately filled up again, and the upper
surface of it leveled.
[Illustration: Plate I]
Nor can ye bury a light body, as a piece of cork under it, but it
presently emerges or swims, as ’twere on the top; nor can ye lay a
heavier on the top of it, as a piece of lead, but it is immediately
buried in sand, and (as ’twere) sinks to the bottom. Nor can ye make
a hole in the side of the dish, but the sand shall run out of it to
a level. Not an obvious property of a fluid body, as such, but this
does imitate; and all this merely caused by the vehement agitation of
the containing vessel; for by this means, each sand becomes to have a
vibrative or dancing motion, so as no other heavier body can rest on
it, unless sustein’d by some other on either side: nor will it suffer
any body to be beneath it, unless it be a heavier than itself.”
Hooke’s experiment can be repeated in a somewhat different form. A
cylindrical metal box, ten inches wide and three inches deep, is fixed
upon a platform which is supported on metal balls so that it moves
easily. It is connected through an eccentric joint with a turning
table as shown in Plate I B. When the wheel is turned rapidly,
the box and the sand which it contains are violently agitated as
Hooke prescribes. The details of the mechanism are best understood by
reference to the figure. A heavy metal ball placed on top of the sand
disappears at once, and light objects, such as ping-pong balls, rise
to the surface. A very ludicrous effect is produced if we bury in the
sand some of the celluloid figures which cannot be made to lie down
because they are heavily weighted at the bottom. The figures slowly
rise out of the sand and finally stand erect (Plate I b and
Fig. 1 a).
[Illustration: Fig. 1 a]
We know now that the motion of the atoms of a body is really its heat:
that the faster they move or vibrate the hotter the body becomes.
Whenever we warm our hands by the fire, we allow the energy radiated by
the fire to quicken up the movements of the atoms of which the hands
are composed. When we cool any substance we check those movements. If
we could still them altogether we should lower the temperature to a
point beyond which it would be impossible to go, the absolute zero, as
it is usually called, 273 degrees centigrade below zero.
As I have said already, we have found two new allies, radioactivity and
X-ray, in our attempt to see the very minute atom. They have increased
the fineness of our vision some ten thousand times. The microscope had
done its best for us; but the smallest thing which it could show us
was composed of billions of atoms. No improvement could be made in the
microscope lenses: technique had reached its highest. The difficulty
was really due to the fact that light is a wave motion and light
waves cannot show us the details of objects unless the objects are
much larger in every way than the length of the wave. We wanted a new
light of very short wave length. It came in the form of the X-rays.
At the same time radioactivity came to show us what a single atom
could do by itself if it were given a tremendous speed. We can now see
the single atom, indirectly, no doubt, but quite usefully, and also
observe something which it does: the X-rays help us with the former and
radioactivity with the latter. I hope to explain to you how both these
agents are adding to our knowledge, and I shall take radioactivity
first.
The atom of radium might be roughly represented in size by one of the
larger balls that lie before you. It is one of the heaviest and largest
of the atoms; a number of them together form a substance which is a
metal like iron or gold. It is, of itself, in no obvious way peculiar
as long as it continues to be an atom of radium, but, for some reason,
which no one understands, there comes a moment when it bursts. A small
portion is hurled away like the shot from a gun, and the remainder
recoils like the gun itself. The remainder is not radium any more, it
is a smaller atom, having entirely different properties. The radium has
turned into a new substance. As a matter of fact, the new substance is
a gas, while the projectile turns out to be an atom whose weight is low
down in the series of atomic weights, the lowest but one in fact; it is
called helium. No one knows what brings about the explosion, nor does
anyone know a way of hastening it, or of hindering it. The radium atom
is just as likely to explode at any given moment if it is in a furnace
as if it is immersed in liquid air. Indeed, its independence of its
surroundings in respect to its time of explosion is shown in a much
stronger light by the fact that combination with other atoms makes no
change. Combination, or molecule-forming, is, no doubt, concerned with
the outside arrangements of the atoms, but the bursting of the atom
comes from inside.
The old alchemist tried to find a means of converting one atom into
another, preferably lead into gold. In the action of radium there
is a transmutation, to use an old word, of the kind of which the
alchemist dreamt. But it is not exactly what he strove for, in two
ways. In the first place, it cannot be controlled by human will--which
is extraordinary, because there are not so many things of which this
can be said. Even when an operation is quite beyond our power to
understand, we can often decide whether or no it shall happen. We
cannot understand how a seed germinates, much less make one that will
do so; but we can lock up seeds in a drawer and prevent them from
germinating as long as we like. But the radium explosion does not wait
on anything which we do.
In the second place, the transmutation does not end in gold: it ends
rather in lead. The gas which consists of atoms of radium that have
shot off one atom of helium is very short-lived: the average life of
each of its atoms is a little less than four days, in contrast to the
average life of the radium atom, which is about two thousand years.
The second explosion “transmutes” the gas atom into a new substance
called Radium A, and on the occasion another helium atom is shot away.
There is a further succession of explosions, at very varying average
intervals, and the final product is actually lead, not gold. The gas
was called the “radium emanation” by Rutherford, who discovered it.
The whole operation is very wonderful, but I want to call attention to
what happens to the projectile when it has left the gun. The velocity
with which it starts is so great that one could never have thought any
particle of matter could have possessed it. When Huygens argued with
Newton on the subject of the nature of light, he condemned Newton’s
idea that light consisted of a flight of corpuscles, on the ground that
material particles could not possibly travel as fast as light had just
been found to move. It is curious that we now find atoms moving with
speeds comparable with, a tenth or twentieth of, that which then seemed
impossible. There are even certain particles, called electrons, also
emitted by radioactive substances, which travel, in some cases, very
nearly as fast as light. It is also curious that the second argument of
Huygens was equally unfortunate in view of the observed phenomena of
radioactivity. He said that it would be impossible, on Newton’s theory,
for two people to look into each other’s eyes because the particles
would meet each other and fall to the ground. We shall presently see
that this argument also is set at naught by the facts of radioactivity.
The velocity with which the helium atom begins its flight is something
like 10,000 miles in a second. In less than a minute it could get to
the moon and back again if the speed were maintained, but the curious
thing is that for all the speed and energy with which it starts it
never gets far when it has to pass through anything material. Even if
it is allowed to finish its course in the air, its speed has fallen to
something of quite ordinary value after it has traversed a course of
two or three inches in length. The course is, in general, perfectly
straight, as we shall presently see in an actual experiment, and this
is the very important point which we must consider with particular
care. At first sight one does not realize how remarkable it is that its
path should be straight: one thinks of a bullet fired through a block
of wood, let us say, and making a cylindrical hole, or of the bullet in
its straight course through the air. But the comparison is unfair. The
bullet is a mass of lead enormously heavier than any molecule which it
meets, and it brushes the air aside. But the helium atom is lighter and
smaller than the atoms of nitrogen or oxygen of which the atmosphere is
mainly composed, and we must think of some more truthful comparison.
Suppose that a number of billiard balls are lying on a billiard table,
and let them represent air molecules. If they are in movement the
picture will be more correct, but the point does not really matter.
Now let us drive a ball across the table, aiming at a point on the
opposite cushion, and watch what happens as the ball tries to get
through the crowd that lies on the table, which crowd may or may not
be in movement. It hits one of the balls and is turned to one side; it
hits several in succession, and soon loses all trace of its original
direction of movement. Shall we now drive it with all the force we can,
and see whether it keeps any more nearly to the straight path? We try,
and find that there is no improvement at all. The straight path cannot
be obtained by any increase of speed, however great.
This picture or model is much more faithful than that of the moving
bullet, and shows more clearly the remarkable nature of the radium
effect. A helium atom must encounter a very large number of air
molecules if it proceeds on a straight-line path, and if the atoms are
of the size we have supposed them to be. In fact, the molecules lie far
more thickly on the path than we can represent by the billiard-table
model. It is possible to calculate how many air molecules, some oxygen,
some nitrogen, would be pierced by a straight line three inches long
drawn suddenly at any moment in the air, and the result is to be
expressed in hundreds of thousands. How can the helium atom charge
straight through this crowd, every member of which is heavier than
itself? It does so, however, and we have to find some explanation.
Perhaps it might be thought that the straightness of the path is only
apparent, and that if we could look into it in sufficient detail we
should see that it was made up of innumerable zigzags made in going
round the molecules met with. But a moment’s reflection shows that the
idea is absurd: the atom would need to possess the intelligence of a
living being to give it the power of recovering a line once lost. If
there were a cake shop on the opposite side of a crowded street, and if
we gave a boy sixpence and directed him to the shop, he would no doubt
pursue a path which was effectively straight, though it would be broken
up by the need of dodging the various people and vehicles which the boy
met with. But one cannot imagine an atom of helium doing anything of
the sort.
There is only one way of explaining the marvel of the straight path: we
must suppose that the helium atom _goes through_ the molecules it
meets, and that somehow it is enabled to do so by the fact that it is
moving at such an unusual speed. It is a very startling idea.
[Illustration: By courtesy of the Cambridge Scientific Instrument Co.
PLATE II.
Shimizu-Wilson ray track apparatus.
The apparatus which shows the tracks of the helium atom shot out by
radium. The chamber as sketched in Fig. 2 is to be seen on the upper
left of the figure. The disc to the left of it is a screen, in which is
a hole. The light from a lantern--not shown--shines through the hole
and lights up the fog tracks. A second screen revolves, and lets the
radium rays, _i.e._ the helium atom, shine into the chamber just
before the expansion is made. On the right is the driving machinery.]
However, no other suggests itself; and, as a matter of fact, it turns
out that we can explain many other things by its aid. Consequently, we
feel sure that we are on the right track.
[Illustration: Fig. 2.—Section of the expansion chamber in Mr. C. T. R.
Wilson’s apparatus for measuring the track of helium atoms (see also
Plate II).
The piston _PP_ is dropped suddenly from the position indicated by
the dotted lines to the position indicated by the full lines; so that
the air in the chamber is suddenly chilled by expansion and fog settles
on the tracks of the helium atoms shot out by the radium at _R_.]
It is time now that we should see this effect with our own eyes: the
conclusion at which we have arrived is so new and so full of meaning
that we would like to have an experimental demonstration if possible,
and convince ourselves of the reality of these straight-line paths. We
owe to Mr. C. T. R. Wilson a beautiful piece of apparatus which gives
us a vivid picture of what happens, and we will make use of it at once.
The experiment is, in my opinion, one of the most wonderful in the
world of science. We are going to see the actual tracks of separate
helium atoms, each of which begins its course at a speed of ten
thousand miles a second and yet completes it after traversing about
three inches of air. But we must first enter upon some explanation of
how the apparatus works; for there are ingenious devices in it.
There is a cylindrical box of brass, with a glass top and a base which
can be raised or lowered so as to alter the depth of the box. There is
a machinery of wheels, cranks and levers by which the bottom of the box
can be suddenly dropped at convenient intervals. Whenever this happens,
the air or other gas which the box contains is chilled by the sudden
expansion. We shall study effects of this kind more carefully in the
next chapter. At the side of the box, in its interior, a minute speck
of radium is mounted on a suitable holder. Every moment some of its
atoms break up and expel atoms of helium, of which a certain number are
shot straight into the box. The diameter of the box is big enough to
allow the atoms to finish their courses in the air within. The average
life of radium is so long that even if the apparatus held together for
two thousand years, half of the radium speck would still be left. Yet
each second, ten, twenty or a hundred atoms disappear in the expulsion
of the helium atoms. Perhaps in no better way can it be shown how many
atoms are concentrated in a small compass.
The air in the chamber is kept damp, consequently the chill due to
expansion tends to produce a fog. Fog when it has to settle prefers to
deposit itself on a solid nucleus of some sort, rather than to form
independent drops in the air. The small particles of dust, if there are
any, are made use of, which is the reason why fogs so readily form in a
dirty atmosphere. But of all things moisture prefers to settle on those
atoms through which the helium atom has passed. The reason is that
the atom is temporarily damaged by the transit: a small portion has
generally been chipped away. The portion removed is what we now call an
“electron”; it is charged with negative electricity, and the atom which
has lost it is correspondingly charged with positive electricity. The
electron set free settles on some neighboring atom, sooner or later;
and in consequence there are two charged atoms, one positive and one
negative, where previously there were no charged atoms at all. The
charged atoms have a great attraction for moisture, and the fog forms
on them in preference to anything else. If, therefore, a helium atom
has just made its straight road through the gas, and has left behind
it numbers of charged atoms on its track, and if, at that moment,
the sudden expansion causes a chill, fog settles along the track. A
bright light is made to illuminate the chamber, so that the fog tracks
are visible as bright straight lines, showing against the blackened
background of the bottom of the cylindrical chamber. They last a few
seconds and then the fog particles slowly disperse. If the helium atom
completes its track just before the fog is formed, the line is sharp
and clear; because the charged atoms have not had time to wander from
the track. But if the track is made some time before the expansion, the
line of fog is more diffuse. It is to be remembered that the helium
atoms are being shot out all the time, day and night; but it is only
when an expansion is made that tracks are made visible.[3]
[Illustration: PLATE III.]
If we watch the successive expansions, we see that the tracks, though
quite straight over large parts of their course, do undergo at times
sudden sharp deflection, especially when they are nearing the end. This
remarkable effect turns out to be most important, and we must refer to
it presently.
Let us now try to picture to ourselves in what way we must modify our
first conception of the atom so that we can explain the effects we now
see. The atoms must be so constituted that when they meet one another
in the ordinary way, as, for example, when molecules of oxygen collide
in the atmosphere, they behave as if each had a domain of its own into
which no other might enter. Or, when they are pressed together, as in
a solid, they occupy as a whole an amount of space which is sufficient
to make room for them all. But when one atom--the helium atom is our
chief example--is hurled against others with sufficient speed, the one
atom goes through the other, as if the defenses round the domains had
been broken down. We find a satisfactory explanation when we imagine
each atom to be like a solar system in miniature. There is to be a
nucleus, corresponding to the sun, and round the nucleus there are
to be satellites or planets, which we call electrons. The nucleus
is charged with positive electricity; each electron is charged with
negative electricity, and all electrons are alike. The positive charge
on the nucleus is just enough to balance the united negative charges of
the electrons. The electrons are supposed to be in movement, just as
the planets are revolving round the sun, but the movements are no doubt
complicated, and their nature need not for the moment concern us at all.
Instead, therefore, of a round hard ball of a certain size, which was
our first rough picture of an atom, we have something like a solar
system in miniature. We can at once see how one atom of this kind can
pass through another, just as we might imagine one solar system passing
through another, without injury to either provided that no one body of
one system made a direct hit on a body of the other and that the motion
was quick enough. The latter condition is necessary because if one
solar system stayed too long inside or in the neighborhood of another
there would certainly be very serious disturbances of the courses of
the planets.
But then, we may ask, how can an atom, if this be its nature, have the
power of keeping another outside its own domain? How can it appropriate
any portion of space to itself, and prevent the intrusion of another
atom when the speed at which they meet is low? The explanation becomes
clear when we consider the special arrangement of the positive and
negative charges. Every atom is surrounded by a shell or cloak of
electrons; and, when two atoms collide, it is their shells which first
come close together. Since like charges of electricity repel one
another, the two atoms will experience a force which tends to keep
them apart: in other words, they will resist encroachment on their
own domains. This is, no doubt, a very rough picture of what actually
happens, and as a matter of fact it is difficult to explain the
strength of the resisting forces on such a simple hypothesis. Still, it
is on the right lines, no doubt. When the two atoms approach each other
at a high speed, the system of electrons and nucleus of one atom slip
through those of the other. A model will help to illustrate the point.
Plate IV a shows a set of bar magnets mounted on spiral springs and
standing erect. The top of the inside magnet is a north pole, and
the tops of the magnets of the outside ring are south poles. The
model represents roughly the central nucleus surrounded by a ring of
electrons. In the model everything is in one plane; in the atom it is
not so, but the point is not important. A single magnet is suspended
by a long thread from a point vertically over the “nucleus” magnet.
Its lower end is a south pole and the length of the thread is such
that the swinging magnet just clears the fixed magnets. Observe now
that if we pull the swinging magnet to one side (_S_ in Fig. 3,
_a_), but not too far, it moves toward the fixed set and is unable
to enter in. It seems to knock at the door at one place after another,
but always recoils. Just so would any electron beat in vain against
the outer defenses of an atom, if it did not beat hard enough. We can
easily imagine that if the single swinging magnet were replaced by
a system of magnets, like our stationary set, the same result would
follow.
[Illustration: PLATE IV.
=A.= Bar magnets on spiral strings.
=B.= Models of atoms with electrons.]
Here we have a picture representing our atoms, as we now think of them,
beating against one another and recoiling; each occupies a certain
domain of space and prevents the intrusion of any other atom.
[Illustration: Fig. 3 (a), (b), and (c).]
But if the swinging magnet is drawn sufficiently far to one side so
that it acquires a greater speed than before by the time it reaches
the stationary set, its momentum will carry it through. If the speed
is very great, it shows no appreciable change in its motion due to
its passing (Fig. 3, _c_); if the speed is rather less, it often
suffers in going through (Fig. 3, _b_). It comes out less vigorous
than when it went in; often it has changed the direction of motion
also, and it has obviously left energy behind, for the magnets of the
stationary set are left quivering. This happens no matter which pole of
the swinging magnet is the lower, and clearly the same effect would be
shown if the single swinging magnet were replaced by a more complicated
set of nucleus and attendant satellites.
The behavior of the model helps us to anticipate what we should find
when atoms of our new design come across one another. If they approach
at a moderate speed, they may rebound from one another; at a high speed
they go through one another, and the higher the speed, the greater the
chance of a passage without any obvious result. But there is always
the chance that the nucleus of the moving atom may go so near to the
nucleus of the atom through which it is passing that it experiences a
perceptible deflection. The smaller the nuclei are, the less likely it
is that this will happen.
You will have guessed already that you have actually seen such
deflections as these in the kinematograph picture, such as also are
shown in Plate III, a to d. The tracks of the helium atom are quite
straight in the main, but there are decided breaks in the straight
lines, usually not more than one or two in each track. They are found
mainly toward the finish. This is what might be expected, since the
motion will then be slower. Several of them appear in Plate III a; a
very good example of this kind of track is reproduced on a large scale
in Plate III c. The upper track shows a slight but sharp deflection at
a little distance from the end of its course, and a more pronounced
deflection farther on. Nearly every track shows some deviations at the
very end. Thus the new conception of atomic structure explains all the
effects in a satisfactory way.
It is strange to think of an atom as being empty as a solar system;
not a round, hard and absolutely impenetrable body, but a combination
of nucleus and electrons which occupies a certain space somewhat as an
army occupies a country. The bodies of the soldiers do not fill the
country from boundary to boundary; but enemy soldiers may not enter,
nevertheless.
These very characteristic pictures are the fruit of much watching and
photographing. Breaks are found at every expansion, but it may be
necessary to wait for a really good one. A very fine picture is shown
in Plate III d. It is due to Mr. P. Blackett. In this case,
helium was used instead of air. The nucleus of the flying helium atom,
in traversing a helium atom belonging to the gas, has made an almost
direct hit on the nucleus of the stationary atom: it has cannoned
off it, as a billiard player would say. Both atoms now move with not
unequal speeds, and both make fog tracks, as the figure shows. In Plate
III c if we look carefully, we see that there is a minute spur
on the last bend of the track already mentioned, which means that in
this case an atom of oxygen or nitrogen has deflected the helium atom
and has recoiled in consequence. Its track is very short, because it
is much heavier than the atom which struck it, and, therefore, the
velocity given to it has been comparatively small.
There is a certain curious feature to be found in some of the
photographs which may well be explained. In some of the tracks there
are gaps, as if the fog settling had failed. This is indeed the actual
fact: there is no moisture to settle, because a helium atom has gone
that way some very short time before and has used up the moisture in
the neighborhood. In Plate III b several tracks due to radium
emanation are shown. They seem to start from anywhere in the chamber
because the atoms of the emanation have wandered about the chamber
before blowing up.
The next question that arises is as to the number of electron
satellites which each atom possesses. Here we come to a very beautiful
and remarkable feature of the new discoveries. It is not necessary
to explain in full how it was discovered; we will be content with
describing it.
In the atom as we now have it the nucleus is charged with positive
electricity, the amount of the charge being just enough to neutralize
the negative charges on the attendant electrons. All electrons, as we
have already seen, are alike. We find that atoms differ in the number
of attendants which they can maintain, and that the statement of that
number describes the atom completely so far as its attitude toward
other atoms is concerned. For instance, the atom of carbon can hold six
electrons; the positive charge on the nucleus is the counterpart of six
standard negative charges. Every atom which can retain six electrons
is a carbon atom: no other definition of the carbon atom is required.
Just so the “seven-electron” atom is nitrogen, the “eight-electron”
is oxygen, and so on. All numbers are found in nature, with very
few exceptions, from the “one-electron” atom—hydrogen—up to the
“ninety-two-electron” atom—uranium. The missing numbers will probably
be found some day; more or less accidentally, it may well be.
We may use models as a rough illustration of the point. The nucleus
(Plate IV b) is represented by a white ball of solid rubber,
the electrons by smaller balls forming the heads of pins which are
stuck into the center ball. The pins may be of different lengths (p.
84).
It is strange that the immense variety in nature can be resolved into
a series of numbers. It was at one time thought that the various sorts
of atoms owed this variety to something more than that; it is a great
surprise to find such a simple kind of difference between atom and
atom. The unchanging feature of any particular sort of atom is the
positive charge of electricity on the nucleus. It is in consequence of
this that the proper number of electrons gather round. We may expect
that they will arrange themselves in some fashion; we shall see later
that they certainly do so. The sort of arrangement they take in each
case, and the nature of the forces put into it, are very difficult
questions, most of which we may well put to one side for the present,
contenting ourselves with one or two simple aspects of the problem.
In the first place, it is interesting to watch the assembling of
the little vertical magnets floating in the glass tank (Plate V
a). They are buoyed up by ping-pong balls, painted black, and,
so that we may see them easily, they carry white ping-pong balls at
top. The magnets are all the same way up, so that naturally they repel
one another and cluster round the edge of the basin. But there is an
electromagnet underneath the bowl, which, when made active, draws the
small magnets together. The arrangement in which they settle finally
is governed partly by the pull toward the center and partly by the
mutual repulsions. Something of this kind must take place in the atom,
but we must not push the analogy too closely, because the forces may
be quite unlike those which are exerted in the model. We must content
ourselves with observing that when there are only a few magnets afloat
they group themselves in a ring; but when the number is increased they
arrange themselves in concentric rings. A pretty effect is produced by
putting in each additional magnet at the edge of the basin and watching
it float away in a stately fashion to take its proper place.
A similar division into concentric shells or groups is found in the
arrangement of the electrons round the central nucleus of the atom. We
will consider this more carefully in the next chapter. The experiment
does not prove that there ought to be such an arrangement, but
certainly suggests it.
We may now see more clearly what happens when the helium atom injures
the atoms through which it passes and renders them attractive to the
particles of moisture that form the fog. It is possible, in fact, for
an atom to be deprived of one of its attendant electrons. Having lost
one, it resists more strongly the loss of a second, still more of a
third. As the helium atom goes on its way, it strips one atom after
another of an attendant, and the electron set free goes off on a course
of its own. But its separate life is very short-lived: it is soon
attached to another atom.
[Illustration: PLATE V.
=A.= Floating magnets.
When the number of floating magnets is small, they form into a single
ring, but when the number is increased, they form concentric rings.
=B.= Crystals in tube containing emanation.
From Prof. F. Soddy’s “Interpretation of the Radium” (John Murray), by
the kind permission of author and publisher.]
The atom that has lost an electron is now positively charged; the
gainer is negatively charged. The two atoms would make things even
again if they came sufficiently close together, and as they move about
in the gas the negatives and the positives do in the end give and take
electrons, and the whole gas is neutral once more.
There is a beautiful experiment with which we may end this chapter.
When the helium atoms strike certain substances they excite a
phosphorescent glow. It is really, when we look into it closely, a set
of minute flashes due to the impacts of the separate atoms; under a
microscope the effect is as when we drop pebbles into a phosphorescent
sea. The glass vessel (Plate V b) contains crystals that
phosphoresce under the stimulus of the swift-moving helium atoms; one
is kunzite, another zinc sulphide, another willemite. In another tube
is a quantity of radium emanation: the gas which, you will remember,
is the immediate descendant of radium itself. When it is released and
is allowed to pass into the tubes containing the crystals the latter
glow in brilliant colors. In the figure the crystals have been made to
photograph themselves by their own phosphorescence.
The radium action has, we see, given us a remarkable insight into the
structure of the atom, for which there is a general reason to be given.
The student of science has long been familiar with the existence of
various atoms and with their properties; he has never seen one, nor
the effects of one. He has handled atoms in crowds only. When the
chemist causes elements to form compounds, or analyzes compounds into
elements, he deals with enormous numbers of atoms in any operation big
enough to see. But in this radioactive effect we observe the action
of one atom at a time, and here lies the secret of the advance. The
speed of the helium projectile, a hundred thousand times the speed with
which the atoms move ordinarily when they form part of a gas, gives
the individual atom the power of making itself felt. When we look at
the fog tracks, we see the actions of separate atoms; we see something
which would have filled the early defenders of the atomic theory with
astonishment and pleasure. One atom of helium passes through one atom
of oxygen, let us say, and comes out on the other side, and both may
bear evidences of the encounter. Effectively we use such evidence to
help us to determine the nature of the atoms. The helium atom is like a
spy that has gone into a foreign country and has come out again with a
tale to tell.