spasmodic collection of facts, and in the truest sense of the word it
has a history. In order that opportunities for this steady progress may
be provided it is very important that this point should be fully
appreciated. Every one, for example, is vaguely conscious that science
played a great part in the War. As a consequence the number of students
of science has greatly increased; manufacturing firms are awakening to
the fact that they must pay more attention to scientific development
and are founding research laboratories. It is very important that this
awakened attention should be well informed, and for that reason it
cannot be pointed out too often that the scientific work which has been
the basis of all material progress can only be turned to definite
material ends in the last stages of its development. Fundamentally
everything rests on the pure attempt to gain knowledge without any idea
of the use to which it may subsequently be put. Without pure science
there is no applied science at all. It is quite right in my opinion that
the researcher in pure science should have with him the hope that what
he does may one day be of direct benefit to others. But it is probable
that he does not in his own mind confine the idea of possible uses to
such material matters as I have mentioned above and as are so prominent
at present. He believes that his work has a less material side whose
value need not be explained to the present audience.
In the general line of progress it is natural to find that there are
certain broad roads along which the main advance has been directed.
Students of physics and chemistry and the subjects which are allied to
them find that they are in general considering either matter, or
electricity, or energy. I make this classification, not from any
philosophical point of view, but simply for present convenience. The
first important principle to which I would like to draw your attention
is that each of these things can be measured quantitatively. If we
accept the weight of a substance as an indirect measure of the amount of
matter present, then we all know we can express the amount of matter in
any given body in terms of a fundamental unit, like a pound or a gramme;
and the idea has been put to immemorial use. In later years we have
learnt that electricity itself is also a quantity and that the amount of
electricity which stands on an electrified body, or flows past a given
point in an electric conductor, as for example the wire connected to an
electric light, can be expressed arithmetically in terms of some unit.
Instruments are made for the purpose of measuring quantities of
electricity in terms of the legal standard. It is one of the functions
of a Government Institution, like the National Physical Laboratory, to
test such instruments and report on their accuracy. International
conferences have been held for the purpose of reducing these units to as
small a number as possible so that people may be able to trade less
wastefully and more conveniently, so that also the barriers between
peoples may be broken down and the interchange of ideas as well as of
materials may be made more easily. Without an arrangement of this kind
it would be impossible to carry on industrial life in which use is made
of electricity. It would be as difficult as to hold a market without the
use of weights and scales, more difficult, in fact, since anyone can
estimate the size of a piece of cloth or the amount of corn in a sack,
but no one has a natural sense by which he can estimate an amount of
electricity.
In just the same way energy can be measured as a quantity in terms of a
fundamental unit. The discovery that this was so was made by Joule and
others towards the middle of the nineteenth century, and lit the road
for further advance as a dark street is lit by the sudden turning-up of
the lamps. All modern industry rests on this principle. We are now so
accustomed to the idea that energy is a quantity that we can hardly
realize a time when it was merely a vague term. If we want an
illustration of how thoroughly we have grasped this idea let us remember
that when we pay our electric-light bill we pay so much money for so
many units of energy supplied; for so much energy, let us note, not for
so much electricity, since we take into account not only the actual
amount of electricity driven through our house wires, but also the
magnitude of the force which is there to drive it. Energy exists in many
forms: energy of motion, heat, gravitational energy, chemical energy,
radiation, and so on. In the transformations of energy which are
continually occurring in all natural processes, there is never any
change in the total amount of energy. This is the famous principle of
the Conservation of Energy. Sometimes it is stated in the form
'Perpetual motion is impossible'.
One of the most important forms of energy is radiation. The constant
outpouring by the sun of energy in this form is vital to us. The fact
was obvious long ago and that is one of the reasons why light and heat
have interested students of science in all ages.
There exist then three main subjects of study--matter, electricity, and
energy. These themselves and their mutual relations have been, and are,
the principal objects of interest to the scientific student, and from
our strivings to understand them we have learnt most of what we know.
All three are quantities and all are expressible in terms of units.
Now there is one point which I have thought would especially interest
you. A very remarkable tendency of modern discovery shows more and more
clearly that not only are these things quantities which we can express
in units of our own choosing, but that Nature herself has already chosen
units for them. The natural unit does not, of course, bear any exact
connexion with our own. This being so, it must be of the utmost
importance that we should know what these natural units are and so be
able to understand what Nature is ready to tell us. Nature has chosen to
speak in a certain language; we must get to know that language.
In the first place we know surely that there are natural units of
matter. This was the great discovery made by Dalton in the beginning of
the nineteenth century. When he found that each of the known elements,
such as copper or oxygen or carbon, consisted ultimately of atoms, all
the atoms of any one element being alike, he laid the foundation on
which the huge structure of modern chemistry has been raised. The
chemist takes one or more atoms of one element, one or more of another,
and may be of a third or fourth, and he puts them together into a
compound which we call a molecule. The molecule for example of ordinary
salt contains always one atom of chlorine and one of sodium. Chlorine
and sodium are elements, salt is a compound. Six atoms of carbon and six
of hydrogen put together in a certain way make benzene. In the same way
every substance that we meet is capable of analysis, showing ultimately
the molecules as made up, according to a definite plan, of so many atoms
of the various elements. In analytical chemistry molecules are dissected
in order to discover the mode of their building; in synthetic chemistry
the atoms are put together to make a molecule which is already known to
have, or even may be anticipated to have, certain properties. This is
the work of the chemist. Sometimes enormous forces are concerned in this
pulling apart and putting together, witness the terrific power of modern
explosives. But the same kind of handling by the chemist may be devoted
to the delicate construction of a molecule which gives a certain colour
to the dyer's vat and so pleases the eye that the great cloth industries
feel the consequence, and nations themselves are affected by the flow of
trade. After all, since the processes of the physical world operate
ultimately through the power and properties of molecules, it is not
surprising that the chemist's work in these and numberless other ways
has such tremendous influence in the world.
Here then by the recognition of the units of matter which Nature has
chosen for herself it has been possible to do great things.
It should be observed that the atom, in spite of its name, is not
something which is incapable of all further division; it is only
incapable of retaining its properties on division. When an atom of
radium breaks down in the unique operation during which its singular
properties are manifested, it dies as radium and becomes two atoms, one
of helium, the other of a different and rare substance. It will interest
you to know that the airships of the future are expected to be filled
with this non-inflammable helium.
The discovery of the atomic nature of electricity came later. Faraday
established the fact that in certain processes there was more than a
hint that electricity was always present in multiples of a definite
unit. In the process called electrolysis the electric current is driven
across a cell full of liquid containing molecules of some substance.
When the electricity passes there is a loosening of the bonds that bind
together the atoms of the molecule, and a separation; atoms of one kind
travel with the electricity across the cell and are deposited where the
current leaves the cell; the other kind travel the opposite way. In this
way for example we deposit silver on metal objects in electro-plating
processes, or separate out the purest copper for certain electrical
purposes. The striking thing which Faraday discovered was that the
number of atoms deposited always bore a very simple relation to the
quantity of electricity that passes. The same current passing in
succession through cells containing different kinds of molecules broke
up the same number of molecules in each cell. It was as if in each
electrolytic cell atoms of matter and atoms of electricity travelled
together. The movement of an atom meant the simultaneous movement of a
definite quantity of electricity. Electricity was, so to speak, done up
in little equal parcels, and an atom of matter on the move, which was
termed an ion, or wanderer, carried, not a vaguely defined amount of
electricity, but one of these definite parcels.
It was not, however, until the later years of the nineteenth century
that the natural unit of electricity was manifested by itself and
without a carrier. At a famous address to the British Association at
York in 1881 Sir William Crookes described the first marvellous
experiments in which this feat had been accomplished, though there was
still to come a long controversy before the interpretation was clearly
accepted. It is now definitely established that there is a fundamental
atom of electricity which we now call the electron. As we all know
electrification is of two kinds--a positive and a negative. The electron
is of the negative kind. There does not appear to be a corresponding
positive atom of electricity, or at least not one that is so singular in
its properties as the electron. Electrons go to the making of all atoms,
just as atoms go to the making of molecules. The atom which is neutral,
that is, shows neither positive nor negative electrification, must
contain positive electricity in some form to balance the electrons which
we know it contains. When we strip an atom, as we know how to do, of one
or more of these electrons, the remainder is positively charged. The
positive ion is any sort of an atom or molecule which has become
positively electrified in this way. An atom which has become positive by
the loss of one or more of its electrons exercises a force on any spare
electrons in its neighbourhood or on any atom carrying a spare electron.
When there are large numbers of atoms seeking in this way to become
neutral once more, as occurs often in Nature, the forces generated may
be tremendous. They are shown, for example, in the lightning-stroke. But
indeed it would seem that all the chemical forces of which we have
already spoken depend ultimately upon the electric state of the atom
concerned.
It is because the force which a positively-charged atom exerts on an
electron is so great and because the electron is so light and easily
moved compared to an atom that the electron has not been isolated at
will until recent years. The isolation in fact depends upon the electron
being endowed with a sufficient speed to carry it through or past the
action of an atom which is seeking to absorb it into its system. A lump
of matter flying in space might enter our solar system with such speed
as to be able to pass through and go on its way almost undeflected. Or
again, it might have a much lower speed and go so much nearer the sun
that it was seriously deflected in its course, as we see in the case of
comet visitors. But if for some reason or other the lump of matter found
itself inside the solar system without the endowment of high velocity it
would certainly be absorbed. Just so an electron can pass through an
atom with or without serious deviation from its line of motion, provided
that motion is rapid enough. Only recently have we been able to exert
electric forces of sufficient strength to set an electron in motion with
the speed it must have if it is to maintain an individual existence Now
we can gather electrons at will, dragging them from the interior of
solid bodies, and hurl them with tremendous speed like a stream of
projectiles. Since in the open air the speed is soon lost by innumerable
collisions with the air-molecules, the effect can only be studied
satisfactorily in a glass bulb from which the air has been evacuated.
Crookes made great improvements in air-pumps during an investigation on
thallium, and consequently was able to obtain the high vacuum required
for the experiment with the electron streams. It was afterwards found by
Röntgen that when an electron stream in an evacuated bulb was directed
upon a target placed within the bulb, a remarkable radiation issued
from the target. Thus arose the so-called X or Röntgen rays. As you all
know they have for many years played a most important part in surgery
and medicine. You may have heard that during the war they were also used
to examine the interior of aeroplane constructions and to look for flaws
invisible from without. Although X-Rays are of the same nature as light
rays they can penetrate where light rays cannot, passing in greater or
less degree through materials which are opaque to visible light and
allowing us to examine the interior which is hidden from the eye.
Every electric discharge is essentially a hurried rush of electrons.
When we rub two bodies together and they become electrified we have in
some way or other torn electrons from one of the bodies and piled them
on the other. The former becomes the positively charged body and the
latter the negative. A film of moisture stops this action. When wool is
spun in factories it tends to become in certain stages of the process
too dry and too free from grease; the yarn then becomes electrified as
it passes over the leather rollers, and when the machine tries to spin
the threads together they fly apart and refuse to join up the minute
hooks with which the wool fibres are furnished. The spinning operation
would come to an end were there not means provided by which the air can
be so filled with moisture that the fibres become damp and the action
ceases. So in some cases a stream of air filled with positive and
negative ions is made to play upon the fibres; the fibres select what
ions they want, and so neutralizing themselves, spinning can proceed
again.
When a current of electricity runs along a wire there is in fact nothing
more than a procession of electrons. The stream of electrons that runs
through the filaments in the lamps that light this room, raising the
filaments to a white heat, are set in motion by the dynamos in the city.
There is a complete wire circuit, including the dynamo, the conductors,
and the lamps. When the dynamos are not working the electrons do not as
a whole move either way, though they are always there. When the dynamo
begins to turn, the electrons set out on their continuous journey.
Electrons are involved in the emission of wireless signals, and in their
receipt. The so-called 'valve', which multiplies minute electric signals
and was so greatly improved during the war, depends entirely on the
action of electrons, and the brilliant experimental work was based on
the newly-acquired knowledge of their properties.
I have told you that under certain circumstances a stream of electrons
may generate X-Rays, in reality a form of light rays. This action is a
very common one, and it is curious that the faster the electron goes the
shorter is the wave-length of the radiation. A very fast electron
generates an X-Ray of so short a wave-length that the penetrating power
of the ray, which goes with the shortness of the wave, is excessive, and
in this way we may have rays which go right through the human body or
even through inches of steel. As the speed of the exciting electron
becomes less, the X-Rays are less penetrating. With still slower
electrons we may generate ordinary light, and it will take a slower
electron to generate red than to generate blue. The slowest electrons we
use in this way have a speed of many hundred miles per second; the
fastest have a speed which nearly approaches that of light, or 186,000
miles a second.
And conversely radiation can set electrons in motion. When X-Rays are
driven into a patient's body electrons are set in motion within, and
moving over certain minute distances, initiate chemical actions which
are necessary to some cure. Or they may go right through the body and
fall on a photographic plate, setting in operation chemical action which
forms a picture on the plate.
There is another occasion of an entirely different kind when the
electron is greatly in evidence and displays effects which are most
astonishing and significant. Every atom of radium or other radio-active
substances sooner or later meets with the catastrophe in which its life
as radium ends and atoms of other substances are formed. At that moment
occurs the emission which is the characteristic property of the
substance. One of the radiations emitted consists of high-velocity
electrons, moving, some of them, nearly as fast as light.
Now it is found that when the speed approaches that of light, 186,000
miles or 3 x 10^{10} centimetres per second, the energy is higher than
it should be if it followed the usual rule, viz. energy is equal to half
the mass multiplied by the square of the velocity. It would seem that an
electron moving with the velocity of light would have infinite energy;
or, to put the matter in another way, the experimenter in his laboratory
can never hope to observe an electron moving so fast; it would be the
end of his laboratory and of himself if ever it turned up.
Linked up with this result is the very strange fact that no one has ever
been able to find any direct evidence of the existence of the ether,
which is postulated in order to carry light-waves. It has been pictured
as a medium through which the heavenly bodies move, and to which their
motions may be referred. But when light is launched into the ether, its
apparent velocity must depend on whether it travels with or against the
drift of the ether through the laboratory where the measurement is made.
The experiment has been performed without the discovery of any such
difference, although the method was amply accurate enough to detect the
effect that might be expected. It was afterwards shown that the negative
result might be explained by supposing that a measure of length varied
in length according to whether it was travelling with or against the
ether. But the continual failure of all such experiments has led to a
remarkable hypothetical development with which the name of Einstein is
firmly connected. It is supposed that some flaw must exist in our
fundamental hypotheses, and that if this were corrected we should then
find that we ought to get the same value for the velocity of light
however and whenever we measured it, and at the same time we should find
that no measurement of the velocity of a body moving relative to the
observer would ever equal the velocity of light. The hypothesis denies
the existence of an absolute standard to which motions can be referred,
and insists that they must all be considered relatively to the observer.
It is called the principle of relativity. Calculations of its
consequences begin with the necessary changes in the fundamentals, such
as Einstein has introduced.[70]
Time does not allow me to say more of the innumerable ways in which
electrons play an essential part in all the processes in the world. We
have long believed that this is so, but the picture has never been so
clear to us as it is now; and with our understanding our power is
increased. Yet once more the illumination of our understanding comes
from our recognition that Nature has preferred the discrete to the
continuous and that electricity is not infinitely divisible but is, like
matter, and even more simply than matter, of an atomic structure. And we
have found the unit and learnt how to handle it.
It is even more strange that it may now be said of energy that there are
signs of atomicity. It may seem absurd to think that the energy which is
transformed in any operation is transformed in multiples of a universal
unit or units, so that the operation cannot be arrested at any desired
stage but only at definite intervals. Indeed we have no right to assert
that this is always true. But undoubtedly there are cases in which the
atomicity of energy is clear enough, as for example in the interchange
of energy between electrons in motion and radiation. It is remarkable
that when radiation sets an electron in motion, the electron acquires a
perfectly definite speed depending only on the wave-length of the
radiation and not on its intensity, and has apparently absorbed from the
radiation a definite unit of energy. Radiation of a particular
wave-length cannot spend its energy in this way except in multiples of a
certain unit, because each of the electrons which it sets in motion has
the same initial energy, which it must have got from the radiation. In
other words, energy of radiation of the particular wave-length can only
be transformed into energy of movement of electrons in multiples of a
certain 'quantum' peculiar to that wave-length. The intensity of the
radiation, that is to say, the amount of energy moving along the beam,
can only affect the number of electrons set in motion and not the speed
of any one of them. During the last few years a very extraordinary
theory has been developed on the basis of these and similar facts. I
doubt if it would be more profitable to give further instances at
present, but I have mentioned it because it seems to show looming on the
horizon of our knowledge another tendency of Nature to make use of the
atomic principle.
I will only add that the whole position of physics is indeed at this
time of extraordinary interest, and at any moment there may be some
great discovery or illuminating thought which will explain the present
startling difficulties and open up new worlds of thought.
FOR REFERENCE
Bragg, _Rays and Crystals_ (Ball & Sons).
FOOTNOTES:
[Footnote 70: Since this address was given, the results of the Eclipse
Expedition to Brazil are considered to have confirmed in a satisfactory
manner one of the most remarkable deductions made by Einstein from the
principles which he maintains. The matter has roused so much interest
that some of the leading exponents of the relativity principle have
published careful accounts intended for students not familiar with it:
it would therefore be superfluous to discuss the matter here.]
IX
PROGRESS IN BIOLOGY DURING THE LAST SIXTY YEARS
PROFESSOR LEONARD DONCASTER, F.R.S.