Recent Developments in European Thought Part 14

[Footnote 55: _Royal Commission, Second Report_ (_Trades and Manufactures_), p. 147.]

[Footnote 56: Ibid., pp. 155-6.]

[Footnote 57: _Midland Mining Commission, First Report_, p. 34.]

[Footnote 58: Ibid., p. 91.]

[Footnote 59: Ibid., p. 44.]

[Footnote 60: _Rural Rides_, ii. 353.]

[Footnote 61: _Commons Committee, Stoppage of Wages_ (_Hosiery, 1854_).

Evidence of Mr. Tremenheere.]

[Footnote 62: _Evidence before the Truck Commissioners_, Q. 33,670.]

[Footnote 63: _Truck Commission, 1871. Report_, p. 16.]

[Footnote 64: _Commons Committee, Stoppage of Wages in the Hosiery Manufacture_ (1854), Q. 80.]

[Footnote 65: _Commons Committee of_ 1816, pp. 64 and 73.]

[Footnote 66: Ibid., p. 38.]

[Footnote 67: Ibid., p. 28.]

[Footnote 68: Speech, March 29, 1825.]

[Footnote 69: Letter to the Chevalier Bunsen, 1834, quoted in Strachey, _Eminent Victorians_, p. 197.]

VIII

ATOMIC THEORIES

PROFESSOR W.H. BRAGG, C.B.E., D.SC., F.R.S.

When a lecture on the progress of Science is given before a conference concerned largely with historical subjects, it is not inappropriate to point out that Science has a history of its own and that its progress makes a connected story. The discovery of new facts is not made in an isolated fashion, nor is it a matter of pure chance, unaffected by what has gone before. On the contrary, scientific progress is made step by step, each new point that is reached forming a basis for further advances. Even the direction of discovery is not entirely in the explorer's control; there is always a next step to be taken and a limited number of possible steps forward from which a choice can be made. The scientific discoverer has to go in the direction in which his discoveries lead him. When discoveries have been made it is possible to think of uses to which they may be put, but in the first instance all discoveries are made without any knowledge whatever of what use may afterwards be made of them.

Consequently scientific progress is a quite orderly advance, not a 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 cla.s.sification, 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 quant.i.tatively. 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 quant.i.ty 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 quant.i.ties of electricity in terms of the legal standard. It is one of the functions of a Government Inst.i.tution, 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 quant.i.ty 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 quant.i.ty that we can hardly realize a time when it was merely a vague term. If we want an ill.u.s.tration 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 princ.i.p.al 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 quant.i.ties 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 quant.i.ties 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 a.n.a.lysis, showing ultimately the molecules as made up, according to a definite plan, of so many atoms of the various elements. In a.n.a.lytical 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 antic.i.p.ated 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 pa.s.ses 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 quant.i.ty of electricity that pa.s.ses. The same current pa.s.sing 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 quant.i.ty 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 a.s.sociation 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 s.p.a.ce might enter our solar system with such speed as to be able to pa.s.s 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 pa.s.s 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 gla.s.s 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 Rontgen 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 Rontgen 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, pa.s.sing 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 pa.s.ses 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 circ.u.mstances 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 ma.s.s 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 a.s.sert 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.