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 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 à market without the use of weights and scales, more difficult, in fact, since any one 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