Recent Developments in European Thought

by Unknown

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.