However, the first step was to recognise that there were three distinct and different rays that were given off by such metals as radium and uranium. Sir Ernest Rutherford christened them, after the first three letters of the Greek alphabet, the Alpha, the Beta, and Gamma rays. We are concerned chiefly with the second group and purpose here to deal with that group only.[3]
[3] The "Alpha rays" were presently recognised as atoms of helium gas, shot out at the rate of 12,000 miles a second.
The "Gamma rays" are _waves_, like the X-rays, not material particles.
They appear to be a type of X-rays. They possess the remarkable power of penetrating opaque substances; they will pa.s.s through a foot of solid iron, for example.
The "Beta rays," as they were at first called, have proved to be one of the most interesting discoveries that science ever made. They proved what Crookes had surmised about the radiations he discovered in his vacuum tube. But it was _not_ a fourth state of matter that had been found, but a new _property_ of matter, a property common to all atoms of matter. The Beta rays were later christened Electrons. They are particles of disembodied electricity, here spontaneously liberated from the atoms of matter: only when the electron was isolated from the atom was it recognised for the first time as a separate ent.i.ty. Electrons, therefore, are a const.i.tuent of the atoms of matter, and we have discovered that they can be released from the atom by a variety of agencies. Electrons are to be found everywhere, forming part of every atom.
"An electron," Sir William Bragg says, "can only maintain a separate existence if it is travelling at an immense rate, from one three-hundredth of the velocity of light upwards, that is to say, at least 600 _miles a second, or thereabouts_. Otherwise the electron sticks to the first atom it meets." These amazing particles may travel with the enormous velocity of from 10,000 to more than 100,000 miles a second. It was first learned that they are of an electrical nature, because they are bent out of their normal path if a magnet is brought near them. And this fact led to a further discovery: to one of those sensational estimates which the general public is apt to believe to be founded on the most abstruse speculations. The physicist set up a little chemical screen for the "Beta rays" to hit, and he so arranged his tube that only a narrow sheaf of the rays poured on to the screen. He then drew this sheaf of rays out of its course with a magnet, and he accurately measured the shift of the luminous spot on the screen where the rays impinged on it. But when he knows the exact intensity of his magnetic field--which he can control as he likes--and the amount of deviation it causes, and the ma.s.s of the moving particles, he can tell the speed of the moving particles which he thus diverts. These particles were being hurled out of the atoms of radium, or from the negative pole in a vacuum tube, at a speed which, in good conditions, reached nearly the velocity of light, i.e. nearly 186,000 miles a second.
Their speed has, of course, been confirmed by numbers of experiments; and another series of experiments enabled physicists to determine the size of the particles. Only one of these need be described, to give the reader an idea how men of science arrived at their more startling results.
Fog, as most people know, is thick in our great cities because the water-vapour gathers on the particles of dust and smoke that are in the atmosphere. This fact was used as the basis of some beautiful experiments. Artificial fogs were created in little gla.s.s tubes, by introducing dust, in various proportions, for supersaturated vapour to gather on. In the end it was possible to cause tiny drops of rain, each with a particle of dust at its core, to fall upon a silver mirror and be counted. It was a method of counting the quite invisible particles of dust in the tube; and the method was now successfully applied to the new rays. Yet another method was to direct a slender stream of the particles upon a chemical screen. The screen glowed under the cannonade of particles, and a powerful lens resolved the glow into distinct sparks, which could be counted.
In short, a series of the most remarkable and beautiful experiments, checked in all the great laboratories of the world, settled the nature of these so-called rays. They were streams of particles more than a thousand times smaller than the smallest known atom. The ma.s.s of each particle is, according to the latest and finest measurements 1/1845 of that of an atom of hydrogen. The physicist has not been able to find any character except electricity in them, and the name "electrons" has been generally adopted.
The Key to many Mysteries
The Electron is an atom, of disembodied electricity; it occupies an exceedingly small volume, and its "ma.s.s" is entirely electrical. These electrons are the key to half the mysteries of matter. Electrons in rapid motion, as we shall see, explain what we mean by an "electric current," not so long ago regarded as one of the most mysterious manifestations in nature.
"What a wonder, then, have we here!" says Professor R. K. Duncan. "An innocent-looking little pinch of salt and yet possessed of special properties utterly beyond even the fanciful imaginings of men of past time; for nowhere do we find in the records of thought even the hint of the possibility of things which we now regard as established fact. This pinch of salt projects from its surface bodies [i.e. electrons]
possessing the inconceivable velocity of over 100,000 miles a second, a velocity sufficient to carry them, if unimpeded, five times around the earth in a second, and possessing with this velocity, ma.s.ses a thousand times smaller than the smallest atom known to science. Furthermore, they are charged with negative electricity; they pa.s.s straight through bodies considered opaque with a sublime indifference to the properties of the body, with the exception of its mere density; they cause bodies which they strike to shine out in the dark; they affect a photographic plate; they render the air a conductor of electricity; they cause clouds in moist air; they cause chemical action and have a peculiar physiological action. Who, to-day, shall predict the ultimate service to humanity of the beta-rays from radium!"
-- 6
THE ELECTRON THEORY, OR THE NEW VIEW OF MATTER
The Structure of the Atom
There is general agreement amongst all chemists, physicists, and mathematicians upon the conclusions which we have so far given. We know that the atoms of matter are constantly--either spontaneously or under stimulation--giving off electrons, or breaking up into electrons; and they therefore contain electrons. Thus we have now complete proof of the independent existence of atoms and also of electrons.
When, however, the man of science tries to tell us _how_ electrons compose atoms, he pa.s.ses from facts to speculation, and very difficult speculation. Take the letter "o" as it is printed on this page. In a little bubble of hydrogen gas no larger than that letter there are _trillions_ of atoms; and they are not packed together, but are circulating as freely as dancers in a ball-room. We are asking the physicist to take one of these minute atoms and tell us how the still smaller electrons are arranged in it. Naturally he can only make mental pictures, guesses or hypotheses, which he tries to fit to the facts, and discards when they will _not_ fit.
At present, after nearly twenty years of critical discussion, there are two chief theories of the structure of the atom. At first Sir J. J.
Thomson imagined the electrons circulating in sh.e.l.ls (like the layers of an onion) round the nucleus of the atom. This did not suit, and Sir E.
Rutherford and others worked out a theory that the electrons circulated round a nucleus rather like the planets of our solar system revolving round the central sun. Is there a nucleus, then, round which the electrons revolve? The electron, as we saw, is a disembodied atom of electricity; we should say, of "negative" electricity. Let us picture these electrons all moving round in orbits with great velocity. Now it is suggested that there is a nucleus of "positive" electricity attracting or pulling the revolving electrons to it, and so forming an equilibrium, otherwise the electrons would fly off in all directions.
This nucleus has been recently named the proton. We have thus two electricities in the atom: the positive = the nucleus; the negative = the electron. Of recent years Dr. Langmuir has put out a theory that the electrons do not _revolve round_ the nucleus, but remain in a state of violent agitation of some sort at fixed distances from the nucleus.
[Ill.u.s.tration: PROFESSOR SIR J. J. THOMSON
Experimental discoverer of the electronic const.i.tution of matter, in the Cavendish Physical Laboratory, Cambridge. A great investigator, noted for the imaginative range of his hypotheses and his fertility in experimental devices.]
[Ill.u.s.tration: _From the Smithsonian Report_, 1915.
ELECTRONS PRODUCED BY Pa.s.sAGE OF X-RAYS THROUGH AIR
A photograph clearly showing that electrons are definite ent.i.ties. As electrons leave atoms they may traverse matter or pa.s.s through the air in a straight path The ill.u.s.tration shows the tortuous path of electrons resulting from collision with atoms.]
[Ill.u.s.tration: MAGNETIC DEFLECTION OF RADIUM RAYS
The radium rays are made to strike a screen, producing visible spots of light. When a magnetic field is applied the rays are seen to be deflected, as in the diagram. This can only happen if the rays carry an electric charge, and it was by experiments of this kind that we obtained our knowledge respecting the electric charges carried by radium rays.]
[Ill.u.s.tration: _Reproduced by permission of "Scientific American."_
PROFESSOR R. A. MILLIKAN"S APPARATUS FOR COUNTING ELECTRONS]
But we will confine ourselves here to the facts, and leave the contending theories to scientific men. It is now pretty generally accepted that an atom of matter consists of a number of electrons, or charges of negative electricity, held together by a charge of positive electricity. It is not disputed that these electrons are in a state of violent motion or strain, and that therefore a vast energy is locked up in the atoms of matter. To that we will return later. Here, rather, we will notice another remarkable discovery which helps us to understand the nature of matter.
A brilliant young man of science who was killed in the war, Mr. Moseley, some years ago showed that, when the atoms of different substances are arranged in order of their weight, _they are also arranged in the order of increasing complexity of structure_. That is to say, the heavier the atom, the more electrons it contains. There is a gradual building up of atoms containing more and more electrons from the lightest atom to the heaviest. Here it is enough to say that as he took element after element, from the lightest (hydrogen) to the heaviest (uranium) he found a strangely regular relation between them. If hydrogen were represented by the figure one, helium by two, lithium three, and so on up to uranium, then uranium should have the figure ninety-two. This makes it probable that there are in nature ninety-two elements--we have found eighty-seven--and that the number Mr. Moseley found is the number of electrons in the atom of each element; that is to say, the number is arranged in order of the atomic numbers of the various elements.
-- 7
The New View of Matter
Up to the point we have reached, then, we see what the new view of Matter is. Every atom of matter, of whatever kind throughout the whole universe, is built up of electrons in conjunction with a nucleus. From the smallest atom of all--the atom of hydrogen--which consists of one electron, rotating round a positively charged nucleus, to a heavy complicated atom, such as the atom of gold, const.i.tuted of many electrons and a complex nucleus, _we have only to do with positive and negative units of electricity_. The electron and its nucleus are particles of electricity. All Matter, therefore, is nothing but a manifestation of electricity. The atoms of matter, as we saw, combine and form molecules. Atoms and molecules are the bricks out of which nature has built up everything; ourselves, the earth, the stars, the whole universe.
But more than bricks are required to build a house. There are other fundamental existences, such as the various forms of energy, which give rise to several complex problems. And we have also to remember, that there are more than eighty distinct elements, each with its own definite type of atom. We shall deal with energy later. Meanwhile it remains to be said that, although we have discovered a great deal about the electron and the const.i.tution of matter, and that while the physicists of our own day seem to see a possibility of explaining positive and negative electricity, the nature of them both is unknown. There exists the theory that the particles of positive and negative electricity, which make up the atoms of matter, are points or centres of disturbances of some kind in a universal ether, and that all the various forms of energy are, in some fundamental way, aspects of the same primary ent.i.ty which const.i.tutes matter itself.
But the discovery of the property of radio-activity has raised many other interesting questions, besides that which we have just dealt with.
In radio-active elements, such as uranium for example, the element is breaking down; in what we call radio-activity we have a manifestation of the spontaneous change of elements. What is really taking place is a trans.m.u.tation of one element into another, from a heavier to a lighter.
The element uranium spontaneously becomes radium, and radium pa.s.ses through a number of other stages until it, in turn, becomes lead. Each descending element is of lighter atomic weight than its predecessor. The changing process, of course, is a very slow one. It may be that all matter is radio-active, or can be made so. This raises the question whether all the matter in the universe may not undergo disintegration.
There is, however, another side of the question, which the discovery of radio-activity has brought to light, and which has effected a revolution in our views. We have seen that in radio-active substances the elements are breaking down. Is there a process of building up at work? If the more complicated atoms are breaking down into simpler forms, may there not be a converse process--a building up from simpler elements to more complicated elements? It is probably the case that both processes are at work.
There are some eighty-odd chemical elements on the earth to-day: are they all the outcome of an inorganic evolution, element giving rise to element, going back and back to some primeval stuff from which they were all originally derived infinitely long ago? Is there an evolution in the inorganic world which may be going on, parallel to that of the evolution of living things; or is organic evolution a continuation of inorganic evolution? We have seen what evidence there is of this inorganic evolution in the case of the stars. We cannot go deeply into the matter here, nor has the time come for any direct statement that can be based on the findings of modern investigation. Taking it altogether the evidence is steadily acc.u.mulating, and there are authorities who maintain that already the evidence of inorganic evolution is convincing enough. The heavier atoms would appear to behave as though they were evolved from the lighter. The more complex forms, it is supposed, have _evolved_ from the simpler forms. Moseley"s discovery, to which reference has been made, points to the conclusion that the elements are built up one from another.
-- 8
Other New Views
We may here refer to another new conception to which the discovery of radio-activity has given rise. Lord Kelvin, who estimated the age of the earth at twenty million years, reached this estimate by considering the earth as a body which is gradually cooling down, "losing its primitive heat, like a loaf taken from the oven, at a rate which could be calculated, and that the heat radiated by the sun was due to contraction." Uranium and radio-activity were not known to Kelvin, and their discovery has upset both his arguments. Radio-active substances, which are perpetually giving out heat, introduce an entirely new factor.
We cannot now a.s.sume that the earth is necessarily cooling down; it may even, for all we know, be getting hotter. At the 1921 meeting of the British a.s.sociation, Professor Rayleigh stated that further knowledge had extended the probable period during which there had been life on this globe to about one thousand million years, and the total age of the earth to some small multiple of that. The earth, he considers, is not cooling, but "contains an internal source of heat from the disintegration of uranium in the outer crust." On the whole the estimate obtained would seem to be in agreement with the geological estimates.
The question, of course, cannot, in the present state of our knowledge, be settled within fixed limits that meet with general agreement.
[Ill.u.s.tration: MAKING THE INVISIBLE VISIBLE
Radium, as explained in the text, emits rays--the "Alpha," the "Beta"
(electrons), and "Gamma" rays. The above ill.u.s.tration indicates the method by which these invisible rays are made visible, and enables the nature of the rays to be investigated. To the right of the diagram is the instrument used, the Spinthariscope, making the impact of radium rays visible on a screen.
The radium rays shoot out in all directions; those that fall on the screen make it glow with points of light. These points of light are observed by the magnifying lens.
A. Magnifying lens. B. A zinc sulphite screen. C. A needle on whose point is placed a speck of radium.
The lower picture shows the screen and needle magnified.]