Part 34 (1/2)

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 s.h.i.+ft 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 s.h.i.+ne 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.]