Alpha rays, these also consist of particles of matter. In the terms used by physicists, they are corpuscular, and not undulatory rays. And the corpuscles, or "electrons," as they are usually called, fly out from the radium at a most portentous speed. The whole subject is new, and estimates as to certain points vary, but Sir Oliver Lodge has said of these electrons, "Three hundred times faster than the fastest-flying star, they are the fastest-moving matter known." Until the discovery of radium, it was thought that the greatest speed ever attained by matter was that of certain of the "runaway stars." Arcturus, for instance, moves at the rate of about a hundred miles a second; but that is a mere dawdle compared with the speed of these electrons.
Now each of these electrons carries with it a tiny charge of electricity. They are all the same size; and the size is the same whether the electron be given out by radium or by thorium or by uranium, or by any other radioactive substance. In virtue of this electric propensity, these substances are able to affect a delicate indicator of electricity; and it was by this means that Madame Curie was enabled to discover radium. She went through two tons of pitchblende, the mineral in which it is found, and ended up with one-tenth of a grain of radium, after some months of hard work. This property of affecting an electric indicator was the only guide and test that she had in tracing this minute quantity of the unknown substance which she suspected to be there. But since then more complications have arisen. I said that the electrons carry a charge of electricity, but it seems probable that each of them is a charge of electricity, or, indeed, is an atom of electricity. And when we come to consider where these electrons come from and what they do before they leave the radium, we shall see that atoms of matter are made of atoms of electricity! As a result of the revelations of radium, not only do we know that one kind of matter may be changed into another, but we have found that matter itself consists of electricity.
Let us try to imagine the size of an atom. Lord Kelvin is our informant on this point, and he calculates that if a drop of water were magnified to the size of the earth, the atoms in it would be somewhere between the size of small shot and cricket-balls. This gives some faint idea of the size of an atom. But now imagine an atom of radium magnified to the size of St. Paul's Cathedral. Under such circumstances it would appear to consist of about one hundred and fifty thousand tiny particles, each of which is one of the electrons we have been speaking of, and the size of those electrons would be about the size of this dot called a full stop, or period (.). Try and realize, if you can, from Lord Kelvin's illustration, what the size of an atom is, and then try to realize that the ratio of an atom to an electron—the ratio of an atom of matter to one of its constituent atoms of electricity—is the ratio of St. Paul's Cathedral to a full stop. Obviously one hundred and fifty thousand full stops would not fill St. Paul's Cathedral. And so far away from one another are the electrons in an atom that the distance is comparable to the distance between the planets of the solar system. Relatively to their size, the electrons are as far from one another in this inconceivably tiny atom as the earth is from Mars, which is an average distance of sixty millions of miles.
This is by no means the only resemblance between an atom—or atomic system, as we should call it—and the solar system. Just as the planets are revolving round a centre, so the electrons in each of the atoms that go to make up those planets are also revolving round an atomic centre—revolving at a speed hundreds of times faster than the speed of the planets which they compose. And it is supposed that the electrons are constantly colliding with one another in their mad race within the atom, and the result of these collisions is to expel some of them from the atomic system. The electrons thus expelled constitute the Beta rays of radium. So small are the electrons, as compared with the atoms of ordinary matter, and so great is their velocity, that they pass through such a substance as the brass tube of the spinthariscope almost as if it were not there. The Alpha rays consist of bigger particles and they are stopped with ease, but the Beta rays need a considerable thickness of matter to arrest them. But
