this mysterious gas—often known as the radium "emanation"—be examined again after an interval of about four weeks, it has changed into a familiar spectrum which is instantly recognizable as that of the gaseous "element" known as helium. So here is the astonishing fact: that the "element" radium decomposes itself and produces another "element," helium. Now the atomic weight of helium is about 2.2, just about one-hundredth part of that of radium (225), so that each atom of radium breaks up—despite the name a-tom, that which cannot be cut up—into about a hundred particles, and when these have had a few weeks in which to settle down, they are recognizable as the atoms of helium. Now it is these particles, flung out at a speed of nearly 200,000 miles a second from the speck of radium in the spinthariscope, that strike the little screen of zinc-sulphide paper, and thereby produce the never-ceasing shower of sparks that you see when you look into the instrument.
And this is the interesting thing, that just before the death of Herbert Spencer, the greatest thinker whom the Anglo-Saxon race has ever produced, there should have been discovered in radium a substance which proves that his great formula of evolution is as applicable to atoms as it is to societies or solar systems. I will not quote that formula here—you will find it in "First Principles,"—but if you know the facts of radium and then read that famous definition of evolution, framed forty years and more before radium was known, you will discover that it fits those facts as well as if it had been framed to describe them. This applicability to all circumstances, new or old, is the hall-mark of a universal truth, and of that alone. We may take this, then, as the most important fact about radium, that it proves the truth of atomic evolution. Not even an atom is immune from the universal law of unceasing change; and the reason why every one should possess a spinthariscope is that this simple little instrument demonstrates evolution going on even in the atom, which the distinguished physicist of not so long ago felt himself justified in describing as bearing upon it the stamp of the "manufactured article." Not manufactured, but evolved.
These rays of radium—consisting of material particles, and not of mere vibrations in the ether, like sunlight, the Roentgen rays., heat rays, electric waves, and most of the others with which we are familiar—have been called by physicists the Alpha rays; but they constitute only one of radium's many activities. There is a great deal more going on in the spinthariscope, though we cannot see it. Omitting for the moment the Beta rays, since they need more detailed discussion, I may mention the Gamma rays, which are also being constantly given off by radium. These are either Roentgen rays or something very like them; and perhaps it is their presence that explains the similarity, in the power of curing certain diseases, between radium and the Roentgen rays. Like these latter, but even in greater degree, the Gamma rays have the most extraordinary penetrating power. They can be detected after passing through five inches of armor plate; and as you look into the spinthariscope, they are passing out of it in all directions. Some of them, for instance, are passing into your eye—though you are unaware of it—and probably go right through your head and proceed on their way without interruption. Not only does radium give out these rays, but it has the power of picking up any Roentgen rays that may be about. If you are looking at a piece of radium in the dark through a fluorescent screen, you will notice that it shines much more brightly than before if Roentgen rays are being generated in the same room, showing that it has the power of picking them up and giving them forth again in an altered form.
And radium is ever giving out a large quantity of those rays which we call heat. Whatever the temperature of its surroundings, it is always a little hotter. So powerful is this action and so nearly inexhaustible, that if you could obtain a sufficient supply of radium—probably half a pound would be quite enough—it would keep a room warm not merely during your lifetime, but for hundreds of generations after you.
And now let us look at the Beta rays, which are also being incessantly given off inside the spinthariscope. Like the
