Nevertheless, the emanation has been detected, and investigated by the electroscope, which measures the radium rays by the power to discharge its electrified gold leaves. “The electroscope is about a million times more sensitive than the most sensitive spectroscope and yet the spectroscope is capable of detecting easily the millionth part of a milligram of matter” (Duncan).
Calculations made by Rutherford show that if a thimbleful of this active gas could be collected, the bombardment of its powerful rays would heat to a red heat, or even might melt down, the walls of the glass containing it. The emanation emits only Alpha rays (or particles) forming helium.
The radium from which the emanation has been abstracted, after the lapse of an hour or so, loses 75% of its activity. During the course of a single month, radium will be found to have restored all its lost emanation. In thirty days it will have regained all its original activity. It was soon discovered that the emanation abstracted from the radium loses its radioactivity at the same rate and according to the same laws as the de-emanated radium regains it. The radium is therefore said to be “in equilibrium with its products.”
Since these processes are wholly outside the sphere of known controllable forces, and cannot be created, altered or destroyed—“since the process is independent of the chemical form of the radium, whether bromide, chloride, sulphate, etc., we are absolutely shut up to the conviction that it is a function of the atom. We are in the presence of an actual decay of the atom. The atom of radium breaks down into atoms of emanation and the atoms of emanation in their turn break down into something else. The activity of emanation decays and falls to half value in about 3.7 days.”
Although the amount of emanation produced from a gram of radium does not amount to more than a needle-point of the gas (= 1.3 cubic millimeter), this is sufficient to raise the temperature of 75 grams of water 1° per hour, which is enough heat to melt more than its own weight of ice in an hour, and to raise it to the boiling-point in the next hour, which is equivalent to 60,000 horse-power days! In other words, the heat evolved by the radium emanation is more than 3,500,000 times greater than that produced in any known chemical reaction: such as, for example, the union of oxygen and hydrogen to form water.
It was soon discovered that if the spectrum of this mysterious gas—or radium emanation—be examined again after an interval of about four weeks, it has changed into a familiar spectrum easily recognized as that of the gaseous element known as helium. Here the chemist comes face to face with the astounding fact that the element radium is decomposed and produces another element, helium—a discovery made by Ramsay and Soddy in the summer of 1903.
In the successive radioactive changes, one Alpha particle (sometimes called “ray”) is ejected from each atom disintegrated by the change—in some cases, at least, accompanied by Beta particles (negative electrons). The Alpha particle, as already stated, is really an atom of helium carrying two atomic charges of positive electricity—twice that of an atom of hydrogen. Strictly speaking, the Alpha particle is only the nucleus of a helium atom, since it has lost two of its negatively charged electrons, which are combined in the ordinary helium atom. The exact velocity of the expelled Alpha particle “varies in the different radioactive elements” (Joly)—say from 10,000 to 18,000 miles each second—a velocity sufficient to carry the particle around the earth in less than two seconds, if unchecked.
But these relatively heavy particles (of atomic size) are actually soon checked, even by seven centimeters (about a third of a foot) of air. The Beta particle (1,845 the mass of a hydrogen atom) “shoots a hundred times as far [as the Alpha particle] and the Gamma rays are a hundred times more penetrating still” (Millikan). But the Alpha particle is sometimes ejected with a velocity nearly 40,000 times that of a rifle bullet,—the velocity of the latter being about half a mile a second. Even the super-guns which bombarded Paris could not eject a projectile with a speed of more than about a mile a second. Rutherford observes that if it were possible to give an equal velocity to an iron cannon ball, the heat generated on a target would be many thousand times more than sufficient to melt the cannon ball and dissipate it into vapor.
The flashes of light seen when the Alpha rays bombard a screen of zinc sulphide, as in Crookes’ spinthariscope, are due to cleavages produced in the zinc sulphide crystals by the impact of the Alpha rays (positive ions). Each impact on a crystal produces a splash of light big enough to be seen by a microscope.
In the phosphorescence caused by the approach of an emanation of radium to zinc sulphate, the atoms throw off the Alpha (helium) particles to the number of five billion each second, with velocities of 10,000 miles or more a second. If the helium projectile should chance to “crash” into an atom of nitrogen or of oxygen, an atom of hydrogen can be knocked out of it, as was discovered by Sir Ernest Rutherford, perhaps the most distinguished of Mme. Curie’s pupils. (Strictly speaking, the disintegration particles are isotropes of helium, of atomic weight 3, the atomic weight of helium being 4.) Despite its large size as compared with an electron (or Beta particle), the Alpha particle passes through a glass wall without leaving a hole behind, and without in any way interfering with the molecules of the glass. It shoots through hundreds of thousands of atoms without ever going near enough to them to be deflected from its course.