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.