The atoms of uranium, for instance, are supposed to be not permanent but unstable systems. According to the hypothesis, about 1 atom in every 10^{18} becomes unstable each second and breaks up with a violent explosion for so small a mass of matter. One, or possibly two alpha particles are expelled with great velocity. This alpha particle corresponds to an atom of helium with an atomic weight of 4, and its loss reduces the original atomic weight to 234 with the formation of a new element, having changed properties corresponding to the new atomic weight. This new element is uranium X_{1}. These new atoms are far more unstable than those of uranium, and the decomposition proceeds at a new rate of 1 in 10^{7} per second. So at a definite, measurable rate this stepwise disintegration proceeds. The explosions are not in all cases equally violent in going from element to element, nor are the results the same. Sometimes alpha particles alone are expelled, sometimes beta, or two of them together, as alpha and beta. The new product may remain with the unchanged part of the original matter. Thus there would be an accumulation of it until its own decay balances its production, resulting eventually in a state of equilibrium. Constitution of the Atom In order to explain the electrical and optical properties of matter, the hypothesis was made that the atom consisted of positively and negatively electrified particles. Later it was shown that negative electrons exist in all kinds of matter. Various attempts were made to work out a model of such an atom in which these particles were held in equilibrium by electrical forces. The atom of Lord Kelvin consisted of a uniform sphere of positive electrification throughout which a number of negative electrons were distributed, and J. J. Thomson has determined the properties of this type as to the number of particles, their arrangement and stability. Rutherford's Atom According to Rutherford, the atom of uranium may be looked upon as consisting of a central charge of positive electricity surrounded by a number of concentric rings of negative electrons in rapid motion. The positively charged centre is made up of a complicated system in movement, consisting in part of charged helium and hydrogen atoms, and practically the whole charge and mass of the atom is concentrated at the centre. The central system of the atom is from some unknown cause unstable, and one of the helium atoms escapes from the central mass as an alpha particle.