bromide and, examining it in the spectroscope, found that it was characterized by a wholly new spectrum, probably the characteristic spectrum of the emanation. But after watching this spectrum for three days they saw the characteristic lines of helium beginning to appear. This seemed to prove with certainty that helium was being continually formed by the disintegration of radium.
The Life of Radium.
It appears, therefore, that all the three heaviest atoms known are slowly disintegrating into simpler atoms. The process is, however, extremely slow. Despite the incessant projection of particles from radium, so strikingly shown by the Crookes spinthariscope, no one has as yet been able to detect with certainty any loss whatever in its weight, nor any diminution in its activity. Yet we may be certain that in fact it is both losing weight and diminishing in activity; for otherwise the principle of the conservation of energy, the corner-stone of modern science, would be violated. From a knowledge of the amount of heat energy given off by radium per hour, viz., 100 calories, and a knowledge of energy represented by each projected particle (this knowledge we possess, since we know the mass and velocity of the alpha particles, the energy contained in the beta particles being wholly negligible in comparison), we can easily estimate certain limits within which we may expect all the radium now in existence to pass out of existence as radium. In the first place we obtain the number of alpha particles projected per second from one gram weight of radium atoms by dividing the 100 gram-calories by the kinetic energy of each alpha particle. The result of this calculation is 200,000,000,000 . Now there are atoms of radium in a gram of radium chloride. Hence if each atom of radium which becomes unstable threw off but one alpha particle, then the fractional part of any given number of radium atoms which become unstable per second would be simply divided by This amounts to but one in fifteen thousand million. On the other hand, if each atom of radium which becomes unstable produces the maximum possible number of alpha particles, viz., 225/2, 235 being the atomic weight of radium and two the atomic weight of the alpha particles, then only one atom in sixteen hundred thousand million would become unstable per second. These two numbers represent then respectively the maximum and minimum possible rates at which the atoms of radium are becoming unstable. At the first rate radium would lose about one one-hundredth of its activity in five years, ninety-nine one-hundredths in 2,200 years and in 9,000 years it would possess no more than one hundred-millionth part of its present activity, i. e., it would no longer be measurably active. Since we have brought forward good evidence in the foregoing para-
