RECENT DISCOVERIES IN RADIATION. 497 



bromide and, examining it in the spectroscope, found that it was charac- 

 terized 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 modem 

 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 leta particles being wholly negligible in com- 

 parison), 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 divi- 

 ding the 100 gram-calories by the kinetic energy of each alpha particle. 

 The result of this calculation is 200,000,000,000 (= 2 X 10"). Now 

 there are 3 X 10^^ 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 

 2 X 10" divided by 3 X 10^^ 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- 

 voi;. Lxiv. — 32. 



