200 RADIUM. 



of the absorbing- substance and not uj)on its chemical nature or phys- 

 ical state. If this law hold for radium, the absorption ]\y it would be 

 on the same scale as the absorption by lead or gold and altogether too 

 small to explain the observed effects. We are thus led to seek for 

 some other explanation. I think that the absence of change in the 

 radium has been assumed without sufticient justification; all that the 

 experiments justify us in concluding is that the rate of change is not 

 sufficiently rapid to be appreciable in a few months. There is, on the 

 other hand, \ery strong evidence that the substances actuall}^ engaged 

 in emitting these radiations can only keep up the process for a short 

 time; then they die out, and the subsequent radiation is due to a dif- 

 ferent set of radiators. 



Take, for example, Beccpierers experiment when he precipitated 

 barium from a radio-activ^e solution containing uranium, and found 

 that the radio-activity was transferred to the precipitate, the solution 

 not being radio-active; after a time, however, the radio-active precipi- 

 tate lost its radio-activit}', while the solution of uranium regained its 

 original vigor. The same thing is very strikingl}^ shown by the 

 remarkable and suggestive experiments made b}' Rutherford and 

 Sodd}' on thorium. The}' separated ordinary radio-active thoria into 

 two parts, transferring practically all the radio-activity to a body 

 called l)y them ''thorium X," tiie mass of which was infinitesimal in 

 comparison with that of the original thoria. The thorium X thus 

 separated lost in a few days its radio-activity, while the original thoria 

 in the same time again ])ecame radio-active. This seems as clear a 

 proof as we could wish foi- that the radio-activity of a given set of 

 molecules is not permanent. The same want of permanence is shown 

 by the radio-active emanations from thorium and radium, and by the 

 induced radio-activity exhibited l)y bodies which have ])een negatively 

 electriticd and exposed to these emanations or to the open air; in all 

 these cases the radio-activity ceases after a few days. I have recently 

 found tiiat the water from deep wells in Cambridge contains a radio- 

 active gas, and that this gas after being liberated from the water 

 gradually loses its radio-activity. The radio-activity of polonium, too, 

 is known not to be permanent. 



The view that seems to me to l)e suggested by these results is that 

 the atom of radium is not stal)le under all conditions, and that among 

 the large number of atoms contained in any specimen of radium there 

 are a few which are in the condition in which stability ceases and 

 which pass into some other configuration, giving out as they do so 

 large a (juantity of energy. I may, perhaps, make ni}^ meaning clearer 

 b}' considering a hypothetical case. Suppose that the atoms of a gas 

 X become unstable when they possess an amount of kinetic energy 100 

 times, say, the average kinetic energy of the atoms at the temperature 

 of the room. There would, according to the Maxwell-Boltzmann law 



