NA TURE 



60 1 



THURSDAY, APRIL 30, 1903. 



RADIUM. 



""P HE discovery by Monsieur and Madame Curie that 

 a sample of radium gives out sufficient energy 

 to melt half its weight of ice per hour has attracted 

 attention to the question of the source from which 

 the radium derives the energy necessary to maintain 

 the radiation ; this problem has been before us ever 

 since the original discovery by Becquerel of the radi- 

 ation from uranium. It has been suggested that the 

 radium derives its energy from the air surrounding it, 

 that the atoms of radium possess the faculty of 

 abstracting the kinetic energy from the more rapidly 

 moving air-molecules while they are able to retain 

 their own energy when in collision with the slowly 

 moving molecules of air. I cannot see, however, that 

 even the possession of this property would explain the 

 behaviour of radium; for imagine a portion of radium 

 placed in a cavity in a block of ice; the ice around the 

 radium gets melted; where does the energy for this 

 come from? By the hypothesis there is no change in 

 the energy of the air-radium system in the cavity, for 

 the energy gained by the radium is lost by the air, 

 while heat cannot flow into the cavity from outside, 

 for the melted ice around the cavity is hotter than the 

 ice surrounding it. 



Another suggestion which has been made is that the 

 air is traversed by a very penetrating kind of Becquerel 

 radiation, and that it is the absorption of this radiation 

 that gives the energy to the radium. We have direct 

 evidence of the existence of such radiation, ' for 

 McCIennan and Burton have recently shown that the 

 ionisation of a gas inside a closed vessel is diminished 

 by immersing the vessel in a large tank full of water, 

 suggesting that part, at any rate, of the ionisation of 

 the gas is due to a radiation which could penetrate 

 the walls of the vessel, but which was stopped to an 

 appreciable extent by the water. To explain the heat- 

 ing effect observed with radium, the absorption of this 

 radiation by radium must be on an altogether different 

 scale from its absorption by other metals. As no 

 direct experiments have been made on radium, it is 

 possible that this may be the case; it is not, however, 

 what we should expect from the experiments which 

 have been made on the absorption of this radiation by- 

 other metals, for these experiments have shown that 

 the absorption depends solely upon the density of the 

 absorbing substance, and not upon its chemical nature 

 or physical state; if this law hold for radium, the 

 absorption by 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 sufficient justification ; all that the experi- 

 ments 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, very 

 strong evidence that the substances actually engaged 

 in emitting these radiations can only keep up the pro- 

 cess for a short time ; then they die out, and the sub- 

 NO. I7-|8, VOL. 67] 



sequent radiation is due to a different set of radiators. 

 Take, for example, Becquerel's experiment when he 

 precipitated barium from a radio-active solution con- 

 taining 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 

 precipitate lost its radio-activity, while the solution of 

 uranium regained its original vigour. The same 

 thing is very strikingly shown by the remarkable and 

 suggestive experiments made by Rutherford and Soddy 

 on thorium ; they separated ordinary radio-active 

 thoria into two parts, transferring practically all the 

 radio-activity to a body called by them thorium X, the 

 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 became radio-active. 

 This seems as clear a proof as we could wish for that 

 the radio-activity of a given set of molecules is not 

 permanent. The same w-ant of permanence is shown 

 by the radio-active emanations from thorium and 

 radium, and by the induced radio-activity exhibited 

 by bodies which have been negatively electrified and 

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

 these eases the radio-activity ceases after a few days. 

 I have recently found that 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 be suggested by these 

 results is that the atom of radium is not stable 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 a large quantity of energy. 

 I may, perhaps, make my meaning clearer by con- 

 sidering 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 of distribution, always be a few atoms 

 in the gas possessing this amount of kinetic energy; 

 these would by hypothesis break up ; if in doing so 

 they gave out a large amount of energy in the form of 

 Becquerel radiation, the gas would be radio-active, 

 and would continue to be so until all its atoms had 

 passed through the phase in which they possessed 

 enough energy to make them unstable; if this energy 

 were 100 times the average energy it would probably 

 take hundreds of thousands of years before the radio- 

 activity of the gas was sensibly diminished. Now in 

 the case of radium, just as in the gas, the atoms are 

 not all in identical physical circumstances, and if 

 there is any law of distribution like the Maxwell- 

 Boltzmann law, there will, on the above hypothesis, 

 be a very slow transformation of the atoms accom- 

 panied by a liberation of energy. In the hypothetical 

 case we have taken the possession of a certain amount 

 of kinetic energy as the criterion for instability; the 

 argument will apply if any other test is taken. 



L> U 



