PRESENT PROBLEMS 267 



tion of practically pure compounds of it, and the determination of its 

 atomic weight, are familiar to all of you. Her discovery of polonium, 

 and Debierne's of actinium, have also attracted much attention. 

 The recognition of the radioactivity of uranium by Becquerel, which 

 gave the first impulse to these discoveries, and of that of thorium by 

 Schmidt, is also well known. 



These substances, however, presented at first more interest for the 

 physicist than the chemist, on account of the extraordinary power 

 which they all possess of emitting "rays." At first, these rays were sup- 

 posed to constitute ethereal vibrations; but all the phenomena were 

 not explicable on that supposition. Schmidt first, and Rutherford 

 and Soddy later, found that certain so-called " rays " really consist of 

 gases; and that while thorium emits one kind, radium emits another; 

 and no doubt Debierne's actinium emits a third. The name " emana- 

 tions " was applied by Rutherford to such radioactive bodies; he and 

 Soddy found that those of radium and thorium could be condensed 

 and frozen by exposure to the temperature of liquid air, and that they 

 were not destroyed or altered in any way by treatment with agents 

 which are able to separate all known gases from those of the argon 

 group, namely, red-hot magnesium-lime, and it was later found that 

 sparking with oxygen in presence of caustic potash did not affect the 

 gaseous emanation from radium. The conclusion therefore followed 

 that in all probability these bodies are gases of the argon group, the 

 atomic weight of which, and consequently the density, is very high ; 

 indeed, several observers, by means of experiments on the rate of 

 diffusion of the gas from radium, believe it to have a density of approx- 

 imately 100, referred to the hydrogen standard. This conclusion has 

 been confirmed by the mapping of the spectrum of the radium emana- 

 tion, which is similar in general character to the spectra of the inactive 

 gases, consisting of a number of well-defined, clearly cut brilliant 

 lines, standing out from a black background. The volume of the gas 

 produced spontaneously from a given weight of radium bromide in 

 a given time has been measured ; and it was incidentally shown that 

 this gas obeys Boyle's law of pressures. The amount of gas thus col- 

 lected and measured, however, was very minute; the total quantity 

 was about the forty-thousandth of a cubic centimeter. 



Having noticed that those minerals which consist of compounds 

 of uranium and thorium contain helium, Rutherford and Soddy made 

 the suggestion that it might not be impossible that helium is the pro- 

 duct of the spontaneous change of the emanation; and Soddy and 

 I were able to show that this is actually the case. For, first, when 

 a quantity of radium salt which has been prepared for some time is 

 dissolved in water, the occluded helium is expelled, and can be recog- 

 nized by means of its spectrum; further, the fresh emanation shows 

 no helium spectrum, but after a few days the spectrum of helium 



