216 PRESENT PROBLEMS OF INORGANIC CHEMISTRY. 



tion of practicall}^ 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 nnich attention. The 

 recognition of the radio-activity of uranium by Ik'cquerel, 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 intei-est for the 

 physicist than the chemist, on account of the extraordinary power 

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

 supposed to constitute ethereal vibrations, but all the phenomena 

 were not explicable on that supposition. Sclnnidt first, and Ruther- 

 ford 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 " emanations " was a])plied by Ivutherford to such radio-active 

 l)odies. 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 b}' treat- 

 ment with agents which are al)le to separate all known gases from 

 those of the argon grou]), namely, red-hot magnesium lime, and it 

 was later found that sparking with oxygen in presence of caustic 

 potash did not aifect the gaseous emanation from radium. The con- 

 clusion therefore followed that in all probability these bodies are 

 gases of the argon group, the atomic weight of which, and conse- 

 quently the density, is very high. Indeed, several observers, by means 

 of experiments on the rate of diffusion of the gas from radium, belieA'c 

 it to have a density of ai)proximately 100, referred to the hydrogen 

 standard. This conclusion has been confirmed by the mapping of the 

 spectrum of the radium emanation, which is similar in general char- 

 acter 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 bi-omide in a given time has been measured; 

 and it was incidentally sliown that this gas obeys Boyle's law of pres- 

 sures. The amount of gas thus collected and measured, however, was 

 very minute; the total (luantity was al)out tlie forty-thousandth of a 

 cubic centimeter. 



Having noticed that those minerals which consist of compounds 

 of uranium and thorium contain helium, Rutherford and Soddy ma(l(^ 

 the suggestion that it might not be impossilile that helium is the 

 product 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 a radium salt which has been ]:)repared for some time is 

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



