216 PRESENT PROBLEMS OF INORGANIC CHEMISTRY. 



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 radio-activity of uranium by Becquerel, ^Yhich gave 

 the first impulse to these discoA^eries. and of that of thorium by 

 Schmidt is also well known. 



These substances, how^ever, presented at first more interest for the 

 physicist than the chemist, on account of the .extraordinary power 

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

 supposed to constitute ethereal vibrations, but all the phenomena 

 Avere not explicable on that supposition. Sclimidt first, and Eiither- 

 ford and Sodd}' 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 " Avas applied by Rutherford to such radio-active 

 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 Avere not destroyed or altered in any Avay by treat- 

 ment Avith agents Avhich are able to separate all knoAvn gases from 

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

 was later found that sparking Avith oxygen in presence of caustic 

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

 clusion therefore folloAved that in all probability these bodies are 

 gases of the argon group, the atomic Aveight of Avhich, and conse- 

 quently the density, is very high. Indeed, several obserA^ers, by means 

 of experiments on the rate of diifusion of the gas from i-adium, belieA^e 

 it to haA^e a density of approximately 100, referred to the hydrogen 

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

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

 acter to the spectra of the inactive gases, consisting of a number of 

 Avell-defined, clearly cut brilliant lines, standing out from a black 

 background. The volume of the gas produced spontaneously from a 

 giA^en Aveight of radium bi-oniide in a giA^en time has been measured; 

 and it Avas incidentally shown that this gas obeys Boyle's law of pres- 

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

 A'ery minute; the total (juanlity Avas about the forty-thousandth of a 

 cubic centimeter. 



Having noticed tliat those minerals which consist of compounds 

 of uranium aiul thorium contain helium, Rutherford and Soddy nuule 

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

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

 I Avere able to shoAv that this is actually the case. For, first, AAdien a 

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

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



