THE TRANSFORMATION OF ELEMENTS 71 



determinations of the chlorine as silver chloride made the atomic 

 weight of radium respectively 226*8, 2257, and 2277 (mean 2267). 

 Until further work has been completed the figure 226*5 ma Y De 

 accepted as approximately correct. 



From certain series of lines in the spectrum, analogous with 

 those of calcium, strontium, and barium, indirect determinations 

 of the atomic weight have been made by Runge and Precht, 

 and by Marshall Watts ; the former deduce an atomic weight 

 of 258, the latter a weight agreeing with Madame Curie's 

 figure. 



Although the time of decay of radium is extremely slow, 

 there is some doubt as to its actual duration ; the matter will 

 be discussed later. The product of the decay is that gas or 

 emanation discovered by Dorn in 1900. It can be pumped off 

 from radium preparations or carried away by a current of air ; 

 obtained in this way its rate of decay has. been examined by 

 a number of investigators. Rutherford and Soddy's first deter- 

 mination was 3*71 days (half life); the Curies obtained the 

 figure 3*99 days, Sackur 3*86. Recently (in 1907) Riimelin, 

 working under Rutherford, obtained the value 3*75 days (the 

 mean of eight experiments; the limits were 3*70-3*80). 



On account of the ease with which small quantities of gas 

 can be separated, tested, and purified in comparison with small 

 quantities of a solid, and of the fact that this is the only emana- 

 tion which has an appreciable life (the half-life period of thorium 

 emanation is 54 seconds, of actinium emanation 4 seconds), it 

 has been examined more completely than any other radioactive 

 substance. 



Rutherford and Soddy, and later, Ramsay and Soddy, 

 demonstrated beyond all doubt its peculiar inertness by 

 subjecting it to a number of extremely violent chemical tests, 

 measuring its radioactivity qualitatively and quantitatively 

 before and after experiment, to determine whether any change 

 had been effected. It is not altered by passage, mixed with 

 oxygen, over platinum black at the highest attainable tem- 

 perature, nor by passage over red-hot lead chromate, nor by 

 red-hot magnesium powder, magnesium lime, or zinc dust. 

 Sparking with oxygen over alkali produces no change ; neither 

 does ignition of phosphorus in that oxygen. It will combine 

 with no other element under any conditions hitherto employed. 

 The only other gases known which behave in a similar manner 



