60 ■ A. W. Steivart on 



to half its quantitj'. Thus, if at the start there were sixteen 

 graninies of niton present, the foHowing figures give the amounts 

 remaining in existence after various periods : — 



Start ... ... 16 grammes. 



After 94 hours .. 8 grammes. 



After 188 hours ... 4 grammes. 



After 282 hours ... 2 grammes. 



After 376 hours ... 1 gramme. 



It is now necessary to deal with the work of Sir William 

 Kamsay and Mr. Soddy upon niton. Hitherto, though the change 

 of one element into anotiier had been well-established — as has 

 been described above — both the parent and the descendant ele- 

 ments liad belonged to the radioactive group, which at that time 

 were regarded as forming a class apart from the ordinary chemical 

 elements. Sir William Ramsay and Mr. Soddy, however, brought 

 the matter into a new orientation by proving that niton, in the 

 course of its disintegration, produced a perfectly well-known 

 element, helium. This formed the first link connecting the 

 radio-elements with the common non-radioactive series of ele- 

 mentary materials known to chemists. 



Suppose that we had, enclosed in a glass tube, some niton, 

 and that we were able to measure the pressure of the gas within 

 the tube from day to day. This was actually done l)y Sir 

 William Ramsay and Mr. Soddy ; and, somewhatto their astonish, 

 ment, they found that the pressure diminished as time passed. 

 Finally, on examining the spectrum of the contents of the tube, 

 they observed that the niton spectrum had disappeared completely, 

 and had been replaced by that of helium. 



Further investigation shows that the walls of the tube have 

 ac(iuired radioactive properties. That these properties are due 

 to a solid deposited on the glass can be proved in various ways. 

 For example, if the glass be scraped with a metal point, the metal 

 in turn acquires the power of discharging an electroscope ; or 

 again, by means of acids it is possil)le to dissolve from the glas§ 



