154 CHEMICAL DISCOVERY AND INVENTION 



attempt to measure when one considers that the limit of sensi- 

 bility of a delicate assay balance is about ^-^ mgrm. 



The atomic weight of niton deduced from five series of experi- 

 ments was as follows : 



I. II. III. IV. V. MEAN. 



227 226 225 220 218 223 



The determinations of the atomic weight of radium show that 

 it is almost exactly 2264, and the loss of one helium atom leads 

 to the value 2224 for the atomic weight of niton, a value which 

 is established by these experiments. 



The same authors have shown that niton is liquefiable by cold, 

 and its boiling-point under atmospheric pressure is 62 or 211 

 absolute. The critical temperature, that is the temperature at 

 which the gas cannot be liquefied by pressure, is 104-5 or 377-5 

 absolute. 



The liquid emanation is colourless, it causes the glass of the 

 containing tube to phosphoresce brightly. On further cooling it 

 sets into a solid which melts at 71. 



The emanation or niton, as it may now be called, though so 

 definite a substance is even less permanent than radium itself, for 

 while the life of radium extends to thousands of years, the period 

 of half-decay being calculated as 2000 years, the half-period of 

 transformation of niton is only 3-85 days. Its immediate products 

 of disintegration are a-rays which escape and a solid active deposit 

 which is still more evanescent, having a half-transformation 

 period of only 3 minutes. This substance, which has been 

 labelled RaA, is transformed again in six stages till a product 

 is obtained which has been identified with the polonium separated 

 from uranium residues by Madame Curie in her original investi- 

 gation. It is believed that the final product of the disintegration 

 of polonium is lead. 



As already mentioned the proportion of radium found in a 

 mineral bears a constant ratio to the amount of uranium present, 

 and this seems to suggest that radium is formed by the dis- 

 integration of uranium. If this is the case it is obvious that 

 if a specimen of a uranium compound were prepared free from 

 radium it should be possible in course of time to recognise the 

 production of radium in such a material. Experiments of this 

 kind have been made, but the results showed that the change 

 was too slow to admit the supposition of an immediate con- 



