ATOMIC WEIGHT OF LEAD 139 



vation of the decay of the radiations of a sealed 

 actinium preparation. 



ATOMIC WEIGHT OF ISOTOPES. 



It is clear that the periodic law connects, not 

 primarily chemical character and atomic weight, but 

 chemical character and atomic charge or atomic 

 number, which alters its value by integers, not 

 continuously, producing the step-by-step changes in 

 chemical character which is at the basis of the 

 analysis of matter into the chemical elements, or 

 heterotopes. This atomic number is, however, the 

 algebraic sum of positive and negative charges, so 

 that the loss of the a-particle with its two positive 

 charges and of two negative electrons as ^-particles 

 leaves its value unchanged and produces an isotope 

 of the element having an atomic weight four units 

 less than the original. Unique chemical character 

 and unique spectrum reaction is no proof of homo- 

 geneity, and so we arrive at the conclusion that the 

 chemical elements, so far considered homogeneous, 

 may be mixtures of isotopes, possessing different 

 atomic structure and stability, revealed when they 

 undergo radioactive change, and in some cases also 

 different atomic weight. This, although within the 

 scope of the Daltonian analysis of matter to detect, 

 nevertheless, until radioactive investigations revealed 

 this possibility, remained overlooked. In two cases, 

 that of the isotopes of lead on the one hand, and of 

 ionium and thorium on the other, this difference of 

 atomic weight in elements spectroscopically and 

 chemically identical has now been established by 

 direct determinations. 



The figure (facing p. 1 34) shows that, so far as these 

 changes have been followed, they all terminate in the 

 place occupied by lead, and, if this is the real, as dis- 



