66 ^4. W. Stewart on 



refinement of experimental skill by numerous workers ; and it 

 has been ])rovcd to be correct. 



It is evident that this work of Professor Soddy's re- 

 volutionises our previous ideas of the elements. Hitherto it has 

 been assumed that all the atoms of an element were identical 

 with each other in every respect ; but it is now conclusively 

 established that two atoms, each of which has all the chemical 

 properties of lead and each of which emits the same spectrum, 

 have weights differing from each other l)y about one per cent. 

 Since the two varieties of lead have identical chemical properties, 

 they must both be placed in the same square in the Periodic 

 Table ; and elements such as these are termed by Professor Soddy 

 " isotopic elements " from isos = equal and topos = a place. The 

 two A'arieties of lead are said to be isotojyes of each other. Thus 

 These isotopes, in some eases, are found to have different 

 radioactive properties, though chemically they are inseparable 

 from each other. Thus thorium is inseparable from uranium - Xj 

 by any chemical means ; but when the two are examined by 

 radioactivity tests it is found that uranium - Xi loses half its 

 activity in thirty-five and a half days, whereas thorium requires 

 many millions of years to decay to the same extent. 



The recent work of Dr. Aston has shown that isotopy is not 

 confined to the radioactive series of elements ; for even such 

 common elements as chlorine are found to be made up of 

 mixtures of atoms of different weights though they are all 

 endowed with identical chemical properties. 



Professor Soddy having thus established the existence of 

 isotopy, it was my good fortune to call attention to the second 

 half of the generalization upon which the modern classification of 

 atoms is based. When a radioactive element parts with an 

 electron in the beta-ray change, its atomic weight is unaltered, 

 since in comparison with atomic weights, the mass of an electron 

 is negligible. None the less, the loss of this electron produces a 

 complete change in the chemical properties of the material. 

 Evidently then, there may be two atoms of equal weights but 



