580 SCIENCE PROGRESS 



tion of a few atomic weights from approximate whole numbers, 

 it may be briefly referred to. In radioactive change two kinds of 

 change are observed. In the one there is expelled from the 

 disintegrating atom an a-particle, which is an atom of helium 

 that has lost two electrons, in the other a /3-particle, which 

 is an electron or unit charge of negative electricity. After 

 the first kind of change, the radio-element changes in chemical 

 nature in a manner corresponding with a shift of two places 

 in the direction of diminishing mass in the periodic table, 

 whilst after the second kind of change the shift is one place 

 in the opposite direction. Hence two /3-ray changes and one 

 a-ray change, in any order of sequence, result in the radio- 

 element reverting to the same place as at first in the periodic 

 table, and though its atomic mass has been reduced by four units, 

 that of the helium atom expelled, its chemical properties are 

 identical with that of its original parent. It is reasonable to 

 suppose therefore that not only the radio-elements but possibly 

 some of the stable elements are mixtures of such " isotopic " 

 elements possessing the same nett atomic charge and occupying 

 " the same place " in the periodic table, but differing by whole 

 numbers in atomic weight. From such evidence as this it was 

 first proved for the sequence of the radio-elements that the 

 successive places in the periodic table correspond with unit 

 changes in the atomic charge (now called the atomic number), 

 as suggested by van der Broeck. Subsequently this was con- 

 firmed by Moseley's work on the wave-lengths of Barkla's 

 characteristic X-radiations for all the elements systematically. 

 On Rutherford's theory, the atomic number, or number the 

 place occupies in the sequence of elements — starting with 

 hydrogen as unity, helium, two, and so on — is the positive 

 charge on the nucleus. 



The positive ray method and gaseous diffusion methods 

 would be almost the only ones capable of distinguishing a 

 heterogeneity of the kind supposed in the theory of isotopes, 

 and many conclusions can scarcely be profitably discussed 

 until the homogeneity of the elements has been systematically 

 re-examined by one or other of these methods. The researches 

 on the variation of the atomic weight of lead from different 

 radioactive minerals, so far as they have yet been pursued, 

 shows that in the case of this one element, at least, the con- 

 clusion drawn from the theory is borne out, 



