CONSTITUTION OP MATTER RUTHERFORD. 197 



radium B is transformed by the loss of a beta particle, the resulting 

 element, radium G, takes up a position in Group V. By this simple 

 rule it has been found possible to define the essential chemical prop- 

 erties of all known radioactive elements. It was found that on this 

 theory one element was missing in the general scheme. This element 

 was discovered a few weeks later by Fajans and Gohring and found 

 to have the general chemical properties predicted for it. 



This generalization is capable of a very simple explanation on the 

 nucleus theory. The loss of an alpha particle of charge 2 lorverR 

 the nuclear charge of the resulting elements two units; the loss of a 

 beta particle, which carries a unit negative charge, raises the nuclear 

 charge by one unit. In other words, the atomic number of an 

 element shifts two units lower after loss of an alpha particle and 

 shifts one unit higher after loss of a beta particle. 



The atomic numbers of the elements in the uranium-radium series 

 can be simply deduced from this rule if the atomic number of one 

 element is known. We shall see later that the atomic number of 

 radium B is 82 and identical with that of lead. The actual atomic 

 numbers of the various elements are given in the circles representing 

 the atoms in figure 4. It is seen that uranium, the heaviest known 

 element, has an atomic number, 92, while the elements radium B, 

 radium D, and the end product, which is believed to be lead, have 

 the same atomic number, viz, 82. The evidence of the correctness of 

 this striking conclusion will now be discussed. 



As a result of a careful examination of the radioactive substances 

 it has been found that in a number of cases elements which are of 

 different atomic weights and exhibit different radioactive proper- 

 ties yet show identical general physical and chemical behavior. 

 For example, the elements radium B, radium D, and lead, 

 of atomic weights 214, 210, and 207, respectively, are so closely 

 allied in chemical and physical properties that all attempts to 

 separate a mixture of any two of them have failed completely. 

 This would be explained if the nuclear charges were identical for 

 those elements, as the generalization, already referred to, indicates. 

 If this be the case they should give identical spectra under similar 

 conditions. Unfortunately, the elements radium B and radium D 

 are in too small quantity to determine their ordinary light spectra, 

 but w^e can compare the X-ray spectrum of lead with that given by 

 radium B under the excitation of its own beta rays. Experiments of 

 this kind were recently made by Dr. Andrade and the writer, and the 

 two spectra were found to be identical within the limits of experi- 

 mental error. It is to be anticipated that their light spectra would 

 also prove to be identical, or nearly so, for, as previously pointed out, 

 the effect of the mass of the nucleus on the spectrum is probably 

 very small. 



