864 Sir E. Rutherford and Dr. E. K da C. Aiidrade on 



element to the next. The frequency of the strongest spec- 

 trum line has been shown to vary as (N — a) 2 where N is a 

 whole number and a a constant (about unit}') for all this 

 group of elements. N changes by unity in passing from 

 one element to the next, and is supposed to represent the 

 number of fundamental units of positive charge carried by 

 the atomic nucleus and may for convenience be called the 

 ** atomic number," since it represents the number of the 

 element when arranged in order of increasing atomic weight 

 supposing that no elements are missing. 



It is well known from the work of Barkla that the heavier 

 atoms emit a second type of characteristic radiation known 

 as the "L"" radiation. Moseley has examined the X-ray 

 spectra of this type for elements of atomic weight from 

 silver to gold, and finds that the spectra of all these ele- 

 ments are similar, but as in the case of the " K" type, the 

 frequency increases by definite steps as we pass from one 

 element to the next. He has shown that the frequency of 

 the chief line of the spectra is nearly proportional to (N — b) 2 , 

 where N as before is the atomic number (or nucleus charge) 

 and b a constant (about 7*4) for the whole groups of elements. 

 On the general theory of the nucleus atom, the nucleus 

 charge determines the chemical and physical properties of 

 the atom, and it is consequently of great importance to 

 determine the value of this constant for the radioactive 

 atoms. Before the publication of this paper, Mr. Moseley 

 kindly informed us of his experimental results, and it 

 became of great interest to determine the nucleus charge 

 of radium B. As we have already seen, the soft radiation 

 from radium B, whose absorption coefficient is //, = 40 in 

 aluminium, was believed to be the " L " type of charac- 

 teristic radiation of radium B, and this is completely borne 

 out by the comparison of the y-ray spectrum of the soft radia- 

 tions of radium B with that of platinum (see page 861) 

 Using Moseley 's formula, and assuming for the atomic 

 numbers the values to be given in a following paragraph, 

 the factor by which the angle of the strong platinum line 

 must be divided to give the angle of the corresponding line 

 of radium B is 1*118 : the value 1*122 used in Table I. was 

 chosen so as to make the experimental lines agree exactly. 



A determination of the nucleus charge of radium B is for 

 another reason of the highest importance, for this radioactive 

 element has been shown by Fleck to have the chemical 

 properties of lead and to be chemically inseparable from it. 

 As is well known, a very comprehensive and far reaching 

 theory of the relation between the chemical and physical 



