662 



SCIENCE 



[N. S. Vol. XLIV. No. 1141 



penetrating rays and the L or soft rays. 

 Moseley subjected a large number of the 

 elements to a bombardment of the cathode 

 rays and determined the vibration frequen- 

 cies of the resulting X-rays. He found 

 that the vibration frequency increases with 

 increase in atomic weight. In the first 

 series of experiments the K series of X-rays 

 from the different elements were reflected 

 from a crystal surface, and the spectra 

 photographed. Each element produced 

 two characteristic lines. On passing from 

 one element to the next higher in atomic 

 weight the two lines were shifted toward 

 the violet end of the spectrum. In this 

 way a remarkable relationship was estab- 

 lished. Moseley found that the vibration 

 frequency is equal to A(N — b) 2 , where A 

 is a constant and b is equal to unity. N is 

 a whole number which increases by unity on 

 passing from one element to the next higher 

 in atomic weight. As aluminium is the 13th 

 element, Moseley gave N a value of 13 for 

 this element, and determined the corre- 

 sponding value of A. "With the value of A 

 thus determined, the other elements gave 

 values for N equal to their respective 

 atomic numbers ; thus, aluminium 13, silicon 

 14, calcium 20, iron 26, cobalt 27, nickel 28, 

 and so on up to silver 47. With elements 

 of higher atomic weights, the L series of 

 rays were used, and the investigations ex- 

 tended to gold, for which N = 79. For 

 these rays, five lines were visible instead of 

 two. The same formula could be used, 

 however, by changing the values of the 

 constants A and b. 



Moseley found that known elements corre- 

 spond with all numbers from 13 to 79 ex- 

 cept three. These elements may be dis- 

 covered later. Moseley suggests that the 

 presence of a new element and its place 

 in the periodic system can be quickly deter- 

 mined by this method. These results show 

 that some fundamental property of the 



atom changes step by step on passing from 

 one element to another in the periodic 

 system. Moseley concluded that as N is 

 equal to the atomic number, it represents 

 the magnitude of the nuclear charge, and 

 that this charge changes by unity on pass- 

 ing from one element to the next. It will 

 be noticed that these results reverse the 

 order of cobalt and nickel, indicating that 

 the magnitude of the nuclear charge is 

 more reliable than the atomic weight as an 

 index of quality. 



The theory that the chemical and phys- 

 ical properties of an element are closely 

 related to the nuclear charge of the atom 

 is supported by recent observations on the 

 radio-elements. 15 Some important general- 

 izations relative to the nature of these ele- 

 ments have been made during the last few 

 years, and the large gap in the periodic 

 system between the elements bismuth with 

 an atomic weight of 208 and uranium with 

 an atomic weight of 238 is now occupied by 

 more than 30 radio-elements which are ap- 

 parently true chemical elements. 



As already observed, radioactivity is a 

 property of the atom. It is caused by a dis- 

 integration of the atoms. There is, how- 

 ever, no gradual disintegration. Each 

 atom of a radio-element is stable until it 

 undergoes a sort of an explosion and ejects 

 an alpha or beta particle, which changes 

 it to a different atom and a different chem- 

 ical element. Each radio-element has its 

 own characteristic radioactive constant 

 which represents the fraction of the whole 

 amount which disintegrates in unit time. 

 The reciprocal of this constant represents 

 the period of average life. This period 

 varies from a very small fraction of a sec- 

 ond to several billions of years for the 

 different radio-elements. 



When a radio-element ejects an alpha 



is For references see Soddy 's ' ' The Chemistry 

 of the Eadio-Elements, " Vols. 1 and 2. 



• 



