534 SCIENCE PROGRESS 



that the frequency of the lines of the X-ray spectra varied as 

 (N — a)' where N is the atomic number of an element and a 

 is a constant for the series. For the K lines a is about i and for 

 the L lines about 7*4. Adopting the Bohr model of an atom 

 and his hypothesis that radiation is emitted in quanta by 

 electrons leaping from one steady orbit to another in the atom, 

 Moseley's experimental law can be justified, provided, as 

 stated above, A'' is an integral number which increases by unity 

 as we proceed step by step along a complete periodic table of 

 elements. As it happens N itself is about half the atomic weight 

 of an element, and this fact brought the earlier work of Barkla 

 and Geiger and Marsden into line with Moseley's results obtained 

 by a different method. Mr. Chadwick, in view of the importance 

 of obtaining fresh evidence on this point, undertook to repeat 

 the experiments on the scattering of a-particles, which probably 

 of all the methods gives a result most directly. The essentially 

 new feature of his apparatus is the arrangement by which the 

 scintillations produced by the a-particles in the scattered beam 

 and those in the direct are counted on the same zinc sulphide 

 screen and under the same conditions. This involves a very 

 considerable increase in the accuracy of comparison of the two 

 beams. His work has been carried out on platinum, silver and 

 copper. For platinum (atomic number 78) he finds for N the 

 value 77*4 ; for silver (atomic number 47) he finds AT" to be 46-3 ; 

 for copper N is 29-3 (atomic number 29). 



As regards the experiments concerned with the disintegra- 

 tion of atoms, reference can be made to the Bakerian Lecture of 

 1920 reprinted in the P.R.S., A 686, 1920 (July), and to a lecture 

 to the London Physical Society, reprinted in their Proceedings 

 for August 1 92 1. Prof. Rutherford's most recent work, 

 however, is summarised in a paper contributed to the Phil. Mag. 

 of last November. It is stated that in the earlier experiments 

 (described in the Phil. Mag. of 191 9 (June) and summarised in 

 the number of Science Progress referred to above) the 

 scintillations due to the H atoms ejected from nitrogen were so 

 few in number and feeble in intensity that it was difficult to 

 decide with certainty whether the maximum range of the H , 

 atoms from nitrogen differed from that for the corresponding I 

 H atoms which were set in motion by recoil when a-particles 

 pass through hydrogen. 



The apparatus has been considerably improved in the 

 meantime. A better form of microscope which aims at a larger 

 field of view than before at the expense of magnifying power 

 (a relatively unimportant feature) and protects the eyes of the 

 counter from injury by 7-radiation, was designed ; special 

 zinc -sulphide screens with a thin and finely powdered layer of 

 the sulphide were used so that the corresponding reduction in 



