4D8 Professor Sir Ernest Rutherford [June 4, 



l)y different kinds of matter. It was early observed, however, that 

 some of the radiations appeared to be complex. This was shown by 

 anomahes in the initial part of the al)Sorption curve. In the mean- 

 time, a notable advance in our knowledge of X-rays had been made 

 l)y the work of Barkla. He found that under certain conditions 

 each element when bombarded by X-rays of suitable penetrating 

 ])Ower gave rise to a strong radiation which was characteristic for 

 that element, e.g., the lighter elements from aluminium to silver 

 emitted characteristic radiations called the " K " series, which 

 increased rapidly in penetrating power with the atomic weight of 

 the radiator. It was found that the heavier elements emitted in 

 addition another characteristic radiation of softer type, which was 

 called the " li " series. These results showed clearly that there must 

 be definite structures within the atom which gave rise to a definite 

 radiation under suitable conditions of excitation. From these results 

 it seemed probable that the y rays from radio-active matter must 

 consist of the characteristic radiations of these heavy elements, 

 nnalogous in type to the corresponding radiations observed in 

 ordinary elements when excited by X-rays or kathode rays. These 

 conclusions were confirmed by a series of investigations made by 

 Rutherford and Eichardson. The y rays were analysed by means 

 of their absorption by aluminium and by lead, the disturbing effects 

 of the primary jS rays being eliminated by means of a strong 

 magnetic field. It was found, for example, that the y rays from 

 radium B, when examined by their absorption in aluminium, consisted 

 of at least two types, one easily absorbed, and the other eighty times 

 more penetrating. By further observations of the absorption of the 

 y rays by lead, Richardson found that the rays from radium B 

 could be divided into at least four definite types, each of which was 

 absorbed exponentially by lead. Similar results were obtained for 

 all the radio-active elements which emitted y rays. In some cases 

 the soft 7 rays, e.g., those from radium B, corresponded to the 

 characteristic radiation of the " L " series, and others to the " K " 

 series. The general results, however, indicated that several addi- 

 tional series of characteristic radiations are present in some cases. 

 It was clear from these experiments that the y rays corresponded to 

 the natural modes of vibration of the inner structure of the radio- 

 active atoms. In the meantime the experiments of W. H. Bragg 

 and W. L. Bragg, and of Moseley and Dai"win, had shown that the 

 characteristic X-radiations of the elements gave definite and well- 

 marked line spectra. These spectra were simply determined by 

 reflecting the rays from crystals. If this were the case, it seemed 

 probable that the y rays from the radio-active atoms would also give 

 line spectra, and thus allow the natural frequencies of vibration of 

 these atoms to be determined. During the past year, a number of 

 experiments have been made to test this point by Rutherford and 

 Andrade, using radium B and radium C as the source of y radiation. 



