180 PROFESSOR E. RUTHERFORD ON THE 



curve (fig. 1) was obtained by means of the a rays, while the four upper curves (fig. 7) 

 were obtained by means of the ft rays. The intermediate curves, for an exposure of 

 1 minute and 5 minutes, do not agree with the results obtained by me for either the 

 a or ft rays, but appear to be a mixture of both. 



The sudden initial drop is a characteristic of the a-ray curve, but I have only 

 observed the rise to a maximum, as in fig. 5, where measurements are made by the 

 ft or y rays. 



Miss BROOKS, working in the laboratory, has re-determined the cause of decay of 

 the excited activity of radium for different times of exposure, separately for both the 

 a and ft rays, and the results will be published shortly.* 



8. Explanation of the Curves. It has been pointed out that the rapid initial drop 

 for curves A and B, fig. 4, is due to a change giving rise to a rays, in which half of 

 the matter is transformed in about 3 minutes. The absence of the drop in the 

 corresponding curves, when measured by the ft rays, shows that the first 3 -minute 

 change does not give rise to ft rays ; for if it gave rise to ft rays, the activity should 

 fall off at the same rate as the corresponding a-ray curve. 



It has been shown that the activity several hours after removal decays in all cases 

 according to an exponential law with the time, falling to half value in 28 minutes. 

 This is the case whether for a short or long exposure, or whether the activity is 

 measured by the , ft, or y rays. This indicates that the final 28-minute change 

 gives rise to all three types of rays. 



It will be shown that these results can be completely explained on the supposition that 

 three successive changes occur in the deposited matter of the following character! : 

 (1) A first change of the matter deposited in which half of the matter is 

 transformed in about 3 minutes, and which gives rise only to a rays ; 



[* Since published, 'Phil. Mag.,' September, 1904.] 



t The view that the complicated rates of decay of the excited activity produced by radium and thorium 

 were due to a double change in the deposit from thorium and a treble change in that of radium was first 

 suggested in 1902 (RUTHERFORD, ' Physik. Zeitsch.,' 3, p. 254, 1902), and discussed later in 'Phil. Mag.,' 

 Jan., 1903, and also by RUTHERFORD and SODDY, 'Phil. Mag.,' April and May, 1903. In 1903, CURIE 

 and DANNE ('Comptes Rendus,' 136, p. 364, 1903) found that the decay curve of the excited activity of 

 radium (measured by the /3 rays) for a long exposure to the emanation could be empirically expressed by 

 the differences of two exponentials. The first definite statement of the changes occurring in the active 

 deposit of radium and the existence of a rayless change occurs in a paper by RUTHERFORD and BARNES, 

 ' Phil. Mag.,' Feb., 1904. A discussion of the evidence in which these conclusions were based was 

 reserved for a later paper. A brief account of the theory of successive changes and the evidence of the 

 existence of a rayless change in both radium and thorium was given in ' Radioactivity,' . pp. 268-274. 

 Somewhat later CURIE and DANNE ('Comptes Rendus,' 138, p. 683, 1904), using the hypothesis of 

 successive changes advanced by RUTHERFORD and SODDY, arrived at a similar conclusion in regard to the 

 changes in radium. In a later paper (' Comptes Rendus,' 138, p. 748, 1904), CURIE and DANNE gave an 

 account of some important experiments on the effect of temperature on the curves of decay of activity, 

 and showed that the product radium B could )>e separated from radium C by volatilization at a suitable 

 temperature. 



