6*20 Ranges of a particles from Radioactive Substances. 



by the scintillation method by G-eiger and Rutherford*. 

 This may be ascribed to the existence o£ two successive a-ray 

 products, or to the emission of two a particles in the disinte- 

 gration of each atom. This latter hypothesis is excluded by 

 the experiments of Marsden and Barrattf, who found no 

 evidence that two a particles were emitted simultaneously. 

 In regard to the first supposition Marsden and Barratt's 

 work also indicates that the period of the second product is 

 more than a few seconds. It seems not impossible that there 

 may exist two successive products, each of long period, which 

 cannot be separated by ordinary chemical methods. For 

 example, if the periods of these two substances were of the 

 same order of magnitude the ranges of the a particles would 

 differ very little and therefore be difficult to distinguish. 



In regard to actinium, it will be observed that the previous 

 notation has been changed in consequence of the discovery of 

 a new a-ray product following the emanation. The new 

 product is called actinium A, the second product actinium 

 B, &c. The reason for the change of nomenclature is given 

 in the following paper by Rutherford and Geiger. It will 

 be seen from the figure that the relation between range and 

 period is again represented by a straight line falling below 

 the corresponding line of the uranium-radium series. Radio- 

 actinium does not lie exactly on the line ; this may be due 

 to a slight error in the range, which is in this case difficult 

 to determine. 



The nomenclature for the active deposit of the thorium 

 emanation has also been changed in a similar manner to that 

 of actinium in consequence of the discovery of another quickly 

 decaying a-ray product, but the ranges of this product and 

 of the emanation have not yet been determined with accuracy. 

 Some preliminary experiments, however, made by one of us 

 indicated that the numbers referring to the products of the 

 thorium series also lie on a straight line when plotted in the 

 same way as has been done for the uranium-radium and 

 actinium series. The details will be discussed later when 

 some experiments now in progress have been completed. 



The connexion indicated above between the period and 

 range is at present only empirical, but it may depend on 

 some simple relation which may ultimately be brought to 

 light. Similar straight lines to those above would be obtained 

 by plotting period against initial velocity of expulsion, since 

 the range is proportional to the third power of the velocity. 



It is of interest that the relation discussed above offers a 



* H. Geiger and E. Kutkerford, Phil. Mag. xx. p. 691 (1910). 



t E. Marsden and J. Barratt, Proc. Phys. Soc. xxiii. p. 367 (1911). 



