CONTEMPORARY ADVANCES IN PHYSICS 95 



difference between the K and the Li extraction-energies agrees with 

 the measured value. This is the usual method. 



There is still another way, which may be explained by describing an 

 experiment performed by C. D. Ellis and W. D. Wooster.^^ They en- 

 closed a sample of radium B mixed with radium C in a rather thick- 

 walled platinum tube, and deposited a thin layer of the same mixture 

 upon the outer surface of the tube. The thin layer contributed the 

 beta-ray spectrum of radium B and radium C. The beta-rays from 

 the substances inside the tube were stopped by its walls, but the 

 gamma-rays went through and expelled electrons from the platinum, 

 which mingled with those from the covering film; so that upon the 

 photographic plate there appeared side by side the spectrum-lines com- 

 posed of electrons extracted from atoms of radium B and radium C by 

 their own gamma-rays, and the spectrum-lines composed of electrons 

 extracted from atoms of platinum by gamma-rays of the identical 

 frequencies. Side by side there appeared, for instance, the lines due to 

 i^-electrons extracted by the same radiation from radium B and from 

 platinum. The electrons from the radioactive substance had less 

 energy than those from the platinum, for more had been spent in ex- 

 tracting them ; the difference between the values of kinetic energy of 

 the electrons was equal to the difference between the values of the K 

 binding-energy for the atoms, with sign reversed; the K binding- 

 energy for platinum is known, that of the other atom is calculated at 

 once.^'- 



The six or eight investigations, performed by these methods upon 

 diverse substances by various physicists during the past two years, 

 have all come to concordant results. The atoms from which the 

 electrons of the beta-ray line-spectra are detached are the atoms of the 

 daughter-substances; the gamma-rays are emitted, or at least they act 

 (and it would be a daring person who would say that they exist for a while 

 before they act!) after the transmutation occurs. The controversy 



fi Proc. Camb. Phil. Soc, 23, pp. 844-848 (1925). There are several important 

 articles in this (November, 1925) number of the Proceedings which deal with the 

 problem of the emission of gamma-rays, secondary X-rays, and electrons emitted 

 from the nucleus or ejected from the circumnuclear family by these rays. 



^2 All the methods require the observer to guess which lines are composed of 

 electrons from the X-class, which of electrons from the Li class, and so forth; and 

 this is the major difficulty of the problem, for there is nothing intrinsically distinctive 

 about the lines. In many cases, especially when there are several gamma-ray 

 frequencies and a multitude of beta-ray lines, it is necessary to proceed by trial and 

 error, assigning a line first to one class of electrons and then to another, and finally 

 adopting the systematization which leaves the smallest number of lines unexplained 

 or at odds. Sometimes only one out of the three L classes yields a perceptible number 

 of electrons; there is a rule, which if general is very valuable, that when the product 

 of h into the frequency of the gamma-ray exceeds the extraction-energies of all the L 

 classes very greatly, then the Li class is the only one out of which electrons enough are 

 extracted to make a noticeable line. 



