SOME CONTEMPORARY ADVAKCES IN PHYSICS— XI 483 



suffices to extract a iv-elcctron from a silver atom; for the photo- 

 graphic film contains silver, and it is the expulsion of electrons from 

 the atoms in it which initiates the photographic process. Proceeding 

 always toward higher frequencies, we see that presently the plates 

 suddenly heconu' w Iiitcr, at another critical frequency \\hicli however 



Fig. 11) 



is not the same for the three elements. The photographic film is not 

 responsible for these "absorption-edges" as they are called; each of 

 them occurs at the particular frequency for which the quantum-energy 

 just suffices to extract a X-electron from an atom of the element which 

 formed the absorbing-layer placed in the path of the beam before it 

 reached the plate. To the right of the absorption-edge we have the 

 lower frequencies, unimpeded by the cadmium (or antimony, or 

 barium) atoms because unable to ionize them; to the left we have the 

 higher frequencies, reduced in intensity by the intercalated matter 

 because some of their energy was drawn ofY to detach electrons. 



From the frequency v at such an absorption-edge, the extraction- 

 energy W of the class of electrons in question for the kind of atom in 

 question is determined by the equation 



hv=W. 



This is a much more delicate way of measuring extraction-energies 

 than the observations upon electronic spectra afiford. Nevertheless 

 the measurements upon the energies of the ejected electrons are of the 

 greatest importance, for they show w^hat is effected by the energy- 

 transformations which set in when one or another of these critical fre- 

 quencies is overpassed. 



Likelihood of Ionization by Electrons Having Mork Than 

 THE Least Ionizing-Energy 



We have seen that electrons projected into a gas of ionizing-energy 

 Vo are able to ionize it if their kinetic energy exceeds Vo, otherwise not 

 (apart from ionizations effected upon atoms in abnormal states). This 



