540 SCIENCE PROGRESS 



electrons ejected from the metallic electrodes (and possibly 

 condensed layers of mercury near the cathode) by the stimu- 

 lated radiation. Had this low value of the ionisation potential 

 been maintained, it would have proved a serious obstacle to 

 acceptance of the Bohr theory. For this theory postulates 

 a number of stationary, non-radiating orbits in which the 

 electrons travel around a central positive nucleus, the total 

 energy of an electron (potential and kinetic) being greater 

 the further out the orbit in which it is moving. Circumstances 

 arise which cause an electron to " jump " from one orbit to 

 another ; if from an outer to an inner, the balance of energy 

 between the two orbits is emitted as radiation with a definite 

 frequency, /, which is determined by the quantum relation 

 hf= the energy emitted ; if, on the other hand, the jump is 

 from inner to outer, energy of a corresponding amount must 

 be absorbed. The further out an orbit is situated from which 

 an electron jumps, and the closer in the orbit to which it 

 jumps, the greater the frequency of the emitted radiation. In 

 particular the greatest frequency for an atom would corre- 

 spond to the passage of a " free " electron (i.e. one outside 

 the influence of the atom and regarded "at infinity") into 

 the innermost orbit, and conversely a jump from an orbit 

 to a state of " freedom " would correspond to ionisation. 

 Now if an atom is ionised, i.e. if it has lost an electron, there 

 is no apparent reason why the return of an electron to the 

 atom should give only one kind of radiation, since, there being 

 several orbits, there are obviously possibilities for the emission 

 of as many monochromatic radiations by an electron entering 

 the atom from without. Consequently had the view that 4-9 

 volts is the ionisation potential of mercury vapour prevailed, 

 we would have had to regard the bombarding electrons at 

 this impact voltage imparting sufficient energy to electrons 

 within certain of the atoms to eject them from these atoms, 

 and that these electrons in returning to the ionised atoms 

 only emitted a monochromatic radiation (2536-72 A.U.), i.e. 

 in all cases returned to a particular orbit. However, as 

 pointed out, it appeared that 4-9 volts was too low a value for 

 the ionising potential, and that probably the course of affairs 

 consists in the bombarding electrons imparting energy only 

 sufficient to carry the electron from an inner orbit to the next 

 one, and the monochromatic radiation corresponds to a return. 



