112 THE PATH OF SCIENCE 



these spectra are very complex. The wave length of the lines 

 emitted can be measured, and certain numerical relationships 

 between them had been deduced as the result of a long study 

 of the problem by many workers. A mechanism for the 

 emission of a spectrum by a given element was still lacking 

 when Bohr took up the problem. In 1913 he suggested that 

 the action in the atom that resulted in the emission of a 

 spectral line was the movement of one of the rotating elec- 

 trons from one orbit to another. Taking Rutherford's pic- 

 ture of the atom, in which the electrons rotate around a 

 nucleus, Bohr assumed that as long as the electron rotated 

 in a given orbit, it would not radiate any energy; but that 

 if it changed its orbit and shifted to a smaller one, energy 

 would be set free and would be emitted as a spectral line. 

 Moreover, the orbits of the electrons would be at discrete 

 definite distances from the nucleus. The radii of these orbits 

 would, in fact, be proportional to the squares of successive 

 whole numbers— 1, 4, 9, 16, etc. Consequently, whenever an 

 electron shifts from one orbit to another, it emits energy of 

 a definite amount, which corresponds, of course, to a definite 

 wave length in the light emitted. 



The idea that energy was emitted by atoms in definitely 

 fixed amounts, corresponding to the change in diameter of 

 the electron orbits, supplied a mechanism for a general law 

 of radiation that had been announced by Max Planck about 

 ten years before— that radiation is emitted in definite units, 

 so to speak, atoms of energy, ^vhich Planck named quanta. 

 Bohr, using Rutherford's idea of the atom, supplied a 

 mechanism for Planck's quantum theory of radiation. The 

 structure of the Bohr-Rutherford atom has undergone some 

 modification since it was originally suggested. It has become 

 established, however, as a basic principle and has been able 

 to explain a great many different phenomena, such as the 

 radiation of hot bodies, the emission of spectra, the absorp- 

 tion spectra of molecules, the chemical structure of com- 

 pounds, the effect upon atoms of radiation, and the radio 

 active elements and their behavior. 



