PROBLEMS OF MODERN SCIENCE 



exceptional conditions, such as at high tempera- 

 tures or under intense electrical stimulation as in 

 vacuum-tube discharges. Another essential part 

 of the content of Bohr's theory is that the atoms 

 are able to pass spontaneously from states of 

 higher to states of lower energy, and the energy 

 thus lost appears as monochromatic radiation whose 

 frequency is determined by the quantum laws. 

 It is this emission which constitutes the line 

 spectra of the element. For every transition 

 between any two stationary states there will be a 

 single line, and all the possible transitions to a 

 given end state will give an infinite series of lines. 

 Such a series constitutes a series spectrum like 

 Balmer's series. In this way Bohr has accounted 

 quantitatively for all the known series spectra of 

 hydrogen, one of which has been discovered since 

 he predicted its existence. It would take us too 

 long to go into the other successes of Bohr's theory, 

 such as the explanation of the series of enhanced 

 lines of helium and of other elements, the fine 

 structure of the lines as modified by the Stark 

 effect, and so on. It must suffice to say here that 

 Bohr has given us the first real theory of spectro- 

 scopic phenomena. 



We have so far assumed that the positively- 

 charged part of the atom consists of a minute but 

 relatively massive nucleus. The most direct and 

 convincing evidence for this lies in the results of 



