Abstract
The mathematical theories which are applicable to the excitation and ionization of atoms in plasmas at thermal equilibrium are described. These processes are controlled by the temperature and electron density in the plasma; some methods of determining these parameters are given. There have been several measurements of arc and spark temperatures in air, but few determinations of electron densities or of temperatures in gases other than air. It is shown that, even with the limited data available, the theories can be applied to practical problems which arise in spectrochemical analysis.