The Mg II spectrum was excited in normal low voltage arcs in mixtures of magnesium vapor with hydrogen, argon, neon, and helium respectively in a tungsten furnace apparatus. The densities of the spectral lines were measured with a Moll microphotometer. The excitation is believed to be due to impacts of the second kind between normal magnesium atoms and metastable atoms or ions of the gas used. It is shown that a small amount of kinetic energy may be added to the inner energy available in the impacts of the second kind and that a larger amount of excess inner energy may be transformed into kinetic energy in such an impact. The probability of an impact of the second kind decreases after the excess energy reaches a certain value.

• Received 9 September 1929

DOI:https://doi.org/10.1103/PhysRev.34.1132