Excitation of extreme ultra-violet spectrum of argon.—Circulating purified argon was excited by electrons from a Wehnelt cathode at various voltages up to 150 volts, with gas pressures of about 0.05 mm and currents of about 100 ma. Such excitation has been shown to arise from single rather than multiple electron impacts. The spectrum was photographed with the aid of a high vacuum spectrograph equipped with a glass grating. 88 lines, all believed due to argon, have been found in the wave-length region 1066 to 461. 20 of these belong to the known arc spectrum. Of the remaining 68 spark lines, 44 have not previously been reported. We fail to excite previously known A III lines, so that we attribute our 68 spark lines to A II.

Analysis of the argon spectrum.—37 spark lines have been classified. 12 of these classifications have been checked by numerous combinations involving lines in the near visible region (6883-2300). Tables of lines, terms and combinations are given. Three series of two members each give the series limit as 223, 363, so that the ionization potential is 27.82±0.05 volts. Adding the first ioniozation potential 15.69 volts gives 43.51 volts as the minimum potential for double ionization. These refer to ionization to the ${}_{}{}^3{}_{}{}^{}P_2^{}{}_{}{}^{}$ limit, the lowest energy state of A III. A tentative term value of -33230 (relative to ${}_{}{}^3{}_{}{}^{}P_2^{}{}_{}{}^{}$) is given to the higher ${}_{}{}^1{}_{}{}^{}D$ limit.

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