Excerpt from On Secondary Spectra and the Conditions Under Which They May Be Produced

Most metallic spectra are greatly altered by the addition of capacity, it is true, but the changes produced are by no means comparable with the change from primary to secondary, while metallic spectra, by every test known, are already secondary spectra before adding capacity. They are series spectra, Show the Zeeman effect and the displacement effect due to pressure, and the new lines brought in by added capacity are of the same (secondary) character. That so many metallic vapors exhibit banded absorption Spectra would Seem to indi cate that metals may have primary spectra as well as nonmetals, but if so they are to be sought in the electrodeless discharge and cathode luminescence where the excitation is very feeble, rather than in the condenser discharge. Nor would the oxyhydrogen flame, banded spectra of the metals studied by Hartley and Ramage be comparable with primary spectra. In these spectra the lines of the series spectra Simply become the heads of the bands in the flame spectra, while the lines of a true secondary spectrum appear to have no relation to the bands of the corresponding primary. Another notable distinction between the spectra of electrlo-negativeand electrolpositive elements is that the former, both primary and secondary, are remarkably invariant, while it is difficult to obtain the spectrum of a metal twice alike.

Why Should a condenser discharge produce a secondary spectrum? The oscillation frequency of a condenser is so much less than the fre quency of collision of an electron in a gas at 1 mm pressure that an electron must collide a great number of times before the impelling force is reversed. Hence the oscillatory nature of the discharge per se could have little effect on the Spectrum of 'a gas. On the other hand, the current density during discharge is thousands of times as great as during the steady flow of the-same current. In testing other gases for the effect of excessive current density, specially designed Plucker tubes were used. One form had a very fine (thermometer) capillary and hollow cylindrical electrodes; another had a third bulb interposed in the middle of the capillary. In this way the current density could be increased a hundred or thousand times in parts of the tubes. Bromine, iodine, and hydrogen easily Show the current den sity effect within the range of current that a common glass tube will carry. The primary spectrum changes continuously over into the secondary spectrum as the current is increased. Nitrogen and the sulphur group failed to Show the secondary spectrum with the largest steady currents that glass tubes could carry. Argon is so sensitive to changes ln current density that, in conjunction with a pocket spectro scope, it might be used as a milliammeter.

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