164 PROCEEDINGS OF THE AMERICAN ACADEMY. 



They may appear upon the plates, but are so wide and so faint that they 

 cannot be detected upon the general shading of the plates. 



That these lines are not merely spark lines introduced into these arc 

 spectra by the supposed spark at the breaking of tbe current through the 

 rotating arc is shown, first, by the fact that they were first observed in 

 the carbon arc, and, second, by the fact that I have seen H a and Hp quite 

 clearly in the magnesium metallic arc, when the poles were not rotating. 

 The lines produced in the stationary arc have much the same character 

 as in the rotating arc, but there is a large amount of continuous spectrum, 

 appearing as a background, in the case of the stationary arc, so that it 

 would be difficult to photograph the hydrogen lines in this way. 



These lines in the arc seem to be due to hydrogen, and not to water 

 vapor coming from the hydrogen generators.* This is shown by the fol- 

 lowing two experiments : (1) I passed the stream of hydrogen through 

 concentrated sulphuric acid and phosphorus pentoxide ; and even after the 

 stream of dry gas had. been running through the hood for three hours, I 

 found the H a line as bright as it was in the damp hydrogen coming 

 directly from the generators. (2) In place of the current of dry hydro- 

 gen, I passed through the hood a stream of air bubbling through warm 

 water, so that this air was charged with moisture to about the same 

 degree as the moist hydrogen coming directly from the generators. In 

 this case I was not able to detect the faintest trace of the H a line. 

 Magnesium poles were used in both the above experiments. 



Other Methods. 



I have examined some of these metals in commercial ammonia gas, 

 such as is used in refrigeration. In this gas the hydrogen lines come out 

 with nearly the same intensity as in hydrogen when copper or aluminium 

 electrodes are used; no hydrogen lines are seen in the sodium arc in 

 ammonia, although the arc works well, and when tin electrodes are used 

 in ammonia a black dust collects in the atmosphere about the arc to such 

 an extent as to shut off practically all the light within thirty seconds after 

 starting the arc. From the standpoint of convenience and safety, the 

 ammonia gas is much to be preferred to hydrogen. 



The copper arc in coal gas shows the H a line very clearly, but the 

 other hydrogen lines are not distinguishable on account of the multitude 

 of comparatively strong carbon lines which the coal gas furnishes in this 

 part of the spectrum. 



* Trowbridge, Phil. Mag., 50, 338 (1900). 



