542 PROCEEDINGS OP THE AMERICAN ACADEMY. 



hitherto observed under any conditions. The same is true of the 

 spectrum of tin in the Wehnelt interrupter. 



5. Certain lines which are of broad, diffuse appearance in the spark 

 and are absent from the arc spectrum of the metal, are present in the 

 Wehnelt spectrum, but much decreased in intensity. The metals which 

 show this phenomenon most sharply are zinc, tin, and lead. 



The first thought is, of course, that we are dealing with a complicated 

 process involving an entire series of temperatures from that sufficient to 

 give the band spectrum of a compound to that necessary to produce the 

 strongest lines of a difficultly fusible metal like platinum or chromium, 

 and that the point and the space about it pass through this series of 

 temperatures at each interruption of the current. The result would be 

 a spectrum which was the sum of the spectra corresponding to the 

 various temperatures through which the system passes. Such a conclu- 

 sion should be evidenced by the presence of all the lines of all the 

 spectra, and if we are to couple each spectrum with a definite tempera- 

 ture we should expect the resulting composite to be complete. But in 

 none of the cases examined are all of the lines of the various spectra 

 present. Some strong lines are and some are not present in normal 

 intensity. So this conclusion, though perhaps the simplest consistent 

 with our present knowledge of temperature and spectra and the relation 

 between them, seems not to explain the facts. 



The interrupting action in the Wehnelt is undoubtedly caused by the 

 intense heating due to the high current density about the small point. 

 The heat vaporizes and then dissociates the water into hydrogen and 

 oxygen, as has been shown by the analyses of Voller and Walter * and 

 others. That spectra like those of the oxyhydrogen flame should re- 

 sult from the explosion of the mixed gases by the hot metal in the cooler 

 parts of the gaseous envelope is not surprising, it is rather remarkable 

 that not more of the characteristic flame spectra were observed. At the 

 same time the great affinity for oxygen of such metals as aluminium, 

 magnesium, calcium, and barium might explain the appearance of the 

 bands belonging to the oxides of these metals. 



The one point which is difficult of explanation by any reasoning based 

 on the assumption of a varying temperature is the absence of some of 

 the characteristic strong lines, while others usually ascribed to the same 

 temperature are present. 



*L. c. 



