404 PROCEEDINGS OF THE AMERICAN ACADEMY. 



which is marked very strong at A 3914.4 in Ilemsalech's * table of ni- 

 trogen bands. The fact that Hutchins is able to intensify the band he 

 describes by making and breaking the arc circuit would seem to indicate 

 that it appears in the arc spectrum primarily in consequence of high 

 electromotive force. 



III. Metallic Impurities. 



The only electrodes which we have employed are the unplated, cored 

 carbons sold by A. T. Thompson, 25 Bromfield St., Boston, for use in 

 projection lanterns. Their size is 1\ X \ inches and they are marked 

 " imported." The metallic impurities which present themselves are 

 practically only aluminium, calcium, copper, iron, and potassium. Pos- 

 sibly others might be detected by very long exposure or by study of 

 portions of the spectrum other than that to which we have limited our- 

 selves, namely, A 4500-A 3200. The strongest lines in this region of 

 the hot spark spectrum are two at A 4047.338 and A 4044.294 belonging 

 to the principal series of potassium. They are faintly represented in the 

 carbon arc ; but no trace of them can be found in the ordinary, or " cold," 

 carbon spark. Is it not rather surprising to find on a spark spectrum 

 plate that the strongest lines are due not only to an impurity, but to an 

 impurity which is introduced apparently by the condition of high tem- 

 perature in the medium ? For, so far as the energy delivered by the 

 spark itself is concerned, this would seem to be enormously greater in 

 the loud and brilliant cold spark than in the quiet and invisible hot spark. 

 We use the expression " high temperature " in this connection only with 

 great hesitation, and then only with reference to the medium after the 

 heating current has been cut off. But this potassium pair persists very 

 distinctly for five seconds after the heating (arc) current has been inter- 

 rupted. Accordingly we find it difficult to imagine any electrical effect, 

 other than heat, which would persist for this length of time, especially, 

 as the electrodes were placed always horizontally so that strong convec- 

 tion currents were sweeping out anything in the nature of electrolytic 

 products. 



It seems not improbable that these effects of the hot spark are brought 

 about through an increased conductivity — and hence a lowered electro- 

 motive force — between the poles of the spark gap. So that, in the 

 series described above, the earlier phases partake of the character of the 



* Hemsalech, Recherches expe'rimentales sur les spectres, etc., p. 126 (Paris, 

 1901). 



