the Occurrence of Enhanced Lines in the Arc. 835 



means o£ a white-hot carbon cathode, the spark lines dis- 

 appear from the spectrum of the metal vapour. Thus 

 the prominence of spark lines in the spectrum of a source 

 appeared to indicate that the temperature of the hitter 

 must be relatively low. In fact the spark lines of lead 

 were found to be strongly emitted by an arc burning under 

 liquid air — namely, at a temperature of about —190° C. 

 Encouraged by these results, we first made a general survey 

 of the arc spectra of fifteen different metals by means of the 

 methods previously described, and then devoted our attention 

 more particularly to the spectrum of magnesium. This 

 element was marked out for special examination not only 

 by reason of its cosmical importance but also because its 

 spectrum contains four distinct types of radiations, all of 

 which' occupy in turn prominent positions in the various 

 laboratory sources of light. It therefore seemed to us that 

 a detailed study of the spectrum of magnesium might lead 

 to results of a more general interest on account of the 

 possibility of investigating in one and the same spectrum 

 the relative behaviour of four different types of radiations 

 under the special conditions prevailing in our experiments. 

 It may be useful for the better understanding of what 

 follows to briefly recall our present knowledge concerning 

 the emission of these four types of magnesium lines com- 

 prised within that region of the spectrum which is of 

 particular interest to the astrophysicist, namely from 

 X 3600 to X 7000 :— 



jp- . . f Flame line X 4571. Observed already at the 



s MP ' ^ temperature of the air-coal gas flame. 



Series triplets at X 3838 and X5184. These lines are 

 absent from the mantle of the air-coal gas flame. 

 X 5184 has been observed in the oxy-hydrogen 

 flame. Both triplets are emitted by the flame 

 of burning magnesium. Their emission is very 

 strong under thermo-electrical excitation in the 

 fringe of a single plate resistance furnace. They 

 are likewise brought out under chemical ex- 

 citation in the blue cone of the air-coal gas 

 flame. Strong in arc and spark. 

 Bydberg's nebulous series, of which the least 

 Third type, «J infrangible line in this region is X5528. With 

 certainty observed only in arc and spark. 

 fSpark lines of which the most prominent and 

 important is X 4481. Not seen in any of the 

 ,' flames nor in the furnace. Traces only in 

 ordinary arc in air between Mg poles. Brought 

 out strongly in arc under water or in hydrogen. 

 [ Verv intense in capacity spark, 

 3H2 



Second type.*( 



Fourth type.-{ 



I 



