1012 Drs. Foote, Meggers, and Mohler on Excitation of 



considerably below 15 volts. On the 10-volt exposure shown 

 we find the following enhanced lines : — 



Line. 



Intensity. 



Notation. 



2791-8 



1 



2^-3® 



2796-802 



9 



1"5©-2<JJ 



2928-36 



1-5 



2IP-2-5® 



The pair 1*5 ©- 2 % X2796 and A, 2802 represent a dis- 

 placement of the single remaining valence electron from the 

 1'5 @ ring to the 2 ty ring. This requires an electron 

 collision of but 4*4 volts. However, the atom must be first 

 simply ionized, requiring an impact of 7*61 volts. Hence 

 this fundamental enhanced line may appear in an arc at the 

 simple ionization potential if the current density is sufficiently 

 high and the vapour pressure low, so that successive impact 

 may take place. 



Referring to fig. 3 it will be seen that, just as in the case 

 of the simple spectrum, the enhanced lines 2791 — 8 and 

 2928—36 belong to series converging at 2^, the first ring- 

 out from 1'5@. These enhanced lines, observed faint 

 compared with 1*5 @ — 2^, are, with high-current density, 

 possibly the result of three successive impacts (or equivalent 

 absorptions of radiation quanta) : the first ionizing the atom 

 requiring 1*21 x 10~ n erg ; the second displacing che 

 remaining valence electron from 1*5© to 2$, requiring 

 0*7 x 10~ n erg ; and the third displacing this electron from 

 2 ^ to an outer orbit. We have then below the second 

 ionization potential of magnesium a single-line enhanced 

 spectrum consisting of the doublet X2796 and X 2802, and 

 with high current density fundamental enhanced lines of 

 series converging at 2 ty. In this respect the simply ionized 

 magnesium atom behaves exactly like the neutral sodium atom, 

 as should be expected. 



The occurrence of enhanced lines in the spectrum of the 

 magnesium arc at reduced pressures was first recorded by 

 Fowler*, who found the enhanced line 4481 A and pairs of 

 lines at 4385, 4391, and 4428, 4434 A very conspicuous in 

 the arc operated on 100 volts with 1 or 2 mm. Hg pressure. 

 He concluded that this modification of arc spectra at reduced 

 pressure might be due to the presence of hydrogen liberated 

 from the heated poles. This conclusion was still adhered t 

 to in 1913, when it was pointed out that the spark-lines of 

 magnesium form two series like those of hydrogen, X4481 



* Fowler, Proc. Hoy. Soc. Lond. lxxii. p. 255 (1903). 

 t Fowler, Proc. Roy. Soc. Lond. lxxxix. p. 133 (1913). 



