40 Dr. Foote and Dr. Mohler on Ionization and 



parts of the apparatus on account of the high temperature, 

 900° C, required. The partial current curves 6, 7, 8, 9 corre- 

 spond to the total current curve 4, and were accordingly used 

 for determining the initial potential. The initial potentials 

 for curves 10 and 11, taken some time later, are a trifle 

 higher. The final values obtained for thallium were 7*3 volts 

 for the ionization potential and 1*07 volts for the resonance 

 potential. 



Discussion of the Results. 



If we apply the quantum relation hv = eY to the expe- 

 rimental data obtained for magnesium, we find that the 

 line X = 4571 corresponds to resonance potential and the 

 convergence or limiting wave-length, \ = 1622, corresponds 

 to ionization potential. The limiting wave-length, v=l*5 S, 

 has the value 1622 A., so that the ionization potential is 

 determined by the same spectral series relation as in the case 

 of other metals so far studied. It was found in the earlier 

 work that the resonance potential is determined by the single 

 line spectrum of the metal; but magnesium would appear to 

 offer an exception to this rule. McLennan* has investigated 

 the single line and absorption spectrum of magnesium, and 

 has concluded that A,=4571 is not the single line spectrum, 

 but rather the line X = 2852. Although special effort was 

 made to detect the presence of X=4571 as a single line, the 

 line 2852 always appeared with the line 4571, and only the 

 line 2852 could be made to appear alone. If the single 

 line spectrum is \=2852, we have the first exception to the 

 rule that resonance potential is determined by the single line 

 spectrum, for our curves show no indication of resonance at 

 multiples of 4*3 volts as would be required by the line 2852. 



The behaviour of the various metals as regards ionization 

 and resonance potentials is apparently determined by their 

 position in the periodic table of the elements. Thus in the 

 case of sodium and potassium, group I., ionization is de- 

 termined by the convergence of the principal series and 

 resonance by the first line of this series. For magnesium, 

 zinc, cadmium, and mercury, group II., ionization is de- 

 termined by the convergence of a combination series 

 1*5 S — mp 2 , where mp 2 is the variable term of the second 

 line of the principal series of triplets, and resonance is given 

 by the first term of this series where m = 2. The 1*5 S 

 term is the limit of the principal series of single lines, 

 and may be derived from the variable term in the second 



* Proc. Roy. Soc. Lond. xcii. p. 574 (1916); xcii. p. 305 (1916). 



