On the Chemistry of the Hottest Stars. 



193 



wave-length, but also to the excess of pressure above one atmo- 

 sphere." The exception referred to is that of calcium, in which 

 case the H and K lines, among others, were shifted only about half 

 as much as the blue line at 4226'9. 



Referring to the exceptional behaviour of calcium, Professor Hale 

 points out that the result " seems to support Lockyer's views as to 

 the dissociation of calcium in the arc and sun," * and further states : 

 " It would thus appear that the temperature of the dissociation of 

 calcium is between that of the Bunsen burner and that of the oxy- 

 coal gas flame. The high molecular weight of calcium has hitherto 

 conflicted with our belief in the presence of this metal in promi- 

 nences. If, however, it be granted that dissociation can be brought 

 about by temperatures even lower than that of the arc, the difficulty 

 is very greatly lessened." 



In an article which I wrote in ' Nature ' on this work,f I pointed 

 out that " it would be very interesting to see if the strontium line at 

 4607 behaves like the calcium g in relation to the lines at 4077 and 

 4215, representing H and K." 



This prediction has been confirmed in a paper by W. J. Hum- 

 phreys,J on " The effect of Pressure on the Wave-lengths of lines in 

 the Spectra of certain Elements," in which the author gives a table 

 of the shifts measured on the strontium lines mentioned above. 

 When working with, pressures varying from 6 to 12 atmos., the shift 

 of the line at 4077 was always approximately half that at 46' )7. 



There can be little doubt that other enhanced lines will follow 

 suit. 



The Question of Series. 



The other branch of new work to which I refer is the investiga- 

 tion of " series " of lines in spectra, which was practically started by 

 Balmer in 1885, and in which, since 1888, we owe so much, to the 

 labours of Messrs. Kayser and Runge and Rydberg. 



I stated in 1878 that the spectrum of a substance was the integra- 

 tion of the spectra of various molecular groupings. 



Messrs. Ruuge and Paschen first showed in 1890§ that the spectra 

 of lithium, sodium, and potassium were the integration of the spectra 

 of various " series." Later they have shown that lead and other 

 metals follow suit. 



Before Balmer's formula was employed, rhythmic structure in 

 spectra had often been noticed. Thus, in 1869, Mascart wrote as 

 follows : — 



* ' Astrophysical J ournal, 5 vol. 3, p. 160. 



f ' Nature,' voL 53, p. 416, March, 1896. 



X ' Astrophysical Journal,' vol. 4, p. 249, November, 1896. 



§ 4 Abh. d. k. Akad. d. Wiss.,' Berlin, 1890. 



