190 



Mr. J. Norman Lockyer. 



reversed over the spots, just as Young saw them at Sherman, while 

 the blue calcium line was not reversed.* The oldest of these photo- 

 graphs which has been preserved bears the date April 1, 1881. 



The experimental results in the case of calcium, therefore indi- 

 cated that the cause of the inversion of intensities in the lines of a 

 substance under different circumstances is due to the varying degrees 

 of dissociation brought about by different temperatures. 



Magnesium. — 1 have already referred to the results which I obtained 

 in 1879,f by passing a spark through a flame charged with different 

 substances. In the case of magnesium the effect of the higher tem- 

 perature of the spark was very marked, some of the flame lines being 

 abolished, while two new ones made their appearance, one of them at 

 4481. The important fact was that the lines special to the flame did 

 not appear among the Fraunhofer lines, while those of the spark did 

 appear. 



Here again the experiments pointed to varying degrees of dis- 

 sociation at different temperatures as the canse of the non-appearance 

 of some of the magnesium lines in the Fraunhofer spectrum, 



From these experiments the results of which were subsequently 

 mapped in relation to the various heat-levels indicated by solar 

 phenomena, I drew the following conclusions in 1879 : — 



" I think it is not too much to hope that a careful study of such 

 maps, showing the results already obtained, or to be obtained, at 

 varying temperatures, controlled by observations of the conditions 

 under which changes are brought about, will, if we accept the idea 

 that various dissociations of the molecules present in the solid are 

 brought about by different stages of heat, and then reverse the pro- 

 cess, enable us to determine the mode of evolution by which the 

 molecules vibrating in the atmospheres of the hottest stars associate 

 into those of which the solid metal is composed. I put this sugges- 

 tion forward with the greater confidence, because I see that help 

 can be got from various converging lines of work."| 



Iron. — The anomalies which have been noted in the case of iron 

 led me to undertake some experimental work with the sparks pro- 

 duced by quantity and intensity coils with and without jars in the 

 circuit. § The outcome of these experiments was to show that the 

 chromospheric representatives of iron were precisely the lines which 

 were brightened on passing from the arc to the spark, while the 

 lines widened in spots corresponded to a lower temperature. 



Attention was at the same time drawn to the gradual simplifica- 

 tion of the iron spectrum by increased temperature as we pass from 

 the arc through the spots to the prominences. 



* ' Roy. Soc. Proc.,' vol. 36, p. 444. 

 t Ibid., vol. 30, p. 26. 

 X Ibid., vol. 30, p. 30 



