36 INORGANIC EVOLUTION. [CHAP. 



them at 448 1 . The important fact was that the lines special to the 

 flame did not appear among the Fraunhofer lines, while some of those 

 of the spark did appear. 



This line at 4481 now takes its place among the enhanced lines like 

 those of iron previously mentioned ; special cases now form pait of the 

 more general one. 



Here again the experiments pointed to varying degrees of dissocia- 

 tion at different temperatures as the cause 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 pheno- 

 mena, 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 process, 

 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 suggestion forward 

 with the greater confidence, because I see that help can be got from 

 various converging lines of work."* 



Calcium. 



In 1876 I produced evidence that the working hypothesis that the 

 molecular grouping of calcium which gives a spectrum having its prin- 

 cipal line at 4226'9 is nearly broken up in the sun, and quite broken 

 up in the spark, explained the facts which are that the low temperature 

 line loses its importance in the spectrum of the sun, in which H and K 

 are by far the strongest lines, 



I summed up the facts regarding calcium as follows : " We have 

 the blue line differentiated from H and K by its thinness in the solar 

 spectrum while they are thick, and by its thickness in the arc while 

 they are thin. We have it again differentiated from them by its ab- 

 sence in solar storms in which they are almost universally seen, and, 

 finally, by its absence during eclipses, while the H and K lines have 

 been the brightest seen or photographed." 



I afterwards attempted to carry the matter further by photograph- 

 ing the spectra of sun spots. In all cases H and K lines were seen 

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



* Proc. Hoy. Soc., 1879, vol. xxx, p. 30. 



