III.] THE PRESENT POSITION. 37 



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, followed 

 suit with those obtained from iron and magnesium, and indicated that 

 the cause of the inversion of intensities in the lines of a substance 

 under different circumstances is due to the varying degrees of dissocia- 

 tion brought about by different temperatures. 



Both in the case of iron, magnesium and calcium, the high tempera- 

 ture lines involved are not seen at all at lower temperatures, and even 

 in the case of calcium, when photographic exposure of 100 hours' dura- 

 tion have been employed. It should be sufficiently obvious to every- 

 body from this that temperature alone is in question. 



Finally, then. The similar changes in the spectra of certain ele- 

 ments, changes observed in laboratory, sun and stars are simply and 

 sufficiently explained on the hypothesis of dissociation. If we reject 

 this, so far no other explanation is forthcoming which co-ordinates and 

 harmonises the results obtained along the different lines of work. Nor 

 is this all : as I shall show later on, there are other branches of 

 physical inquiry which suggest the same hypothesis. 



