1878.] A nalysis in connexion with the Spectrum of the Sun. 169* 



It is abundantly clear that if the so-called elements, or more pro- 

 perly speaking their finest atoms — those that give us line spectra — are 

 really compounds, the compounds must have been formed at a very 

 high temperature. It is easy to imagine that there may be no superior 

 limit to temperature, and therefore no superior limit beyond which 

 such combinations are possible, because the atoms which have the 

 power of combining together at these transcendental stages of heat do 

 not exist as such, or rather they exist combined with other atoms, like 

 or unlike, at all lower temperatures. Hence association will be a com- 

 bination of more complex molecules as temperature is reduced, and of 

 dissociation, therefore, with increased temperature there may be no 

 end. 



That is the first point. 

 The second is this : — 



We are justified in supposing that our " calcium," once formed, is a 

 distinct entity, whether it be an element or not, and therefore, by 

 working at it alone, we should never know whether the temperature 

 produces a single simpler form or more atomic condition of the same 

 thing, or whether we actually break it up into x + y, because neither x 

 nor y will ever vary. 



But if calcium be a product of a condition of relatively lower tem- 

 perature, then in the stars hot enough to enable its constituents to 

 exist uncompounded, we may expect these constituents to vary in 

 quantity ; there may be more of x in one star and more of y in another ; 

 and if this be so, then the H and K lines will vary in thickness, and 

 the extremest limit of variation will be that we shall only have H 

 representing, say, x in one star, and only have K representing, say, y in 

 another. Intermediately between these extreme conditions we may 

 have cases in which, though both H and K are visible, H is thicker in 

 some and K is thicker in others. 



Professor Stokes was good enough to add largely to the value of my 

 paper as it appeared in the " Proceedings " by appending a note point- 

 ing out that " When a solid body such as a platinum w r ire, traversed 

 by a voltaic current, is heated to incandescence, we know that as the 

 temperature increases not only does the radiation of each particular 

 refrangibility absolutely increase, but the proportion of the radiations 

 of the different refrangibilities is changed, the proportion of the higher 

 to the lower increasing with the temperature. It would be in accord- 

 ance with analogy to suppose that as a rule the same would take 

 place in an incandescent surface, though in this case the spectrum 

 would be discontinuous instead of continuous. Thus, if A, B, C, D, E 

 denote conspicuous bright lines of increasing refrangibility, in the 

 spectrum of the vapour, it might very well be that at a comparatively 

 low temperature A should be the brightest and the most persistent : at 

 a higher temperature, while all were brighter than before, the relative 



