226 



NATURE 



\_7an, 9, 1879 



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

 more properly 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 tem- 

 perature, 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 

 combination 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 temperature, 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 thick- 

 ness, 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. Interme- 

 diately between these extreme conditions we may have 

 cases in which, though both H and K are risible, H is 

 thicker in some and K is thicker in others. 



Prof. Stokes was good enough to add largely to the 

 'Value of my paper as it appeared in the Proceedings by 

 appending a note pointing out that "When a solid body 

 such as a platinum wire, traversed by a voltaic current, 

 is heated to incandescence, we know that as the tem- 

 perature increases not only does the radiation of each 

 particular refrangibility absolutely increase, but the pro- 

 portion 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 compara- 

 tively low temperature A should be the brightest and the 

 most persistent ; at a higher temperature, while all were 

 brighter than before, the relative brightness might be 

 changed, and C might be the brightest and the most 

 persistent, and at a still higher temperature E." 



On these grounds Prof. Stokes, while he regarded the 

 facts I mentioned as evidence of the high temperature of 

 .tbe sun, did not look upon them as conclusive evidence of 

 the dissociation of the molecule of calcium. 



Since that paper was sent in, however, the appeal to 

 the stars to which I referred in it has been made, and 

 made with the most admirable results, by Dr. Huggins. 



The result of that appeal is that the line which, accord- 

 ing to Prof. Stokes' view, should have prerailed over all 

 others, as Sirius is acknowledged to be a hotter star than 

 our sun, is that, if it exists at all in the spectrum, it is so 

 faint that it was not recognised by Dr. Huggins in the 

 first instance. 



In Sirius, indeed, the H line due to one molecular 

 grouping of calcium is as thick as are the hydrogen lines as 

 mapped by Secchi, while the K line, due to another mole- 

 c^ilar grouping, which is equally thick in the spectrum of 

 the sun, has not yet made its appearance. 



In the sun, where it is as thick as H, the hydrogen 

 lines have vastly thinned. 



While this paper has been in preparation. Dr. Huggins 

 has been good enough to communicate to me the results 

 of his most important observations, and I have also had 

 an opportunity of inspecting several of the photographs 

 which he has recently taken. The result of the recent 

 work has been to show that H and h are of about the 

 same breadth in Sirius. In a Aquilas while the relation of 

 H to // is not greatly changed, a distinct approach to the 

 solar condition is observed, K being now unmistakably 

 present, although its breadth is small as compared with 

 that of H. I must express my obligations to Dr. Huggins 

 for granting me permission to enrich my paper by refer- 

 ence to these unpubhshed observations. His letter, which 

 I have permission to quote, is as follows : — 



*' It may be gratifying to you to learn that in a photo- 

 graph I have recently taken of the spectrum of a Aquilae 

 there is a line corresponding to the more refrangible of 

 the solar H lines [that is K], but about half the breadth 

 of the line corresponding to the first H lines. 



" In the spectra of a LjTse and Sirius the second line 

 is absent." 



Prof. Young's observations of the chromospheric lines, 

 to which I shall afterwards refer, give important evidence 

 regarding the presence of calcium in the chromosphere 

 of the sun. He finds that the H and K lines of calcium 

 are strongly reversed in every important spot, and that 

 in solar storms H has been observed injected into the 

 chromosphere seventy-five times, and K fifty times, while 

 the blue line at W. L. 4226-3, the all-important line at_the 

 arc-temperature, was only injected thrice. 



Further, in the eclipse observed in Siam in 1875, the 

 H and K lines left the strongest record in the spectrum of 

 the chromosphere, while the line near G in a photographic 

 region of much greater intensity was not recorded at all. 

 In the American eclipse of the present year the H and K 

 lines of calcium were distinctly visible at the base of the 

 corona, in which for the first time the observers could 

 scarcely trace the existence of any hydrogen. 



To sum up, then, the facts regarding calcium, we 

 have first of all the H-line differentiated from the others 



K H 



BLUE 

 LINE 



RED 

 LINE 



SIRIUS 



SUN 



i 



ARC 



FLAME 



Fig. 4.— The Molecular Groupings of Calcium. 



by its almost solitary existence in Sirius. We have the 

 K-line differentiated from the rest by its birth, so to 

 speak, in a Aquilae, and the thickness of its line in the 

 sun, as compared to that in the arc. We hare 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 differ- 

 entiated from them by its absence in solar storms in 

 which they are almost universally seen, and finally, by its 

 absence during eclipses, while the H and K lines hare 

 been the brightest seen or photographed. Last stage of 

 all, we have calcium, distinguished from its salts by the 

 fact that the blue line is only visible when a high tempe- 

 rature is employed, each salt having a definite spectrum 

 of its own, in which none of the lines to which I have 

 drawn attention appear, so long as the temperature is 

 kept below a certain point. 



