1878.] Analysis in connexion with the Spectrum of the Sun. 177 



tlie line wlrich our observation led us to believe was due to calcium in 

 1875, was traced to that element in this year's eclipse. The observa- 

 tions also show the curious connexion that, at the time when the 

 hydrogen lines were most brilliant in the corona, the calcium lines 

 were not detected ; next, when the hydrogen lines, being still brilliant, 

 the h line was not present (a condition of things which, in all proba- 

 bility, indicated a reduction of temperature), calcium began to make 

 itself unmistakably visible ; and finally, when the hydrogen lines are 

 absent, H and K become striking objects in the spectrum of the corona. 



To come back to h, then, I have shown that Dr. Frankland and 

 myself, in 1869, found that it only made its appearance when a high 

 tension was employed. We have seen that it was absent from among 

 the hydrogen lines during the eclipse of 1875. 



I have now to strengthen this evidence by the remark that it is 

 always the shortest line of hydrogen in the chromosphere. 



I now pass to another line of evidence. 



1 submit to the Society a photograph of the spectrum of indium, in 

 which, as already recorded by Thalen, the strongest line is one of the 

 lines of hydrogen (h), the other line of hydrogen (near Gr) being 

 absent. I have observed the C line in the spark produced by the 

 passage of an induced current between indium poles in dry air. 



As I am aware how almost impossible it is to render air perfectly 

 dry, I made the following differential experiment. A glass tube with 

 two platinum poles about half an inch apart was employed. Through 

 this tube a slow current of air was driven after passing through a 

 U -tube one foot high, containing calcic chloride, and then through 

 sulphuric acid in a Wolff's bottle. The spectrum of the spark passing 

 between the platinum electrodes was then observed, a coil with five 

 Grove cells and a medium-sized jar being employed. Careful notes 

 were made of the brilliancy and thickness of the hydrogen lines as 

 compared with those of air. This done, a piece of metallic indium, 

 which was placed loose in the tube, was shaken so that one part of it 

 rested against the base of one of the poles, and one of its ends at a 

 distance of a little less than half an inch from the base of the other 

 pole. The spark was then passed between the indium and the pla- 

 tinum. The red and blue lines of hydrogen were then observed, both 

 by my friend Mr. Gr W. Hemming, Q.C., and myself. Their brilliancy 

 was most markedly increased. This unmistakable indication of the 

 presence of hydrogen, or rather of that form of hydrogen which gives 

 us the h line alone associated into that form which gives us the blue 

 and red lines, showed us that in the photograph we were not dealing 

 with a physical coincidence, but that in the arc this special form of 

 hydrogen had really been present ; that it had come from the indium, 

 and that it had registered itself on the photographic plate, although 

 ordinary hydrogen persistently refuses to do so. Although I was 



