of the H and K Lines of the Spectrum of Calcium. 437 



B. Spark taken with one electrode only of metallic calcium, the 

 other electrode being of platinum. In this case the effect of a 

 smaller density of the calcium vapour is clearly shown in the greatly 

 reduced wingedness of the lines. It will be remarked that the 

 diminished density has had the greatest influence on the pair at 3737 

 and 3706; these lines are now much less strong than the blue line, 

 which still holds its own, and remains about as strong as H and K. 

 The lines of the more refrangible pair are no longer diffuse at the 

 edges. 



C. Spark taken between platinum electrodes moistened with a 

 strong solution of calcium chloride. Here the effect of a smaller 

 quantity of vapour begins to tell strongly upon the intensity of the 

 blue line relatively to H and K. It may now be estimated at less 

 than one-fourth of the intensity of H. At the same time, H and K 

 have almost completely lost their diffuse character, and have become 

 thinner and more defined. 



D. The electrodes as left in the former experiment were slightly 

 washed Avith pure water, leaving a trace only of calcium chloride. 

 There is, as might- be expected, an advance in the enfeeblement of 

 the blue line and of the more refrangible pair, relatively to H and K. 



E. The electrodes were again slightly washed with pure water, so 

 that a still smaller trace of calcium chloride must have remained 

 upon them. The enfeeblemeut of the blue line and of the pair has 

 now become very great, while H and K, though thinner, remain 

 strong. 



F. The electrodes were once more washed with pure water, 

 reducing still further the trace of calcium chloride which remained 

 upon the platinum wires. The blue line has now practically dis- 

 appeared, and the refrangible pair become very thin. The H and 

 K lines have become thin and defined, as they usually present them- 

 selves in the prominences. 



Gr. The electrodes remaining as they were left after the last experi- 

 ment (F),the spark was taken upon a background consisting of a 

 faint solar spectrum. The blue line has now completely disappeared, 

 leaving H and K strong. 



H. Once more the electrodes were washed, with the expectation of 

 having removed completely the last remaining trace of calcium. To 

 our surprise, when the photograph was developed, the lines H and 

 K came out alone. The more refrangible pair had now faded oat as 

 well as the blue line. H and K were now thin, and extended but 

 a short distance in the spectrum. 



It must be remembered that the only condition which was varied 

 during this set of experiments was the amount or density of the 

 calcium vapour. The changes of relative intensity, and the modifi- 

 cations of the calcium, spectrum produced thereby as shown in the 



