440 Behavwur of H and K Lines of the Spectrum of Calcium. 



may show in our instruments for some distance after the hydrogen 

 and the other light matter associated with it, have become too subtle 

 to furnish a spectrum that we can detect. 



The relative behaviour of the lines of the calcium spectrum as they 

 present themselves in the different orders of stellar spectra when 

 interpreted by the terrestrial experiments described in this paper, 

 will throw much light on many of the important questions which 

 are still pending in celestial physics. In forming conclusions as to 

 the state of the stellar atmospheres from the different densities which 

 may be indicated by the modifications of the calcium spectrum, it 

 must be borne in mind that, as I have said elsewhere : — 



"The conditions of the radiating photosphere and those of the 

 gases above it, on which the character of the spectrum of the star 

 depends, will be determined not alone by temperature, but also by 

 the force of gravity in these regions ; this force will be fixed by the 

 star's mass and its stage of condensation, and will become greater as 

 the star continues to condense."* 



It may be, though on this point we have as yet no sufficient data, 

 that though the stars are built up of matter essentially similar to that 

 of the sun, the proportion of the different elements is not the same in 

 stars which have condensed in parts of the heavens widely distant 

 from each other, or at epochs greatly separated in time. 



It does not seem desirable to discuss any of these questions at the 

 present time, as we hope before long to offer some explanation of 

 the, to some extent analogous, relative behaviour of the lines of 

 some other substances as observed in the sun and. stars. 



[The following letter from our friend Professor Liveing, which he 

 has given us permission to publish, contains an account of early 

 experiments on the spectrum of calcium which not only support, by 

 a different method of working, the conclusions of our paper, but also 

 seem to suggest the possible occurrence of the line H without the line 

 K. In our experiments both lines were always present, the line K 

 being longer and stronger than H ; conditions of the calcium lines 

 which are in agreement with the photographs of the prominences 

 taken by Hale and by Deslandres. 



" I have been looking up some observations of Dewar's and mine 

 en the H and K lines of calcium made in 1879. We found that 

 when we used, for the arc, carbon poles which had been heated for 

 two davs in chlorine to remove metals, the calcium lines were not at 

 first visible in the arc, but after a time H was seen alone and not 

 strong; after a further time, K was seen, and then other calcium 

 lines came out. No doubt the calcium had been pretty well removed 

 from the carbon rods to some depth, but not entirely from the 

 * A idress, ' Brit. Assoc. Beport,' 1891, p. 15- 



