438 Dr. W. Huggins and Mrs. Huggins. Relative Behaviour 



succession of photographs on the plate, correspond closely to the 

 behaviour of calcium at different levels near the sun's limb, and in 

 the atmospheres of stars of different orders. There can remain little 

 doubt that the true interpretation of the changes in appearance of 

 the calcium lines in the celestial bodies is to be found in the different 

 states of density of the celestial gases from which the lines are 

 emitted or by which they are absorbed. 



A similar set of experiments was made with iron electrodes. 

 Precisely similar results as to the relative enfeeblement of the lines, 

 as with calcium chloride on platinum electrodes, were obtained. Of 

 course the iron lines were also present. As might be anticipated, in 

 consequence of the simultaneous presence of the iron vapour, the 

 lines of calcium were thinner than when platinum was used. 



Outside the range of wave-lengths which could be conveniently 

 given on the plate, far on in the ultra-violet, there is a pair of strong 

 lines which behave very much as H and K. It remains visible in 

 photograph H, when the pair at 3737 and 3706 have disappeared. 

 This pair is situated at 3158'98 and 3179*45. 



It is desirable to point out again that all the photographs on the 

 plate and the far ultra-violet lines, were obtained with a spark of 

 quite unusually small intensity, which was purposely made as little 

 hot as possible, in order to emphasize the important fact that the deter- 

 mining condition of the spectral changes uuder discussion is not one 

 of increase of temperature. 



In the modifications of the calcium spectrum arising from varia- 

 tions in the relative intensities of the lines which have been discussed 

 in this paper, and which correspond to those observed in the celestial 

 bodies, there does not appear to us any reason for assuming, much 

 less any direct evidence in favour of, a true dissociation of calcium, 

 that is, of its resolution into chemically different kinds of matter. 



It would be remarkable if, by decomposition through increase of 

 temperature, a large number of lines of a spectrum should become 

 relatively enfeebled, and that as the result of decomposition a 

 spectrum should become simpler, and not as analogy would suggest, 

 more complex. 



It is of importance to keep in mind that the recent chemical use 

 of the word dissociation is not equivalent to true decomposition, i.e., 

 to a resolution of the original substance into two or more chemically 

 different kinds of matter. It may, and does often mean not more 

 than a different arrangement of the parts of the molecule, while 

 those parts are all chemically matter of the same kind as the 

 original molecule. As in the case of the resolution of a com- 

 pound molecule of peroxide of nitrogen into two identical half 

 molecules ; or, in the separation of a molecule of elementary 

 iodine into two hilf molecules or atoms of identical chemical 



