of the H and K Lines of the Spectrum of Calcium. 43$ 



characters. Such dissociations are well known, and are not of 

 infrequent occurrence, and may, indeed, take place in connection 

 with some of the spectral changes of a substance observed under 

 different conditions. On the other hand, a true decomposition of a 

 chemical element, that is, a breaking up of the molecule into simpler 

 and quite other kinds of matter, though a notion familiar to chemists 

 since Prout's time, and regarded as theoretically possible, is, as yet, 

 unknown as a matter of fact. 



Conclusions. 



These experiments seem to us to furnish an adequate and consistent 

 explanation of the behaviour of the calcium lines at and near the 

 sun's limb. Near the photosphere where the absorption mainly 

 takes place, by which the dark lines of the solar spectrum are 

 formed, there would be, we should expect, a much greater density of 

 calcium vapour than at a higher level, and we find the Fraunhofer 

 line at 4226*9 strong but much less broad than H and K. The 

 recent photograph .of the reversing layer shows that the broad 

 shading of H and K is not produced there, but probably, as Prof. 

 Jewell concludes from his measures, lower down where the gas is 

 still denser, which is in agreement with photograph A on the plate. 



Higher up in the chromosphere, in the prominences, and possibly 

 in the lower coronal regions, the decrease of the density of the gases 

 composing them must be rapid, and the temperature gradient as 

 determined by expansion must be also rapid. We have clearly to do, 

 in these regions, with calcium vapour in a rarer state, and except so 

 far as the molecules may have carried up within themselves to some 

 extent the higher heat of a lower level, or through imperfect trans- 

 parency, the gases may have received heat from the sun's radiation, 

 it must be at a much lower temperature than near the photosphere. 

 Now, the changes of the calcium spectrum which take place in these 

 regions, are those which correspond in our experiments to a very 

 small amount of calcium vapour, and a spark of small intensity. 



On account of the violent commotion which must exist through 

 the strong convection currents at the sun's limb, we should not be 

 surprised to find some calcium vapour, notwithstanding its greater 

 density, carried high up together with the lighter substances such as 

 hydrogen and helium. Our experiments show how strongly the H 

 and K lines may come out when a trace only of calcium vapour is 

 present, and so, it seems to us, offer a possible explanation of the great 

 height at which these lines may be sometimes recognised. At no 

 very great distance from the surface of the sun the gases must 

 become too tenuous to give a visible spectrum ; but it may well be that 

 the brilliant radiations of even very rare calcium gas at H and K 



VOL. lxi. 2 I 



