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Mr. J. Norman Lockyer. 



chemical simplicity, the lines of these substances will become more 

 important. But this argument is not philosophical, because we have 

 no right to assume such a change. These gases already exist in tLe 

 order named in the sun at the temperature of the electric spark, and, 

 in fact, in the sun hydrogen and the cleveite gases give us no traces 

 of their existence at any great height above the chromosphere ; the 

 gas that does exist in these elevated regions is one about which we 

 know nothing, so far, terrestrially, and of which no trace has yet been 

 found in the spectrum of the hottest stars. 



The only change which we can imagine on the usual hypothesis, as 

 resulting from the increase of temperature, is that with the increase 

 in volume there will be a reduction in density, and all the lines will 

 be equally enfeebled. But this is exactly what does not happen. 



If now we turn to the other hypothesis, that, namely, of dissocia- 

 tion, we see at once, in the light of laboratory experiments, that with 

 every considerable increase of temperature in all such masses of 

 vapour arid gas as that which now constitutes the solar chromosphere, 

 a fundamental change in the appearance of the spectrnm must be 

 brought about, complex molecules would be broken up into simpler 

 ones, and the result of this action would bring new lines into the 

 spectrum, indicating the vibration of the molecules produced. Xow 

 let us come to facts. Were the temperature of the chromosphere to be 

 increased, if dissociation takes place at this temperature, the dissocia- 

 tion products must become visible, and we must look for them aniong* 

 those lines which expand at the expense of those which contract and 

 disappear. Is any such experiment as this going on even at this 

 moment ? The answer is beyond question. The evidence is 

 complete that the temperature in the reversing layers of a Cygni is 

 higher than that of the reversing layers of the sun. What do we 

 find ? Of lines disappearing we have arc lines of iron, some 

 thousands in number, calcium, magnesium, strontium, and so on. 

 Of lines increasing in importance we have the small number repre- 

 senting the enhanced lines of iron, the lines of hydrogen, and some 

 others which we cannot at present associate with the name of any 

 known substance. Here, then, we get a series of phenomena which, 

 on the hypothesis we are discussing, is simply and sufficiently 

 explained by the statement that on passing from the temperature of 

 the sun to a Cygni, among other changes brought about, the 

 complicated line spectrum of iron is giving way to a more simple 

 one consisting of the enhanced lines. Further enquiries show that 

 the other metallic spectra are behaving in the same way. Looking 

 for the lines which increase in importance, while the others are 

 reduced, we find the lines of hydrogen increasing in importance. 



