4 REPORT 1875. 



In our own atmosphere the continual disturbances prevent this arrangement from 

 taking place ; but in the sim's enormously extended atmosphere (if, indeed, our lumi- 

 nary be not nearly all gaseous) it appears to hold, inasmuch as the upper portion 

 of this atmosphere, dealing with known elements, apparently consists entirely of 

 hydrogen. 



Various other vapours are, however, as we have seen, injected from below the 

 photosphere into the solar atmosphere on the occasion of great disturbances ; and 

 Mr. Lockyer has asked the question, whether we have not here a true indication of 

 the relative densities of these various vapoiu-s derived from the relative heights to 

 which they are injected on such occasions. 



This question has been asked, but it has not yet received a definite solution ; for 

 chemists tell us that the vapour-densities of some of the gases injected into the sun's 

 atmosphere on the occasion of disturbances are, as far as they know from terrestrial 

 observations, different from those which would be indicated by taking the relative 

 heights attained in the atmosphere of the sun. Mr. Lockyer has attempted to bring 

 the question a step nearer to its solution by showing that the vapours at the tem- 

 perature at which their vapour-densities have been experimentally determined are 

 not of similar molecular constitution ; whereas in the sun we get an indication, from 

 the fact that all the elements give us line spectra, that they are in similai- mole- 

 cular states. 



Without, however, attempting to settle this question, I may remark that we have 

 liere an interesting example of how two branches of science, physics and chemistry, 

 meet together in solar research. 



It had already been observed by Kirchhoff that sometimes one or more of the 

 spectral lines of an elementary vapour appeared to be reversed in the solar spec- 

 trum, while the other lines did not experience reversal. Mr. Lockj'er succeeded in 

 obtaining an explanation of this phenomenon. This explanation was found by means 

 of the method of localization already mentioned. 



Hitherto, when taking the spectrum of the electric spark between the two me- 

 tallic poles of a coil, the aiTangements were such as to give an average spectrum of 

 the metal of these poles ; but it was fomid that when the method of localization 

 was employed, diifereut portions of the spark gave a different number of lines, the 

 regions near the terminals being rich in lines, while the midway regions give com- 

 parativelv few. 



If we imagine that in the midway regions the metallic vapour given off by the 

 spark is in a rarer state than that near the poles, we are thus led to regard the short 

 lines which cling to the poles as those which require a greater density or nearness 

 of the vapour particles before they make their appearance ; while, on the other hand, 

 those which extend all the way between the two poles come to be regarded as those 

 which will continue to make their appearance in vapour of great tenuity. 



Now it was remarked that these long lines were the very lines which were 

 reversed in the atmosphere of the sun. Hence when we observe a single coinci- 

 dence between a dark solar line and the bright line of any metal, we are further 

 led to inquire whether this bright line is one of the long lines which will 

 continue to exist all the way between two terminals of that metal when the 

 spark passes. 



If this be the case, then we may argue with much probability that the metal in 

 question really occurs in the solar atmosphere ; but if, on the other hand, the 

 fiiincidence is merely between a solar dark line and a short bright one, then we 

 are led to imagine that it is not a true coincidence, but something which will 

 probably disappear on further examination. This method has already afforded us 

 a means of determining the relative amount of the various metallic vapours in the 

 sun's atmosphere. Thus, in some instances all lines are reversed, whereas in others 

 the reversal extends only to a few of tlie longer lines. 



Several new metals have thus been added to the list of those previously detected 

 in the solar atmosphere ; and it is now certain tliat the v.ipours of hydrogen, 

 potassium, sodium, rubidium, barium, strontium, calcium, magnesium, aluminium, 

 iron, manganese, chromium, cobalt, nickel, tiUiiiiiuu, lead, copper, cadmium, zinc, 

 uranium, cerium, vanadium, and palladium occur in our lumiuar3% 



I have .spoken hitherto uulj^ of telescopic spectroscopy : but photography has 



