NATURE 



[Nov. 6, 1879 



Simply, it amounted to this. The new work had made 

 us acquainted with the fact that there were coincidences 

 in the lines of metallic spectra of two perfectly distinct 

 kinds. 



The lines of one kind we could explain, on the hypo- 

 thesis that the elements are truly elementary, by suppos- 

 ing that in the case, let us say, of coincident lines in the 

 spectrum of iron and cobalt, the common line was due to 

 an impurity either of iron in the cobalt ,"or of cobalt in 

 the iron. Most spectroscopic workers were of the true 

 faith in this matter ; they accepted the dicta of the che- 

 mist, and not only was the work which had shown how 

 the phenomena observed might be thus explained received 

 with favour, but no one, so far as I know, inquired 

 whether there was any other " might be " in the matter. 

 It is more than probable, however, that the future will 

 have much to say on this very point ; but with this set of 

 coincidences I am not dealing in this paper. 

 So much for the one set of coincidences. 

 The other set was as different as possible. In this 

 category there was, on the impurity hypothesis, no 

 possible explanation forthcoming without changing 

 ground. In fact, the separation of the coincidences into 

 two classes was brought about by this very circumstance, 

 since all the coincidences which, in accordance with a 

 general law established for a constant temperature some 

 years before, could be attributed to impurity had, 

 as a matter of fact, been eliminated from the maps 

 at a prior stage of the investigation. Further, be it 

 noted that all the photographs represented the work of 

 similar temperatures, for they were all taken with electric 

 arcs, for the production of which the same number of 

 Grove's cells was used in all cases. 



Since therefore these lines which were common to 

 two or more spectra, could not be traced to impurities, 

 what was their probable origin ? Their number was so 

 great that to attribute them to physical coincidences, 

 and to rest and be thankful accordingly, would have 

 been to take the very pith and marrow out of the 

 science of spectrum analysis, which we have heard so 

 often is based absolutely upon different substances giving 

 us spectra with special lines for each. The matter then 

 was worthy of serious investigation. 



Using the analogy presented by the spectroscopic 

 behaviour of known compounds when simplified by heat, 

 a simple explanation of these common lines lay on the 

 surface. This explanation is as follows : — 



The temperature of the sun and the electric arc is 

 high enough to dissociate some of the so-called chemical 

 elements, and give us a glimpse of the spectra of their 

 bases, just as in the case of the various salts of calcium 

 there is a temperature which just allows us to get a 

 glimpse of a line indicating the metal calcium common 

 to them all. 



Hence it was allowable to term the coincident lines of 

 the second order " basic lines," since they might point to 

 the existence of a base common to the substances in the 

 spectra of which they appeared. Davy, before he dis- 

 covered potassium, used, as I have since found, the word 

 "basic " to express the same idea. 



I propose in the present paper to refer to some of 

 the facts collected along one line of work to which my 

 subsequent studies of these lines has led me, with a view 



to show that their true basic nature can now no longer be 

 open to doubt. 



Naturally the first thing to do was to see if these 

 basic lines varied in their behaviour from other lines of 

 spectra taken at random. Supposing them to represent 

 mere chance coincidences — "physical coincidences," as 

 they have been called, or again, lines so near together that 

 our means cannot separate them — there is no reason why 

 they should vary together when the temperature is changed ; 

 while, if they be truly basic, they must vary with tem- 

 perature. Further, they must vary in such a way that 

 other conditions being equal, they shall become stronger 

 when the temperature is increased, and become fainter 

 when the temperature is reduced. 



Now what was the best mode of attacking this problem? 

 I was unable to see a more expeditious one than that 

 presented to us by the sun. The following consideration 

 will show how we might hope for help in this quarter. 



We are accustomed to say that the sun is surrounded 

 by an enormous atmosphere, and that this atmosphere has 

 in it the vapours of metals, such as iron, magnesium, &c, 

 with which metals we are familiar on this planet. This 

 statement has been based on the near agreement pre- 

 sented by the places of the lines in the spectrum of the 

 substances as studied in our laboratories and the Fraun- 

 hofer lines themselves. The matching of these spectra 

 is nothing like so perfect, and the conclusion drawn, 

 therefore, is nothing like so firmly based, as is gene- 

 rally imagined; but this point need not occupy our 

 attention at present ; what it is important for us to 

 bear in mind is this : whatever be the chemical 

 nature of this atmosphere, it will certainly be hotter at 

 bottom — that is, nearer the photosphere — than higher up. 

 Hence, if temperature plays any part in moulding the 

 conditions by which changes in the resulting spectrum are 

 brought about, the spectrum of the atmosphere close to 

 the photosphere will be different from that of any higher 

 region, and therefore from the general spectrum of the 

 sun, which practically gives us the summation of all the 

 absorptions of all the regions from the top of the atmo- 

 sphere to the bottom. 



Now as a matter of fact we have the opportunity, when 

 we observe the spectrum of a sun-spot or a prominence, 

 of determining the spectrum of an isolated mass of 

 vapours in the hottest region open to our inquiries, and 

 seeing whether it is like or unlike the general spectrum 

 of the sun. What then are the facts ? 



It is as unlike as possible : the intensities of the lines 

 are inverted to a wonderful extent. More than this there 

 is a constant difference between the spectra of sun-spots 

 and the spectra of metallic prominences, though we see 

 these phenomena generally at about the same niveau in the 

 sun's atmosphere. This may arise from the fact that in 

 the case of the spots we deal generally with a greater 

 thickness of the vapours. 



To get the best idea of this inversion I have prepared 

 maps of the spectra of the chief chemical substances 

 showing the behaviour of the various lines under the 

 various conditions. The result is very striking ; indeed 

 it is striking to quite an unexpected degree. The whole 

 character of the spectrum of iron, for instance, is 

 changed when we pass from the iron lines seen among 

 the Fraunhofer lines to those seen among the spot- 



