August 4, 1881] 



NATURE 



521 



scope was no bad guide in that matter, and, thanks to his valu- 

 able researches, we are now able to photograph as well, if not 

 better, at the extreme red end of the spectram than we did at 

 that timj — years a^o now — in the blue. 



Well, then, four years were consumed in the accumulation 

 of these facts. I do not now intend to call attention to the 

 whole of them, but I will take some instances, directing 

 s.^ecial attention to what happened with regard to the spec- 

 trum of iron. This ^ is the final map produced up to a 

 certain point. We have first the solar spectrum ; below this 

 are mapped all the lines of iron observed on one of the photo- 

 graphs which we obtained, including of course all impurities ; 

 and then follow the spectra of manganese, cobalt, nickel, 

 chromium, uranium, cerium, and so on through the whole story. 

 When that work had been completed in that manner we had to 

 get rid of the impurities by the process which I have already 

 explained, and at last we got what is called a purified spectrum, 

 in which, along the horizon labelled iron we had only those 

 lines left which we could not by any application of the principle 

 which has been explained be shown to be due to the admixture of 

 any other substance whatever. What then was the total result ? 

 The accompanying table (p. 320) will show the sort of corner in 

 which we found ourselves after all this work had been accom- 

 plished. It gives the list of the iron lines which, after making 

 every allowance for the existence of impurities, were found to 

 coincide with lines in other substances. 



It will be seen, for instance, that the two short lines 390600 

 and 395423 coincided, the first with short lines in uranium, 

 zirconium, and yttrium, the second with short lines in uranium, 

 molybdenum, and tungsten. Similarly there are two short- 

 line coincidences with zirconium, and no less than six with 

 vanadium, and so on. The total gives the coincidence of 

 the lines of all the elements under the conditions that I have 

 draw n attention to. So that the sum total of lliis really very 

 laborious inquiry with regard to iron was that in the region between 

 39 and 40, the region including H and K on that map, where, 

 before the introduction of photography, scarcely any iron lines had 

 been seen, and where only five solar lines I think had been 

 mapped, photography gave us a total of nearly 300 lines in the 

 solar spectrum, and it gave us sixty-two lines of iron. 



Of those sixty-two lines of iron only eighteen went straight ; 

 by which I mean that the remainder had short-line coincidences 

 with the lines of other subs-tances. So that the idea first thrown 

 out by Kirchhoff, Angstrom, and Thalen of the possibility of the 

 coincidence of lines among the metallic elements was enormously 

 intensified. It will be .'-een that the thing is absolutely reversed 

 in the case of iron, and it might be the case als:> in ether sub- 

 stances. The fact of a line not being coincident witli a line in 

 another substance was the exception, and not the rule. The 

 ratio in the case of iron being as 44 to 18. 



It is amusing in the light of recent criticisms to go back to the 

 old observations and to see with what pertinacity for the first 

 two years we stuck to the possibility that the solar line or the 

 iron line we were dealing with was a double line, and then, 

 after we had to give that idea up, as the coincidences became 

 of three, four, five, and sixfold complexity, we came to the 

 conclusion that we were dealing with a common impurity. 

 That of course was a point we could not settle until we had 

 gone through all the chemical elements which were known to 

 us, and it was going through so many substances which took up 

 so much time. 



But there was another question which became striking, in this 

 excessively minute anatomy of even a very small portion of the 

 solar spectrum, for I should say that the small range of the spec- 

 trum represented here forms a portion of a map h hich, when com- 

 pleted, will be ihe sixteenth of a mile long, so that after all 

 we were dealing with an excessively small portion of the total 

 work which had to be done. Having there mapped that small 

 region, where without photography it would have been difficult 

 to see any lines at all, we got in almo>t twenty cases from one 

 end to the other, instances in which there was absolutely no 

 relationship at all between the brightness of the iron line on our 

 photographs and the darkness of the corresponding solar line. 



These were carefully noted as "anomalous reversals," a term 

 we coined in the laboratory at the time, and which we still use, 

 although the word anomalous always suggests a very large 

 amount of ignorance. 



In more ways than one, then, this work landed us in rather 

 worse confusion than w e were in before. What we had to face was 



I This map is too large and detailed to reproduce here. 



(i) the variation in intensity as we passed from earth to sun, a 

 variation so great that in some cases terrestrial lines w ere missing in 

 the sun, and in others feeble terrestrial lines were greatly infer- 

 rupted ; and (2) the coincidence of lines in several spectra. That 

 is, here and there along the spectrum we found the lines massed as 

 it were even if the coincidence was but apparent, and It really did 



Fig. 32.' 



seem time to consider what the effect would be, supposing that a 

 dissociation was really going on under our eyes without our 

 knowing or imagining anything about it. Why, it may be said, 

 did you pitch on dissociation ? For the reason that the startling 

 results really after all contained nothing that was new — nothing 

 that was novel about them the least in the world, if we regarded 

 them with an absolutely unbiased and receptive mind. Dissocia- 

 tion would undoubtedly account for all the variations of intensity 

 observed on passing from one temperature to another, as already 

 exemplified in the case of Ihe calcium lines, and moreover 

 the short common lines, should they turn out to be truly 

 common, which we were getting in the case of all sub- 

 stances, might be simply the equivalents of those short common 

 lines of calcium which for years past we had watched 

 coming out of the salts of calcium when decomposition 

 was taking place. No new theory was necessary. The appeal 

 to the law of continuity, as I said before, was really open to us, 

 and it seemed to be our duty to appeal to it, and it was also easy 

 to see, before really one has inquired into the matter, that 

 if nature had built up the inorganic world in the way we 

 now know she has built up the organic world, that precisely 

 these facts and none other would be those she would pre.ent 

 to us. 



" Let us assume a series of furnaces A-D, of which A is the 

 hottest (Fig. 32). 



"Let us further assume that in A there exists a substance a, by 

 itself competent to form a compound body /3 by union with itself, 

 or with something else when the temperature is lowered. 



" Then we may imagine a furnace B in which this compound 

 body exists alone. The spectrum of the compound 5 woiild be 



the only one visible in B, as the spectrum of tlie assumed ele- 

 mentary body a would be the only one visible in A. 



"A lower temperatiu-e furnace C will provide us %vith a more 

 compound substance 7, and the same considerations will hold 

 good. 



^ The figures between the hypothetical spectra point to the gradual change 

 in the interstices of the lines as the spectrum is observed near the tempeiature 

 of each of the furnaces. 



