3^8 



NATURE 



\_A iigust 4, J \ 



other line — the all-important one at low temperatures — is feeble 

 and unimportant. So that both on the solar evidence aud on tlie 

 evidence of all these spectra, whatever the explanation may be, 

 there is the undoubted fact that fundamental changes of intensity 

 in the lines are produced by some cause or other, and if Kirch- 

 hoff'.s statement about the matching of lines is true for one tem- 

 perature it is faKe for all the others. 



II. In my reference to stellar spectra I mentioned the word 

 "fluted" spectrum. Before Kirchhoff had published his first 

 paper two very eminent Germans — Pliicker and Hittorf — w ere 

 working at spectrum analysis at Bonn, and they found that in 

 the case of a great many simple substances what are called lluted 

 spectra were to be observed as well as line spectra. 



The accompanying diagram (Fig. 29) of the fluted spectrum 

 of iodine will show the difference between these fluted spectra 

 and the line spectra, on which we have been exclusively occupied 

 up to the present. 



We observe that the chief novelty is an absolute rhythm in the 

 spectrum ; instead of lines irregularly distributed over the 

 spectrum, we have groups which are beautifully rhythmic in 

 their structure. The next diagram (Fig. 30) shows us the radia- 

 tion spectrum of .i particular molecular grouping of carbon vapour, 

 that alsi' is beautifully rhythmic ; the rhythm of each of the 

 elementary flutings exactly resembling; that of the iodine. 



These observations were among the first to su^^gest the idea 

 that the same chemical element could have two completely dis- 

 tinct spectra. They were eminently suggestive, for if two, why 

 not many ? 



In my reference to the " long and short " method of observa- 

 tion I stated that it enabled us to note what happens when a 

 known compound bodyi^ decomposed. With ordinary compounds, 

 such as chloride of calcium and sd on, one can watch the preci e 

 moment at which the compound is broken up — when the calcium 

 begins to come out ; and we can then determine the relative 

 amount of dissociation by tlie number and thickness of the lines 

 of calcium which are produced. Similarly with regard to these 

 flutings we can take i ]dine vapour, which gives us thi^ fluted 

 spectrum, and we can then increase the temperature suddenly, so 

 that we no longer get the fiuled spectrum at all, or we may increae 

 it so gently that the lines of iodine come out one by one in exactly 

 the same way that the lines of calcium came out from the chloriile 

 of calcium. We end by destroying the compound of calcium 

 in the one case, and by destroying the fluted spectrum in the 

 other, leaving, as the result in both cases, the bright lines of the 

 constituents — in the one case calcium and chl jrine; in the other 

 case iodine itself. I have by no means exhausted the list of 

 difficulties which were gradually presented to us when we consi- 

 dered that both in the sun and in our laboratories spectrum 

 analysis brought befjre us the results of unique, absolutely 

 similar "chemicil atoms." Not only were there differences, 

 but the differences worked iu different ways, whether we passed 

 from low to high temperatures in laboratory work, or from the 

 general spectrum or the flame spectrum in the sun. 



But I have said enough for my present purpose ; details on 

 the points I have referred to and on others must be gone into 

 afterwards. 



How then was one to attempt to grapple with these difficul- 

 ties ? Was it the time to found new theories ? or to rest and be 

 thankful ? Was it not better to appeal to what was known — to 

 proceed in accordance with Newton's laws of philosophising, and 

 start no new principle unless one were absolutely bound to do 

 so : to appeal in fact to the law of continuity, and to suppose 

 that the explanation of a very large p:irt at all events, of this new 

 matter, lay in the fact that, all unconsciously, spectroscopists had 

 been working under more transcendental conditions as regards 

 temperature than had ever been employed before, and that the 

 natural result was tliat this higher temperature had done for the 

 matter on which they had experimented exactly what all lower tem- 

 peratures had been found to do. That is to say, that they had been 

 broken up. ■ In other «ords, it lent great probability to the view 

 that when we subjected, say iron— because it is a good thing to 

 keep to one specific substance — to one of these transcendental 

 temperatures, we were no longer dealing with the spectrum of iron, 

 but with the spectrum of the constituents of iron revealed to us 

 by a temperatture at which no experiments had been made 

 before. 



And one was the more struck by the probability of this being 

 at all events an approximation to the truth by those stellar 

 spectra to which I have referred, and by the knowledge v\e 

 possessed, that in the case of a star of the simplest spectrum we 



were dealing with the highest possible temperature. So the 

 idea was thrown out that th:se stars were really simpler in their 

 structure ; that their immense temperature had not allowed a 

 complex evolution of higher complex forms of chemical matter 

 to take place ; and that we had there the primordial germs of 

 matter, so to speak, or at all events something nearer to the 

 beginning of things thai; anything that we had in this cool 

 planet of ours, or anything that we were likely to find easily 

 here, in consequence of the various difficulties which harass every 

 kind of experimentation. It was imagined that we might 

 picture to ourselves a sort of celestial dissociation in the 

 heavenly bodies which would place those stars, the spectra of 

 which have been seen, in a different order ; that the first star 

 with lines should be a star of the simplest spectrum, the next 

 star with lines should be that which mostly resembled our sun, 

 and that the last in order should be that one in which the lined 

 spectrum had utterly disappeared in favour of the fluted spec- 

 trum. If this were granted for the stars, why not attach all 

 this to the sun ? Because, as has already been mentioned, all these 

 lines which were seen in the spectra of the hottest stars were 

 precisely those Hues which were seen most intense in the hottest 

 parts of the sun ; and it did really seem as if in that way we 

 could eventually sooner or later — most likely laler, for Art is 

 very long— get some light on the subject. 



I at once say that this idea which was thrown out in the year 

 1S73 °" spectroscopic evidence had been anticipated by the 

 foremost philosopher amongst English chemists of his time ; I 

 mean the late Sir Benjamin Brodie. ' From, considerations of 

 a perfectly different kind he had come to the conclusion that 

 our chemical philosophy was not anything like so firmly based 

 as was generally imagined, and that, given a higher temperature, 

 the elementary bodies would cease to be elementary — that the 

 adjective "elementary" applied to them was merely the measure 

 of our inability to dissociate them ; and to watch the progress of 

 dissociation when we got them at a temperature at our com- 

 mand. By a stroke of genius he, before anything was known 

 about the chemistry of the sun, w ent to the sun for that trans- 

 cendental temperature he was in search of ; thus showing that 

 he had an absolutely pure and accurate conception of the whole 

 thing as I believe it to be — but that is anticipating matters. He 

 suggested that the constituents of our elementary bodies might be 

 found in the hottest parts of the solar atmosphere existing as 

 independent forms. The whole merit of that conception there- 

 fore is due to Sir Benjamin .Brodie, and d.itts from the year 

 1S67. 



Now we can easily understand, seeing that much of the 

 spectroscopic work which had been done up to 1874 had had 

 for its object the connecting — intermingling, so to speak — of solar, 

 stellar, and terre-trial chemistry, that it was nst a pleasant thing 

 to find that the path seemed about to be such a very rugged one — 

 that we seemed after all not to be in the light, but in the dark, 

 and the very practical question was, w hat was to be done ? 

 Would it have been wise to have considered, tlicn, the whole 

 question of the dissociation of elementary bodies ? I think it 

 would not have been wise ; the data were insufficient. The 

 true thing to be done was, I think, to endeavour to accumulate 

 a vast number of new facts and then to see what would happen 

 w'hen a sufficiently long base of facts had been obtained. 

 What did we want? We chiefly wanted to settle those questions 

 of the variations of spectra seen in our l.iboratories, and the 

 variations observed v\ hen we passed from the spectrum, say of 

 iron on the earth, to the spectrum of iron in solar spots and 

 storms. The coincidence of lines of different bodies which had 

 been referred to by Angstrom and Kirchhoff also required inves- 

 tigation. What more ready means of doing that— what more 

 perfect means were there than those placed at our disposal 

 by photography ? Photography has no personal equation, it 

 has no inducement to cook a remit either iu one direction 

 or the other, and it moreover has this excellent thing about it, 

 that the results can be multiplied a thousandfold and can be 

 recorded in an absolutely easy and safe manner. There were 

 other reasons why photography should be introduced. We see 

 at once that it it was quite easy to introduce the process of puri- 

 fication of the spectra to which I have already drawn attention, 

 by merely comparing a series of photographs ; the A, B, C of 

 my diagram (Fig. 26) being represented, say, by iron, cobalt, and 

 nickel, or any other substances. Again, it was quite pos;ible 

 by the use of the electric lamp to very considerably increase the 



■ " Ideal Chemistry. " Lecture delivered to the Chemical Society in 1867, 

 repuhlished 1S80. (Macmillan). 



