Jan. 9, 1879] 



NATURE 



227 



Iron 



With regard to the iron spectrum I shall limit my 

 remarks to that portion of it visible on my photographic 

 plates between H and G. It may be described as a very 

 complicated spectrum so far as the number of lines is 

 concerned in comparison with such bodies as sodium and 

 potassium, lead, thallium, and the like, but unlike them 

 again it contains no one line which is clearly and unmis- 

 takably reversed on all occasions. Compared, however, 

 with the spectrum of such bodies as cerium and uranium 

 the spectrum is simplicity itself. 



Now among these lines are two triplets, two sets of 

 three lines each, giving us beautiful examples of those 

 repetitions of structure in the spectrum which we meet 

 with in the spectra of almost ail bodies, some of which 

 have already been pointed out by Mascart, Comu, and 

 myself. Now the facts indicate that these two triplets 

 are not due to the vibration of the same molecular 

 grouping which gives rise to most of the other lines. 

 They are as follows. In many photographs in which 

 iron has been compared with other bodies, and in 

 •others again in which iron has been photographed as 

 existing in different degrees of impurity in other bodies, 

 these triplets have been seen almost alone, and the rela- 

 tive intensity of them, as compared with the few remain- 

 ing lines, is greatly changed. In this these photographs 

 resemble one I took three years ago, in which a large 

 coil and jar were employed instead of the arc, which 

 necessitated an exposure of an hour instead of two 

 minutes. In this the triplet near G is very marked, 

 the two adjacent lines more refrangible near it, which 

 are seen nearly as strong as the triplet itself in some of 

 the arc photographs I possess, are only very faintly 

 visible, while dimmer still are seen the lines of the triplet 

 between H and h. 



There is another series of facts in another line 

 of work. In solar storms, as is well known, the iron 

 lines sometimes make their appearance in the chromo- 

 sphere. Now, if we were dealing here with one mole- 

 cular grouping, we should expect the lines to make 

 their appearance in the order of their lengths, and 

 we should expect the shortest lines to occur less fre- 

 quently than the longest ones. Now, precisely the opposite 

 is the fact. One of the most valuable contributions to 

 solar physics that we possess is the memoir in which 

 Prof. C. A. Young records his observation of the chromo- 

 spheric lines, made on behalf of the United States 

 Government, at Sherman, in the Rocky Mountains. The 

 glorious climate and pure air of this region, to which I 

 can personally testify, enabled him to record phenomena 

 which it is hopeless to expect to see under less favourable 

 conditions. Among these were injections of iron vapour 

 into the chromosphere, the record taking the form of 

 the niunber of times any one line was seen during the 

 whole period of observation. 



Now two very faint and short lines close to the triplet 

 near G were observed to be injected thirty times, while 

 one of the lines of the triplet was only injected twice. 



The question next arises, Are the triplets produced by one 

 molecular grouping or by two ? This question I also think 

 the facts help us to answer. I will first state by way of re- 

 minder that in the spark photograph the more refrangible 

 triplet is barely visible, while the one near G is very 

 strong. Now if one molecular grouping alone were in 

 question this relative intensity would always be preserved 

 however much the absolute intensity of the compound 

 system might vzxy, but if it is a question of two molecules 

 we might expect that in some of the regions open to our 

 observ'ation we should get evidence of cases in which the 

 relative intensity is reversed or the two intensities are 

 assimilated. What might happen does happen ; the rela- 

 tive mtensity of the two triplets in the spark photograph 

 IS grandly reversed in the spectrum of the sun. The 

 lines barely visible in the spark photograph are among 



the most prominent in the solar spectrum, while the 

 triplet which is strong in that photograph is represented 

 by Fraunhofer lines not half so thick. Indeed, while the 

 hypothesis that the iron lines in the region I have indicated 

 are produced by the vibration of one molecule does not 

 include all the facts, the hypothesis that the vibrations 

 are produced by at least three distinct molecules includes 

 all the phenomena in a most satisfactory manner. 



Lithium 



Before the maps of the long and short lines of some 

 of the chemical elements compared with the solar spectra, 

 which were published in the Phil. Trans, for 1873, 

 "Plate IX.," were communicated to the Society, I very 

 carefully tested the work of prior observers on the non^ 

 coincidence of the red and orange lines of that metal 

 with the Fraunhofer lines, and found that neither of 

 them were strongly if at all represented in the sun, and 

 this remark also applies to a line in the blue at wave- 

 length 4,603. 



The photographic lithium hne, however, in the violet, 

 has a strong representative among the Fraunhofer lines. 



Applying, therefore, the previous method of stating the 

 facts, the presence of this line in the sun differentiates it 

 from all the others. For the differentiation of the red 

 and yellow lines I need only refer to Bunsen's spectral 

 analytical researches, which were translated in the Phil. 

 Mag., December, 1875. 



In Plate IV. two spectra of the chloride of lithium are 

 given, one of them showing the red line strong and the 

 yellow one feeble, the other showing merely a trace of 

 the red line, while the intensity of the yellow one is much 

 increased, and a line in the blue is indicated. Another 

 notice of the blue line of lithium occurs in a discourse 

 by Prof. Tyndall, reprinted in the Chemical News, and a 

 letter of Dr. Frankland's to Prof. Tyndall, dated Novem- 

 ber 7, 1 86 1. This letter is so important for my argument, 

 that I reprint it entire from the Philosophical Magazine, 

 vol. xxii. p. 472 : — 



" On throwing the spectrum of lithium on the scrftea 

 yesterday, I was surprised to see a magnificent blue 

 band. At first I thought the lithic chloride must be 

 adulterated with strontium, but on testing it with Stein- 

 heil's apparatus it yielded normal results without any 

 trace of a blue band. I am just now reading the report 

 of your discourse in the Chemical News, and I find that 

 you have noticed the same thing. Whence does this bluo 

 line arise ? Does it really belong to the lithium, or are 

 the carbon points or ignited air guilty of its production ? 

 I find there blue bands with common salt, but they have 

 neither the definiteness nor the brilliancy of the lithiimi 

 band. When lithium wire bums in air it emits a some- 

 what crimson light ; plunge it into oxygen, and the light 

 changes to bluish white. This seems to indicate that 

 a high temperature is necessaiy to bring out the blue 

 ray." 



"Postscript, Nov. 22, 1861. — I hare just made some 

 further experiments on the lithium spectrum, and they 

 conclusively prove that the appearance of the blue line 

 depends entirely on the temperature. The spectrum of 

 lithic chloride, ignited in a Bunsen's burner flame, does 

 not disclose the faintest trace of the blue line ; replace 

 the Bunsen's burner by a jet of hydrogen (the tempera- 

 ture of which is higher than that of the Bunsen's burner) 

 and the blue hne appears, faint, it is true, but sharp and 

 quite unmistakable. If oxygen now be slowly turned 

 into the jet, the brilliancy of the blue line increases until 

 the temperature of the flame rises high enough to fuse 

 the platinum, and thus put an end to the experiment." 



These observations of Profs. Tyndall and Frankland 

 differentiate this blue line from those which are observed 

 at low temperatures. The hne in the violet to which I 

 have already referred, is again differentiated from all the 

 rest by the fact that it is the only line in the spectrum of 



