Feb. 25, 1875] 



NATURE 



335 



determine tlie presence of new materials in the solar atmosphere, 

 and it is seen at once that to the last published table of solar 

 elements — that of Thalen — must be added zinc, aluminium, and 

 possibly strontium, as a result of the new method." 



In order to pursue the inquiry under the best conditions; com- 

 plete maps of the long and short lines of all the elements 

 are necessary. It is, however, not absolutely necessary for 

 the purposes of a preliminary inquiry to wait for such a complete 

 .set of maps, for the lists of lines given by the various observers 

 may be made to serve as a means of differentiating between the 

 longest and shortest lines, because I have also shown that the 

 lines given at a low temperature, by a feeble percentage com- 

 position, or by a chemical combination of the vapour to be 

 observed, are jirecisely those lines which appear longest when 

 the complete spectrum of the pure dense vapour is studied. 



Now with regard to the various lists and maps published by 

 various observers, it is known (l) that very different temperatures 

 were employed to produce the spectra, some investigators using 

 the electric arc with great battery power, others the induction 

 s]>ark with and without the jar ; (2) that some observers employed 

 in certain cases the chlorides of the metals the spectra of which 

 tliey were investigating, others used specimens of the metals 

 themselves. 



It is obvious, then, that these differences of method could not 

 fail to produce differences of result; and accordingly, in referring 

 to various maps and tables of spectra, we find that some include 

 large numbers of lines omitted by others. A reference to 

 these tables in connection with the methods employed shows at 

 once that the large lists are those of observers using great battery 

 power or metallic electrodes, the small ones those of observers 

 using small battery power, or the chlorides. If the lists of the 

 latter class of obseners be taken, we shall have only the longest 

 lines, while those omitted by them and given by the former class 

 will be the shortest lines. 



In cases therefore in which I had not mapped the spectrum by 

 the new method of observation referred to in my paper, I have 

 taken the longest lines as thus approximately determined ; for it 

 seemed desirable, in view of the vei^y large number of unnamed 

 lines, to search at once for the longest elemental lines in the solar 

 spectrum without waiting for a complete set of maps. 



A preliminary search having been determined on, I endeavoured 

 to get some guidance by seeing if there was any quality which 

 differentiated the elements already traced in the sun from those 

 not traced ; and to this end I requested my assistant, Mr. R. J. 

 Friswell, to prepare two lists showing broadly the chief chemical 

 characteristics of the elements traced and not traced. This was 

 done by taking a number of the best known compounds of each 

 element (such, for instance, as those formed with o.xygen, sul- 

 phur, chlorine, bromine, or hydrogen), stating after each whether 

 the compounds in question were unstable or stable. Where any 

 compound was known not to exist, that fact was indicated. 



Two tables were thus prepared, one containing the solar, the 

 other the more important non-solar elements (according to our 

 knowledge at the time). 



These tables gave me, as the differentiation sought, the fact that 

 in the main the known solar elements formed stable oxygen-com- 

 ]iounds. 



I have said in the main, because the differentiation was not 

 absolute, but it was sufficiently strong to make me commence 

 operations by searching for the outstanding strong oxide-forming 

 elements in the sun. 



The result up to the present time has been that strontiicin, 

 cadmiutn, lead, co/'per, ctritiiit, and uranium,* in addition to those 

 elements in Thalens' last list, would seem with considerable 

 probability to exist in the solar reversing layer. Should the 

 presence of cerium and uranium be subsequently confirmed, 

 most of the iron group of metals will thus have been found in 

 the sun. 



As another test, certain of those elements which form unstable 

 compounds with oxygen were also sought for, gold, silver, mer- 

 cury being examples. None of these were found. 



The same result occurred when the lines due to the jar-spark 

 taken in chlorine, bromine, iodine, and those of some of the other 

 non-metals were sought, these being distinguishable as a group 

 by formation of compounds with hydrogen. 



Now other researches, not yet completely ready for publica- 

 tion, have led me to the following conclusions ; — 



I, The absorption of some elementai-y and compound gases is 

 limited to the most refrangible part of the spectnim when the 



* Potassium has since been added. 



gases are rare, and creeps gradually into the visible violet part 

 and finally to the. red end of the spectrum, as the pressure is in- 

 creased. 



II. Both the general and selective absorption of the' photo- 

 spheric light are gieater (and therefore the temperature of the 

 photosphere of the sun is higher) than has been supposed. 



III. The lines of compounds of a metal and iodine, bromine 

 &c., are observed generally in the red end of the spectrum, and 

 this holds good for absorption in the case of aqueous vapour. 



Such spectra, like those of the metalloids, are separated spec- 

 troscopically from those of the metallic elements by their columnar 

 or banded structure. 



IV. There are in all probability no compound? ordinarily pre- 

 sent in the sun's reversing layer. 



V. When a metallic compound vapour, such as is referred to 

 in III., is dissociated by the spark, the band spectrum dies out, 

 and the elemental lines come in, according to the degree of tem- 

 perature employed. 



Again, although our knowledge of the spectra of stars is 

 lamentably incomplete, I gather the following facts from the 

 work already accomplished with marvellous skill and industry by 

 Secchi of Rome. 



Vf. The sun, so far as the spectrum goes, may be regarded as 

 a representative of class (0) intermediate between stars (a) 

 with much simpler spectra of the same kind, and stars (7) with 

 much more complex spectra of a different kind. 



VII. Sii-ius, as a type of a, is (i) the brightest (and therefore 

 hottest ?).star in our northern sky ; (2) the blue end of its spectrum 

 is open ; it is only certainly known to contain hydrogen, the 

 other metallic lines being exceedingly thin, thus indicating a 

 small proportion of metallic vajiours ; while (3) the hydrogen 

 lines in this star are enormously distended, showing that the chro- 

 mosphere is largely composed of that element. 



There are other bright stars of this class. 



Vni. As types of 7 the red stars may be quoted, the spectra of 

 which are composed of channelled spaces and bands. Hence 

 the reversing layers of these stars jirobably contain metalloids, or 

 compounds, or both, in great qu?ntity ; and in their spectra not 

 only is hydrogen absent, but the metallic lines are reduced in 

 thickness and intensity, which in the light of V., ante, may indicate 

 that the metallic vapours are being associated. It is' fair to assume 

 that these stars are of a lower temperature than our sun. 



I have asked myself whether all the above facts cannot be 

 grouped together in a working hypothesis which assumes that 

 in the reversing layers of the sun and stars various degrees of 

 "celestial dissociation" are at work, which dissociation prevents 

 the coming together of the atoms which, at the temperature o( 

 the earth and at all artificial temperatures yet attained here, 

 compose the metals, the metalloids, and compounds. 



On this working hypothesis, the so-called elements not present 

 in the reversing layer of a star will be in course of formation in 

 the coronal atmosphere and in course of destruction as their vapour- 

 densities carry them down ; and their absorption will not only be 

 small in consequence of the reduced pressure of that region, 

 but what absorption there is will probably be limited wholly or 

 in great part to the invisible violet end of the spectrum in the 

 case of such bodies as the pure gases and their combinations, 

 and chlorine. (See I. ante.) 



The spectroscopic evidence as to what may bej called 

 the plasticity of the molecules of the metalloids, including of 

 course oxygen and nitrogen, but excluding hydrogen, is so over- 

 whelming, that even the absorption of iodine, although generally 

 it is transparent to violet light, may (as I have found in a repe- 

 tition of Dr. Andrews' experiments en the dichroism of iodine, 

 in which I observed the spectrum) in part be driven into the 

 violet end of the spectrum, for iodine in a solution in water or 

 alcohol at once gives up its ordinary absorption properties, and 

 stops violet light.* 



A preliminary comparison of the ordinary absorption spectrum 

 of a stratum of 6 ft. of chlorine renders it not improbable that 

 chlorine at a low temperature is the cause of some of the 

 Fraunhofer lines in the violet, although, as said before, I have 

 not yet obtained certain evidence as to the reversal of the bright 

 lines of chlorine seen in the jar-spark. 



There is also an apparent coincidence between some of the 

 faint Frauenhofcr lines and some of the lines of the low tempera- 

 ture absorption-spectrum of iodine. 



Should subsequent researches strengthen the probability of this 



■esult by observing the absorption of I 



