4i6 



NATURE 



[March 5, 1896 



while the other line (the larger one corresponding to one 

 of Lockyer's ' long lines') is unaffected. But this does not 

 prove that iron vapour is dissociated in the sun. It 

 merely shows that the apparently similar portions of the 

 two lines m the solar spectrum are produced at diiTerent 

 elevations in the solar atmosphere. The stronger iron 

 line will be aft'ected in a sun-spot as much as the other 

 one, but it is the portion of the line produced at the 

 same level as the other line, and may be masked com- 

 pletely, or very largely, by the emission line produced at 

 a higher level, while the second absorption line in the 

 solar spectrum may be entirely unaffected, being 

 produced at a still higher altitude." 



" This also explains why some of the lines (the short 

 lines generally) of an element may be most prominent in 

 sun-spot spectra, while others (generally the long lines) 

 are those most frequently seen in prominences or in the 

 chromosphere." 



My thirty years' work at solar physics leaves me with 

 such an oppressive feeling of ignorance that I willingly 

 concede to Mr. Jewell a knowledge so much greater than 

 my own as to give him a perfect right to dismiss all my 

 work in two lines ; but I am compelled to point out that 

 he has not carefully read what I have published. 



A comparison of the facts brought together in Figs. 112 

 and 1 14 of my " Chemistry of the Sun," for instance, 

 drives his last paragraph into thin air : it is distinctly 

 shown that we have to do with the short lines in the 

 chromosphere and with the long lines in spots, the exact 

 opposite of his statement. Mr. Jewell is not running 

 counter to my views in supposing that different pheno- 

 mena are produced at different elevations. I thought I 

 had abundantly proved in my eclipse observation of 1882 

 ("Chemistry of the Sun," p. 363), that the iron lines, to 

 take a concrete instance, are produced at different heights 

 in the solar atmosphere ; and that was one among many 

 reasons which compelled me to abandon the thin revers- 

 ing layer suggested by Dr. Frankland and myself in 1869 

 in opposition to Kirchhoffs view ; but surely the moi'e we 

 consider the solar atmosphere as let out in flats, with cer- 

 tain families of iron lines free to dwell in each and to flit 

 a discretion, the more a dissociation hypothesis is wanted. 

 And beyond all this, we have to take into account that at 

 the sun-spot maximum no iron lines at all are seen 

 amongst the most widened lines, while at the minimum 

 we have little else. 



Another very interesting part of Mr. Jewell's paper 

 refers to the phenomena of absorption. There is room 

 for plenty of work here. As I pointed out in 1879, we 

 get unequal widenings, " trumpetings," and a whole host 

 of unexplained phenomena.^ It is clear that the enormous 

 -dispersion at Mr. Jewell's command will largely help 

 matters. 



I now pass to Messrs. Humphreys and Mohler's paper. 



Messrs. Humphreys and Mohler have used an electric 

 arc enclosed in a cast-iron cylindrical vessel, which enabled 

 them to vary the pressure up to fourteen atmospheres. 

 One hundred photographs have been taken, and the 

 shifts of some lines of twenty-three elements have been 

 measured. The accompanying rough diagram, bringing 

 together specimens of their observations, will indicate 

 the kind of result they have obtained. 



The pressures in atmospheres are shown to the left. 

 The shift towards the red in thousandths of an Angstrom 

 unit are shown below. The shifts have been reduced 

 to what they would be at A 4000, in the neighbourhood of 

 which most of the work was done. I must refer to the 

 paper itself for the method of measurement adopted. 



The displacement or shift varied greatly for different 

 ■elements. It was always towards the red, and directly 

 proportional to the wave-length and the excess of pressure 

 •over one atmosphere. 



Only one exception to this general statement is given ; 



1 " Chemistry of the Sun," pp. 380-387. 



NO. 1375, VOL. 53] 



it refers to calcium. " The lines H and K, among others, 

 shift only about half as much as g (the blue line at 4226), 

 and the group at wave-length 5600. That g should dififer 

 in this respect from H and K is not very surprising, since 

 it is known to differ greatly from them in many other 

 respects." 



On this exceptional behaviour of these lines of calcium, 

 I quote the following, from a note by Prof. Hale, which 

 appears in the same number of the journal. 



" The difference in behaviour of H and K and the blue 

 line of calcium discovered by Messrs. Jewell, Humphreys, 

 and Mohler, seems to support Lockyer's views as to the 

 dissociation of calcium in the arc and sun. The remark- 

 able variations of the calcium spectrum with temperature 

 have long been known principally through the investi- 

 gations of Lockyer. The writer has shown that the H 

 and K lines are produced at the temperature of burning 

 magnesium and in the oxy-coal-gas flame. They could 

 not be photographed in the spectrum of the Bunsen 



/f^ JO 



burner, though an exposure of sixty-four hours was given. 

 Since these experiments were made, I have been informed 

 by Prof. Eder that his own efforts to photograph the 

 lines in the Bunsen burner were no more successful, 

 though an optical train of quartz and fluor-spar was em- 

 ployed. It would thus appear that the temperature of 

 the dissociation of calcium is between that of tlie Bunsen 

 burner and that of the oxy-coal-gas flame. The high 

 molecular weight of calcium has hitherto conflicted with 

 our belief in the presence of this metal in prominences. 

 If, however, it be granted that dissociation can be brought 

 about by temperatures even lower than that of the arc, 

 the difficulty is very greatly lessened." 



I may add that it will be very interesting to see if the 

 strontium line at 4607 behaves like the calcium g in 

 relation to the lines at 4077 and 4215 repiesenting H 

 and K. 



I have said enough in the present paper to show the 

 extreme importance of these new results. So much care 

 has been taken, that there is little doubt that subsequent 



