540 



NATURE 



[April 2>, 1886 



THE SUN AND STARS'" 

 V. 

 Mclaliic Promiiitnces 

 X/ERV impressive indeed are the phenomena when we pass to 

 that other class representing prominences no longer of the 

 quiet sort. These are at times observed shooting up almost in- 

 stantaneously — the exact rate of motion I will state by and by — 

 to enormous heights ; and not only are they seen to shoot up 

 into the atmosphere with very great velocity and with every 

 indication of the most violent disturbance, but the alteration in 

 the lines of hydrogen in the spectrum indicates most violent 

 lateral motions. These phenomena unfortunately have been 

 called eruptions, and, as it very often happens, when we get a 

 word like that coined it means more than it is intended tb mean 

 by the author of it ; and more or less on the strength of this 

 word "eruption," we have theories trying to explain these 

 prominences on the idea that they are ejected, possibly from a 

 volcano — a real solar volcano — at some distance below the photo- 

 sphere. I think we have no right to call them eruptions at all. 

 In the first place they are not like any volcanic eruption that 

 man has ever seen. 



When^we get the chromosphere agitated preparatorily to one 

 of these tremendous outbursts — one of these metallic prominences, 

 at they are called — the lines which we see are ditferent from those 

 in the table which I have given. The Italian observers, to 

 whom we are indebted chiefiy for our knowledge on this part of 

 the subject, have recorded three lines, which they call tlu' 

 " elementary'metallic prominence spectrum." These are — 



4943 No Fraunhofer lines corresponding. 



5031 



5315'9 ■= 1474 

 Althouijh these energetic prominences are eventually very often 

 fuUijof lines of various vapours these three lines always precede 

 them when the action commences. There is one point about 

 this matter to which I must call attention, and that is that of 

 these three lines one — the 1474 line — is not the line with the 

 same namej to which I have already drawn your attentiuii, 

 and about which we know absolutely nothing, but it is a line 

 of iron almost c lincident with it, which the temperature of tlie 



iS.-iMe 



: prominence, Vuung. Ka 



spark brings out, though it is invisible in the arc. The 

 other two are lines which do not even appear amongst 

 the Fraunhofer lines at all, and about which, therefore, 

 we know nothing. We have me.ans, both by actual obser- 

 vation in the case of the uprise of the prominences into the 

 solar air and in the change of the wave-lengths of the lines in 

 ' A Course of Lectures to Working Men delivered by J. Norman Lockyer, 

 F.R.S., at the Museum of Practical Geology. Revised from shorthand 

 notes. Continued from p. 502. 



the case of any lateral motion, of determining how fast these 

 violent prominences vise and are driven by solar winds. Well, 

 these metallic prominences have been seen to mount upwards at 

 the rate of 250 miles a second, that is very nearly 1,000,000 

 miles an hour ; so that, if these gases continued their flight they 

 would reach the top of the solar atmosphere, if the solar atmo- 

 sphere were 1,000,000 miles deep from the top down to the 

 photosphere, in about an hour's time. There are indica- 

 tions that these prominences, instead of rising vertically, as 

 we may imagine them to do, are at times shot out sideways — 

 almost tangentially. In that case, of course, the spectroscope 

 enables us to determine the velocity. 100 miles a second, 

 either towards or from the eye, is by no means an uncommon 

 velocity, and there are also indications that, in the neighbour- 

 hood of the photosphere where these enormous prominences 

 take their rise, vividly incandescent hydrogen at a considerable 

 pressure is rushing up from the interior part of the sun. 



In the case of some of these violent prominences the spectrum 

 at the base appears to be full of lines, but we know enough 

 about the subject now to know that many, if indeed not most, 

 of those lines are not Fraunhofer lines at all, not lines with 

 which we are familiar, but new ones. In fact, the same thing 

 happens in the prominences as happens in the spots. To show 

 that this is so I again refer to some very important work 

 done by Prof. Voung in the United States sume years ago. He 

 went to a station in the Rocky Mountains, at a height of Sooo 

 feet, to observe these prominences. Of course the higher we 

 go the purer the air, and the better we can see. As the result 

 of one month's work he brought back a very valuable catalogue 

 of lines which he had seen in such prominences as I have 

 attempted to describe. 



Let us consider one particular substance. It is always 

 well in these matters to be as definite as possible, and if the 

 l>romiuences contained that particular substance, say, for in- 

 stance, barium, in the same conditions in which we find it on this 

 earth, we should imagine that the spectrum of barium in the 

 prominence would be very much like the spectrum of barium in 

 the electric spark. To see whether that was so or not, what my 

 .assistants and myself did was this. We prepared a map showing 

 the lines of barium over a long reach of the spectrum, and we 

 drew the lines so that the longest represented the strongest 

 according to our highest authority in these matters. Prof. Thalen. 

 Alongside of these we m.ade another map showing the particular 

 lines which had been seen by Prof. Young, and we assumed that 

 the line which was strongest at the sun would be the line which 

 he would most probably see most frequently, and therefore we 

 made the line which he s.aw the greatest number of times the 

 longest line. That being premised, you will see there is no rela- 

 tion whatever between these two spectra. In the first place a 

 great number of the lines of barium seen in the laboratory 

 are left out of account altogether in the prominence spectrum, 

 and when the other lines are considered we find that the 

 intensities in the sun are quite different ; and that, I may say, is a 

 very fair indication of what as a matter of fact we have observed 

 with regard to a number of these substances. Calcium, iron, 

 nickel, cobalt, and several other metals which we have tested in 

 the same way, give us exactly the same result. 



But there is more important work to do than that. Since 

 Prof. Voung made those admirable observations in America, the 

 Italian observers, Profs. T.acchini and Ricco, have been observing 

 metallic prominences every day. It has been our duty at Ken- 

 sington to map every line which these industrious Italian gentle- 

 men h.ave observed ever since 1871, and in these maps, as in 

 those of spot-spectra, we have the lines of the various elements 

 seen in the sun, in the arc, and in the spark. Now, of all those 

 lines, we only get a very small number in the prominences. In the 

 case of iron, for instance, in the F — b region, we may say there are 

 only two lines ; one iron line was left out by mistake by Prof. 

 Thalen in his map of the solar spectrum, and the Italian obser- 

 vations of the sun suggested to us at Kensington that Thalen, at 

 Upsala, had made an omission in the spectrum of iron. This we 

 found to be the case. All the other lines are clean swept out of the 

 record. We get none of them in the spectrum of prominences. 



At a ceitain date, I believe about the end of necember 1873, 

 the lines in question suddenly ceased to appear in the spectrum 

 of j^rominences. The Italian observers, who had observed them 

 constantly day by day for three years, suddenly found them g<^ne, 

 but other new lines were seen. I shall show by and by that 

 there was a very good reason why that should have happened. 

 But the important point now is that it really did happen. 



