34 



NA TURE 



[November io, 1898 | 



-stars, taken by my assistants at South Kensington ; the 

 complete harmony of the results obtained along various 

 lines of other work carries conviction with it. 



We find ourselves here in the presence of minute details 

 exhibiting the workings of a chemical law, associated dis- 

 tinctly with temperature : and more than this, we are also 

 in the presence of high temperature furnaces, entirely 

 shielded by their vastness from the presence of those dis- 

 tracting phenomena which we are never free from in the 

 most perfect conditions of experiment we can get here. 



What, then, is the chemical law? It is this. In the 

 very hottest stars we deal with the gases hydrogen, 

 helium, and doubtless others still unknown, almost e.\- 

 ■clusively. .-\t the ne.\t lowest temperatures we find these 

 gases being replaced by metals in the state in which they 

 are observed in our laboratories when the most powerful 

 jar-spark is employed. .•Xt a lower temperature still the 

 gases almost disappear entirely, and the metals e.xist in 

 the state produced by the electric arc. Certain typical 

 -stars showing these chemical changes may be arranged 

 as follows : 



Star* getting ho 



Hottest st.-trs. 

 ~- Bellatrix ^ 



Stars 



oling. 



Rlgel 

 a Cygni 

 7 Cygni 

 « Orionis 



if Tauri j8 Persei 



7 Lyra 



Castor 



Procyon 

 Arcturus and Sun 



This, then, is the result of our first inquiry into the 

 •existence of the various chemical elements in the atmo- 

 spheres of stars generally. We get a great diversity, 

 and we know that this diversity accompanies changes of 

 temperature. We have also found that the sun, which 

 we independently know to be a cooling star, and Arcturus, 

 are identical chemically. 



We have now dealt with the presence of the various 

 chemical elements, generally, in the atmospheres of 

 stars. The next point we have to consider is whether 

 the absorption which the spectrum indicates for us takes 

 place from top to bottom of the atmosphere, or only in 

 certain levels. 



In many of these stars the atmosphere may be millions 

 ■of miles high. In each the chemical substances in the 

 hottest and coldest portions may be vastly ditterent ; the 

 region, therefore, in which this absorption takes place, 

 which spectroscopically enables us to discriminate star 

 from star, must be accurately known before we can obtain 

 the greatest amount of information from our inquiries. 



Our next duty then clearly is to study the sun — a star 

 so near us that »e can examine the different parts of its 

 atmosphere, which we cannot do in the case of the more 

 ■distant stars. l!y doing this we may secure facts which 

 will enable us to ascertain in what parts of the atmosphere 

 the absorption takes place which produces the various 

 phenomena on which the chemical classification has been 

 based. 



It is obvious that the general spectrum of the sun, like 

 that of stars generally, is built up of all the absorptions 

 ■which can' make themselves fell in every layer of its 

 atmosphere from bottom to top, that is from the photo- 

 sphere to the outermost part of the corona. Let me 

 remind you that this spectrum is changeless from year to 

 year. 



Now sun-spots are disturbances produced in the 

 photosphere ; and the chromosphere, with its disturb- 

 ances, called prominences, lies directly above it. Here, 

 then, we are dealing with the lowest part of the sun's 

 atmosphere. We find first of all that in opposition to the 

 changeless general spectrum, great changes occur with 

 the sun-spot period, both in the spots and chromosphere. 

 The spot spectrum is indicatecf, as was found in 1866, 

 by the widening of certain lines ; the chromospheric 

 NO. 1515, VOL. 59] 



ifri 



spectrum, as was found in 1868, by the appearance at th< 

 sun's limb of certain bright lines. In both cases th( 

 lines atifected seen at any one time are relatively fe\i^ 

 in number. 



In the spot spectrum, at a sun-spot minimum, we find 

 iron lines chiefly affected ; at a maximum they are 

 chiefly of unknown or unfamiliar origin. At the present 

 moment the affected lines are those recorded in the 

 spectra of vanadium and scandium, with others nevei 

 seen in a laboratory. That we are here far away from 

 terrestrial chemical conditions is evidenced by the faci 

 that there is not a gramme of scandium available foi 

 laboratory use in the world at the present time. 



Then we have tlie spectrum of the prominences and the 

 chromosphere. Th.it spectrum we are enabled to observe 

 every day when the sun shines, as conveniently as we 

 can observe that of sun-spots. The chromosphere is full ol 

 marvels. At first, when our knowledge of spectra was very 

 much more restricted than now, almost all the lines obi 

 served were unknown. In 1S68 I saw a line in the 

 yellow, which 1 found behaved very much like hydrogen, 

 though I could prove that it was not due to hydrogen ; 

 for laboratory use the substance which gave rise to it I 

 called helium. Next year I saw a line in the green at 1474 

 of Kirchhoff's scale. That was an unknown line, but in 

 some subsequent researches I traced it to iron. From 

 that day to this we have observed a large number of, 

 lines. They have gradually been dragged out from the] 

 region of the unknown, and many are now recognisei 

 as enhanced lines, to which I have already called atten- 

 tion as appearing in the spectra of metals at a very high 

 temperature. 



But useful as the method of observing the chromo-A 1 

 sphere without an eclipse, which enables us 1 1 



"... to feel from world to world," 



as Tennyson has put it, has proved, we want an eclipse 

 to see it face to face. 



A tremendous flood of light has been thrown upon it by 

 the use of large instruments constructed on a plan de- 

 vised by Respighi and myself in 1871. These give us an 

 image of the chromosphere painted in each one of its 

 radiations, so that the exact locus of each chemical layer 

 is revealed. One of the instruments employed during 3 

 the Indian eclipse of this year is that used in photograph-f 

 ing the spectra of .stars, so that it is now easy to place* 

 photographs of the spectra of the chromosphere obtained 

 during a total eclipse and of the various stars side 

 by side. 



I have already pointed out that the chemical classifica- 

 tion indicated that the stars next above the sun in 

 temperature are represented by y Cygni and Procyon, 

 one on the ascending, the other on the descending branch 

 of the temperature curve. 



Studying the spectra photographed during the eclipse 

 of this year we see that practically the lower part of the 

 sun's atmosphere, if present by itself, would give us the 

 lines which specialise the spectra of y Cygni or Procyon. 



I recognise in this result a veritable Rosetta stone 

 which will enable us to read the celestial hieroglyphics 

 presented to us in stellar spectra, and help us to study the 

 spectra and to get at results much more distinctly and 

 certainly than ever before. 



One of the most important conclusions we draw from 

 the Indian eclipse is that, for some reason or other, the 

 lowest hottest part of the sun s atmosphere does not 

 write its record among the lines which build up the 

 general spectrum so effectively as does a higher one. 



There was another point especially important on 

 which we hoped for information, and that was this. 

 Up to the employment of the prismatic camera in- 

 sufficient attention had been directed to the fact that 

 in observations made by an ordinary spectroscope, no 

 true measure of the height to which the vapours or gases 



