92 



NA TURE 



[May 27, 1897 



(3) In the case of iron, calcium, and magnesium, there are 

 four distinct temperature steps which are marked by spectral 

 changes : {a) the flame spectrum, {b) the arc spectrum, {c) the 

 spark spectrum, {d) a spectrum consisting solely of those lines 

 which are enhanced in passing from the arc to the spark. 



(4) The order of temperature of certain stars, as determined 

 from a comparison of the extensions of the continuous spectrum 

 into the violet or ultra-violet, is precisely the same as that which 

 follows from a comparison of the metallic spectra at the four 

 stages of temperature. 



(5) The variations of the metallic lines furnish the most con- 

 venient means of determining relative stellar temperatures, for 

 the reason that photographs with special exposures are un- 

 necessary. 



(6) Having ascertained the relative temperature of a star in 

 this way, and assuming that all the absorbing vapours are at the 

 same temperature, the presence or absence of any other metallic 

 substance can be determined by looking for the lines which are 

 longest in its spectrum at that temperature. In the case of the 

 hottest stars, the fourth stage spectrum must be the term of 

 comparison. 



(7) Accepting the new results with regard to the lines en- 

 hanced in the spark, several lines in the spectra of the hottest 

 stars, for which no origins could previously be assigned, can 

 now be ascribed to metallic substances at the fourth stage of 

 temperature. 



(8) The lines of the cleveite gases appear only in the hotter 

 stars, as indicated by the extension of the continuous radiation 

 into the ultra-violet. -They increase in intensity with increased 

 temperature in certain stars. 



(9) The order of stellar temperatures, determined from the 

 increasing intensity of'the lines of the cleveite gases, is identical 

 with that determined from the decreasing intensity of the 

 metallic lines in the case of those stars which show both series 

 of lines. 



(10) Different substances are spectroscopically visible through 

 different ranges of stellar temperatures. The hydrogen lines are 

 visible in stars ranging in temperature from that of o Orionis to 

 that of Bellatrix, while those of the cleveite gases do not appear 

 below the temperature of a Cygni. The enhanced lines of 

 calcium appear at temperatures as low as a Orionis, and persist, 

 with reduced intensity, to the temperature of Bellatrix ; those of 

 iron do not appear at temperatures lower than that of o Cygni, 

 and disappear altogether at the temperature of Bellatrix ; while 

 the enhanced line of magnesium appears at the temperature 

 of a Cygni, and remains feebly visible at the temperature of 

 Bellatrix. 



(11) It follows, then, that the enhanced metallic lines may be 

 absent from a stellar spectrum, either because the temperature is 



, too low or too high. 



(12) In the case of those stars which previous investigations 

 have shown to be cooling, the metallic line phenomena are in- 

 verted. The enhanced lines first become visible, then the arc 

 lines ; while the enhanced lines disappear at a certain stage 

 in the process of cooling the arc lines continue to become 

 stronger. 



(13) The lines of the cleveite gases show a similar inversion 

 on the downward side of the temperature curve. Strongly repre- 

 sented in the hottest stars, they thin out very rapidly in cooling 

 stars, and disappear before the arc lines have begun to show 

 themselves. 



(14) Utilising the iron lines as a method of bringing together 

 stars of approximately equal temperature, it is found that at each 

 stage the stars are divisible into two groups, which, in accordance 

 with my previous work, correspond to increasing and decreasing 

 temperatures respectively. 



(15) As determined in this way, stars of increasing temperature 

 difi'er from those of decreasing temperature at the same stage of 

 heat : (i) in the greater continuous absorption in the violet or 

 ultra-violet, (2) in the generally greater intensity and breadth 

 of the metallic lines, (3) in the smaller thickness of the hydrogen 

 lines, (4) in the greater thickness of the helium lines at those 

 stages in which they are visible. 



(16) These differences are all explained on the meteoritic 

 hypothesis. 



(17) There are stars, near and at the top of the curve, which 

 cannot be arranged in order of temperature by the criteria 

 referred to in (15), for the reason that the iron lines have dis- 

 appeared, and the lines of hydrogen and cleveite gases show 

 little variation. 



NO. 1439, VOL. 56] 



(18) The arrangement of stars about the top of the curve 

 will depend upon the conditioning of certain lines, at preseiv 

 of unknown origin ; the necessary criteria, therefore, requii^ 

 further investigation. 



(19) The known facts with regard to changes in the line 

 spectrum of an element can be easily explained on the hypo- 

 thesis of successive dissociations analogous to those observed in 

 the case of undoubted compounds. 



(20) Similarly, the differences in the lines representative of 

 a metal such as iron in the spectra of sun-spots, prominences, 

 chromosphere, or different stars, are explained by supposing 

 that there are different molecular groupings at each stage of 

 temperature. 



(21) The change from a continuous spectrum to one consisting 

 of flutings, and afterwards to one of lines, is now acknowledged 

 to be due to the existence of different molecular combinations. 



(22) The recent investigations of Humphreys and Mohler on 

 the shifts produced in metallic lines, when the vapours are ob- 

 served at different pressures, confirm my view that the line 

 spectrum of a metal integrates for us the vibrations of several 

 sets of molecules. 



(23) It is argued that the existence of "series" of lines in 

 the spectra of some chemical elements is another indication 

 of molecular complexity, each series probably representing the 

 vibrations of similar molecules. 



(24) The behaviour of the magnesium lines in stellar spectra 

 is ascribed by Dr. Scheiner to differences of temperature, in 

 accordance with my experimental results of 1879. 



(25) The experiments on the spectrum of mercury which 

 have l)een made by Eder and Valenta have revealed variations 

 which according to them favour the dissociation hypothesis. 



(26) On various grounds, the view that the differences in 

 stellar spectra represent fundamental differences of chemical 

 composition is untenable. The fact that many stars which are 

 widely separated in space give identical spectra, indicates that 

 they not only contain the same "elements," but that the 

 " elements" exist in the same proportions in all. 



(27) On the non-dissociation hypothesis, the action of heat 

 on the sun's chromosphere could not produce such a spectrum 

 as that which we know to be associated with hotter stars, since 

 the relative proportions of different vapours could not be 

 changed. The only change which can be imagined to take 

 place on this hypothesis is a reduction of intensity of all the 

 lines due to reduced pressure. 



(28) On the dissociation hypothesis, increased temperature 

 would bring about fundamental changes in the spectrum due to 

 molecular simplifications, and in this way the effect of an in- 

 crease of temperature on the .sun's chromosphere, as indicated 

 by hotter stars, can be predicted, and receives a simple and 

 sufficient explanation. 



(29) The disappearance of the enhanced iron lines in the 

 hottest stars, and the simultaneous intensification of the lines 

 of hydrogen, helium, and gas X, bring us face to face with 

 the fact that iron is a compound, into the ultimate formation 

 of which one or all of these gases enters. 



(30) The ultimate molecules of the chemical elements dis- 

 cussed in the present paper may be provisionally arranged in 

 the following order of resistance to the effects of temperature : — 



^''^ He} Doubtful which. 

 H 

 Ca 

 Mg 

 Fe 



(31) Each step in advance which has been made since 1873, 

 has demonstrated more and more that there is really such 

 a "celestial dissociation" going on as that which I then 

 suggested. 



UNIVERSITY AND EDUCATIONAL 

 INTELLIGENCE. 

 Oxford. — The sixth Robert Boyle Lecture of the Junior 

 Scientific Club will be delivered in the Examination Schools, 

 on Tuesday, June i, at 8.30, by Captain Abney, C.B., F. R.S., 

 who has chosen for his subject " The Scientific Requirements 

 of Colour Photography." 



Mr. W. Garstang, Fellow and Lecturer of Lincoln College, 



