Sept. I, 1881] 



NATURE 



409 



which we are familiar. In Aldebaran, for instance, we may infer 

 the presence of hydrogen, sodium, magnesium, iron, calcium, 

 tellurium, antimony, bismuth, and mercury ; some of which are 

 not yet known to occur in the sun. As might have been expected, 

 the composition of the stars is not uniform, and it would appear 

 that they may be arranged in a few well-marked classes, 

 indicating differences of temperature, or in other words, of age. 

 Some recent photographic spectra of stars obtained by Huggins 

 go very far to justify this view. 



Thus we can make the stars teach us their own composition 

 with light which started from its source before w^e were born — 

 light older than our Association itself 



But spectram analysis has even more than this to tell us. The 

 old methods of observation could determine the movements of 

 the stars so far only as they were transverse to us ; they afforded 

 no means of measuring motion either directly towards or away 

 from us. Now Doppler suggested in 1S41 that the colours of 

 the stars would assist us in this respect, because they would be 

 affected liy their motion to and from the earth, just as a steam- 

 whistle is raised or lowered as it approaches or recedes from us. 

 Every one has observed that if a train whistles as it i)asses us, the 

 sound appears to alter at the moment the engine goes by. This 

 arises, of course, not from any change in the whistle itself, but 

 because the number of vibrations which reach the ear in a given 

 time are increased by the speed of the train as it approaches, and 

 diminished as it recedes. So like the sound, the colour would 

 be affected by such a movement ; but Doppler's method was 

 practically inapplicable, becau e the amount of effect on the 

 colour would be utterly insensible ; and even if it were otherwise 

 the method could not be applied, because, as we did not know 

 the true colour of the stars, we have no datum line by which to 

 measure. 



A change of refrangibiUty of light, how ever, does occur in 

 consequence of relative motion, and Huggins successfully applied 

 the spectroscope to solve the jroblem. He took in the first 

 place the spectroscope of Sirius, and chose a line known as F, 

 which is due to hydrogen. Now, if Sirius was motionles', or 

 rather if it retained a const.int distance from the earth, the line 

 F would occupy exactly the same position in the spectrum of 

 Sirius, as in that of the sun. On the contrary, if Sirius were ap- 

 proaching or receding from us, this line would be slightly shifted 

 either towards the blue or red end of the spectrum. He found 

 that the line had moved very slightly towards the red, indicating 

 that the distance between us and Sirius is increasing at tlie rate 

 of'about tw'enty miles a second. So also Betelgeux, Kigel, 

 Castor, and Regulus are increasing their distance ; w hile, on the 

 contrary, that of others, as for instance of Vega, Arcturus, and 

 Pollux, is diminishing. The results obtained by Huggins on 

 about tw enty stars have since been confirmed and extended by 

 Mr. Christie, now Astronomer-Royal in succession to Sir G. 

 Aity, who has long occupied the post with so much honour to 

 himself and advantage to science. 



To examine the spectrum of a shooting star would seem even 

 more difficult ; yet Alexander Herschel has succeeded in doing 

 so, and finds that their nuclei are incandescent solid bodies ; he 

 has recognised the lines of potassium, sodium, lithium, and other 

 substances, and considers that the shooting itars are bodies 

 similar in character and composition to the ttony masses which 

 sometimes reach the earth as aerolites. 



No element has yet been found in any meteorite, w hich was 

 not previously known as exiting in the earth, but the phenomena 

 which they exhibit indicate that they must have been formed 

 under conditions very diiferent from those which prevail on the 

 earth's surface. I may mention, for instance, the peculiar form 

 of crystallised silica, called by Maskelyne, Asmanite ; and the 

 whole class of meteorites, consisting of iron generally alloyed 

 with nickel, which Daubree terms Holosiderites. The interest- 

 ing discovery, however, by Nordenskjold, in 1S70, at Ovifak, of 

 a number of blocks of iron alloyed w itli nickel and cobalt, in 

 connection with ba-alts containing disseminated iron, has, in the 

 words of Judd, "afforded a very important link, placing the 

 terrestrial and extra-terrestrial rocks in closer relations with one 

 another." 



We have as yet no sufficient evidence to justify a conclusion 

 as to whether any substances exist in the heavenly bodies which 

 do not occur in our earth, though there are many lines which 

 cannot yet be satisfactorily referred to anv terrestrial element. 

 On the other hai.d, some substances which occur on our earth 

 have not yet been detected in the sun's atmosphere. 



Such discoveries as these seemed, not long ago, entirely beyond 



our hopes. M. Comte, indeed, in his " Cours de Philoiophie 

 Positive," as recently as 1S42, laid it down as an axiom regard- 

 ing the heavenly bodies, that "Nous concevons la possibilite de 

 determiner leurs formes, leurs distances, leurs grandeurs et leurs 

 mouvements, tandis que nous ne saurions jamais etudier par 

 aucun moyen leur composition chimique ou lenr structure miner- 

 alogique." Yet within a few years this supposed impossibility 

 has been actually accomplished,, showing how unsafe it is to 

 limit the possibilities of science. 



It is hardly necessary to point out that, while the spectrum has 

 taught us so much, we have still even more to learn. Why should 

 some substances give few, and others many, lines ? Why should 

 the same substance give different lines at different temperatures ? 

 What are the relations between the lines and the physical or 

 chemical properties? 



We may certainly look for much new knowledge of the hidden 

 actions of atoms and molecules from future researches with the 

 spectroscope. It may even, perhaps, teach us to modify our 

 views of the so-called simple substances. Prout long ago, struck 

 by the remarkable fact that nearly all atomic weights are simple 

 multiples of the atomic weight of hydrogen, suggested that 

 hydrogen must be the primordial substance. Brodie's researches 

 also naturally fell in with the supposition that the so-called 

 simple substances are in reality complex, and that their con- 

 stituents occur separately in the hottest regions of the solar 

 atmosphere. Lockyer considers that liis researches lend great 

 probability to this view. The whole suljject is one of interne 

 interest, and we may rejoice ih.at it is occupying the attention, 

 not only of such men as Abney, Dewar, Hartley, Liveing, 

 Roscoe and Schuster in our own countiy, bnt also of many foreign 

 observers. 



When geology so greatly extended our ideas of past time, the 

 continued heat of the sun became a question of greater interest 

 than ever. Helmholtz has shown that, while adopting the 

 nebular hypothesis, we need not assume that the nebulous 

 matter was originally incandescent ; but that its present high 

 temperature may be, and probably is, mainly due to gi-avitation 

 between its parts. It follows that the potential energy of the 

 sun is far from exhausted, and that with continued shrinking it 

 will continue to give out light and heat, with little, if any, 

 diminution for several millions of years. 



Like the sand of the sea, the stars of heaven have ever been 

 used as effective symbols of number, and the improvements in 

 our methods of observation have added fresh force to our original 

 impressions. We now know that our earth is but a fraction of 

 one out of at least 75,000,000 worlds. 



But this is not all. In addition to the luminous heavenly 

 bodies, we cannot doubt that there are countless others, invisible 

 to us from their greater distance, smaller size, or feebler light ; 

 indeed we know that there are many dark bodies w hich now 

 emit no light or comparatively little. Thus in the case ot 

 Frocyon, the existence of an invisible body is proved by the 

 movement of the visible star. Again I may refer to the curious 

 phenomena presented by Algol, a bright star in the head of 

 Medusa. This star shines without change for two days and 

 thirteen hours ; then, in three hours and a half, dwindles from 

 a star of the second to one of the fourth magnitude ; and then, 

 in another three and a half hour--, reassumes its original bril- 

 hancy. These changes seem certainly to indicate the presence 

 of an opaque body, which intercepts at regular intervals a part 

 of the light emitted by Algol. 



Thus the floor of heaven is not only " thick inlaid with 

 patines of bright gold," but studded also with extinct stars ; 

 once yirobably as brilliant as our own sun, but now dead and 

 cold, as Helmholtz tells us that our sun itself will be, some 

 seventeen millions of years hence. 



The general result of astronomical researches has been thus 

 eloquently summed up by Proctor: — "The sidereal system is 

 altogether more complicated and more varied in structure than 

 has hitherto been supposed : in the same region of the stellar 

 depths co-exist stars of m.any orders of real magnitude ; all 

 orders of nebula;, gaseous or stellar, planetary, ring-formed, 

 elliptical, and spiral, exist within the limits of the galaxy ; and 

 lastly, the whole system is alive with movements, the laws of 

 which may one day be recognised, though at present they appear 

 too complex to be understood." 



We can, I think, scarcely claim the establishment of the undu- 

 latory theory of light as falling within the last fifty years ; for 

 though Brewster, in his " Report on Optics," published in our 

 first volume, treats the que-tion as open, and expresses himself 



