4o8 



NATURE 



\Sept. \, 



atolN, and it was impossi'ile to imagine so great a number of 

 crater';, all so nearly of the same altitude. Darwin showed, more- 

 over, that so far from the rin^ of c irals resting on a corresponding 

 ridge of rock, the lagoon";, on the contrary, now occupy the place 

 which was once the highest land. He pointed out that smie 

 lagoon^, as, for instance, that of Vanikoro, contain an island in 

 the middle ; while other islands, such as Tahiti, are surrounded 

 by a margin of smooth water, separated from the ocean by a coral 

 reef. Now, if we suppose that Tahiti were to sink slowly, it 

 would gra lually approximate to the condition of Vanikoro ; and 

 if Vanikoro gradually sank, the central island would disappear, 

 while on the contrary the growth of the coral might neutralise the 

 subsidence of the reef, so that we should have simply an at ill, 

 with its lagoon. The same considerations explain the origin of 

 the "barrier reef;," such as that which runs, for nearly one 

 thousand miles, along the north-east coast of Australia. Thui 

 Darwin's theory explained the form and the approximate identity 

 of altitude of these coral islands. But it did more than this, 

 because it showed us that there were great areas in process of 

 subsiilence, which though slow, was of great importance in 

 phyiical geography.' 



Much information ha^ also been acquired with reference to the 

 abysses of the ocean, especially from the voyages of the Porcitpinc 

 and the Ckallengcr. The greatest depth yet recorded is near 

 the Ladrone Island--, where a sounding of 4575 fathoms was 

 obtained. 



Ehrenberg long ago pointed out the similarity of the calcareous 

 mud now accumulating in our recent seas to the Chalk, and showed 

 that the green sands of the ge :iIogist are largely made up of casts 

 of firaminifera. Clay, however, had been 1 joked on, until the 

 recent expeditions, as e;sentially a product of the disintegratim 

 of older rocks. Not only, however, are a large proportion of 

 siliceous and calcareous rocks either directly or indirectly derived 

 from material which has once formed a portion of living organ- 

 isms, but Sir WyviUe Thonson maintains that this is the case 

 with some clays also. In that case the striking remark of 

 Linnieus that "fossils are not the children but the parents of 

 rocks," will have received remarkable confirmation. I should 

 have thought it, I confess, probable that these clays are, to a 

 considerable extent, composed of volcanic dust. 



It would appear that calcareous deposits resembling our chalk 

 do not occur at a greater depth than 3000 fathoms ; they have 

 not been met with in the abys-es of the ocean. Here the bottim 

 consists of exceedingly fine clay, sometimes coloured red by 

 oxide of iron, sometimes chocolate by manganese oxide, and 

 containing with Foraminifera occasionally large numbers of 

 siliceous Radiolaria. These strata seem to accumulate with 

 extreme slowness : this is inferred from the comparative abun- 

 dance of whales' bones and tishes' teeth ; and from the 

 presence of minute spherical particles, supposed by Mr. Murray 

 to be of cosmic origin — in fact, to be the dust of meteo- 

 rites, which in the course of ages have fallen on the ocean. 

 Such particles no doubt occur over the whole surface of the 

 earth, but on land they soon oxidise, and in shallow water they 

 are covered up by other deposits. Another interesting result of 

 recent deep-sea explorations has been to show that the depths of 

 the ocean are ni mere barren solitudes, as was until recent years 

 confidently believed, but, on the contrary, present us many 

 remarkable forms of life. We have, however, as yet but 

 thrown here and there a ray of light down i ito the ocean 

 abysses : — 



'* Nor can so short a time sufficient be, 

 To fathom the vast depths of Nature's sea." 



In Astronomy, the discovery in 1845 of the planet Neptune, 

 made independently and almost simultaneously by Adams and by 

 Le Verrier, was certainly one of the very greatest triumphs of 

 mathematical genius. Of the minor planets four only were 

 known in 1831, whilst the number now on the roll amounts to 

 220. Many astronomers believe in the existence of an intra- 

 ruercurial planet or planets, but this is still an open question. 

 The Solar System lixs al^o been enriched by the discovery of an 

 inner ring to Saturn, of satellites to Mars, and of additional 

 s.atellites to Saturn, Uranus and Neptune. 



The most unexpected progress, however, in our astronomical 

 knowledge during the past half-century has been due to Spectrum 

 Analysis. 



The dark lines in the spectrum were first seen by WoUaston, 

 who noticed a few of them ; but they were independently dis- 



' I ought to mention that Darwin's views have recently been questioned 

 by Semper and Murray. 



covered by Fraunhofer, after whom they are justly named, and 

 who, in 1814, mapped no fewer than 576. The first steps in 

 "spectrum analysis," properly so called, iwere made by Sir J. 

 Herschel, Fox Talbot, and by Wheatstone, in a paper read before 

 this Association in 1S35. The latter showed that the spectrum 

 emitted by the incandescent vapour of metals was formed of 

 bright lines, and that these lines, while, as he then supposed, 

 constant for each metal, differed for different metals. "We 

 have here," he said, " a mode of discriminating metallic bodies 

 more readily than that of chemical examination, and which may 

 hereafter be employed f jr useful purposes." Nay, not only can 

 bodies thus be more readily discriminated, but, as we now 

 know, the presence of extremely minute portions can be detected, 

 As sinrjinriith of a grain being in some cases easily perceptible. 

 It is also easy to see that the presence of any new simple sub- 

 stance might be detected, and in this manner already several new 

 elements have been discovered, as I shall mention when we coine 

 to Chemistry. 



But spectru n analysis has led to even grander and more un- 

 expected triumphs. Fraunhofer himself noticed the coincidence 

 betueen the double dark line D of the solar spectrum and a 

 double line which he observed in the spectra of ordinary flames, 

 whde Stokes pointed out to Sir W. Thomson, who taught it in 

 his lectures, that in both cases these lines were due to the 

 presence of sodium. To Kirchhoff aud Bunsen, however, is 

 due the independent conception and the credit of having first 

 systematically investigated the relation which exists between 

 Fraunhofer's lines and the bright lines in the spectra of incan- 

 descent metals. In order to get some fixed measure by which 

 they might determine and record the lines characterising any 

 given substaice, it occurred to them that they might use for 

 comparison the spectrum of the sun. They accordingly arranged 

 their spectroicope so that one half of the slit was lighted by 

 the sun, and the other by the luminous gases they proposed to 

 examine. It immediately struck the u th.at the bright lines in the 

 one corresponded with the dark lines in the other— the bright line 

 of sodium, for instance, with the line or rather lines D in the sun's 

 spectrum. The conclusion was obvious. There was sodium in 

 the sun ! It must indeed have been a glorious moment when 

 that thought flashed across them, and even by itself well worth 

 all their labour. 



Kirchhoff and Bunsen thus proved the existence in the sun of 

 hydrogen, sodium, magnesium, calcium, iron, nickel, chromium, 

 manganese, titanium, and cobalt; since which Angstrom, Thalen, 

 and Lockyer have considerably increased the list. 



But it is not merely the chemistry of the heavenly bodies on 

 which light is thrown by the spectroscope ; their physical structure 

 and evolutional history are also illuminated by this wonderful 

 instrument of research. 



It used to be supposed that the sun was a dark body enveloped 

 in a luminous atmosphere. The reverse now appears to be the 

 truth. The body of the sun, or ]jhotosphere, is intensely brilliant ; 

 round it lies the solar atmosphere of comparatively co 3I gases, 

 which cause the dark lines in the spectrum ; thirdly, a chromo- 

 sphere, — a sphere principally of hydrogen, jets of which are 

 said sometimes to reach to a height of 100,000 miles or more, 

 int D the outer coating or corona, the nature of which is still very 

 d iubtful. 



Forinerly the red flames which represent the higher regions of 

 the chromosphere could be seen only on the rare occasions of a 

 total solar eclipse. Janssen and Lockyer, by the application of 

 the spectroscope, have enabled us to study this region of the sun 

 at all times. 



It is, moreover, obvious that the powerful engine of investiga- 

 tion afforded us by the spectroscope is by no means confined to 

 the substances which form part of our system. The incandescent 

 body can thus be examined, no matter how great its distance, so 

 long only as the light is strong en lugh. That this method was 

 theoretically applicalJe to the light of the stars was indeed 

 obvious, but the practical difficulties were very great.^ Sirius, 

 the brightest of all, is, in round numbers, a hundred millions of 

 millions of miles from us ; and, though as big as sixty of our 

 suns, his light when it reaches us, after a journey of sixteen 

 years, is at most one two-thousand-millionth part as bright. 

 Nevertheless as long ago as 1S15 Fraunhofer recognised the fixed 

 lines in the light of four of the stars, and in 1S63 Miller and 

 Huggins in our own country, and Rutherford in America, 

 succeeded in determining the dark lines in the spectrum of some 

 of the brighter stars, thus showing that these beautiful and 

 mysterious lights contain many of the material substances with 



