Page:Encyclopædia Britannica, Ninth Edition, v. 10.djvu/229

Rh E.-RTlI’S PLACE IN SOLAR SYSTEDL] sium. There occur also carbon, silicon, phosphorus, sulphur, oxygen, mtrogen, and hydrogen. In some of their com- binations these elements, as found in the meteoric stones, differ from their mode of occurrence in the accessible parts of the earth. Iron, for example, occurs as native metal, alloyed with a variable proportion (6 to 10 per cent.) of metallic nickel. But in other respects they closelyresemble some of the familiar materials of the eartl1’s rocky crust. Thus we have such minerals as pyrite, apatite, olivine, augite, hornblende, and labradorite. No more reliable proof could be desired that seine at least of the other members of the solar system are formed of the same materials as compose the earth. But in recent years a far more precise and generally applicable method of research into the composition of the heavenly bodies has been found in the spectroscope. By means of this instrument, the light emitted from self- luminous bodies can be analysed in such a way as to show what elements are present in their intensely hot luminous vapour. When the light of a burning metal is examined with a properly—arranged prism, it is seen to give a dark band or s_/mctrmn which is traversed by certain vertical bright lines. This is termed a rculiatiomspeclrzmz. Each element appears to have its own characteristic arrange- ment of lines, which retain the same relative position, intensity, and colours. Moreover, gases a11d the vapours of solid bodies are found to intercept those rays of light which they themselves emit. The spectrum of b11rning sodium, for example, shows two bright yellow lines. If therefore white light from some other source passes through the vapour of sodium, these two bright lines become dark lines, that portion of the light being exactly cut off which would have been given out by the sodium itself. This is called an absmy)t2'0n—speclrmn. By this method of examination it has been ascertained that many of the elements of which our earth is composed exist in the state of incandescent vapour in the atmosphere of the sun. Among these are some of our most familiar metals—iron, zinc, copper, nickel, with sodium, magnesium, barium, calcium, and vast quantities of free hydrogen. lloreover, as Mr Lockyer has pointed out, these elements appear to succeed each other in relation to their respective densities. Thus the coronal atmosphere which, as seen in total eclipses, extends to so prodigious a distance beyond the orb of the sun, consists mainly of sub-incandescent hydrogen and another element which may be new. Beneath this external vaporous envelope lies the chromosphere where the vapours of incandescent hydrogen, calcium, and magnesium can be detected. Further inward the spot—zone shows the presence of sodium, titanium, &c.; while still lower, a layer (the reversiazg layer) of intensely hot vapours, I ying probably next to the inner brilliant photosphere gives spectroscopic evidence of the existence of incandescent iron, manganese, cobalt, nickel, copper, and other well-known terrestrial metals.‘ The spectrosope has likewise been successfully applied by Mr Huggins and others to the observation of the ﬁxed stars and nebulte, with the result of establishinga similarity of elements between our own system and other bodies in sidereal space. In the radiation spectra of nebulm Mr Huggins ﬁnds the hydrogen lines very prominent; and he conceives that they maybe glowing masses of that element. Sir William Thomson and Professor Tait have suggested, on the other hand, that they are more probably clouds of stones in rapid motion, perhaps in an atmosphere of hydrogen. Among the ﬁxed stars absorption spectra have ‘ On the coiistitiition of the sun see Roscoe's Spectrum Anal]/sz'.s; Lockyer s Solar I’h_7/szcs, 1873; and memoirs in Proc. of Roy. Soc., by B. Stewart, Loewy, and De la Rue. GEOLOGY 215 been recognized, pointing to a structure resembling that of our sun, viz., a solid or liquid incandescent nucleus, sur- rounded with an atmosphere of glowing vapour? Accord- ing to Mr Lockyer, these stars or nebulae which have the highest temperature have the simplest spectra, and in pro- portion as they cool their materials become more and more differentiated into what we call elements. He remarks that the most brilliant or hottest stars show in their spectra only the lines of gases, as hydrogen. Cooler stars, like our sun, give indications of the presence, in addition, of the more stable metals—magnesium, sodium, calcium, iron. A still lower temperature he regards as marked by the appearance of the other metals, metalloids, and compounds, so that the older a star or planet is the more will it lose free hydrogen, till, when it comes to the condition of our earth, all its free hydrogen will have disappeared.3 According to this view the atoms of all the elements existed originally in the nebula. dissociated fron1 each other by reason of the intense heat- As the nebula gravitated towards its nucleus and cooled, the atoms came together, and the elements appeared in a certain order, beginning with hydrogen, and passing on through the metals a11d metalloids into compounds such as we ﬁnd on our globe. The sun would thus be a star considerably advanced in the process of diﬁerentiation or association of its atoms. It contains, so far as we know, no metalloids or compounds, while stars like Sirius show the presence only of hydrogen, with but a feeble proportion of metallic vapours 3 and on the other hand, the red stars indicate by their spectra that their metallic vapours have entered into combination, whence it is inferred that their temperature is lower than that of our sun. Further conﬁrmation of these views as to the order of planetary evolution is furnished by the form and structure of the earth. leference has already been made to the fact that the outer crust of our planet possesses only about half the density of the whole mass. It consists largely of metalloids—oxygen, silicon, carbon, sulphur, chlorine. On the other hand, lavas and mineral veins, which are believed to have been supplied from some considerable depth, con- tain abundance of metallic ingredients. The _form of the globe likewise points to a former ﬂuid condition. As the result of computations from ten measured arcs of the meridian made by different observers between the latitudes of Sweden and the Cape of Good Hope, Bessel obtained the following data for the dimensions of the earth :——- Equatorial diameter ....... ..41,S-17,192 feet, or 7925604 miles. Polar diameter .............. ..4l,707,314 7899114 ,, Amount of polar ﬂattening, 139,768 26471 ,, The equatorial circumference is thus a little less than 25,000 miles, and the difference between the polar and equatorial diameters (nearly 26,17 miles) amounts to about §,‘,—6th of the equatorial diameter.4 More recently, however, it l1as been shown that the oblate spheroid indicated by these measurements is not a symmetrical body, the equa- torial circumference being an ellipse instead of a circle. The diameter of which the vertices touch the surface of the globe i11 longitudcs 14° 23' E. and 194° 23' E. of Greenwich is nearly two miles longer than that at right angles to it.5 In obedience to the inﬂuence of rotation on its axis, our planet would tend to assume exactly such a ﬂattening at the poles as it has been proved to possess. This was dis- covered and demonstrated by Newton, and the amount of 9 Huggins, Proc. Roy. Soc., 1863-66, and Brit. Assoc. Lecture (Nottingham, 1866) ; Huggins and Miller, Phil. T-r¢ms., 1864. 3 Lockyer, Comptcs Rendus, Dec. 1873. 4 Herschel, Astronomy, 1». 139. 5 A. R. Clarke, Jllcm. Roy. .—l.stron. .S'oc., xxix.; Herschel,_Aslron., p. 691. See also a. more recent paper by Colonel Clarke, Phzl. J1ag., August 1878.