l62 



NATURE 



[June 14, 1900 



study of spectroscopy at once a sound philosophic basis for 

 spectrum-definitions, and a new territory of interesting astrono- 

 mical investigations in the sun's glowing atmosphere, upon 

 which it was not remiss or slow to grow up in strength and 

 stature, expanding itself largely in new observational, practical 

 and theoretical directions, in the next following interval of ten 

 or fifteen years. 



From the graphic mementos which he kept, as we have seen, 

 of the total solar eclipse of 185 1, and from his successful 

 attempts, described in 1858 in his well-known and most attrac- 

 tively illustrated volume on Teneriffe, to prove by visits (in that 

 year, and again in 1868) to the Island and Peak of Teneriffe, the 

 practical benefit to be obtained in astronomical observations, of 

 avoiding in great part the atmosphere's absorbing action on 

 the light of stars and planets by establishing observatories on 

 mountain heights, it cannot be doubted that these discoveries 

 with spectroscopes concerning the bright, ruddy light-flakes seen 

 round the sun, or round the moon's disc when the sun is totally 

 eclipsed, and regarding the particular rays of the sun's light 

 which undergo absorption in the earth's atmosphere, would, as 

 important contributions to our extremely circumscribed know- 

 ledge of the materials and physical conditions of planetary and 

 stellar atmospheres, greatly impress and interest him. 



In the preface of his " Spectre Normale du Soleil," Angstrom 

 pointed out that the spectrum of the aurora, as he had frequently 

 observed it in the winter months of 1867-8, consisted almost 

 entirely, as Dr. O. Struve at Pulkova, on hearing from Prof. 

 Angstrom of this discovery also confirmed it in May 1868, and 

 as Prof. Angstrom had previously found to be the case with the 

 spectrum of very bright appearances of the zodiacal light at 

 Upsala in March 1867, of a nearly solitary bright yellow line. 

 Many exact corroborations of this line's conspicuousness, and 

 detections of several less constant bright and faint auroral lines 

 were thereupon made by observers of a series of fine red auroras 

 which at the time of the marked maximum of sunspot frequency 

 in 1870 appeared during the years from 1869 to 1871. A paper 

 recommending Prof. Swan's well-known blue gas-flame, or blow- 

 pipe flame spectrum with its five well-determined line or band- 

 edge positions as a most suitable one for reference in mapping 

 auroral spectra, was sent in 1870 by the late Prof. Piazzi Smyth 

 to the Royal Astronomical Society in London ; but owing to his 

 describing the flame-spectrum as Prof. Swan had done in 1856, 

 and as did also Angstrom in the introduction to his " Solar Spec- 

 trum Atlas " in 1868, as the spectrum of hydrocarbons, or of 

 acetylene, it remained unpublished on account of the doubtfully 

 correct chemical apellation given in the paper to this important 

 set of spectrum-bands. Yet the same chemical origin, describing 

 it as probably that of acetylene, was attributed to this spectrum 

 both by Profs. Liveing . and Dewar at Cambridge, and by 

 Angstrom and Thalen in their " Spectres des Metalloides " at 

 Upsala, in 1875 5 ^""^ V^qL Smyth never felt induced to resign 

 the view which he held in such good company, by the contrary 

 opinion steadfastly maintained by many not less skilled and ex- 

 perienced and at least as chemically well versed spectroscopists, 

 that the blue candle-flame's spectrum of delicately fluted bands 

 was not really due to any chemical compound of carbon with 

 other elements, but to carbon itself in one of the modes of mole- 

 cular aggregation into which, like the materials of some other 

 metalloids at least, the substance of carbon in its volatilised 

 state is liable, by temperature or by some sufficient chemical or 

 electrical powers of dissociation to be broken up. 



Another very similar band-spectrum to the Bunsen-flame one, 

 agreeing in the positions of its two brightest (citron and green) 

 band-edges pretty closely with two corresponding bright band- 

 edges of the latter spectrum, but differing from it throughout in 

 its more numerous remaining bands' positions, and with all its 

 bands evenly shaded, instead of (as in the other spectrum) both 

 fluted and shaded off towards the blue direction, only too often 

 seen mingling with the latter spectrum to some extent in nearly 

 all electrically excited vacuum-tubes, can be very readily pro- 

 duced in its natural purity with ordinary induction-sparks in 

 carbon oxide or di-oxide vacuum-tubes ; and it was described on 

 . that account, in their " Spectres des Metalloides" in 1875, by 

 Angstrom and Thalen, and after some hesitation about its pos- 

 sible chemical nature in his first paper on " Gaseous Spectra in 

 Vacuum-tubes under Small Dispersion" in 1880,^ it was after- 



1 Edinburgh Philosophical Transactions, vol. xxx. (1883), pp. 99, 104. 

 In a letter to Nature (vol. xx. p. 75), in May 1879, on " End-on Vacuum- 

 tubes brought to bear on the Carbon and Carbohydrogen question," the late 

 Prof. Piazzi Smyth also adopted at first without reserve the view of this 

 spectrum that it is produced by carbon simply. 



NO. 



1598, VOL. 62] 



wards regarded also by the late Prof. Piazzi Smyth as belonging 

 to carbonic oxide. Appearing as these two spectra do almost 

 ubiquitously as impurities in ordinary gas-vacuum-tubes, their 

 precise discrimination from each other, and the resolution of 

 their many hazy bands into as many ranks of scores upon scores 

 of accurately measured linelets, was a work of exact spectro- 

 metry in which the great light-intensities of his vacuum-tubes 

 and the powerful train of prisms finally used by Prof. Smyth for 

 the maps of gaseous spectra which he constructed in 1884,^ 

 accomplished some of the most wonderfully perfect and beautiful 

 achievements. The much debated experimental evidence as to 

 the chemical origins of these two spectra, moreover, prepared the 

 way for some most embracing views of the modes of production 

 of stellar and celestial spectra, which, besides providing astro- 

 nomers with the means of classifying stars and the lesser lights 

 of nebulce and comets methodically, also afforded chemists an 

 imposing outline of problems for consideration, of apparently 

 successive stages of subdivision of the elementary forms of matter 

 from dense into light-atomed elements. 



In Sir J. N. Lockyer's hands the condensed spark of a 

 Leyden-jar introduced into the vacuum-tube circuit (which Prof. 

 Smyth never used, having decided to confine himself to weak- 

 spark or low temperature excitations only in his spectrum- 

 measures), supplied a method of transition from the oxy-carbon 

 spectrum in carbon-oxide and dioxide vacuum-tubes, directly to 

 the hydrocarbon or blue gas-flame one,^ showing that only a 

 rise of temperature was needed, from that of the nearly con- 

 tinuous induction-spark or simple brush-discharge in rarefied 

 gas-tubes, to the vastly hotter disruptive spark (instantly vola- 

 tilising gold-leaf or thin metallic wires), of a Leyden-jar and 

 air-gap in the outer circuit, to furnish a new spectrum, not, we 

 must conclude, by any chemical change of substance, but by 

 disgregation, it seemed evident, of cool and dense into hot light 

 molecules of pure carbon, which could thus be made at pleasure 

 to give either of these two spectra in succession. The flame, 

 and tube spectra, or the hydrocarbon and carbonic oxide ones 

 are therefore now usually referred to, by Sir J. N. Lockyer, as 

 the "hot carbon" and the "cool carbon" spectrum, respec- 

 tively.^ But all the best means that can be used to obtain, on 

 the one hand, an evenly ascending scale of temperature and 

 uniform intensities of action of discharges of the electric arc and 

 spark (the only sufficient known means which can be used to 

 reach the high temperatures demanded), and on the other hand 

 the requisite chemical purity of the substances submitted to 

 spectroscopic examination, are so very liable to unsuspected 

 failures from the many lurking sources of deception which most 

 insidiously waylay and falsify the observations and conclusions, 

 that although, on both sides, these sources of error have been 

 unremittingly sought out and often most startlingly disclosed and 

 very skilfully eliminated, it is difficult to say yet whether the 

 distinctive attributes in which the substances which give the 

 different banded carbon spectra really differ fundamentally from 

 each other, are either, as was at first supposed, simply chemical, 

 or else, according to a subsequent suggestion, attributing to pure 

 carbon spectroscopic properties which are at least not at vari- 

 ance with those of oxygen, hydrogen, sulphur, selenium and 

 phosphorus, of an entirely structural kind ; that is to say, 

 gaseously allotropic, or molecularly disgregational under the 

 action of increasing temperature. New discoveries and fresh 

 discussions of these bands must doubtless be awaited before we 

 can be definitely sure to what extent the views expressed by 

 different observers as to the chemically compound or elementary 

 dissociated natures of the material sources of special series of 

 shaded or fluted bands seen in banded carbon spectra can be 

 fully trusted. 



Besides the two chief ranks already mentioned, there is another 



1 " Micrometrical Measures of Gaseous Spectra under high Dispersion," 

 Edinburgh Philosophical Transactions, vol. xxxii. Pt. 3 (1886). The end- 

 on vacuum-tubes used in these measures and in those of the earlier paper, 

 were devised by the late Prof. Smyth himself, as described in a paper, 

 " End-on Vacuum-tubes in Private Spectroscopy," read before the Royal 

 Scottish Society of Arts in 1^79. The eminent spectroscopist of Ghent, in 

 Belgium, Dr. van Monckhoven, had, however, invented and used such 

 tubes a few years earlier. An ingenious arrangement of electrodes which 

 he applied to them in 1882 (one electrode at the foot, and one at the summit 

 of each upright leg), for passing two discharges of different strengths, either 

 simultaneously or alternately through the connecting capillary tube, in a 

 research on the effects of temperature and pressure in widening gas-spectrum 

 lines, was described in an interesting paper by Dr. van Monckhoven, in 

 Comptes rendiis, vol. xcv. (2me semestre, 1882). 



'^ "Carbon and Carbon-compounds," by Prof. A. S. Herschel, Nature, 

 vol. xxii. p. 320, August, 1880. 



3 " Researches on the Spectra of Meteorites." Proceedings of the Royal 

 Society, vol. xliii. pp. 118, 133, Map 3, November, 1887. 



