618 



SPECTRUM 



the son, of nitric peroxide, and of Sinn- ; that of the 

 nebula in Orion almost rivals in simplicity tin' vi-iKle 

 spectrum of sodium vapour. The case of Thallium 

 (c|.v. ) is of peculiar historic interest, since it was the 

 observation of its very characteristic line spectrum 

 which led to its discovery. Of even greater interest 

 historically is the spectrum of sodium, which may 

 be observed by burning common salt in a spirit- 

 flame. Fraunnofer observed that the two bright 

 yellow lines so characteristic of the sodium spectrum 

 coincided in position with the double line known 

 as D iu the solar spectrum. A very careful test of 

 this coincidence was made by Professor Miller, 

 following upon which Stokes (in 1850) gave for the 

 first time the physical explanation of the pheno- 

 menonviz, that'the Fraunhofer double D is pro- 

 duced by the absorptive action of sodium vapour 

 in the sun's atmosphere. Foucault (in 1849) had 

 already obtained on evident darkening of the D 

 lines when the ray of sunlight was passed through 

 the electric arc, which gave in its spectrum the 

 bright sodium lines ; but he failed to grasp the 

 significance of the experiment. Ten years later 

 Kirchhoff made a similar experiment, and to him 

 we owe the complete statement of the principle on 

 which spectrum-analysis is based. (For the im- 

 portant work of Balfour Stewart in this connection, 

 see HEAT.) The principle is denned by Kirchhoff 

 thus : the ratio of the emissive and absorptive 

 }x>wers for any given radiation is the same for all 

 bodies at the same temperature. If we imagine 

 the existence of an ideal black body which is at 

 once a perfect absorber and a perfect radiator, 

 we may, following Tait in his development of 

 Stewart, express the principle in this wise : for 

 any given temperature the emissivity of a radi- 

 ating body is equal to its absorptivity. Here 

 emissivity is the emissive power of the chosen 

 body compared with that of the ideal block 

 Ixxly ; and similarly absorptivity is the ratio of 

 the absorptive powers of the chosen body and 

 the black body for the same radiation at the same 

 temperature. Suppose we have a body A exposed 

 to radiation r from a body B. If A were black 

 the whole radiation would oe absorbed. As it is, 

 however, the body A will absorb only er, where e 

 is the emissivity. Again, if K is the measure of 

 the radiation which a black body at the tempera- 

 ture of A would radiate, .ell will measure the 

 radiation of A. Hence the amount of radiation 

 which reaches us from A, and through A from B, 

 will be eR + (r - er) = r - e (r - R). Hence 

 there will be a real resultant absorption by A as 

 the rays from B pass through it if, and only if, r is 

 greater than H i.e. in accordance with experience, 

 if B is at a higher temperature than A. The ulti- 

 mate basis of the argument is the Second Law of 

 Thermodynamics (q.v.); and it should be noted that 

 the principle fails to apply to cases of phosphor- 

 escence or fluorescence. Thus we conclude that 

 the Fraunhofer lines in the solar spectrum are due 

 to the absorptive action of the comparatively cool 

 atmosphere of the sun upon the radiation which 

 comes from the hotter interior parts. At the 

 instant of a total eclipse of the sun, when the hot 

 interior is screened off, the spectrum of the cooler 

 but still self-luminous envelo]ie is seen to consist 

 of several bright linos. With the exception of one 

 peculiar l : :ic in the yellow, these are all coincident 

 in (Minition with certain of the dark Praunhofer 

 lines. The most conspicuous of the lines that so 

 become reversed are the four hydrogen lines. The 

 It line due to magnesium, the double D, and some 

 of the iron line- have also been observed reversed 

 at the instant of totality. 



The identification of the dark lines in solar and 

 stellar spectra with the bright lines in the spectra 

 of the various elementary substances raised to a 



high enough temperature is one of the most im- 

 portant labours of the spectroscopist. A list 

 of the elements which have leen proved to exist 

 in the solar atmosphere will be found in the article 

 SUN. In fig. 4 a small portion of the sun's 

 spectrum near the 

 6 line is given, 

 showing the 

 identification of 

 certain constitu- 

 ents of the sun's 

 atmosphere with 

 iron, magnesium, 

 nickel, and cal- 

 cium. 

 The character of 



i 



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' NtOJKEL CALCIUM 



"MAGNESIUM 



I 



L. 



the spectrum of a 

 given substance 

 changes with tem- 

 perature and pres- Fig. 4. 

 sure. For exam- 

 ple, although hydrogen, like all gases, gives at 

 ordinary pressures a bright line spectrum with sharp 

 thin lines, these lines become broader and broader 

 as the pressure is increased, until at very liif,'h 

 pressures the spectrum becomes almost continuous 

 like that given by a glowing white hot solid. 

 Thus we learn that a highly compressed gas at a 

 high temperature ceases to give the discontinuous 

 bright line spectrum so characteristic of it at low 

 pressures. One tolerably safe conclusion to draw 

 is that stars which all have continuous spectra 

 crossed by dark absorption lines or bonds consist 

 of a highly condensed nucleus ; whereas true 

 nebulse, which show bright line spectra (see the 

 table of spectra), are luminous because of the 

 presence of glowing gas in a comparatively atten- 

 uated condition. In the case of Comet* (a.v.) the 

 spectrum is faintly continuous with bright lines 

 crossing it a mingling of solar reflected light with 

 the proper gaseous spectrum of the comet itself. 

 The planete give in like manner the spectrum of 

 sunlight modified more or less by the absorptive 

 character of their atmospheres. 



If a ray of sunlight, or a ray from the electric or 

 lime light, is passed through various liquids, very 

 characteristic absorption bands are obtained across 

 the otherwise continuous spectrum. For example, 

 arterial and venous bloods give absorption spectra, 

 which are readily distinguishable one from the 

 other. The second spectrum in the plate is an 

 absorption spectrum produced by passing the electric 

 ray through peroxide of nitrogen. It shows the 

 banded characteristics of such spectra. 



A very remarkable application of spectrum- 

 analysis 'is to the measurement of the rate of 

 approach or recession of any heavenly body. If 

 we are approaching a star the waves of light will 

 meet us at a somewhat quicker rate than if we 

 were relatively steady with regard to it. That is. 

 the waves of Tight will appear to be shorter hence 

 all the lines in the spectrum will IK; displaced 

 towards the violet end. On the other hand, if we 

 are receding from the star, the spectrum lines vill 

 appear to be shifted towards tne red end. For 

 example, in the spectrum of Sinus, the F line is 

 very slightly shifted towards the red by an amount 

 which is measurable in a fine spectroscope. The 

 interpretation is that Sirins is receding from the 

 solar system with a velocity of about 20 miles per 

 secondl Arcturus. on the other hand, is approach- 

 ing our system with a speed of 55 miles per second. 

 Similar displacements of lines are observed in the 

 spectra of certain sun-spote, which are thereby 

 proved to consist of downrushes of gas. 



Throughout this article we have confined our 

 attention to the visible part of solar spectrum. 

 But this extends much further than is apparent 



