August 20, 1891] 



NATURE 



373 



is not peculiar to the solar system, but is common to all the stars 

 which are visible to us. 



In the case of a star such as Capella, which has a spectrum 

 almost identical with that of the sun, we feel justified in con- 

 cluding that the matter of which it is built up is similar, and 

 that its temperature is also high, and not very different from the 

 solar temperature. The task of analyzing the stars and nebulae 

 becomes, however, one of very great difficulty when we have to 

 do with spectra differing from the solar type. We are thrown 



t' back upon the laboratory for the information necessary to enable 

 us to interpret the indications of the spectroscope as to the 

 I chemical nature, the density and pressure, and the temperature 

 } of the celestial masses. 



What the spectroscope immediately reveals to us are the 

 waves which were set up in the ether filling all interstellar space, 

 years or hundreds of years ago, by the motions of the molecules 

 of the celestial substances. As a rule, it is only when a body 

 is gaseous and sufficiently hot that the motions within its mole- 

 cules can produce bright lines and a corresponding absorption. 

 The spectra of the heavenly bodies are, indeed, to a great ex- 

 tent absorption spectra, but we have usually to study them 

 through the corresponding emission spectra of bodies brought 

 into the gaseous form and rendered luminous by means of flames 

 or of electric discharges. In both cases, unfortunately, as has 

 been shown recently by Profs. Liveing and Dewar, Wiiliner, 

 E. Wiedemann, and others, there appears to be no certain direct 

 relation between the luminous radiation as shown in the spectro- 

 scope and the temperature of the flame, or of the gaseous 

 contents of the vacuum tube — that is, in the usual sense of the 

 term as applied to the mean motion of all the molecules. In 

 both cases, the vibratory motions within the molecules to which 

 their luminosity is due are almost always much greater than 

 would be produced by encounters of molecules having motions 

 of translation no greater than the average motions which 

 characterize the temperature of the gases as a whole. The 

 temperature of a vacuum tube through which an electric dis- 

 charge is taking place may be low, as shown by a thermometer, 

 quite apart from the consideration of the extreme smallness of 

 the mass of gas, but the vibrations of the luminous molecules 

 must be violent in whatever way we suppose them to be set up 

 by the discharge ; if we take Schuster's view that comparatively 

 few molecules are carrying the discharge, and that it is to the 

 fierce encounters of these alone that the luminosity is due, then 

 if all the molecules had similar motions, the temperature of the 

 gas would be very high. 



So in flames where chemical changes are in progress, the 

 vibratory motions of the molecules which are luminous may be, 

 in connection with the energy set free in these changes, very 

 different from those corresponding to the mean temperature of 

 the flame. 



Under the ordinary conditions of terrestrial experiments, 

 therefore, the temperature or the mean vis viva of the molecules 

 may have no direct relation to the total radiation, which, on the 

 other hand, is the sum of the radiation due to each luminous 

 molecule. 



These phenomena have recently been discussed by Ebert from 

 the standpoint of the electro-magnetic theory of light. 



Very great caution is therefore called for when we attempt to 

 reason by the .nid of laboratory experiments to the temperature 

 of the heavenly bodies from their radiation, especially on the 

 reasonable assumption that in them the luminosity is not ordin- 

 arily associated with chemical changes or with electrical dis- 

 charges ; but is due to a simple glowing from the ultimate con- 

 version into molecular motion of the gravitational energy of 

 shrinkage. 



In a recent paper Stas maintains that electric spectra are 

 to be regarded as distinct from flame spectra, and from 

 researches of his own, that the pairs of lines of th^ sodium spec- 

 trum other than D are produced only by disruptive electric dis- 

 charges. As these pairs of lines are found reversed in the solar 

 spectrum, he concludes that the sun's radiation is due mainly to 

 electric discharges. But Wolf and Diacon, and later. Watts, 

 observed the other pairs of lines of the sodium spectrum when 

 the vapour was raised above the ordiiiary temperature of the 

 Bunsen flame. Recently, Liveing and Dewar saw easily, be- 

 sides D, the citron and green pairs, and sometimes the blue pair 

 and the orange pair, when hydrogen charged with sodium vapour 

 was burning at different pressures in oxygen. In the case of 

 sodium vapour, therefore, and presumably in all other vapours 

 and gases, it is a matter of indifference whether the necessary 



NO. I 138, VOL. 44] 



vibratory motion of the molecules is produced by electric dis- 

 charges or by flames. The presence of lines in the solar spec- 

 trum which we can only produce electrically, is an indication, 

 however, as Stas points out, of the high temperature of the 

 sun. 



We must not forget that the light from the heavenly bodies 

 may consist of the combined radiations of different layers of gas 

 at different temperatures, and possibly be further complicated to 

 an unknown extent by the absorption of cooler portions of gas 

 outside. 



Not less caution is needed if we endeavour to argue from the 

 broadening of lines and the coming in of a continuous spectrum 

 as to the relative pressure of the gas in the celestial atmospheres. 

 On the one hand, it cannot be gainsaid that in the laboratory 

 the widening of the lines in a Pliicker's tube follows upon in- 

 creasing the density of the residue of hydrogen in the tube, when 

 the vibrations are more frequently disturbed by fresh encounters, 

 and that a broadening of the sodium lines in a flame at ordinary 

 pressure is produced by an increase of the quantity of sodium in 

 the flame ; but it is doubtful if pressure, as distinguished from 

 quantity, does produce an increase of the breadth of the lines. 

 An individual molecule of sodium will be sensibly in the same 

 condition, considering the 'relatively enormous number of the 

 molecules of the other gases, whether the flame is scantily or 

 copiously fed with the fodium salt. With a small quantity of 

 sodium vapour the intensity will be feeble except near the 

 maximum of the lines ; when, however, the quantity is increased, 

 the comparative transparency on the sides of the maximum will 

 allow the light from the additional molecules met with in the 

 path of the visual ray to strengthen the radiation of the mole- 

 cules farther back, and so increase the breadth of the lines. 



In a gaseous mixture it is found, as a rule, that at the same 

 pressure or temperature, as the encounters with similar molecules 

 become fewer, the spectral lines will be affected as if the body 

 were observed under conditions of reduced quantity or tem- 

 perature. 



In their recent investigation of the spectroscopic behaviour of 

 flames under various pressures up to fortyatmospheres, Profs. Live- 

 ing and Dewar have come to the conclusion that, though the pro- 

 minent feature of the light emitted by flames at high pressure 

 appears to be a strong continuous spectrum, there is not the 

 slightest indication that this continuous spectrum is produced by 

 the broadening of the lines of the same gases at low pressure. 

 On the contrary, photometric observations of the brightness of 

 the continuous spectrum, as the pressure is varied, show that it 

 is mainly produced by the mutual action of the molecules of a 

 gas. Experiments on the sodium spectrum were carried up to a 

 pressure of forty atmospheres without producing any definite 

 effect on the width of the lines which could be ascribed to the 

 pressure. In a similar way the lines of the spectrum of water 

 showed no signs of expansion up to twelve atmospheres ; though 

 more intense than at ordinary pressure, they remained narrow 

 and clearly defined. 



It follows, therefore, that a continuous spectrum cannot be 

 considered, when taken alone, as a sure indication of matter in 

 the liquid or the solid state. Not only, as in the experiments 

 already mentioned, such a spectrum may be due to gas when 

 under pressure, but, as Maxwell pointed out, if the thickness of 

 a medium, such as sodium vapour, which radiates and absorbs 

 different kinds of light, be very great, and the temperature high, 

 the light emitted will be of exactly the same composition as that 

 emitted by lamp-black at the same temperature, for the radia- 

 tions which are feebly emitted will be also feebly absorbed, and 

 can reach the surfscc from immense depths. Schuster has shown 

 that oxygen, even in a partially exhausted tube, can give a con- 

 tinuous spectrum when excited by a feeble electric discharge. 



Compound bodies are usually distinguished by a banded spec- 

 trum ; but, on the other hand, such a spectrum does not neces- 

 sarily show the presence of compounds — that is, of molecules 

 containing different kinds of atoms— but simply of a more com- 

 plex molecule, which may be made up of similar atoms, and be, 

 therefore, an allotropic condition of the same body. In some 

 cases— for example, in the diffuse bands of the absorption spec- 

 trum of oxygen— the bands may have an intensity proportional 

 to the square of the density of the gas, and may be due either 

 to the formation of more complex molecules of the gas with in- 

 crease of pressure, or it may be to the constraint to which the 

 molecules are subject during their encounter with one another. 



It may be thought that at least in the coincidences of bright 

 lines we are on the solid ground of certainly, since the length of 



K 2 



