212 



SCIENCE 



[Vol. XVIII. No. 454 



Of not less interest are the experiments of Enut Angstrom, 

 which show that the carbonic acid and aqueous vapor of the 

 atmosphere reveal their presence by dark bands in the in- 

 visible infra-red region, at the positions of bands of emission 

 of these substances. 



It is now some thirty years since the spectroscope gave us 

 for the first time certain knowledge of the nature of the 

 heavenly bodies, and revealed the fundamental fact that ter- 

 restrial matter is not peculiar to the solar system, but is com- 

 mon to all the stars which are visible to us. 



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

 trum almost identical with that of the sun, we feel justified 

 in concluding that the matter of which it is built up is simi- 

 lar, and that its temperature is also high, and not very dif- 

 ferent from the solar temperature. The task of analyzing 

 the stars and nebulae becomes, however, one of very great 

 difiiculty when we have to do with spectra diifering from 

 the solar type. We are thrown back upon the laboratory 

 for the information necessary to enable us to interpret the 

 indications of the spectroscope as to the 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 molecules can produce bright lines and- a corre- 

 sponding absorption. The spectra of the heavenly bodies 

 are, indeed, to a great extent 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 

 Professors Liveing and Dewar, Wiillner, E. Wiedemann, 

 and others, there appears to be no certain direct relation be- 

 tween the luminous radiation as shown in the spectroscope 

 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 mo- 

 tions of translation no greater than the average motions 

 which chara:cterize the temperature of the gases as a whole. 

 The temperature of a vacuum tube through which an electric 

 discharge is taking place may be low, as shown by a ther- 

 mometer, quite apart from the consideration of the extreme 

 smallness of the mass of gas, but the vibrations of the lumi- 

 nous 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 dis- 

 charge, and that it is to the fierce encounters of these alone 

 that the luminosity is due, then if all the molecules had simi- 

 lar 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 tem- 

 peratui'e of the flame. 



Under the ordinary conditions of terrestrial experiments, 

 therefore, the temperature or the mean vis viva of the mole- 

 cules may have no direct relation to the total radiation, 

 which, on the other hand, is the sum of the radiation due to 

 eacli 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 aid of laboratory experiments to the tem- 

 perature of the heavenly bodies from their radiation, espe- 

 cially on the reasonable assumption that in them the lumi- 

 nosity is not ordinarily associated with chemical changes or 

 with electrical discharges, but is due to a simple glowing 

 from the ultimate conversion 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 re- 

 seai'ches of his own, that the pairs of lines of the sodium 

 spectrum other than D are produced only by disruptive 

 electric discharges. As these pairs of lines are found re- 

 versed in the solar spectrum, he concludes that the sun's ra- 

 diation 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 vapor was raised above 

 the ordinary temperature of the Bunsen flame. Recently, 

 Liveing and Dewar saw easily, besides D, the citron and 

 green pairs, and sometimes the blue pair and the orange 

 pair, when hydrogen charged with sodium vapor was burn- 

 ing at different pressures in oxygen. In the case of sodium 

 vapor, therefore, and presumably in all other vapors and 

 gases, it is a matter of indifference whether the necessary 

 vibratory motion of the molecules is produced by electric 

 discharges or by flames. The presence of 'lines in the solar 

 spectrum which we can only produce electrically is an indi- 

 cation, 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 fur- 

 ther complicated to an unknown extent by the absorption of 

 cooler portions of gas outside. 



Not less caution is needed if we endeavor 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 celes- 

 tial atmospheres. On the one hand, it cannot be gainsaid 

 that in the laboratory the widening of the lines in a Pliick- 

 er's tube follows upon increasing the density of the residue 

 of hydrogen in the tube, when the vibrations are more fre- 

 quently disturbed by fresh encounters, and that a broadening 

 of the sodium lines in a flame at ordinary pressure is pro- 

 duced 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 scan- 

 tily or copiously fed with the sodium salt. With a small 

 quantity of sodium vapor the intensity will be feeble except 

 near the maximum of the lines; when, however, the quan- 

 tity 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 molecules 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 quan- 

 tity or temperature. 



In their recent investigation of the spectroscopic behavior 



