ADDRESS. 



charges. 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 between 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 motions of transla- 

 tion no greater than the average motions which characterise the tempera, 

 ture 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 thermo- 

 metrically, 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 correspond- 

 ing 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 aum 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 aid 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 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 re- 

 garded as distinct from flame spectra, and, from researches 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 

 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 ordinary temperature of the Bunsen flame. Recently, 

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



