Physical Constitution of the Sun, 327 



de Boisbaudran*, with regard to Wiillner's experiments on the va- 

 riation of the spectra with pressure and temperature, that these 

 results must be applied only with the greatest care to the phe- 

 nomena of the pressure-variations of the sojar atmosphere, inas- 

 much as these alterations in the spectra are influenced far more 

 by changes of temperature than by changes of pressure. Even 

 if this suggestion should be established by future experiments, 

 the results developed in the foregoing memoir would only be 

 very slightly affected; for the constitution of the function 

 (formula V.) which has served us for the determination of the 

 temperature of the atmosphere is such that the pressure^, at 

 which the hydrogen spectrum becomes continuous, may be 

 altered within very wide limits without necessitating a corre- 

 sponding alteration in the temperature. Thus, if we employ the 

 two extremes of the assumed pressures 1 : 10, we find that the 

 corresponding values for the temperatures stand in the ratio of 1 

 to 1*15 only. 



Nevertheless it must be admitted that the exact elimination of 

 the influences which pressure and temperature exert on the con- 

 stitution of the spectrum is to be regarded as a problem the 

 solution of which is of the greatest importance for Stellar 

 Physics. 



Perhaps by the application of the well-known laws of the heat 

 of the current and of Gay-Lussac's law, it may prove possible so 

 to regulate the pressure of the gas by alteration of the level of 

 the mercurial column, that the increase of pressure produced by 

 the elevation of temperature of a powerful discharge may be com- 

 pensated for by a reduction of pressure before the discharge. In 

 this way the pressure could be kept constant, and without know- 

 ing the temperature itself the effects of the changes of tempera- 

 ture on the spectra could be readily observed. In this case the 

 loss of heat by conduction and radiation experienced during the 

 short period of the discharge is neglected, and the quantity of 

 heat evolved in the circuit is considered to be approximately pro- 

 portional to the temperature of the incandescent gaseous mass. 

 If this mass is known, it would be possible to calculate a superior 

 limit for the absolute temperature of the incandescent gas, pro- 

 vided the duration of the discharge and the quantity of heat de- 

 veloped during such discharge were determined. 



* Comptes Rendus, vol. Ixx. p. 1091 (May 16, 1870). 



