8 EEPOKT 1891. 



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 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 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 increasing 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 sodium 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 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 quantity or temperature. 



In their recent investigation of the spectroscopic behaviour of flames 

 under various pressures up to forty atmospheres, Professors Liveing and 

 Dewar have come to the conclusion that though the prominent feature of 

 the Kght emitted by flames at high pressure appears to be a strong con- 

 tinuous spectrum, there is not the slightest indication that this continu- 

 ous 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 



