i8yi.] On Double Spectra. 3 



that, although the appearance of the spectrum may be very- 

 different under different circumstances, yet the position of 

 the lines does not depend on the temperature. 



" Even the alteration of the mass of the incandescent gas 

 is sufficient to effect a change in the character of the spec- 

 trum. If the thickness of the film of vapour whose lines 

 are being examined be increased, the luminous intensities of 

 all the lines increase, but in different ratios. The intensity 

 of the bright lines increases more slowly than that of the 

 less visible rays. The impression which a line produces on 

 the eye depends on its breadth as well as its brightness. 

 Hence, it may happen that one line being less bright 

 although broader than a second is less visible when the 

 mass of incandescent gas is small, but becomes more dis- 

 tinctly seen than the second line when the thickness of the 

 vapour is increased. Indeed, if the luminosity of the whole 

 spectrum be so lowered that only the most striking of the 

 lines are seen, it may happen that the spectrum appears to 

 be totally changed when the mass of the gas is altered. 

 Change of temperature appears to produce an effect similar 

 to this alteration in the mass of the glowing vapour, no 

 deviation in the maxima of light being observed, but the 

 intensities of the lines increasing so differently that those 

 most visible at a high temperature are not those most 

 readily seen at a low temperature." 



These conclusions of Bunsen and Kirchhoff are now known 

 to be true only within certain limits. The spectrum of a 

 substance may be very considerably altered by change of 

 temperature, and these changes in the spectrum do not 

 consist merely in the alteration of the relative intensities of 

 the lines, but are caused both by the addition of new lines 

 and by the actual disappearance of lines present in the 

 spectrum produced at the lower temperature. We have in 

 the lithium and sodium spectra examples in which the change 

 caused by increase of temperature consists simply in the 

 addition of new lines, and the higher the temperature 

 the greater becomes the complexity of the spectrum. A 

 bead of lithium chloride in the Bunsen flame gives a spec- 

 trum consisting of only one red line, whose wave-length 

 is about 6684 ten-millionths of a millimetre, which cor- 

 responds to 32 of the scale to which the spectra accom- 

 panying this article are drawn. If the temperature be 

 slightly raised by employing the blowpipe, an orange line at 

 44J (wave length, 6107) makes its appearance. At the higher 

 temperature of the oxyhydrogen jet a blue line at 105 (wave 

 length, 4605) is added, while at the intense temperature 



