4 On Double Specifa. [January, 



obtained by using the electric light the spectrum gives a 

 fourth line at 86. The sodium spectrum at the temperature 

 of the Bunsen flame consists only of the double yellow line 

 of the same refrangibility as the solar line D,but if the sodium 

 compound be ignited in the electric arc the spectrum contains 

 four other lines, each also double. 



The high temperature spectrum of sodium is represented 

 in Fig. I, on the plan proposed by Bunsen. The position 

 of the bright bands on the illuminated millimetre scale 

 of the spectroscope is shown by the position of the black 

 lines, while the intensity is indicated by the relative 

 height of the lines. In regard to the other conclusion 

 of Bunsen and Kirchhoff, that all compounds of a metal 

 give the same spectrum, we now know that this is true only 

 if the metal is one whose compounds are decomposed even 

 at the low temperature of the flame. It is well known that 

 a sufficiently high temperature causes the decomposition of 

 many chemical substances into their elements, and that 

 these re-combine when the temperature is allowed to fall 

 again. Thus, sodium carbonate and sodium nitrate give the 

 same spectrum, because each is decomposed in the flame 

 yielding metallic sodium by the incandescent vapour of 

 which the yellow line is produced. 



But we have compounds which do not split up into their 

 elements in the flame, although they are decomposed in the 

 intense heat of the electric spark. Such a substance is 

 copper chloride, which volatilises in the flame without 

 being decomposed, and gives a spectrum which is altogether 

 different from the true spectrum of copper obtained when 

 the electric spark is allowed to pass between copper poles. 

 There is no doubt that the spectrum is that of the com- 

 pound copper chloride. Fig. z shows this spectrum com- 

 pared with that of copper. 



The spectra of barium, strontium, and calcium show 

 differences at different temperatures, which are probably to 

 be explained in the same way. A reference to Fig. 3 will 

 show the great difference observed when the calcium spec- 

 trum is produced by taking the electric spark in an atmo- 

 sphere containing calcium, and when it is produced by 

 bringing a bead of calcium chloride into the Bunsen flame. 

 Figs. 4 and 5 show the spectra of strontium and barium 

 respectively under the same circumstances. It is sup- 

 posed that in the flame spectrum the incandescent sub- 

 stance to which the lines of the spectrum are due is 

 calcium or strontium oxide, so that we have the spectrum 

 of a compound, but when the electric spark is used, the 



