1871J On Double Spectra. 5 



oxide is decomposed, and we have the true spectrum of 

 the metal. It should be remarked here that the true metal 

 spectrum consists invariably of sharply defined lines, while 

 a compound gives a spectrum containing broad bands, which 

 show a family resemblance amongst themselves and are 

 often repetitions of each other. This is seen in Figs. 2, 3, 

 4, and 5. The spectra of those metals whose salts are easily 

 decomposed in the flame — for example, sodium, lithium, 

 and thallium — give spectra containing only lines, and the 

 only change producible by increase of temperature is the 

 addition of new lines, while, in the case of metals whose 

 salts are not so easily decomposed, the increase of tempera- 

 ture not only adds new lines, but also splits up the bands 

 into groups of fine lines. 



We observe the same thing amongst the non-metallic 

 elements in the case of cyanogen. If the flame of cyanogen 

 burning in air be examined with the spectroscope, a magni- 

 ficent spectrum is seen, which is obviously made up of two 

 different spectra; one stretching from the light green into the 

 blue exhibits a series of groups of lines, in each of which the 

 brightest line is towards the red, and each group fades away 

 on the side towards the blue ; but the red end of the spectrum 

 shows a series of groups of lines of exactly the opposite 

 character — the brightest line of each group being on the 

 side towards the blue, and each group fading away on the 

 red side. We shall see afterwards that the blue portion of the 

 spectrum is due to the element carbon, but the red end is pro- 

 duced by the compound cyanogen ; and if the cyanogen be 

 burnt in oxygen instead of air, the two of these groups most 

 towards the blue become replaced by carbon lines, while if 

 the gas be ignited by the electric spark, the whole of the 

 cyanogen bands disappear, and the spectrum consists alto- 

 gether of carbon lines. 



It is a coincidence which doubtless has its own signifi- 

 cance that, in all cases where by increase of temperature the 

 bands of the compound are made to give way to the lines of 

 the element, the change takes place earliest in the blue end 

 of the spectrum, and proceeds gradually towards the red. 

 The flame-spectrum of strontium contains one line SrS of 

 the metal which is seen also in the spark-spectrum. The 

 calcium flame-spectrum also contains one line (135) which 

 remains unchanged on increasing the temperature to that of 

 the spark, and in both cases these lines of the metal terminate 

 the spectrum towards the blue end.* The flame-spectrum 



*I owe to my friend Mr. Aldis the probable explanation of this peculiarity 

 as also of the way in which.the carbon bands shade off uniformly towards the blue. 



