io On Double Spectra. [January, 



the brightest, and each succeeding line is less brilliant than 

 the one before ; so that the group rises sharply out of dark- 

 ness on the left, and fades gradually away on the right. The 

 group 8, which contains four lines, presents the same gra- 

 dation of intensity ; e contains four lines of nearly equal 

 intensity, the fourth being double; and /consists of a broad 

 band, then a fine bright line, and then a band fading away 

 on the most refracted side. When the spectrum is obtained 

 very brightly, there may be observed in addition six very fine 

 bright lines of equal intensity, which gave the readings 86, 

 87'5> 89, 91, 93, 95. The band 128 — 133 is also seen to be 

 shaded by a large number of nearly equidistant fine dark 

 lines; and the least refrangible band of the group/ (121 — 126) 

 is resolved into lines. 



This spectrum may be obtained from the flame of any 

 hydrocarbon, though in many caess, owing to the faintness 

 of the spectrum, only some of the groups can be recognised. 

 In the flame of an ordinary Bunsen burner 8 and e are easily 

 seen, 7 and /are much fainter, and the red group cannot be 

 detected. 



When the temperature is sufficiently high, we have, 

 instead of the group /, two other groups, £ and 6, which are 

 also represented in the chromolithograph. Group f contains 

 seven lines (105 — 113) and group 6 contains six (136 — 142). 



The lines of the second carbon spectrum were first 

 observed by Pliicker in 1859, and supposed by him to be the 

 lines of the compound carbonic acid, but the fact that they 

 really constitute a spectrum of carbon, since they can be ob- 

 tained either from carbonic Oxide or from olefiant gas, was 

 first noted by Roscoe in 1864. This spectrum is represented 

 in Fig. 2 of the chromolithograph. 



The third carbon spectrum is that of the flame which 

 issues from the converter in the Bessemer steel process, in 

 which air is forced through molten iron. It is represented 

 in the chromolithograph, Fig. 3, and is remarkable because 

 it consists of groups of lines, in each of which the brightest 

 line is the most refrangible — an aspect which is exactly the 

 reverse of that so noticeable in the ordinary carbon-spectrum, 

 where each group has its strongest line on the left hand. 

 These lines are unquestionably produced by carbon in 

 some form or other. They disappear when all the carbon 

 has been burned out of the iron, and their disappearance 

 forms the most delicate test by which to determine the right 

 point to stop the blast. 



The fourth carbon spectrum is obtained when the spark 

 from an induction coil is taken in carbonic acid, and a 



