SPECTEA OF IGNITED GASES AND VAPOUES. 
15 
defined but soon merging into a single one. Like the less refracted part of the spec- 
trum, the most refracted part is developed only at a higher ignition of the vapour of 
the sulphur. At its violet extremity (we do not give here a full description of the 
middle part) we observe at the same distance from one another five well-defined fainter 
bright lines. Then follows, after an expanded violet band, a group of four bright lines, 
the second of which is accompanied by a more refracted, the fourth by a less refracted 
faint line. The fourth line especially is distinct to a degree seldom observed at so high 
a refraction and so great a power of the discharge. After two bands of faint light, 
there is seen at the end of the spectrum a group of four slightly expanded bright lines, 
preceded by an expanded violet band. 
39. Like sulphur, selenium has two spectra — one of the first, another of the second 
order. 
40. Ignited carbon, even in a state of greatest division, gives a continuous spectrum. 
41. We select, among the various compound gases which, if decomposed in flame, give 
the spectrum of carbon, in the first place cyanogen. The gas was procured by heating 
cyanide of mercury introduced into a retort of glass by means of a lamp. The flame of 
it may be fed either with oxygen or with air. 
When a jet of cyanogen mixed with oxygen is kindled, in the interior part of the flame 
a most brilliant cone of a whitish-violet light is seen, the limit between the ignited and 
the cold part of the jet. This cone exhibiting the spectrum of vapour of carbon 
best developed, we conclude that the cyanogen must be decomposed into carbon and 
nitrogen, the carbon being in the gaseous condition a moment before its combination 
with oxygen takes place*. 
42. In order to prevent explosion of the mixture of cyanogen and oxygen, it is pre- 
ferable that the jets of the two gases meet from opposite sides before the slit of the spec- 
tral apparatus, forming there, if kindled, a brilliant, flat, vertical surface. The jet of 
cyanogen might be obtained directly from the retort, by the heating of tvhich it may be 
regulated. Thus we get, all being properly arranged, a splendid and richly coloured 
spectrum. Especially we distinguish eight groups of bright lines , which, being all of 
the same general character, indicate at first sight the existence of vapour of carbon. We 
shall denote these groups, starting from the less refracted and proceeding to the more 
refracted ones, by a, b, c, d, e, f, g , h. The group a is formed by five, b by six, c by 
four, d by five, e by seven, f by three, g by seven, and h by three bright lines. But these 
lines, of a measurable breadth and a quite different appearance, are not to be confounded 
with the bright lines which, in the case of nitrogen and sulphur, for instance, constitute 
spectra of the second order. In each group the first line is the brightest ; the following, 
which are nearer to one another, decrease in intensity, and under less favourable circum- 
stances the last ones are not seen. Hence the groups, according to an expression of 
Mr. Attfield, have the appearance of a portico. The red group (a) is not always seen 
distinctly (less distinctly in the present case than in the case of other gaseous com- 
* Mr. Attfield has the merit of haying first stated that spectra hitherto attributed to compound gaseous 
substances, are to he referred to the vapour of carbon itself (Philosophical Transactions for 1862, p. 221). 
