SPECTRA OF IGNITED GASES AND VAPOIJES. 
19 
51. If the heating-power of the discharge be too strong, spectral tubes enclosing 
oxide of carbon at a higher tension showed only three large shaded fields, without 
any traces of the characteristic groups. The first two of these fields are coincident with 
the second and third of the former fields ; the third occupies the place of the fourth 
and fifth former fields united into one. Here the shading of the three large fields 
not being disturbed by any additional appearance, the transversal shading lines were 
observed most distinctly even in making use of four prisms and employing a magni- 
fying power of 36. In observing especially the light and less refracted part of the first 
field close to its extremity, these lines, on account of their extreme subtleness, are 
scarcely to be perceived ; when they begin to become well defined they are very near to 
each other; but towards the more refracted part of the field their distance increases 
simultaneously with their breadth, till, at some distance from the bright extremity, the 
dark expanded lines are resolved into small shaded bands*. 
52. Spectral tubes containing carbonic acid instead of oxide of carbon gave essen- 
tially the same spectra. The increased quantity of oxygen of the decomposed gas may 
be observed by means of the interposed jar. In such tubes there was no carbon depo- 
sited, not even after a long passage of the discharge. 
53. All compound gases enclosed in our spectral tubes are decomposed by the heat 
produced by the discharge of Ruhmkorff’s large induction coil ; but instantly after the 
discharge passes, the recomposition takes place. The recomposition is prevented only 
by a sudden cooling of the elementary gases obtained by the decomposition. Thus, for 
instance, spectral tubes enclosing cyanogen are scarcely fitted for observation, the inte- 
rior surface of their capillary part being instantaneously blackened by the deposited 
carbon. No carburetted hydrogen resists final decomposition by the passing current. 
We add only a few observations, made by means of spectral tubes. 
54. The spectrum of the light hydrocarbon gas, C 2 H 4 , obtained without the Leyden 
jar, at once showed the expanded bright lines of hydrogen and an imperfect spectrum 
of vapour of carbon, especially the brightest lines of the characteristic groups b , c, 
and d. By intercalating the jar, the hydrogen-spectrum, approaching to a continuous 
one, became quite predominant. 
Olefiant gas , C 4 H 4 , of a primitive tension of about 70 millims., gave, without the jar, 
a scarcely visible spectrum ; by intercalating the jar, the three hydrogen-lines Ha, H/3, 
Hy appeared well defined, and the spectrum of vapour of carbon, with its groups a, b, 
c , d , and its shaded large fields, well developed. 
Methyl , C 2 H 3 , showed, without the jar, at once Ha, H/3, Hy, and the characteristic 
groups e and g ; with the interposed jar these two groups disappeared, and were replaced 
by the groups a, b, c, and d. 
Acetylene, C 4 H 2 , though according to Berthelot and Morren formed from its 
* The same spectrum, but fainter, is obtained under quite different conditions. We have already noticed, 
in the introductory remarks, that in a spectral tube evacuated to the last degree by Geissler’s exhauster, 
vaporized carbon is indicated by its spectrum. The spectrum obtained is that described above (8). 
D 2 
