424 MR WILLIAM SWAN ON THE PRISMATIC SPECTRA OF THE 
tions equal weight, and to take simply the arithmetical mean of the whole. The 
mean results of the two series, and the number of observations in each, being 
tabulated separately, the reader will be able to form some judgment regarding 
the probable accuracy of the final determinations obtained from the two series 
combined. In Fig. 1, which is a graphic construction of the observations in 
Tables I. and III., the lines were drawn by the engraver through points laid 
down by me on the copper to a scale,—adopted to suit the size of the plate,—of 
one inch to 2200”. I have ascertained the errors in the positions of these lines 
to amount, in one case only to ‘01 inch (corresponding to 22”), and to be generally 
much less; so that the spectra are represented in the figure with tolerable 
fidelity. 
In addition to the observations of the carbohydrogen and solar spectra con- 
tained in Table Il., where the deviation for each line of either spectrum was 
separately determined by the theodolite or micrometer, I have also made simul- 
taneous observations of the spectra of sun light and of olefiant gas. The gas, which 
was prepared by heating alcohol with sulphuric acid, was conducted through 
wash bottles containing caustic potash and sulphuric acid, to a gas holder; from 
which it afterwards passed, through a tube filled with pieces of quicklime, to a 
platinum jet where it was burned. 
The lines in the spectrum of olefiant gas are very distinct, being well seen 
without using the blowpipe; but like the lines in the other carbohydrogen spectra, 
they are not sufficiently luminous to be seen when projected on the solar spectrum, 
unless the latter is made so faint, that its lines have disappeared. I succeeded, 
however, in observing the spectra simultaneously, by intercepting the sun light 
which fell upon one half of a narrow slit, and illuminating the whole slit with 
the flame of olefiant gas. The gas spectrum then appeared immediately over that. 
of the sun, and the dr7ghier lines in it were well seen, especially when the flame 
was urged by the blowpipe. The intervals between the lines of the gas spectrum 
and the nearest lines of the solar spectrum, given in Table V., were measured by 
the micrometer, with a magnifying power of 21; and the observations for the 
brighter line 6, y, and 6, agree well with those of Table III. 
The line a was rarely visible in the spectrum of olefiant gas, and its appear- 
ance was only momentary, which confirms the opinion already stated, that it does 
not properly belong to the carbohydrogen spectra. To the proof already adduced 
in support of this opinion, [ may here also add, that ] have found it permanently 
absent in the flames of carbonic oxide, and of light carburetted hydrogen.* The 
continued invisibility of so brilliant a line of the spectrum, coupled with its 
* T have found that the column of heated air rising from the flame of a spirit lamp with a salted 
wick, is most energetic in communicating yellow light to the exterior envelope of the flame of the 
Bunsen lamp. This effect is apparently confined to the outer, or oxidizing portion of the flame, 
where there is no excess of hydrogen, to decompose the chloride of sodium; and the experiment is 
interesting, as tending to prove that the yellow light may be caused by simple incandescence, without 
the actual combustion of sodium. 
