SPECTRA OF IGNITED OASES AND VAPOURS. 
25 
opposing a comparatively small resistance to the passing current, we found it useful to 
intercalate at the same time a Leyden jar and a stratum of air. Thus, indeed, by regu- 
lating as well the density of the vapour as the thickness of the stratum, we obtained 
the best-developed spectrum. 
The least quantity of mercury, if vaporized, becomes visible by the passing current. 
Especially when mixed with other metals like arsenic, antimony, &c., we may detect 
even the least traces of it, which would entirely elude chemical analysis. Thus, for 
instance, we observed that arsenic, whatever may be its origin, is not free from mercury. 
After introducing a small quantity of it, which we heated by an alcohol-lamp when we 
placed it before the slit of the spectral apparatus, in a few moments four lines of great 
brightness, among which was a double yellow one, rose from a dark ground, but before 
the spectrum was fully developed it was abruptly replaced by another quite as brilliant. 
The first spectrum obtained belongs to vapour of mercury, first developed by evapora- 
tion, the second to arsenic, which increasingly vaporized at a higher temperature dis- 
putes the conduction of the discharge with the mercury, the vapour of which, according 
to its small existing quantity, reaches only a very low limit. The spectrum of arsenic 
remaining alone, gradually increased in brilliancy by the development and expansion of 
its bright lines. In cooling the spectral tube, by taking off the lamp, the spectrum of 
arsenic lost its extreme brilliancy; well-defined bright lines, the number of which 
gradually diminished, rose from a dark ground, and were replaced again by the spectral 
lines of mercury, till finally all light was extinguished. 
68. The metals of alkalies, sodium, potassium, lithium, thallium show, even at the 
lower temperature of Bunsen’s lamp, a spectrum of the second order, consisting of bright 
lines, the number of which is increased by the higher temperature of the current, while 
the principal ones are expanded. 
69. Barium, strontium, calcium show, even in Bunsen’s lamp, shaded bands, and a 
bright chief single line at the same time. This line, green in the case of barium, bluish 
violet in the case of strontium, violet in the case of calcium, fully exhibits the character 
of the bright lines in the spectra of the second order. The bands, if well developed, 
constitute a spectrum of the first order. We examined especially the spectrum of 
barium, by introducing its chloride into the hydrogen-flame. In making use of two 
prisms and employing a magnifying power of eighteen, we distinctly obtained the shading 
of the bands resolved into dark lines, finer and closer to one another than in former 
similar cases. Thus we proved that the band-spectrum of baryta is in every respect a 
spectrum of the first order. 
70. Spectra of the first order were observed in the case of a few heavy metals only. 
Among these metals we mention in the first instance lead. We obtain its spectrum in 
Bunsen’s lamp, but in order to get it beautifully developed we must make use of the 
oxyhydrogen flame. The spectra we obtained were identically the same whatever com- 
pound of lead was introduced into that flame. We especially examined its combinations 
with chlorine, bromine, iodine, and oxygen. In all cases we observed larger bands, 
MDCCCLXV. e 
