948 
EEV. T. E. EOBINSON ON SPECTEA OE ELECTEIC LIGHT. 
If the secondary coils be arranged for quantity, the red end is not changed, the violet 
is much brighter. 
On rarefying the gas in which the discharges are made (E,) there is at first no change, 
except perhaps a little diminution of brightness, till at a certain pressure (varying with 
the media and diameter of the tube) the spectrum fades away. Sometimes, as with 
CuO (^, e. copper electrodes in oxygen), all vanishes except a trace of the lines at 
0‘75inch; in SbN all but a trace of in NiN, at 2'8 inches, all but a suspicion of 
violet light, and sometimes perfectly dark bands take the place of bright lines. In 
others the change is not so striking. In Al, Air, at I '2 inch, the chief violet lines 
remain tolerably conspicuous ; ^ can be seen, but n and » vanish : in Pt, Air, tube 1 inch 
diameter, at 6 inches the first half fades to a neutral-tinted haze with faint alterna- 
tions of brightness, and the rest has eleven definite but faint bands. In general, how- 
ever, it may be considered the rule that from 3 inches to 1 inch these spectra almost 
vanish. I will only mention another, PbH. In hydrogen spectra, at C.P. the most 
distinctive characters are a very bright red line (found in all spectra, but not so bright) ; 
a very bright and broad one at the confines of green and blue, and a similar one in the 
violet. Of these the last diappears at 5 '3 inches; the red at 3‘I inches; the blue 
not totally till 0*15 inch ; but this is doubtful, as it may have been confounded when 
waning with another which was near its place. It deserves notice that this disappear- 
ance of the peculiar hydrogen bands is also produced when the hydrogen is diluted with 
air. The electrodes were cobalt ; 20 cubic inches of hydrogen were mixed with 10 of 
air, and the spectrum of the mixture examined; its volume was reduced to 20, 10 more 
of air added, and so on ; the degree of dilution was easily found from the number of 
additions ; and it was found that the violet band vanished when H is 0*2 of the mixture, 
the blue when 0*09. These correspond to hydrogen under the pressures 6 inches and 
2-7 inches: the red could not be determined, as air has the same band. 
If in any case we rarefy beyond the limits of these transition spectra, bright fines 
reappear, but not all in the same places or with the same characters. Ordinarily the 
brilliant lines of the C.P. spectra are wanting, though sometimes fines which are faint in 
them assume this type in the others. The red, yellow, and green seem difiused in 
cloudy light ; and the violet system is replaced by a set of broad cloudy bands neaily 
equidistant, and more conspicuous than the less refrangible ones. The red band, which 
almost always begins the C.P. spectra, is often wanting, and when it does occur is insu- 
lated in darkness, but at the other end both spectra are nearly of the same length. 
These E spectra are in general much less luminous, and show little distinction ot 
colours unless the metal of the electrodes be easily vaporized. That of tellurium is 
very bright, containing thirteen brilliant fines ; and under every circumstance of pres- 
sure or discharge, those of potassium and sodium show the dazzling orange bands. 
As might be expected from their greater faintness, they contain fewer fines, or at least 
fewer are visible ; but it is remarkable that of this number a considerable proportion is 
not found in the C.P. spectra. The percentage deduced from fifteen metals is 
