342 Harriey AnD RamaGE—Banded Flame-Spectra of Metals. 
bands are degraded towards the more refrangible end of thespectrum. The lines 
and bands have been described more or less perfectly by several observers, but the 
most complete account has been given by Liveing and Dewar.* They describe a 
flame-spectrum of magnesium, consisting of flutings which they attribute to a 
combination of hydrogen with magnesium, and suppose it may be a chemical com- 
pound formed only within certain limits of temperature and dependent for its 
stability on the pressure of the gaseous element, like the instance of the hydrides 
of palladium, sodium, and potassium, investigated by Troost. 
In attempting to photograph the flame-spectra of zinc and cadmium, the corre- 
sponding triplets in the visible spectrum were easily obtained; and in the spectrum 
of cadmium, a strong line of wave-length 3261 was also photographed. Traces 
of weak band-spectra were photographed in the violet part of the spectrum in 
each case; but it was only after many attempts that photographs were obtained 
upon which the bands were seen to be strongly marked. 
The lines of impurities, such as lead, indium, thallium, &c., are much stronger 
in the photographs of some samples of ‘‘ pure” metals than even those character- 
istic of the ‘‘ pure” metals themselves. This is a proof of the low emissive power 
of the radiant molecules of the elements in the oxyhydrogen flame. It may 
perhaps be attributed to a smaller degree of volatility and greater heat of com- 
bustion, with less volatility of the oxide. The heat of combustion cannot, 
however, be the explanation in the case of mercury; neither can the volatility of 
the oxide. ‘There may be a more profound cause for such a difference in these 
spectra, arising out of a peculiar mode of vibration set up within the ether by the 
monatomic molecules of the metals. 
It is possible that a flame of a higher temperature will be required to produce 
the bands in a stronger degree. This is known to be the case with the lines 
of mercury.t The bands of zinc and cadmium were photographed with greatest 
success by heating the respective metals on cyanite supports in the oxyhydrogen 
flame, the spectra being received on a ‘‘ Cadett Lightning Spectrum plate”; the 
time of exposure was for zinc 30 minutes, and for cadmium 20 minutes. The 
bands in these spectra are degraded towards the ultra-violet, and the lines of 
which they are composed are nebulous. The spectra are very complex. There 
appear to be two well-marked bands in each spectrum, and the two spectra are 
very similar in constitution. In addition to the two bands, weaker lines, which 
are components of other bands, are visible, and can be traced to the edge of the 
plate, wave-length 3530. 
We were unable to photograph either lines or bands of mercury when its 
oxide, used as a convenient source of the metal, was heated in the oxyhydrogen 
flame. Both Mitscherlich and Liveing have observed lines in the spectrum of 
* Proc. Roy. Soc., vol. 32., 1881, p. 189. } Mitscherlich, Phil. Mag., 1864, p. 178. 
