Hartley and Ram age — Banded Flame-S'pectra of Metcds. 351 



(5). It is manifest that elements belonging to the same group in the j^eriodic 

 system of classification exhibit banded spectra which are similarly constituted, and 

 hence similarly constituted molecules of the elements have similar modes of vibra- 

 tion, whether at the lower temperature of the flame or at the higher temperature of 

 the arc. 



Banded spectra are thus shown to be connected with the periodic law. 



NOTE. 



It has not appeared necessary to modify the view expressed by one of the authors in the first part of 

 " Flame-Spectra at High Temperatures," namely, that the banded speotraof the elements are not primarily 

 due to au allotropic condition of their vapours, nor solely to a lower temperature of the flame as compared 

 with arc and spark spectra, but to the greater vapour pressure of these substances at the lower tempera- 

 ture, combined with the incandescence of their vapours. It is quite possible to imagine that such elements 

 as lead, antimony, bismuth, and tellurium, under considerable pressures, yield continuous spectra, while, by 

 lowering the pressure, the principal edges of the bands would alone appear, and even, these would vanish 

 until only lines remained. We have only to contrast what occurs in the combustion of hydrogen with 

 oxygen at atmospheric pressures and under the pressure of thirty or forty atmospheres, wherein line- 

 spectra of feeble radiant power become continuous and highly luminous spectra. It will be well to recall 

 the instances from which the original deductions were drawn, namely, the case of bismuth containing 

 lead and silver as impurities. The line-spectra of lead and silver appear as if laid upon the band-spectrum 

 of bismuth, but no band of lead or silver can be traced. Since the temperature of the flame is not reduced, 

 the only difference affecting the metals constituting impurities is one of vapour-pressure. The entire 

 pressure of the mixed vapours is divided proportionally amongst the constituents of the mixture. If, for 

 instance, the lead be one per cent, by weight of the mixed vapours, the partial pressure will be i^tr of an 

 atmosphere. Let us consider now the case of a substance like mercury or arsenic, both of which are very 

 easily vapourised. The former, which is the best example, since mercury at the temperature of the 

 oxyhydrogen flame is not oxidised, would evaporate so freely that it would pass out of the flame without 

 attaining the temperature necessary for its incandescence. In the case of arsenic, we have this difference 

 that, though the substance is easily vaporised, it is also at the same time oxidised, so that, in addition to 

 the heat of the flame, we have the heat of combustion of the element causing incandescence. Substances, 

 such as silver and gold with very high boiling points, which are not oxidisable metals, easily attain a 

 temperature at which the vapour is incandescent, because the quantity of substance in the flame is not 

 large. Other metals, which are easily oxidisable and have high boiling points, evolve a large amount of 

 heat on combustion. Magnesium, zinc, and aluminium are conspicuous examples, and hence their vapours 

 are rendered incandescent by the act of oxidation. Deductions as to the vapour-pressure of the substance 

 in the flame were drawn also from the spectrum of manganese and alloys of manganese and iron. In tool- 

 steel only the principal lines in the violet and a faint indication of bands in the yellowish green are seen ; 

 in spiegel-eisen the lines and the bands are well seen, while, in ferro-manganese and in pure manganese 

 metal, the bands are the principal features, the spectra consisting almost entirely of bands ; indeed in 

 pure manganese even the group of lines in the violet becomes a band. 



It may also be remarked that the yellow lines of sodium become a band when the quantity of substance 

 increases in the flame, until in fact, finally a continuous spectrum is the result in which a strong band 

 appears — "W. N. Haetlet. 



TRANS. ROY. DUB. SOC, N.S. VOL. VII., PART XII. 3 2 



