the Nature of Spectra. 85 



forces conditioning in these the vibrations of the aether are 

 of an intrinsically different nature from those in the mole- 

 cules built up out of the atoms of the same elements; but when 

 the grouping of the atoms in the compounds and in the mo- 

 lecules of the elements composing them are similar, the general 

 appearance of the spectra given by both will also be the same, 

 even if the individual lines in them do not coincide. Thus it 

 is with the absorption-spectra of iodine vapour, vapour of bro- 

 mine, and the vapour of monochloride of iodine closely in- 

 vestigated by Roscoe and Thorpe. In like manner must com- 

 pounds of analogous composition also furnish similar spectra 

 — as, for example, the haloid compounds of the earth-metals, 

 mercury, &c. 



If we can succeed in determining for the band spectra of 

 the elements or compounds the fundamental vibrations, and if 

 the motions are conditioned by the forces in action between 

 the atoms, we must be able to determine these in absolute and 

 relative measure from the absolute weight and mutual distances 

 of the atoms, which are at any rate determinable by the newer 

 theory of gases. Intimations for such investigations we find 

 in the displacements which, according to Mitscherlich's, Lecoq 

 de Boisbaudran's, and others' observations, certain groups of 

 lines undergo on the transition from a chlorine to a bromine 

 or an iodine compound. From the thermal processes alone 

 which accompany the formation and decomposition of the com- 

 pounds this is indeed not possible, because, even when we let 

 the elements act upon one another in the gaseous state, the 

 quantities of heat that appear are still conditioned by the in- 

 determinable separation of the atoms which form the molecules 

 of the elements. 



As we ascribe the band spectra to combined, but the line 

 spectra to the atoms isolated at higher temperatures, it can be 

 alleged that cwteris paribus the former more readily occur 

 with bodies which are with more difficulty subject to che- 

 mical actions, and therefore to decompositions, than with less- 

 stable bodies. G. Wiedemann has pointed out that by an elec- 

 tric discharge which corresponds to the compensation of equal 

 differences of potential, equal amounts of heat are generated 

 when it passes through different gases which are present in 

 the same capillary tube and under equal pressure. If these 

 gases (for example, hydrogen and nitrogen) possess equal 

 specific heat even at the temperatures to which they are heated 

 by the discharges, and disintegration does not occur, the tem- 

 peratures attained must also be equal. Nevertheless nitrogen, 

 which is chemically more stable, shows the band spectrum, 

 hydrogen the line spectrum. Chlorine, bromine, and iodine, 



