a gradual disintegration of the series of lines into simpler arrangements, caused probably by reason of a similarity existing between the ultimate constituents of our elements. This explanation is made more plausible from a study of the Zeeman magnetic effect upon similarly charged particles.
Even under the ordinarily obtainable conditions of the laboratory a great similarity may be noted between the series of lines in the spectrum of one element and the series of all other elements belonging to the same family. Thus with a gradual increase in atomic weights there occurs a corresponding gradual shifting of the series toward the red end of the spectrum. Increase in atomic complexity is ever seen to have a marked effect upon the vibratory motion of the simplest particles such that vibratory frequence is retarded. Among compounds, as well as with the elementary substances, this influence of mass is clearly shown in their spectra. Owing to the great tendency among most compounds to undergo ready decomposition when heated an examination of their spectra is restricted to the absorption spectra alone. The relations for absorption spectra having already been noted, it need hardly be further stated that the absorption bands in the spectra of compounds indicate at once the color of the compounds themselves and, what is most important of all, anything that can be brought to bear upon the interpretation of these bands and their positions should give us an insight into the cause of color as existent among compounds generally. In the examination of absorption spectra of compounds, the best results are obtained when the substances can be dissolved in some solvent which exerts but little or no absorption action for light. Among the best examples of such solvents are water, methyl alcohol (wood-spirit), and ethyl alcohol, none of which will absorb rays of a wave-length over 2,000 A.U. The absorption spectrum of a compound dissolved in a medium of this nature is identical with its absorption spectrum observed in the free state.
Among the first to obtain any positive results whatsoever in the examination of the absorption spectra of compounds was W. N. Hartley. He studied the solutions of metallic nitrates and found that the absorption in these cases was slightly modified with increase in atomic weight of the metal present, and concluded, therefore, that that portion now termed the nitrate ion—or negatively charged portion of a nitrate when dissociated by a solvent—has no effect upon the band. Not, however, until 1879, when Hartley and Huntington turned their attention to the study of absorption bands in the ultra-violet regions of the spectrum, could any hypothesis of a definite nature be formulated as regards the relation of these bands to chemical constitution. Their method of observation, which has been in use up to the present time, depended entirely upon obtaining a series of photographs of the spark spectrum as viewed through layers of a solution at varying concentra-