i22 POPULAR SCIENCE MONTHLY 



a gradual disintegration of the series of lines into simpler arrange- 

 ments, 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 ex- 

 amination 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 com- 

 pound dissolved in a medium of this nature is identical with its absorp- 

 tion 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 ab- 

 sorption 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 atten- 

 tion 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- 



