CHAPTER VI. 



SUMMARY AND GENERAL DISCUSSION OF THE MOST 



IMPORTANT RESULTS. 



MAPPING OF SPECTRA. 



The first problem to be solved in a study of absorption spectra is the 

 recording of the spectra themselves. The method employed in this series of 

 investigations is that of photographing the absorption spectrum of a solution 

 placed in a beam of light having a continuous spectrum. Such a photograph 

 may be called a spectrogram or a map of the given absorption spectrum. 



The study of the spectrograms of colored solutions shows that this color 

 is due to a selective absorption of the solution in some part of the spectrum, 

 and the extent of this selective absorption may be over regions of the spectrum 

 hundreds of Angstrom units in width, or over regions only a fraction of an 

 Angstrom unit wide a width that is not much greater than that of a spark 

 or arc line. In fact, bands of almost all widths and degrees of diffuseness are 

 found in addition to the absorption, which is one-sided. If the region of one* 

 sided absorption lies in the red, it is probable that it has another edge in the 

 infra-red. Practically all solutions show an encroaching general absorption 

 in the ultra-violet as the amount of the solution in the path of the light is 

 increased or as the temperature is raised. 



The plates and the description of these plates in Publications Nos. 60, 

 110, 130, and the present monograph of the Carnegie Institution of Washing- 

 ton give a pretty thorough representation of the details of the absorption 

 spectra of most of the typical solutions of the colored inorganic salts. 



It can be said, in general, that the absorption spectra of the various salts 

 of the same element are very much alike; indeed, so much alike that it is only 

 when considerable dispersion is used that any differences can be noted. It 

 can also be said that the absorption spectra of the same salt in various sol- 

 vents are very similar. For these reasons we are justified in assuming that 

 the color of solutions of colored salts dissolved in colorless solvents is due 

 primarily to the metal or metallic radicle of the colored salt. The various 

 spectra that are characteristic of a given kind of salts, say the uranous salts, 

 will, therefore, be called the uranous spectra. 



In the mapping of the absorption spectra of solutions of increasing con- 

 centration or of increasing depth, it is always found that the absorption bands 

 widen and become more intense as the amount of salt in the path of the beam 

 of light is increased. It can also be stated, as an approximately general law, 

 that the more diffuse a band is the greater will be the widening under these 

 conditions. Examples of this kind o are shown by the uranous bands, which 

 may widen from fifty or a hundred Angstrom units to a width of thousands of 

 Angstrom units. The widening of single, very sharp bands with increasing 

 amounts of the salt in the path of the light is invariably small. As an example 

 we can take the X 4271 neodymium chloride water band. 



When characteristic absorption spectra of salts of the rare earths, of 

 cobalt, chromium, etc., are studied with the use of a high-dispersion spectro- 



87 



