THE NITRATES AND PHOSPHATES. 127 



this respect the systematic relation of fluorescence and absorption is 

 not so completely brought out as in the spectra of the chlorides. 



The observed intervals between bands are not so nearly constant in 

 the case of the absorption series as in the fluorescence series. It seems 

 to us probable that this is due to the greater uncertainty in the wave- 

 length determinations; for on account of the lack of sharpness of the 

 absorption bands and their greater width, as compared with fluorescence 

 bands, the accuracy that is attainable in determining their location is 

 considerably less than in the fluorescence spectrum. 



It will be noted also that the interval between bands is different for 

 different series. For series e the average interval is 73.1 ; for series d' it 

 is 71.7; for series h, 70.3; while for the two pairs of lines in the ultra- 

 violet, which have been designated as series ft and 7, the interval is in 

 one case 69.0 and in the other 68.9. This change in interval as we pass 

 from one series to another, which is too great to be accounted for by 

 experimental errors, appears of especial significance when it is remem- 

 bered that in the fluorescence spectrum the interval is the same for all 

 the series. 



One of the most puzzling points brought out by the detailed study of 

 the observed spectra is the fact that a considerable number of the 

 absorption bands are spaced with the interval corresponding to the 

 fluorescence series, and in some cases appear to form a continuation 

 of these series. In such cases the reciprocal wave-lengths for these 

 bands are included in table 40, but are preceded by the letter A. Thus, 

 there are four absorption bands, in addition to the reversible band, 

 which apparently belong to series F. If it is assumed that they do 

 form a part of this series, the average interval for the whole series comes 

 out exactly the same as for the fluorescence bands alone. Series J 

 appears to include two absorption bands, and series B, E, and G each 

 show one band. On the whole, however, we are inclined to look upon 

 these cases as the result of accidental coincidences and to believe that 

 the fluorescence series do not extend into the absorption region beyond 

 the reversible band. 



The interval of about 70, which appears to be characteristic of the 

 absorption spectrum, is also found in the fluorescence spectrum. Thus 

 the bands of series C are displaced from those of series E by intervals 

 ranging from 69.0 to 70.2. A similar relation appears to exist between 

 series J and series H, the average displacement being 70.1. Between 

 series B and A the average shift is 70.6, between series F and H, 68.1, 

 between series D and B, 69.1. With the exception of series I the series 

 thus seem to be grouped in pairs, the interval between pairs being in 

 the neighborhood of 70. It seems not unlikely that a companion series 

 exists for I also, since faint bands were occasionally observed at 1,826.0 

 and 1,911.7 (interval 85.7) which are displaced by the intervals 71.3 

 and 71.4 from the corresponding bands of series I. 



