THE NITRATES AND PHOSPHATES. 133 



SUMMARY OF SECTION I. 



(1) In the case of each of the nitrates, the fluorescence spectrum is 

 made up of series in which the intervals between bands are constant and 

 the same for all of the series. The interval increases slightly, but 

 unmistakably, as the amount of water of crystallization decreases. For 

 the hexahydrate the interval is 86.0, for the trihydrate 86.8, for the 

 dihydrate 88.1, and for the anhydrous salt 88.5. 



(2) Numerous constant-interval series occur in the absorption spec- 

 trum, the interval being approximately 71. But the interval does not 

 appear to be the same for different series, even when these occur in the 

 spectrum of the same salt. No systematic variation with the amount 

 of water of crystallization could be detected. 



(3) Nearly all of the series in the absorption spectrum have their 

 origin in the " reversing region," the first member of the absorption 

 series being in coincidence with the last member of a fluorescence series 

 and constituting a " reversible" band. 



(4) There is some slight resemblance between the different hydrates 

 as regards the grouping of the bands (see fig. 77). In each case, for 

 example, a certain short interval appears with a frequency consider- 

 ably above the average. In the case of the hexahydrate and the 

 dihydrate this interval is about 8 ; in the case of the trihydrate is almost 

 exactly 7. The interval 14, in the case of the trihydrate, and 16, in the 

 case of the other two salts, is also of unusually frequent occurrence. 



(5) It is clear from inspection of figure 78, in which a characteristic 

 group of bands is shown for each of the hydrates studied, that the 

 different spectra are not in the least similar in their general appear- 

 ance. It might at first appear that the three hydrates have one series 

 in common, viz, that designated as series F. But while the central 

 band of this series does occupy practically the same position in each of 

 the three spectra, the fact that the interval between bands is different 

 for the different salts causes the bands to fall more and more out of 

 step as we proceed in either direction from the center of the spectrum. 



On the whole, the spectra of the different hydrates differ from one 

 another fully as much as do the spectra of two different uranyl salts. 

 This result is surprising, since it is customary to think of water of 

 crystallization as rather loosely attached and therefore incapable of 

 exerting a great influence upon properties which depend upon the 

 internal structure of the molecule. Indeed, were the amount of water 

 of crystallization the only difference between the forms of uranyl 

 nitrate under consideration, we should look upon the attachment as 

 more intimate than has generally been supposed. 



There is, however, another distinction, that of crystalline structure, 

 and, as will appear from the subsequent sections of this chapter, the 

 crystal form has a profound influence upon the character of the fluores- 

 cence and absorption spectra of the uranyl salts. 



