VII. THE NITRATES AND PHOSPHATES; INFLUENCE OF 

 WATER OF CRYSTALLIZATION AND OF CRYSTAL FORM. 



I. URANYL NITRATE AND EFFECT OF WATER OF CRYSTALLIZATION. 



The spectra of the different uranyl salts are so similar in their general 

 characteristics that we can scarcely doubt that the nature of these 

 spectra is chiefly determined by the radical UO 2 . Apparently the 

 uranyl radical contains a group of electrons whose arrangement is 

 such as to permit of vibrations that give this type of spectrum; and 

 although U02 is not stable in the chemical sense and must be com- 

 bined with some acid in order to form a stable compound, yet the 

 effect of the acid radical is merely to modify the constants of this 

 vibrating system in the U0 2 radical without changing the type of 

 vibration. 



It is natural to expect that the addition of water of crystallization 

 would produce a similar effect, and it is our intention to present in this 

 section of Chapter VII the results of a study of the influence of water 

 of crystallization upon the fluorescence and absorption spectrum in 

 the case of uranyl nitrate. The nitrate is particularly suited for such 

 an investigation because of the fact that several different hydrates are 

 formed. The crystals grown from a water solution contain 6 molecules 

 of water. In an acid solution crystals are formed with 3 molecules of 

 water. In both cases crystals may be obtained which are large enough 

 to permit of observations being made with a single crystal. By methods 

 described later, small crystals containing only 2 molecules of water are 

 readily obtained. It is a matter of some difficulty to push the dehy- 

 dration further, but specimens have been prepared for us by Mr. 

 D. T. Wilber which we have reason to believe are either anhydrous or 

 formed of a mixture of the anhydrous salt and the monohydrate. 



The fluorescence of the nitrate, like that of the other uranyl salts, 

 with the exception of the double chlorides, the resolution of the bands 

 of whose spectra into groups of five at +20 has been described in 

 Chapter VI, is unresolved at ordinary temperatures. Careful spectro- 

 photometric measurements of what appear to be unresolved bands 

 reveal, however, indications of overlapping components, as has already 

 been shown in Chapter III. 



At the temperature of liquid air the resolution into narrow bands 

 characteristic of the uranyl spectra in general takes place, and it is to 

 these resolved spectra that the following discussion refers. 



In the case of the hexahydrate, wave-lengths were in most cases 

 determined photographically. \ isual observations, however, were 

 also made, although these could not be extended throughout the whole 

 spectrum. The agreement between measurements made by the two 

 methods was surprisingly good. In the case of weak bands lying near 



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