THE NITRATES AND PHOSPHATES. 



141 



Again, in the case of the uranyl nitrate, the crystals of the hexa- 

 hydrate are of the rhombic system, while those of the trihydrate and 

 dihydrate are of the triclinic system. In spite of slight shifts due to 

 changing molecular weight, the strong bands of the two spectra pro- 

 duced by the crystals of the triclinic system agree fairly well, while the 

 strong bands of the spectrum produced by the rhombic crystal reside 

 in entirely different positions. 



There is one more bit of evidence which adds weight to the above 

 view. The chemical formulae of the two potassium salts are more 

 nearly alike than those of the two ammonium salts, since the di- 

 ammonium salt has 2 molecules of water of crystallization, while the 

 other salts have none, yet there is a greater difference between the 

 first and second spectra than there is between the second and fourth 

 spectra. 



SUMMARY OF SECTION II. 



(1) The spectra of the double uranyl nitrates resemble those of the 

 previously studied uranyl salts in that the bands can be arranged in 

 series having constant frequency intervals. 



(2) These intervals, while constant for any given series, are different 

 for different series. 



(3) In the mono-ammonium uranyl nitrate and the mono-potassium 

 uranyl nitrate the ratio of the interval of a fluorescence series to the 

 interval of the absorption series which joins that fluorescence series is 

 approximately a constant. 



(4) Although the grouping of the bands shows a strong family 

 resemblance in the four spectra, yet the absolute position of a group 

 is largely determined by the crystal system. 



III. RESOLUTION ON COOLING AND ITS DEPENDENCE ON 

 CRYSTALLINE STRUCTURE. 



The crystal system of any uranyl compound is an important factor 

 in determining the character of its fluorescence and absorption spectra, 

 as we have endeavored to show in the foregoing section. There is 

 equally good evidence that resolution is dependent on the existence of 

 a crystalline condition. 



TABLE 51. Bands of fluorescence in canary glass. 1 



1 R. C. Gibba, Physical Review (1), vol. 30, p. 382. 



