INTIMATE STRUCTURE ON COOLING. 



73 



other, so as to greatly increase the depth of the transmitting substance, 

 it has been found possible 1 to greatly extend the absorption spectrum 

 toward the red. 



Since the crystals are of a greenish-yellow color, they become rapidly 

 transparent as the light admitted is changed from blue to yellow; hence 

 the use of increasingly thicker layers to bring out the absorption bands. 

 To a certain extent the crystal acts as a^screen to absorb the blue light 

 which would cause fluorescence; nevertheless it was found necessary 

 to interpose orange or yellow screens of different densities to eliminate 

 fluorescence in a region where ordinarily it is at a maximum. At first 

 colored glasses obtained from Dr. H. P. Gage, of the Corning Glass 

 Company, were used as filters ; later, solutions of potassium bichromate 

 of varying concentration. It is evident that the screening must be 

 constantly changed when light from the arc is used as a background for 

 bands of increasingly longer wave-length. It was thought that a beam 

 of monochromatic light could be used as a background and thus 

 obviate exciting the crystal to fluorescence, but a preliminary study 

 indicated that such a beam of dispersed light could not be made of 

 sufficient intensity to bring out the dimmer bands. 



FIG. 61. Fluorescence bands are indicated by lines above the horizontal. Old absorption bands 

 are indicated by dotted bands below the line; new absorption bands by solid bands below the 

 horizontal. The plot shows only a portion of the complete spectra of the following salts at 

 +30 C: (1) potassium uranyl chloride; (2) ammonium uranyl chloride; (3) rubidium uranyl 

 chloride; (4) csesium uranyl chloride. 



In figure 61 is pictured a portion of the fluorescence and absorption 

 spectrum of each of the double chlorides studied. Fluorescence bands 

 are designated by lines above the horizontal line. The older, well- 

 established absorption bands are designated by dotted lines below the 

 horizontal and the new bands by solid lines below the horizontal. The 

 relative positions of the fluorescence and absorption bands are readily 

 seen. These bands appear to be of two distinct classes: 



1 Howes, H. L., Physical Review (2), xi, p. 66. 1918. 



