PROFESSOR STOKES ON THE CHANGE OF REFRANGIBILITY OF LIGHT. 517 
Compounds of Uranium. 
137. Towards the end of last autumn, when the lateness of the season afforded 
but few opportunities for observation, I learned from different sources that the kind 
of yellow glass which has been already mentioned as possessing in so high a degree 
the property of internal dispersion was coloured with oxide of uranium. This ren- 
dered it interesting to examine other compounds of uranium ; and I accordingly 
procured some crystallized nitrate of the peroxide, which, with a few other com- 
pounds formed from it, and some of the natural minerals which contain uranium, 
were examined by methods which have been already explained. 
138. The crystals of the nitrate were not sufficiently large and perfect to admit of 
observation by the methods applicable to fluids and clear solids, but they could be 
readily observed by means of a linear spectrum. They proved to be sensitive in a 
very high degree, dispersing a green light which had the same very remarkable com- 
position that has been already described in the case of the yellow glass. On placing 
a crystal in the continuation of the same linear spectrum with the glass, and viewing 
the whole through a prism, the five bright bands of which the derived spectrum given 
by each of the two media usually consisted, appeared to correspond to one another as 
regards their position in the spectrum. With great concentration of light I have 
seen an additional band of greater refrangibility in the spectrum of the crystals. 
139. Some crystals of nitrate of uranium were gently heated so as to expel a good 
part at least of the water of crystallization. The residue after some time became 
opake and nearly white. In this state it was still more sensitive than the crystals. 
The dispersed light was not exactly of the same tint, but more nearly white ; and the 
derived spectrum was found on being analysed to contain, in addition to the bright 
bands usually seen in the derived spectrum of the crystals, another blue band still 
more refrangible. The fused mass gradually attracted moisture from the air, its 
colour changed to that of the crystals, and the most refrangible of the bright bands 
disappeared from the derived spectrum. Although when the incident light was very 
much concentrated 1 have seen this band even in the crystals, it was faint com- 
pared with the preceding bands, whereas in the case of the whitish mass its intensity 
was not very different from that of the others. It appears therefore that the quality 
as well as the quantity of the dispersed light was altered by depriving the crystals of 
a part of their water. 
140. A solution of nitrate of uranium in water is decidedly sensitive, though not 
sufficiently so to exhibit much dispersive reflexion. When the dispersed beam is 
analysed it is resolved into bright bands. When the solution is examined in a pure 
spectrum, the mode of dispersion is found to agree with that of canary glass. The 
dispersion commences abruptly at the same part of the spectrum as in the case of the 
glass, and after a rather narrow band in which light is copiously dispersed, there 
follows a remarkable minimum of sensibility, just as in the glass (see Art. 76.), where 
the dispersed light is almost imperceptible. After this the dispersion is resumed. 
