522 PROFESSOR STOKES ON THE CHANGE OF REFRANGIBILITY OF LIGHT. 
line carbonates, added in solution to a solution of nitrate of uranium, give yellow pre- 
cipitates which are redissolved in an excess of the precipitant. The solutions thus 
obtained with the carbonates of potassa and soda, which were of a greenish yellow 
colour, were found to be totally insensible. They exhibited however four of those 
singular absorption bands so characteristic of salts of peroxide of uranium. Of these 
the third fell a little short of G, its more refrangible edge nearly coinciding with that 
fixed line ; the first and second were situated between F and G, the distance of the 
first beyond F being somewhat greater than the interval between two consecutive 
bands. The fourth, which was situated beyond G, was fainter than the others. 
The second and third were the most conspicuous of the set. 
158. The absorption bands due to peroxide of uranium afford an easy mode of 
detecting that substance in solution. For this purpose the solutions mentioned in 
the preceding paragraph are much preferable to the nitrate, for they produce much 
stronger bands when only a small quantity of uranium is present. The absorption 
bands of nitrate of uranium are visible, as might have been expected, in presence of 
a large quantity of nitrate of copper*. 
Optical Tests of Uranium in Blowpipe Experiments. 
159. When a bead of microcosmic salt is fused with oxide of uranium, and brought 
to its highest state of oxidation, it is yellow by transmitted light. Such a bead is 
sensitive in a very high degree, quite as much so as canary glass. When the light 
falls sideways on it, and it is held against black cloth or a dark object, it exhibits 
plainly the green colour due to internal dispersion. When properly examined by 
means of sunlight its sensibility is evident at once, and when the dispersed light is 
viewed through a prism it is resolved into bright bands. One of the most convenient 
modes of examining such minute objects consists in reflecting the sun’s light hori- 
zontally through a large lens, intercepting by means of absorbing media all the rays 
except those of very high refrangibility, placing the object to be examined in the 
condensed beam, and viewing it through a prism. So delicate is this test when 
applied to uranium, that on one occasion, when engaged in examining a bead 
coloured green by chromium, which had been fused in the exterior flame, I observed 
the appearance given by uranium. This turned out to be actually due to uranium, 
of which a mere trace was accidentally present without my knowledge. 
160. The green communicated to microcosmic salt by uranium after exposure to 
the reducing flame has a very peculiar composition, by means of which the presence 
of uranium may be instantly detected. For this purpose it is suflUcient to view 
through a prism the inverted image of the flame of a candle formed by the bead, the 
latter being so held as to be seen projected on a dark object. The observation is 
perfectly simple, and occupies only a few seconds. The spectrum exhibits an isolated 
band at the red extremity, followed by a very intense dark band of absorption. A 
* See note D. 
