602 PEOFESSOE STOKES ON THE LONG SPECTEHM OF BLECTEIC LIGHT, 
likely to have been obtained by one working by photography, and I have therefore 
thought it well to draw up a paper embodying these results, and thus forming, as it 
were, a supplement to the paper by Dr. Miller. 
Preimration of a Screen hy means of a Salt of Uranium. 
Few substances are more powerfully fluorescent than several of the salts of sesquioxide 
of uranium ; and a piece of glass coloured by uranium and polished along at least two 
planes at right angles to each other is exceedingly convenient, from its powerful fluo- 
rescence and its permanence, for a screen on which to receive a spectrum. Nevertheless 
such a screen, which must be viewed in particular directions in order to get the strongest 
effect, is in many cases less convenient than a screen would be which was prepared by 
means of a highly fluorescent powder treated like a water colour, which could be 
viewed in all directions indiflerently. This is especially the case in taking measures by 
a method which will be mentioned presently. Besides, I And an excellent piece of such 
glass defective in fluorescent power as regards the extreme lines shown by aluminium ; 
and some specimens are defective to a much greater extent, which is doubtless due to 
impurities. Accordingly I have long regarded it as a desideratum to obtain by preci- 
pitation an insoluble or very sparingly soluble salt of sesquioxide of uranium which 
should be as fluorescent as the best salts of that base, and which might be treated like 
a w^ater colour. I have now succeeded in preparing such a salt, though not by direct 
precipitation. 
The ordinary phosphate obtained by precipitation, the composition of which, inde- 
pendently of w^ater of hydration, is P 05(11203)2110, is only slightly fluorescent. If, 
however, this salt, with as much water as remains when it is washed by decantation, be 
put into a saucer, a little free phosphoric or sulphuric acid added, and then crystals of 
phosphate of soda, phosphate of ammonia, microcosmic salt, or borax be added in excess, 
the original salt is gradually changed into one which is powerfully fluorescent. The 
change seems to take place most rapidly with borax ; but as an excess of this salt is 
liable slowly to decompose the fluorescent salt first formed, it is better to employ a 
phosphate. The quantity of acid should be sufficient to leave a decided acid reaction 
when the liquid is fully saturated by the alkaline phosphate employed. The change 
may be watched by observing from time to time the fluorescence of the salt by daylight, 
with the aid of absorbing media. It is complete in a few days at furthest, w^hen the 
salt is ready to be collected. 
This requires precaution, as the salt is quickly decomposed by dilute acids (and 
accordingly by its own mother-hquor if diluted), and even, though more slowly, by pure 
water, with the formation apparently of the original phosphate. It is also decomposed, 
at least in time, by alkaline carbonates, with the formation of a beautiful yellow non- 
fluorescent salt resembhng the precipitate given by alkaline carbonates in salts of sesqui- 
oxide of uranium. The salt may be collected by adding at once, instead of water, a 
saturated solution of borax, in quantity at least sufficient to destroy the acid reaction. 
