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THE SPECTRUM 319 large glass jar of slightly ammoniated water, in the rays of the lantern. Beautifully brilliant arborescent green streams descend through the water. Chrysoline does the same, and is perhaps a purer green than the uranine. A few drops of almost any red ink will show the phenomena fairly well. Green, or rather greenish-yellow, is the commonest and strongest fluorescent colour, and for designs painted in it there is ample choice. The best way is to prepare a thick size or thin glue from gelatine, and dissolve fresh some uranine in the fluid. This can be laid on with a broad pen or brush, and then dried, so as to give a little body of colour. Such will shine brilliantly in almost invisible blue light. Barium-platino-cyanide will be equally brilliant, but is far more expensive. Tin's is best rubbed up with gum-water. A substance called thallene by Professor Morton, prepared by him from petroleum residues, has probably the brightest fluorescence, of the same yellow-green. Professor Morton kindly wrote to me that this is best prepared by grinding up with rather thin varnish of gum-damar in benzol. Slight and unknown impurities often impart splendid fluorescence to various organic compounds, which are destitute of it when really pure. Other colours are more difficult to get, and I have not yet obtained any of them brilliant enough to use in designs. In cells, chlorophyll has been already mentioned for red. Magdala rose fluoresces orange-red, but was expensive, and is now almost impossible to obtain, being gone out of fashion, which is capricious as to these aniline colours. The best red I yet know of was brought to my notice by Mr. Sidney Jewsbury, of Manchester, and is a solution of azo-resorufin, (C 2 4 H 16 N 2 0 7 ) in slightly ammoniated alcohol—methylated will do. Little must be used, one grain is enough for a large cell. In this substance the fluorescence is rather masked by the natural colour of the solution being also red; but by treating with bromine, a compound is obtained giving a blue