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 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 



