Mr. W. Crookes. 



[May 31, 



a large filter. A few ounces of water were poured on, and after 

 passing through, poured back, and the exhaustion repeated several 

 times. The aqueous extract was then evaporated to dryness, ignited 

 with sulphuric acid, ground in a mortar with small successive quan- 

 tities of water, and precipitated with ammonic oxalate. The precipi- 

 tate, ignited with sulphuric acid, showed ti«e citron band very fairly, 

 far more intensely than it was seendn the original calcic sulphate. 



These experiments are conclusive in proving that the citron band 

 is not due to calcium, but to some other element, probably one of the 

 earthy metals, occurring, in very minute quantities but widely dis- 

 tributed along with calcium, and' I at once commenced experiments to 

 find a more abundant supply of the body sought for. Amongst other 

 substances tested I may note the following as giving a more or less 

 decided citron band in' the spectrum when' treated with sulphuric 

 acid in the manner indicated above : — Crystallised barytic chlorate, 

 heavy spar, common limestone, strontic nitrate, native strontic car- 

 bonate, crystallised uranic nitrate, commercial magnesic sulphate, 

 commercial potassic sulphate, tobacco' ash, wagnerite (magnesic 

 phosphate and fluoride), zircon, cerite, and commercial cenc 

 oxalate. 



Some specimens of zircon appeared sufficiently rich 1 to make it 

 probable that here might be found an available source of the citron 

 band yielding body. I found it in crystals from Green River, North 

 Carolina, from Ceylon, from Expailly, from Miask (Oural), and from 

 Brevig, and having a good supply of North Carolina zircons, these 

 were worked up by a process given' in detail in- the paper. 



I may condense a' year's work on zircon, — over' 10 lbs. weight of 

 crystals from North Carolina having been worked up — by stating 

 that the result was' comprised in about 300 grs.- of an earthy 

 residue, and about 2 ozs, of oxalate, chiefly calcic; the former gave 

 the citron band very well. 



The zirconia prepared 1 from these zircons, when tested, sometimes 

 showed the citron band, and at other times none. A zirconia rich m 

 citron band, fractionally precipitated by ammonia, yielded precipitates 

 of increasing richness, the last fraction showing the citron band 

 strongly. 



The calcic oxalate obtained from 1 zircon gave unsatisfactory results, 

 so attention was directed to the earthy residue: This was found to be 

 of highly complex character, containing thoria, ceria, lanthana, 

 didymia, yttria, and probably some of the newly-discovered rarer 

 earths. 



The position of the citron band in the spectrum falls exactly on the 

 strongest absorption band of didymium, so that a piece of didymium 

 glass or cell of solution of the nitrate entirely obliterates the citron 

 band. This naturally suggested that the band was due to didymium. 



