of Compounds of Zirconia and the Oxides of Uranium. 453 



I will not now enter into a description of the various chemical 

 and physical facts which seemed to warrant the conclusion that zir- 

 cons sometimes contain a new earth ; but taking these into considera- 

 tion, there seemed to be every reason to believe that spectra which thus 

 differed so much from those of any previously known substance were 

 characteristic of this new earth. Judging from the facts then known, 

 it was more probable that spectra of such a new type were due to a 

 new element, than that they were due merely to a combination of 

 two such elements as zirconium and uranium. Some of these che- 

 mical and physical facts can now be explained by the presence of 

 uranium ; but besides this and several of the more common earths 

 and oxides, I have detected in some zircons erbium, didymium, 

 yttria, and another substance which exists in such small quantity that 

 I have not yet been able to ascertain whether or not it is the suspected 

 new earth. These accidental constituents do not, indeed, occur in 

 sufficient quantity to be of importance, except as modifying the phy- 

 sical and optical properties, the didymium giving its usual charac- 

 teristic absorption-bands (zircons from Sveneroe, Norway), and the 

 manganese the same spectrum as that of garnets (zircons from an 

 unknown locality in Siberia*). This method, however, fails to give 

 evidence of a new earth ; for, since the publication of my former paper, 

 I have proved that the very abnormal spectra which seemed suffi- 

 cient to establish its existence are really due to compounds of zir- 

 conia with the oxides of uranium, which have such a powerful action 

 on light that an almost inappreciable amount is sufficient to pro- 

 duce the spectra to great perfection — in fact, so small an amount 

 that the total quantity which misled me was only a few thousandths 

 of a grain ; and its presence might easily have remained unsuspected, 

 if I had not made a number of experiments which at first did not 

 seem to have much connexion with the subject. 



In studying the spectra of crystalline blowpipe-beads, it seemed 

 desirable to examine those made with carbonate of soda, with or 

 without a little borax. This when melted dissolves certain oxides ; 

 and though it crystallizes on cooling, so as to be only partially 

 translucent, yet with strong direct sunlight well-marked spectra may 

 be seen. For example, in the oxidizing flame uranic oxide is easily 

 dissolved by carbonate of soda alone ; and when quickly cooled, an 

 orange-coloured bead is obtained, probably containing uranate of 

 soda in a vitreous condition, which gives a single well-marked ab- 

 sorption-band in the green (see spectrum No. 4) ; with so small a 

 quantity of the oxide, that in a bead -J- inch in diameter shows the 

 spectrum to the best advantage, and even T o ^ 00 grain can be easily 

 detected. We need not be surprised that this spectrum differs so 

 much from the usual type of uranic salts (Nos. 1, 2, and 3), since 

 in this case the oxide plays the part of an acid. It may be only an 

 accidental coincidence ; but this difference is analogous to the change 

 which commonly occurs on adding an alkali to neutral solutions of 

 vegetable coloursf. When gently reheated it seems as if the uranate 



* For both of these I am indebted to my kind friend Mr. David Forbes, 

 t See my paper in Proc. Eoy. Soc. 1867, vol. xv. p. 433. 



