278 Notices of the Labours of Continental Chemists: Cerium, ^c. 



larizing apparatus, the diameter in air of the darkest part of 

 the first ring is about 7° 30'. When this mineral is coloured, 

 as is usually the case, the optical constant belonging to the 

 extraordinary ray cannot be determined, on account of the 

 absorption of the light polarized in the plane of the axis. 



In Dioptase, according to observations made with a very 

 perfect and transparent crystal, for which I am indebted to 

 Mr. Heuland, for the brightest part of the solar spectrum the 

 index of refraction of the ordinary ray is 1*667; in an extra- 

 ordinary ray perpendicular to the axis the velocity of light in 

 air divided by its velocity within the crystal is 1*723. 



In Anatase, for the brightest part of the solar spectrum, the 

 index of refraction of the ordinary ray is 2*554; in an ex- 

 traordinary ray perpendicular to the axis, the velocity of 

 light in air divided by its velocity within the crystal is 2*493. 



St. John's College, Sept. 9, 1842. W. H. MlLLER. 



XLVIII. Notices of the Results of the Labours of Continental 

 Chemists. By Messrs. W. Francis and H. Croft. 



[Continued from p. 21.] 



On Cerium and some of its Salts, and on Didymium. 



\ N examination respecting the true atomic weight of cerium 

 ■^*- has been made by M. A. Beringer in the laboratory of 

 Professor Wohler ; new experiments on this subject were ne- 

 cessary on account of the discovery of lanthanium. It will 

 however be useless to insert this treatise in these reports, inas- 

 much as a notice has appeared in PoggendorfF's Annals, vol. 

 lvi. p. 503, from which we learn that Mosander has discovered 

 a third metal mixed with cerium and lanthanium, which he 

 calls Didymium. It is scarcely possible to separate the oxide 

 of this metal; Mosander, although he has known this body one 

 year and a half, has as yet been unable to isolate it in a pure 

 form. Oxide of didymium causes the brown colour of the 

 so-called oxide of cerium, and also the rose or amethyst 

 tinge of some salts of yttria. The perfectly pure oxides of 

 lanthanium and cerium are probably quite colourless. In 

 the usual mode of preparing oxide of lanthanium by means 

 of dilute nitric acid, the whole of it is never extracted, but 

 part remains with the oxide of cerium. Mosander is engaged 

 with the examination of the three bodies, and from him we 

 may expect a full description. Beringer has examined metallic 

 cerium (impure), the double sulphates, and some other salts. 

 — (Antialen der Chemie und Pharmacie, vol. xlii. p. 134.) 



