The detection of cerium in a mixture of earths is a comparatively simple matter, as it has several distinctive reactions. The brown colour of the peroxy-compounds has been suggested as a convenient test by several authors. This may be observed when ammonia is added to a cerous salt in presence of hydrogen peroxide. In the presence of a large excess of foreign earths, very dilute ammonia should be added, drop by drop, with continuous shaking, until a small permanent precipitate remains; this will be rich in the weakly basic ceric hydroxide, and on addition of the peroxide solution will show the colour clearly.[226] For very small quantities of cerium, the neutral solution is added to warm concentrated potassium carbonate solution, and one or two drops of dilute hydrogen peroxide added to the clear liquid; the yellow colour is then very characteristic.[227] [226] Marc, _Ber._ 1902, ~35~, 2370. [227] Meyer, _Zeitsch. anorg. Chem._ 1904, ~41~, 94. Biltz and Zimmerman[228] employ the reducing powers of cerous hydroxide; ammoniacal silver nitrate is added to the neutral solution of the cerous salt, and the mixture warmed. Dilute solutions (1-2 mgms. per litre) give a brown colour, concentrated solutions a black precipitate. The oxidation of an ammoniacal solution of the tartrate by air or hydrogen peroxide, by which an intense yellowish brown colour is developed, has been recently suggested by Wirth[229] as a very delicate test for the element. [228] _Ber._ 1907, ~40~, 4979. [229] _Abstr. Chem. Soc._ 1913, ~104~, ii. 712. _Spectrum analysis._--Cerous salts show no absorption, ceric salts general absorption of the violet end of the spectrum. Arc spectrum--see Exner and Haschek,[230] Eder and Valenta,[231] and Cooper.[232] The emission spectrum of cerium is especially rich in lines; for identification, the following may be used: [230] _Die Spektren der Elemente, etc._, Leipzig and Vienna, 1911. [231] _Sitzungsber. kaiserl. Akad. Wiss. Wien_, 1910, ~119~, II_a_, 531. [232] _Astrophys. J._ 1909, ~29~, 352. 4150·11 4186·78 4222·78 4296·88 4337·96 4382·32 4386·95 4460·40 4479·52 4487·06 4527·51 4528·64 4539·90 4562·52 4572·45 4594·11 4628·33 5512·72 The _estimation_ of cerium cannot be carried out accurately by gravimetric methods in the presence of other earths; volumetric methods, however, will give reasonably accurate results, if the necessary precautions are taken. In Bunsen’s method the ignited oxides are treated with hydrochloric acid in presence of potassium iodide, the iodine set free from the hydriodic acid by reduction of the cerium dioxide being estimated by means of sodium thiosulphate, in the usual way. This method gives very inaccurate results, since in the presence of cerium dioxide, other oxides of the group can be converted into higher oxides which will also liberate iodine under these conditions.