OXIDES OF URANIUM. 643 



The other oxides are dissolved out of the ignited mass by hy- 

 drochloric acid, the protoxide of uranium remaining as a very 

 fine, dark grey powder, which is received on a filter and washed 

 well. Protoxide of uranium, obtained in this manner, may 

 be dissolved, with the aid of heat, in concentrated sulphuric acid. 



The peroxide of uranium is easily reduced by hydrogen. 

 Arfwedson obtained the metal by passing hydrogen over the 

 double chloride of uranium and potassium at a red heat, in the 

 form of little regular octohedrons of a brilliant metallic lustre, and 

 of a dark grey colour, almost black. The density of this metal 

 is 9.00. It combines with oxygen in two proportions, forming 

 a protoxide, U O, and peroxide U 2 O 3 . 



Protoxide of uranium, Uranous oxide, U O, 2811.4 or 225.26. 

 This oxide is obtained when an alkali is added to a solution of 

 any of its salts, as a greenish grey hydrate, which soon becomes 

 yellowish, and is finally converted into uranic oxide by the 

 oxygen of the air. Carbonate of ammonia added in excess to 

 a uranous salt, redissolves the precipitate and forms a green 

 solution. The uranous chloride, U Cl, forms a green syrupy 

 solution which does not crystallize. The uranous sulphate 

 yields, by evaporation, green prismatic crystals. This oxide is 

 employed in painting upon porcelain, and yields a black of the 

 greatest purity.* 



Peroxide of uranium, Uranic oxide, U 2 O 3 , 2911.4 or 233.26. 

 When an alkali is added to a salt of this oxide, a compound 

 of uranic oxide and the alkali is precipitated and not the hy- 

 drated oxide itself; but the latter may be obtained by the oxi- 

 dation of uranous oxide. The uranic hydrate is of a yellow 

 colour, has an acid reaction upon litmus, and is decomposed 

 by heat, allowing water and oxygen to escape, while uranous 

 oxide remains. Uranic oxide forms insoluble compounds with 



* The atomic weight of uranium, indicated by the specific heat of the raetaJ, is 

 only 677.8, or one-fourth of the number hitherto received, according to new 

 researches of M. Regnault, (An. de Ch. et de Ph. t. 73, p. 71.) The oxide of ura- 

 nium, at present considered as the protoxide, comes then to be U 4 O. M. Regnault 

 has added greatly to the value of specific heat, as an element in fixing atomic 

 weights, by confirming Dulong's general results by new experiments, and also by 

 removing several of the exceptions to the law, that all simple atoms have the same 

 capacity for heat, noticed at page 124. The elements enumerated below have the 

 same capacity for heat, in the following atomic proportions, by Regnault's experi- 

 ments : 



Bismuth . 1330 Cobalt . 369 Antimony . 806.4 



Silver . . 675.5 Selenium 494.6 Phosphorus . 196.1 



Uranium . 677.8 Arsenic 470 Carbon . . 152. ! 



