CHROMIUM, MOLYBDENUM, TUNGSTEN, URANIUM, ETC. 301 



oxide, R(U0 2 )H 2 P 2 O 8 ,7H 2 0, where R=Cu or Ca, uranium- vitriol 

 U(S0 4 ) 2 ,H 2 O, samarakite, and seschynite, are very rarely found, and 

 then only in small quantities. Of more frequent and abundant 

 occurrence is the non-crystalline, earthy brown uranium ore known as 

 pitchblende (sp. gr. 7*2), which is mainly composed of the intermediate 

 oxide, TJ 3 Og= U0 2 ,2UO 3 . This ore is found at Joachimsthal in Bohemia 

 and in Cornwall. It usually contains a number of different impuri- 

 ties, chiefly sulphides and arsenides of lead and iron, as well as lime 

 and silica compounds. In order to expel the arsenic and sulphur it is 

 roasted, ground, washed with dilute hydrochloric acid, which does not 

 dissolve the uranoso-uranic oxide, U 3 O 8 , and the residue is dissolved 

 in nitric acid, which transforms the uranium oxide into the nitrate, 

 U0 2 (N0 3 ) 2 . 



It must be observed that the oxide of uranium, first distinguished 

 by Klaproth (1789), was for a long time regarded as able to give 

 metallic uranium under the action of charcoal and other reducing agents 

 (with the aid of heat). But the substance thus obtained was only the 

 uranium dioxide, UO 2 . The compound nature of this dioxide, 12 or the 

 presence of oxygen in it, was demonstrated by Peligot (1841), by igniting 

 it with charcoal in a stream of chlorine. He thus obtained a volatile 

 uranium tetrachloride* UC1 4 , 13 which, when heated with sodium, gave 



13 Uranium dioxide, or uranyl, UO , which 'is contained in the salts UO<jX 2 , has 

 the appearance and many of the properties of a metal. Uranic oxide may be regarded as 

 uranyl oxide, (U0 2 )O, its salts as salts of this uranyl ; its hydroxide, (UOo)H 2 O 2 , is consti- 

 tuted like CaILjO 2 . The green oxide of uranium, uranoso-uranic oxide (easily formed from 

 uranic salts by the loss of oxygen), U 5 Og = UOo,2UO 3 , when ignited with charcoal or 

 hydrogen (dry) gives a brilliant crystalline substance of sp. gr. about ll'O (Urlaub), whose 

 appearance resembles that of metals, and decomposes steam at a red heat with the 

 evolution of hydrogen ; it does not, however, decompose hydrochloric or sulphurio 

 acid, but is oxidised by nitric acid. The same substance (i.e. uranium dioxide UO^) is 

 also obtained by igniting the compound (UO.yjILjC^ in a stream of hydrogen, according" 

 to the equation UO..K.Cl 4 + H i = UO 2 + 2HCl + 2KCl. It was at first regarded as the 

 metal, In 1841 Peligot found that it contained oxygen, because carbonic oxide and 

 anhydride were evolved when it was ignited with charcoal in a stream of chlorine, and 

 from 272 parts of the substance which was considered to be metal he obtained 882 parts 

 of a volatile product containing 142 parts of chlorine. From this it was concluded that 

 the substance taken contained an equivalent amount of oxygen. As 142 parts of chlorind 

 correspond with 32 parts of oxygen, it followed that 272-82 = 240 parts of malal were 

 combined in the substance with 82 parts of oxygan, and also in the chlorine compound 

 obtained with 142 parts of chlorine. These calculations have been made for the now 

 accepted atomic weight of uranium (U = 240, see Note 14). Peligot took another atomic. 

 weight, but this does not alter the principle of the argument. 



15 Uranium tetrachloride, uranous chloride, UC1 4 , corresponds with uranous oxide 

 as a base. It was obtained by Peligot by igniting uranic oxide mixed with charcoal in a- 

 stream of dry chlorine: UO 3 + 8C + 2Clj-UCl 4 -h3CO. This green volatile compound 

 (Note 12) crystallises in reg_ular octahedra, is very hygroscopic, easily soluble in water, 

 with the development of a considerable amount of heat, and no longer separates out 

 from its solution in an anhydrous state, but disengages hydrochloric acid when 



