542 CHEMISTRY [CERIUM GROUP. of oxychloride bo present it is decomposed by water even in the cold. Tungsten pentachloride crystallizes in black, highly deliquescent needles ; it melts at 248 C., and boils at 376 C., and its vapour density at 350 C. corre sponds with the formula WC1 5 . It has already been pointed out that the existence of pentachlorides of tungsten and molybdenum, and of a vanadium tetrachloride, is apparently anomalous ; at present, these chlorides, chlorine dioxide, C1O 2 , nitrogen monoxide and dioxide, and, perhaps, uranium pentachloride are the only instances known to us in which apparently an odd number of affinities are disengaged (see p. 473). Tungsten tetrachloride is a crystalline, greyish-brown substance ; when strongly heated it decomposes into the volatile pentachloride and the non-volatile dichloride, which is a loose grey powder, destitute of crystalline struc ture. Tungsten also forms two oxychlorides, WOC1 4 and WO 2 C1 2 ; the former is obtained in beautiful ruby-red, and the latter in yellow crystals. The formula of the monoxy- chloride, WOC1 4 , has been established by determining its vapour density. Molybdenum trioxide has marked acid properties, but the remaining oxides are feebly basic; both oxides of tungsten exhibit acid functions. Solutions of molybdous salts corresponding to the oxide Mo. 2 3 are almost black, and solutions of molybdic salts corresponding to the oxide Mo 2 O 4 are reddish-yellow ; the molybdates are mostly white. Molybdenum trioxide, Mo0 3> melts at a. red heat to a dark yellow liquid, which on cooling forms a straw- yellow mass, breaking up into crystalline scales ; it is scarcely soluble in water, but dissolves in hydrochloric, nitric, and sulphuric acids. Like silica and other feebly acid oxides it combines with basic oxides in a large number of different proportions, forming molybdates.. Tungsten trioxide, WO 3 , is a canary-yellow substance, insoluble in water and in most acids ; it melts at a high temperature, and, like molybdenum trioxide, may be volatilized by heat ; it dissolves in alkaline solutions, forming tungstates, of which a great variety may be obtained by the union of tungsten trioxide with basic oxides in various proportions. The tungstates are mostly colourless. When a hot solution of an alkaline tungstate is mixed with an acid a yellow precipitate of tungstic acid, H 2 WO 4 , is obtained, which is insoluble in water, and forms insoluble salts with all metals, except the alkali metals ; but by decomposing tungstates by the stronger acids at ordinary temperatures a soluble variety of the acid, called metatungstic acid, is produced, which forms soluble salts with nearly all metals. Uranium. The atomic weight of uranium has not yet been ascertained, and its position among the elements is therefore uncertain, especially as it does not exhibit marked analogy to any other element. The generally received atomic weight is 120, but Mendeljeff has proposed to double this ; L. Meyer, however, considers that from the high specific gravity of the metal it is more probable that its atomic weight is 180, and that it occupies a position in the series near to tungsten. A redetermination of the equivalent of this element and an investigation of its compounds are much required. Uranium is like iron in colour ; it tarnishes when exposed to the air, and in the pulverulent state takes fire at a comparatively low temperature, and burns with great brilliancy, forming a green oxide, U 3 O 8 ; it is dissolved by hydrochloric acid, forming a green solution, which has powerful reducing properties. The highest oxide has the composition UO 3 . Uranium forms a volatile stable which crystallizes in dark-green tetrachloride, 3 UC1 4 Name. Symbol. At, wt. Sp. gr. At, vol. Lanthanum La 139 6-1 22-6 Cerium Ce 141 6-7 21-0 Didymium Di 147 6-5 22-6 Yttrium Y (?) 89-5 Erbium Er (?) 170-5 Thorium Th (?) 178-5 77 23-1 octahedra, and an unstable chloride regarded as the pentachloride, UC1 5 ; both are dissolved and decomposed by water. When uranium, or either of its oxides, is dissolved in nitric acid, a solution of uranic nitrate is formed from which the salt may be crystallized in beautiful yellow prisms of the composition (UO 2 )(NO 3 ) 2 , 6H 2 O ; on the addition of potassium hydroxide to a solution of uranic nitrate a yellow insoluble precipitate of potassium uranate of the composition K 2 U 2 O 7 is produced. An oxychloride, U0 2 C1 2 , corresponding to the nitrate, is obtained on pass ing chlorine over an ignited mixture of charcoal with an oxide of uranium. In all these formulae, U = 240. LANTHANUM CERIUM DIDYMIUM YTTRIUM ERBIUM THORIUM. Excepting thorium, these elements are closely related in properties ; their compounds are of extremely rare occur rence. Lanthanum, cerium, and didymium are always associated together, and on account of their occurrence in the mineral cerite are usually termed cerite metals ; they appear to occupy a position intermediate between the alkaline earth metals and the metals of the aluminium group. The atomic weights of yttrium and erbium, the so-called gadolinite metals, have not been determined ; their compounds much resemble those of the cerite metals. The atomic weight of thorium is also unknown ; from the properties of its compounds, however, it is not improbable that it is a member of the aluminium group. Lanthanum, cerium, and didymium have been obtained by the electrolysis of their chlorides. Cerium is an extremely ductile metal, resembling iron in colour and lustre ; it melts more readily than silver, but much less readily than anti mony. It retains its lustre for a considerable period in dry air, but takes fire much more readily than magnesium, and burns with great brilliancy. It slowly decomposes cold water, and rapidly dissolves in hydrochloric acid. Lanthanum and didymium closely resemble cerium in their general chemical behaviour. Lanthanum is a slightly harder, less ductile metal than cerium, and appears to melt nearly at the same temperature. It readily oxidizes super ficially even in dry air, although it takes firo at a much higher temperature than cerium. Didymium more closely resembles lanthanum than cerium, but has a distinct yellowish colour. Only one oxide of lanthanum is known, namely, La 2 O 3 ; it is white, and unalterable by heat, and combines directly with water, forming the hydroxide La 2 (OH) 6 , which is a soft, white powder resembling calcium hydroxide. The lanthanum salts of colourless acids are all colourless. Cerium forms the two oxides Ce 2 O 3 and Ce 2 O 4 . Cerous oxide, Ce 2 O 3 , has a(?) greyish-blue colour, and on exposure to the air is converted into yellow eerie oxide, Ce 2 O 4 . Cerous hydroxide, Ce 2 (OH) 6 , precipitated from the solution of a cerous salt by an alkali, is white ; on exposure to the air it is converted into a mixture of cerous carbonate and eerie hydroxide ; the latter is also obtained on passing chlorine into water in which cerous hydsoxide is suspended. Cerie hydroxide dissolves in hydrochloric acid with evolution of chlorine, forming a solution of cerous chloride, Ce. 2 Cl c , but it forms with sulphuric acid a brown- red solution of eerie sulphate, which is a powerful oxidizing agent. Cerous salts in solution are colourless, but a few
possess a pale rose colour in the solid state.