The melting points of these elements have not been determined; cerium is said to fuse considerably above 500° but under 900°, and zirconium at a higher temperature than cerium. Carbon, titanium, and thorium, have not been melted. sparks are passed from tures. CH4O5; no other form of carbon gives this acid. Exhibits allo tropy very markedly. Atom is tetra valent in gas eous molecules. Group IV. continued. TITANIUM. Burns brilliant- atures. Decomposes Atom is tetra- Carbon is 468 General formulae and characters of compounds. characterised by the enormous number of compounds which it forms by combining with some or all of the elements hydrogen, oxygen, and nitrogen. The chlorides MC, except CeCl, and the fluorides MF, have been gasified and their vapour-densities determined; the oxides CO and CO2, and the sulphide CS,, have also been gasified; the formulae of these compounds are therefore molecular; the formulae of the other compounds are the simplest that express their compositions. None of the elements of the family except carbon forms any compound with hydrogen: the hydrides of carbon are exceedingly numerous. 3 Oxides: CO, ( TiO); (? C,O,), Ti,O,, Ce,O,; MO,, M = any element of the family. Sulphides: CS, CS,, (CS), TiS., Ce,S,, ThS.. Haloid compounds: MX,, M = any element of the family; C,Cl, &c., Ti̟¿Cl, Ce̟Cl ̧; TiCl. Acids: H,CO,Aq; salts of the form M,TiO, and M.ZrO ̧ are known (M = an alkali metal); the hydrates TiO,.xH2O and ZrO,.H.O are acidic. Salts carbon does not form salts by replacing the hydrogen of acids; M.2X (X=SO,, 2NO,, &c.) M = Ti, Zr, Ce, Th; Ce,3X, and Ti,3X. 469 Oxides. The dioxides MO, are produced by heating the elements in oxygen; the dioxides of titanium, zirconium, cerium, and thorium, are also obtained by precipitating solutions of salts of these elements by ammonia, and drying and heating the hydrated oxides so obtained. Carbon dioxide is most easily obtained by decomposing a metallic carbonate by an acid, e.g. CaCO,+ 2HClAq= CO2+ H2O+ CaCl,Aq. The dioxides, with the exception of CO,, are solids, insoluble in water, some of them insoluble also in most acids; carbon dioxide is a colourless, odourless, gas which can be condensed to a liquid, and, at a very low temperature, to a snow-like solid. 2 Carbon dioxide dissolves freely in water; the solution reddens blue litmus and interacts with alkalis to form salts MCO from these alkali carbonates, carbonates of most metals can be obtained. An aqueous solution of carbon dioxide probably contains carbonic acid, H,CO,; but this compound has not been isolated; the sulphur compound thiocarbonic acid, H,CS,, is known as a solid. By precipitating solutions of salts (M2X) of titanium, zirconium, cerium, and thorium, hydrated dioxides MO..xH2O are obtained. Many of these hydrated oxides seem to exist; the following, obtained by drying under different conditions, are among the more important, TiO,. H2O, TiO,.2H,O; ZrO2. H2O, ZrO,. 2H,O; ThO,. 2H2O; 2CeO2.3H2O. These hydrated dioxides are soluble in acids, and from these solutions salts of the form M. 2X (X=SO,, 2NO,, &c.) are obtained. The solution of CeO2.xH2O in acids seems to contain cerous salts Ce,. 3X, as well as ceric salts Ce2X. Most of the dioxides MO,.xH2O exhibit acidic properties. None of them dissolves in solutions of alkalis to form salts. When however titanium dioxide, TiO,, is fused with sodium carbonate, a quantity of carbon dioxide is evolved corresponding with that calculated on the assumption that sodium titanate Na,TiO, is produced; the fused mass is separated by water into NaOHAq and NaHTIO,. Zirconium dioxide, ZrO2, fused with Na,CO, behaves similarly to TiO,; the zirconates Na,ZrO, and Na,ZrO are said to have been obtained. Thorium dioxide, ThO,, does not decompose Na,CO when heated with this salt, nor does it form salts by fusion with solid potash or soda. The fact that when hydrated cerium dioxide, CeO2.xH2O, is precipitated by adding potash to a solution of ceric sulphate, Ce(SO4)2, the whole of the potash cannot be removed from the precipitate by washing with water indicates the possible formation of an unstable potassium cerate. Carbon monoxide, CO, is obtained by heating the dioxide with carbon (CO, + C = 2CO), or by the interaction of (1) formic acid, or (2) oxalic acid, with concentrated hot sulphuric acid; (1) H,CO,+H,SO,= CO + H,O.H,SO4, (2) H,C,O,+ H,SO, = CO + CO,+ H,O. H,SO4. This oxide is a poisonous gas; salts of formic acid H,CO2 can be obtained from it, and as the oxide is produced from the acid, the oxide is sometimes called formic anhydride. The sesquioxides Ti0, and Ce,O, are obtained by reducing the higher oxides MO; in the case of titanium this is done by heating TiO, to redness in dry hydrogen, in the case of cerium it is better to heat cerous oxalate, Ce2(C2O4)37 in hydrogen. Ti2O, is oxidised to TiO, by continued heating in air; Ce2O, is very easily oxidised by mere exposure to air. Both oxides dissolve in acids forming salts; a large series of cerous salts Ce2. 3X is known; only titanous sulphate, Ti(SO,),.8H2O, seems to have been obtained as a definite solid. 3 The sulphides, with the exception of CS,, have not 470 been much examined. Titanium disulphide, Tis,, is a dark coloured stable solid, obtained by the interaction of titanium tetrachloride and sulphuretted hydrogen at high temperatures. Thorium disulphide, ThS2, resembles TiS,; it is obtained by passing hydrogen mixed with hydrogen sulphide over heated Tho. Cerium sesquisulphide, CeS, is a golden coloured solid formed by passing sulphuretted hydrogen over hot CeO2. None of these sulphides exhibits any tendency to form thiosalts by interacting with alkali sulphides, or by fusion with alkalis. Carbon disulphide is a mobile, very refractive, liquid, boiling at 47°; its vapour is very inflammable. It is obtained by the direct union of its elements at high temperatures. This sulphide is the anhydride of thio-carbonic acid H,CS,; when CS, interacts with concentrated Na,SAq sodium thio-carbonate Na,CS, is formed; the acid H,CS, is obtained as a dark M. E. C. 22 471 472 yellow oily liquid by decomposing one of its alkaline salts by dilute HClAq. Thio-carbonic acid and its salts are very easily decomposed, to CS, and HS in the case of the acid, and to H,S and carbonates in the case of the salts. The haloid compounds of the carbon family of the form MX, where X = Cl or Br, are prepared, except in the case of the carbon compounds, by strongly heating an intimate mixture of the oxides MO, and carbon in a stream of chlorine, or bromine; tetrachloride of cerium has not yet been prepared. TiCl, is a liquid boiling at 136°, the other tetrachlorides are solids; they have all been gasified without decomposition. These tetrachlorides all combine with various other chlorides, and in some cases with other compounds, to form double compounds; e.g. TiCl, PC1; 3Tic.4NOCI; TiC1⁄4.4NH,, ZrCI,. 2NaCl; 2ThCl,. 8NH,Cl. The tetrafluorides MF, M = Ti, Zr, Ce, or Th, are obtained by dissolving the hydrated dioxides MO,.xH2O in aqueous hydrofluoric acid and evaporating. The tetrafluorides of titanium, zirconium, and thorium form compounds with potassium fluoride of the form 2KF. MF,; as in some cases the corresponding hydrogen compounds 2HF. MF, have been isolated and the potassium compounds have been obtained by neutralising aqueous solutions of these hydrogen compounds, it seems better to regard the compounds in question as the potassium salts of fluotitanic, fluozirconic, and fluothoric acids, H2MF (M = Ti, Zr, Th). A double fluoride of cerium and potassium is known, but its composition is different from that of the K,MF, salts; it is represented by the formula 2CeF. 3KF.2H,0. 6 6 Cerous chloride, or cerium sesquichloride, Ce,Cl, is obtained by heating in chlorine a mixture of the corresponding oxide, Ce2O, and carbon. = Carbon forms several haloid compounds; CX,, C2X ̧, C2X1, where X Cl or Br; and CI. Carbon tetrachloride CCI, trichloride CCl, and dichloride C,Cl, have been gasified without decomposition. The first of these compounds is obtained by the interaction of chlorine and chloroform, CHCl,; the second by the interaction of chlorine and ethylene dichloride, CH Cl; and the third by reducing C,Cl, by means of hydrogen evolved in contact with the carbon trichloride. Salts. Salts of carbon, i.e. compounds obtained by replacing the hydrogen of acids by carbon, are unknown. Titanium forms two series of salts; titanous salts represented by |