[13]Sodium carbonate 72·33 Sodium hydrate 1·93 Sodium chloride 8·30 Sodium sulphate 3·95 Sodium sulphite ·63 Silica 7·09 Carbon 4·70 Oxide of iron and alumina ·50 Other constituents (by difference) ·57 ────── 100·00 The whole of the soda present as sulphur compounds is not lost, as a large proportion of it is present as sodium sulphite, most of which is converted into caustic soda by the causticising process. [Footnote 13: A certain quantity of potash derived from the mineral constituents of esparto and straw is always present in recovered soda.] A certain amount of soda is carried forward, partly mechanically and partly volatilised, to the flue leading to the chimney. This accumulates, and may be from time to time removed in the form of fine dust. It contains, besides carbonate of soda, much sulphate and chloride. In two different samples examined by the authors, the amounts of soda (Na_{2}O) present were 25·0 and 27·1 per cent. _Causticising._—The next process consists in converting the sodium carbonate in the recovered soda into caustic soda. This operation is known as “causticising,” and consists in heating a solution of the soda with lime. The decomposition which takes place is shown in the following equation:— Sodium Lime. Water. Caustic Calcium Carbonate. Soda. Carbonate. Na_{2}CO_{3} + CaO + H_{2}O = 2 NaOH + CaCO_{3}. The recovered soda should be dissolved in separate vessels. Perhaps the best form of apparatus is a series of lixiviating tanks such as are used for dissolving the alkali in black ash. {188} By this means a nearly perfect exhaustion of the mass can be effected with a minimum of labour. Special tanks are sometimes made for the purpose, provided with mechanical stirrers.