-Caustic. 835
frequently in the shape of a tent or a waggon boiler.Sometimes they are built of bricks. In any case,they are provided with a false bottom of laths orshelves, on which the soda crystals are piled up tothe roof of the box, in lumps of about a foot square,and of such thickness as they come out of thecrystallizing cones. Only the inferior crystal isgenerally used in the bicarbonate chambers, as bothits chemical impurity and its slightly yellowishcolour do not prevent it from making perfectly purebicarbonate. The carbonic acid is very quicklyabsorbed; and as the resulting NaIIC0 3 crystallizeswithout any water, 9 of the 10 equivalents of thewater of crystallization, combined with sodium car-bonate, become disengaged, and water collects at thebottom below the shelves, fromw'hence it is gener-ally conveyed away by an U-shaped tube, whosebend forms a trap for the gas. This liquor has insolution a certain amount of sodium bicarbonate orbicarbonate, along with practically all the foreignsalts (sodium chloride and sulphate), whose propor-tion in crystal soda is, however, only slight, and whichare much more soluble than bicarbonate. Thismother liquor is either added to the tank liquor,which it helps to carbonate, or boiled down byitself, and made into weak alkali (of about 36 percent, available soda).
The time requisite for completely converting thesodium carbonate into sodium bicarbonate varies verymuch, according to the size of the chambers, and indeedin the same chamber it is not equal; as a rule, theoperation lasts from eight to ten days. The bicar-bonate chambers are always provided with a chargingdoor and a vent for the air; they are also generallyconnected in such a way that the gas entering intoone passes through a second, and even a third chamber,to be fully utilized. Openings are provided, gener-ally plugged up, through which the workmen tryfrom time to time whether the operation is com-pleted by thrusting iron rods into the mass; whenthey can do so without feeling the resistanceof unconverted soda crystals, the process is con-sidered complete. Other marks for it are the ceas-ing of the flow of liquor, and testing of a samplefrom the centre of the lumps with merourie chloride,which must not form a yellow precipitate, but onlya white cloud. During its progress considerableheat is evolved; and as this is prejudicial to theformation of bicarbonate, iron chambers are pre-ferable to those made of brickwork. The bicar-bonate, when the door is opened, still shows theshape of the crystals from which it has been made ;but, instead of being transparent, they are perfectlyopaque, and have the colour and other appear-ances of snow. As the salt comes out, it is still verydamp; and it requires drying at a gentle heat, notexceeding 45° C. It is piled up in large stoves,sometimes 20 feet square, and exposed to the above-mentioned temperature for eight or ten days, bymeans of iron pipes heated by a hot-air apparatus,or by any convenient apparatus. Lastly, it is groundto an impalpable powder in mills entirely resemblingordinary flour mills. Great care is required in
this operation, as any heating of the stuff in themill would injure it by causing a loss of carbonioacid. After grinding, the powder is sifted througha cylinder sieve covered with brass-wire gauze of
360 meshes per square inch, and packed in smallbarrels of 1 ewt. each. Attempts have been madeto dry it at ordinary temperature, by piling thedamp bicarbonate on a large number of shelves invery thin layers, and connecting the whole systemat one end with the atmosphere, at the other endwith the chimney. This process is, however, too slow.
The yield of bicarbonate from fresh soda crystalsis about 50 per cent. When the crystals areeffloresced it is, of course, larger. Such efflorescedcrystals also absorb the carbonic acid much morerapidly than those containing the full amount ofwater of crystallization. Mohr has proposed toavoid the formation of mother liquor completely,and hasten at the same time the absorption of thegas, by mixing 1 part of crystallized with 3 of cal-cined soda, as this mixture contains just enoughwater to supply the hydrogen in NaHC0 3 ; but thisprocess is never practised now on' a large scale,because the calcined soda ash, or even refinedalkali, is too impure and does not yield sufficientlygood bicarbonate. Moreover, the absorption of car-bonic acid is in that ease too rapid, and the heatingcaused in consequence of this may spoil the pro-duct. Other sources of carbonic acid than thatabove described have also been proposed, such asthat escaping in nature in some places from volcanicrocks, that evolved in alcoholic fermentation, andthat produced in lime burning. The two formercan only have quite local applications, and the latteris too troublesome to purify and cool sufficiently forthe purpose.
The bicarbonate of commerce, as it is made inalkali works and principally used as baking powder,is never quite free from sesquicarbonate; but itought not to be- more than 2 per cent, short of thequantity of carbonic acid pure bicarbonate contains(52-38 per cent.). If 1-91 grams, are weighed intothe carbonic acid apparatus, each -01 loss of weightshows 1 per cent, of pure NaHCO g . '
Sodium Hydrate. —Caustic soda, Hydrated oxideof sodium, Sodium hydroxide —NallO. This body isproduced by adding water to sodium monoxide(Na ? 0 + II 2 0 = 2NaIIO), or by decomposing asolution of sodium carbonate by calcium hydroxide.
The clear solution is decanted from the precipitateand evaporated to dryness. This latter reaction isadopted in manufacturing caustic soda on a largoscale.
Hydrate of sodium is a white, opaque, brittle sub-stance, of a fibrous texture; it fuses below redness.
In properties it greatly resembles the correspondingpotassium hydroxide, but is less volatile at a redheat. Specific gravity 2'00.
When exposed to the air it rapidly attracts mois-ture, becomes liquid, but soon dries up again byabsorption of carbonic acid, the mass becoming con-verted into carbonate, which is not deliquescent.
A solution of sodium hydrate dissolves wood