Other forms of cone are given in Figs. 264 to 269. In Fig. 270 is shown a convenient disposition of these small cones with a tramway running round each set to convey the crystals to the bench or warehouse. Occasionally rectangular pans of wrought iron are used, but these can hardly bear the picking and hammering required in order to remove the block crystal, The circular cone shown in Figs. 262 and 263 is the best. The yield from such a crystallizer is about 2/ tons.
Soda crystals are packed in casks holding from 1 to 10 cwt. As a rule, 5 casks go to the ton, unless some particular size is indicated or bargained for. One cwt. casks fetch 20s. per ton extra price. The labour in crystal making is very small—half a dozen hands being capable of turning out 150 tons per week. The whole is usually let out to a " ganger " at about 2s. per ton, which includes all labour except the boiler and engine man. Cost and selling prices of soda crystals at the present time are about the same-21. 15s. per ton. The price fluctuates very much with that of soda ash. Within the last few years, the selling price in Loudon has been as high as 61.10s. per ton.
In Franco and Belgium, it is customary to use very small crystallizing cones, not more than 2 ft. In diameter—more usually 18 in. square—and 9 inches deep. From these small cones better and perer crystals are obtained, but the labour is immensely greater. They are arranged iu tiers, one above another, and only allowed to stand about forty-eight hours. The cone is then lifted i up in the hands, the mother liquor poured out, and the crystals are loosened by placing the cone in warm water. They are thee tilted out in one irregular block. Very often, too, the French crystals are dried in a warmed chamber without being allowed to effloresce.
The fact that English crystals after a short exposure to the air become opaque, and covered with a fine powdery sub dance, ie no sign of inferior or deteriorated quality. The appearance is simply caused by a certain amount of the water of crys tallization being given off and the original alkali forming upon the surface. If any thing, the real value of the crystals is enhanced thereby.
Bicarbonate of Sodium. salt may be prepared by passing a stream of carbon dioxide through a cold solution of the neutral carbonate, or by exposing the crystals to an atmo sphere of the gas. Bicarbonate separates in very fine crystalline grains.
This salt has only a feebly alkaline taste. It reddens litmus, and, exposed to a low red heat, loses carbon dioxide, and is converted into the neutral carbonate. It requires 10 parts of water at 15° for solution, giving off carbon dioxide when heated to 70°.
Bicarbonate of soda forms another branch of the alkali manufacture. It has been proposed to
obtain it by mixing four parts of soda crystals with one part of commercial s,squicartionate of ammonia. Upon being heated, the mixture gives off ammonia, and sodium bicarbonate remains. The usual method, howovor, is to act upon soda crystals with carbon dioxide, the process being as follows :—Pieces of chalk or limestoue of any quality, but in not too large lumps, aro loosely thrown into an underground well, or cistern, built of stone, and made tight with a good bedding of clay. In the well-cover is a man-bole, which serves as an entrance when clearing out is required, and for introducing the chalk or limestouo. The weak hydrochloric acid from the roaster condenser—acid that is too weak to be utilized in the bleaching powder department—enters the cistern through a pipe near the bottom. As it rises through the chalk, it becomes saturated, carbon dioxide is evolved, and finally a neutral solution of chloride of calcium overflows through a pipe set in the side of the cistern near the top. The gas is taken off through a pipe stemmed into the cover, and conveyed to boxes in which the soda crystals are packed. These boxes, or chambers, are of various descrip tions and material—stone, wood, or iron. The interior is provided with a false perforated bottom, or series of shelves, upon which the crystals are piled, the carbon dioxide permeating the whole mass. Absorption of the gas immediately takes place with considerable generation of heat, and disen gagement of nearly all the water of crystallization, which collects at the bottom of the box, and is conveyed away by a U pipe, or any convenient luting apparatus. The operation is allowed to proceed until a rod passed through convenient holes in the box meets with no resistance from hard lumps of crystal soda. The finished bicarbonate is then removed in the form of opaque white lumps, retaining the shape of the original crystals. It is dried at a gentle beat in a chamber the temperature of which is kept to about 35° by any suitable arrangement of hot-air pipes. It is finally ground in an ordinary 'flour mill, and sifted through a flue brass or copper gauze, containing not less than 300 meshes to the inch. The finished product is a fine white impalpable powder, and is packed in 1 cwt. barrels, or 5 cwt. casks. The drying and grinding must be care fully effected to prevent loss of carbon dioxide. The mother liquor from the shelves contains a certain amount of bicarbonate and nearly all the foreign salts of the crystals. It is either added to the tank liquor, or boiled down and furnaced as weak alkali. For the bicarbonate process it is usual to employ the inferior crystals, ur crystals that have been in any way damaged.