This is now always made, on a large scale, by the manufacturer. Formerly it was prepared from the ashes of sea-weeds, usually called kelp, or from that of plants growing by the sea-side and cultivated for the purpose, which is known in commerce by the name of barilla. At present it is procured almost exclusively by the decomposition of sulphate of soda. This salt, having been prepared by acting on common salt with dilute sulphuric acid, is exposed to heat in a reverberatory furnace with carbonate of lime and bituminous coal, which react upon it so as to produce a mixture, consisting of carbonate of soda, caustic soda, sulphuret of calcium, undecomposed sulphate of soda, etc. From this the soluble substances are separated by lixiviation, and, having been procured in the solid state, are again exposed to heat with carbonaceous matter, as coal-dust, saw-dust, etc. The caustic soda is thus carbonated, and the remaining sulphate converted first into sulphuret, and finally into carbonate, which now becomes the chief constituent. in this state the preparation is called soda ash. From this the carbonate of soda is obtained by lixiviation and evaporation, and is subsequently purified by solution and crystallization.

Properties

The salt is in fine large transparent rhombic prisms, which speedily effloresce on exposure to the air; and, as found in the shops, they are in various conditions between the two extremes of perfect crystal and an opaque white powder. Exposed to heat, they effloneys. The elimination of the bicarbonate of soda is very slow compared with that of the other alkalies. it escapes with the urine in the state of bicarbonate. The bicarbonate of potassa is longer tolerated than that of soda, because its elimination is much more rapid. The introduction of either of these bicarbonates with the food has always occasioned the elimination of a notable quantity of carbonate or bicarbonate of ammonia. The escape by the kidneys of carbonate of ammonia, when administered, is very rapid; and its elimination by any other organ could not be detected. it could never be found in the expired air. No disturbance of the nervous system consequent on its exhibition was observed; and the intestines on post-mortem examination were perfectly sound. The organic ingredients of the blood were not sensibly affected in quantity by the alkalies; nor did the urea appear to be diminished. The health of the dogs remained perfectly sound, though they lost some flesh in consequence of the diminished amount of food consumed, from its mixture with the alkalies. (Arch. Gén., 6e sér., i. 352.) - Note to the third edition.

resce more rapidly, and at a red heat give up all their water of crystallization. The salt is inodorous, and has a saline, acrid, and alkaline taste, but less disagreeable than that of the corresponding salt of potassa. It is soluble in twice its weight of cool water, and insoluble in alcohol. It has an alkaline reaction.

The crystals of carbonate of soda contain one equivalent of carbonic acid, one of soda, and ten of water; and the proportion of water of crystallization is 64 per cent.; so that the perfectly dried salt is nearly three times as strong as the perfectly crystallized.

Medical Effects and Uses

These are so nearly the same as those of carbonate of potassa, that I must content myself with referring to what has been stated of that salt. it is used for the same purposes internally and externally. in consequence of its less disagreeable taste, it is usually preferred to the carbonate of potassa as an antacid; but it is less relied on for alkalizing the system. it has been supposed, however, to be specially useful in the resolution of goitre. in consequence of its inequality of strength, according to the degree of its efflorescence, it is impossible to give a precise dose of the salt as ordinarily found in the shops. From fifteen grains to a drachm of the crystallized, and five grains to a scruple of the perfectly dried salt, may be given twice or three times a day. For external use, the same preparation and proportions may be employed as of carbonate of potassa. (See page 812.)

Dried Carbonate of Soda (Sodae Carbonas Exsiccata, U.S., Br.) has been directed in the Pharmacopoeias, in consequence of the inequality of the partially effloresced salt. it is prepared by exposing the crystals to heat until entirely deprived of their water. The dose has been stated above. One advantage of the preparation is, that it may be given in pills if thought advisable.