SODA (SALT of), a name for the fossile alkali, or basis of sea-salt. In the eastern countries this alkali is found native, either in a pretty pure state, or mixed with earthy substances, from which it is easily separated by means of water; and Dr. Heberden found it in considerable plenty on the peak of Teneriffe. It is also obtained from the ashes of the plant kali, and from those of the alga marina, which last form our kelp; but this is the most impure state in which it ever is obtained. The best sort of that artificially procured is made by burning the plant kali; and is prepared at Alicante in Spain, from an annual procumbent kali with short leaves like those of horse-leek. These ashes are called soude or sode in France, where they are used as one of the common kinds of potash. They are brought to this country under the name of Spanish ashes, bariglia or barilla, in hard spongy masses, partly whitish or grey, and partly blackish. From these the saline part is extracted pure by powdering and dissolving in water. This, however, contains also a portion of vegetable alkali, and some sea-salt. A work has lately been erected in England for decomposing sea-salt and extracting the pure alkali from it, which is commonly sold in the shops under the name of English pearl-ashes; but it

is much less pure than that which comes from Alicante; being mixed with a large quantity of sea-salt. The process for decomposing the salt in this work is said to be mixing it with lime, and taking off the saline efflorescence which appears on the surface.

This mineral alkali will saturate fully as much acid as the vegetable, though its attraction for acids is not so great. It is certain, that if a quantity of vegetable alkali be added to a solution of Glauber's salt, or to sea-salt, they will be decomposed, and the mineral alkali will be obtained in its pure form: but whether this would also take place if the vegetable alkali were deprived of its fixed air, has not been ascertained; indeed it has been observed, that the two alkalis when rendered caustic are much less easily distinguished than when combined with fixed air. The mineral alkali easily then distinguishes itself by its superior mildness and readiness to crystallize. For the combinations of the salt of soda, see CHEMISTRY, no 124, 185, 231, 270, 275, 296.