1322. Soda is found in the ashes of sea-weed, and may be procured by certain chemical agencies from common salt. Common salt consists of the metal named sodium, combined with chlorine; and pure soda consists of the same metal united to oxygen. When water is present, which can afford oxygen to the sodium, soda may be obtained in several modes from salt. The same reasoning will apply to the operation of soda and its compounds, as of the other alkalies and salts; and when common salt acts as a manure, it is probably by supplying inorganic matter to the plant in the same manner as gypsum, phosphate of lime, and the alkalies. In small quantities it is sometimes an useful manure, and it is probable that its efficacy depends upon many combined causes. It is not unlikely, that the same causes as those which act in modifying the operation of gypsum influence the effects of salt. Most lands in these islands, particularly those near the sea, probably contain a sufficient quantity of salt for all the purposes of vegetation; and in such cases the supply of it to the soil will not only be useless, but may be injurious. In great storms the spray of the sea has been carried more than fifty miles from the shore; so that from this source salt must be often supplied to the soil. Salt is found in almost all sandstone rocks, and it must exist in the soil derived from these rocks. It is a constituent likewise of almost every kind of animal and vegetable manure.