[Kinds of earth:—(i) rock, of which there are two species; (ii) earthenware; (iii) a certain stone of a black colour; (iv) soda; (v) salt; (vi) compounds of earth and water, including a. glass and fusile stones, and b. wax and incense.—These compounds, like compressed earth or water, are soluble by fire only, which penetrates the water in them. Earth and water, however, in their natural state are soluble, the former by water only, the latter by fire and air.] As to the kinds of earth, that which is filtered through water passes into stone in the following manner: