While the ashes of some plants, as the upland plants, generally yield potassa; others, as many marine plants, the _salicornia europea_, _salsola tragus_, _salsola kali_, _&c._ afford soda by incineration. It will be sufficient, however, to observe, that the ashes of all plants contain alkali, in more or less quantity, which depends on various circumstances; and that the alkali may be extracted by lixiviation, and, in some instances, may even be seen among the ashes, in a semivitrified mass. The white ashes, which are formed by the combustion of animal matter, as osseous or bony substances, we may remark, do not afford potassa or soda, but only phosphate of lime, and some uncombined earths. Bones, nevertheless, may, like wood, be carbonized, although the charcoal formed is of a different nature. For the preparation of phosphorus from bone-ash, see the article Phosphorus. _Sec. XXXIX. Of Clay._ Clay is an argillo-siliceous substance, of a colour more or less yellow, and containing a variable quantity of silica and alumina, with oxide of iron. There are a variety of clays; the common potter's clay, pipe clay, porcelain clay, &c. Some contain, and others are free from iron. Those that contain this metal burn _red_; while those which remain, or become white in the process of burning, are free from it. The use of clay in fire-works is confined nearly altogether to rockets. In the driving of sky-rockets, &c. the _charge_ must always be driven one diameter above the piercer, and on it there is sometimes rammed one-third of a diameter of clay, through the middle of which a hole is bored to the composition, so that, when the charge is burnt to the top, it may communicate its fire through the hole, to the stars in the head. This, however, is not always the case. See _Rockets_. The clay for fire-works, is usually prepared of the common kind, which contains neither stones nor sand. It must be first baked in an oven, until perfectly dry, and then pulverized, and sifted through a common hair sieve. In China, the Chinese mostly employ, for this purpose, their white porcelain clay. _Sec. XL. Of Quicklime._ Lime, as it is found in nature, is combined with carbonic and sulphuric acids, and less frequently with some of the other acids, as the nitric, fluoric and phosphoric. Calcareous carbonates are the most abundant; in which we include marble, limestone, and chalk; and the sulphate, or gypsum, may be considered the next. Lime constitutes the basis of marine shells; for, when burnt, they furnish quicklime. Its union with nitric acid is well known, forming the calcareous nitre of the saltpetre caves of Kentucky, &c. We have mentioned this combination under the head of nitre.