in physiology, denotes the conversion of wood, bones, and other substances, into stone.

The fossile bodies found petrified are principally either of vegetable or animal origin; and are more or less altered from their original state, according to the different substances they have lain buried among in the earth; some of them having suffered very little change, and others being so highly impregnated with crystalline, sparry, pyritical, or other extraneous matter, as to appear mere masses of stone or lumps of the matter of the common pyrites; but they are generally of the external dimensions, and retain more or less of the internal figure, of the bodies into the pores of which this matter has made its way.

The animal-substances thus found petrified are sea-shells; the teeth, bony palates, and bones, of fish; the bones of land-animals, &c. These are found variously altered, by the infumation of stony and mineral matter into their pores; and the substance of some of them is now wholly gone, there being only stony, sparry, or other mineral matter remaining in the shape and form.

As to the manner in which petrification is accomplished, we know very little. It has been thought by many philosophers, that this was one of the rare processes of nature; and accordingly such places as afforded a view of it, have been looked upon as great curiosities. However, it is now discovered, that petrification is exceedingly common; and that every kind of water carries in it some earthy particles, which being precipitated from it, become stone of a greater or lesser degree of hardness; and this quality is most remarkable in those waters which are much impregnated with selenitic matter. Of late, it has also been found by some observations on a petrification in East Lothian, Scotland, that iron contributes greatly to the process; and this it may do by its precipitation of any aluminous earth which happens to be dissolved in the water by means of an acid; for iron has the property of precipitating this earth, though it cannot precipitate the calcareous kind. The calcareous kinds of earth, however, by being soluble in water without any acid, must contribute very much to the process of petrification, as they are capable of a great degree of hardness by means only of being joined with fixed air, on which depends the solidity of our common cement or mortar used in building houses. See the articles CEMENT and MORTAR. See also SAND, SELENITES, ROCK, STONE, and WATER.