Carbon is very widely distributed throughout all the kingdoms of nature; the diamond represents its purest condition, crystallized in the form of a regular octahedron. Plumbago, or graphite, the "black lead" of our pencils, is another form which is nearly pure, and is sometimes crystalline. This element, combined with oxygen and various earths and minerals, forms carbonates, as chalk and limestone, and combined with oxygen it occurs as carbonic acid in the air and in many mineral waters. Two varieties of carbon are now officinal, in the form of charcoal prepared from wood and from bones.