Why are tar, pitch, and turpentine called naval stores?--and what determines the locality in which they are made? What is varnish, and for what purposes is it used? FOR STUDY AND REFERENCE Obtain specimens of crude rubber, vulcanized rubber, and hard rubber; note carefully the characteristics of each. Burn a very small piece of cheap white rubber-tubing in an iron spoon or a fire-shovel; note the character of the residue. Obtain specimens of gutta-percha, resin, pitch, turpentine, shellac, copal, dammar, and creosote for study and inspection. CHAPTER XII COAL AND PETROLEUM The economic history of nearly every country that has achieved eminence in modern times dates from its use of coal and iron; and indeed the presence of these substances in workable deposits means almost unlimited power. The present era is sometimes called the Age of Steel, but the possibilities of producing steel in enormous quantities, at less than one-fifth its price at the beginning of the nineteenth century, depended mainly upon the use of mineral coal instead of charcoal in its manufacture. Coal consists of accumulations of vegetable matter that were formed in prior geological ages. Under the action of heat and moisture, and also the tremendous pressure of the rock layers that afterward covered them, the vegetable matter was converted to mineral coal. The aggregate coal-fields of the United States are not far from two hundred thousand square miles in extent, but of this area not much more than one-half is workable. In Europe there are estimated to be about one hundred thousand square miles of coal-lands, of which about half are productive at the present time. Of this Great Britain has 12,000 square miles, Spain 4,000, France 2,000, Germany 1,800, and Belgium 500. In Canada there are about 20,000 square miles of coal-land; a part of this is included in the Nanaimo field on the Pacific coast, but the most important are the Nova Scotia beds, which form about the only supply for the British naval stations of America. China has extensive coal-fields. In character coal is broadly divided into two classes--anthracite or hard, and bituminous or soft, coal. Anthracite coal occurs in folded and metamorphic rocks. It is hard and glassy, and does not split into thin layers or leaves. The beds have been subjected to intense heat and pressure, and the coal has but a very small amount--rarely more than five per cent.--of volatile matter; it burns, therefore, with little or no smoke and soot, and on this account is very desirable as a fuel in cities. Two areas in Colorado and New Mexico produce small quantities of pure anthracite; practically all the commercial anthracite comes from three small basins in Pennsylvania. In quality it is known as "red ash" and "white ash," the former being the superior.