Beyond the coal stage, however, lies the more or less distinctively modern one of the utilization of gas and oil for fuel. The existence of great natural centers, or underground stores, of gas and oil is probably no new fact. We read in the histories of classic chroniclers of the blazing gases which were wont to issue from the earth, and to inspire feelings of superstitious awe in the minds of beholders. Only within a few years, however, have geologists been able to tell us much or anything regarding these reservoirs of natural fuel which have become famous in America and in the Russian province of Baku. For example, it is now known that three products--gas, oil, and salt or brine--lie within natural receptacles formed by the rock strata in the order of their weight. This law, as has well been said, forms the foundation of all successful boring experiments, and the search for natural fuel, therefore, becomes as easy and as reliable a duty as that for artesian water or for coal. The great oil fever of the West was attended at first, as Professor M'Gee tells us, with much waste of the product. Wells were sunk everywhere, and the oil overflowed the land, tainting the rivers, poisoning the air, and often driving out the prospectors from the field of discovery. In Baku accidents and catastrophes have, similarly, been of frequent occurrence. We read of petroleum flowing from the ground in jets 200 feet high, and as thick as a man's body; we learn how it swept away the huge cranes and other machinery, and how, as it flowed away from the orifices, its course was marked by the formation of rivers of oil many miles in length. In America the pressure of rock gas has burst open stills weighing over a ton, and has rushed through huge iron tanks and split open the pipes wherewith it was sought to control its progress. The roar of this great stream of natural gas was heard for miles around as it escaped from the outlet, and when it was ignited the pillar of flame illumined the surrounding country over a radius extending in some cases to forty miles. It is clear that man having tapped the earth's stores of natural fuel, stood in danger of having unloosed a monster whose power he seemed unable to control. Yet, as the sequel will show, science has been able to tackle with success the problems of mastering the force and of utilizing the energy which are thus locked up within the crust of the globe.