works were operated entirely with wood. The same practice prevailed in Europe, and for many years wood was preferred to coal. This made it necessary to establish glass-houses near the forest districts, for in the absence of railroads and of steam navigation it was impracticable to carry so bulky a fuel for any great distance. With the substitution of coal a new condition was introduced, and the question of fuel became for the time of less moment than the supply of crude materials. These could only be obtained in certain localities, while the fossil fuel was available in many. In our own day, and within the last half dozen years, another and much greater disturbance of the industrial equilibrium has been brought about by the displacement of coal by natural gas. It is no exaggeration to say that, as far as quantity and the perfection of the processes of manipulation are concerned, the development of the glass industry has been greater, since the introduction of natural gas than in all previous time. The immense advantage of the fluid fuel over the solid, both in economy of operation and superiority of product, has made the geography of the glass industry and of natural gas nearly identical. One might almost use a geological map of the United States for a chart of the glass-making districts. Wherever the Trenton limestone and the upper coal measures are near the outcrop, one may reasonably expect to find glass-houses scattered over the surface.
It does not follow, of course, that glass-making at the present day is limited entirely to the natural gas country. There are occasional glass-houses in various localities, and there are districts so favorably located in other respects that they can overcome the disadvantages of the solid fuel and still rank as recognized centers of the industry. Such a center is found in the large bottle-making establishments in southern New Jersey. But these works are generally quite old, and already had expensive plants in operation before the utilization of natural gas.
The character of the fuel has thus given rise to three distinct eras in the industry—that of wood, of coal, and of gas. The use of petroleum has been too limited and for too secondary purposes to mark a distinct chapter.
The fourth essential element is labor. While it is, in a technical sense, the most important element of all, it has had much less influence than the material factors in deciding the history of the industry. Since its emancipation from serfdom, labor has displayed a portability which has made it available in any quarter of the globe. A large degree of dexterity, if not of intelligence, is needed in the glass-worker; but if one is to judge from the mixed nationality of our American representatives of the craft, he does not belong to any country, and is ready to go wherever he is wanted. It is easier to bring him to the work than to take