$2,600,000. It will be noticed that while the number of establishments only doubled during this interval of twenty years, the value of the product was nearly quadrupled. At the time of the tenth census—that is, in 1880—there were fifty-one factories, yielding an annual output of about $6,000,000. The importance of Pittsburg as a glass center can best be appreciated by considering these figures relatively to the whole American output. The one district of Allegheny County produced a little over a quarter of the entire glass manufactured in this country, while the State, as a whole, made a trifle over two fifths of the total. The product was window glass, hollow ware, and green glass, no plate glass appearing in the State returns up to that time.
While the other conditions were also favorable, the chief cause of this marked development in Pennsylvania has undoubtedly been her fuel. During the first ten years of the century her forests alone were used to any extent, but the substitution of coal for wood went on continuously for the succeeding seventy years, until in 1880 it was everywhere the chief fuel, wood being employed only in heating the annealing ovens and for other minor purposes. Up to 1880, however, the development of the industry consisted for the most part in the improvement of already existing devices. The furnaces were made larger, the chemicals were purer, the melting pots more capacious. The coal was burned to better advantage, and consequently the batch was more thoroughly fused. Greater differentiation of the processes was being slowly brought about. In window-glass factories separate furnaces were provided for melting and blowing. In the handling of the glass there were similar improvements. The continuous rod leer was coming into use, while bottles and other hollow ware were annealed in iron trucks and no longer needed separate handling. All these were substantial gains; yet up to 1880 the fact remained that no very radical changes had been introduced into general glass-making practices—we do not here refer to the subsequent working of the material—and it was undeniable that the American product was in many respects inferior to the imported. We could not at that time successfully compete with Belgium even in the matter of window glass.
But during the past decade there has come a change so radical and so far-reaching in its results that more glass history has been condensed into these busy ten years than is to be found in the previous eighty years. The natural-gas well has been a veritable Aladdin's lamp to the glass industry. The fuel itself has been known for many years. As early as 1775 Washington had a "burning spring" on the tract of land deeded to him in the Kanawha Valley for military service, which he desired to make public property; but through some technicality the grant