the other hand, the processes of manufacture by which the latter glass is produced have been so far perfected that its use is now permitted to many who would hitherto have thought themselves unable to afford it.
A third form of window glass, the so-called crown glass, must also be mentioned for the sake of completeness, though it has little commercial importance, and less in America than in England. Both sheet and crown glass owe their origin to the blower's breath. Though they are less brilliant than the plate, their methods of fabrication are much more interesting, since they involve a far greater amount of manual dexterity on the part of the artisans. It is, indeed, difficult to know which to admire the more, the chemistry or the physics of the operation; the nicety with which the glass-maker regulates the proportions of his charge so as to produce this beautifully clear substance, or the skill with which he subsequently handles the finished glass and adapts it to our uses.
Sheet glass forms the window-pane of the multitude. The possibility of making it of excellent quality and in large sizes is due almost entirely to the substitution of gaseous for solid fuel. No other among our numerous American industries has been so benefited by the utilization of natural gas. European sheet glass was up to this time unquestionably superior to our own. A larger experience and more approved furnaces made it possible for the foreign manufacturers, and particularly those of France and Belgium, to solve with greater success the knotty problems connected with glass-making. In many places they had already substituted gas for coal, and obtained the happiest results. With the advent of natural gas the position of the American producer was suddenly changed. He had at his command the most desirable of fuels, and one that was at the same time very cheap and almost totally free from sulphur. As a result, he soon equaled and now surpasses his transatlantic rivals.
But the manufacture of window glass is essentially difficult. Even when the troublesome question of fuel has been satisfactorily settled, there remain many other substantial difficulties which must be met and conquered. From the mixing of the crude materials to the annealing of the finished product, the glass-maker must be alert and intelligent. It is a very easy matter simply to make glass. Sand, metallic bases, and heat are the only elements needed. But to make good glass—glass that is clear, transparent, colorless; that simulates the purest water of a mountain-stream—this requires skill and patience. From beginning to end the process is one of painstaking and delicate manipulation.
In the genesis of a pane of glass, the first step is naturally the provision of such stuff as it is made of. While glass is theoreti-
