GLASS. Definition and trial art has had no more beautiful and useful material to aid its progress throughout the ages than glass, and many sciences could scarcely have existed, or could not have developed far, without its assistance. A perfect glass is surpassed in its brilliancy, pellucidity, refractive ness and colorless transparency only by the diamond itself. Its essential and dis tinguishing features are its freedom from air bubbles, specks of foreign matter and stria. It can be produced either color less or tinted with any hue, and either transparent or opaque. Its opacity may be either partial or complete. It is smooth and shining on surface and in fracture. It retains its shining surface upon being reheated, and is capable of a high degree of polish when cold. It can be welded by contact while in a semi-molten state, and fractured instantane ously—at a certain stage of manipulation— by chilling. It is tenacious and elastic, and can be blown as thin as gold leaf or spun as fine as the web of silk.
As the medium for one of the world's oldest handicrafts, much that is worth saying about glass cannot be said without reviewing the operations of its ancient artificers. The volcanic erupt, obsidian, an impure semi-trans parent, vitreous substance, in color varying from a greenish gray to almost black, probably served as a native material from which articles for ornament and use were fashioned by the ancients before the event—accident or incident —occurred which gave the world its artificial substitute and complement, glass. This native glass — obsidian — invariably found in the neighborhood of some extinct volcano, was used by the Egyptians in the fabrication of works of art, and in some few known instances in articles of utility, but the artificial product, when once brought under control, effectually put a limit to its usefulness.
It is known that the Romans and the early Mexicans fashioned objects from obsidian, but in all probability the former used it as a variety of glass, and the latter from the fact that glass itself was unknown to them.
The period of the invention of glass cannot now be traced and how it was discovered is a matter of surmise, but of its importance there can be no question. What glass is, and what its possibilities are, no words could define more concisely than do those used by Dr. Johnson* in one of his papers to The Rambler. *Who,D he says, °when he first saw the sand and ashes by casual intenseness of heat melted into a metalline form, rugged with excrescences and clouded with impurities, would have imag ined that in this shapeless lump lay concealed so many conveniences of life as would in time constitute a great part of the hapiness of the world? Yet by some such fortuitous lique faction was mankind taught to procure a body at once in a high degree solid and transparent, which might admit the light of the sun and exclude the violence of the wind, which might extend the sight of the philosopher to new ranges of existence, and charm him at one time with the unbounded extent of the material creation, and at another with the endless sub ordination of animal life, and, what is yet of more importance, might supply the decay of nature and succor old age with subsidiary sight. Thus was the first artificer of glass employed, though without his own knowledge or expecta tion. He was facilitating and prolonging the enjoyment of light, enlarging the avenues of science, and conferring the highest and most lasting pleasures; he was enabling the student to contemplate nature, and the beauty to behold herself." The essential constituents of glass are silica and alkali, and in accepting the authority of the ancient historian that these elements were present in the °sandy beach under Mount Car me') and the "cargo of natron conveyed by Phcenician merchants from Egypt to Syria)" in an unrecorded a.c. period, the camping incident, and the cooking fire accident may be accepted as contributing to the °sand and ashes, by casual intenseness of heat° being °melted into a metalline form') and giving the world a ma terial absolutely essential to its progress.