PROFESSOR OF PHYSICS AND CHEMISTRY IN THE PHILADELPHIA MANUAL TRAINING SCHOOL.
THERE are few objects of manufacture which better than glass illustrate the immense preponderance in value of human labor over crude material. It is a substance which might serve economists as a parallel to their favorite illustration of the comparative values of a steel watch-spring and the bit of iron-bearing earth from which it is wrought.
In the case of glass, the crude materials are so plentifully distributed in nature as to be almost valueless. The basis of the compound, sand, is so very abundant that it has furnished the symbol, in more than one parable, for quantity without limit. Like the unnumbered sands of the sea was a vast promise to the children of men. Somewhat less abundant than the sand are the other chemicals which it is necessary to mix with it in order to produce that double silicate which goes under the general name of glass. They are, however, far from being either scarce or expensive. The alkaline ingredient, the carbonate of soda, is made from common salt, a mineral whose wide distribution in nature is at once apparent when one recalls the fact that the sea, thirty or forty times in bulk the total elevated mass of the earth, is one vast storehouse of the substance; that salt springs or brines abound at our very doors—in New York State, in Michigan, and in Virginia; and that vast deposits of the solid rock-salt are to be found in Louisiana and Prussia. The third ingredient, the lime, is simply calcined limestone, a rock which forms whole ranges of hills, and is found in every corner of the globe. For the production of the fine flint glass, or crystal, which forms the special subject of the glass-worker's skill, it is also necessary to add a fourth ingredient, red lead or minium. As this is the oxide of an easily