Page:Popular Science Monthly Volume 34.djvu/619

 is proposed that little more shall be seen than falls to the lot of the flesh-and-blood visitor. It is, perhaps, wiser that the uncertain light and the steam and smoke shall be permitted to cover with their convenient mantle those technical details that would fail to attract general interest. In such matters it is uncomfortable to have your guide too knowing if he insists on sharing all his knowledge with you.

To define glass physically would be a superfluous task. Every one is informed of its hardness and solidity. A series of annoying accidents has demonstrated beyond doubt its exceeding brittleness. The ragged-edged splinters that result from such occasions suggest that the solid is amorphous, or without regular crystalline form.

To define the material chemically may be less unnecessary. It is a mixture of different silicates—that is to say, of mixtures of silicic acid with the bases soda, potash, lime, magnesia, alumina, iron, and lead. Considering that we are to be non-technical, this is rather a formidable list, but it must not be thought that any one glass contains all of these ingredients. Every true glass consists of at least two metallic bases united with the silicic acid, and generally, by virtue of the impurities of the crude material, traces of several more. So we have grown into the habit of designating the different kinds of glass by the names of the two predominant bases. Window glass, for instance, is known as a lime-soda glass; table crystal as a lead-potassium glass, and so on through the list. This system of nomenclature is open to the objection that the name of the product and its composition do not correspond in all the glass-producing countries, but these technical discrepancies seem unavoidable. The physical properties of the glass follow very closely its chemical constitution. Many of the silicates employed in glass-making are entirely infusible alone, but, when given suitable associates, are quite manageable. The weight of the glass is also directly dependent upon the metallic bases with which the silica is combined. Crystal is made heavy by the lead present, while window glass, having only light bases in its make-up, has a correspondingly small weight. It is little more than two and a half times as heavy as water. Each chemical change has its physical counterpart.

In spite, however, of the relative cheapness of glassware, we have still a pane of glass for the rich and another for the poor. Both products, the plate and the sheet glass, have essentially the same composition, but they differ very much in the purity of the crude materials used, and in the method of fabrication. Of recent years the improvements in the manufacture of sheet glass have been so marked that it is now frequently introduced into buildings of the better class in place of the more expensive plate. On