GLASS-MAKING. 601 



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 sui^erfluous 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 corre- 

 spond 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 chemi- 

 cal 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 build- 

 ings of the better class in place of the more expensive plate. On 



