Page:Popular Science Monthly Volume 7.djvu/576

558 observe the interior, without entirely uncovering the bath. A fire-truck, p is charged with live fuel, heats the bath to the desired temperature. The glass is introduced into the preparatory oven by an opening in the outer wall, and thence it is moved through a second open-in on to the floor of the oven, a. The workman who watches the glass through a spy-hole, when he finds it at the proper heat, pushes it by an iron rod to the slope, d, whence it slides into the bath and is received on the basket, k. When the glass has cooled to the temperature of the bath, the lid is removed, and the basket, k, is raised out of the bath with the tempered glass.

In tempering sheet-glass the arrangements of both oven and bath are slightly modified, as shown in Fig. 3. In place of the sloping exit



for articles from the oven to the bath, M. de la Bastie has a rocking table, E, which is hinged underneath to the mouth of the oven, and which also forms the floor of the oven. When the glass has been sufficiently heated, the workman, by means of a lever, tilts the table, and the glass slides gently down an easy incline on to a table set at a corresponding incline in the bath. If it is not of importance that the transparency of the glass should be preserved, no special precautions are taken to prevent the dust from the furnace settling on its face. Where, however, clearness is required, the glass is heated in a muffle, perfect transparency being obtained. The process of tempering or toughening, exclusive of the time required for heating the glass, occupies but a minute or so, the glass being immersed in the bath and at once withdrawn and set aside to cool. The cost per article, as may be supposed, is merely nominal.

Glass which has been treated in this manner undergoes a physical transformation as complete as it is remarkable. Its appearance is in no way altered, either as regards transparency or color—if colored glass be so treated—and its ring or sound is not in any way affected.