GLASS-MAKING. 625 



the end is drawn into a fine jet. This is hastily plunged beneath 

 the surface of a Bath of pure mercury while the whole thermome- 

 ter is still hot. As it cools, the air inside shrinks and the mercury 



Fig. 5. Calibrating Thermometers. 



rises into the outer bulb. The permanent bulb is then carefully 

 warmed, the tube being in a horizontal position so that the ex- 

 panding air may freely escape. The tube is then held vertically 



