GLASS-MAKING.
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the end is drawn into a fine jet. This is hastily plunged beneath the surface of a Bath of pure mercury while the whole thermometer is still hot. As it cools, the air inside shrinks and the mercury
![](http://upload.wikimedia.org/wikipedia/commons/thumb/b/b2/PSM_V39_D643_Calibrating_thermometers.jpg/500px-PSM_V39_D643_Calibrating_thermometers.jpg)
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 expanding air may freely escape. The tube is then held vertically