Chap, xxxvi.] MERCURIAL THERMOMETER. 467 



boiling, so that air and moisture are expelled from 

 the tube, which is filled with mercurial vapour. At 

 this moment the upper end below the funnel is sealed 

 by a hot flame. This instrument will measure varia- 

 tions of temperature, but in this form will not 

 indicate corresponding values. The stem must, there- 

 fore, be graduated. For this purpose two standard 

 temperatures are taken, viz. the temperature of the 

 freezing point, and that of the boiling point. The 

 former is found by immersing the thermometer in a 

 mixture of ice and water till the mercury reaches 

 a level at which it remains. At this point a mark is 

 made on the stem. The boiling point is obtained not 

 by immersing the thermometer in boiling water, but 

 by surrounding it with steam from boiling water. 

 Water boils at different temperatures in different 

 vessels, but the temperature of steam is constant. 

 The thermometer is, therefore, suspended in a vessel 

 at the bottom of which water is kept boiling. It is 

 surrounded by a steam jacket, and when the level of 

 the mercury has become stationary a mark is made on 

 the stem. Both marks ought to be obtained at 

 standard pressure, for water will boil at a lower tem- 

 perature if the pressure be diminished. The space 

 between the two marks is now divided off by marks 

 into lengths of equal volumes, and the graduation 

 continued above the mark for the boiling point, and 

 below that for the freezing point. The instrument 

 will now indicate degrees of temperature which can 

 be used for comparison. If the thermometer is 

 brought into contact with two bodies successively, 

 and the reading of the level of the mercury is the 

 same in both cases, both bodies are at the same 

 temperature ; if there is a difference in the reading, 

 the number of degrees of difference on the scale 

 indicates the difference of temperature. 



To the thermometer thus constructed different 



