SCIENCE OF COMMON THINGS. 189 



Use of mercury in thermometers. How thermometers are constructed. 



1353 Why is mercury preferable to all other liquids for the purposes 

 of the thermometer f 



Because it 'boils at a higher temperature than, any 

 other liquid, except certain oils ; and, on the other 

 hand, it freezes at a lower temperature than all other 

 liquids, except some of the most volatile, such as ether 

 and alcohoL 



Thus, a mercurial thermometer will have a wider range than any other 

 liquid thermometer. It is also attended with this convenience, that the 

 extant of temperature included between melting ice and boiling water- 

 stands at a considerable distance from the limits of its range, or its 

 freezing and boiling points. 



1253 Of what does the mercurial thermometer consist f 



The mercurial thermometer consists essentially of a 

 glass tube with a bulb at one extremity, and which, 

 having been filled with mercury at a certain tempera- 

 ture, introduced through the open end, has been her- 

 metically sealed while full, so that no air can after- 

 wards enter it. 



As the tube and mercury in it gradually become cooled, the inclosed 

 fluid contracts, and consequently sinks, leaving above it a vacant space 

 or vacuum, through which it may again expand on the application of 

 heat. 



1354 As thermometers are constructed of different dimensions and 

 capacities, how are they graduated to indicate the same temperature under 

 the same circumstances, as the freezing point, for example? 



The thermometers are first immersed in melting snow 

 or ice. The mercury will be observed to stop in each 

 thermometer-tube at a certain height ; these heights 

 are then marked upon the tubes. Now it has been 

 ascertained that at whatever time and place the instru- 

 ments may be afterwards immersed in melting snow or 

 ice, the mercury contained in them will always fix itself 

 at the point thus marked. This point is called the 

 freezing point of water. 



1355 How is the boiling point ascertained ? 



It has been found that at whatever time or place the 

 instruments are immersed in pure water, when boiling, 

 provided the barometer stands at the height of thirty 

 inches, the mercury will always rise in each to a certain 



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