234 WELLS'S NATURAL PHILOSOPHY. 



nations in volume, or magnitude, through a considerable range of tempe- 

 rature are exactly uniform and proportional with every increase and dim- 

 inution of heat. Mercury, moreover, 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 vola- 

 tile, 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 extent of temperature included between melting 

 ice and boiling water stands at a considerable distance from the limits of ita 

 range, or its freezing and boiling points. 



Describe the ^35. TliG mercurial thermometer consists es- 

 Someter *'""" sentially of a glass tube with a bulb at one 

 end, partially filled with mercury. The mer- 

 cury introduced through an opening in the end of the 

 tube is afterward boiled, so as to expel all air and moist- 

 ure, and fill the tube with its own vapor. The open end 

 of the tube is then closed, by fusing the glass, and as the 

 mercury cools it contracts, and collects in the bulb and 

 lower part of the tube, leaving a vacuum above, through 

 which it may again expand and rise on the application of 

 heat. In this condition the thermometer is complete, 

 with the exception of graduation. 



536. As thermometers are constructed of different dimen- 

 IToT7 are tner- . .... ^ j i 



momcters gra- sions and capacities, it IS necessary to nave some nxed rules 



duated ? f-Qj. graduating them, in order that they may always indicate 



the same temperature under the same circumstances, as the freezing-point, for 



example. To accomplish this end the following plan has been adopted : — 



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 instruments may be afterward 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. 



Another fixed point is determined by immersing the instruments in boiling 



•water. It has been found that at whatever time or place the instrumenta 



are immersed in pure water, when boiling, provided the barometer stands it 



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



height. This, therefore, forms another fixed point on the scale, and is called 



the boiling point. 



Thus far all thermometers are constructed alike. In the 



th"7raometM° thermometer most generally used, and which is known as 



of Fahrenheit Fahrenheit's, the intervals on the scale, between the freezing 



graduated? ^^^ boiling points, are divided into 180 equal parts. This 



