86 



Introduction to the Study of Science 



uniformly between the freezing point and the boiling point, 

 and because it becomes solid at freezing temperature and vapor- 

 izes at boiling. The substances commonly utilized in ther- 

 mometers of the " liquid " kind are mercury and alcohol. Al- 

 cohol is adapted to temperatures that fall below that at which 

 mercury freezes or becomes a solid, as it does in the high lati- 

 tudes and very high altitudes. Both mercury and alcohol 

 expand and contract uniformly throughout a wide range of 

 changes in temperature. They are sensitive to very slight 

 differences in temperature and show relatively 

 large changes in volume as a result. They re- 

 spond promptly to such changes and are also 

 easily visible or can be made so by using a 

 glass tube with small bore. They report quickly 

 and accurately, therefore, the actual tempera- 

 ture of any body. 



Exercise. Other important features may be 

 noted. Why, for instance, is glass used for the tube ? 

 How does glass respond to changes in temperature, 

 as compared with mercury? Why is the bore of 

 the tube very small and uniform? Why is there a 

 bulb at the lower end and why is it much larger than 

 the bore of the tube ? What other substances would 

 do as well as mercury and alcohol in the liquid 

 thermometer? Explain your answer. What must 

 be done to make alcohol easily visible in the tube ? 



On the Fahrenheit scale (Fig. 24), which is com- 

 monly used, the freezing point is designated as 

 32 and the boiling point 212. This gives an in- 

 terval of 180 between the two fixed points. The 

 zero, 0, is 32 below the freezing point, 

 a German scientist (1686-1736), in making the 

 one of the fixed points the temperature of the 

 which he called 100. The other fixed point 

 mixture of ice, sal ammoniac, and water, and this 

 zero. He divided the interval into 100 equal 



FIG. 24. Fah- 

 renheit and Centi- 

 grade scales. 



Fahrenheit, 

 scale, took as 

 human body, 

 he found by a 

 he called 0, 



