HEAT 75 



Mercury freezes at 40 below zero and boils at 360 

 Centigrade and so cannot be used for very low or very 

 high temperatures. Hydrogen gas thermometers are 

 used for extremely low and high temperatures. 



54. Uses of Thermometers. Thermometers can, of 

 course, be used only to determine the temperature 

 of a substance. To determine the temperature with 

 a mercury thermometer is to compare the molecular 

 activity of a substance with that of mercury. When 

 the molecules of mercury become more active they 

 need more room, and the mercury moves up the 

 thermometer tube. The amount of this expansion can 

 be read by the degree marks on the tube. The tem- 

 perature of the air in the schoolroom is measured by 

 a thermometer on the wall. What kind is it? What 

 is the most comfortable temperature of the air for 

 the schoolroom and home? Physicians determine the 

 temperature of their patients by placing a very small 

 thermometer under the tongue or in the arm pit. The 

 temperature of a healthy person's body is about 

 98.4 F. If it rises very much above or falls below 

 the normal temperature, there is something physically 

 wrong with the individual. 



In all kinds of manufacturing in which substances are 

 heated, the approximate temperature must be known, 

 and in most cases the exact temperature must be ascer- 

 tained. The temperature of the ovens for baking is 

 carefully watched; also the temperature of the sirup in 

 sugar refineries; and especially is the temperature ob- 

 served in the manufacture of fine steel for razors and 

 watch springs. The purity of substances such as butter 

 and olive oil can be determined by noticing their char- 

 acteristics at a given temperature. Pure butter will 



