58 CHEMISTRY AND HEAT 



The temperature that is indicated by the Fahrenheit ther- 

 mometer may be converted into absolute temperature by 

 adding it to 460. Thus, a temperature of 85 by the Fahren- 

 heit thermometer corresponds to the absolute temperature 

 of 85+460 = 545. On the centigrade scale the absolute zero 

 is 273 J below the zero point. On the Reaumur scale it is 

 2 18 1 below zero. When the thermometer indicates tem- 

 peratures below the zero point of its graduation, the indicated 

 temperature must be subtracted from 460, 273 i or 218 , 

 respectively, to find the absolute temperature; that is, absolute 

 zero is -460 F., -273| C., and -218f R. 



Conversion of Temperatures. A degree on the Fahrenheit 

 scale is equal to TS& = I of a degree centigrade and to /& = $ of 

 a degree Reaumur. Temperatures according to any one of 

 these scales, therefore, may be converted into the correspond- 

 ing temperatures on the other scales by using the following 

 simple formulas: 



Temp. F. = f Temp. C.+32 =| Temp. R. + 32 . 



Temp. C. = | (Temp. F.-32)=f Temp. R. 



Temp. R. = a (Temp. F.-32)=^ Temp. C. 



The table on page 57 shows the equivalents of centigrade 

 temperatures on the Fahrenheit scale. 



COEFFICIENTS OF LINEAR EXPANSION 



The coefficient of expansion of a body is its expansion per 

 degree rise of temperature. The coefficient of surface expan- 

 sion is double, and that of cubic expansion three times the 

 coefficient of linear expansion. The table on page 59 shows 

 the coefficient of linear expansion for various substances. 



For example, a 30-ft. steel rail in warming from 20 F. below 

 zero to 100 F. will expand (20+100) X .0000059!) X30X 12 

 38 in. 



MEASUREMENT OF HEAT 



British Thermal Unit. The unit most commonly used for 

 the measurement of heat is the British Thermal Unit, abbre- 

 viated B. T. U. This is the amount of heat required to raise 

 the temperature of 1 Ib. of pure water 1 F. at or near the tem- 

 perature 39.1 F., whi,ch is the point of maximum density of 

 water. Heat is a form of energy, and it may be transformed 



