18 



THE PRODUCTION AND USE OF LIGHT 



180 degrees. It is, therefore, evident that 100 degrees on the 

 centigrade scale equal 180 degrees on the Fahrenheit scale, or 

 1C. equals 1.8F. But we must always remember that the 0C. 

 is at the freezing point of water while 0F. is at the freezing 

 point of the mixture of ice and salt, and that the freezing 

 point of water is 32F. (Fig. 11). 



T e. 



BOOMS' 



-to- 

 ne 



KK 



t/o 



li 

 ffi 



FIG. 11. Thermometer scales. 



EXAMPLE. Change 70F. to C. Solution: 70F. means 70 Fahren- 

 heit degrees above the melting temperature of the mixture of salt and 

 ice. 70 -32 = 38. This temperature is, therefore, 38 Fahrenheit 

 degrees above the freezing point of water. But each centigrade degree 

 equals 1.8 Fahrenheit decrees. Therefore, it is as many centigrade 

 degrees above the freezing point of water as 1.8 is contained times 

 in 38 which is 21%. Therefore, 70F. = 2lJ^C. 



EXAMPLE. Change -10C. (10 degrees below zero, centigrade) 

 to F. Solution: 10C. means 10 degrees below the freezing point 



