62 HEAT 



produced by a certain "freezing mixture" (cf. 93). The 

 melting point of ice, on the Fahrenheit thermometer, is 

 32, and the boiling point of water 212. 



The number of degrees between 32 and 212 is 180; consequently 

 100 Centigrade degrees equal 180 Fahrenheit degrees. Each Fahren- 

 heit degree is, therefore, 5 / 9 of a Centigrade degree, and each Centi- 

 grade degree is 9 / 5 of a Fahrenheit degree. If we multiply the number 

 of degrees shown by a Centigrade thermometer by 9 / 5 , and then add 

 32, we get the Fahrenheit reading. 

 Fahr. = (Cent.X 9 A)+32. 



If we subtract 32 from the Fahrenheit reading, and then multiply 

 the remainder by 5 / 9 , we get the Centigrade reading. 

 Cent. = 5 / 9 (Fahr. 32). 



The Centigrade scale is used almost everywhere on the continent of 

 Europe, and practically everywhere, the world over, for scientific 

 work. For temperatures below 39.1 C., such as are found in the 

 Arctics, alcohol is used instead of mercury (cf. 57). 



64. Temperature and Heat. We must know the 

 difference between quantity of heat and degree of heat. 

 We know whether a body is hot or cold, generally speak> 

 ing, by its degree of heat, that is, its temperature. The 

 temperature of a body depends upon the rapidity with 

 which its molecules are moving, and not upon whether 

 there are many molecules or few. But the heat which a 

 body possesses depends upon the speed of the molecules 

 and also upon the number of molecules, that is, upon both 

 the temperature of the body and its mass. We shall see 

 later ( 73) that it also depends upon the substance of 

 which the body is composed. 



The unit of heat quantity is called a calorie, just as the unit of mass 

 is called a gram. .A calorie is the amount of heat needed to warm 

 1 gram of water 1 C. It is also the amount given off by 1 g. of water 



