PRESENT FUNDAMENTAL CONCEPTIONS OF PHYSICS. 487 



of a weight unit of a body by 1° C. ; 011 the contrary there are needed, 

 to this end, tests by mixture like the above, or other similar methods. 



In order to be able to give amounts of heat iu numbers a heat-unit 

 must be selected. For the heat-unit (or calorie) that amount of heat is 

 chosen which has to be imparted to the weight-unit, that is, the 1 leg. of 

 water, in order to raise its temperature 1° C* If 1 leg. of water needs 

 1 heat-unit to raise its temperature from 0° C. to 1° C, then 2, 3, 4, &c. 

 7.y/.s\ of water will need for the same temperature 2, 3, 4, &c. heat-units. 

 If, for instance, a body contains 100 such heat-units, we say its amount 

 of heat is 100. Any apparatus which permits an accurate measurement 

 of such amounts of heat is called a calorimeter. In the example herein 

 quoted, the vessel containing the mixture, carefully guarded against 

 cooling and provided with accurate thermometers, would be the calori- 

 meter. 



If similar calorimetric mixture-tests as the one above cited are made 

 with different substances and water it will be found that equal weights 

 of different substances require different amounts of heat in order to raise 

 their temperature by 1° C. Iron and nickel require only ^ ; tin, silver, 

 and antimony only £- ; bismuth, lead, gold, platinum, and quicksilver, 

 only 3^ part of the amount of heat which the same weight of water 

 requires to raise its temperature 1° C. The quantity of heat which is 

 required to raise the temperature of a weight-unit (/'. c. 1 leg.), of a body 

 is called its specific heat or heat capacity. The specific heat of water is 

 taken as the unit in quoting the specific heat of various substances. 

 According to the preceding examples, the specific heat of water is 1 ; 

 that of iron and nickel ^ ; that of tin, silver, and antimony .}- ; that of 

 bismuth, lead, gold, platinum, and quicksilver 3^. From this it will be 

 seen that water possesses the greatest specific heat, and that metals 

 possess only a small amount of it. A small amount of heat can conse- 

 quently raise their temperature perceptibly. Iu receiving equal amounts 

 of heat the temperature of that body will rise most rapidly which has 

 the least specific heat. 



The knowledge of the specific heat of bodies is of high importance, 

 theoretically and practically. In regard to the latter, the following ex- 

 ample will illustrate what has been said: Supposing there were equal 

 weight amounts of water and quicksilver in separate vessels which, hav- 

 ing the same temperature from the beginning, were to be warmed in the 

 same space of time by small flames which produce an equal amount of 

 heat iu a time-unit, 33 such flames would have to be placed under the 



* [More generally stated, a " heat-unit" is the amount of heat necessary to raise a 

 weight-unit of pure water at maximum density — a unit of temperature. The heat- 

 unit ahove given is that employed by French physicists. The British heat-unit is only 

 about one-fourth as large; being expressed by one pound of water raised in temperature 

 one degree Fahrenheit. The reason for the limitation to the temperature of maximum 

 density of water (about 39° F. or about 1 ; C.) is that the heat-capacity (specific heat) 

 of liquids increases slightly with their temperature. — Ed.'] 



