SPECIFIC HEAT. 23 



substitute for the hot water in this experiment an equal measure 

 of mercury at 130, on mixing it with the measure of water at 70 

 the temperature of the whole will not be 100, but more nearly 

 90. Here the mercury is cooled from 130 to 90, or loses 

 forty degrees of heat; which have been transferred to the water, 

 but which raise the temperature of the latter only twenty de- 

 grees, or from 70 to 90. To heat the measure of water at 70 

 to 1 00, we must mix with it two, or a little more than two 

 equal measures, of mercury at 130% although one measure of 

 water at 130 would answer the purpose. If, therefore, two 

 measures of mercury, by losing thirty degrees of temperature, 

 heat up only one measure of water by thirty degrees, it follows 

 that hot mercury possesses only half the heat of equally hot 

 water ; or that water requires double the quantity of heat that 

 is required by mercury, to raise it a certain number of degrees. 

 This is expressed by saying that water has twice the capacity 

 for heat that mercury possesses. 



It is more convenient to express the capacities of different 

 bodies for heat, with reference to equal weights than equal 

 measures of the bodies. On accurate trial, it is found that a 

 pound of water absorbs thirty times more heat than a pound 

 of mercury, in being heated the same number of degrees : the 

 capacity of water for heat is, therefore, thirty times greater than 

 that of mercury. The capacities of these two bodies are in the 

 relation of 1000 to 33 ; and it is convenient to express the 

 capacities for heat of all bodies, in relation to that of water 

 as 1000. Such numbers are the specific heats of bodies. 



The best method of determining the capacity for heat, con- 

 sists in allowing different substances to cool the same number 

 of degrees in circumstances which are exactly similar; to inclose 

 thenij for instance, in a polished silver vessel, containing the 

 bulb of a thermometer in its centre, and to place this vessel 

 under a bell-jar in which a vacuum is made. The time which 

 the different substances take to cool, enables us to calculate 

 the quantity of heat which they give out. By this exact method, 

 Messieurs Dulong and Petit determined the capacity for heat of 

 the following substances : 



Specific heat. 



Water 1000 



Sulphur 1SS 



Glass . .117 



