66 HEAT. 



125 1 



1 gives out, or in rising 1 takes in, -^p = ^ calory, about. This 



DO x oZ'o Ai 



is expressed by saying that the specific heat of lead is . These 



o2 



illustrations prepare us for the following definitions : 



The Specific Heat Of a Substance is the number of calories 



needed to raise 1 gramme of the substance 1 C. 



If the specific heat of a substance over a range t is s, the quantity of 



heat required to raise m grammes of the substance t is mst calories. 



Water Equivalent and Capacity for Heat. Since mst calories 



would raise ms grammes of water through the same range t, the 

 quantity ms is termed the Water Equivalent of the m grammes. When 

 the range is 1 the quantity of heat required is termed the Capacity for 

 Heat. The two expressions have the same meaning in practice. 



We shall now give an account of the chief methods of determining 

 specific heats. The details of the methods, though of the utmost- 

 importance in obtaining exact results, need not be fully described here. 

 These may be best understood from the accounts given by the original 

 workers. Our aim is to point out the general principles. 



The method most easily applied is 



The Method of Mixtures. Suppose that we are to find the 

 specific heat s of a certain solid. Then a known mass M of it is raised 

 to a known temperature t', and dropped into a known mass of water W 

 at a known lower temperature t. The experimenter observes the 

 temperature 6 at which the mixture stands when the two have come 

 to thermal equilibrium. If all the heat lost by the solid could be 

 assumed to have gone into the water, and to remain there, then 

 expressing the equality 



Heat lost by solid = Heat gained by water 

 we have M*(*' - 0) = W(0 - 1) 



w e-t 



8= M X * 



But in practice the heat does not all go into the water and remain there. 

 Some of it goes into the containing vessel or Calorimeter, into the 

 thermometer, and into the stirrer necessary to mix the water up 

 thoroughly. Some of it passes out through the calorimeter, where it 

 is partly given to the air, and partly radiated out into the surrounding 

 space. Corrections must be determined and applied on both these 

 accounts. We may understand how they are made by considering an 

 example. Let us suppose that we are to find the specific heat of a 

 specimen of brass. It is advisable to have the brass either in a spiral 

 roll, or in a coil of wire, or in pieces, in order that its surface shall 

 be large, and that it shall quickly part with its heat to the water 

 when immersed. The brass may conveniently be heated to the tempera- 

 ture of boiling water in a steam-jacketed chamber or, for rough work, 

 in a test-tube immersed in boiling water and the temperature may 

 be taken as that of boiling water at the atmospheric pressure at the 



H 760 

 time. This will be nearly 100+ -~ , where H is the barometric 



it 



height in millimetres. 



