102 PHYSICS. 
f. Specific Heat of Bodies. 
One substance, when compared with another, has a greater or less 
capacity for heat, according as a greater or less amount of heat is necessary 
to produce a given change of temperature in it; the amount of heat thus 
necessary is called the specific heat of bodies. In some substances the 
capacity for heat varies. Thus, for instance, it requires more heat to 
elevate the temperature of platinum from 212° to 213° F., than to elevate 
it from 32° to 33° F. As, however, the capacity for heat possessed by 
water is constant, this is taken as the unit for all determinations. To 
determine the specific heat of a body, the following three different 
methods may be employed :— 
1. The method of melting of ice, in which the calorimeter of Lavoisier 
and Laplace (jig. 43) is employed. The instrument, represented in 
section, consists of three vessels of sheet iron, one inside of the other. The 
interval, a, between the outer and middle vessels is filled with. pieces of ice 
(not pounded finely), as also is the interval, b, between the middle and 
inner one; the water formed in melting flows off through the cocks d and e. 
If the body to be investigated be brought into the inner vessel, it becomes 
cooled to 32° F’., the heat given off serving to melt the ice in b. The 
specific heat of the body is estimated from the mass and original 
temperature of the body placed in c, and the amount of ice melted. The 
ice or snow in the external space, @, serves only to keep off the 
surrounding heat. 
2. The method of mixtures consists in heating a given weight of the 
body to be examined to a certain temperature, and then immersing it in 
water, whose temperature is elevated by the cooling of the body; from the 
quantity of the water, and the elevation of temperature produced in it, the 
specific heat of the body may be ascertained. 
3. Method of cooling. A body cools, other circumstances being equal, 
the slower as its specific heat is greater. On this principle Dulong and 
Petit determined the specific heat of many bodies by means of the 
apparatus represented on pl. 19, fig.44. Here a is a leaden receiver which 
may be exhausted of air; in the middle of its cover is a metallic nut, c, in 
which the thermometer, d, is fixed; the cylindrical mercury vessel of the 
latter is placed in a small silver vessel, e (shown in the figure between 
figs. 27 and 37), which is suspended by strings, and contains the substance 
to be examined. If the latter be a solid body it is reduced to powder and 
tightly pressed in the silver vessel. This, with the body inclosed, is now 
heated from 15° to 20° C., and introduced into the leaden receiver, a, 
which itself is immersed in a water-bath of given temperature. The 
receiver, a, is now exhausted of air, and observation made of the length of 
time necessary for the thermometer to fall 50° from a temperature 
exceeding that of the water by 10°C. From this interval of time, and the 
amount of the body, its capacity for heat may be ascertained. This 
method, however, gives no very trustworthy results. 
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