108 PHYSICS. 
It has been found by experiments with this apparatus that the rate of 
cooling is not uniform, that is, that bodies do not cool equally in each 
successive minute. The greater the excess of heat possessed by bodies 
above that of surrounding bodies, the more rapidly does cooling take place. 
The loss of heat of a body is, however, only proportional to the excess of 
temperature when the latter amounts to about 100°—115°F. 
h. Conduction of Heat. 
Heat passes from one body to another, not only by conduction, but also 
by immediate contact; all bodies do not possess, however, the same 
conducting power. Some bodies allow heat to pass with great facility 
from one particle to another; these are called good conductors. Others 
may be inflamed at one point, while in another quite near to it, the 
temperature may be but slightly increased. Such are bad conductors. 
Metals form the best conductors; spongy or very porous bodies the 
worst. 
If several rods of different material, but of the same size, be coated at the 
upper end with wax, and set on a hot plate, the relative rapidity of melting 
which will be observed in the wax, will indicate the relative conducting 
power of the different materials. 
If an elongated body. as a metallic rod, be connected at one end with a 
source of heat, this heat will gradually diffuse itself throughout the entire 
mass: it will, however, be greatest in the vicinity of the source, and 
decrease inversely as the square of the distance from it. In similar rods 
of different metals, the conducting power is as the square of that distance 
from the source of heat, at which, other things being equal, equal excesses 
of temperature have been observed. 
In liquids and gases heat is diffused principally by currents. As the heated 
strata become specifically lighter, and therefore rise to the surface, the 
displaced strata occupy their place and become heated in turn. Liquids, 
and still more gases, are much poorer conductors than metals; hence it 
follows that porous bodies, powdered substances, and even metals in a state 
of minute division, conduct heat much worse than those which are dense, 
on account of the pores being constantly filled with air or other gases. 
1. Sources of Heat. 
The principal source of heat is the sun, and next to this, chemical 
combinations, combustion particularly, that is, the rapid combination of 
bodies with the oxygen of the air. The heat produced in such combustion 
is estimated by the degree to which equal quantities of the combustibles 
elevate the temperature of equal quantities of water. The most satis- 
factory experiments on this subject have been instituted by Rumford, 
Lavoisier, Laplace, and Despretz. 
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