SECT. XXV. PROPAGATION OF HEAT. 225 



in another. The internal structure of crystalized mat- 

 ter must be very peculiar, thus to modify the expansive 

 power of heat, and so materially to influence the trans- 

 mission of caloric and the visible rays of the spectrum. 



Heat is propagated with more or less rapidity through 

 all bodies ; air is the worst conductor, and consequently 

 mitigates the severity of cold climates by preserving the 

 heat imparted to the earth by the sun. On the con- 

 trary, dense bodies, especially metals, possess the power 

 of conduction in the greatest degree, but the transmis- 

 sion requires time. If a bar of iron twenty inches long 

 be heated at one extremity, the caloric takes four min- 

 utes in passing to the other. The particle of the metal 

 that is first heated communicates its caloric to the sec- 

 ond, and the second to the third ; so that the temperature 

 of the intermediate molecule at any instant is increased 

 by the excess of the temperature of the first above its 

 own, and diminished by the excess of its own tempera- 

 ture above that of the third. That however will not 

 be the temperature indicated by the thermometer, be- 

 cause as soon as the particle is more heated than the 

 surrounding atmosphere, it loses its caloric by radiation, 

 in proportion to the excess of its actual temperature 

 above that of the air. The velocity of the discharge is 

 directly proportional to the temperature, and inversely 

 as the length of the bar. As there are perpetual varia- 

 tions in the temperature of all terrestrial substances and 

 of the atmosphere, from the rotation of the earth, and 

 its revolution round the sun, from combustion, friction, 

 fermentation, electricity, and an infinity of other causes, 

 the tendency to restore the equability of temperature 

 by the transmission of caloric must maintain all the 

 particles of matter in a state of perpetual oscillation, 

 which will be more or less rapid according to the con- 

 ducting powers of the substances. From the motion of 

 the heavenly bodies about then* axes, and also round the 

 sun, exposing them to perpetual changes of temperature, 

 it may be inferred that similar causes will produce like 

 effects in them too. The revolutions of the double stars 

 show that they are not at rest ; and though we are to- 

 tally ignorant of the changes that may be going on in the 

 nebulae and millions of other remote bodies, it is hardly 

 15 



