HEAT. 147 



will be perceived that the wax does not begin to melt on 

 them all at the same time. That on the silver will 

 melt first, and that on the glass last. Therefore, some 

 bodies heat sooner than others. When bodies become 

 hot in this manner, it is said to be conducted through 

 them. 



All solid bodies are conductors of heat. The metals 

 are the best conductors, next come stony substances. 

 When a solid is sufficiently heated to change its state, it 

 is no longer a conductor. Ice will conduct heat at any 

 temperature below the freezing point of water, but at 32 

 it is no longer a conductor ; because any addition of heat 

 changes it to water. 



In solids, the heat moves from particle to particle, 

 but not in liquids and gaseous bodies ; because their 

 particles move freely among themselves. As we have 

 explained in a former chapter, bodies are increased in 

 bulk by heat, and are of course lighter ; therefore, when 

 heat is applied to a liquid, it heats a stratum of it next 

 the source of heat, and it becomes lighter, and if below 

 another, it will rise, while another takes its place, and in 

 this way, the whole becomes heated : of course, it makes 

 a great difference to what part of a liquid we apply the 

 heat. If we apply it to the top, it can make its way 

 downward but slowly, in the same way solids are heated ; 

 but if it be applied at the bottom, it makes its way up- 

 ward, in consequence of the movement of the particles, 

 independent of any conducting power. Liquids then, 

 have the power of carrying heat. 



Experiment. If we mix some light substance, of the 

 same specific gravity of water, with a portion of this 

 liquid in a glass tube, or oil flask, and heat be applied to 

 it, the particles will be seen in motion ; in the middle of 

 the tube they will be seen to ascend, and near the sides 

 to descend. These motions are caused by the heated 

 particles going to the sides and giving out heat, thus 

 becoming heavier, they fall to the bottom, and push up 

 lighter ones, which, in their turn, lose a part of their 

 heat, and fall also. Upon this experiment, and similar 

 ones, Count Rumford founded this general rule: the 

 more the internal motions of a liquid are impeded, the 

 longer time does it require to heat them to any given 

 temperature. 



