CHAPTER VIII. 



SPECIFIC HEAT FUSION LATENT HEAT CONDUCTION AND CONVECTION 



OF HEAT CALORESCENCE. 



WE have considered the effects of heat upon water, and touched upon one orr 

 two kindred experiments. But we have some other subjects to discuss,. 

 two in particular ; viz., Specific Heat t and Latent Heat. 



The specific heat of any substance is " the number of units of heat 

 required to raise one pound of such substance one degree." We can explain 

 this farther. When heat is communicated to a body it has two or three 

 functions to perform. Some of it has to overcome the resistance of the air 

 in expanding the body, more of it expands, and the remainder increases 

 the temperature of the body. So some heat disappears as heat, and is* 

 turned into energy, " molecular potential energy," as it is called, and the- 

 rest remains. Of course in objects the molecules vary very much in weight 

 and in their mutual attraction, and the heat requisite to raise equal weights- 

 of different substances through the same number of degrees of temperature- 

 will vary. This is called capacity for heat,, or specific heat. The capacity 

 of different metals for heat can easily be shown. The specific heat of water 

 is very high, because its capacity for heat is great. We can cool a hot irons 

 in very little water, and it takes thirty times as much heat to raise a given- 

 weight of water a certain number of degrees, as it would to raise the same 

 weight of mercury to the same temperature. Water has greater specific 

 heat, generally speaking, than other bodies, and it is owing to this cir- 

 cumstance that the climate is so affected by ocean currents. 



Nearly all substances can be melted by heat, if we go far enough, or 

 frozen, if we could take the heat away. Solid can be made liquid, and these^ 

 liquids can be made gases and fly off in vapour. Similarly, if we could 

 only get heat away sufficiently from the atoms of a substance we could' 

 freeze it We cannot freeze alcohol, nor make ice from air, nor can we liquify 

 it, for we are unable to take away its heat sufficiently. But we can turn 

 water into steam, and into ice ;. or ice into water, and then into steam. But 

 there is one body we cannot melt by heat, that is carbon. In the hottest 

 fire coal will not melt, it becomes soft. We call this melting fusion, and 

 every body has its melting point, or fusing point, which is the same at all 

 times if the air pressure be the same. 



It is a curious fact that when a body is melting it rises to a certain 

 temperature (its fusing point), and then gets no hotter,. no matter whether or 



