HEAT. 51 



extreme tenuity and elasticity supposed to be diffused throughout 

 the universe, and indeed permeating all matter. 



Similarly as waves of water are generated by dropping a stone into 

 it, and as sound waves are produced in air by causing it to vibrate, 

 so heat waves are produced in the ether whenever it is disturbed by 

 the rapid molecular motion of a heated body. 



While our views regarding the nature of ether are of a hypothetical 

 character, there can be no doubt of the existence of these heat waves. 

 Indeed, their length, their velocity, and many other features have 

 been fully determined ; they obey largely the same laws which have 

 been established for light waves i. e., when striking against a body 

 they are generally reflected, transmitted, diffused, or absorbed. It 

 is this absorption of heat, as we call it, which causes the heating- 

 effects through space. It may be compared to the motion that can 

 be imparted to an object floating on still water. By disturbing the 

 water, as by dropping a stone into it at a distance from the floating 

 object, concentric ripples or waves pass from the point where the 

 water was disturbed, and in striking the floating object cause it to 

 move. Similarly waves of heat pass from a heated body through 

 the ether in every direction, and in striking against a body cause its 

 molecules to move faster i. e., render it warmer. It is by this 

 process that heat is transmitted from the sun to the earth. 



Change in molecular state. As was pointed out above in the discus- 

 sion of the nature of heat, it requires energy to bring about a change 

 in the relative position of the molecules of a substance, which is 

 called internal work. This is true for solids and liquids, but not for 

 gases, since there is practically no cohesion between the particles of a 

 gas, and so no work is required to change the position of the particles. 

 Several familiar phenomena involve change in the molecular state. 



Fusion or melting-. This name is applied to the process by which 

 a solid passes into the liquid state. When a liquid passes into the solid 

 state, the process is known as solidification. As a rule, when solids 

 are heated, they begin to melt at a definite temperature, which is known 

 as the fusion-point or melting-point. Conversely, when liquids are 

 cooled, they begin to solidify at a definite temperature, which is iden- 

 tical with the melting-point of the solid. Moreover, the temperature 

 remains constant as long as fusion or solidification continues. 



Solids that are individuals (not mixtures) and are perfectly pure as 

 far as they melt without decomposition, have definite or sharp melting- 



