184 THE WORLD-ENERGY 



necessity, develop within itself the opposite mode of 

 force. Whence it may be said that, though we may in 

 a certain degree imagine the action of gravity apart 

 from repulsion, yet it is impossible to conceive such 

 action in the sense of thinking it. 



There is, besides, no little significance for our present 

 purpose in the fact that, in modern science, heat is 

 treated throughout as exclusively a mode of repulsion. 

 The entire discussion of the subject proceeds upon, and 

 is but a development of, the proposition that "heat 

 expands all bodies." The chief topics regarding the 

 action of heat are: Expansion; change of state; rela- 

 tion between tension and density of vapors. 



Finally, as regards the distribution of heat, it need 

 only be said that conduction of heat is simply the pro- 

 gressive transfer of the energy of motion from particle to 

 particle in a series; while convection begins in conduction, 

 which, in turn, brings about a difference in specific grav- 

 ity in the fluid medium, the result being currents con- 

 veying the energy of motion (whence convection currents) 

 and distributing that energy by contact that is, by 

 conduction again. So, also, radiation itself appears to 

 be but a modified form of conduction, and the more 

 when we recognize that all "bodies" are in reality 

 indefinitely extended, and, hence, mutually inclusive. 



But let us now pass to the consideration of another 

 mode of molecular energy, which is also intimately 

 related to heat. 



b. CHEMICAL ACTION. 



We have already noticed that molecular combination 

 depends upon the varying degrees of relation between 



