44 CORRELATION OF PHYSICAL FOKCES. 



vest itself of this idea, and to view the phenomena simply in 

 their dynamical relations. To assist us in so viewing them, 

 let us first parallel with purely mechanical actions, certain 

 simple effects of heat, where change of state (I mean such 

 change as from the solid to the liquid, or liquid to the gase- 

 ous state) is not concerned. Thus, place within a receiver a 

 bladder, and heat the air within to a higher temperature 

 than that without it, the bladder expands ; so, force the air 

 mechanically into it by the air-pump, the bladder expands ; 

 cool the air on the outside, or remove its pressure mechani- 

 cally by an exhausting pump, the bladder also expands ; con- 

 versely, increase the external repellent force, either by heat 

 or mechanical pressure, and the bladder contracts. In the 

 mechanical effects, the force which produced the distension 

 is derived from, and at the expense of, the mechanical power 

 employed, as from muscular force, from gravitation, from the 

 reacting elasticity of springs, or any similar force by which 

 the air-pump may be worked. In the heating effects, the 

 force is derived from the chemical action in the lamp or 

 source of heat employed. 



Let us next consider the experiment so arranged that the 

 force, which produces expansion in the one case, produces a 

 correlative contraction in the other : thus, if two bladders, 

 with a connecting neck between them, be half-filled with air, 

 as the one is made to contract by pressure the other will di- 

 late, and vice versa ; so a bladder partly filled with cold air, 

 and contained within another filled with hot air, expands, 

 while the space between the bladders contracts, exhibiting a 

 mere transfer of the same amount of repulsive force, the 

 mobility of the particles, or their mutual attraction, being 

 the same in each body ; in other words, the repulsive force 

 acts in the direction of least resistance until equilibrium is 

 produced ; it then becomes a static or balanced, instead of a 

 dynamic or motive force. 



Let us now consider the case where a solid is to bo 



