1871.] Heat , and Force. 85 



engine, the horse, the dog, the swallow, the wasp, the gnat ? 

 At any rate, we may observe that the very phrase is certainly 

 a misnomer, and a misnomer of such a kind as to have a fatal 

 effect in producing a false conception of things. For 

 mechanical energy just as often produces cold as heat ; it 

 may produce either heat, or cold, or neither. In fact, as a 

 general rule, though with notable exceptions, every pushing 

 or compressing force produces heat, and every pulling or 

 expanding force cold. Place a weight on a pillar, and the 

 weight produces heat in the pillar ; hang it on a wire, and it 

 cools the wire. Place it on a pillar, which pillar is itself 

 hanging by its lower end on wires, and it will produce 

 neither heat nor cold. The heat produced in the pillar may 

 be made exactly to counterbalance the cold produced in the 

 wire. In the same way, in a fire-syringe, use force to press 

 down the piston, it produces heat — heat enough to kindle 

 tinder; but use the same force to pull up the piston, and it 

 produces cold. Combine two fire-syringes together, one 

 within the other, or in any other way, and let equal forces 

 push one piston down and pull the other up ; neither heat 

 nor cold will be the final result. So, also, put a pressure on 

 water at a temperature above its greatest density, and it 

 produces heat. Below that temperature it produces cold. 

 At that temperature it produces neither heat nor cold. 

 Hence we see the same pushing force produces at one time 

 heat and at another cold ; and, similarly, a pulling force, 

 tending to expand water, may produce either heat or cold, 

 according to the temperature of the water. The phenomena 

 which point to —273° C. as the absolute deprivation of all heat 

 possibly only tend to show that at that temperature a further 

 condensation of air would produce not heat but cold, and 

 that further cold would expand, not condense, air. There is 

 just as much mechanical energy in a lump of ice which 

 will produce 100 units of cold as there is in a lump 

 of coal which will produce 100 urn^s of heat ; there is 

 as much stored-up power in a glacier as in a coal-mine. 

 When our coal is exhausted we may quarry the icebergs of 

 the poles and make them do the work which coal now does 

 for us. No amount of heat in a body can produce any effect 

 till that body comes into contact or communication with 

 some other body hotter or colder than itself. So that, in 

 reality, force is produced, not by heat or cold, but by the 

 restoration of the equilibrium of two bodies or parts of 

 bodies unequally heated, and mechanical energy produces 

 neither heat nor cold (except accidentally), but simply a dis- 

 turbance of the equilibrium in the heat of two bodies or 



