TRANSFORMATION AND EQUIVALENCE OF FORCES. 207 



masses to each other, but as occurring among the units of 

 which such sensible masses consist. If we cease to think of 

 Heat as that particular sensation given to us by bodies in 

 certain conditions, and consider the phenomena otherwise 

 presented by these bodies, we find that motion, either in 

 them or in surrounding bodies, or in both, is all that we 

 have evidence of. With one or two exceptions which are 

 obstacles to every theory of Heat, heated bodies expand; 

 and expansion can be interpreted only as a movement of the 

 units of a mass in relation to each other. That so-called 

 radiation through which anything of higher temperature 

 than things around it, communicates Heat to them, is clearly 

 a species of motion. Moreover, the evidence afforded by the 

 thermometer that Heat thus diffuses itself, is simply a move- 

 ment caused in the mercurial column. And that the molecu- 

 lar motion which we call Heat, may be transformed into visi- 

 ble motion, familiar proof is given by the steam-engine; in 

 which " the piston and all its concomitant masses of matter 

 are moved by the molecular dilatation of the vapour of 

 water." Where Heat is absorbed without apparent 



result, modern inquiries show that decided though unob- 

 trusive changes are produced: as on glass, the molecular 

 state of which is so far changed by heat, that a polarized ray 

 of light passing through it becomes visible, which it does not 

 do when the glass is cold ; or as on polished metallic surfaces, 

 which are so far changed in structure by thermal radiations 

 from objects very close to them, as to retain permanent im- 

 pressions of such objects. The transformation of Heat into 

 electricity, occurs when dissimilar metals touching each 

 other are heated at the point of contact : electric currents be- 

 ing so induced. Solid, incombustible matter introduced 

 into heated gas, as lime into the oxy-hydrogen flame, be- 

 comes incandescent; and so exhibits the conversion of Heat 

 into light. The production of magnetism by Heat, if it can- 

 not be proved to take place directly, may be proved to take 

 place indirectly through the medium of electricity. And 



