40 CORRELATION OF PHYSICAL FORCES. 



shall presently notice, the effects of what is called heat are 

 simply an expansion of the matter acted upon, and that the 

 matter so expanded has the power by its own contraction of 

 communicating expansion to all'bodies in contiguity with it. 

 Thus, if the body be a solid, for instance, iron, a liquid, say 

 water, or a gas, say atmospheric air each of these, when 

 heated, is expanded in every direction ; in the two former 

 cases, by increasing the heat to a certain point, we change 

 the physical character of the substance, the solid becomes a 

 liquid, and the liquid becomes a gas ; these, however, are 

 still expansions, particularly the latter, when, at a certain 

 period, the expansion becomes rapidly and indefinitely greater. 

 But what is, in fact, commonly done in order to heat a sub- 

 stance, or to increase the heat of a substance? it is merely 

 approximated to some other heated, that is, to some other 

 expanded substance, which latter is cooled or contracted as 

 the former expands. Let us now divest the mind of the impres- 

 sion that heat is in itself anything substantive, and suppose 

 that these phenomena are regarded for the first time, and 

 without any preconceived notions on the subject ; let us in- 

 troduce no hypothesis, but merely express as simply as we 

 can the facts of which we have become cognisant ; to what 

 do they amount ? to this, that matter has pertaining to it a 

 molecular repulsive power, a power of dilatation, which is 

 communicable by contiguity or proximity. 



Heat thus viewed, is motion, and this molecular motion 

 we may readily change into the motion of masses, or motion 

 in its most ordinary and palpable form : for example, in the 

 steam engine, the piston and all its concomitant masses of 

 matter are moved by the molecular dilatation of the vapour of 

 water. 



To produce continuous motion there must be an alternate 

 action of heat and cold ; a given portion of air, for instance, 

 heated beyond the temperature of the circumambient air, is 

 expanded. If now it be made to act on a movable piston, it 



