28 CORRELATION OF PHYSICAL FORCES. 



guity 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, how- 

 ever, 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 substance, 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 impression 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 introduce 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 continuity 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, thepiston 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, foi instance, 

 heated beyond the temperature of the circumambient air, is 

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

 moves this to a point at which the tension or elastic force of 

 the confined air equals that of the surrounding air. If the 

 confined air be kept at this point, the piston would remain 

 stationary ; but if it be cooled, the external air exercising then 

 a greater relative degree of pressure, the piston returns 

 towards its original position ; just as it will be seen, when we 

 come to the magnetic force, that a magnet placed in a parti- 

 cular position produces motion in iron near it ; but to make 



