in examples of the most felicitous processes of induction, from which the gen- 

 eral reader may obtain a view of that beautiful logic, the light of which Bacon 

 first let in on the obscurity in which he found physics involved. And, finally, 

 the whole range of our domestic experience presents a series of familiar and 

 pointed illustrations of the principles to which it leads. 



The first and most common effect of heat is to increase the size of the body 

 to which it is imparted. This effect is called dilatation, or expansion ; and the 

 body so affected is said to expand, or be dilated. If heat be abstracted from a 

 body, the contrary effect is produced, and the body contracts. These effects 

 are produced in different degrees, and estimated by different methods, according 

 as the bodies which suffer them are solids, liquids, or airs. 



The dilatation of solids is very minute, even by considerable additions of heat ; 

 that of liquids is greater, but that of air is greatest of all. 



The force with which a solid dilates is equal to that with which it would 

 resist compression ; and the force with which it contracts is equal to that with 

 which it would resist extension. Such forces are, therefore, proportional to 

 the strength of the solid, estimated with reference to the power with which 

 they would resist compression or extension. 



The force with which liquids dilate is equivalent to that with which they 

 would resist compression ; as liquids are nearly incompressible, this force is 

 very considerable. 



As air is capable of being compressed with facility, its dilatation by heat is 

 easily resisted. If such dilatation be opposed by confining air within fixed 

 bounds, then the effect of heat, instead of enlarging its dimensions, will be to 

 increase its pressure on the surface by which it is confined. 



The works of clocks and watches swell and contract with the vicissitudes 

 of heat and cold to which they are exposed, When the pendulum of a clock 

 or balance-wheel of a watch is thus enlarged by heat, it swings more slowly, 

 and the rate is diminished. On the other hand, when it contracts by cold, its 

 vibration is accelerated, and the rate is increased. Various contrivances have 

 been resorted to to counteract these effects. When boiling water is poured 

 into a thick glass, the unequal expansion of the glass will tear one part from 

 another, and produce fracture. The same vessel contains a greater quantity of 

 cold than of hot water. 



If a kettle, completely filled with cold water, be placed on a fire, the water, 

 when it begins to get warm, will swell, and spontaneously flow from the spout 

 of the kettle until it ceases to expand. 



If a bottle well corked be placed before the fire, especially if it contain fer- 

 mented liquor in which air is fixed, the air confined in it will acquire increased 

 pressure by the heat imparted to it, and its effort to expand will at length be so 

 great that the cork will shoot from the bottle, or the bottle itself will burst. 



Thus we perceive that the magnitude of a body depends on the quantity of 

 heat which has been imparted to it, or abstracted from it ; and as it must be in 

 a state of continual variation, with respect to the heat which it contains, it fol- 

 lows that it must be in a state of continual variation with respect to its magni- 

 tude. We can, therefore, never pronounce on the magnitude of any body with 

 exactness, unless we are at the same time informed of its situation with respect 

 to heat. Every hour the bodies around us are swelling and contracting, and 

 never for one moment retain the same dimensions ; neither are these effects 

 confined to their exterior dimensions, but extend to their most intimate com- 

 ponent particles. These are in a constant state of motion, alternately ap- 

 ( proaching to and receding from one another, and changing their relative posi- 

 tions and distances. Thus, the particles of matter, sluggish and inert as they 

 appear, are in a state of constant motion and apparent activity. 



