HEAT. 55 



substance, that is, in another portion of the same substance 

 in which the particles are more distant from each other. 

 The amount of approximation or recession of the particles of 

 a body, in other words, its change of bulk by a given change 

 of temperature, being thus in a given substance an index of 

 the relative proximity of its particles, may it not be so of all 

 bodies ? The proposition is very ingeniously argued by Dr. 

 Wood, but the argument is based upon certain hypotheses as 

 to the sizes and distances of atoms, which must be admitted 

 as postulates by those who adopt his conclusions. Dr. 

 Wood seeks by means of this theory to explain the heat pro- 

 duced by chemical combination, and I shall endeavour to give 

 a sketch of his mode of reasoning when I arrfve at that part 

 of my subject. 



Although the comparative effects of specific heat may not 

 be satisfactorily explicable by any known theory, the absolute 

 effect of heat upon each separate substance is simply expan- 

 sion, but when bodies differing in their physical characters 

 are used, the rate of expansion varies, if measured by the 

 correlative contractions exhibited by the substances produc- 

 ing it. Though I am obliged, in order to be intelligible, to 

 talk of heat as an entity, and of its conduction, radiation, &c., 

 yet these expressions are, in fact, inconsistent with the dyna- 

 mic theory which regards heat as motion and nothing else ; 

 thus conduction would be simply a progressive dilatation or 

 motion of the particles of the conducting substance, radiation 

 an undulation or motion of the particles of the medium 

 through which the heat is said to be transmitted, &c. ; and it 

 is a strong argument in favour of this theory, that for every 

 diversity in the physical character of bodies, and for every 

 change in the structure and arrangement of particles of the 

 same body, a change is apparent in the thermal effects. 

 Thus gold conducts heat, or transmits the motion called heat, 

 more readily than copper, copper than iron, iron than lead, 

 and lead than porcelain, &c. 



