HEAT. 41 



another body. His mode of reasoning, if I rightly conceive 

 it, may be concisely put as follows : 



As bodies contract by cold, it is clear that, in a given 

 body, the lower the temperature the nearer are the particles ; 

 and, as the coefficient of expansion increases with the tempe- 

 rature, the lower the temperature of the substance be, the less 

 the particles require to move, or approach to or recede from 

 each other, so as to compensate the correlative recession or 

 approach of the particles in a hotter portion of the same sub- 

 stance, 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 produced by 

 chemical combination, and I shall endeavour to give a sketch 

 of his mode of reasoning when I arrive 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 corre- 

 lative contractions exhibited by the substances producing 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 dynamic 

 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 un- 

 dulation 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 diver- 



