54 CORRELATION OF PHYSICAL FORCES. 



crease in temperature increase in specific heat. The ratio 

 of this increase in specific heat is greater with solids than 

 with liquids, although the. latter are more dilatable ; an 

 effect probably depending upon the commencement of fusion. 

 Again, those metals whose rate of expansion increases most 

 rapidly when they are heated, increase most in specific heat ; 

 and their specific heat is reduced by percussion, which, by 

 approximating their particles, makes them specifically more 

 dense. When, however, we examine substances of very 

 different physical characters, we find that their specific heats 

 have no relation to their density or rate of expansion by 

 heat ; their differences of specific heat must depend upon 

 their intimate molecular constitution in a manner accounted 

 for (as far as I am aware) by no theory of heat hitherto 

 proposed. 



In the greater number, probably in all solids and liquids, 

 the expansion by heat is relatively greater as the temperature 

 is higher ; or, preserving the view of expansion and contrac- 

 tion, if two equal portions of the same substance be juxta- 

 posed at different temperatures, the hotter portion will con- 

 tract a little more than the colder will expand ; from this 

 fact, viz. that the coefficient of expansion increases in a given 

 body with the temperature, and from other considerations, 

 Dr. Wood has argued, with much apparent reason, that the 

 nearer the particles of bodies are to each other, the less they 

 require to move to produce a given expansion or contraction 

 in those of 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 tem- 

 perature, 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 



