40 CORRELATION OF PHYSICAL FORCES. 



dynamical view, and more satisfactorily than by having re- 

 course to the hypothesis of latent matter. Many, however, 

 of the phenomena of heat are involved in much mystery, 

 particularly those connected with specific heat or that relative 

 proportion of heat which equal weights of different bodies 

 require to raise them from a given temperature to another 

 given temperature, which appears to depend in some way 

 hitherto inexplicable upon the molecular constitution of 

 different bodies. 



The view of heat which I have taken, viz. to regard it 

 simply as a communicable molecular repulsive force, is sup- 

 ported by many of the phenomena to which the term specific 

 or relative heat is applied ; for example, bodies as they 

 increase 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 pro- 

 bably 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 approxi- 

 mating 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 juxtaposed 

 at different temperatures, the hotter portion will contract 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 



