160 COEKELATION OF PHYSICAL FOBCE9. 



into the steam will consume more fuel than if it be confined 

 and kept at a temperature above its boiling point. 



Why chemical action produces heat, or what is the action 

 of the molecules of matter when chemically uniting, is a 

 question upon which many theories have been proposed and 

 which may possibly be never more than approximately re- 

 solved. 



Some authors explain it by the condensation which takes 

 place ; but this will not account for the many instances where, 

 from the liberation of gases, a great increase of volume en- 

 sues upon chemical combustion, as in the familiar instance 

 of the explosion of gunpowder : others explain it as resulting 

 from the union of atmospheres of positive and negative elec- 

 tricity which are assumed to surround the atoms of bodies ; 

 but this involves hypothesis upon hypothesis. Dr. Wood has 

 lately thrown out a view of the heat of chemical action which 

 is more in accordance with a dynamic theory of heat, and 

 as such demands some notice. Starting with his proposition, 

 which I have previously mentioned, ; that the nearer the par- 

 ticles of bodies are to each other the less they require to 

 move to produce a given motion in the particles of another 

 body/ his argument, if I rightly understand it, assumes some- 

 thing of this form. 



In the mechanical approximation of the particles of a 

 homogeneous body heat results ; the particles a a of the body 

 A would, by their approximation, produce expansion in the 

 neighbouring body B, the more so in proportion as they them- 

 selves were previously nearer to each other. In chemically 

 combining, a a the particles of A are brought into very close 

 proximity with b b the particles of B ; heat should therefore 

 result, and the greater because the proximity may fairly be 

 assumed to be greater in the case of chemical combination 

 than in that of mechanical compression. In cases, then, 

 where there is no absolute diminution of bulk ensuing on 

 chemical combination, if the greater proximity of the com- 



