144 CORRELATION OF PHYSICAL FORCES. 



chemical action of the .ordinary combustion of coal and oxy- 

 gen, the expenditure of fuel will be in proportion to the ex- 

 pansibility of the substances heated ; water passing freely 

 into the state of 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 ques- 

 tion upon which many theories have been proposed, and which 

 may possibly be never more than approximately resolved. 



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 

 ensues 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 propo- 

 sition, which I have previously mentioned, ' that the nearer 

 the particles 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 assumes something 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 

 themselves were previously nearer to each other. In chemi- 

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

 close proximity with b b the particles of B ; heat should there- 

 fore 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 combining par- 

 ticles be such that the correlative expansion ought to be greater 



