RICHARDS. SOURCE OF CHEMICAL HEAT. 407 



17.66, 17.1, and about 14.1 respectively. The first small contraction is 

 attended with an evolution of 0.65 kilojoules, and the second larger one 

 with the evolution of 114 kilojoules of heat energy. In those cases 

 where there is a transition from a more compressible union to a stabler, 

 less compressible one, involving more work of compression, solidification 

 would involve increase of volume, as in the case of water. 



II. A Plausible Interpretation. 



It becomes now an interesting question to determine, if possible, the 

 mechanism by which this work is converted into heat. One is reminded 

 at once of the compression of a gas, where the work of compression re- 

 appears quantitatively as heat energy. But the compression under con- 

 sideration differs from the other in detail, because in the present case the 

 attraction of the two substances for one another seems to be the cause of 

 their mutual compression ; and this mutual compression takes place not 

 from the outside, but throughout the whole substance. 



Those who shun the atomic hypothesis and consider substance only in 

 the mass, will rest contented without further attempt at interpretation ; 

 but those who hold that the hypothesis is a useful tool, to be thrown aside 

 when newer invention has devised a better one, will be tempted to go 

 further. 



The case, considered hypothetically, seems to be this : "When two dif- 

 ferent atoms possessing mutual affinity approach one another, they are 

 drawn closer than they can be to their respective fellows, and in the 

 process evolve heat. The " repulsion " which is often supposed to sur- 

 round an atom, and prevent it from touching any other, seems to be par- 

 tially overcome by the potential energy of affinity. But of what nature 

 is this " repulsion" ? Ordinarily it is assumed to be due to the frequent 

 impacts of a hard atom in the centre of the space; but no evidence is 

 afforded of the existence of a free space. Indeed, it seems inconceivable 

 that solids should retain their structure, or should be capable of retaining 

 gases or liquids, if they are so loosely built up. A pile of sand would 

 be stable compared to such a fabric. 



The present research points to quite a different interpretation of the 

 facts. The space occupied by a solid seems to have a chemical signifi- 

 cance as well as a physical one; it seems, indeed, to be as essential a 

 property of the material as any other property. Since the significance of 

 the total volume is a chemical one, the "free space" around each indi- 

 vidual atom must also have a chemical as well as a physical significance. 



