AIM OF THERMO-CHEMICAL INVESTIGATIONS. 771 



Every action which causes a change in the internal structure of the 

 molecules is a chemical action. These changes may be of various 

 sorts : either there takes place simply a change of position among the 

 atoms of the molecule and there is then formed a body which is 

 isomeric or metameric with the original substance ; or the molecule is 

 broken up into several molecules, and the process is then one of dis- 

 sociation, or of simple decomposition ; or, further, several molecules 

 unite together to form a single molecule, in which case the process 

 is spoken of as one of condensation or addition ; or, finally, molecules 

 act upon each other with interchange of atoms and formation of new 

 molecules, which is the phenomenon of mutual decomposition, the most 

 common sort of chemical action. 



From the doctrine of the conservation of matter, it follows that the 

 mass of the bodies which take part in any chemical reaction does not 

 change, and is fully accounted for in the products of the reaction : 

 this principle forms the foundation for quantitative chemical investi- 

 gations into the composition of substances. 



From the principle of the conservation of energy, it follows, in like 

 manner, that no energy is lost or created, and that, consequently, the 

 whole of the energy originally present in the bodies, which act upon 

 each other, appears again in the products of the chemical reaction, 

 although often in a different manner than before. This principle forms 

 the foundation of all quantitative thermo-chemical investigations. 



The energy of a molecule is always the same at the same tempera- 

 ture ; any increase or diminution of the energy of the molecules, with- 

 out change of their internal structure, manifests itself as an elevation 

 or a lowering of the temperature of the substance in question. 



By chemical action, the structure of the molecule is changed, the 

 atoms become grouped together in a different way, new relations mani- 

 fest themselves among them, and the energy of the new molecule be- 

 comes different from that of the original one. The simplest case is 

 that where the only change is in the grouping and the motions of the 

 atoms of the same molecule, that is where an isomeric compound is 

 formed ; in such a case the atoms of the molecule pass from one con- 

 dition of equilibrium to another, and, according as the new condition 

 of equilibrium answers to a greater or less stability than the original, 

 there occurs either an evolution or an absorption of energy, and the 

 temperature of a corresponding body changes ; in the first case it rises, 

 in the second case it falls. 



The chemical action, the passage from one isomeric condition to 

 the other, is therefore accompanied by an evolution, or by a disappear- 

 ance of heat, according as the attractions of the atoms are more fully 

 or less fully satisfied in the body in its new condition. 



The case is similar in other chemical actions ; if the molecules of 

 the substances produced by the reaction contain at the same temper- 

 ature a smaller total energy than the molecules of the bodies acting 



