upon each other, then the chemical action is accompanied by an evolution of heat; in the opposite case, an absorption of heat takes place. The greater the difference, the greater also will be the change of temperature accompanying the process, so that, if the action takes place rapidly, the temperature may reach that of a red heat, as a result of which the chemical action assumes the character of combustion in the more limited sense of the word.
The aim of quantitative thermo-chemical investigation is now, in the first place, to measure those quantities of heat which are evolved or absorbed in chemical actions. It is true that these values furnish no direct information as to the magnitude of the forces which are concerned in the chemical action, partly because they are merely an expression for the difference between the energy of the molecules decomposed and that of the molecules formed, partly because they are often affected by other actions which accompany the chemical process; they furnish, however, the material for theoretical investigations—for the higher aim of thermo-chemistry is to establish the dynamical laws of chemical action and to afford an insight into the mysterious region of the constitution of chemical compounds—that is, of the molecules.
Up to the present time, an almost impenetrable veil has enveloped the internal structure of the molecules and the true nature of the atoms: we know, at the most, the relative number of the different atoms in the molecule, the relative mass of the molecules, and of the individual atoms, and the presence of certain groups of atoms (radicals) in the molecules; but we know almost nothing about the nature of the forces which dominate in the molecule, and which cause the formation and decomposition of compounds.
Experience teaches that the different atoms exert an influence upon each other, which seems to be independent of their mass and which appears now as attraction, now as repulsion, and that the combining capacity of the atoms does not extend beyond a certain limit; still, up to the present time, no satisfactory explanation has been offered for these, the chief phenomena of chemistry. Chemical processes consequently do not as yet admit of a mathematical discussion in their entire extent, as is the case, for example, with the phenomena of physics and astronomy; for the general mathematical discussion of chemical phenomena we lack that which is most important as a basis, namely, a knowledge of the fundamental laws which govern the actions of the atoms. With each decade, however, chemistry approaches nearer and nearer the exact sciences, and already many laws of wider or narrower application are being established on the basis of experiment. The extremely rich and varied material of chemistry now arranges itself in large groups, the members of which follow certain common rules or laws, with reference to their formation and decomposition, and whose properties may be, to a certain extent, deduced from the composition of their molecules.