We can calculate the heat of formation from its ions for any substance dissolved in a given liquid, from a knowledge of the temperature coefficient of ionization, by means of an application of the well-known **thermodynamical** process, which also gives the latent heat of evaporation of a liquid when the temperature coefficient of its vapour pressure is known.

The cell, together with this balancing electromotive force, is thus a reversible system in true equilibrium, and the **thermodynamical** reasoning applicable to such systems can be used to examine its properties.

In the modified Joule-Thomson equation (17), both c and n have simple theoretical interpretations, and it is possible to express the **thermodynamical** properties of the substance in terms of them by means of reasonably simple formulae.

This not only verifies that the second law of thermodynamics is obeyed, but enables us to identify T with the absolute **thermodynamical** temperature.

Though the experimental and theoretical developments were not necessarily dependent on each other, and by far the larger proportion of the subject which we now term " Spectroscopy " could stand irrespective of Gustav Kirchhoff's **thermodynamical** investigations, there is no doubt that the latter was, historically speaking, the immediate cause of the feeling of confidence with which the new branch of science was received, for nothing impresses the scientific world more strongly than just that little touch of mystery which attaches to a mathematical investigation which can only be understood by the few, and is taken on trust by the many, provided that the author is a man who commands general confidence.