If we keep to the present era, evolution has a fixed direction—­that which leads to an increase of entropy; and it is possible to enquire, in any given system to what physical manifestations this increase corresponds.  We note that kinetic, potential, electrical, and chemical forms of energy have a great tendency to transform themselves into calorific energy.  A chemical reaction, for example, gives out energy; but if the reaction is not produced under very special conditions, this energy immediately passes into the calorific form.  This is so true, that chemists currently speak of the heat given out by reactions instead of regarding the energy disengaged in general.

In all these transformations the calorific energy obtained has not, from a practical point of view, the same value at which it started.  One cannot, in fact, according to the principle of Carnot, transform it integrally into mechanical energy, since the heat possessed by a body can only yield work on condition that a part of it falls on a body with a lower temperature.  Thus appears the idea that energies which exchange with each other and correspond to equal quantities have not the same qualitative value.  Form has its importance, and there are persons who prefer a golden louis to four pieces of five francs.  The principle of Carnot would thus lead us to consider a certain classification of energies, and would show us that, in the transformations possible, these energies always tend to a sort of diminution of quality—­that is, to a degradation.

It would thus reintroduce an element of differentiation of which it seems very difficult to give a mechanical explanation.  Certain philosophers and physicists see in this fact a reason which condemns a priori all attempts made to give a mechanical explanation of the principle of Carnot.

It is right, however, not to exaggerate the importance that should be attributed to the phrase degraded energy.  If the heat is not equivalent to the work, if heat at 99 deg. is not equivalent to heat at 100 deg., that means that we cannot in practice construct an engine which shall transform all this heat into work, or that, for the same cold source, the output is greater when the temperature of the hot source is higher; but if it were possible that this cold source had itself the temperature of absolute zero, the whole heat would reappear in the form of work.  The case here considered is an ideal and extreme case, and we naturally cannot realize it; but this consideration suffices to make it plain that the classification of energies is a little arbitrary and depends more, perhaps, on the conditions in which mankind lives than on the inmost nature of things.