In the second place, it may happen, and it probably will happen, as it happened in the last century in the case of electricity, that some new form of energy will be discovered belonging to the universal order as to the living order. This will be a conquest of general physics as well as of biology. And finally we may rigorously and provisionally admit a last category of _vital energies properly so called_. It is difficult to give much precision to the idea of _vital energies properly so called_. It will be easier to measure them by means of equivalents than to indicate their nature. Besides, this is the ordinary rule in the case of physical agents. We can measure them, although we know not what they are. _Characteristics of Vital Energies._—We see why we cannot exhibit with precision, _a priori_, the nature of vital energies. In the first place, they are expressed by what takes place in the tissues in activity, and this cannot at present be identified with the known types of physical, chemical, and mechanical phenomena. This is a first, intrinsic reason for not being able to distinguish them readily, since what takes place is not distinguished by the phenomenal appearances to which we are accustomed. There is a second, intrinsic reason. These vital phenomena are intermediary, as we shall see, between manifestations of known energies. They lie between a chemical phenomenon which always precedes them, and a thermal phenomenon which always follows them. They are lost sight of, as it were, between manifestations which strike our attention. Generally speaking, intermediary energies often escape us even in physics. Only the extreme manifestations are clearly seen. In the presence of the organism we are, as it were, in electric lighting works which are run by a fall of water, and at first we only see the mechanical energy of the falling water, of the turbine and dynamo at work, and the photic energy of the lamps which give the light. Electrical energy, an intermediary, which has only a transient existence, does not impose itself on our attention.