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 which have already been studied and recognized in general physics. They are the same energies: chemical, thermal, mechanical, with their characteristics of mutability, their lists of equivalents, and their actual and potential stales.

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