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 *morphoses being governed by two laws equally inevitable, the conservation of matter and the conservation of energy. The first of these laws expresses the fact that matter is indestructible, and passes from one phenomenal determination to another at a rate of equivalence measured by weight; the second, that energy is indestructible, and that it passes from one phenomenal determination to another at a rate of equivalence fixed for each category by the discoveries of the physicists.

The idea of energy has become the point of departure of a science, Energetics, to the establishment of which a large number of contemporary physicists, among whom are Ostwald, Le Châtelier, etc., have devoted their efforts. It is the study of phenomena, regarded from the point of view of energy. I have said that it claims to co-ordinate and to embrace all other sciences.

The first object of energetics should be the consideration of the different forms of energy at present known, their definition and their measurement. This is what we have just done in broad outline.

In the second place, each form of energy must be regarded with reference to the rest, so as to determine if the transformation of this into that is directly realizable, and by what means, and, finally, according to what rate of equivalence. This new chapter is a laborious task which would compel us to traverse the whole field of physics.

Of this long examination we need only concern ourselves here with three or four results which will be