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 garment; in a word, that something active and permanent subsists in the passage from one condition to another, and that what has changed is only the aspect, the appearance.

This constant something which is perceived beneath the inconstancy and the variety of forms, and which circulates in a certain manner from the antecedent phenomenon to its successor, is energy.

But still this is only a very vague view, and it may seem arbitrary. It may be made more exact by examples borrowed from the different categories of natural phenomena. There are energetic modalities in relation with the different phenomenal modalities. The different orders of phenomena which may be presented—mechanical, chemical, thermal, electrical—give rise to corresponding forms of energy.

When to a mechanical phenomenon succeeds a mechanical, thermal, or electrical phenomenon, we say, embracing transformation in its totality, that there has been a transformation of mechanical energy into another form of energy, mechanical, thermal, or electrical, etc.

This idea becomes more precise if we examine successively each of these cases and the laws which regulate them.

§ 3..

Mechanical energy is the simplest and the oldest known.

''Mechanical Elements: Time, Space, Force, Work, Power''.—Mechanical phenomena may be considered under two fundamental conditions—time and space,