This form has the peculiarity that not only can it be re-transformed into mechanical motion this mechanical motion, moreover, having the same vis viva as the original one but also that it is capable only of this change of form. Mechanical potential energy can never produce heat or electricity, unless it has been converted first into real mechanical motion. To use Clausius term, it is a reversible process. The second form in which mechanical motion disappears is in friction and impact which differ only in degree. Friction can be conceived as a series of small impacts occurring successively and side by side, impact as friction concentrated at one spot and in a single moment of time.
Friction is chronic impact, impact is acute friction.
The mechanical motion that disappears here, disappears altogether as such . It can never be restored immediately out of itself. The process is not directly reversible. The motion has been transformed into qualitatively different forms of motion, into heat, electricity into forms of molecular motion. Hence, friction and impact lead from the motion of masses, the subject matter of mechanics, to molecular motion, the subject matter of physics.