But heat can also be produced by the friction of liquids, in which there could be no question of changes in structure, or of the liberation of latent heat.

Heat can also be produced by the impact of imperfectly elastic bodies as well as by friction. This is the case, for instance, when we produce fire by striking flint against steel, or when an iron bar is worked for some time by powerful blows of the hammer.

Each individual fact, taken by itself, can indeed arouse our curiosity or our astonishment, or be useful to us in its practical applications.

Hermann von Helmholtz Quotes