“Matter is inert.” That is what you read in every treatise on physics—what does it mean? Here is a very simple experiment that will prove this truth to anyone.

Pile up ten draughtsmen, as shown in Fig. 1. Before this pile place another piece on edge, and pressing its circumference with the forefinger, let it glide from underneath so that it strikes the pile with considerable force. The piece so thrown must, you will think, upset the whole pile of draughts; but no: the piece thus sharply sent forward will strike only one piece of the pile, and this alone will be dislodged without putting the others out of their equilibrium, and the whole column above will settle down together on the bottom piece.

In effect, the force of the impulse, making itself felt on the piece that is touched, the latter leaves the pile without transmitting its movement to the other pieces, which, following another physical law, that of gravity, descend vertically to fill the place left vacant.

The experiment may be varied by using a knife and striking with it a sharp horizontal blow on one of the pieces. The piece struck will fall out of the pile without disturbing the symmetry of the others.