Common matches are ignited and cold hands warmed by the heat developed by friction. It is said that some savages kindle fires by skilfully rubbing together well-chosen pieces of wood. In the case of the axles of railway cars and ordinary carriages, this conversion of mechanical energy into heat is not so difficult as its prevention. Lubricants are used to diminish the friction and prevent the waste of energy due to the undesirable transformation. A railway train is really stopped by the conversion of its motion into heat. When this has to be done quickly, the change is hastened by increasing the friction by means of the brakes. Examples of this change are matters of every day experience. (a.) Attach a brass tube 10 cm. long, about 2 cm. in diameter and closed at the bottom, to a whirling table. Partly fill the tube with alcohol and cork the open end. Press the tube between two pieces of board hinged together. The boards should have two grooves for the reception of the tube; the inner faces of the boards may be covered with leather. When the machine is set in motion, the friction warms and soon boils the alcohol. The vapor drives out the cork with explosive violence.