A small iron rod placed upon an anvil may be heated to redness by repeated blows of a hammer. The energy of the moving mass is broken up, so to speak, and distributed among the molecules, producing that form of molecular motion that we call heat. The same transformation was illustrated in the kindling of a fire by the "flint and steel" of a century ago. It may be experimentally illustrated by the "air-syringe." (a.) The air-syringe consists of a cylinder of metal or glass and an accurately fitting piston. By suddenly driving in the piston, the air is compressed and heat developed. A bit of gun cotton previously placed in the cylinder may thus be ignited. If the cylinder be made of glass, and a bit of ordinary cotton dipped in sulphuric ether be used, repeated flashes of light may be produced by successive combustions of ether vapor. The fumes of one combustion must be blown away before the next combustion is attempted.