The heat used in warming a house is usually obtained by burning coal or wood. Coal is believed to be formed from the remains of plants that grew in former geologic times. These plants grew through the help of the radiant energy of the sun. The following are transformations of energy that have occurred: The radiant energy of sunlight was transformed into the _chemical_ energy of the plants. This remained as chemical energy while the plants were being converted into coal, was mined, brought to the stove or furnace and burned. The burning transformed the chemical energy into heat energy in which form we use it for warming rooms. Take the energy used in running a street car whose electrical energy comes from a waterfall. The energy of the car itself is mechanical. Its motor, however, receives electrical energy and transforms it into mechanical. This electrical energy comes along a wire from a dynamo at the waterfall, where water-wheels and generators transform into electrical energy the mechanical energy of the falling water. The water obtained its energy of position by being evaporated by the heat of the radiant energy of the sun. The vapor rising into the air is condensed into clouds and rain, and falling on the mountain side, has, from its elevated position, potential energy. The order of transformation, therefore, is in this case, radiant, heat, mechanical, electrical, and mechanical. Can you trace the energy from the sun step by step to the energy you are using in reading this page? =112. Forms of Energy.=--A steam-engine attached to a train of cars employs its energy in setting the cars in motion, _i.e._, in giving them kinetic energy and in overcoming resistance to motion. But what is the source of the energy of the engine? It is found in the coal which it carries in its tender. But of what kind? Surely not kinetic, as no motion is seen. It is therefore potential. What is the source of the energy of the coal? This question leads us back to the time of the formation of coal beds, when plants grew in the sunlight and stored up the energy of the sun's heat and light as _chemical_ energy. The sun's light brings to the earth the energy of the sun, that central storehouse of energy, which has supplied nearly all the available energy upon the earth. Five _forms_ of energy are known, viz., mechanical, heat, electrical, radiant, and chemical. =113. Energy Recognized by its Effects.=--Like force, energy is invisible and we are aware of the forms only by the effects produced by it. We recognize _heat_ by _warming_, by expansion, by pressure. We recognize _light_ by _warming_, by its affecting vision. We recognize _electrical_ energy by its heat, light, motion, or magnetic effect. We recognize _mechanical_ energy by the _motion_ that it produces. We recognize _chemical_ energy by knowing that the source of energy does not belong to any of the foregoing.