Another form in which to present these relations is to make a distinction between _free_ energy and energy _at rest_. If we have a heat quantity the temperature of which is higher than that of the surrounding objects, it can be used to do work only until its temperature has dropped to that of the surrounding objects. Although energy in abundance is still present, there is no longer any energy _capable of change_, or _free_ energy. Since differences of temperature, like other differences of intensity, have a constant tendency to diminish, the amount of free energy on earth is constantly decreasing, and yet it is only this free energy that has value. For since all phenomena depend upon change of energy, and change of energy is possible only through free energy, _free energy is the condition of all phenomena_. =51. Electricity and Magnetism.= While the knowledge of heat energy goes back to the most ancient periods of civilization, electrical and magnetic energies are relatively young acquisitions. The highly developed technical application of both with the rich harvests they have yielded belongs exclusively to most recent times. Both these forms of energy, like those discussed above, are connected in the main with ponderable "matter," but in a much slighter and less regular measure. While it is not possible as yet to render any given body free of heat (although lately the absolute zero point has been considerably approximated), freedom from electrical and magnetic energy is the normal condition of most bodies. This is connected with the peculiarity that electrical and magnetic properties are decidedly bi-symmetrical or _polar_. This property is not found in any other form of energy, and can serve as the special scientific characteristic of electricity and magnetism. This peculiarity shows itself in the concepts of positive and negative magnetism, and positive and negative electricity, and is due to the fact that two equal opposite quantities of electricity or magnetism, when added together, do not produce double their value, but nullify each other.[G] [G] For the sake of the layman it must be observed that those "quantities" are not energy magnitudes but factors of the electrical and magnetic energies. Energy itself in its various forms is an _exclusively positive magnitude_, and the result of the additions of their various amounts is always the sum, never the difference, of their numerical values. By the negative sign is understood the energy _expended_ in contradistinction to the energy _received_. It is therefore nothing more than the indication of a mathematical operation.