It can thus be seen that neither matter nor energy can be created. All we can do is to manipulate certain types of matter in a way that liberates whatever energy had been in existence, in one form or another, since the beginning of time. All the energy that we had been using on earth until the advent of the atomic age had originally come from the sun. Coal, as already said, is a petrified plant that had stored up the energy of the sun in the form of chemical energy millions of years ago, before man made his appearance on the earth. Oil comes from organic matter that also had stored up light and heat from the sun in the form of chemical energy. Water power and wind power are also made possible by the sun’s heat, since all water would freeze and no winds would blow were it not for the sun’s heat energy keeping the waters flowing and the air moving, the latter by creating differences in the temperature of air masses. There are two forms of energy that we take advantage of which are not due directly to the sun’s radiations—gravitation and magnetism—but the only way we can utilize these is by employing energy derived from the sun’s heat. In harnessing Niagara, or in the building of great dams, we utilize the fall of the water because of gravitation. But as I have already pointed out, without the sun’s heat water could not flow. To produce electricity we begin with the chemical energy in coal or oil, which is first converted into heat energy, then to mechanical energy, and finally, through the agency of magnetism, into electrical energy. The radiations of the sun, of the giant stars millions of times larger than the sun, come from an entirely different source, the greatest source of energy in the universe, known as atomic or, more correctly, nuclear energy. But even here the energy comes as the result of the transformation of matter. The difference between nuclear energy and chemical energy is twofold. In chemical energy, such as the burning of coal, the matter lost in the process comes from the outer shell of the atoms, and the amount of matter lost is so small that it cannot be weighed directly by any human scale or other device. In nuclear energy, on the other hand, the matter lost by being transformed into energy comes from the nucleus, the heavy inner core, of the atom, and the amount of matter lost is millions of times greater than in coal, great enough to be weighed. An atom is the smallest unit of any of the elements of which the physical universe is constituted. Atoms are so small that if a drop of water were magnified to the size of the earth the atoms in the drop would be smaller than oranges.