Of the great stream of free energy poured out from the sun into cosmic space the earth receives an extremely small portion (corresponding to the bit of space it occupies in the heavenly sphere as seen from the sun), and the plants collect and store up only a very small fraction of this portion received by the earth. Measurements have shown that in most favorable circumstances a plant leaf changes only about 1/50 of the radiant energy it receives into chemical energy. If we consider that only a small part of the surface of the earth is covered with plants and that during the winter no energy from the sun is stored up at all, we perceive what infinite possibilities for development there still are in arresting and storing up free energy. The part stored up by the plants flows from these into the countless streams, brooks, and veins of the other organisms, to end finally as used-up energy, or energy at rest. This energy is at rest, it is true, only in relation to the earth's surface. We do not know whether the radiation from the earth, which at present amounts to about as much as the radiation from the sun to the earth, is in its turn somewhere utilized. While the free energy is poured out in such a stream in one direction, the ponderable substances of which the organisms are made up _circulate_ through plants and animals and back again. This is especially true of _carbon_, which is freed from its combination with oxygen, that is, from carbonic acid, by the sun energy transformed in the plants. While carbon serves to build up the plant body and represents its supply of chemical energy, the oxygen is returned to the air. These two substances are again chemically combined in the various organisms and the quantities of energy which were necessary for their decomposition are again available for the manifold functions of life. The product of the chemical combination, carbonic acid, returns to the air and is ready for renewed decomposition in the plants. Thus, the entire mechanism of life can be compared to a water-wheel. The free energy corresponds to the water, which must flow in one direction through the wheel in order to provide it with the necessary amount of work. The chemical elements of the organisms correspond to the wheel, which constantly turns in a circle as it transfers the energy of the falling water to the individual parts of the machine. =56. The Soul.= Our observations so far have shown the organisms to be extremely specialized individual instances of physico-chemical machines. Now we have to take into consideration a property which seems markedly to distinguish them from the lifeless machines, and which we have already encountered in the very beginning of our treatise.