Other nitrogenous residues are contained in the urine of animals, and several other excretory products may be formed. But in all these cases we can easily find subsidiary energy-transformations effected by bacteria, as in the above scheme. This, then, is the positive, or direct half, of that reversible Carnot cycle with which we are comparing life. In it energy falls in potential (or intensity, or level), and in this fall of potential transformations are produced--exhibit themselves, is perhaps a better way of putting it. We will consider these transformations later; in the meantime it should be noted that in this fall of potential is a degradation of chemical energy. Compounds, carbon dioxide, water, and nitrate are produced which are chemically inert. It is no use to say that carbon dioxide may react with (say) glowing magnesium, water with metallic sodium, and nitrate with (say) glowing carbon. A condition of chemical equilibrium would result from purely inorganic becoming on our earth in which there was no metallic sodium or magnesium or incandescent carbon; in which the metals would become inert oxides, and the carbon would become dioxide. The formation of these compounds represents a limit to energy-transformations. Note also that all these energy-transformations are conservative; the total quantity remains unchanged throughout, and is the same at the end as at the beginning. But _entropy has been augmented: unavailable energy has increased at the expense of available energy_. Consider now the indirect, or reversed, Carnot cycle. We begin with the inert matter, resulting from the metabolism of the animal, carbon dioxide, water, nitrate, and a few more mineral substances. We have the energy of solar radiation. By virtue of the living chlorophyll plastid in the cells of the green plant, this solar radiation uses the carbon dioxide and water as raw materials in the elaboration of starch. At the same time it absorbs nitrate, with some other inert mineral substances from the soil, and takes these into its tissues. The starch formed in the chlorophyll is converted into soluble sugar, which circulates through the vessels of the plant and is associated with the nitrogenous salt in the elaboration of proteid. Proteid, oils, fats and resins, and to a greater extent carbohydrates, are thus built up by the plant and _accumulate_, for mechanical work is not done by it, nor is heat dissipated--or at least these processes occur to an insignificant extent. } Metabolism --------------> are synthesised to-- Carbon dioxide } of the { proteid Water } green { fat Nitrate } plant { carbohydrate Chemical energy at --------------> Chemical energy low potential Work done on at high potential. the system