processes; for, if the organism is able by their use to keep its own temperature at a higher point than that of its immediate surroundings, and also to develop mechanical force, then, if not all, at least a majority of these processes, are accompanied by a rise of temperature. As is well known, however, the highest temperatures are obtained by combustion—that is, by the combination of other bodies with oxygen. Since oxygen is continually inhaled and consumed by animals during life, we are obliged to consider this as the source of heat and force. We have here a problem which is open to discussion, namely, whether the energy liberated by the combustion was originally contained in the oxygen or in the other substances. It appears as if the latter assumption was generally accepted; at least, statements are often met with, such as, for instance, that coal contains the heat of the sun which has been stored up during thousands of years. Although we can not, at present, with the means at our disposal, definitely solve this problem, it can at least be shown that the statement has little in its favor. The decomposition of carbonic acid by the influence of the light and heat of the sun is effected in such a manner that the carbon is employed in the formation of the compounds of which the plant is built up, while the oxygen escapes into the atmosphere., Now, we know that solids contain the least energy, because it must be supplied to them in the form of heat in order to convert them into the liquid or gaseous state, while, on the contrary, heat must be withdrawn from gases to condense them to liquids or solids. Oxygen is one of the most permanent gases, and must therefore possess an enormous amount of energy, while carbon, on the other hand, being one of the most difficultly diffusible and volatile bodies, can only contain a little energy. This makes it extremely probable that the force of the sun, taken up by the plants, is not stored in their bodies, but in the free oxygen of the atmosphere. Hence the latter is to be considered as the inexhaustible source of power on which man and animals draw, and in the carbon we possess a valuable aid for making this energy, contained in the oxygen, available.
After this digression, let us return to the chemical processes in the animal body. The production of heat and mechanical force, as well as the large quantity of oxygen consumed, indicates that the greater part of these processes must consist of oxidations, and accordingly we find that, among the products of excretion, carbonic acid and water form the principal parts, two bodies which can not be further oxidized. Besides these (to which sulphuric acid and phosphoric acid may be added), other substances are excreted in small quantity, which admit of further oxidation, but which have not suffered it in the organism. I will only mention urea, uric acid, and creatinine. All these compounds are products of chemical action, and the question is raised, How are they produced in the body? to what reactions do they owe their origin? to a single one, or to a whole series? Here we