more readily. One of the most important of these is urea, and by this we can show that the problem is capable of solution. The high physiological importance of this body is due to the fact that it contains the greatest part of the nitrogen introduced with the food, and removes it from the body. Its composition is very simple, its synthesis has been effected in many ways, and there appears to be no difficulty in explaining its formation in the animal organism. At first it was supposed that it is formed by direct oxidation of the albuminoids, but all attempts to prove this experimentally have failed. Later it was found that nitrogenous bodies, other than albuminoids, particularly the products of decomposition of the same, such as glycocol, asparagin, even ammonia, were converted into urea in the organism; and, as glycocol and ammonia each contain only one atom of nitrogen, while urea contains two, evidently by synthesis. Accordingly, two hypotheses were proposed to explain the formation of urea in the organism: the one assumed that by the oxidation of the nitrogenous bodies cyanic acid is first formed, which, combining with ammonia, forms ammonium cyanate, and is transformed into urea; the other, which is principally based on the experiments in which ammonium carbonate is introduced into the organism, assumed a separation of water from this salt, which would of course give m-ea. But neither of these hypotheses is tenable: for, on the one hand, no chemist has ever obtained cyanic acid by the oxidation of nitrogenous substances under the conditions which are found, or may at any rate be assumed, in the organism; on the other hand, the ammonium carbonate which is introduced with the food can not be resorbed as such, for it is decomposed by the acid juices of the stomach, neither can this substance be formed in the organism from carbonic acid and ammonia. From the facts which have been observed, however, a third hypothesis may be deduced which is more probable than either of the former. It has been shown that by the oxidation of nitrogenous organic bodies, particularly glycocol, leucine, and tyrosine, in alkaline solution and at a blood-heat, carbonic acid is always formed; also that, by the union of carbonic acid and ammonia in aqueous solution and in presence of the strongest bases, carbonic acid is produced. We must therefore assume that in the blood or in other parts of the body, wherever nitrogenous compounds are oxidized, carbonic acid is constantly formed. From this salt the formation of urea must take place by the elimination of water, a reaction which has long ago been effected by heating the salt to 140° with absolute alcohol. In the organism this elimination of water must evidently take place in a different manner, and especially at the temperature of the body. This may be effected in two ways: either the water is eliminated as such, or its elements are removed one after another by two different reactions. The latter assumption is the most probable, for it will make it readily perceptible that the reaction takes place in an aqueous solution. If it is asked in what manner the
