PROBLEMS OF PHYSIOLOGICAL CHEMISTRY. 521 



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 phys- 

 iological importance of this body is due to the fact that it contains 

 the greatest part of the nitrogen introduced with the food, and re- 

 moves 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 gly- 

 cocol 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 cya- 

 nate, 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 niti'ogenous 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 organ- 

 ism 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, car- 

 bonic 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 com- 

 pounds 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 



