THE PRECURSORS OF UREA 669 



ammonium carbamate is produced. It has been found, however, 

 that when solutions of ammonia (NH 4 OH) are mixed with 

 soluble carbonates, little or no carbamate but carbonate is pro- 

 duced, unless the ammonia solution be in great excess. Ammonium 

 carbamate is also produced when proteid, mono-ammo acids, 

 tartaric or oxalic acid, are oxidised according to the method of 

 Hofmeister with a permanganate in the presence of ammonia 

 solution. Ammonium carbamate can be converted into urea by 

 heating it to 135 C. Drechsel carried out the conversion at a 

 low temperature by passing an alternating electric current through 

 its solution ; this produces an alternating oxidation and reduction 

 according to the following formulae 



NH 4 O . CO . NH 2 + O = NH 2 O . CO . NH 9 + H 9 O. 

 NH 2 O . CO . NH 2 + H 2 = NH 2 . CO . NH 2 +^H 2 O^ 



There is no necessary antagonism between this view of the origin 

 of urea and that of Hofmeister. Ammonium carbamate might arise 

 by a process of oxidative synthesis, and this be subsequently 

 dehydrated into urea. 



There can be little doubt that carbamates are a constituent of 

 the normal body ; but all the quantitative estimations made are 

 now known to be unreliable because Drechsel' s method of esti- 

 mation is open to the objection that carbamate may be formed 

 during the manipulations. Macleod and Haskins have lately 

 published what promises to be an accurate method of estimation, 

 but it has not yet been applied to the elucidation of this subject. 

 Carbamates have been found in traces in the blood serum and 

 urine of normal dogs, and in considerable quantities in the alkaline 

 urine of the horse. Large quantities have been found in the urine 

 of dogs which had been fed on lime until their urine was alkaline 

 to litmus. Apart from the theoretical possibility. that ammonium 

 carbamate could be a precursor of urea, the only experimental 

 evidence which supports it is that afforded by Pawlow, 

 Massen, Hahn, and Nencki. They showed that, after an Eck's 

 fistula, an anastomosis between the portal vein and inferior vena 

 cava, had been established in dogs, and especially if the hepatic 

 artery was tied as well, large quantities of carbamate were present 

 in the urine and the urea was reduced. 



(4) Schmiedeberg' s view ; Origin of Urea from Ammonium Car- 

 bonate. It has been shown that ammonium carbonate can be 



