EXCRETION 



389 



its solutions in the form of long colourless needles, or four-sided 

 prisms with pyramidal ends. It can be prepared from urine as des- 

 cribed in the Practical Exercises (p. 4 2 3-) Urea can also be obtained 

 artificially by heating its isomer, ammonium cyanate (NH 4 O CN), 

 to TOO C. This reaction is of great historical interest, as it forms 

 the final step in Wohler's famous synthesis of urea, the first example 

 of a complex product of the activity of living matter being formed 

 from the ordinary materials of the laboratory. Heated in watery 

 solution in a sealed tube to 180 C., urea is entirely split up into 

 carbon dioxide and ammonia, a change which can also be brought 

 about, as already mentioned, by the action of micro-organisms. 

 Nitrous acid, hypochlorous acid, and salts of hypobromous acid carry 

 the decomposition still further, carbon dioxide, nitrogen, and water 

 being the products of their oxidizing action on urea. Thus: 

 CO. 2(NH 2 ) + sNaBrO = sNaBr + 2H 2 O + CO 2 + N 2 . This reaction 



FIG. 119. KREATIN. 



FIG. 1 20. KREATININ-ZINC-CHLORIDE. 



is the basis of the hypobromite method of estimating the quantity of 

 urea in urine (Practical Exercises, p. 424). 



Uric acid (C 5 H 4 N 4 O 3 ) exists in large amount in the urine of birds. 

 The excrement of serpents consists almost entirely of uric acid. In 

 man and mammals the quantity is comparatively small in health, but 

 is increased after a meal, particularly one containing substances rich 

 in nucleo-proteids, e.g., the thymus of the calf. 



The xanthin bases are a group of substances allied to uric acid, 

 and including, besides xanthin itself (C 5 H 4 N 4 O 2 ), hypoxanthin 

 (C 5 H 4 N 4 O), guanin (C 5 H 5 N 5 O), and other bodies. They exist in 

 very small amount in urine, but, like uric acid, are increased in 

 amount by the ingestion of nucleo-proteids. 



Hippuric acid (C 9 H 9 NOs) occurs in considerable quantity in the 

 urine of herbivora ; in the urine of carnivora and of man only in 

 traces ; in that of birds not at all. Its amount is much more 

 dependent on the presence of particular substances in the food than 

 that of the other organic constituents of urine. Anything which 



