110 



ESSENTIALS OF CHEMICAL PHYSIOLOGY 



These crystals may be readily obtained in an impure form by 

 adding excess of the respective acids to urine which has been con- 

 centrated to a third or a quarter of its bulk.^ 



Under the influence of an organised ferment, the torula or micro- 

 coccus ureae, which grows readily in stale urine, urea takes up water, 

 and is converted into ammonium carbonate [CON2H4 + 211,0= 

 (NH4)2C03]. Hence the ammoniacal odour of putrid urine. 



By means of nitrous acid, urea is broken up into carbonic acid, 

 water, and nitrogen, CON2H4 + 2HN02=C02 + 3H20 + 2N2. This 

 may be used as a test for urea. Add fuming nitric acid (i.e. nitric acid 

 containing nitrous acid in solution) to a solution of urea, or to urine ; 

 an abundant evolution of gas bubbles takes place. 



Fig. 41. — «, nitrate ; h, oxalate of urea. 



Hypobromite of soda decomposes urea in the following way :- 

 CON2H4 + 3NaBrO = CO2 + N2 + 2H2O + 3NaBr 



[urea] 



[sodium] [carbonic [nitrogen] 

 hypobromite] acid] 



[water] 



[sodium 

 bromide] 



This reaction is important, for on it one of the readiest methods for 

 estimating urea depends. There have been various pieces of appa- 

 ratus invented for rendering the analysis easy ; but the one described 

 in the practical exercise at the head of this lesson appears to be the 

 best. If the experiment is performed as directed, nitrogen is the 

 only gas that comes off, the carbonic acid being absorbed by excess 

 of soda. The amount of nitrogen is a measure of the amount of 

 urea. 



The quantity of urea excreted is somewhat variable, the chief cause 

 of variation being the amount of proteid food ingested. In a man in a 



• The preparation of urea nitrate and urea oxalate is postponed to the next . 

 lesson, when other microscopic crystals will also be under examination. 



