CHKMICAL IIASIS OF THK ANIMAL BODY. 159 



nitric acid containing nitrous fumes; if urea is present, nit: 

 and carbonic acid will be obtained. To a fourth part pun- nitric 

 acid in excess and a little mercury an- adiled, and the mixture is 

 warmed. In presence of urea a mlmi //!** mixture of gases (X and 

 CO S ) is given off. A tifth jiortioii is treated, after evaporation to 

 dryness, in the way above described for the application of the 

 biuret reaction, and a sixth part is tested with furfurol. 



Quantitative determination. The methods are based on some 

 of the reactions above described. They consist of (i) Precipita- 

 tion by a standardised solution of mercuric nitrate (Liebig). 

 (ii) Decomposition into carbonic acid and nitrogen by means of 

 sodium hypohromite, and measurement of the volume of nitrogen 

 (Knop-Hiifner). (iii) Conversion into carbonic acid and ammonia 

 by heating in a sealed tube with an ammoniacal solution of barium 

 chloride, and determination of the weight of barium carbonate 

 obtained. ( Hunsen.) 



Although simple in principle, the above methods, and especially 

 the first, require the careful observance of certain precautions to 

 ensure accuracy. The needful precautions have recently been 

 most assiduously investigated, more particularly by Pttiiger and 

 his pupils, and of these and of the application of the methods a 

 full account is given in Neubauer and Vogel's exhaustive work /'/ 

 '//>' des ffarn*. 



The determination of the total nitrogen in urine is also of great 

 importance, and is now usually carried out by Kjeldahl's method. 1 

 This consists in converting all the nitrogen of a measured portion 

 of urine int.. ammonia by boiling with fuming sulphuric acid and 

 the sul)se.|uent addition of potassium permanganate. The am- 

 monia is then expelled from the acid solution by distillation with 

 an excess of caustic soda or potash, the ammonia l.eing received 

 into a measured volume of standardised acid, whose diminution of 

 acidity due to the absorption of ammonia is finally determined by 

 titration with standard alkali. 



The synthesis of urea by molecular transformation of ammonium 

 cyanate indicates an undoubtedlyclo-,. ivliitimishipof un-a to cyanic 

 acid, and then- an- other reactions which enforce the same idea. 

 Thus by the union of water with cyanamide, which is readily 

 affected by treatment with 50 p.c. sulphuric acid, urea is obtained: 

 ON. N 1 1 . -f H,O = (NH,), CO. It is further stated that when 

 potassium cyanate and acid potassium tartrate an dissolved in 

 water and the mixture is kept for some time, a not inconsiderable 

 amount of urea is formed along with some carbonic acid, 2 thus 

 affording experimental support of Salk\\ki's view 3 that urea 



1 /.I. f. nnnl. <'h,m. It.l xxtl. (188.1). S. 366. 

 3 Hoppe-S.-vl.-r. Pkytie < - *09. 



Zt. f. phyiiol. L'hem. Bd. I. (1877), 8. 41. 



