CH. XXXVII.] FORMATION OF UREA 555 



mated, and 1 gramme of urea subtracted for every 1-3 gramme of sodium chloride 

 formed. 



These two methods give nearly identical results ; the former is the easier to 

 perform, and the results are sufficiently accurate for ordinary purposes. 



A more accurate determination can be best made by the method introduced by 

 Morner and Sjoquist. The following reagents, etc., are wanted : (i.) A saturated 

 solution of barium chloride containing 5 per cent, of barium hydrate : (ii.) A mixture 

 of alcohol and ether in the proportion 2:1; (iii.) The apparatus, etc., necessary for 

 carrying out Kjeldahl's method of estimating nitrogen. 5 c.c. of urine are mixed 

 with 5 c.c. of the barium mixture, arid 100 c.c. of the ether-alcohol mixture. By 

 this means all nitrogenous substances except urea are precipitated. Twenty-four 

 hours later this is filtered off, and the precipitate is washed with 50 c.c. of the ether- 

 alcohol mixture. The washings are added to the filtrate, and a little magnesia is 

 added to drive off ammonia. The fluid is then evaporated down at 55 C. until its 

 volume is about 10 c.c., and the nitrogen in this estimated by Kjeldahl's method. 

 The nitrogen found is multiplied by 2'143, and the result is the amount of the urea. 



Kjeldahl's method of estimating nitrogen consists in boiling the material under 

 investigation with strong sulphuric acid. The nitrogen present is by this means 

 converted into ammonia. Excess of soda is then added, and the ammonia distilled 

 over into a known volume of standard acid. The amount of diminution of acidity 

 in the standard enables one to calculate the amount of ammonia, and thence the 

 amount of nitrogen. 



The quantity of urea is variable, the chief cause of variation 

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

 nitrogenous equilibrium, taking daily 100 grammes of proteid in his 

 food, the quantity of urea secreted daily is about 33 to 35 grammes 

 (500 grains). The normal percentage in human urine is 2 per cent. ; 

 but this also varies, because the concentration of the urine varies 

 considerably in health. In dogs it may be 10 per cent. The 

 excretion of urea is usually at a maximum three hours after a 

 meal, especially after a meal rich in proteids. The urea does not 

 come, however, direct from the food ; the food must be first assimi- 

 lated, and become part of the body, before it can break down to form 

 urea. Food increases the elimination of urea because it stimulates the 

 tissues to increased activity; their waste nitrogenous products are 

 converted into urea, which, passing into the blood, is directly excreted 

 by the kidneys. The greater the amount of proteid food given, the 

 more waste products do the tissues discharge from their protoplasm, 

 in order to make room for the new proteid which is built into its 

 substance. Eecent experiments by Chittenden and others have shown 

 that nitrogenous equilibrium can be maintained on a diet containing 

 only half the usual amount of proteid. In such people the excretion 

 of urea falls correspondingly, the other nitrogenous constitutents of 

 the urine remaining fairly constant. 



Muscular exercise has little immediate effect on the amount of 

 urea discharged. In very intense muscular work there is a slight 

 immediate increase of urea, but this is quite insignificant when com- 

 pared to the increase of work. This is strikingly different from what 

 occurs in the case of carbonic acid ; the more the muscles work, the 

 more carbonic acid do they send into the venous blood, which is 



