552 



THE URINE 



[CH. XXXVII. 



certain circumstances the urine becomes less acid and even alkaline ; 

 the most important of these are as follows : 



1. During digestion. Here there is a formation of free acid in 

 the stomach, and a corresponding liberation of bases in the blood, 

 which, passing into the urine, diminish its acidity, or even render it 

 alkaline. This is called the alkaline tide ; the opposite condition, the 

 acid tide, occurs after a fast for instance, before breakfast. 



2. In herbivorous animals and vegetarians. The food here con- 

 tains excess of alkaline salts of acids like tartaric, citric, malic, etc. 

 These acids are oxidised into carbonates, which, passing into the urine, 

 give it an alkaline reaction. 



Specific Gravity. This should be taken in a sample of the 

 twenty-four hours' urine with a urinometer. 



The specific gravity varies inversely as the quantity of urine 

 passed under normal conditions from 1015 to 1025. A specific 

 gravity below 1010 should excite suspicion of hydruria; one over 

 1030, of a febrile condition, or of diabetes, a disease in which it may 

 rise to 1050. The specific gravity has, however, been known to sink 

 as low as 1002 (after large potations, urina potus), or to rise as high 

 as 1035 (after great sweating) in perfectly healthy persons. 



Composition. The following table gives the average amounts of 

 the urinary constituents passed by a man in the twenty-four hours : 



The most abundant constituents of the urine are water, urea, and 

 sodium chloride. In the foregoing table one must not be misled by 

 seeing the names of the acids and metals separated. The acids and 

 the bases are combined to form salts, such as urates, chlorides, 

 sulphates, phosphates, etc. 



Urea. 



Urea, or Carbamide, CO(NH 2 ) 2 , is isomeric (that is, has the same 

 empirical, but not the same structural formula) with ammonium 

 cyanate (NHJ CNO, from which it was first prepared synthetically 



