144 



ESSENTIALS OF CHEMICAL PHYSIOLOGY 



to free acid, as the uric and hippuric acids in the urine are combined 

 as urates and hippurates respectively. The acidity is due to acid salts, 

 especially acid sodium phosphate. In 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 such as tartaric, citric, malic, &c. 



FIG. 42. Urinometer floating in uriuo 

 in a testing glass. 



FIG. 43. Crystals of urea : , four-sided 

 prisaas ; b, indefinite crystals, such 

 as are usually formed from alcohol 

 solutions. 



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 good urinometer (see fig. 42). 



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 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 (taking an ordinary 



