326 FUNCTIONS OF NUTRITION. 



of color then increase, and its acidity returns; all these properties 

 becoming more strongly marked during the afternoon and evening. 

 Toward night it is again deeply colored and strongly acid, and its 

 specific gravity often 1028 or 1030. 



These variations are liable to modification from temporary causes. 

 The color, acidity, and specific gravity of the urine may be diminished 

 at any time by large draughts of liquid or the use of diuretic mineral 

 waters; or they may be increased by abstinence from drink or by 

 copious perspiration. Its acidity is also liable to vary from the use 

 of food, such as summer fruits or vegetables, containing salts of the 

 organic acids, namely, lactates, acetates, malates, and tartrates. These 

 salts, when introduced into the system, are replaced by carbonates of 

 the same bases and appear under that form in the urine, reducing for 

 the time its acidity, or even causing its alkalescence. 



It is evident, therefore, that when the specific gravity and acidity 

 of the urine are to be tested, it will not be sufficient to rely upon the 

 examination of a single specimen. Its normal variation in specific 

 gravity may reach the limits of 1015 as a minimum and 1030 as a 

 maximum ; but either of these would be unnatural if continued for 

 twenty-four hours. All the specimens of urine passed during the day 

 should therefore be collected and examined together. The mean specific 

 gravity thus obtained will represent its normal daily density. 



Its daily volume is also to be taken into account. The total amount 

 of solids discharged by the urine in health is from 50 to 60 grammes 

 per day; and this quantity is dissolved in about 1200 cubic centimetres 

 of water. This gives an average daily quantity and an average specific 

 gravity of the urine, as the measure of the excretory process during 

 twenty-four hours. 



Both the quantity of the urine and its mean specific gravity are 

 liable to vary in the same individual from day to day ; but when this 

 is due to physiological or temporary causes, the variations of quantity 

 and specific gravity are in inverse ratio to each other. Usually the 

 water of the urine is more than sufficient to hold all its solid matters 

 in solution ; and its proportion may therefore be lessened without 

 the production of turbidity or the formation of a deposit, the urine 

 merely becoming deeper in color, and of higher specific gravity. If 

 the quantity of drink be diminished, or if the exhalation from the 

 lungs and skin, or the intestinal discharges, be increased, a smaller 

 quantity of water will pass off by the kidneys ; and the urine will be 

 diminished in quantity, while its specific gravity is increased. The 

 urine is sometimes reduced in this way to 500 or 600 cubic centi- 

 metres per day, its mean specific gravity rising at the same time to 

 1030. On the other hand, if the fluid ingesta be unusually 'abundant, 

 or if the perspiration be diminished, the surplus water will pass off by 

 the kidneys; the amount of urine in twenty-four hours being .increased 

 to 1500 or 1600 cubic centimetres, and its mean specific gravity reduced 

 to 1020 or 1015. These changes depend simply on the fluctuating 



