ELEMENTAEY PHYSIOLOGICAL CHEMISTRY 



241 



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dispensable if we wish to ascertain whether any one of its constituents 

 is being excreted in normal amount, a mere determination of the per- 

 centage in an isolated sample being of very slight value. For accurate 

 work (e.g. in making observations in metabolism) the method employed 

 is to collect the total urine for the 24 hours in a graduated urine jar 

 fitted with a glass lid, and then to remove a measured 

 sample for analysis. The amount of urine is in- 

 creased by the imbibition of large quantities of liquid 

 and by certain drugs called diuretics which act on 

 the renal circulation ; it is diminished by excessive 

 sweating or diarrhoea, and by failure of the heart's 

 action. 



Specific Gravity. This is determined by a special 

 form of hydrometer a urinometer graduated so 

 that the zero mark 1000 corresponds to distilled 

 water (Fig. 149). 



EXPERIMENT I. Fill a urine testing glass with 

 urine and place the urinometer in it, and read off the 

 graduation which is on a level with the surface of the 

 urine. 



The average density varies between 1015 and 

 1025, but a highly concentrated urine e.g. after 

 severe sweating may reach 1035, or a very dilute 

 one e.g. after huge potations 1002, and still be 

 healthy. A specific gravity over 1030, however, 

 usually indicates the presence of sugar or the 

 existence of high fever, and one much below 1010 

 is suspicious of some renal trouble. 



It is important to remember that it is only dis- 

 solved substances which affect the density. 



Reaction. Healthy urine usually reacts acid to 

 litmus. 



EXPERIMENT II. Test the reaction of urine with 

 litmus. 



This acidity is not due to free acids but to acid salts. 



EXPERIMENT III. Add some urine to about 5 c.c. of a solution of 

 congo red in a test-tube, and the colour of the reagent 

 remains unchanged. Congo red is turned blue by free acids, 

 but not by acid salts (see Digestion, p. 220). The salt which pro- 

 duces the acid reaction in urine is acid phosphate of sodium 

 NaH 2 P0 4 . There is also a certain amount of the alkaline phosphate 

 Na 2 HP0 4 present, and under certain conditions, this becomes increased 



Q 



Fio. 149. The urino- 

 meter. 



