EXCRETION OF MATTER FROM THE BODY 395 



the degree of acidity varies with the concentration. But the 

 acidity of the urine may also be varied by different con- 

 ditions. It is increased when there is an increased oxidation 

 of proteids, for the sulphuric acid and phosphoric acid thus 

 formed have to be neutralised by combining with the 

 alkalies. It is decreased by taking alkalies, or by taking 

 such vegetable salts as the citrate, malate or tartrate of 

 soda, because these are oxidised in the body and excreted 

 as the carbonates. The NaH 2 P0 4 may then be changed 

 to Na 2 HPO 4 . 



Urine is normally transparent; but when it has stood for 

 a few hours, a cloud of a mucin-like substance is seen floating 

 in it. When it is alkaline it is turbid from the separation 

 of a white deposit of phosphates of lime and magnesia. In 

 the alkaline urine of herbivora the white deposit is chiefly 

 composed of carbonate of lime. 



A brick-red deposit of urates tends to fall as the urine 

 cools when it is concentrated and very acid. 



The smell of urine is characteristic, and it may be 

 modified by the ingestion of many different substances. 

 * 



II. Composition. 



Since the relative amounts of water and solids vary 

 within such wide limits, the percentage composition of urine 

 is of little moment. Under average conditions the water 

 constitutes about 96 per cent., and the solids about 4 per 

 cent. Of these solids, rather more than half are organic, 

 rather less than half are inorganic. Since water and solids 

 are derived from the water and solids taken by the indi- 

 vidual, the amounts excreted depend upon the amounts 

 taken, and must be considered in connection with them. 

 Thus if a man takes little fluid, he will pass little water in 

 the urine. If he takes little food, a small quantity of solids 

 will be excreted by the kidneys. Since excretion and inges- 

 tion must be studied in relationship to one another, it is 

 convenient to compare them during a definite period of 

 time, and the natural division into days of twenty-four 

 hours is generally adopted. 



Under ordinary conditions the amount of solid food taken 



