DISEASES OF THE URINARY SYSTEM. 257 



According to our own investigations the average of a large 

 number of estimates of the specific gravity of the urine is 

 1041-2. 



The actual amount of urine passed per day is estimated 

 by Colin at 15—25 litres (44-033— 61-646 pints) ; this esti- 

 mate gives an average 20 litres a day. This amount is some- 

 what in excess of the average — 18-91 litres — found by us in 

 a number of cases. 



The quantity of water in 1,000 parts of urine has been 

 estimated at 880 — 930 (Von Bibra and others). 



The chief constituents of the urine are water, products of 

 nitrogenous decomposition, of which urea is the most im- 

 portant, hippuric and uric acids^ inorganic salts, extractives 

 and colouring matters. 



The actual amount of solids in 100 parts of urine of the 

 horse we have estimated to average 8-955. 



Urea. — Urea is by far the most important of the solid 

 constituents of the urine. It is derived from the disinte- 

 gration or metamorphosis of albuminous and albuminoid 

 substances of the food and of the tissues, and contains most 

 of the nitrogen which was originally incorporated with the 

 substances from which it is derived. 



Urea, or carbamide (CO(NH2)2) is isomeric with am- 

 monium cyanate (NH^CNO, or CH^N^O). It is excessively 

 soluble in water, from which it crystallizes in white silky 

 needles or transparent four-sided prisms, the ends of which 

 are often formed by one or two inclined planes. 



It has a bitter saline taste, is colourless, soluble in alcohol, 

 but insoluble in ether. 



Nitrate of urea may be obtained in considerable quantities 

 by the addition of excess of nitric acid to concentrated urine. 



This salt crystallizes in rhombic prisms or plates, having a 

 characteristic imbricated arrangement and mother-of-pearl 

 lustre. 



17 



