EXCRETION. 475 



children, relatively to their body-weight, is much greater than by adults; 

 Thus the quantity of urea excreted per kilogram of weight was found to 

 be, in a child, 0.8 grm.; in an adult only 0.4 grm. Regarded in this 

 way, too, the excretion of carbonic acid gives similar results, the pro- 

 portions in the child and adult being as 82 : 34. 



The quantity of urea does not necessarily increase and decrease with 

 that of the urine, though on the whole it would seem that whenever the 

 amount of urine is much augmented, the quantity of urea also is usually 

 increased; and it appears that the quantity of urea, as of urine, may be 

 especially increased by drinking large quantities of water. In various 

 diseases the quantity is reduced considerably below the healthy stan- 

 dard, while in other affections it is above it. 



Quantitative Estimation. There are two chief methods of estimating the 

 amount of urea in the urine. (1.) By decomposing it by means of an alkaline 

 solution of sodium hypobromite, or hypochlorite, and calculating the amount 

 in a measured quantity, by collecting and measuring the amount" of nitrogen 

 evolved under such circumstances. Urea contains nearly half its weight of 

 nitrogen, hence the amount of the gas collected may be taken as a measure of 

 the urea decomposed, remembering that 1 litre of nitrogen at the standard 

 temperature and pressure weighs 14 X .08936, or 1.251 grms. The percentage 

 of urea can thus be readily calculated from the volume of nitrogen evolved 

 from a measured quantity of the urine, but this calculation is avoided by 

 graduating the tube in which the nitrogen is collected with numbers which 

 indicate the corresponding percentage of urea. The reaction is CON 2 H 4 + 

 SNaBrO + 2NaHO = 3NaBr -f 3H 2 O -f Na a CO 3 + N 2 . (2. ) By precipitating the 

 urea by adding to a given amount of urine, freed from sulphates and phos- 

 phates, a standard solution of mercuric nitrate from a burette, until the whole 

 of it has been thrown down in an insoluble form ; then reading off the exact 

 amount of the mercuric nitrate solution, which it was necessary to use. As 

 the amount of urea which each cubic centimetre of the standard solution will 

 precipitate is previously known, it is easy to calculate the amount in the sam- 

 ple of urine taken. The precipitate which is formed was generally said to be 

 composed of mercuric oxide and urea. Some, however, now consider that it 

 is a mixture of mercuric nitrate itself and urea. 



Uric Acid (CsH^Os). Uric or lithic acid is rarely absent from 

 the urine of man or animals, though in the feline tribe it seems to be 

 sometimes entirely replaced by urea. 



Properties. Uric acid when pure is colorless, but when deposited 

 from the urine is yellowish-brown. It crystallizes in various forms, of 

 which the most common are smooth transparent, rhomboid plates, 

 diamond-shaped plates, hexagonal tables, etc. (fig. 301). It is odorless 

 and tasteless. It is very slightly soluble in cold water, and a little more 

 so in hot water, quite insoluble in alcohol and ether. It dissolves freely 

 in solution of the alkaline carbonates and other salts. 



