366 HANDBOOK OF PHYSIOLOGY. 



is exclusively used, less when the diet is mixed, and least of all with a 

 vegetable diet. As a rule, men excrete a larger quantity than women, 

 and persons in the middle periods of life a larger quantity than infants 

 or old people. The quantity of urea excreted by children, relatively to 

 their body-weight, is much greater than by adults. Thus the quantity 

 of urea excreted per kilogram of weight was, in a child, 0.8 grm. ; in 

 an adult only 0.4 grm. Regarded in this way, the excretion of carbonic 

 acid gives similar results, the proportions 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 standard, 

 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 calcu- 

 lating the amount in a measured quantity, by collecting and measuring 

 the amount of nitrogen evolved under such circumstances. Urea con- 

 tains nearly half its weight of nitrogen, hence the amount of the gas 

 collected may be taken as a measure of the urea decomposed. The per- 

 centage of urea can of course be readily calculated from the volume of 

 nitrogen evolved from a measured quantity of the urine, but this calcu- 

 lation 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 + 3NaBrO + 2NaI10^:3]S'aBr + 3H 2 + Na 2 C0 3 + N 2 . 

 (2.) By precipitating the urea by adding to a given amount of urine, 

 freed from sulphates and phosphates, 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 ni- 

 trate 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 sample of 

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

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

 is a mixture of mercuric nitrate itself and urea. 



(2.) Uric Acid (C 5 H 4 N 4 3 ). Uricorlithic 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 (Fig. 

 It is odorless and tasteless. It is slightly soluble in cold water, 



