SPECIFIC GRAVITY OF URIXE. 449 



stances, presents the acid reaction and other qualities of 

 the urine of Carnivora, its ordinary alkalinity, being re- 

 stored only on the substitution of a vegetable for the animal 

 diet (Bernard). Human urine is not usually rendered 

 alkaline by vegetable diet, but it becomes so after the free 

 use of alkaline medicines, or of the alkaline salts with car- 

 bonic or vegetable acids ; for these latter are changed into 

 alkaline carbonates previous to elimination by the kidneys. 

 Except in these cases, it is very rarely alkaline, unless 

 ammonia has been developed in it by decomposition com- 

 mencing before it is evacuated from the bladder. 



The average specific gravity of the human urine is about 

 IO2O. Probably no other animal fluid presents so many 

 varieties in density within twenty-four hours as the urine 

 does ; for the relative quantity of water and of solid 

 constituents of which it is composed is materially influ- 

 enced by the condition and occupation of the body during 

 the time at which it is secreted, by the length of time 

 which has elapsed since the last meal, and by several 

 other accidental circumstances. The existence of these 

 causes of difference in the composition of the urine has. 

 led to the secretion being described under the three 

 heads of urina sannuinis, urina potus, and urina cibi. The 

 first of these names signifies the urine, or that part of it 

 which is secreted from the blood at times in which neither 

 food nor drink has been recently taken, and is applied 

 especially to the urine which is evacuated in the morning 

 before breakfast. The urina potus indicates the urine 

 secreted shortly after the introduction of any considerable 

 quantity of fluid into the body : and the urina cibi the por- 

 tions secreted during the period immediately succeeding a 

 meal of solid food. The last kind contains a larger quantity 

 of solid matter than either of the others; the first or 

 second, being largely diluted with water, possesses a com- 

 paratively low specific gravity. Of these three kinds, the 

 morning urine is the best calculated for analysis, since it 

 represents the simple secretion unmixed with the elements. 



G G 



