378 PHYSIOLOGY CHAP. 



subject should have been investigated long before chemistry had 

 developed into an exact science. The earliest attempts of Van 

 Helmont to determine the nature and origin of the urinary con- 

 stituents date from the commencement of the seventeenth century. 

 Towards the middle of the same century, Brand, the alchemist- 

 physician of Hamburg, first obtained phosphorus from the urine, 

 while Kunkel, shortly after, described it more exactly. At the 

 beginning of the eighteenth century the famous Boerhaave made 

 the first analysis of urine, which is now only of historical interest, 

 but was thought marvellous at the time it was drawn up. 



The most important nitrogenous constituent of urine, which 

 received the name of urea, was first recognised by Hilaire Rouelle 

 (1775), the younger brother of that Gr. Eouelle who was Lavoisier's 

 teacher. But the actual discovery of urea was due to Cruikshank 

 (1797), who first obtained crystals of urea nitrate. Scheele and 

 Bergmann (1775-88) discovered the compound now termed i<rir 

 acid, in calculi of the bladder. Fourcroy and Vauquelin (1799) 

 made a more profound study of the chemistry of urine, and 

 particularly of the composition of the urinary calculi. The first 

 quantitative analysis of urine, by Berzelius (1809), is quoted in 

 the classical Text-book of Johannes Miiller. Comparison of this 

 with the analyses of modern times shows the enormous advances 

 made in the chemistry of the urine, owing to the labours of a long 

 succession of workers, between the beginning and the close of the 

 nineteenth century. 



We must limit ourselves to a summary of the characters and 

 composition of urine, referring for more minute details to recent 

 text-books of chemical physiology, and the special monographs 

 which deal with this subject from the standpoint of theoretical 

 and practical medicine. 



Human urine, when first given off from the bladder, is normally 

 a clear straw-coloured fluid, with a peculiar, somewhat aromatic 

 odour, saltish-bitter taste, acid reaction, and mean specific gravity 

 of 1020. 



The amount of urine excreted in the 24 hours varies greatly 

 under different conditions : the age and weight of the individual, 

 the diet, the quantity of fluid imbibed, the season or external 

 temperature, muscular rest or exercise, etc. In a normal, well- 

 nourished adult it may fluctuate, owing to these or other circum- 

 stances, from 1300 to 1600 grms. in man, from 900 to 1200 grins, 

 in woman. 



The specific gravity of the urine varies with the amount 

 secreted, and the content of solid substances dissolved in it. 



Under normal conditions the specific gravity, as measured by 

 the urinometer, varies between 1016 and 1025. To calculate with 

 approximate accuracy from these values the amount of solids dis- 

 solved in 1000 c.c. urine, it is only necessary to multiply the two 



