302 THE MICROSCOPIST. 



pends largely upon the amount taken in as food, but may 

 be stated generally as from 400 to 500 grains a day, or 

 3J grains per pound weight of the body. The specific 

 gravity of the urine usually gives an indication of the 

 quantity of urea excreted, since it is about one-third of 

 the amount of solid matter. 



If urea be suspected in excess, a drop of urine (concen- 

 trated and cold) may be put on a slide and a drop of nitric 

 acid added. On covering with thin glass and placing 

 under J-inch objective, the characteristic rhomboidal crys- 

 tals of nitrate of urea will be seen (Plate XXVI, Fig. 

 240). 



Volumetric analysis is the best means of ascertaining 

 the quantity of urea as of other chemical ingredients, but 

 this falls rather within the province of the professional 

 chemist than of the microscopist. The practitioner may 

 estimate approximately by weighing the crystals of nitrate 

 of urea formed by adding nitric acid to double the quan- 

 tity of urine \vhich has been concentrated to half its bulk. 



CHLORIDES. 



The chlorides are always present in normal urine. They 

 are diminished, and sometimes nearly suppressed, in sev- 

 eral febrile diseases, especially in pneumonia. The quan- 

 tity may be roughly estimated by acidulating the urine 

 with a few drops of nitric acid, and then adding a strong 

 solution of nitrate of silver. The density or abundance 

 of the precipitate, as compared with a sample of normal 

 urine, indicates the quantity; or the precipitate maybe 

 weighed after being dried and fused in a porcelain capsule. 

 Albumen, if present, must be separated before testing for 

 chlorides, as it is also thrown down by nitrate of silver. 



