90 CHEMICAL PHYSIOLOGY. 



14. Erdmann's Float consists of a glass vessel loaded with 

 mercury, so that it will float vertically. It is used to facilitate 

 the reading off of the burette. It has a horizontal line engraved 

 round its middle, and must be of such a width as to allow it just 

 to float freely in the burette. Read off the mark on the burette 

 which coincides with the ring on the float. (Fig. 20.) 



15. The Lime, Magnesia, Iron, and other inorganic urinary 

 constituents are comparatively unimportant, and have 110 known 

 clinical significance. 



LESSON XV. 

 ORGANIC CONSTITUENTS OF THE URINE. 



1. Urea (CON 2 H 4 ) is the most important organic constituent 

 in urine, and is the chief end-product of the oxidation of the 

 nitrogenous constituents of the tissues and food. It crystallises 

 in silken four-sided prisms, with obliquely cut ends (rhombic 

 system), and when rapidly crystallised, in delicate white needles. 

 It has no effect on litmus ; odourless, weak cool-bitter taste, like 

 saltpetre. It is very soluble in water, alcohol, and almost 

 insoluble in ether. It is an isomer of ammonium cyanate. It 



{-VTTT 

 NE 



Urea represents the final stage of the metamorphosis of albu- 

 minous substances within the body. More than nine-tenths of 

 all the N. taken in is excreted in the form of urea. 



2. Preparation. Take 20 cc. of fresh filtered human urine, add 

 20 cc. of baryta mixture Lesson XVI., 2 (c.) to precipitate 

 the phosphates. Filter, evaporate the filtrate to dryness in an 

 evaporating chamber, and extract the residue with boiling 

 alcohol. Filter off the alcoholic solution, place some of it on 

 a slide, and allow the crystals of urea, usually long, fine, 

 transparent needles, to separate out. Examine them micro- 

 scopically. 



3. Combinations. Urea combines with acids, bases, and salts. 

 Evaporate human urine to one-sixth its bulk, and divide the 



