466 PHYSIOLOGICAL CHEMISTRY. 



magnesium phosphates, which salts upon heating are decomposed into soluble 

 monophosphates and insoluble triphosphates, thus : 



4CaHP0 4 : : Ca(H 2 P0 4 ) 2 + Ca 3 (PO 4 ) 2 

 Dicalcium Monocalcium Tricalcium 



phosphate. phosphate. phosphate. 



To decide the nature of the precipitate a few drops (10 to 15) of 

 nitric acid are allowed to flow gently down the side of the tube into 

 the urine. The precipitate will readily disappear when caused by 

 phosphates, but will be permanent when albumin is present. 



Instead of heating, as above described, merely the upper half of the 

 urine, the total quantity of the urine (acidulated by a few drops of 

 acetic acid) may be heated, and the test-tube set aside for several hours 

 (after having added 10 to 15 drops of nitric acid), in order to allow 

 the albumin to subside, when it can be more distinctly seen and its 

 quantity noticed. 



b. Nitric acid test. A test-tube is filled to the depth of about half 

 an inch with colorless nitric acid, and an equal quantity of urine is 

 allowed to flow down the side of the test-tube in such a manner that 

 the specifically lighter urine forms a distinct and separate layer over 

 the nitric acid. (If the urine be allowed to flow from a pipette, as 

 shown in Fig. 43, the formation of the two strata is easily accom- 

 plished.) In case albumin is present, a white band or zone of vary- 

 ing thickness (according to the quantity of albumin present) appears 

 at the point of contact. 



If the urine be highly concentrated, a similar white zone is formed 

 between the acid and urine, due to the separation of insoluble acid 

 urates ; the difference between the separated u rates and albumin is 

 that the latter forms a sharply defined zone, whilst the urates diffuse 

 into the urine above. Moreover, the urates dissolve on the applica- 

 tion of heat. Finally, the separation of acid urates may be avoided 

 by diluting the urine with an equal volume of water and placing this 

 diluted urine upon the nitric acid. 



c. Picric acid test for albumin. This test has the advantage that 

 neither phosphates nor urates can be mistaken for albumin. It con- 

 sists in slowly dropping urine into a test-tube filled to about one- 

 fourth with a highly colored solution of picric acid in water. In the 

 presence of albumin a white cloud or sharply defined white turbidity 

 is formed, and on warming the liquid the albumin collects into balls 

 which rise to the surface of the liquid. 



d. Potassium ferrocyanide test. 5 to 10 c.c. of cold urine are acidu- 

 lated with 5 to 10 drops of acetic acid, and to the mixture are added 



