388 PHYSIOLOGICAL CHEMISTRY. 



line hydrate. This action is analogous to the formation of the 

 soluble scale compounds of iron, where the precipitation of 

 ferric hydrate is also prevented hy tartaric or other organic 

 acids. 



Fehling's solution is very apt to decompose, and if not re- 

 cently made should be tested by boiling some of it alone and 

 some of it mixed with about three volumes of water in test- 

 tubes; if no precipitate occurs in either case, the fluid may 

 safely be used. 



This is done by heating about 10 c.c. of Fehling's solution in 

 a test-tube, arid adding drop by drop the suspected urine ; if 

 the latter contains larger quantities of sugar a yellow or red 

 precipitate of cuprous hydrate and oxide will be produced very 

 readily ; if but small quantities are present, an equal volume 

 of urine may be added to the solution, and the boiling repeated 

 several times before the reaction takes place. 



Botger's bismuth test consists in adding to a mixture of equal 

 volumes of urine and potassium (or sodium) hydrate solution 

 a few grains of subnitrate of bismuth and boiling for half a 

 minute. If sugar be present, a gray or dark brown, finally 

 black, precipitate of metallic bismuth is formed. If but very 

 little sugar is present, the undecomposed excess of bismuthic 

 nitrate mixes with the metallic bismuth, imparting to it a gray 

 color ; the test should then be repeated with a smaller amount 

 of the bismuth salt. 



If the urine contains hydrogen disulphide (sometimes pro- 

 duced by decomposition of certain urinary constituents), black 

 bismuthic sulphide will be formed, which may be mistaken for 

 metallic bismuth; albumin itself may be the cause of the forma- 

 tion of alkaline sulphides ; the previous complete separation of 

 albumin is therefore indispensable. 



The fermentation test is based upon the decomposition of sugar 

 by the action of yeast with generation of carbon dioxide. The 

 test is made by adding to about 50 or 75 c.c. of urine (contained 

 in a large test-tube or small flask) a few c.c. of ordinary baker's 

 or brewer's yeast. The vessel containing the urine is provided 

 with a perforated cork, through which is passed one limb of a 

 bent glass tube, long enough to reach nearly to the bottom of 

 the vessel, which should be completely filled with urine. If the 

 urine be acid, it should be rendered slightly alkaline by a little 



