PRACTICAL EXERCISES 



491 



tube in a water-bath at 100 C. The solution becomes cherry-red, 

 and a precipitate gradually separates, which may be dissolved in 

 amyl alcohol. The solution shows with the spectroscope a band 

 between D and E. 



(2) Orcin Reaction. Use orcin instead of phloroglucin in (i). 

 The solution becomes reddish-blue on warming, and shows a band 

 between C and D, near D. The colour quickly changes from violet 

 to blue, red, and finally green. A bluish-green precipitate separates, 

 which is soluble in amyl-alcohol. Glycuronic acid gives all the above 

 reactions of pentoses. 



Bile-Salts (Hay's Test). Put a little finely-divided sulphur, in 

 the form of flowers of sulphur, on the top of a glass of urine. If bile- 



FIG. 182. MITSCHERLICH'S POLARIMETER. 

 (Half-Shadow Instrument.) (Simple Form.) 



salts are present the sulphur will sink to the bottom. If there are 

 no bile-salts it will float on the top. The difference is due to an altera- 

 tion in the surface tension of the urine produced by the bile-salts. 

 We must exclude the presence of acetic acid, alcohol, ether, chloro- 

 form, turpentine, benzine and its derivatives, phenol and its deriva- 

 tives, anilin and soaps, all of which also cause such an alteration in 

 the surface tension of urine that the sulphur sinks to the bottom. 

 The urine should be fresh, and if it has to be kept it should be pre- 

 served from decomposition by cyanide of mercury, which does not 

 alter the surface tension. The reaction has the great advantage 

 over other tests of being easily carried out at the bedside. 



