5 20 



EXCRETION 



are closed by glass discs secured by brass screw caps. The glass discs 

 must be slid on, so as to exclude all air. The tube having been intro- 

 duced between the polarizer and analyzer, the sharp vertical line which 

 indicates the division between the two half -fields is focussed with the 

 eye-piece, and then the analyzer is rotated till the two halves are again 

 equally shadowed. The angle of rotation, a, is read off on the graduated 

 arc, which is provided with a vernier. 



Pentoses reduce Fehling's solution, but do not give the yeast test. 

 They give the following characteristic tests, which may be performed 

 with gum arabic, a substance containing arabinose, one of the pentoses: 



(i) Phloroglucin Reaction. Warm in a test-tube some pure concen- 

 trated hydrochloric acid to which an equal volume of distilled water 

 has been added. Add phloroglucin until a little remains undissolved. 



Fig. 197. MitsoherlicVs Polarimeter. (Half -shadow instrument.) (Simple f or .n.) 



Add a small quantity of gum arabic, and keep the test-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-salts 

 are present the sulphur will sink to the bottom. If there are no bile- 



