NUCLEIC ACIDS AND NUCLEOPROTEINS 135 



pose the copper compound continuing the addition until the precipitate becomes 

 uniformly black and then in small successive portions, testing a drop of the prod- 

 uct after each addition by bringing it into contact with a drop of lead acetate 

 on a filter paper. To the boiling fluid add acetic acid (in the case of the extract 

 of pig's spleen and other solutions containing guanine the acetic acid should be 

 replaced by dilute sulphuric) until the insoluble copper sulphide collects and filter 

 the hot fluid as quickly as possible. 



C. Treatment of the Filtrate from Pig's Spleen. When the filtrate from the 

 copper sulphide is cold make strongly alkaline with ammonia and precipitate the 

 purine compounds with a slight excess of ammoniacal silver nitrate. Filter, wash 

 thoroughly with cold water. Pierce the paper, wash the precipitate into a flask 

 with boiling water and decompose the silver precipitate with hydrochloric acid. 

 When enough acid has been added the silver chloride will settle as a heavy case- 

 ous precipitate leaving clear interstitial fluid. Filter and heat the filtrate to 

 boiling. Treat with an excess of ammonia (enough to make about 1-2 per cent). 



FIG. 39. GUANINE CHLORIDE. 

 (Reproduced from crystals furnished by Professor Walter Jones.) 



Allow to cool and filter off the guanine which precipitates. Wash the guanine 

 with i per cent ammonia and then suspend it in a little hot water and add a few 

 drops of 20 per cent sulphuric acid to dissolve it. At the boiling point add a little 

 animal charcoal, boil and filter. Make strongly alkaline with ammonia. Snow 

 white guanine is precipitated. Dissolve the precipitate in 20 volumes of boiling 



5 per cent hydrochloric acid. Upon cooling beautiful needle-shaped crystals of 

 guanine chloride separate. (See Fig. 39.) 



Evaporate the filtrate from the guanine to dryness on the water-bath to expel 

 ammonia. Moisten with hydrochloric acid and again evaporate. Treat the 

 residue with warm water. Does it dissolve almost completely indicating the 

 absence of xanthine and uric acid? Test a drop of the solution with picric acid. 

 If no precipitate is obtained adenine is absent. Evaporate to dryness, moisten 

 with alcohol and again evaporate. Dissolve the residue in about 30 parts of hot 



6 per cent nitric acid. On cooling, characteristic whetstone crystals of hypo- 

 xanthine nitrate form. ((See Fig. 40, page 136.) The xanthine nitric acid 

 color test should be practically negative on this product. 



