900 



APPENDIX. 



Salts of uric acid. Of these the most important are the acid urates of sodium, 

 potassium and ammonium. The sodium salt crystallizes in many different forms- 

 [Fig. 239J, these not being characteristic, since they are almost the same for the 

 corresponding compounds of the other two bases. It is very insoluble in cold water 

 (1 in 1100 or 1200), more soluble in hot (1 in 125). It is the principal constituent 

 of several forms of urinary sediment, and constitutes a large part of many calculi ; 

 the excrement of snakes contains it largely. The potassium resembles tlie sodium 

 salt very closely, as also does the compound with ammonium ; the latter occurs gen- 

 erally in the sediment from alkaline urine. [Fig. 240.] 



[FIG. 239. 



[FiG. 240. 



FIG. 239. Urate of Soda, a a, from a gouty concretion ; 6 b, artificially prepared by adding 

 liq. sodse to the amorphous urate deposit.] 



FIG. 240. The Normal Deposit from Ammoniacal Urine, showing Crystals of Ammoniaco- 

 Magnesian Phosphate, Amorphous Phosphate of Lime, and Spheres of Urate of Ammonia.] 



Preparation. Usually from guano or snake's excrement. From guano by boil- 

 ing with caustic potash (1 part alkali to 20 of water) as long as ammonia is evolved. 

 In the filtrate a precipitate of acid urate of potassium is formed by passing a, 

 current of carbonic anhydride; this salt is then washed, dissolved in a caustic 

 potash, and decomposed by carefully pouring its solution into an excess of hydro- 

 chloric acid. 



The presence of uric acid is recognized by the following tests : The substance 

 having been examined microscopically, a portion is evaporated carefully to dryness 

 with one or two drops of nitric acid. The residue will, if uric acid is present, be 

 of a red color, which on the addition of ammonia turns to purple. This is the mu- 

 rexide test, and depends on the presence of alloxan and alloxantin in the residue. 

 Schiff 1 has given a delicate reaction for uric acid. The substance is dissolved in 

 sodic carbonate and dropped on paper moistened with a silver salt. If uric acid 

 be present a brown stain is formed, due to the reduction of the carbonate of silver. 

 An alkaline solution of uric acid can, like dextrose, reduce cupric sulphate, with 

 precipitation of the cuprous oxide. 



Uric acid resists very largely the action of even strong acids and alkalies, exhibit- 

 ing in this respect a marked difference from urea. It might therefore perhaps be 

 supposed that urea residues do not pre-exist in uric acid ; nevertheless by oxida- 

 tion uric acid does give rise not only to ordinary urea, but also, and at the same 

 time, to the compound ureas (ureides) spoken of above. Thus, by oxidation with 

 acids, 



Uric acid. Alloxan. Urea. 



C 5 H 4 N 4 3 + H 2 + - C 4 N 2 H A + CN 2 H 4 0. 



Now alloxan, as was stated above, is a compound urea, viz., mesoxalyl-urea, and 

 by hydration can be converted into mesoxalic acid and urea, thus : 



1 Ann. d. Chern. u. Pharm., Bd. cix. S. 65. 



