APPENDIX TO CHAPTER II w-AMlNO-ACIDS 139 



must be halved. The free acid is slightly soluble in cold water (0-15 

 per cent, at 18 according to Siegfried) and readily soluble in hot 

 water. The melting point depends greatly on the rate of heating ; 

 after crystallisation from dilute acetone Barger and Ewins found 

 2 35-236 (uncorr.). Hunter gives 231-232 (corn), Jaffe 212-213, 

 Siegfried 229. Hunter obtained the acid in slender, beautifully iri- 

 descent needles or tetragonal prisms. With sodium p-diazobenzene 

 sulphate it gives the red coloration of histidine. The acid is pre- 

 cipitated from solution by silver nitrate ; the precipitate dissolves in 

 excess of ammonia and in nitric acid. 



The barium salt, (C 6 H 5 O 2 N 2 ) 2 Ba . 8H 2 O, crystallises in needles and 

 loses 6H 2 O at 100 and the rest at 150. 



The nitrate, C 6 H 6 O 2 N 2- HNO 8 , is the most characteristic salt. It 

 is sparingly soluble in dilute nitric acid and crystallises in small 

 sickle-shaped plates frequently united to cross- or rosette-shaped aggre- 

 gates (figured by Hunter, p. 541); m.p. 198 with explosive decom- 

 position (Barger and Ewins). 



The picrate, C 6 H 6 O 2 N 2 . C 6 H 3 O 7 N 3 , forms golden yellow prisms; 

 m.p. 213-214, 224-225 (corr.). 



The picrolonate, C 6 H 6 O 2 N 2 . C 10 H 8 O 5 N 4 , crystallises from dilute 

 alcohol ; m.p. 268 (corr.). 



The phosphotungstate forms small rectangular plates from dilute 

 acetone or from hot water. 



Kynurenic Acid. 



To obtain kynurenic acid, Kretschy [1881] fed a dog of 34 

 kilos, weight daily with I kilo, of horse meat, 70 grm. of bread and 

 I litre of water. At first the daily production of the acid was cri 

 grm. but after I month O'8 grm. The best method, however, is to 

 give tryptophane by the mouth. The urine is acidified and the pre- 

 cipitate formed in twenty-four hours is filtered off and purified by 

 dissolving in ammonia, acidifying slightly with acetic acid and leaving 

 for twenty-four hours to allow a brown impurity to precipitate. After 

 filtration the solution is acidified with 4 per cent, hydrochloric acid. 

 Adherent uric acid may be removed by Hopkins's method and the 

 kynurenic acid may be finally recrystallised from 800 parts of boiling 

 alcohol (Homer [1913]). The pure acid forms long glistening 

 needles, of the formula C 10 H 7 O 3 N, H 2 O. The water of crystallisation 

 is given off at 140-145. The highest melting point obtained by 

 Miss Homer was 288-289 (uncorr.). The acid is practically insoluble 

 in cold water and 100 parts of boiling water only dissolve 0*09 parts ; 



