GASTRIC DIGESTION. 45 



on standing or gently warming; on supersaturating with 

 caustic soda this color changes to orange (xanthoproteic 

 reaction). 



4. Acidify the solution with a few drops of acetic acid and 

 then add some potassium ferrocyanide solution: marked 

 cloudiness which disappears on heating (often not com- 

 pletely). 



5. Add to a portion of the solution half its volume of 

 caustic soda and then, drop by drop, a dilute solution 

 of copper sulphate. The copper hydroxide, which first 

 precipitates, dissolves on shaking with a purplish-violet color: 

 biuret reaction. An excess of copper sulphate changes the 

 color of the solution to a blue violet. The biuret reaction is 

 not characteristic of albumoses, as albumin also gives it. 



A part of the albumose solution is then diluted to ten 

 times its volume (0.5 per cent.). 



1. Biuret Reaction, Posner's Modification. Float a dilute 

 solution of copper sulphate on the diluted albumose solution, 

 containing half its volume of sodium hydroxide solution (so 

 that the copper sulphate solution runs down the wall of the 

 tube, held in an inclined position, and the fluids do not mix). 

 The characteristic color develops at the surface of contact of 

 the two fluids or proceeds from this. Recommended for 

 dilute solutions. With such solutions we may also use to 

 advantage an ammoniacal solution of copper sulphate or 

 Fehling's solution. 



2. Add to small portions of the solution (1) mercuric 

 chloride solution, (2) a solution of tannin, (3) some drops of 

 hydrochloric and phosphotungstic acids: insoluble precipi- 

 tates. 



3. Add to a small portion of the solution a few drops of 

 Millon's reagent and heat: red precipitate. 



