8 A MANUAL OF PHYSIOLOGY 



(3) To another portion add Milloris reagent ;* a white precipitate 

 comes down, which is turned reddish on boiling. If only traces of 

 protein are present, no precipitate is caused, but the liquid takes 

 on a red tinge. The reaction is due to tyrosin. It is given by all 

 aromatic substances which contain the group C G H 6 with at least one 

 H replaced by OH. 



(4) Adamkiewicz's reaction (Hopkins' s modification). To a small 

 quantity of the albumin solution add the same bulk of dilute gly- 

 oxylic acid.f Mix, and to the mixture add an equal volume of 

 strong pure sulphuric acid. A purple colour is obtained. The 

 substance in the protein molecule which gives the reaction is tryp- 

 tophane (p. 332). 



(5) The Formaldehyde Reaction. Add to the albumin solution a 

 few drops of a very dilute solution of formaldehyde (i : 2,500), and 

 then allow some strong (commercial) sulphuric acid to run from a 

 pipette into the bottom of the test-tube. A purple ring appears at 

 the surface of contact. This reaction depends on the presence of 

 tryptophane in the protein. 



Precipitation Reactions. 



(6) Acidify another portion strongly with acetic acid, and add a 

 few drops of a solution of potassium ferrocyanide. A white pre- 

 cipitate is obtained. Peptones do not give this reaction. 



(7) Heat a portion to 30 C. on a water-bath. Saturate with 

 crystals of ammonium sulphate ; the albumin is precipitated. Filter, 

 and test the nitrate for proteins by (2). None, or only slight traces, 

 will be found. The sodium hydroxide must be added in more than 

 sufficient quantity to decompose all the ammonium sulphate. It 

 will be best to add a piece of the solid hydroxide. Peptones are not 

 precipitated by ammonium sulphate, but all other proteins are. 



(8) Add alcohol to a small quantity of the solution. The protein 

 is precipitated. It can be redissolved at first, but rapidly becomes 

 insoluble. 



2. Special Reactions of Certain Proteins (i) Heat-Coagulable 

 Proteins : (a) Albumins. -(a) Heat a little of the solution of egg- 

 albumin in a test-tube ; it coagulates. With another sample deter- 

 mine the temperature of coagulation, first very slightly acidulating 

 with a 2 per cent, solution of acetic acid. 



To determine the Temperature of Coagulation. Support a beaker 

 by a ring which just grips it at the rim. Nearly fill the beaker with 

 water, and slide the ring on the stand till the lower part of the beaker 

 is immersed in a small water-bath (a tin can will do quite well). In 

 this beaker place a test-tube, and in the test-tube a thermometer, 

 both supported by rings or clamps attached to the same stand. Put 

 into the test-tube at least enough of the albumin solution to com- 

 pletely cover the bulb of the thermometer, and heat the bath, stirring 



* Millon's reagent consists of a mixture of the nitrates of mercury with 

 nitric acid in excess, and some nitrous acid. To make it, dissolve mercury 

 in its own weight of strong nitric acid, and add to the solution thus obtained 

 twice its volume of water. Let it stand for a short time, and then decant 

 the clear liquid, which is the reagent. 



f A solution containing glyoxylic acid in the requisite strength can be 

 prepared by treating half a litre of a saturated solution of oxalic acid with 

 40 grammes of 2 per cent, sodium amalgam in a tall cylinder. When all 

 the hydrogen has been evolved, the solution is filtered, and diluted with 

 twice its volume of water. Oxalic acid and sodium binoxalate are also 

 present in the solution. 



