172 PRACTICAL PHYSIOLOGY 



phous masses or powders, but some of them, e.g. haemoglobin, are 

 easily obtainable as crystals, and of the others many can now be 

 made to crystallise. Thus egg-albumin can be made to crystallise by 

 allowing a solution of it, in which the solvent just barely dissolves the 

 albumin, to stand exposed to the air (Fig. 140). Slow evaporation of 

 the water takes place, in consequence of which the albumin is no longer 

 held in solution, and is precipitated as crystals (see Advanced Course). 

 III. Rotation. All proteids are laevo-rotatory. 



CHEMICAL PROPERTIES AND REACTIONS. 



I. Colour Reactions. These are very important as tests for proteids. 

 They are : 



(1) Biuret Reaction (Piotrowski's test). 



EXPERIMENT IV. Place a trace of a weak solution of CuSO 4 in a 

 test tube (this is best done by placing some of the solution in the 

 test tube and then pouring it out, sufficient of it remaining adherent 

 to the glass). Add to this about 5 c.c. of a weak solution of egg-white, 

 and then, drop by drop, a 20 per cent, solution of KOH until a violet 

 colour is produced. 



EXPERIMENT V. Repeat this experiment with a solution of peptone. 

 A rose-pink colour is developed. 



This violet or pink colour is given by all proteids, and it seems to 

 depend on the presence of a HCNO group. It is called the biuret 

 reaction, because biuret (the substance left after heating urea crystals 

 in a dry test tube) gives it (see "Urea"). The chemical equation of 

 the reaction is unknown. 



(2) Xanthoproteic Reaction. 



EXPERIMENT VI. To about 5 c.c. of a solution of egg-white add a 

 drop or two of HN0 3 (con.) ; a white precipitate results. Warm this, 

 and the white precipitate changes to a yellow curd. Cool under the 

 tap, and then add a few drops of strong ammonia, when the yellow will 

 change to a brilliant orange. 



(3) Millon's Reaction. 



EXPERIMENT VII. Millon's reagent consists of a solution of mer- 

 curous and mercuric nitrates in concentrated nitric acid. Add a few 

 drops of it to a solution of diluted egg-white ; a white coagulum results, 

 which on boiling changes into a brick-red curd. 



Both these reactions (viz. xanthoproteic and Millon's) seem to 

 depend on the presence of some hydroxyl derivative of benzene in the 



