COLOR-REACTIONS. 103 



teids it is a question whether it can be regarded as a true solu- 

 tion. 



Action on Polarised I/ight. All proteids are levorotatory 

 (see p. 89), but the degree of rotation varies considerably. The 

 following table gives the specific rotatory power of several of the 

 proteids : 



Proteid. Value of (d)d. 



Serum-albumin 56 to - 68 



Egg-albumin - 35.5 " - 38.08 



Lactalbumin - 36 " - 37 



Serum-globulin - 59.75 



Fibrinogen - 43 



Alkali-albumin - 62.2 



Syntonin (from myosin) 72 



Casein (dissolved in MgSO 4 solution) .... -80 



Various proteoses 70 to 80 



Color-reactions. Certain color-changes which take place 

 when proteids are treated with various reagents have been made 

 use of to detect their presence or absence. Although chemists 

 have determined the elements which go to make up proteids, they 

 have not as yet determined their constitution. They have ascer- 

 tained that proteids undergo changes or decomposition in the body, 

 as a result of which carbonic acid, water, and urea are finally 

 formed, and that there are various intermediate products, such as 

 leucin, creatin, uric acid, and ammonia. It has also been deter- 

 mined that by chemical means proteids can be decomposed into a 

 great variety of substances : some of these are ammonia, carbonic 

 acid, amins, leucin, and aromatic compounds. Of this last class, 

 the aromatic compounds, there are three groups: 1. The phenol 

 group, including tyrosin, phenol, and cresol ; 2. The phenyl group, 

 including phenylacetic and phenylpropionic acids ; and 3. The 

 indol group, in which are indol and skatol. It is upon this class 

 of substances, the aromatic compounds or radicles, that the color- 

 reactions of proteids depend. 



Xanthoproteic Reaction. When a solution of proteid, to which a 

 few drops of nitric acid have been added, is boiled, it becomes yel- 

 low, and if ammonia is added the yellow color changes to orange. 



Millon's Reaction. Proteids when heated with Millon's reagent 

 give a white precipitate which becomes brick red on cooling. This 

 reagent is prepared by dissolving mercury in nitric acid and add- 

 ing water. The precipitate which forms is allowed to settle, and 

 the fluid is the reagent. Very small amounts of proteids give the 

 red color without the precipitate. 



Piotrowski's or Biuret Reaction. When many proteids are mixed 

 with an excess of concentrated solution of sodium hydrate and 

 one or two drops of a dilute solution of cupric sulphate, a violet 

 color is produced which becomes deeper on boiling ; with peptones 

 and proteoses the color produced is rose red. Biuret is the substance 



